A review on controlling the formation of intermetallic compounds in Ti/Al dissimilar metal laser weld seams

A review on controlling the formation of intermetallic compounds in Ti/Al dissimilar metal laser weld seams | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A review on controlling the formation of intermetallic compounds in Ti/Al dissimilar metal laser weld seams Zhi-Ming Zheng, Jia Zhang, Ya-Zhe Xing This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6054738/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract Titanium alloys and aluminum alloys are two lightweight alloys with a wide range of applications in aerospace and transportation. Ti/Al connectors combine the excellent properties of titanium alloys and aluminum alloys, which can markedly reduce the component weight and lower the cost. Laser welding has the advantages of high energy density, rapid heating concentration, short residence time at high temperatures. Therefore, it is considered to have unique advantages for joining Ti/Al dissimilar metals. However, due to the significant differences in the thermophysical properties and crystal microstructure of titanium and aluminum, it is highly probable that large residual stresses will be generated after welding. This can facilitate the formation and expansion of cracks. Meanwhile, titanium and aluminum are metallurgically incompatible systems due to their minimal mutual solubility at room temperature. The formation of brittle intermetallic compounds during the welding results in an increase in the brittleness of the joint, rendering it highly susceptible to fracture under stress. Consequently, the modulation of the formation of Ti/Al intermetallic compounds represents a pivotal approach to improving the quality of Ti/Al laser welded joints. In this paper, the current research status on the modulation of interfacial intermetallic compounds in Ti/Al laser welded joints is reviewed from three aspects: laser welding heat input, pre-weld pretreatment and post-weld heat treatment, and interlayer. Finally, on the basis of analyzing the existing problems in the current intermetallic compound modulation research, the future research and development direction of modulating intermetallic compounds in Ti/Al welded joints is proposed. Ti/Al intermetallic compounds Laser welding Heat input Pretreatment Post-weld heat treatment Interlayer Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction In recent years, there has been an increasing demand for dissimilar metal welded structures in industries, which not only to offer excellent performance, but also to meet the specific needs of the affiliated industry [1]. Welded structures of dissimilar metals can make full use of the respective advantages of two materials [2, 3], which not only reduces the overall weight of the components, saves the excessive use of rare metals, improves the overall structural strength and corrosion resistance, but also improves energy efficiency and reduces solid waste emission. Therefore, dissimilar metal welded structures are widely used in aerospace, automotive, power and chemical fields [4]. Table 1 Applications of Ti/Al dissimilar structures Fields Applications Aerospace Honeycomb sandwich structures for aircraft wings [18, 19] Spaceship piping system [20] Transportation Cabin heatsinks [20] Passenger seat rails [21, 22] Automotive Exhaust systems [23] Table 2 Applications, advantages, and disadvantages of different Welding methods for Ti and Al. Welding methods Applications Advantages Disadvantages Diffusion welding Butt welding of Ti/Al Turbine wheel Excellent mechanical properties and corrosion resistance Sensitive to temperature and pressure Friction stir welding Butt welding of Ti/Al Heat exchanger Low thermal crack and porosity Reduce the formation of IMCs Design limitation for tooling by material and joint type Tooling wear Cold cracks and holes Explosive welding Butt welding of Ti/Al Ti/Al composite plates Low interfacial resistance Small heat-affected zone Restricted operating environment Generate noise and airwaves Brazing Butt welding of Ti/Al Lower thermal stress Suitable for workpieces of all shapes and sizes Lower welding efficiency High requirements for brazing materials Ultrasonic welding Multi-layer lap welding of Ti/Al Medical devices Short welding time and energy input Destruction of the metal structure by ultrasonic vibration Sensitive to surface roughness Raising welding temperature by high-frequency vibration Laser welding Butt and lap welding of Ti/Al High energy density High precision Small heat-affected zone High-speed welding Large depth-to-width ratio Remoting welding Hot cracking and porosity IMCs formation Expensive equipment Laser absorptivity affects actual heat input Table 3 The physical properties of Ti and Al at room temperature [50] Ti Al Ionization energy(eV) 6.8 6 Density (kg⋅m 3 ) 4500 2700 Elastic modulus (N/ m 3 ) 0.4 0.2425 Melting point (K) 1941 933 Boiling point(K) 3558 2793 Viscosity (kg⋅ m − 1 ⋅s − 1 ) 0.0052 0.0013 Surface tension (N/m) 1.65 0.914 Thermal conductivity coefficient (W⋅m − 1 ⋅K − 1 ) 21.9 237 Thermal diffusivity (m 2 /s) 2.15×10 − 6 3.65×10 − 5 Specific heat capacity (J⋅Kg⋅K − 1 ) 519 917 Latent heat of fusion (kJ/kg) 419 398 Thermal expansion coefficient (×10 6 ⋅K − 1 ) 8.5 23.5 Among many dissimilar metal structures, Ti/Al dissimilar structures have attracted great attention due to unique advantages. Titanium alloy exhibits high specific strength, good tenacity, and excellent corrosion and high temperature resistance. It is one of the most promising alloys in aerospace [5–9], which known as the aerospace metal and the future metal . Nevertheless, the availability of titanium alloy is limited, and its processing performance is suboptimal. Aluminium alloys are known as flying metal and widely used in aerospace and transportation due to their lightweight, low cost, excellent thermal conductivity and processing performance, but it has a low melting point and poor strength. In order to meet the practical needs of metal structures in lightweight, functional integration and low cost, the combination of titanium alloys and aluminium alloys has induced increasing attention [13–15]. As shown in Table 1 , Ti/Al dissimilar structures have been widely used in aerospace, transportation and automotive industries [16, 17]. As illustrated in Fig. 1 , the Ti/Al dissimilar structure is employed in the passenger seat rails of an Airbus aircraft. Thus, the joining of Ti/Al dissimilar metals has become one of the hot spots for domestic research. Some researchers have used diffusion welding [24–28], friction stir welding [29–35], explosive welding [36, 37], brazing [38–42] and ultrasonic welding [4,43–44] to join titanium and aluminium alloys. Their respective strengths and weaknesses are shown in Table 2 . As illustrated in Table 2 , they are not suitable for mass production due to their complex processes, high cost, low productivity, confined thickness and joint assembly. Laser welding can improve the flexibility and adaptability of joints effectively owing to high energy density, fast cooling, small heat affected zone, high productivity, precise control of heat input and heating position [45–46], which is regarded as Green Manufacturing Technologies for the 21st Century [47]. It has unique advantages not only for same metals but for dissimilar metals during joining [48–50]. And the use of laser to join titanium alloys and aluminium alloys has also been widely studied. However, due to the significant differences in physical and chemical properties of titanium and aluminium, as demonstrated in Table 3 , there are several problems during welding. To begin with, they would oxidise at high temperatures and absorb hydrogen, oxygen and nitrogen readily, causing weld defects such as porosity and white spots. Meanwhile, there is a difference for their melting point, when the temperature reaches the melting point of titanium, alloy elements in aluminium would evaporate and burn, resulting in an uneven distribution to weld components. What’s more, it is inclined to form brittle IMCs rather than solid solutions due to their low mutual solubility at room temperature and metallurgical incompatibility [51–54], which increases the brittleness of joints. In addition, it is easy to generate residual stresses due to difference in thermal conductivity and coefficient of linear expansion after welding. And it is prone to form and spread for cracks in joint, which influenced by brittle IMCs [55]. One of the most critical issues for improving the quality of Ti/Al dissimilar joints is the modulation of brittle IMCs formation [56–57], because the massive formation of IMCs would lead to brittle failure and a reduction in mechanical properties. Thus, inhibiting formation of IMCs in order to reduce their content at the Ti/Al interface, or reducing brittleness and stress of microstructures by decreasing thickness and changing distribution of IMCs to improve the strength and tenacity of joints, are of great importance in ensuring and improving the quality and service performance of Ti/Al dissimilar joints. The heat input of base metal is uneven during welding because of the base metal is heated locally. The fusion zone (FZ) absorbs most of the heat, while the heat affected zone (HAZ) and other regions absorb less heat, resulting in a large temperature gradient inside the joint, which not only leads to an uneven interaction between the interfaces [58,59], but also affects the growth pattern and morphology of microstructure at the interface during cooling. Moreover, it is an important factor for the actual contact area during welding, which can affect the formation of IMCs and the quality of joints. It would affect atomic diffusion and composition at the interface locally, which in turn influences the tissues formed at the interface. There is an inverse relation between contact area and stress. In other words, the contact area is larger, the internal stress is lower and the joint is more stable. The martensite with high hardness would appear in fusion zone and heat affected zone due to fast cooling after welding, forming uneven tissues. The post-weld heat treatment can not only modulate the tissues at the Ti/Al interface, but also improve the mechanical properties of joints. The addition of interlayer can reduce the mismatch of thermal expansion and stress at the Ti/Al interface during welding. It can change the metallurgical reaction at the Ti/Al interface, strengthening elements of interlayer can also improve the composition of brittle phase and increase the ductility of joints. In this paper, the current research status and principles on the modulation of interfacial intermetallic compounds in Ti/Al laser welded joints is reviewed from three aspects: laser welding heat input, pre-weld pretreatment and post-weld heat treatment, and interlayer. Finally, on the basis of analyzing the existing problems in the current intermetallic compound modulation research, the future research and development direction of modulating intermetallic compounds in Ti/Al welded joints is proposed. 2. Effect of thermal input on formation of IMCs The laser with high power density irradiates the base metal and creates a keyhole in the weld seam during welding titanium alloy and aluminium alloy. The transfer of heat occurs through the outer wall of the hole cavity in all directions, melting the surrounding metal to form a molten pool. During this process, titanium and aluminium atoms migrate towards each other through the Ti/Al contact interface, which results in the metallurgical joining of Ti/Al dissimilar metals and the formation of IMCs. The formation of IMCs is related closely to atomic migration during the melting stage of the weld and crystal growth during the solidification stage. During the melting stage, the atoms in the molten pool migrate by both diffusion and convection, with diffusion playing a dominant role and convection occurring only in local areas [60]. Atomic diffusion is mainly dominated by temperature, which changes diffusion distance and relative concentration of Ti/Al atoms in the molten aluminium and molten titanium directly, which in turn determines the composition of the IMC layers. Song [61] et al. found that TiAl 3 formed and grew on the aluminium side of the interface, suggesting that the formation of TiAl 3 is controlled by the diffusion process of titanium atoms through the TiAl 3 /Ti interface and the TiAl 3 phase. Jiang [60] et al. found that Ti 3 Al and Ti 2 Al in the interfacial IMC layers were located in the mixing region near titanium, and TiAl 3 and TiAl were located in the mixing region near aluminium. Figure 2 shows the binary phase diagram of Ti-Al [62], according to Fig. 2 , β-Ti precipitates from the liquid titanium during the initial stage of solidification within the melten pool, and transforms into α-Ti and Ti 3 Al depending on the aluminium content. The limited solubility of liquid aluminium for titanium results in the titanium content and temperature determining the precipitation of TiAl 3 . Meanwhile, the Gibbs free energy of TiAl 3 is lower than that of TiAl and Ti 3 Al [19], so it forms as 3Al + Ti→TiAl 3 firstly. As the solidification rate increases in the latter stage, local convective mixing at the interface is limited and heat transfer is faster near the substrate on both sides of the molten pool, making solidification to begin from the sides towards the centre. TiAl is not precipitated directly from liquid phase aluminium directly. It is formed by diffusion of titanium atoms in the form TiAl 3 + 2Ti→3TiAl. And titanium atoms combine with TiAl by diffusion to form Ti 2 Al as TiAl + Ti→Ti 2 Al simultaneously. Consequently, the composition and thickness of the IMCs layer can be adjusted by modulating the temperature of the molten pool through alterations to the heat input. In addition, the plasticity of the weld can be improved by controlling the heat input to alter the growth pattern of the IMCs during solidification. The occurrence of significant temperature gradients is a consequence of the high heating and cooling rates, which results in an uneven distribution of heat absorption in the fusion zone, heat-affected zone and base metal of the joint. Appropriate heat input can reduce the temperature gradient on either side of the interface during solidification, allowing the IMCs grains to grow in a dendritic fashion to form a more ductile serrated structure and increase the interface bonding area, which is beneficial to hinder crack propagation and improve the strength of the joints [51]. The formation of IMCs is influenced by the heat input, which is primarily controlled by the laser beam and welding parameters. Their respective effects on the formation of IMCs will be analysed in the following sections. 2.1 Effect of laser beam on formation of IMCs 2.1.1 Dual beam laser Tomashchuk et al. [63] reported the joining of T40 titanium alloy and AA5754 aluminum alloy with V-shaped groove and filler wires by using a dual spot tandem laser beam. The findings indicated that the dual beam laser could extend the interaction zone encompassed by the energy, with two spots exhibiting a symmetrical power distribution and both reaching 50% of the initial power, creating more uniform heat input to the seams. Concurrently, the 60° V-bevel facilitates the directional wetting of the titanium surface by molten aluminum, thereby establishing a continuous TiAl 3 layer with a thickness ranging from 2 to 5 µm. The joint shear strength achieved is 152 MPa, representing a 38% enhancement compared to the conventional single-beam laser plus planar butt-joint method. However, the positive correlation between laser line energy and IMCs thickness established by Tomashchuk does not take the effect of interspot distance on transient heat flow into account. The change in spot distance triggers fluctuations in the interfacial temperature, leading to non-uniform growth of IMCs, which contradicts the initial conclusion. Furthermore, beam splitting may result in an insufficient heat input at the bottom of the weld, leading to the formation of unfused joints. This subsequently reduces the mechanical properties of the joint. On the other hand, Tomashchuk attributed the increase in joint strength to the thickness control of IMCs, but did not quantify the effect of residual stress distribution on joint strength. V-bevels have been shown to induce elevated residual stresses within the joint, which have the potential to counteract the benefits achieved through thickness optimization of IMCs. This, in turn, can result in a decline in long-term fatigue performance of the joints. In order to address these issues, Dual Laser Beam Bilateral Synchronous Welding (DLBSW) has become a feasible solution due to its high efficiency, small deformation, and narrow seams. And it has been successfully employed in the lightweighting of aircraft structures [64, 65]. Chen et al. [66.67], Tian et al. [68] and Zhan et al. [69] investigated the temperature field and residual stresses using finite element simulation for T-joints of alloys such as 2219 and TC4. The results demonstrated that the joint strength can reach 80–90% of the base metal strength, and that the ductility of the joint can also be enhanced by adjusting the welding speed in DLBSW. The researches by Zhao [70, 71] et al. and Liu [72] et al. also showed that DLBSW could improve the mechanical properties of T-joints at low power and welding speed. Based on their finding, Zhang [73] et al. applied DLBSW to the joining of Ti/Al dissimilar metals. It was found that the fluctuation range of IMCs thickness was reduced from ± 2.1 µm to ± 0.4 µm for conventional unilateral welding when the energy ratio of both sides was 1:1 and the laser beam was offset to the aluminum side by 0.6 mm. The tensile strength of the joint reaches 139 MPa, and the fracture mode of the joint changes from localized brittle fracture to ductile fracture. As shown in Table 4 , the average thickness of the interfacial IMC layers is less than 4 µm, with a serrated distribution at the same power on both sides. In the event that the power differs between the two sides, the maximum thickness of the interfacial IMCs layer exceeds 15 µm, exhibiting a serrated distribution in addition to the presence of rods and layers, as demonstrated in Fig. 3 . Table 4 Average thickness of IMC layers under different laser powers [73] Laser power(W) Laser offset(mm) Average thickness(µm) Left Middle Right 800/800 0.6 2.14 1.73 2.38 800/800 0.4 3.92 2.98 3.6 1000/600 0.6 16.5 3.66 2.44 As the thickness ( d ) of IMCs layer formed at the interface is controlled by the reaction temperature ( T ) and the reaction time ( t ), $$\:\begin{array}{c}\text{d}\text{=}\sqrt{\text{Kt}} （\text{1}）\end{array}$$ $$\:\begin{array}{c}\text{K}\text{=}{\text{K}}_{\text{0}}\text{exp}\left(\text{-}\text{Q}/\text{RT}\right)（\text{2}）\end{array}$$ where K is the diffusion coefficient, K 0 is the proportionality constant, Q is the diffusion activation energy, R is the gas constant, and T is the reaction temperature. As shown in Fig. 4 , if the power is the same on both sides, the top and bottom of the seam will be heated for a longer time and their temperatures will be higher than those in the middle of the seam, so the thickness of the IMCs layer at the top and bottom will be greater than that in the middle. Moreover, the temperature gradient between the IMCs grains and the solid-liquid interface is greater, resulting in significant supercooling. As the weld cools at a rapid rate, the grains grow into small dendrites and penetrate the liquid phase at different rates, forming IMCs with serrated and rod-shaped structures. The dual beam laser is capable of regulating the thermal input to the joint and effectively improve the thermal distribution at the Ti/Al interface, forming IMC layers with a more even thickness and serrated shape. This, in turn, leads to an improvement in joint strength. However, the thickness of base metal is key to welding quality during using dual beam laser. When welding thick plates (> 2 mm), the difference in the surface absorptivity of titanium and aluminum results in the actual heat input on both sides not being symmetrical, resulting in an increase in the thickness of the local IMCs layer. In the joining of thin plates (< 2 mm), DLBSW tends to induce warping and deformation of the plates, and the joints need to be annealed of stress-relieved after welding, which increases the process flow and time. 2.1.2 Laser oscillations To begin with, the oscillation of the beam along specific paths during welding produces a stirring effect on the molten pool, which increases the fluidity of the molten pool and makes the heat absorbed by the joint more even, improving the quality of seam formation effectively. Moreover, the molten pool is subjected to thermal cycling during beam oscillation repeatedly, which prolongs the solidification time of the molten pool and allows sufficient time for the gas to escape, thereby reducing the occurrence of defects such as porosity in welds. [74–78]. On the other hand, the molten pool can reduce the elemental segregation, break the dendrites and refine the grains significantly after stirring and repeated thermal cycling. Many researchers have conducted a multitude of studies on the laser oscillating welding of aluminium alloys. The main patterns of laser oscillation are‘S’-shaped oscillation [79], circular oscillation [80–85], ‘8’-shaped oscillation [83, 86], ‘∞’-shaped oscillation [79, 84, 87–89], three-dimensional oscillation [90], sinusoidal oscillation [91, 92], ‘S’-shaped plus sinusoidal oscillation [93] and so on. In improving the properties of Ti/Al dissimilar joints using oscillating laser, Zhou [94,95] et al. found that when the oscillation frequency was 150 Hz and the amplitude was 1.2 mm, the thickness of the interfacial IMCs layer decreased to 2.3 µm from 8.5 µm at an oscillation frequency of 0, and the TiAl 3 at the interface showed a discontinuous distribution. At this point, the maximum load of the joint reaches 1852 N, which is 76.38% higher than when the oscillation frequency is 0. This is due to the fact that the stirring effect of the laser oscillation on the molten pool inhibits the diffusion of aluminum atoms into the titanium matrix, thus suppressing the generation of aluminum-rich compounds, as shown in Fig. 5 . Meanwhile, the oscillation of the laser enhances the molten pool flow, promotes the uniform diffusion of the elements, and reduces the peak interface temperature from 1120°C to 860°C, thus suppressing the increase in the thickness of the TiAl 3 layer. The study further indicates through molecular dynamics simulations that the oscillating laser can reduce the Ti/Al interfacial energy from 1.45 J/m² to 0.92 J/m², which effectively reduces the driving force for nucleation of IMCs. In order to further reduce the thickness of IMCs layer, Chen [96] et al. used an S-shape plus sinusoidal laser oscillation. The high power of the laser is applied to the aluminium alloy with low melting point, the low power is applied to the titanium alloy with high melting point, and the aluminium alloy melts and spreads towards the titanium alloy side to form a joint. With an oscillation frequency of 30 Hz and a laser beam offset by 1.2 mm to the aluminum side, as shown in Fig. 6 , the maximum thickness of interfacial IMCs layer was reduced from 7.4 µm to 4.9 µm due to the lower power on the titanium alloy side slowing down the temperature gradient at the interface, optimising the interfacial heat distribution, and suppressing the overgrowth of IMCs. The thickness of the IMCs layer at different locations in the weld was less than 2 µm, resulting in a joint strength of 173 MPa. The oscillation frequency plays a pivotal role in determining the thickness of IMCs layer in laser oscillation welding. The thickness of IMCs layer at the interface decreases significantly as the oscillation frequency increases. As the frequency of the beam increases, the intensity of the stirring of the molten pool also rises. This results in a more homogeneous distribution of heat at the interface, with no localized heat concentration. Meanwhile, the atoms in the molten pool diffuse more even under the influence of stirring, which reduces the local segregation and consequently makes the thickness of IMCs decrease. However, as the frequency of laser oscillation continues to increase, the heat flow to the molten pool would affect the seam formation and, as a consequence, decrease the stability of the keyhole. It also increases porosity and affects joint performance adversely. Furthermore, when the oscillation frequency is excessively high, the inadequate thickness of the IMCs layer of the joint hinders the formation of an effective connection between titanium and aluminum, leading to a substantial degradation in joint performance. In addition, the disparity in thermal conductivity between titanium and aluminum, in conjunction with the heat flow induced by mechanical stirring during laser oscillation, gives rise to asymmetric heat flow within the molten pool. This results in a substantial temperature gradient between the center and the edge of the molten pool. Under this effect, the thickness of IMCs at the edge of the molten pool will be much larger than that in the interior, making the edge of the molten pool the starting point of joint fracture. 2.1.3 Pulsed laser It can be divided into continuous wave and pulsed wave for laser. Compared with continuous wave, there are several advantages for pulsed wave in welding. First of all, it should be noted that the material has solidified, which means that changes in the keyhole during one pulse period will not affect subsequent cycles due to the rapid solidification of the molten pool. This makes the keyhole more stable. Then, the minimum residence time of the metal in the molten state can be obtained by adjusting the pulse frequency and period, which can not only inhibit the growth of IMCs but reduce the cracks and porosity in seams. Liu [97] et al. employed finite element analysis to investigate the thermal distribution of the Ti/Al interface at varying pulse frequencies and the impact of pulse period on the thickness of IMCs layer. The results indicated that there is a notable difference in the thickness of IMCs at each part of the interface due to different temperature residence times above the isotopic transition temperature (882°C) of titanium at different pulse periods, as demonstrated in Fig. 7 . This can be attributed to the isotopic transformation of titanium, which directly affects its solubility rate in aluminum. Below 882°C, titanium exists in a close-packed hexagonal (HCP) structure, designated as α-Ti. At temperatures above 882°C, titanium assumes a body-centered cubic (BCC) structure, designated as β-Ti. The dissolution rate of β-Ti for aluminium atoms is greater than that of α-Ti for aluminium atoms, resulting in faster growth of IMCs above 882°C. As the total pulse time increases, the pulse duration above 882°C also increases. Furthermore, the Ti/Al interface remains above 882°C for a longer period of time. Therefore, it is effective to reduce the thickness of IMCs layer at the interface by reducing the pulse period during pulsed laser welding. However, the study by Liu et al. was based on a linear relationship between the effect of pulse frequency on heat input, but failed to take the nonlinear effects of pulsed-wave laser waveforms (e.g., square and sinusoidal) on the solidification rate of the molten pool into consideration. Therefore, the effects of pulse frequency and pulse waveform on the formation of IMCs at the Ti/Al interface need to be further investigated in depth. Meanwhile, the elevated cooling rate of pulsed lasers relative to continuous-wave lasers can precipitate thermal cracking of the joints when the power is excessively high. 2.2 Effect of welding parameters on formation of IMCs Two key parameters in continuous wave laser welding are laser power and welding speed, which directly determine the heat input during welding. The beam offset distance is the distance between the beam and the centre line of the seam during welding. By adjusting the offset distance within a certain range, it is possible to alter the thermal input of the joint. Therefore, the heat input of the joint can be adjusted by modifying the laser power, welding speed, and beam offset distance. Subsequently, the thickness and distribution of the IMCs can be modulated. 2.2.1 Laser power Chen [100] et al from the Harbin Institute of Technology used dual-spot laser to join TC4 titanium alloy and 6061 aluminum alloy, and they investigate the effect of laser power on IMCs at the interface. The results demonstrated that the thickness and distribution of the IMCs layer undergo a change with an increase in laser power. As shown in Table 5 , as the laser power increases, the average thickness of the IMCs layer also increases. A constant power input results in the greatest thickness of the IMCs layer at the top regions of the interface. When the laser power is 1900W, the joint strength can reach 241 MPa, when the laser power is 2500W, the joint strength is 185 MPa. Meanwhile, as the laser power was increased, the shape of the IMCs layer underwent a transformation, initially adopting a lamellar shape, subsequently evolving into a serrated morphology, and ultimately assuming a club-shaped structure. As illustrated in Fig. 8 , the distribution of interface IMCs is observed to vary with differing laser powers. This result is also identical to that reported by Zhang [73] et al. Table 5 Effect of laser power on the average thickness of IMC layers [100] Laser power(W) Average thickness(µm) Top Middle Bottom 1600 0.22 0.3 0.00 1900 0.6 0.4 0.35 2200 3.00 1.22 1.15 2500 12.00 4.00 2.10 The energy of the beam is transferred from top to bottom during welding, so the top regions of the interface are hotter than the middle and bottom regions, and their residence time at high temperature is longer, which makes the thickness of IMCs layer in the top regions is much greater than that in the middle and bottom regions. A lower power results in a decrease in heat absorption at the bottom of the interface, which in turn leads to a reduction in temperature and a corresponding reduction in the rate of interatomic diffusion. Meanwhile, the reduction in supercooling resulted in the IMCs developing into a lamellar structure. As the laser power increases gradually, the temperature gradient at the interface increases, and so does the subcooling. During the rapid cooling of the molten pool, fine dendrites are formed at the interface, and these fine dendrites grow into the liquid phase with different growth rates, forming a serrated shape eventually. As laser power continues to increase, the top regions of the interface absorb too much heat and become hotter, increasing the dynamics of atomic diffusion. The growth rate of fine dendrites in the liquid phase also increases with increasing supercooling. The titanium atoms become more diffusible and will diffuse in a particular direction, forming IMCs with club-shaped and acicular-shaped. Consequently, the formation of IMCs at the upper part of the interface is more sensitive to changes in laser power. Sun [101] et al. employed laser spot welding to get lap joints of TC4 and 5052 by varying the laser power at different defocusing distances. The results showed that, for a certain defocusing distance, the lower power results in the formation of two separate pools of titanium alloy at the top and aluminium alloy at the bottom of the joint respectively, which are not in contact with each other. This avoids the direct mixing of the two metals in the liquid phase, thereby suppressing the formation of IMCs. The thickness of the TiAl 3 formed at the interface increases with rising of power and changes from a serrated shape to an acicular shape. By increasing the laser power further, a large amount of liquid titanium enters the weld, forming a pinned structure. The titanium and aluminum at the edges of the structure will undergo a transition from metallurgical bonding to diffusion bonding, resulting in the formation of two different IMCs, TiAl 2 near the titanium side and TiAl 3 near the aluminum side. It is key to determine the reliability of the joints for the thickness of the IMCs layer. Anil [102] et al. proposed that the interfacial temperature, flow rate and diffusion rate are the key factors affecting the thickness and microstructure of IMCs. The actual power absorbed by the metal surface is less than the laser power due to the reflection of the beam by the metal surface during welding, and there is a linear relation between absorbed power and laser power. A research of the growth process of IMCs by Zhao [103] et al. showed that, the IMCs grow horizontally at the interface by reaction diffusion firstly and thicken in the vertical direction of the interface subsequently. So, there is also a linear relationship between actual absorbed power and thickness of the IMCs. Therefore, the thickness of the IMC layers at the interface can be reduced by reducing the laser power appropriately while other parameters remain constant. However, it remains challenging to regulate the uniformity of the thickness of IMCs at the interface through adjustments in laser power. The inhomogeneous distribution of laser power in the joints is a primary factor contributing to the inhomogeneous thickness of the IMCs layer. The focusing characteristics of the laser beam results in the highest power density at the center of the weld and lower power density at the edges of the weld. This inhomogeneous distribution of power leads to significant temperature differences at different locations of the weld. At the center of the weld, due to the higher temperature, the formation of the IMCs layer is faster and the thickness is larger, while at the edge of the weld, due to the lower temperature, the formation of the IMCs layer is slower and the thickness is smaller, which leads to the non-uniformity of the thickness of the IMCs layer in the weld. 2.2.2 Welding speed Tomashchuk et al. [104] studied the distribution of IMCs at the Ti/Al interface after high speed welding. The results demonstrated that high speed welding minimizes the interaction time between titanium and aluminum, prevents the overmixing of the two metals in the liquid state, and reduces the mixing area, which subsequently reduces the area of the IMCs. Zhou [105] also demonstrated that an increase in welding speed results in a reduction in the heating time of the molten pool, the temperature of the molten pool, and the heat per unit area within the molten pool. This causes insufficient incentive for the atoms to diffuse into each other. Meanwhile, high welding speeds would increase the solidification rate of the molten pool, resulting in insufficient time for the IMCs to grow, thus reducing their thickness. This finding was confirmed by Lee [106] et al, too. Their research showed that the distribution of IMCs at the interface will change due to increasing of welding speed, as shown in Fig. 9 . Because the dendritic IMCs at the interface are mainly transformed from the titanium-containing phase, and as the welding speed increases, the coarse and sparse dendrites are transformed into fine, dense dendrites. Moreover, the IMCs with island shape in the fusion zone would transform into a granular shape, which forms a dispersed distribution. A research by Leo [107] et al. also demonstrated that an increase in welding speed has a significant impact on the distribution of IMCs. Higher welding speeds result in a linear and approximately regular distribution of IMCs at the interface, which tends to be smooth. In contrast, lower welding speeds result in a significant degree of mixing between the two metals, which gives rise to a curved and protruding distribution of IMCs. However, the above studies failed to form IMCs with uniform thickness even though the distribution of IMCs could be modulated by increasing the welding speed. Due to the large difference in viscosity and surface tension between liquid titanium and liquid aluminum, a complex flow field would be formed in the molten pool during the welding process. When only the welding speed is varied, the mixing and convection in the molten pool is not uniform due to the difference in fluidity between liquid titanium and aluminum, resulting in inconsistent conditions for the formation of IMCs. Liquid aluminum exhibits superior fluidity, readily spreading in the molten pool. Conversely, liquid titanium demonstrates inferior fluidity, resulting in delayed diffusion, leading to uneven thickness of IMCs layer. In addition, the filling and spreading behavior of the liquid metal changes as the welding speed changes. At higher welding speeds, liquid aluminum can fill the weld gap more rapidly due to its high fluidity. Conversely, liquid titanium will be underfilled due to its low fluidity, resulting in the formation of weld defects at the interface. At lower welding speeds, liquid titanium will collect in localized areas for a longer period of time, resulting in an increase in the thickness of the IMCs layer at that location. Therefore, a single change in welding speed does not result in a layer of IMCs with uniform thickness. 2.2.3 Laser offset As shown in Fig. 10 , in addition to applying the laser beam to the centre line of the weld directly, it is also possible to focus the beam on the side of the base metal with a certain distance from the centre line of the weld, which known as laser offset welding (LOW) [107]. Casalino et al. [56, 108–109] offset the beam towards titanium alloy. Once a stable keyhole was formed on the titanium side, heat was transferred from the titanium alloy to the aluminum alloy, resulting in the melting of the aluminum alloy and the formation of the joint. It demonstrated that the distance at which the laser beam is deflected on the titanium alloy side has a direct impact on the proportion of the two base materials melted in the joint. Additionally, the average thickness of the IMCs layer increases with a reduction in the deflection distance. The same conclusion was reached by Song [110] et al. from Tsinghua University. Their research indicated that the heat transfer from the titanium alloy to the interface is greater when the offset distance is minimal, resulting in elevated interface temperatures and the formation of brittle phases, including TiAl 3 , TiAl 2 , Ti 3 Al, and TiAl. The thickness of IMCs layer at each part of the interface decreases gradually as the offset distance increases. The top of the interface formed club shape and acicular shape of TiAl and TiAl 3 , while the middle and bottom parts formed layer of TiAl 3 with a thickness of 1 µm. As the laser is deflected towards the titanium alloy side, the heat absorbed at the Ti/Al interface increases gradually as the deflection distance decreases. This results in a rise in temperature, which in turn enhances the interatomic diffusion motion. A significant quantity of heat is transferred from the titanium side to the aluminum side, resulting in an increase in the quantity of aluminum melted and a gradual increase in the proportion of molten aluminum in the fusion zone. A reduction in the beam offset distance will result in an increase in the interface temperature and a prolongation of the cooling period required for the weld. This will consequently afford a greater opportunity for the brittle phase grains to develop. And the growth morphology of the brittle phase grains is influenced by the change in temperature gradient at each part of the interface, resulting in the formation of different shapes eventually. However, the joint did not form a complete metallurgical bond in the results of Song et al. The interface was fusion welded at the top of the interface and brazed at the bottom of the interface. The performance of the joint would be degraded due to discontinuity of the interface. Furthermore, the range of beam offsets selected for the study by Song et al. is limited, excluding the distribution of IMCs at the joint interface when the offset exceeds 0.7 mm. Consequently, the generalizability of the conclusions is constrained. Guo [111] et al. offset the laser beam towards the aluminium alloy side and simulated the temperature field of the joint using finite element to investigate the effect of the offset distance on the distribution of IMCs at the Ti/Al interface. It was found that the interfacial IMCs are sensitive to the offset distance. When the offset distance was less than 0.5 mm, coarse TiAl 3 and TiAl formed at the interface and many microcracks appeared inside the joint. As the offset distance increases, TiAl 3 appears as long strips and blocks with an average thickness of approximately 15 µm. As the offset distance increases to 2 mm, the heat absorbed by the interface is reduced, the temperature is lowered and the atoms cannot diffuse with each other sufficiently, reducing the effective bonding area of the interface. The beam offset to the aluminum alloy would result in an uneven temperature distribution on both sides of the Ti/Al interface due to the differing thermal conductivities of the two metals. Aluminum exhibits a thermal conductivity of 237 W·m − 1 ·K − 1 , which is greater than that of titanium (15.7 W·m − 1 ·K − 1 ). The highest temperature at the top of the interface has exceeded the melting point of the titanium alloy, but the middle and bottom temperatures have not yet reached the melting point, resulting in a large amount of melting of the aluminium alloy and less melting of the titanium alloy, causing the concentration of aluminium atoms in the molten pool to be greater than the concentration of titanium atoms. As the offset distance to the aluminium alloy increases, the quantity of aluminum melted increases, and the concentration of aluminium atoms in the molten pool continues to increase. During the cooling process, TiAl 3 is prone to formation due to its high stability under high temperatures and high enthalpy, as well as its Gibbs free energy being smaller than that of Ti 3 Al and TiAl. [112–114]. What is more, TiAl 3 exhibits high hardness and poor plasticity, which increases the brittleness of the joint and reduces the toughness of the joint. The fusion proportion of titanium alloys and aluminium alloys can be controlled by varying the offset distance on either side of the weld, which not only reduces the average thickness of the IMCs layer, but also changes it from a brittle continuous layer shape to a ductile serrated shape. However, when attempting to offset towards titanium, if the distance is insufficient, it can result in an increased proportion of molten aluminum. Furthermore, the vigorous mixing of two metals would lead to an increase in the thickness of the brittle phase layers at the interface. During offsetting towards aluminium alloys, the quality of the weld seam is significantly influenced by the presence of porosity, which is a consequence of the inherent properties of the aluminum alloy. [115, 116]. The energy efficiency of laser beam would decrease due to the high reflectivity of the aluminium alloy to the beam. Therefore, it is imperative to address this matter in order to reduce reflectivity and enhance processing stability during offset operations with aluminum alloys. The laser power has a direct impact on the heat input and molten pool temperature. The melting points of titanium and aluminium are significantly different, and high power increases the melt pool temperature and prolongs the residence time of the liquid metal. This, in turn, promotes the interdiffusion of titanium and aluminium, thus accelerating the formation of IMCs. As demonstrated in Table 6 , an augmentation in power results in a thickening of the IMCs layer. This phenomenon can be attributed to an escalation in the dissolution of titanium and aluminium in the molten pool. Consequently, this leads to a protracted diffusive reaction between the two elements at the interface. This, in turn, results in a diminution in the strength of the joint. Concurrently, at elevated power levels, the interface predominantly generates either jagged or continuous layers of IMCs, while at low power, IMCs are distributed in islands or scattered. The metallurgical process of the molten pool is affected by welding speed, which alters both the duration of heat input and the cooling rate. In high-speed welding, the existence time of the molten pool is reduced and elemental diffusion is inhibited. Conversely, in low-speed welding, the diffusion time is prolonged and the growth of IMCs is promoted. It has been demonstrated that, at lower speeds, the IMCs layer is thicker due to the greater diffusion time, facilitating the formation of a continuous layer. Conversely, at higher speeds, the IMCs layer is thinner, resulting in a discontinuous distribution. As demonstrated in Table 6 , the strength of joints at elevated welding speeds is found to exceed that of low welding speeds. However, it should be noted that employing an excessively high welding speed may result in the presence of unfused joints or inadequate fusion depth. The distance at which the laser beam is directed towards the titanium or aluminium side is a critical factor in determining the melting ratio of titanium and aluminium and the degree of mixing of the elements in the molten pool. As demonstrated in Table 6 , when the process is biased towards the titanium side, there is an increase in the amount of titanium melting, a decrease in the amount of aluminium melting, and an increase in the concentration of titanium in the molten pool. This facilitates the formation of titanium-rich IMCs (e.g., Ti 3 Al). Consequently, the IMCs layer will be concentrated on the titanium side of the interface, exhibiting a thin thickness. In circumstances where the bias is oriented towards the aluminium side, there is an observed increase in the amount of aluminium that melts, whilst simultaneously there is a decrease in the amount of titanium that melts. Concurrently, the concentration of aluminium in the melt pool is increased, thereby facilitating the formation of aluminium-rich IMCs (e.g. TiAl 3 ). During this process, the thickness of the IMCs layer is increased and becomes more widely distributed, thus enhancing the brittleness of the joint. It is therefore vital to increase the power of the laser when welding towards the titanium side in order to compensate for its high melting point, or to decrease the power or increase the speed of the laser when welding towards the aluminium side in order to reduce the excessive melting of aluminium, which has a significant effect on the strength of the joint. 3. Effect of pretreatment and post-weld heat treatment on formation of IMCs 3.1 Pretreatment of Ti/Al connector before welding Zhao [117] et al. used pulsed laser pretreatment (PLP) to process the butt interface of titanium alloys before welding and compared it with the interface after polishing process (PP) to investigate the effect of PLP on the formation of interfacial IMCs. As illustrated in Fig. 11 , the results demonstrate that after the interface has been treated with PLP, not only has the thickness of IMCs layer been diminished, but also their distribution has undergone a notable transformation in comparison to that of PP. And there is a change from a three-layer sequential distribution (TiAl 3 、TiAl and Ti 3 Al) to a bilayer mixed distribution(TiAl 3 、TiAl 3 ་TiAl་Ti 3 Al). PLP increases the surface roughness of titanium alloy, resulting in a significant increase in the effective contact area at the Ti/Al interface, which is 2.916 times greater than that of PP, thus reducing internal stresses between the interfaces. The contact surfaces exhibited significant bending at the microscopic due to the increased interfacial roughness. Furthermore, the curved interfaces prevented the growth of the IMCs, reducing their thickness by 74%. The reason for this is that the flat titanium surface treated with PP first comes into contact with a large number of aluminium atoms to form TiAl 3 , the deeper the diffusion goes into the titanium alloy, the greater the difficulty for the aluminium atoms to diffuse, resulting in a gradual decrease in their concentration. On the contrary, the concentration of titanium atoms gradually increases as the diffusion goes deeper into the titanium alloy. In this process, TiAl and Ti 3 Al are generated sequentially and ultimately resulting in a three-layer sequential distribution. For the interface after PLP, its high roughness creates a large number of pits in the surface and results in a reduction in the number of titanium atoms present on the surface, thereby limiting their participation in the reaction. A significant number of aluminium atoms will penetrate the interior of the pits, causing an uneven distribution of titanium atoms and resulting in a mixed distribution of TiAl 3 , TiAl and Ti 3 Al, with alternating layers of these phases in the inner layer. However, the inhibition of IMCs growth by constructing a pit-structure on the titanium surface by PLP as proposed by Zhao et al. is based on the fact that the geometrical parameters of the pit-structure (e.g., depth, spacing) remain stable during the welding. In fact, during the welding thermal cycle, the pit-structure can dynamically collapse or locally fuse due to the variation of welding heat input, weakening its effectiveness in suppressing IMCs. Furthermore, following the PLP, the oxide film that has been newly formed on the titanium surface will exhibit localized thickness variations. This will result in fluctuations in the wetting angle of aluminum, which in turn will significantly alter the wetting behavior of aluminum during the subsequent welding process. This, in turn, will lead to a substantial difference in the thickness of the interfacial IMCs layer. 3.2 Post-weld heat treatment Several researchers have investigated the effect of post-weld heat treatment on the Ti/Al interfacial microstructures and mechanical properties of joints in some non-fusion welding methods, such as friction stir welding and explosion welding. Li [118] et al. reported the effect of heat treatment on the interfacial microstructures of Ti/Al friction stir welded joints, and they found that the even distribution of IMCs could improve the bond strength of the two alloys. Fronczek [119] et al. used the high temperature and pressure formed by explosion welding to join titanium alloy and aluminium alloy. It showed that TiAl 3 at the interface starts to form at 552°C in joint. Based on the idea that the temperature of post-weld heat treatment would have an effect on the IMCs. Leo [107] et al. conducted low-temperature heat treatments of two Ti/Al joints with different line energies at 350°C and 450°C respectively, in order to investigate the effect of heat treatments at different temperatures on interfacial IMCs. The results demonstrated that the thickness of the interfacial IMC layers exhibits a pronounced increase when the heat treatment temperature reaches 450°C. Meanwhile, at the local parts of interface, brittle phases, primarily composed of TiAl 3 , nucleate and grow on the original microstructures. Additionally, the particles in the fusion zone exhibit varying degrees of coarsening, as shown in Fig. 12 . It can be demonstrated that TiAl 3 is thermodynamically stable at lower heat treatment temperatures [28], its nucleation and growth is induced by atomic diffusion at higher temperatures. When the heat treatment temperature was increased to 450°C, TiAl 3 reached thermodynamic equilibrium and titanium atoms were able to diffuse intracrystalline through the IMCs layer [118], providing the necessary raw materials for TiAl 3 growth, leading to an increase in the TiAl 3 area of interface [120]. On the other hand, the IMCs at the interface of the joints with lower line energies were transformed from continuous layers to fragmented clubs after heat treatment at 350°C, and no growth of the original IMCs layer was found. Therefore, an appropriate heat treatment temperature could improve the mechanical properties of the joints, transforming the IMCs from highly brittle continuous layers to clubs and reducing the concentration of interfacial stresses. A further study by Leo [121] et al. demonstrated that the thickness of IMCs did not begin to increase until a heat treatment temperature of 530°C. The thickness of the IMCs layer has been observed to decrease at a heat treatment temperature of 350°C, as reported in the study by Leo et al. However, due to the difference in thermal conductivity between titanium and aluminum, the temperature field of the joint is not uniformly distributed as the temperature rises. The temperature rise and thermal gradient will also be greater on the aluminum side than on the titanium side due to higher thermal conductivity of aluminum. In the context of an inhomogeneous thermal gradient, the thickness of the IMCs layer is subject to localized fluctuations. In addition, the increase in joint strength is partly due to the change in the morphology of IMCs and partly possibly due to the reduction of residual stresses in the joints after heat treatment, whereas the study by Leo et al. did not quantify the change in residual stresses introduced by the heat treatment. Therefore, the changes in residual stresses should be taken the mechanism of post-weld heat treatment to enhance the strength of Ti/Al joints into account. As demonstrated in Table 7 , the strength of the joints that have undergone pretreatment is significantly higher than that of the untreated joints. This finding provides a viable solution for enhancing the performance of Ti/Al dissimilar joints in actual service conditions. The application of heat treatment has been demonstrated to have a negligible effect on enhancing joint strength, and in some cases, there is a decline in strength. This phenomenon may be attributed to the elevated formation temperature of IMCs. Furthermore, it has been observed that heat treatment at low temperatures merely serves to alleviate joint stresses, without exerting an effect on IMCs. 4. Effect of the Interlayer on formation of IMCs The formation of IMCs can be suppressed to a certain extent and their morphology and distribution can be changed by modulating the heat input to the joints, as well as by pretreatment and post-weld heat treatment of the joints. However, these methods do not completely prevent the formation of brittle phases at Ti/Al interface. Therefore, if it is possible to transform brittle phases into IMCs with tenacity, or change their morphology to transform them into reinforced phases, the performance of the joints can be greatly improved. The addition of interlayers into Ti/Al joints has proven to be an effective method. 4.1 Alloy interlayer 4.1.1 Aluminium-based alloys Aluminium-based alloys have the advantage of a low melting point and excellent wettability, and their main elements are aluminium and silicon, so the addition of Ti/Al joints does not introduce other elements. As a result, aluminum-based alloys are widely used for joining titanium alloys and aluminum alloys. Zhou [122] et al. used AlSi12 as an interlayer to join TC4 and 5A06.Their research showed that the addition of AlSi12 resulted in the enrichment of silicon atoms at the Ti/Al interface, and the diffusion of silicon atoms was also enhanced, which in turn changed the weld composition. Meanwhile, due to the increase in the concentration of silicon atoms at the interface, the thickness of the interfacial IMCs decreases significantly with the addition of AlSi12 compared to the joints without the addition of AlSi12, suggesting that the increase in the silicon content at the interface plays an inhibitory role in the growth of IMCs. On the one hand, the addition of AlSi12 interlayer to the joints effectively promotes the formation of Ti(Al,Si) compounds, and these IMCs play the role of binder at the interface, which improves the bonding strength between titanium and aluminum. On the other hand, the Si in AlSi12 assists in refining the particles of IMCs, ensuring uniform distribution in the weld. This, in turn, enhances the microstructure uniformity and stress distribution of the joints, thereby mitigating the risk of crack initiation and extension due to the inhomogeneous growth of IMCs. However, the AlSi12 interlayer is more sensitive to process parameters due to its lower melting point. Its role in modulating IMCs needs to be realized under suitable conditions of laser power, welding speed, and shielding gas atmosphere. When the process parameters deviate from the appropriate range, the modulation effect of the AlSi12 interlayer is significantly diminished. This has been shown to result in the occurrence of welding defects, thereby increasing the instability of the welding process. Lv [123] et al. used an Al-Cu-La alloy as an interlayer added to a Ti/Al joint. It was shown that the addition of the lanthanum to the original Al-Cu alloy changed the IMC layers at the Ti/Al interface from the original single TiAl 3 to a composite layer of TiAl 3 and Ti 2 Al 20 La. And the formation of Ti 2 Al 20 La caused the hardness of the original IMCs to drop by more than half, reducing the brittleness of the joint markedly. The lanthanum in the Al-Cu-La alloy is easily polarized at the grain boundaries, which inhibits the growth of IMCs and makes the IMCs appear as fine particles, thus improving the bonding strength of the interface and the plasticity of the joint. Moreover, the addition of Cu and La changed the solute diffusion path and reaction kinetics during welding, improved the distribution of IMCs, avoided the stress concentration and crack initiation caused by the over-concentration of local IMCs, and improved the overall mechanical properties of the joint. However, the Al-Cu-La alloy interlayer is also sensitive to the process parameters. In the event that these parameters are not properly controlled, the joints are susceptible to weld defects, including porosity and unfusion, which adversely affect the quality of the joints. Moreover, the elevated cost of Al-Cu-La alloys, in comparison to Al-Si system interlayer materials, results in a further escalation in production costs and a restriction of their extensive application in large-scale industrial production. When zirconium is added to Al-Cu alloys [124], the content of TiAl 3 at the interface gradually decreases with increasing heat input. What is more, two new phases, L-(Ti,Zr)Al 3 and H-(Ti,Zr)Al 3 , are formed at the interface, which improves the tenacity of TiAl 3 and enhances properties of the joint. However, the preparation and use of Al-Cu-Zr alloy interlayers would greatly increase welding costs. In comparison with the prevalent Al-Si interlayer materials, Al-Cu-Zr alloys are more expensive and necessitate more stringent welding process parameters, which contributes to their relative infrequent utilization in large-scale industrial production. Therefore, composite interlayers can be developed by compositing Al-Cu-Zr alloys with other materials, such as ceramic and high-entropy alloys. This approach not only enhances the efficacy of IMCs modulation but also leads to a substantial reduction in cost. 4.1.2 High-entropy alloys Gu [125] et al. used the high-entropy alloy (HEA) CoNiCuNb0.5V1.5 as an interlayer connecting TC4 and 6082. The results showed that after the addition of CoNiCuNb0.5V1.5 to the joints, solid solutions with FCC structure and solid solutions containing vanadium elements were formed at the interfaces of aluminium alloy with HEA and titanium alloy with HEA. At the interface between HEA and aluminum alloy, vanadium atoms combine with aluminum atoms to form VAl 3 with a thickness of only 0.8 µm. And at room temperature and low pressure (below 20 Gpa), VAl 3 exhibits superior tenacity in comparison to TiAl 3 [126]. At the interface between HEA and titanium alloy, HEA and titanium atoms diffused with each other to form a mixed layer with solid solution structure, and no brittle phase was detected at the interface. The conspicuous alteration in interfacial microstructures can be attributed to two key effects: the high-entropy mixing effect and the kinetic sluggish diffusion effect, which are both induced by the addition of HEA to the interface.To begin with, the high-entropy mixing effect of HEA improves the compatibility between different elements and promotes the formation of ductile solid solutions. In addition, elements contained in HEA also causes severe lattice distortion in the crystal structure of HEA. The kinetic sluggish diffusion effect induced by this lattice distortion hinders interatomic diffusion, thus preventing the mixing of titanium atoms and aluminum atoms and inhibiting the formation of brittle IMCs. However, due to the difference in composition between aluminium alloy and HEA, there can be a large difference in flowability and surface tension between the two in the molten state. It will cause an inadequate mixing between aluminium alloy and HEA during welding. And in the cooling process, the weld would produce a certain degree of segregation, resulting microstructures is not uniform. Furthermore, the intricate composition of HEA gives rise to a diverse array of IMCs, which complicates the attainment of precise prediction and regulation of the products. Meanwhile, the fabrication of HEA entails a complex process, with the inclusion of valuable components resulting in elevated costs. This, in turn, imposes constraints on the widespread utilization of HEA interlayers. 4.2 Metal interlayer When joining titanium alloys and aluminium alloys by laser welding–brazing, it is necessary to add some metallic elements to the brazing material to enhance its wettability and spreadability to inhibit the formation of IMCs and also to improve the spreading of the brazing material or aluminium liquid on the titanium alloy substrate. Wang [127] et al. added zinc foil as an interlayer to Ti/Al joints and investigated its effect on interfacial IMCs. The results showed that the addition of zinc can significantly improve the wettability and spreading of Al-Si brazing material on the substrate. Meanwhile, a new intermetallic compound TiZn 16 was formed at the interface. The formation of TiZn 16 effectively reduces the content of TiAl 3 at the interface and improves the strength of the joint. However, the thermal stability of TiZn 16 is poor. As the heat input of the joint increase continuedly, the formation of a high temperature field within the joint would accelerate the diffusion of zinc atoms, leading to a persistent decrease in the amount of TiZn 16 at the interface until it disappears. And the brittleness of the joints also increases with decreasing TiZn 16 . Zn foils exhibit high surface activity and strong affinity for titanium and aluminum. During the welding process, Zn atoms exhibit a marked preference for reacting with Ti atoms, resulting in the formation of fine TiZn 16 grains, improving interfacial bond strength and plasticity. Moreover, Zn foils are comparatively inexpensive and straightforward to process, offering economic and practical advantages over complex alloy interlayers. However, Zn has a relatively low boiling point and is highly susceptible to evaporation when exposed to high-energy lasers, which may lead to the loss of zinc foil and the instability of the welding. Concurrently, the TiZn 16 exhibits diminished thermal stability, leading to a progressive decline in its content throughout the welding thermal cycle. This impedes the enhancement of the joint strength. To overcome the problem of poor thermal stability due to the low melting point of the interlayer, selecting metal interlayer with high melting point becomes a feasible solution. Majumdar [53] et al. found that two brittle phases, TiAl 3 and TiAl, were formed at the Ti/Al interface, which made the weld more brittle and resulted in a poor ability of the weld to withstand thermal stresses. What is more, cracks will appear at the interface between the fusion zone and the aluminum alloy during the cooling due to the high content of TiAl 3 on the side close to the aluminum alloy in the fusion zone. Majumdar concluded that the formation of interfacial cracks is very sensitive to the aluminum content. For this, Majumdar added thin Nb sheet as an interlayer to the joint for slowing down the temperature gradient at the interface and reducing the amount of aluminium melted. The results showed that niobium atoms and titanium atoms are able to diffuse with each other and form a solid solution, improving the tenacity of the weld effectively. Moreover, a portion of unmelted niobium in the fusion zone prevents the melting of aluminium, thereby reducing the proportion of aluminium in the fusion zone and inhibiting the formation of TiAl 3 . However, in order to minimise thermal cracking of the joint, Majumdar et al. chose to weld with lower heat input. The insufficient heat input prevents the thin niobium sheet to melt completely in the pool, resulting in discontinuities and stress concentrations in the joint after cooling. Gu [128] et al. also used Nb as an interlayer and deflected the beam towards the aluminium alloy to obtain a joint with completely unmelted niobium. As shown in Fig. 13 , two different interfaces were formed in the joints, a diffusion bonded interface between the titanium alloy and niobium, a metallurgically bonded interface between the aluminium alloy and niobium. As the melting point of niobium is much higher than that of aluminium, when the aluminium melts, the niobium has not melted at all. Completely unmelted niobium prevents interdiffusion between titanium atoms and aluminium atoms, inhibits the formation of TiAl, TiAl 2 and TiAl 3 , and produces the Nb -containing ductile phases NbAl 3 , Nb 2 Al and an aluminium-based solid solution at the interface between the aluminium alloy and niobium. However, the completely unmelted niobium in the joint will be an area of stress concentration. It is from here that cracks can start and grow under external forces. In addition, the coefficients of linear expansion of titanium, aluminium and niobium are different. And joints at high temperatures would crack at the interface between niobium and the two alloys as a result of thermal stress, causing the joint to break ultimately. The implementation of Nb foil as an interlayer has been demonstrated to effectively impede direct contact between titanium and aluminum, thereby mitigating the formation of brittle IMCs. Moreover, Nb demonstrated the capacity to diffuse into the titanium matrix, thereby forming a stable Nb-containing solid solution layer. Concurrently, its reaction with aluminum, resulting in the formation of IMCs, hindered the development of Ti/Al IMCs. What’s more, Nb atoms can enter the fusion zone and assume a role in grain refinement during the solidification, thereby enhancing the plasticity and strength of the joints. On the other hand, Nb has a high melting point, which enables it to maintain good stability at high temperatures and does not evaporate or decompose easily, ensuring the continuity of its modulating effect on IMCs. However, the unmelted Nb foil in the joint can also become a site of stress concentration during cooling, which can easily result in the formation of cracks. Moreover, the coefficient of linear expansion of aluminum (23.5×10 6 ⋅K − 1 ) is three times higher than that of niobium (7.3×10 6 ⋅K − 1 ), so the Nb/Al interface would be extremely susceptible to fracture during cooling due to the rapid cooling rate. Zhang et al. used tungsten foil as an interlayer to connect TC4 titanium alloy and 6061 aluminium alloy in order to transform the crystal structure of TiAl 3 from a brittle tetragonal structure (D0 22 ) to a ductile face-centred cubic structure (L1 2 ) [129, 130]. The results showed that some tungsten atoms enter the TiAl 3 at the interface and take the place of some of these titanium atoms to form Al 3 (Ti,W) after the tungsten is added to the joint. The presence of elemental segregation of tungsten at the Ti/Al interface causes a partial transformation of the TiAl 3 crystal structure from D0 22 to L1 2 . According to research form Jahnatek [132] et al. and Zhang [133] et al. for D0 22 and L1 2 structure of TiAl 3 . D0 22 is a stable structure, whereas L1 2 is a sub-stable structure. This leads to the conclusion that TiAl 3 in the L1 2 structure cannot be present in large quantities and its proportion at the interface is extremely low. Meanwhile, the extent of segregation of tungsten elements at the interface is constrained. Therefore, the tungsten interlayer has a limited effect on changing the crystal structure of TiAl 3 , and it is unable to significantly enhance the brittleness of the weld. Moreover, tungsten is classified as a refractory metal, with a melting point of 3422°C, which is significantly higher than the melting points of titanium and aluminum. The process of incorporating tungsten foil necessitates precise control over process parameters, thereby significantly restricting its applications. First principles researches have demonstrated that the addition of elements such as zinc, copper and silver to TiAl 3 -based alloys can result in the formation of a stable L1 2 phase [134]. The larger radii of zinc, copper and silver atoms facilitate their preferential replacement of aluminium atoms in the crystal structure of TiAl 3 , thereby reducing the axial ratio of the TiAl 3 cell and lowering the average strength of the Ti-Al covalent bond. This transformation of L1 2 from a sub-stable to a stable structure is a consequence of the aforementioned changes. The addition of a zirconium interlayer to the weld has also been demonstrated to be an effective means of improving the plasticity of the weld and increasing the strength of the joint. The addition of zirconium elements into the Ti/Al interface is accompanied by an increase in their content, which transforms TiAl 3 from the brittle tetragonal structure D0 22 to the more plastic crystal structure D0 23 [135]. However, Karpets et al. do not indicate a detailed mechanism for transformation. In order to obtain optimal performance of the Ti/Al joint, Zhang et al. [136] incorporated silver elements into the weld. In order to avoid stress concentrations in the joint after welding, silver mesh is employed as an interlayer to guarantee that the silver can be fully melted into the weld seam. The results showed that silver atoms can replace aluminium atoms at specific positions in the TiAl 3 lattice, thereby changing the crystal structure of TiAl 3 to form a local superstructure. And, the addition of silver mesh also reduces the difference in mechanical properties between the aluminium substrate and TiAl 3 in the joint, reducing the tendency of the joint to crack under stress. On the one hand, the MgAg and MgAgAl formed in the fusion zone after the addition of the silver mesh play a strengthening role as the second phase. Meanwhile, the combination of MgAg and MgAgAl with the aluminium substrate produces solid solution strengthening to weld. On the other hand, the addition of the silver caused a large number of particles favourable to heterogeneous nucleation of aluminium grains to appear in the molten pool, effectively refining the grains in the fusion zone. However, the addition of silver mesh did not reduce the thickness of the IMCs layer and change the morphology of TiAl 3 , nor did the transition from D0 22 -TiAl 3 to L1 2 -TiAl 3 take place. It only enhanced the performance of the joints through the strengthening of the fusion zone, and the fracture still occurs at the interface with the fusion zone and aluminium substrate. However, the relatively poor wettability of the silver mesh with titanium and aluminum resulted in the silver mesh not spreading and fusing well during the welding process, which weakened its modulation effect on IMCs. The melting point of silver is 962°C, which is significantly lower than that of titanium. In the process of laser welding, elevated temperatures can lead to the premature melting or even evaporation of silver, resulting in the destruction of the structure of the silver mesh. In addition, the implementation of silver mesh as an interlayer contributes to a substantial escalation in welding costs, consequently leading to its limited utilization in large-scale industrial production. Zhang [137] et al. used titanium mesh as an interlayer connecting TC4 titanium alloy and 6061 aluminium alloy. It was shown that the titanium mesh altered the distribution of TiAl 3 in the weld seam, while enhancing the wettability and spreading of molten aluminium on the surface of the titanium substrate. As shown in Fig. 14 , the addition of the titanium mesh increases the reaction area between the molten aluminium and titanium, as well as the content of titanium atoms in the fusion zone. At the contact interface, diffusion of titanium atoms and aluminium atoms results in the formation of a thin layer of TiAl 3 , while on the aluminium side, granular TiAl 3 is produced in the fusion zone. A proportion of the particles will enter the fusion zone as a consequence of the heat flow in the molten pool and the surrounding grains during the growth of TiAl 3 at the interface. This results in a significant increase in the number of TiAl 3 particles present in the fusion zone. With the fusion zone cooling gradually, the granular TiAl 3 becomes a plasmonic point for heterogeneous nucleation of aluminium grains. Furthermore, these granular TiAl 3 become the second phase in the fusion zone, where it refines the grains, thereby strengthening the weld. However, Zhang et al did not investigate the change in thickness of the interfacial layer of IMCs after the addition of titanium mesh. The thickness of the IMCs layer is critical to the strength of the weld bond after welding. The reinforcement of the aluminium side of the fusion zone alone results in an overall inhomogeneity of the fusion zone, causing fracture to occur at the interface between the fusion zone and the titanium substrate. Moreover, the addition of titanium mesh did not change the type of the original IMCs, and only played a role in refining the grain in the fusion zone on the aluminum side, while the performance of the Ti/Al dissimilar joints was determined by the bonding strength at the interface. Therefore, the utilization of titanium mesh as an intermediate layer to modulate IMCs at the weld interface is ineffective. The interlayer, upon incorporation of Ti/Al dissimiliar joints, has been shown to enhance the strength of the joint through three mechanisms. To begin with, reaction path modulation has been demonstrated, whereby the elements in the interlayer preferentially react with Ti or Al to generate low-brittle IMCs, thus reducing the brittleness of the joint. Secondly, the interlayer functions as a diffusion barrier, blocking the direct contact between Ti and Al and inhibiting the mutual diffusion of the two elements. Finally, the interlayer functions as an intermediate band of stress buffer, absorbing thermal stress through plastic deformation and thereby preventing crack initiation. As demonstrated in Table 8 , the incorporation of the interlayer has been shown to enhance the strength of the joint to a substantial degree. Furthermore, the findings of the aforementioned study by et al. also demonstrated that the addition of the interlayer resulted in an improvement in the fracture mode of the joint. Prior to the incorporation of the interlayer, the fracture occurs within the IMCs layer, exhibiting characteristics of brittle disintegration. Subsequent to the addition of the interlayer, the fracture shifts to the interlayer/substrate interface or to the substrate itself, exhibiting ductile fracture characteristics. The addition of an interlayer has been demonstrated to affect a transition in the morphology of IMCs, thereby transforming them from continuous layers to diffusely distributed particles. This transition is concomitant with a shift in the IMCs from a brittle to a tough state, thus effectively addressing the issue of inadequate joint strength that is attributable to the brittleness of IMCs. The addition of an interlayer has been demonstrated to affect a transition in the morphology of IMCs, thereby transforming them from continuous layers to diffusely distributed particles. This transition is concomitant with a shift in the IMCs from a brittle to a tough state, thus effectively addressing the issue of inadequate joint strength that is attributable to the brittleness of IMCs. However, the incorporation of certain interlayers proved ineffective. This can be attributed primarily to the failure to select elements capable of forming solid solutions or low-brittle IMCs with both titanium and aluminium. Additionally, the melting points of the interlayers are considerably higher than those of titanium and aluminium, resulting in premature melting or unfusion (e.g. niobium). In order to optimise the preparation of interlayers for the purpose of inhibiting IMCs in Ti/Al dissimilar joints, and consequently improve the strength of said joints, it is necessary to minimise the thickness of interlayers. This can be achieved by means of optimising the preparation process, for example by using plating or pre-positioning of foils. However, it should be noted that an interlayer thickness of less than 10 µm is not effective in blocking diffusion. Secondly, the selection of interlayers with high temperature stability is crucial to prevent oxidation or volatilisation at elevated temperatures. Further optimisation of the material system and process compatibility of the interlayer is imperative to enhance its position in Ti/Al dissimiliar joints. 5. Summary and prospects It is challenging to prevent the formation of IMCs in the weld seam during the laser welding. The formation and distribution of these compounds can significantly impact the properties of Ti/Al dissimilar joints. Consequently, one of the principal concerns in Ti/Al dissimilar metal laser welding is the modulation of IMCs formation and distribution. The formation of IMCs in welds is primarily influenced by the welding heat and mass transfer processes. Based on the thermodynamics and kinetics of weld solidification, it is possible to homogenise the thermal distribution at the Ti/Al interface, promote atomic diffusion, reduce local segregation and shorten the high temperature residence time. This can be achieved by adapting the welding process, for example by using a dual beam laser, pulsed laser and laser oscillations. On the other hand, optimising the welding parameters, such as reducing the laser power, increasing the welding speed and offset distance, can reduce interfacial heat accumulation and modulate the growth and distribution of IMCs at the interface. In addition, the addition of an alloy or metal interlayer at the Ti/Al interface not only inhibits the formation of IMCs at the interface and alters their distribution, but also generates new ductile phases, thereby effectively improving the load-bearing properties of the joints. However, the quantitative mechanism of elemental diffusion at the interface and the growth mechanism of IMCs under thermodynamic coupling during the laser welding remain incompletely clear. Consequently, future research on Ti/Al laser welding may yield the development of new exogenous assisted processes, which could be achieved by combining a range of energy fields, including electric, magnetic, ultrasonic, and airflow fields. It has been demonstrated that these applied energy fields can be employed in laser welding of homogeneous metals with the objective of refining the grain structure in the fusion zone, reducing the occurrence of elemental segregation and crack susceptibility, and enhancing the joint properties. However, there is a paucity of relevant research investigating the impact of these applied energy fields on the formation of IMCs during Ti/Al welding. Meanwhile, the molecular dynamics diffusion model of the Ti/Al interface, in addition to the growth model of IMCs, can be modeled using simulation. The thermodynamic and kinetic conditions for the formation of IMCs can be investigated through the use of numerical simulations. This will enable a deeper understanding of the mechanisms involved in IMCs formation and growth, which will in turn inform the regulation of IMCs formation and distribution in weld micro-regions through the implementation of appropriate processes and other means. This will ultimately improve the performance of the joints, thus laying a solid foundation for a wider application of the Ti/Al dissimilar structures in the equipment manufacturing. Abbreviations IMC Intermetallic compound FZ Fusion zone HAZ Heat affected zone DLBSW Dual laser beam bilateral synchronous welding LOW Laser offset welding PP Polishing process PLP Pulsed laser pretreatment HEA High-entropy alloy Declarations Funding This work was financially supported by the Key Research and Development Program of Shaanxi (Grant No.2022GY-408). Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Authorship contribution statement Zhi-Ming Zheng : Writing - original draft, Visualization, Writing - review & editing. Jia Zhang : Writing - review & editing, Visualization. Ya-Zhe Xing : Writing-review & editing, Supervision, Conceptualization. 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Casalino, Low temperature heat treatments of AA5754-Ti6Al4V dissimilar laser welds: Microstructure evolution and mechanical properties, Optics & Laser Technology 100 (2018) 109-118. G. Casalino, M. Mortello, P. Peyre, Yb–YAG laser offset welding of AA5754 and T40 butt joint, J. Mater. Process. Technol. 223 (2015) 139–149. G. Casalino, S. D’Ostuni, P. Guglielmi, P. Leo, M. Mortello, G. Palumbo, A. Piccininni, Mechanical and microstructure analysis of AA6061 and Ti6Al4V fiber laser butt weld, Optik 148 (2017) 151-156. Z. Song, A. Wu, W. Yao, G. Zou, J. Ren, Y. Wang, Effect of beam offset on the microstrutures and properties of titanium/aluminum dissimilar alloy laser welded joints, Transactions of The China Welding Institution 2013,34(01):105-108+118. (In Chinese) S. Guo, Y. Peng, J. Zhu, Q. Gao, Q. Zhou, C. Cui, Microstructure and Mechanical Properties of Laser Welded Ti/Al Alloys[J]. Chinese Journal of Lasers, 2018, 45(11): 1102010. (In Chinese) J.C. Rawers, W.R. 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Casalino, Analysis of the Process Parameters, Post-Weld Heat Treatment and Peening Effects on Microstructure and Mechanical Performance of Ti–Al Dissimilar Laser Weldings, Metals 11(8) (2021) 1257. X. Zhou, X. Cao, F. Zhang, Z. Chen, J.a. Duan, Effects of AlSi12 interlayer on microstructure and mechanical properties of laser welded 5A06/Ti6Al4V joints, Welding in the World 65(7) (2021) 1389-1402. S.X. Lv, X.J. Jing, Y.X. Huang, Y.Q. Xu, C.Q. Zheng, S.Q. Yang, Investigation on TIG arc welding-brazing of Ti/Al dissimilar alloys with Al based fillers, Science and Technology of Welding and Joining 17(7) (2012) 519-524. S. Lv, Q. Cui, Y. Huang, X. Jing, Influence of Zr addition on TIG welding–brazing of Ti–6Al–4V to Al 5A06, Materials Science and Engineering: A 568 (2013) 150-154. X. Gu, L. Zhang, Laser lap welding of TC4 titanium alloy to 6082 aluminum alloy using a CoNiCuNb0.5 V1.5 high entropy alloy filler, Materials Letters 312 (2022) 131562. F.-S. Meng, Z. Yao, M. Všianská, M. Friák, M. Šob, Theoretical investigations on structural, elastic, thermodynamic and electronic properties of Al3Ti and Al3V compounds in L12 structure under high pressure, Materials Research Express 6(5) (2019) 056536. H. Wang, X. Yuan, T. Li, K. Wu, Y. Sun, C. Xu, TIG welding-brazing of Ti6Al4V and Al5052 in overlap configuration with assistance of zinc foil, Journal of Materials Processing Technology 251 (2018) 26-36. X. Gu, M. Cui, J. Chen, D. Sun, X. Gu, L. Liu, Laser welding of 6082 aluminum alloy to TC4 titanium alloy via pure niobium as a transition layer, Journal of Materials Research and Technology 13 (2021) 2202-2209. Y.V. Milman, D.B. Miracle, S.I. Chugunova, I.V. Voskoboinik, N.P. Korzhova, T. N. Legkaya, Y.N. Podrezov, Mechanical behaviour of Al3Ti intermetallic and Ll(2) phases on its basis, Intermetallics 9 (2001) 839–845. B.T. Tan, S.N. Wu, F. Anariba, P. Wu, A DFT study on brittle-to-ductile transition of D0(22)-TiAl3 using multi-doping and strain-engineered effects, J. Mater. Sci. Technol. 51 (2020) 180–192. Z. Zhang, J. Huang, Y. Dai, X. Zhang, C. Yao, Effect of tungsten mesh interlayer on microstructure and mechanical performance of A6061/Ti6Al4V dissimilar joints, Materials Characterization 182 (2021) 111569. M. Jahnatek, M. Krajci, J. Hafner, Response of fcc metals and L1(2) and D0(22) type trialuminides to uniaxial loading along 100 and 001: ab initio DFT calculations, Philos. Mag. 91 (2011) 491–516. X.Y. Zhang, Y.C. Huang, Y. Liu, X.W. Ren, A comprehensive DFT study on the thermodynamic and mechanical properties of L1(2)-Al3Ti/Al interface, Vacuum 183 (2021) 13. H. Hu, X.Z. Wu, R. Wang, Z.H. Jia, W.G. Li, Q. Liu, Structural stability, mechanical properties and stacking fault energies of TiAl3 alloyed with Zn, Cu, Ag: first-principles study, J. Alloys Compd. 666 (2016) 185–196. M.V. Karpets, Y.V. Milman, O.M. Barabash, N.P. Korzhova, O.N. Senkov, D. B. Miracle, T.N. Legkaya, I.V. Voskoboynik, The influence of Zr alloying on the structure and properties of Al3Ti, Intermetallics 11 (2003) 241–249. Z. Zhang, J. Huang, C. Yao, X. Zhang, Effect of Ag alloying on the microstructure and mechanical properties of laser welded-brazed Ti/Al dissimilar joints, Materials Science and Engineering: A 848 (2022) 143359. Z. Zhang, J. Huang, J. Fu, P. Nie, S. Zhang, Microstructure and mechanical properties of laser welded-brazed titanium/aluminum joints assisted by titanium mesh interlayer, Journal of Materials Processing Technology 302 (2022) 117502. Tables Tables 6 to 8 are available in the Supplementary Files section. Supplementary Files Table678.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revisions Needed 18 Jul, 2025 Reviewers agreed at journal 03 Jul, 2025 Reviewers invited by journal 03 Jul, 2025 Editor invited by journal 12 May, 2025 Editor assigned by journal 09 May, 2025 First submitted to journal 07 May, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6054738","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":480195016,"identity":"fd1ce53b-ce21-42ae-a00e-4051bf167542","order_by":0,"name":"Zhi-Ming 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Xing","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Ya-Zhe","middleName":"","lastName":"Xing","suffix":""}],"badges":[],"createdAt":"2025-02-18 09:28:18","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6054738/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6054738/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86195984,"identity":"2c113b91-f0a7-4de7-870f-8b3b9cb9976b","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":218615,"visible":true,"origin":"","legend":"\u003cp\u003ePassenger seat rails in Airbus cabins [21]\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/54b790afae01d2d71befd440.png"},{"id":86196130,"identity":"5e40d898-ae71-4be6-93db-019f3fb48801","added_by":"auto","created_at":"2025-07-07 21:35:39","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":83085,"visible":true,"origin":"","legend":"\u003cp\u003eTi-Al binary phase diagram [62]\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/c26318979d99939ccdc5b631.png"},{"id":86195985,"identity":"0cbfda6c-da8e-44d7-a840-d3bf229764c1","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":630515,"visible":true,"origin":"","legend":"\u003cp\u003eInterfacial IMCs microstructures of Ti/Al dissimilar joints welded at different parameters and zones: (a-c) 800 W/800 W-0.6 mm; (d-f)800 W/800 W-0.4 mm;\u003c/p\u003e\n\u003cp\u003e(g-i) 1000 W/600 W-0.6 mm [73]\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/21656911e8d41226f4b7a109.png"},{"id":86196131,"identity":"c7a0ab7e-1ee5-45e5-ac45-42b2369ba117","added_by":"auto","created_at":"2025-07-07 21:35:39","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":272739,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of IMCs formation at interface: (a) same power; (b) different power [73]\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/180dfe33e142f3ba761c3819.png"},{"id":86195993,"identity":"a9d1f50f-48c7-4a27-8ffa-3a135dc2136b","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":967845,"visible":true,"origin":"","legend":"\u003cp\u003eElement distribution of Ti/Al joint cross section with no oscillation, 150 Hz and 200 Hz [94]\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/5cc38c971cbcfcc188e1dc45.png"},{"id":86195988,"identity":"cea7e43b-e825-4b0c-bb60-922f2c24fcb8","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":131701,"visible":true,"origin":"","legend":"\u003cp\u003eInterfacial IMCs at different oscillation frequencies: (a–c) 30Hz 1.1mm, (d–f) 25Hz 1.1mm, (g–i) 28Hz 1.2mm [96]\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/b0103b7d02d05ad05ebee8d2.png"},{"id":86195997,"identity":"05aa7baf-fabc-4d50-a76c-84aeb2947560","added_by":"auto","created_at":"2025-07-07 21:27:40","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":211756,"visible":true,"origin":"","legend":"\u003cp\u003ePulse duration and average thickness of IMC layers above 882 °C [97]\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/493bfc018a220f188393b8d0.png"},{"id":86195990,"identity":"abf1174b-c8ce-4436-a09e-e6a96bcad183","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":598231,"visible":true,"origin":"","legend":"\u003cp\u003eInterfacial IMCs under different laser powers: (a-c) 1600 W (d-f) 1900 W (g-i) 2200W (j-1) 2500W [100]\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/bde38c2f3abe7d8bcd1680eb.png"},{"id":86195999,"identity":"7ecfae1c-9057-48f8-b7f0-e277af2fb15b","added_by":"auto","created_at":"2025-07-07 21:27:40","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":224053,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructures at speeds of 10 m/min and 50 m/min at the Ti/Al interface [106]\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/8397291e4b854f8810cb3920.png"},{"id":86196018,"identity":"b2139249-d883-48a0-9c0c-c9e3b503357c","added_by":"auto","created_at":"2025-07-07 21:27:40","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":57366,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of laser offset welding\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/3145d7f2ff523ca7b0cd984d.png"},{"id":86196016,"identity":"8244c112-9732-430f-83f9-9f66be428f79","added_by":"auto","created_at":"2025-07-07 21:27:40","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":905335,"visible":true,"origin":"","legend":"\u003cp\u003eTi/Al interface after polishing process (PP) and pulsed laser pretreatment (PLP) [117]\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/63c3d4b12d39782ade3a6bc1.png"},{"id":86196005,"identity":"19c8c313-5013-48c9-a527-58355ed79089","added_by":"auto","created_at":"2025-07-07 21:27:40","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":680027,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructures of A (72 J/mm) and B (36 J/mm) joints after heat treatment at 350°C and 450°C [107]\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/4de819c51f3814c5020a8fcc.png"},{"id":86195987,"identity":"df42904c-c642-4b0e-9d46-dee598cffe40","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":343956,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cross-section of the joint with Nb foil, (b) Ti/Nb diffusion interface, (c) Al/Nb metallurgical Interface [128]\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/0dd9b4490d0f05a8e670e9f4.png"},{"id":86196011,"identity":"d4c8eda6-01b5-4ece-a275-41bfccb68990","added_by":"auto","created_at":"2025-07-07 21:27:40","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":445723,"visible":true,"origin":"","legend":"\u003cp\u003eFormation process of TiAl\u003csub\u003e3\u003c/sub\u003e at the Ti/Al interface after addition of titanium mesh [137]\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/32d95dfebc71131c649b71f4.png"},{"id":86196613,"identity":"80a41293-148c-4d07-9c66-ce149e102602","added_by":"auto","created_at":"2025-07-07 21:51:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7063464,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/91a4f8fc-4d53-4416-bb82-8aad1840e190.pdf"},{"id":86195982,"identity":"8138b631-445c-4de3-a7f7-6c158d360cdd","added_by":"auto","created_at":"2025-07-07 21:27:39","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":29476,"visible":true,"origin":"","legend":"","description":"","filename":"Table678.docx","url":"https://assets-eu.researchsquare.com/files/rs-6054738/v1/b1cda742e1f926c8098bc2ca.docx"}],"financialInterests":"","formattedTitle":"A review on controlling the formation of intermetallic compounds in Ti/Al dissimilar metal laser weld seams","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn recent years, there has been an increasing demand for dissimilar metal welded structures in industries, which not only to offer excellent performance, but also to meet the specific needs of the affiliated industry [1]. Welded structures of dissimilar metals can make full use of the respective advantages of two materials [2, 3], which not only reduces the overall weight of the components, saves the excessive use of rare metals, improves the overall structural strength and corrosion resistance, but also improves energy efficiency and reduces solid waste emission. Therefore, dissimilar metal welded structures are widely used in aerospace, automotive, power and chemical fields [4].\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eApplications of Ti/Al dissimilar structures\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFields\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eApplications\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eAerospace\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHoneycomb sandwich structures for aircraft wings [18, 19]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSpaceship piping system [20]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eTransportation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCabin heatsinks [20]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePassenger seat rails [21, 22]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAutomotive\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eExhaust systems [23]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eApplications, advantages, and disadvantages of different Welding methods for Ti and Al.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWelding methods\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eApplications\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAdvantages\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDisadvantages\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDiffusion welding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eButt welding of Ti/Al\u003c/p\u003e\u003cp\u003eTurbine wheel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eExcellent mechanical properties and corrosion resistance\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSensitive to temperature and pressure\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFriction stir welding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eButt welding of Ti/Al\u003c/p\u003e\u003cp\u003eHeat exchanger\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLow thermal crack and porosity\u003c/p\u003e\u003cp\u003eReduce the formation of IMCs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDesign limitation for tooling by material and joint type\u003c/p\u003e\u003cp\u003eTooling wear\u003c/p\u003e\u003cp\u003eCold cracks and holes\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExplosive welding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eButt welding of Ti/Al\u003c/p\u003e\u003cp\u003eTi/Al composite plates\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLow interfacial resistance\u003c/p\u003e\u003cp\u003eSmall heat-affected zone\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRestricted operating environment\u003c/p\u003e\u003cp\u003eGenerate noise and airwaves\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBrazing\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eButt welding of Ti/Al\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLower thermal stress\u003c/p\u003e\u003cp\u003eSuitable for workpieces of all shapes and sizes\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eLower welding efficiency\u003c/p\u003e\u003cp\u003eHigh requirements for brazing materials\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUltrasonic welding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMulti-layer lap welding of Ti/Al\u003c/p\u003e\u003cp\u003eMedical devices\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eShort welding time and energy input\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDestruction of the metal structure by ultrasonic vibration\u003c/p\u003e\u003cp\u003eSensitive to surface roughness\u003c/p\u003e\u003cp\u003eRaising welding\u003c/p\u003e\u003cp\u003etemperature by\u003c/p\u003e\u003cp\u003ehigh-frequency vibration\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLaser welding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eButt and lap welding of Ti/Al\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHigh energy density\u003c/p\u003e\u003cp\u003eHigh precision\u003c/p\u003e\u003cp\u003eSmall heat-affected zone\u003c/p\u003e\u003cp\u003eHigh-speed welding\u003c/p\u003e\u003cp\u003eLarge depth-to-width ratio\u003c/p\u003e\u003cp\u003eRemoting welding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eHot cracking and porosity\u003c/p\u003e\u003cp\u003eIMCs formation\u003c/p\u003e\u003cp\u003eExpensive equipment\u003c/p\u003e\u003cp\u003eLaser absorptivity affects actual heat input\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThe physical properties of Ti and Al at room temperature [50]\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAl\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eIonization energy(eV)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDensity (kg\u0026sdot;m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4500\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2700\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eElastic modulus (N/ m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.2425\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMelting point (K)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1941\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e933\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBoiling point(K)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3558\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2793\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eViscosity (kg\u0026sdot; m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026sdot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.0052\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.0013\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSurface tension (N/m)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.914\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eThermal conductivity coefficient (W\u0026sdot;m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026sdot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e21.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e237\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eThermal diffusivity (m\u003csup\u003e2\u003c/sup\u003e/s)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.15\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.65\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpecific heat capacity (J\u0026sdot;Kg\u0026sdot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e519\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e917\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLatent heat of fusion (kJ/kg)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e419\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e398\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eThermal expansion coefficient (\u0026times;10\u003csup\u003e6\u003c/sup\u003e\u0026sdot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e23.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAmong many dissimilar metal structures, Ti/Al dissimilar structures have attracted great attention due to unique advantages. Titanium alloy exhibits high specific strength, good tenacity, and excellent corrosion and high temperature resistance. It is one of the most promising alloys in aerospace [5\u0026ndash;9], which known as the \u003cem\u003eaerospace metal\u003c/em\u003e and the \u003cem\u003efuture metal\u003c/em\u003e. Nevertheless, the availability of titanium alloy is limited, and its processing performance is suboptimal. Aluminium alloys are known as \u003cem\u003eflying metal\u003c/em\u003e and widely used in aerospace and transportation due to their lightweight, low cost, excellent thermal conductivity and processing performance, but it has a low melting point and poor strength. In order to meet the practical needs of metal structures in lightweight, functional integration and low cost, the combination of titanium alloys and aluminium alloys has induced increasing attention [13\u0026ndash;15]. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Ti/Al dissimilar structures have been widely used in aerospace, transportation and automotive industries [16, 17]. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the Ti/Al dissimilar structure is employed in the passenger seat rails of an Airbus aircraft. Thus, the joining of Ti/Al dissimilar metals has become one of the hot spots for domestic research.\u003c/p\u003e\u003cp\u003eSome researchers have used diffusion welding [24\u0026ndash;28], friction stir welding [29\u0026ndash;35], explosive welding [36, 37], brazing [38\u0026ndash;42] and ultrasonic welding [4,43\u0026ndash;44] to join titanium and aluminium alloys. Their respective strengths and weaknesses are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. As illustrated in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, they are not suitable for mass production due to their complex processes, high cost, low productivity, confined thickness and joint assembly. Laser welding can improve the flexibility and adaptability of joints effectively owing to high energy density, fast cooling, small heat affected zone, high productivity, precise control of heat input and heating position [45\u0026ndash;46], which is regarded as \u003cem\u003eGreen Manufacturing Technologies for the 21st Century\u003c/em\u003e [47]. It has unique advantages not only for same metals but for dissimilar metals during joining [48\u0026ndash;50]. And the use of laser to join titanium alloys and aluminium alloys has also been widely studied. However, due to the significant differences in physical and chemical properties of titanium and aluminium, as demonstrated in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, there are several problems during welding. To begin with, they would oxidise at high temperatures and absorb hydrogen, oxygen and nitrogen readily, causing weld defects such as porosity and white spots. Meanwhile, there is a difference for their melting point, when the temperature reaches the melting point of titanium, alloy elements in aluminium would evaporate and burn, resulting in an uneven distribution to weld components. What\u0026rsquo;s more, it is inclined to form brittle IMCs rather than solid solutions due to their low mutual solubility at room temperature and metallurgical incompatibility [51\u0026ndash;54], which increases the brittleness of joints. In addition, it is easy to generate residual stresses due to difference in thermal conductivity and coefficient of linear expansion after welding. And it is prone to form and spread for cracks in joint, which influenced by brittle IMCs [55]. One of the most critical issues for improving the quality of Ti/Al dissimilar joints is the modulation of brittle IMCs formation [56\u0026ndash;57], because the massive formation of IMCs would lead to brittle failure and a reduction in mechanical properties. Thus, inhibiting formation of IMCs in order to reduce their content at the Ti/Al interface, or reducing brittleness and stress of microstructures by decreasing thickness and changing distribution of IMCs to improve the strength and tenacity of joints, are of great importance in ensuring and improving the quality and service performance of Ti/Al dissimilar joints.\u003c/p\u003e\u003cp\u003eThe heat input of base metal is uneven during welding because of the base metal is heated locally. The fusion zone (FZ) absorbs most of the heat, while the heat affected zone (HAZ) and other regions absorb less heat, resulting in a large temperature gradient inside the joint, which not only leads to an uneven interaction between the interfaces [58,59], but also affects the growth pattern and morphology of microstructure at the interface during cooling. Moreover, it is an important factor for the actual contact area during welding, which can affect the formation of IMCs and the quality of joints. It would affect atomic diffusion and composition at the interface locally, which in turn influences the tissues formed at the interface. There is an inverse relation between contact area and stress. In other words, the contact area is larger, the internal stress is lower and the joint is more stable. The martensite with high hardness would appear in fusion zone and heat affected zone due to fast cooling after welding, forming uneven tissues. The post-weld heat treatment can not only modulate the tissues at the Ti/Al interface, but also improve the mechanical properties of joints. The addition of interlayer can reduce the mismatch of thermal expansion and stress at the Ti/Al interface during welding. It can change the metallurgical reaction at the Ti/Al interface, strengthening elements of interlayer can also improve the composition of brittle phase and increase the ductility of joints. In this paper, the current research status and principles on the modulation of interfacial intermetallic compounds in Ti/Al laser welded joints is reviewed from three aspects: laser welding heat input, pre-weld pretreatment and post-weld heat treatment, and interlayer. Finally, on the basis of analyzing the existing problems in the current intermetallic compound modulation research, the future research and development direction of modulating intermetallic compounds in Ti/Al welded joints is proposed.\u003c/p\u003e"},{"header":"2. Effect of thermal input on formation of IMCs","content":"\u003cp\u003eThe laser with high power density irradiates the base metal and creates a keyhole in the weld seam during welding titanium alloy and aluminium alloy. The transfer of heat occurs through the outer wall of the hole cavity in all directions, melting the surrounding metal to form a molten pool. During this process, titanium and aluminium atoms migrate towards each other through the Ti/Al contact interface, which results in the metallurgical joining of Ti/Al dissimilar metals and the formation of IMCs. The formation of IMCs is related closely to atomic migration during the melting stage of the weld and crystal growth during the solidification stage. During the melting stage, the atoms in the molten pool migrate by both diffusion and convection, with diffusion playing a dominant role and convection occurring only in local areas [60]. Atomic diffusion is mainly dominated by temperature, which changes diffusion distance and relative concentration of Ti/Al atoms in the molten aluminium and molten titanium directly, which in turn determines the composition of the IMC layers. Song [61] et al. found that TiAl\u003csub\u003e3\u003c/sub\u003e formed and grew on the aluminium side of the interface, suggesting that the formation of TiAl\u003csub\u003e3\u003c/sub\u003e is controlled by the diffusion process of titanium atoms through the TiAl\u003csub\u003e3\u003c/sub\u003e/Ti interface and the TiAl\u003csub\u003e3\u003c/sub\u003e phase. Jiang [60] et al. found that Ti\u003csub\u003e3\u003c/sub\u003eAl and Ti\u003csub\u003e2\u003c/sub\u003eAl in the interfacial IMC layers were located in the mixing region near titanium, and TiAl\u003csub\u003e3\u003c/sub\u003e and TiAl were located in the mixing region near aluminium. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the binary phase diagram of Ti-Al [62], according to Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, β-Ti precipitates from the liquid titanium during the initial stage of solidification within the melten pool, and transforms into α-Ti and Ti\u003csub\u003e3\u003c/sub\u003eAl depending on the aluminium content. The limited solubility of liquid aluminium for titanium results in the titanium content and temperature determining the precipitation of TiAl\u003csub\u003e3\u003c/sub\u003e. Meanwhile, the Gibbs free energy of TiAl\u003csub\u003e3\u003c/sub\u003e is lower than that of TiAl and Ti\u003csub\u003e3\u003c/sub\u003eAl [19], so it forms as 3Al\u0026thinsp;+\u0026thinsp;Ti\u0026rarr;TiAl\u003csub\u003e3\u003c/sub\u003e firstly. As the solidification rate increases in the latter stage, local convective mixing at the interface is limited and heat transfer is faster near the substrate on both sides of the molten pool, making solidification to begin from the sides towards the centre. TiAl is not precipitated directly from liquid phase aluminium directly. It is formed by diffusion of titanium atoms in the form TiAl\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2Ti\u0026rarr;3TiAl. And titanium atoms combine with TiAl by diffusion to form Ti\u003csub\u003e2\u003c/sub\u003eAl as TiAl\u0026thinsp;+\u0026thinsp;Ti\u0026rarr;Ti\u003csub\u003e2\u003c/sub\u003eAl simultaneously. Consequently, the composition and thickness of the IMCs layer can be adjusted by modulating the temperature of the molten pool through alterations to the heat input. In addition, the plasticity of the weld can be improved by controlling the heat input to alter the growth pattern of the IMCs during solidification. The occurrence of significant temperature gradients is a consequence of the high heating and cooling rates, which results in an uneven distribution of heat absorption in the fusion zone, heat-affected zone and base metal of the joint. Appropriate heat input can reduce the temperature gradient on either side of the interface during solidification, allowing the IMCs grains to grow in a dendritic fashion to form a more ductile serrated structure and increase the interface bonding area, which is beneficial to hinder crack propagation and improve the strength of the joints [51].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe formation of IMCs is influenced by the heat input, which is primarily controlled by the laser beam and welding parameters. Their respective effects on the formation of IMCs will be analysed in the following sections.\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Effect of laser beam on formation of IMCs\u003c/h2\u003e\u003cdiv id=\"Sec4\" class=\"Section3\"\u003e\u003ch2\u003e2.1.1 Dual beam laser\u003c/h2\u003e\u003cp\u003eTomashchuk et al. [63] reported the joining of T40 titanium alloy and AA5754 aluminum alloy with V-shaped groove and filler wires by using a dual spot tandem laser beam. The findings indicated that the dual beam laser could extend the interaction zone encompassed by the energy, with two spots exhibiting a symmetrical power distribution and both reaching 50% of the initial power, creating more uniform heat input to the seams. Concurrently, the 60\u0026deg; V-bevel facilitates the directional wetting of the titanium surface by molten aluminum, thereby establishing a continuous TiAl\u003csub\u003e3\u003c/sub\u003e layer with a thickness ranging from 2 to 5 \u0026micro;m. The joint shear strength achieved is 152 MPa, representing a 38% enhancement compared to the conventional single-beam laser plus planar butt-joint method. However, the positive correlation between laser line energy and IMCs thickness established by Tomashchuk does not take the effect of interspot distance on transient heat flow into account. The change in spot distance triggers fluctuations in the interfacial temperature, leading to non-uniform growth of IMCs, which contradicts the initial conclusion. Furthermore, beam splitting may result in an insufficient heat input at the bottom of the weld, leading to the formation of unfused joints. This subsequently reduces the mechanical properties of the joint. On the other hand, Tomashchuk attributed the increase in joint strength to the thickness control of IMCs, but did not quantify the effect of residual stress distribution on joint strength. V-bevels have been shown to induce elevated residual stresses within the joint, which have the potential to counteract the benefits achieved through thickness optimization of IMCs. This, in turn, can result in a decline in long-term fatigue performance of the joints.\u003c/p\u003e\u003cp\u003eIn order to address these issues, Dual Laser Beam Bilateral Synchronous Welding (DLBSW) has become a feasible solution due to its high efficiency, small deformation, and narrow seams. And it has been successfully employed in the lightweighting of aircraft structures [64, 65]. Chen et al. [66.67], Tian et al. [68] and Zhan et al. [69] investigated the temperature field and residual stresses using finite element simulation for T-joints of alloys such as 2219 and TC4. The results demonstrated that the joint strength can reach 80\u0026ndash;90% of the base metal strength, and that the ductility of the joint can also be enhanced by adjusting the welding speed in DLBSW. The researches by Zhao [70, 71] et al. and Liu [72] et al. also showed that DLBSW could improve the mechanical properties of T-joints at low power and welding speed. Based on their finding, Zhang [73] et al. applied DLBSW to the joining of Ti/Al dissimilar metals.\u003c/p\u003e\u003cp\u003eIt was found that the fluctuation range of IMCs thickness was reduced from \u0026plusmn;\u0026thinsp;2.1 \u0026micro;m to \u0026plusmn;\u0026thinsp;0.4 \u0026micro;m for conventional unilateral welding when the energy ratio of both sides was 1:1 and the laser beam was offset to the aluminum side by 0.6 mm. The tensile strength of the joint reaches 139 MPa, and the fracture mode of the joint changes from localized brittle fracture to ductile fracture.\u003c/p\u003e\u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the average thickness of the interfacial IMC layers is less than 4 \u0026micro;m, with a serrated distribution at the same power on both sides. In the event that the power differs between the two sides, the maximum thickness of the interfacial IMCs layer exceeds 15 \u0026micro;m, exhibiting a serrated distribution in addition to the presence of rods and layers, as demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eAverage thickness of IMC layers under different laser powers [73]\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLaser power(W)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eLaser offset(mm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eAverage thickness(\u0026micro;m)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLeft\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMiddle\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRight\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e800/800\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.38\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e800/800\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.92\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.98\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1000/600\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e16.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e3.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAs the thickness (\u003cem\u003ed\u003c/em\u003e) of IMCs layer formed at the interface is controlled by the reaction temperature (\u003cem\u003eT\u003c/em\u003e) and the reaction time (\u003cem\u003et\u003c/em\u003e),\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}\\text{d}\\text{=}\\sqrt{\\text{Kt}} （\\text{1}）\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}\\text{K}\\text{=}{\\text{K}}_{\\text{0}}\\text{exp}\\left(\\text{-}\\text{Q}/\\text{RT}\\right)（\\text{2}）\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere K is the diffusion coefficient, K\u003csub\u003e0\u003c/sub\u003e is the proportionality constant, Q is the diffusion activation energy, R is the gas constant, and T is the reaction temperature. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, if the power is the same on both sides, the top and bottom of the seam will be heated for a longer time and their temperatures will be higher than those in the middle of the seam, so the thickness of the IMCs layer at the top and bottom will be greater than that in the middle. Moreover, the temperature gradient between the IMCs grains and the solid-liquid interface is greater, resulting in significant supercooling. As the weld cools at a rapid rate, the grains grow into small dendrites and penetrate the liquid phase at different rates, forming IMCs with serrated and rod-shaped structures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe dual beam laser is capable of regulating the thermal input to the joint and effectively improve the thermal distribution at the Ti/Al interface, forming IMC layers with a more even thickness and serrated shape. This, in turn, leads to an improvement in joint strength. However, the thickness of base metal is key to welding quality during using dual beam laser. When welding thick plates (\u0026gt;\u0026thinsp;2 mm), the difference in the surface absorptivity of titanium and aluminum results in the actual heat input on both sides not being symmetrical, resulting in an increase in the thickness of the local IMCs layer. In the joining of thin plates (\u0026lt;\u0026thinsp;2 mm), DLBSW tends to induce warping and deformation of the plates, and the joints need to be annealed of stress-relieved after welding, which increases the process flow and time.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.1.2 Laser oscillations\u003c/h2\u003e\u003cp\u003eTo begin with, the oscillation of the beam along specific paths during welding produces a stirring effect on the molten pool, which increases the fluidity of the molten pool and makes the heat absorbed by the joint more even, improving the quality of seam formation effectively. Moreover, the molten pool is subjected to thermal cycling during beam oscillation repeatedly, which prolongs the solidification time of the molten pool and allows sufficient time for the gas to escape, thereby reducing the occurrence of defects such as porosity in welds. [74\u0026ndash;78]. On the other hand, the molten pool can reduce the elemental segregation, break the dendrites and refine the grains significantly after stirring and repeated thermal cycling. Many researchers have conducted a multitude of studies on the laser oscillating welding of aluminium alloys. The main patterns of laser oscillation are\u0026lsquo;S\u0026rsquo;-shaped oscillation [79], circular oscillation [80\u0026ndash;85], \u0026lsquo;8\u0026rsquo;-shaped oscillation [83, 86], \u0026lsquo;\u0026infin;\u0026rsquo;-shaped oscillation [79, 84, 87\u0026ndash;89], three-dimensional oscillation [90], sinusoidal oscillation [91, 92], \u0026lsquo;S\u0026rsquo;-shaped plus sinusoidal oscillation [93] and so on.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn improving the properties of Ti/Al dissimilar joints using oscillating laser, Zhou [94,95] et al. found that when the oscillation frequency was 150 Hz and the amplitude was 1.2 mm, the thickness of the interfacial IMCs layer decreased to 2.3 \u0026micro;m from 8.5 \u0026micro;m at an oscillation frequency of 0, and the TiAl\u003csub\u003e3\u003c/sub\u003e at the interface showed a discontinuous distribution. At this point, the maximum load of the joint reaches 1852 N, which is 76.38% higher than when the oscillation frequency is 0. This is due to the fact that the stirring effect of the laser oscillation on the molten pool inhibits the diffusion of aluminum atoms into the titanium matrix, thus suppressing the generation of aluminum-rich compounds, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Meanwhile, the oscillation of the laser enhances the molten pool flow, promotes the uniform diffusion of the elements, and reduces the peak interface temperature from 1120\u0026deg;C to 860\u0026deg;C, thus suppressing the increase in the thickness of the TiAl\u003csub\u003e3\u003c/sub\u003e layer. The study further indicates through molecular dynamics simulations that the oscillating laser can reduce the Ti/Al interfacial energy from 1.45 J/m\u0026sup2; to 0.92 J/m\u0026sup2;, which effectively reduces the driving force for nucleation of IMCs.\u003c/p\u003e\u003cp\u003eIn order to further reduce the thickness of IMCs layer, Chen [96] et al. used an S-shape plus sinusoidal laser oscillation. The high power of the laser is applied to the aluminium alloy with low melting point, the low power is applied to the titanium alloy with high melting point, and the aluminium alloy melts and spreads towards the titanium alloy side to form a joint. With an oscillation frequency of 30 Hz and a laser beam offset by 1.2 mm to the aluminum side, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the maximum thickness of interfacial IMCs layer was reduced from 7.4 \u0026micro;m to 4.9 \u0026micro;m due to the lower power on the titanium alloy side slowing down the temperature gradient at the interface, optimising the interfacial heat distribution, and suppressing the overgrowth of IMCs. The thickness of the IMCs layer at different locations in the weld was less than 2 \u0026micro;m, resulting in a joint strength of 173 MPa.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe oscillation frequency plays a pivotal role in determining the thickness of IMCs layer in laser oscillation welding. The thickness of IMCs layer at the interface decreases significantly as the oscillation frequency increases. As the frequency of the beam increases, the intensity of the stirring of the molten pool also rises. This results in a more homogeneous distribution of heat at the interface, with no localized heat concentration. Meanwhile, the atoms in the molten pool diffuse more even under the influence of stirring, which reduces the local segregation and consequently makes the thickness of IMCs decrease.\u003c/p\u003e\u003cp\u003eHowever, as the frequency of laser oscillation continues to increase, the heat flow to the molten pool would affect the seam formation and, as a consequence, decrease the stability of the keyhole. It also increases porosity and affects joint performance adversely. Furthermore, when the oscillation frequency is excessively high, the inadequate thickness of the IMCs layer of the joint hinders the formation of an effective connection between titanium and aluminum, leading to a substantial degradation in joint performance. In addition, the disparity in thermal conductivity between titanium and aluminum, in conjunction with the heat flow induced by mechanical stirring during laser oscillation, gives rise to asymmetric heat flow within the molten pool. This results in a substantial temperature gradient between the center and the edge of the molten pool. Under this effect, the thickness of IMCs at the edge of the molten pool will be much larger than that in the interior, making the edge of the molten pool the starting point of joint fracture.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e2.1.3 Pulsed laser\u003c/h2\u003e\u003cp\u003eIt can be divided into continuous wave and pulsed wave for laser. Compared with continuous wave, there are several advantages for pulsed wave in welding. First of all, it should be noted that the material has solidified, which means that changes in the keyhole during one pulse period will not affect subsequent cycles due to the rapid solidification of the molten pool. This makes the keyhole more stable. Then, the minimum residence time of the metal in the molten state can be obtained by adjusting the pulse frequency and period, which can not only inhibit the growth of IMCs but reduce the cracks and porosity in seams. Liu [97] et al. employed finite element analysis to investigate the thermal distribution of the Ti/Al interface at varying pulse frequencies and the impact of pulse period on the thickness of IMCs layer. The results indicated that there is a notable difference in the thickness of IMCs at each part of the interface due to different temperature residence times above the isotopic transition temperature (882\u0026deg;C) of titanium at different pulse periods, as demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThis can be attributed to the isotopic transformation of titanium, which directly affects its solubility rate in aluminum. Below 882\u0026deg;C, titanium exists in a close-packed hexagonal (HCP) structure, designated as α-Ti. At temperatures above 882\u0026deg;C, titanium assumes a body-centered cubic (BCC) structure, designated as β-Ti. The dissolution rate of β-Ti for aluminium atoms is greater than that of α-Ti for aluminium atoms, resulting in faster growth of IMCs above 882\u0026deg;C. As the total pulse time increases, the pulse duration above 882\u0026deg;C also increases. Furthermore, the Ti/Al interface remains above 882\u0026deg;C for a longer period of time. Therefore, it is effective to reduce the thickness of IMCs layer at the interface by reducing the pulse period during pulsed laser welding.\u003c/p\u003e\u003cp\u003eHowever, the study by Liu et al. was based on a linear relationship between the effect of pulse frequency on heat input, but failed to take the nonlinear effects of pulsed-wave laser waveforms (e.g., square and sinusoidal) on the solidification rate of the molten pool into consideration. Therefore, the effects of pulse frequency and pulse waveform on the formation of IMCs at the Ti/Al interface need to be further investigated in depth. Meanwhile, the elevated cooling rate of pulsed lasers relative to continuous-wave lasers can precipitate thermal cracking of the joints when the power is excessively high.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Effect of welding parameters on formation of IMCs\u003c/h2\u003e\u003cp\u003eTwo key parameters in continuous wave laser welding are laser power and welding speed, which directly determine the heat input during welding. The beam offset distance is the distance between the beam and the centre line of the seam during welding. By adjusting the offset distance within a certain range, it is possible to alter the thermal input of the joint. Therefore, the heat input of the joint can be adjusted by modifying the laser power, welding speed, and beam offset distance. Subsequently, the thickness and distribution of the IMCs can be modulated.\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1 Laser power\u003c/h2\u003e\u003cp\u003eChen [100] et al from the Harbin Institute of Technology used dual-spot laser to join TC4 titanium alloy and 6061 aluminum alloy, and they investigate the effect of laser power on IMCs at the interface. The results demonstrated that the thickness and distribution of the IMCs layer undergo a change with an increase in laser power. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, as the laser power increases, the average thickness of the IMCs layer also increases. A constant power input results in the greatest thickness of the IMCs layer at the top regions of the interface. When the laser power is 1900W, the joint strength can reach 241 MPa, when the laser power is 2500W, the joint strength is 185 MPa. Meanwhile, as the laser power was increased, the shape of the IMCs layer underwent a transformation, initially adopting a lamellar shape, subsequently evolving into a serrated morphology, and ultimately assuming a club-shaped structure. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, the distribution of interface IMCs is observed to vary with differing laser powers. This result is also identical to that reported by Zhang [73] et al.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEffect of laser power on the average thickness of IMC layers [100]\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLaser power(W)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAverage thickness(\u0026micro;m)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTop\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMiddle\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eBottom\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1600\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1900\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.35\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.15\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2500\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.10\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe energy of the beam is transferred from top to bottom during welding, so the top regions of the interface are hotter than the middle and bottom regions, and their residence time at high temperature is longer, which makes the thickness of IMCs layer in the top regions is much greater than that in the middle and bottom regions. A lower power results in a decrease in heat absorption at the bottom of the interface, which in turn leads to a reduction in temperature and a corresponding reduction in the rate of interatomic diffusion. Meanwhile, the reduction in supercooling resulted in the IMCs developing into a lamellar structure. As the laser power increases gradually, the temperature gradient at the interface increases, and so does the subcooling. During the rapid cooling of the molten pool, fine dendrites are formed at the interface, and these fine dendrites grow into the liquid phase with different growth rates, forming a serrated shape eventually. As laser power continues to increase, the top regions of the interface absorb too much heat and become hotter, increasing the dynamics of atomic diffusion. The growth rate of fine dendrites in the liquid phase also increases with increasing supercooling. The titanium atoms become more diffusible and will diffuse in a particular direction, forming IMCs with club-shaped and acicular-shaped. Consequently, the formation of IMCs at the upper part of the interface is more sensitive to changes in laser power.\u003c/p\u003e\u003cp\u003eSun [101] et al. employed laser spot welding to get lap joints of TC4 and 5052 by varying the laser power at different defocusing distances. The results showed that, for a certain defocusing distance, the lower power results in the formation of two separate pools of titanium alloy at the top and aluminium alloy at the bottom of the joint respectively, which are not in contact with each other. This avoids the direct mixing of the two metals in the liquid phase, thereby suppressing the formation of IMCs. The thickness of the TiAl\u003csub\u003e3\u003c/sub\u003e formed at the interface increases with rising of power and changes from a serrated shape to an acicular shape. By increasing the laser power further, a large amount of liquid titanium enters the weld, forming a pinned structure. The titanium and aluminum at the edges of the structure will undergo a transition from metallurgical bonding to diffusion bonding, resulting in the formation of two different IMCs, TiAl\u003csub\u003e2\u003c/sub\u003e near the titanium side and TiAl\u003csub\u003e3\u003c/sub\u003e near the aluminum side.\u003c/p\u003e\u003cp\u003eIt is key to determine the reliability of the joints for the thickness of the IMCs layer. Anil [102] et al. proposed that the interfacial temperature, flow rate and diffusion rate are the key factors affecting the thickness and microstructure of IMCs. The actual power absorbed by the metal surface is less than the laser power due to the reflection of the beam by the metal surface during welding, and there is a linear relation between absorbed power and laser power. A research of the growth process of IMCs by Zhao [103] et al. showed that, the IMCs grow horizontally at the interface by reaction diffusion firstly and thicken in the vertical direction of the interface subsequently. So, there is also a linear relationship between actual absorbed power and thickness of the IMCs. Therefore, the thickness of the IMC layers at the interface can be reduced by reducing the laser power appropriately while other parameters remain constant.\u003c/p\u003e\u003cp\u003eHowever, it remains challenging to regulate the uniformity of the thickness of IMCs at the interface through adjustments in laser power. The inhomogeneous distribution of laser power in the joints is a primary factor contributing to the inhomogeneous thickness of the IMCs layer. The focusing characteristics of the laser beam results in the highest power density at the center of the weld and lower power density at the edges of the weld. This inhomogeneous distribution of power leads to significant temperature differences at different locations of the weld. At the center of the weld, due to the higher temperature, the formation of the IMCs layer is faster and the thickness is larger, while at the edge of the weld, due to the lower temperature, the formation of the IMCs layer is slower and the thickness is smaller, which leads to the non-uniformity of the thickness of the IMCs layer in the weld.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Welding speed\u003c/h2\u003e\u003cp\u003eTomashchuk et al. [104] studied the distribution of IMCs at the Ti/Al interface after high speed welding. The results demonstrated that high speed welding minimizes the interaction time between titanium and aluminum, prevents the overmixing of the two metals in the liquid state, and reduces the mixing area, which subsequently reduces the area of the IMCs. Zhou [105] also demonstrated that an increase in welding speed results in a reduction in the heating time of the molten pool, the temperature of the molten pool, and the heat per unit area within the molten pool. This causes insufficient incentive for the atoms to diffuse into each other. Meanwhile, high welding speeds would increase the solidification rate of the molten pool, resulting in insufficient time for the IMCs to grow, thus reducing their thickness.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThis finding was confirmed by Lee [106] et al, too. Their research showed that the distribution of IMCs at the interface will change due to increasing of welding speed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Because the dendritic IMCs at the interface are mainly transformed from the titanium-containing phase, and as the welding speed increases, the coarse and sparse dendrites are transformed into fine, dense dendrites. Moreover, the IMCs with island shape in the fusion zone would transform into a granular shape, which forms a dispersed distribution. A research by Leo [107] et al. also demonstrated that an increase in welding speed has a significant impact on the distribution of IMCs. Higher welding speeds result in a linear and approximately regular distribution of IMCs at the interface, which tends to be smooth. In contrast, lower welding speeds result in a significant degree of mixing between the two metals, which gives rise to a curved and protruding distribution of IMCs.\u003c/p\u003e\u003cp\u003eHowever, the above studies failed to form IMCs with uniform thickness even though the distribution of IMCs could be modulated by increasing the welding speed. Due to the large difference in viscosity and surface tension between liquid titanium and liquid aluminum, a complex flow field would be formed in the molten pool during the welding process. When only the welding speed is varied, the mixing and convection in the molten pool is not uniform due to the difference in fluidity between liquid titanium and aluminum, resulting in inconsistent conditions for the formation of IMCs. Liquid aluminum exhibits superior fluidity, readily spreading in the molten pool. Conversely, liquid titanium demonstrates inferior fluidity, resulting in delayed diffusion, leading to uneven thickness of IMCs layer. In addition, the filling and spreading behavior of the liquid metal changes as the welding speed changes. At higher welding speeds, liquid aluminum can fill the weld gap more rapidly due to its high fluidity. Conversely, liquid titanium will be underfilled due to its low fluidity, resulting in the formation of weld defects at the interface. At lower welding speeds, liquid titanium will collect in localized areas for a longer period of time, resulting in an increase in the thickness of the IMCs layer at that location. Therefore, a single change in welding speed does not result in a layer of IMCs with uniform thickness.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e2.2.3 Laser offset\u003c/h2\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e, in addition to applying the laser beam to the centre line of the weld directly, it is also possible to focus the beam on the side of the base metal with a certain distance from the centre line of the weld, which known as laser offset welding (LOW) [107]. Casalino et al. [56, 108\u0026ndash;109] offset the beam towards titanium alloy. Once a stable keyhole was formed on the titanium side, heat was transferred from the titanium alloy to the aluminum alloy, resulting in the melting of the aluminum alloy and the formation of the joint. It demonstrated that the distance at which the laser beam is deflected on the titanium alloy side has a direct impact on the proportion of the two base materials melted in the joint. Additionally, the average thickness of the IMCs layer increases with a reduction in the deflection distance. The same conclusion was reached by Song [110] et al. from Tsinghua University. Their research indicated that the heat transfer from the titanium alloy to the interface is greater when the offset distance is minimal, resulting in elevated interface temperatures and the formation of brittle phases, including TiAl\u003csub\u003e3\u003c/sub\u003e, TiAl\u003csub\u003e2\u003c/sub\u003e, Ti\u003csub\u003e3\u003c/sub\u003eAl, and TiAl. The thickness of IMCs layer at each part of the interface decreases gradually as the offset distance increases. The top of the interface formed club shape and acicular shape of TiAl and TiAl\u003csub\u003e3\u003c/sub\u003e, while the middle and bottom parts formed layer of TiAl\u003csub\u003e3\u003c/sub\u003e with a thickness of 1 \u0026micro;m.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs the laser is deflected towards the titanium alloy side, the heat absorbed at the Ti/Al interface increases gradually as the deflection distance decreases. This results in a rise in temperature, which in turn enhances the interatomic diffusion motion. A significant quantity of heat is transferred from the titanium side to the aluminum side, resulting in an increase in the quantity of aluminum melted and a gradual increase in the proportion of molten aluminum in the fusion zone. A reduction in the beam offset distance will result in an increase in the interface temperature and a prolongation of the cooling period required for the weld. This will consequently afford a greater opportunity for the brittle phase grains to develop. And the growth morphology of the brittle phase grains is influenced by the change in temperature gradient at each part of the interface, resulting in the formation of different shapes eventually. However, the joint did not form a complete metallurgical bond in the results of Song et al. The interface was fusion welded at the top of the interface and brazed at the bottom of the interface. The performance of the joint would be degraded due to discontinuity of the interface. Furthermore, the range of beam offsets selected for the study by Song et al. is limited, excluding the distribution of IMCs at the joint interface when the offset exceeds 0.7 mm. Consequently, the generalizability of the conclusions is constrained.\u003c/p\u003e\u003cp\u003eGuo [111] et al. offset the laser beam towards the aluminium alloy side and simulated the temperature field of the joint using finite element to investigate the effect of the offset distance on the distribution of IMCs at the Ti/Al interface. It was found that the interfacial IMCs are sensitive to the offset distance. When the offset distance was less than 0.5 mm, coarse TiAl\u003csub\u003e3\u003c/sub\u003e and TiAl formed at the interface and many microcracks appeared inside the joint. As the offset distance increases, TiAl\u003csub\u003e3\u003c/sub\u003e appears as long strips and blocks with an average thickness of approximately 15 \u0026micro;m. As the offset distance increases to 2 mm, the heat absorbed by the interface is reduced, the temperature is lowered and the atoms cannot diffuse with each other sufficiently, reducing the effective bonding area of the interface. The beam offset to the aluminum alloy would result in an uneven temperature distribution on both sides of the Ti/Al interface due to the differing thermal conductivities of the two metals. Aluminum exhibits a thermal conductivity of 237 W\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is greater than that of titanium (15.7 W\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The highest temperature at the top of the interface has exceeded the melting point of the titanium alloy, but the middle and bottom temperatures have not yet reached the melting point, resulting in a large amount of melting of the aluminium alloy and less melting of the titanium alloy, causing the concentration of aluminium atoms in the molten pool to be greater than the concentration of titanium atoms. As the offset distance to the aluminium alloy increases, the quantity of aluminum melted increases, and the concentration of aluminium atoms in the molten pool continues to increase. During the cooling process, TiAl\u003csub\u003e3\u003c/sub\u003e is prone to formation due to its high stability under high temperatures and high enthalpy, as well as its Gibbs free energy being smaller than that of Ti\u003csub\u003e3\u003c/sub\u003eAl and TiAl. [112\u0026ndash;114]. What is more, TiAl\u003csub\u003e3\u003c/sub\u003e exhibits high hardness and poor plasticity, which increases the brittleness of the joint and reduces the toughness of the joint.\u003c/p\u003e\u003cp\u003eThe fusion proportion of titanium alloys and aluminium alloys can be controlled by varying the offset distance on either side of the weld, which not only reduces the average thickness of the IMCs layer, but also changes it from a brittle continuous layer shape to a ductile serrated shape. However, when attempting to offset towards titanium, if the distance is insufficient, it can result in an increased proportion of molten aluminum. Furthermore, the vigorous mixing of two metals would lead to an increase in the thickness of the brittle phase layers at the interface. During offsetting towards aluminium alloys, the quality of the weld seam is significantly influenced by the presence of porosity, which is a consequence of the inherent properties of the aluminum alloy. [115, 116]. The energy efficiency of laser beam would decrease due to the high reflectivity of the aluminium alloy to the beam. Therefore, it is imperative to address this matter in order to reduce reflectivity and enhance processing stability during offset operations with aluminum alloys.\u003c/p\u003e\u003cp\u003eThe laser power has a direct impact on the heat input and molten pool temperature. The melting points of titanium and aluminium are significantly different, and high power increases the melt pool temperature and prolongs the residence time of the liquid metal. This, in turn, promotes the interdiffusion of titanium and aluminium, thus accelerating the formation of IMCs. As demonstrated in Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, an augmentation in power results in a thickening of the IMCs layer. This phenomenon can be attributed to an escalation in the dissolution of titanium and aluminium in the molten pool. Consequently, this leads to a protracted diffusive reaction between the two elements at the interface. This, in turn, results in a diminution in the strength of the joint. Concurrently, at elevated power levels, the interface predominantly generates either jagged or continuous layers of IMCs, while at low power, IMCs are distributed in islands or scattered. The metallurgical process of the molten pool is affected by welding speed, which alters both the duration of heat input and the cooling rate. In high-speed welding, the existence time of the molten pool is reduced and elemental diffusion is inhibited. Conversely, in low-speed welding, the diffusion time is prolonged and the growth of IMCs is promoted. It has been demonstrated that, at lower speeds, the IMCs layer is thicker due to the greater diffusion time, facilitating the formation of a continuous layer. Conversely, at higher speeds, the IMCs layer is thinner, resulting in a discontinuous distribution. As demonstrated in Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the strength of joints at elevated welding speeds is found to exceed that of low welding speeds. However, it should be noted that employing an excessively high welding speed may result in the presence of unfused joints or inadequate fusion depth. The distance at which the laser beam is directed towards the titanium or aluminium side is a critical factor in determining the melting ratio of titanium and aluminium and the degree of mixing of the elements in the molten pool. As demonstrated in Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, when the process is biased towards the titanium side, there is an increase in the amount of titanium melting, a decrease in the amount of aluminium melting, and an increase in the concentration of titanium in the molten pool. This facilitates the formation of titanium-rich IMCs (e.g., Ti\u003csub\u003e3\u003c/sub\u003eAl). Consequently, the IMCs layer will be concentrated on the titanium side of the interface, exhibiting a thin thickness. In circumstances where the bias is oriented towards the aluminium side, there is an observed increase in the amount of aluminium that melts, whilst simultaneously there is a decrease in the amount of titanium that melts. Concurrently, the concentration of aluminium in the melt pool is increased, thereby facilitating the formation of aluminium-rich IMCs (e.g. TiAl\u003csub\u003e3\u003c/sub\u003e). During this process, the thickness of the IMCs layer is increased and becomes more widely distributed, thus enhancing the brittleness of the joint. It is therefore vital to increase the power of the laser when welding towards the titanium side in order to compensate for its high melting point, or to decrease the power or increase the speed of the laser when welding towards the aluminium side in order to reduce the excessive melting of aluminium, which has a significant effect on the strength of the joint.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"3. Effect of pretreatment and post-weld heat treatment on formation of IMCs","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Pretreatment of Ti/Al connector before welding\u003c/h2\u003e\n \u003cp\u003eZhao [117] et al. used pulsed laser pretreatment (PLP) to process the butt interface of titanium alloys before welding and compared it with the interface after polishing process (PP) to investigate the effect of PLP on the formation of interfacial IMCs. As illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e, the results demonstrate that after the interface has been treated with PLP, not only has the thickness of IMCs layer been diminished, but also their distribution has undergone a notable transformation in comparison to that of PP. And there is a change from a three-layer sequential distribution (TiAl\u003csub\u003e3\u003c/sub\u003e、TiAl and Ti\u003csub\u003e3\u003c/sub\u003eAl) to a bilayer mixed distribution(TiAl\u003csub\u003e3\u003c/sub\u003e、TiAl\u003csub\u003e3\u003c/sub\u003e་TiAl་Ti\u003csub\u003e3\u003c/sub\u003eAl).\u003c/p\u003e\n \u003cp\u003ePLP increases the surface roughness of titanium alloy, resulting in a significant increase in the effective contact area at the Ti/Al interface, which is 2.916 times greater than that of PP, thus reducing internal stresses between the interfaces. The contact surfaces exhibited significant bending at the microscopic due to the increased interfacial roughness. Furthermore, the curved interfaces prevented the growth of the IMCs, reducing their thickness by 74%. The reason for this is that the flat titanium surface treated with PP first comes into contact with a large number of aluminium atoms to form TiAl\u003csub\u003e3\u003c/sub\u003e, the deeper the diffusion goes into the titanium alloy, the greater the difficulty for the aluminium atoms to diffuse, resulting in a gradual decrease in their concentration. On the contrary, the concentration of titanium atoms gradually increases as the diffusion goes deeper into the titanium alloy. In this process, TiAl and Ti\u003csub\u003e3\u003c/sub\u003eAl are generated sequentially and ultimately resulting in a three-layer sequential distribution. For the interface after PLP, its high roughness creates a large number of pits in the surface and results in a reduction in the number of titanium atoms present on the surface, thereby limiting their participation in the reaction. A significant number of aluminium atoms will penetrate the interior of the pits, causing an uneven distribution of titanium atoms and resulting in a mixed distribution of TiAl\u003csub\u003e3\u003c/sub\u003e, TiAl and Ti\u003csub\u003e3\u003c/sub\u003eAl, with alternating layers of these phases in the inner layer.\u003c/p\u003e\n \u003cp\u003eHowever, the inhibition of IMCs growth by constructing a pit-structure on the titanium surface by PLP as proposed by Zhao et al. is based on the fact that the geometrical parameters of the pit-structure (e.g., depth, spacing) remain stable during the welding. In fact, during the welding thermal cycle, the pit-structure can dynamically collapse or locally fuse due to the variation of welding heat input, weakening its effectiveness in suppressing IMCs. Furthermore, following the PLP, the oxide film that has been newly formed on the titanium surface will exhibit localized thickness variations. This will result in fluctuations in the wetting angle of aluminum, which in turn will significantly alter the wetting behavior of aluminum during the subsequent welding process. This, in turn, will lead to a substantial difference in the thickness of the interfacial IMCs layer.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Post-weld heat treatment\u003c/h2\u003e\n \u003cp\u003eSeveral researchers have investigated the effect of post-weld heat treatment on the Ti/Al interfacial microstructures and mechanical properties of joints in some non-fusion welding methods, such as friction stir welding and explosion welding. Li [118] et al. reported the effect of heat treatment on the interfacial microstructures of Ti/Al friction stir welded joints, and they found that the even distribution of IMCs could improve the bond strength of the two alloys. Fronczek [119] et al. used the high temperature and pressure formed by explosion welding to join titanium alloy and aluminium alloy. It showed that TiAl\u003csub\u003e3\u003c/sub\u003e at the interface starts to form at 552\u0026deg;C in joint. Based on the idea that the temperature of post-weld heat treatment would have an effect on the IMCs. Leo [107] et al. conducted low-temperature heat treatments of two Ti/Al joints with different line energies at 350\u0026deg;C and 450\u0026deg;C respectively, in order to investigate the effect of heat treatments at different temperatures on interfacial IMCs. The results demonstrated that the thickness of the interfacial IMC layers exhibits a pronounced increase when the heat treatment temperature reaches 450\u0026deg;C. Meanwhile, at the local parts of interface, brittle phases, primarily composed of TiAl\u003csub\u003e3\u003c/sub\u003e, nucleate and grow on the original microstructures. Additionally, the particles in the fusion zone exhibit varying degrees of coarsening, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eIt can be demonstrated that TiAl\u003csub\u003e3\u003c/sub\u003e is thermodynamically stable at lower heat treatment temperatures [28], its nucleation and growth is induced by atomic diffusion at higher temperatures. When the heat treatment temperature was increased to 450\u0026deg;C, TiAl\u003csub\u003e3\u003c/sub\u003e reached thermodynamic equilibrium and titanium atoms were able to diffuse intracrystalline through the IMCs layer [118], providing the necessary raw materials for TiAl\u003csub\u003e3\u003c/sub\u003e growth, leading to an increase in the TiAl\u003csub\u003e3\u003c/sub\u003e area of interface [120]. On the other hand, the IMCs at the interface of the joints with lower line energies were transformed from continuous layers to fragmented clubs after heat treatment at 350\u0026deg;C, and no growth of the original IMCs layer was found. Therefore, an appropriate heat treatment temperature could improve the mechanical properties of the joints, transforming the IMCs from highly brittle continuous layers to clubs and reducing the concentration of interfacial stresses. A further study by Leo [121] et al. demonstrated that the thickness of IMCs did not begin to increase until a heat treatment temperature of 530\u0026deg;C.\u003c/p\u003e\n \u003cp\u003eThe thickness of the IMCs layer has been observed to decrease at a heat treatment temperature of 350\u0026deg;C, as reported in the study by Leo et al. However, due to the difference in thermal conductivity between titanium and aluminum, the temperature field of the joint is not uniformly distributed as the temperature rises. The temperature rise and thermal gradient will also be greater on the aluminum side than on the titanium side due to higher thermal conductivity of aluminum. In the context of an inhomogeneous thermal gradient, the thickness of the IMCs layer is subject to localized fluctuations. In addition, the increase in joint strength is partly due to the change in the morphology of IMCs and partly possibly due to the reduction of residual stresses in the joints after heat treatment, whereas the study by Leo et al. did not quantify the change in residual stresses introduced by the heat treatment. Therefore, the changes in residual stresses should be taken the mechanism of post-weld heat treatment to enhance the strength of Ti/Al joints into account.\u003c/p\u003e\n \u003cp\u003eAs demonstrated in Table \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e, the strength of the joints that have undergone pretreatment is significantly higher than that of the untreated joints. This finding provides a viable solution for enhancing the performance of Ti/Al dissimilar joints in actual service conditions. The application of heat treatment has been demonstrated to have a negligible effect on enhancing joint strength, and in some cases, there is a decline in strength. This phenomenon may be attributed to the elevated formation temperature of IMCs. Furthermore, it has been observed that heat treatment at low temperatures merely serves to alleviate joint stresses, without exerting an effect on IMCs.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Effect of the Interlayer on formation of IMCs","content":"\u003cp\u003eThe formation of IMCs can be suppressed to a certain extent and their morphology and distribution can be changed by modulating the heat input to the joints, as well as by pretreatment and post-weld heat treatment of the joints. However, these methods do not completely prevent the formation of brittle phases at Ti/Al interface. Therefore, if it is possible to transform brittle phases into IMCs with tenacity, or change their morphology to transform them into reinforced phases, the performance of the joints can be greatly improved. The addition of interlayers into Ti/Al joints has proven to be an effective method.\u003c/p\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e4.1 Alloy interlayer\u003c/h2\u003e\n \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\n \u003ch2\u003e4.1.1 Aluminium-based alloys\u003c/h2\u003e\n \u003cp\u003eAluminium-based alloys have the advantage of a low melting point and excellent wettability, and their main elements are aluminium and silicon, so the addition of Ti/Al joints does not introduce other elements. As a result, aluminum-based alloys are widely used for joining titanium alloys and aluminum alloys. Zhou [122] et al. used AlSi12 as an interlayer to join TC4 and 5A06.Their research showed that the addition of AlSi12 resulted in the enrichment of silicon atoms at the Ti/Al interface, and the diffusion of silicon atoms was also enhanced, which in turn changed the weld composition. Meanwhile, due to the increase in the concentration of silicon atoms at the interface, the thickness of the interfacial IMCs decreases significantly with the addition of AlSi12 compared to the joints without the addition of AlSi12, suggesting that the increase in the silicon content at the interface plays an inhibitory role in the growth of IMCs.\u003c/p\u003e\n \u003cp\u003eOn the one hand, the addition of AlSi12 interlayer to the joints effectively promotes the formation of Ti(Al,Si) compounds, and these IMCs play the role of binder at the interface, which improves the bonding strength between titanium and aluminum. On the other hand, the Si in AlSi12 assists in refining the particles of IMCs, ensuring uniform distribution in the weld. This, in turn, enhances the microstructure uniformity and stress distribution of the joints, thereby mitigating the risk of crack initiation and extension due to the inhomogeneous growth of IMCs. However, the AlSi12 interlayer is more sensitive to process parameters due to its lower melting point. Its role in modulating IMCs needs to be realized under suitable conditions of laser power, welding speed, and shielding gas atmosphere. When the process parameters deviate from the appropriate range, the modulation effect of the AlSi12 interlayer is significantly diminished. This has been shown to result in the occurrence of welding defects, thereby increasing the instability of the welding process.\u003c/p\u003e\n \u003cp\u003eLv [123] et al. used an Al-Cu-La alloy as an interlayer added to a Ti/Al joint. It was shown that the addition of the lanthanum to the original Al-Cu alloy changed the IMC layers at the Ti/Al interface from the original single TiAl\u003csub\u003e3\u003c/sub\u003e to a composite layer of TiAl\u003csub\u003e3\u003c/sub\u003e and Ti\u003csub\u003e2\u003c/sub\u003eAl\u003csub\u003e20\u003c/sub\u003eLa. And the formation of Ti\u003csub\u003e2\u003c/sub\u003eAl\u003csub\u003e20\u003c/sub\u003eLa caused the hardness of the original IMCs to drop by more than half, reducing the brittleness of the joint markedly. The lanthanum in the Al-Cu-La alloy is easily polarized at the grain boundaries, which inhibits the growth of IMCs and makes the IMCs appear as fine particles, thus improving the bonding strength of the interface and the plasticity of the joint. Moreover, the addition of Cu and La changed the solute diffusion path and reaction kinetics during welding, improved the distribution of IMCs, avoided the stress concentration and crack initiation caused by the over-concentration of local IMCs, and improved the overall mechanical properties of the joint.\u003c/p\u003e\n \u003cp\u003eHowever, the Al-Cu-La alloy interlayer is also sensitive to the process parameters. In the event that these parameters are not properly controlled, the joints are susceptible to weld defects, including porosity and unfusion, which adversely affect the quality of the joints. Moreover, the elevated cost of Al-Cu-La alloys, in comparison to Al-Si system interlayer materials, results in a further escalation in production costs and a restriction of their extensive application in large-scale industrial production.\u003c/p\u003e\n \u003cp\u003eWhen zirconium is added to Al-Cu alloys [124], the content of TiAl\u003csub\u003e3\u003c/sub\u003e at the interface gradually decreases with increasing heat input. What is more, two new phases, L-(Ti,Zr)Al\u003csub\u003e3\u003c/sub\u003e and H-(Ti,Zr)Al\u003csub\u003e3\u003c/sub\u003e, are formed at the interface, which improves the tenacity of TiAl\u003csub\u003e3\u003c/sub\u003e and enhances properties of the joint. However, the preparation and use of Al-Cu-Zr alloy interlayers would greatly increase welding costs. In comparison with the prevalent Al-Si interlayer materials, Al-Cu-Zr alloys are more expensive and necessitate more stringent welding process parameters, which contributes to their relative infrequent utilization in large-scale industrial production. Therefore, composite interlayers can be developed by compositing Al-Cu-Zr alloys with other materials, such as ceramic and high-entropy alloys. This approach not only enhances the efficacy of IMCs modulation but also leads to a substantial reduction in cost.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\n \u003ch2\u003e4.1.2 High-entropy alloys\u003c/h2\u003e\n \u003cp\u003eGu [125] et al. used the high-entropy alloy (HEA) CoNiCuNb0.5V1.5 as an interlayer connecting TC4 and 6082. The results showed that after the addition of CoNiCuNb0.5V1.5 to the joints, solid solutions with FCC structure and solid solutions containing vanadium elements were formed at the interfaces of aluminium alloy with HEA and titanium alloy with HEA. At the interface between HEA and aluminum alloy, vanadium atoms combine with aluminum atoms to form VAl\u003csub\u003e3\u003c/sub\u003e with a thickness of only 0.8 \u0026micro;m. And at room temperature and low pressure (below 20 Gpa), VAl\u003csub\u003e3\u003c/sub\u003e exhibits superior tenacity in comparison to TiAl\u003csub\u003e3\u003c/sub\u003e [126]. At the interface between HEA and titanium alloy, HEA and titanium atoms diffused with each other to form a mixed layer with solid solution structure, and no brittle phase was detected at the interface.\u003c/p\u003e\n \u003cp\u003eThe conspicuous alteration in interfacial microstructures can be attributed to two key effects: the high-entropy mixing effect and the kinetic sluggish diffusion effect, which are both induced by the addition of HEA to the interface.To begin with, the high-entropy mixing effect of HEA improves the compatibility between different elements and promotes the formation of ductile solid solutions. In addition, elements contained in HEA also causes severe lattice distortion in the crystal structure of HEA. The kinetic sluggish diffusion effect induced by this lattice distortion hinders interatomic diffusion, thus preventing the mixing of titanium atoms and aluminum atoms and inhibiting the formation of brittle IMCs.\u003c/p\u003e\n \u003cp\u003eHowever, due to the difference in composition between aluminium alloy and HEA, there can be a large difference in flowability and surface tension between the two in the molten state. It will cause an inadequate mixing between aluminium alloy and HEA during welding. And in the cooling process, the weld would produce a certain degree of segregation, resulting microstructures is not uniform. Furthermore, the intricate composition of HEA gives rise to a diverse array of IMCs, which complicates the attainment of precise prediction and regulation of the products. Meanwhile, the fabrication of HEA entails a complex process, with the inclusion of valuable components resulting in elevated costs. This, in turn, imposes constraints on the widespread utilization of HEA interlayers.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n \u003ch2\u003e4.2 Metal interlayer\u003c/h2\u003e\n \u003cp\u003eWhen joining titanium alloys and aluminium alloys by laser welding\u0026ndash;brazing, it is necessary to add some metallic elements to the brazing material to enhance its wettability and spreadability to inhibit the formation of IMCs and also to improve the spreading of the brazing material or aluminium liquid on the titanium alloy substrate. Wang [127] et al. added zinc foil as an interlayer to Ti/Al joints and investigated its effect on interfacial IMCs. The results showed that the addition of zinc can significantly improve the wettability and spreading of Al-Si brazing material on the substrate. Meanwhile, a new intermetallic compound TiZn\u003csub\u003e16\u003c/sub\u003e was formed at the interface. The formation of TiZn\u003csub\u003e16\u003c/sub\u003e effectively reduces the content of TiAl\u003csub\u003e3\u003c/sub\u003e at the interface and improves the strength of the joint. However, the thermal stability of TiZn\u003csub\u003e16\u003c/sub\u003e is poor. As the heat input of the joint increase continuedly, the formation of a high temperature field within the joint would accelerate the diffusion of zinc atoms, leading to a persistent decrease in the amount of TiZn\u003csub\u003e16\u003c/sub\u003e at the interface until it disappears. And the brittleness of the joints also increases with decreasing TiZn\u003csub\u003e16\u003c/sub\u003e.\u003c/p\u003e\n \u003cp\u003eZn foils exhibit high surface activity and strong affinity for titanium and aluminum. During the welding process, Zn atoms exhibit a marked preference for reacting with Ti atoms, resulting in the formation of fine TiZn\u003csub\u003e16\u003c/sub\u003e grains, improving interfacial bond strength and plasticity. Moreover, Zn foils are comparatively inexpensive and straightforward to process, offering economic and practical advantages over complex alloy interlayers. However, Zn has a relatively low boiling point and is highly susceptible to evaporation when exposed to high-energy lasers, which may lead to the loss of zinc foil and the instability of the welding. Concurrently, the TiZn\u003csub\u003e16\u003c/sub\u003e exhibits diminished thermal stability, leading to a progressive decline in its content throughout the welding thermal cycle. This impedes the enhancement of the joint strength.\u003c/p\u003e\n \u003cp\u003eTo overcome the problem of poor thermal stability due to the low melting point of the interlayer, selecting metal interlayer with high melting point becomes a feasible solution. Majumdar [53] et al. found that two brittle phases, TiAl\u003csub\u003e3\u003c/sub\u003e and TiAl, were formed at the Ti/Al interface, which made the weld more brittle and resulted in a poor ability of the weld to withstand thermal stresses. What is more, cracks will appear at the interface between the fusion zone and the aluminum alloy during the cooling due to the high content of TiAl\u003csub\u003e3\u003c/sub\u003e on the side close to the aluminum alloy in the fusion zone. Majumdar concluded that the formation of interfacial cracks is very sensitive to the aluminum content. For this, Majumdar added thin Nb sheet as an interlayer to the joint for slowing down the temperature gradient at the interface and reducing the amount of aluminium melted. The results showed that niobium atoms and titanium atoms are able to diffuse with each other and form a solid solution, improving the tenacity of the weld effectively. Moreover, a portion of unmelted niobium in the fusion zone prevents the melting of aluminium, thereby reducing the proportion of aluminium in the fusion zone and inhibiting the formation of TiAl\u003csub\u003e3\u003c/sub\u003e. However, in order to minimise thermal cracking of the joint, Majumdar et al. chose to weld with lower heat input. The insufficient heat input prevents the thin niobium sheet to melt completely in the pool, resulting in discontinuities and stress concentrations in the joint after cooling.\u003c/p\u003e\n \u003cp\u003eGu [128] et al. also used Nb as an interlayer and deflected the beam towards the aluminium alloy to obtain a joint with completely unmelted niobium. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e, two different interfaces were formed in the joints, a diffusion bonded interface between the titanium alloy and niobium, a metallurgically bonded interface between the aluminium alloy and niobium. As the melting point of niobium is much higher than that of aluminium, when the aluminium melts, the niobium has not melted at all. Completely unmelted niobium prevents interdiffusion between titanium atoms and aluminium atoms, inhibits the formation of TiAl, TiAl\u003csub\u003e2\u003c/sub\u003e and TiAl\u003csub\u003e3\u003c/sub\u003e, and produces the Nb -containing ductile phases NbAl\u003csub\u003e3\u003c/sub\u003e, Nb\u003csub\u003e2\u003c/sub\u003eAl and an aluminium-based solid solution at the interface between the aluminium alloy and niobium. However, the completely unmelted niobium in the joint will be an area of stress concentration. It is from here that cracks can start and grow under external forces. In addition, the coefficients of linear expansion of titanium, aluminium and niobium are different. And joints at high temperatures would crack at the interface between niobium and the two alloys as a result of thermal stress, causing the joint to break ultimately.\u003c/p\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003eThe implementation of Nb foil as an interlayer has been demonstrated to effectively impede direct contact between titanium and aluminum, thereby mitigating the formation of brittle IMCs. Moreover, Nb demonstrated the capacity to diffuse into the titanium matrix, thereby forming a stable Nb-containing solid solution layer. Concurrently, its reaction with aluminum, resulting in the formation of IMCs, hindered the development of Ti/Al IMCs. What\u0026rsquo;s more, Nb atoms can enter the fusion zone and assume a role in grain refinement during the solidification, thereby enhancing the plasticity and strength of the joints. On the other hand, Nb has a high melting point, which enables it to maintain good stability at high temperatures and does not evaporate or decompose easily, ensuring the continuity of its modulating effect on IMCs. However, the unmelted Nb foil in the joint can also become a site of stress concentration during cooling, which can easily result in the formation of cracks. Moreover, the coefficient of linear expansion of aluminum (23.5\u0026times;10\u003csup\u003e6\u003c/sup\u003e\u0026sdot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is three times higher than that of niobium (7.3\u0026times;10\u003csup\u003e6\u003c/sup\u003e\u0026sdot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), so the Nb/Al interface would be extremely susceptible to fracture during cooling due to the rapid cooling rate.\u003c/p\u003e\n \u003cp\u003eZhang et al. used tungsten foil as an interlayer to connect TC4 titanium alloy and 6061 aluminium alloy in order to transform the crystal structure of TiAl\u003csub\u003e3\u003c/sub\u003e from a brittle tetragonal structure (D0\u003csub\u003e22\u003c/sub\u003e) to a ductile face-centred cubic structure (L1\u003csub\u003e2\u003c/sub\u003e) [129, 130]. The results showed that some tungsten atoms enter the TiAl\u003csub\u003e3\u003c/sub\u003e at the interface and take the place of some of these titanium atoms to form Al\u003csub\u003e3\u003c/sub\u003e(Ti,W) after the tungsten is added to the joint. The presence of elemental segregation of tungsten at the Ti/Al interface causes a partial transformation of the TiAl\u003csub\u003e3\u003c/sub\u003e crystal structure from D0\u003csub\u003e22\u003c/sub\u003e to L1\u003csub\u003e2\u003c/sub\u003e. According to research form Jahnatek [132] et al. and Zhang [133] et al. for D0\u003csub\u003e22\u003c/sub\u003e and L1\u003csub\u003e2\u003c/sub\u003e structure of TiAl\u003csub\u003e3\u003c/sub\u003e. D0\u003csub\u003e22\u003c/sub\u003e is a stable structure, whereas L1\u003csub\u003e2\u003c/sub\u003e is a sub-stable structure. This leads to the conclusion that TiAl\u003csub\u003e3\u003c/sub\u003e in the L1\u003csub\u003e2\u003c/sub\u003e structure cannot be present in large quantities and its proportion at the interface is extremely low. Meanwhile, the extent of segregation of tungsten elements at the interface is constrained. Therefore, the tungsten interlayer has a limited effect on changing the crystal structure of TiAl\u003csub\u003e3\u003c/sub\u003e, and it is unable to significantly enhance the brittleness of the weld. Moreover, tungsten is classified as a refractory metal, with a melting point of 3422\u0026deg;C, which is significantly higher than the melting points of titanium and aluminum. The process of incorporating tungsten foil necessitates precise control over process parameters, thereby significantly restricting its applications.\u003c/p\u003e\n \u003cp\u003eFirst principles researches have demonstrated that the addition of elements such as zinc, copper and silver to TiAl\u003csub\u003e3\u003c/sub\u003e-based alloys can result in the formation of a stable L1\u003csub\u003e2\u003c/sub\u003e phase [134]. The larger radii of zinc, copper and silver atoms facilitate their preferential replacement of aluminium atoms in the crystal structure of TiAl\u003csub\u003e3\u003c/sub\u003e, thereby reducing the axial ratio of the TiAl\u003csub\u003e3\u003c/sub\u003e cell and lowering the average strength of the Ti-Al covalent bond. This transformation of L1\u003csub\u003e2\u003c/sub\u003e from a sub-stable to a stable structure is a consequence of the aforementioned changes. The addition of a zirconium interlayer to the weld has also been demonstrated to be an effective means of improving the plasticity of the weld and increasing the strength of the joint. The addition of zirconium elements into the Ti/Al interface is accompanied by an increase in their content, which transforms TiAl\u003csub\u003e3\u003c/sub\u003e from the brittle tetragonal structure D0\u003csub\u003e22\u003c/sub\u003e to the more plastic crystal structure D0\u003csub\u003e23\u003c/sub\u003e [135]. However, Karpets et al. do not indicate a detailed mechanism for transformation. In order to obtain optimal performance of the Ti/Al joint, Zhang et al. [136] incorporated silver elements into the weld. In order to avoid stress concentrations in the joint after welding, silver mesh is employed as an interlayer to guarantee that the silver can be fully melted into the weld seam. The results showed that silver atoms can replace aluminium atoms at specific positions in the TiAl\u003csub\u003e3\u003c/sub\u003e lattice, thereby changing the crystal structure of TiAl\u003csub\u003e3\u003c/sub\u003e to form a local superstructure. And, the addition of silver mesh also reduces the difference in mechanical properties between the aluminium substrate and TiAl\u003csub\u003e3\u003c/sub\u003e in the joint, reducing the tendency of the joint to crack under stress. On the one hand, the MgAg and MgAgAl formed in the fusion zone after the addition of the silver mesh play a strengthening role as the second phase. Meanwhile, the combination of MgAg and MgAgAl with the aluminium substrate produces solid solution strengthening to weld. On the other hand, the addition of the silver caused a large number of particles favourable to heterogeneous nucleation of aluminium grains to appear in the molten pool, effectively refining the grains in the fusion zone. However, the addition of silver mesh did not reduce the thickness of the IMCs layer and change the morphology of TiAl\u003csub\u003e3\u003c/sub\u003e, nor did the transition from D0\u003csub\u003e22\u003c/sub\u003e-TiAl\u003csub\u003e3\u003c/sub\u003e to L1\u003csub\u003e2\u003c/sub\u003e-TiAl\u003csub\u003e3\u003c/sub\u003e take place. It only enhanced the performance of the joints through the strengthening of the fusion zone, and the fracture still occurs at the interface with the fusion zone and aluminium substrate.\u003c/p\u003e\n \u003cp\u003eHowever, the relatively poor wettability of the silver mesh with titanium and aluminum resulted in the silver mesh not spreading and fusing well during the welding process, which weakened its modulation effect on IMCs. The melting point of silver is 962\u0026deg;C, which is significantly lower than that of titanium. In the process of laser welding, elevated temperatures can lead to the premature melting or even evaporation of silver, resulting in the destruction of the structure of the silver mesh. In addition, the implementation of silver mesh as an interlayer contributes to a substantial escalation in welding costs, consequently leading to its limited utilization in large-scale industrial production.\u003c/p\u003e\n \u003cp\u003eZhang [137] et al. used titanium mesh as an interlayer connecting TC4 titanium alloy and 6061 aluminium alloy. It was shown that the titanium mesh altered the distribution of TiAl\u003csub\u003e3\u003c/sub\u003e in the weld seam, while enhancing the wettability and spreading of molten aluminium on the surface of the titanium substrate. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e14\u003c/span\u003e, the addition of the titanium mesh increases the reaction area between the molten aluminium and titanium, as well as the content of titanium atoms in the fusion zone. At the contact interface, diffusion of titanium atoms and aluminium atoms results in the formation of a thin layer of TiAl\u003csub\u003e3\u003c/sub\u003e, while on the aluminium side, granular TiAl\u003csub\u003e3\u003c/sub\u003e is produced in the fusion zone. A proportion of the particles will enter the fusion zone as a consequence of the heat flow in the molten pool and the surrounding grains during the growth of TiAl\u003csub\u003e3\u003c/sub\u003e at the interface. This results in a significant increase in the number of TiAl\u003csub\u003e3\u003c/sub\u003e particles present in the fusion zone. With the fusion zone cooling gradually, the granular TiAl\u003csub\u003e3\u003c/sub\u003e becomes a plasmonic point for heterogeneous nucleation of aluminium grains. Furthermore, these granular TiAl\u003csub\u003e3\u003c/sub\u003e become the second phase in the fusion zone, where it refines the grains, thereby strengthening the weld.\u003c/p\u003e\n \u003cp\u003eHowever, Zhang et al did not investigate the change in thickness of the interfacial layer of IMCs after the addition of titanium mesh. The thickness of the IMCs layer is critical to the strength of the weld bond after welding. The reinforcement of the aluminium side of the fusion zone alone results in an overall inhomogeneity of the fusion zone, causing fracture to occur at the interface between the fusion zone and the titanium substrate. Moreover, the addition of titanium mesh did not change the type of the original IMCs, and only played a role in refining the grain in the fusion zone on the aluminum side, while the performance of the Ti/Al dissimilar joints was determined by the bonding strength at the interface. Therefore, the utilization of titanium mesh as an intermediate layer to modulate IMCs at the weld interface is ineffective.\u003c/p\u003e\n \u003cp\u003eThe interlayer, upon incorporation of Ti/Al dissimiliar joints, has been shown to enhance the strength of the joint through three mechanisms. To begin with, reaction path modulation has been demonstrated, whereby the elements in the interlayer preferentially react with Ti or Al to generate low-brittle IMCs, thus reducing the brittleness of the joint. Secondly, the interlayer functions as a diffusion barrier, blocking the direct contact between Ti and Al and inhibiting the mutual diffusion of the two elements. Finally, the interlayer functions as an intermediate band of stress buffer, absorbing thermal stress through plastic deformation and thereby preventing crack initiation. As demonstrated in Table \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e, the incorporation of the interlayer has been shown to enhance the strength of the joint to a substantial degree. Furthermore, the findings of the aforementioned study by et al. also demonstrated that the addition of the interlayer resulted in an improvement in the fracture mode of the joint. Prior to the incorporation of the interlayer, the fracture occurs within the IMCs layer, exhibiting characteristics of brittle disintegration. Subsequent to the addition of the interlayer, the fracture shifts to the interlayer/substrate interface or to the substrate itself, exhibiting ductile fracture characteristics. The addition of an interlayer has been demonstrated to affect a transition in the morphology of IMCs, thereby transforming them from continuous layers to diffusely distributed particles. This transition is concomitant with a shift in the IMCs from a brittle to a tough state, thus effectively addressing the issue of inadequate joint strength that is attributable to the brittleness of IMCs. The addition of an interlayer has been demonstrated to affect a transition in the morphology of IMCs, thereby transforming them from continuous layers to diffusely distributed particles. This transition is concomitant with a shift in the IMCs from a brittle to a tough state, thus effectively addressing the issue of inadequate joint strength that is attributable to the brittleness of IMCs. However, the incorporation of certain interlayers proved ineffective. This can be attributed primarily to the failure to select elements capable of forming solid solutions or low-brittle IMCs with both titanium and aluminium. Additionally, the melting points of the interlayers are considerably higher than those of titanium and aluminium, resulting in premature melting or unfusion (e.g. niobium). In order to optimise the preparation of interlayers for the purpose of inhibiting IMCs in Ti/Al dissimilar joints, and consequently improve the strength of said joints, it is necessary to minimise the thickness of interlayers. This can be achieved by means of optimising the preparation process, for example by using plating or pre-positioning of foils. However, it should be noted that an interlayer thickness of less than 10 \u0026micro;m is not effective in blocking diffusion. Secondly, the selection of interlayers with high temperature stability is crucial to prevent oxidation or volatilisation at elevated temperatures. Further optimisation of the material system and process compatibility of the interlayer is imperative to enhance its position in Ti/Al dissimiliar joints.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"5. Summary and prospects","content":"\u003cp\u003eIt is challenging to prevent the formation of IMCs in the weld seam during the laser welding. The formation and distribution of these compounds can significantly impact the properties of Ti/Al dissimilar joints. Consequently, one of the principal concerns in Ti/Al dissimilar metal laser welding is the modulation of IMCs formation and distribution. The formation of IMCs in welds is primarily influenced by the welding heat and mass transfer processes. Based on the thermodynamics and kinetics of weld solidification, it is possible to homogenise the thermal distribution at the Ti/Al interface, promote atomic diffusion, reduce local segregation and shorten the high temperature residence time. This can be achieved by adapting the welding process, for example by using a dual beam laser, pulsed laser and laser oscillations. On the other hand, optimising the welding parameters, such as reducing the laser power, increasing the welding speed and offset distance, can reduce interfacial heat accumulation and modulate the growth and distribution of IMCs at the interface. In addition, the addition of an alloy or metal interlayer at the Ti/Al interface not only inhibits the formation of IMCs at the interface and alters their distribution, but also generates new ductile phases, thereby effectively improving the load-bearing properties of the joints. However, the quantitative mechanism of elemental diffusion at the interface and the growth mechanism of IMCs under thermodynamic coupling during the laser welding remain incompletely clear.\u003c/p\u003e\u003cp\u003eConsequently, future research on Ti/Al laser welding may yield the development of new exogenous assisted processes, which could be achieved by combining a range of energy fields, including electric, magnetic, ultrasonic, and airflow fields. It has been demonstrated that these applied energy fields can be employed in laser welding of homogeneous metals with the objective of refining the grain structure in the fusion zone, reducing the occurrence of elemental segregation and crack susceptibility, and enhancing the joint properties. However, there is a paucity of relevant research investigating the impact of these applied energy fields on the formation of IMCs during Ti/Al welding. Meanwhile, the molecular dynamics diffusion model of the Ti/Al interface, in addition to the growth model of IMCs, can be modeled using simulation. The thermodynamic and kinetic conditions for the formation of IMCs can be investigated through the use of numerical simulations. This will enable a deeper understanding of the mechanisms involved in IMCs formation and growth, which will in turn inform the regulation of IMCs formation and distribution in weld micro-regions through the implementation of appropriate processes and other means. This will ultimately improve the performance of the joints, thus laying a solid foundation for a wider application of the Ti/Al dissimilar structures in the equipment manufacturing.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eIMC \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Intermetallic compound\u003c/p\u003e\n\u003cp\u003eFZ \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Fusion zone\u003c/p\u003e\n\u003cp\u003eHAZ \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Heat affected zone\u003c/p\u003e\n\u003cp\u003eDLBSW \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Dual laser beam bilateral synchronous welding\u003c/p\u003e\n\u003cp\u003eLOW \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Laser offset welding\u003c/p\u003e\n\u003cp\u003ePP \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Polishing process\u003c/p\u003e\n\u003cp\u003ePLP \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Pulsed laser pretreatment\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHEA \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; High-entropy alloy\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThis work was financially supported by the Key Research and Development Program of Shaanxi (Grant No.2022GY-408).\u003c/p\u003e\u003cp\u003e\u003cb\u003eConflict of Interest\u003c/b\u003e The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\u003cp\u003e\u003cb\u003eAuthorship contribution statement Zhi-Ming Zheng\u003c/b\u003e: Writing - original draft, Visualization, Writing - review \u0026amp; editing. \u003cb\u003eJia Zhang\u003c/b\u003e: Writing - review \u0026amp; editing, Visualization. \u003cb\u003eYa-Zhe Xing\u003c/b\u003e: Writing-review \u0026amp; editing, Supervision, Conceptualization.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eThe authors confirm that all the data supporting the findings of this study are available within the article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eK. 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Zhang, Microstructure and mechanical properties of laser welded-brazed titanium/aluminum joints assisted by titanium mesh interlayer, Journal of Materials Processing Technology 302 (2022) 117502.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 6 to 8 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Ti/Al intermetallic compounds, Laser welding, Heat input, Pretreatment, Post-weld heat treatment, Interlayer","lastPublishedDoi":"10.21203/rs.3.rs-6054738/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6054738/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTitanium alloys and aluminum alloys are two lightweight alloys with a wide range of applications in aerospace and transportation. Ti/Al connectors combine the excellent properties of titanium alloys and aluminum alloys, which can markedly reduce the component weight and lower the cost. Laser welding has the advantages of high energy density, rapid heating concentration, short residence time at high temperatures. Therefore, it is considered to have unique advantages for joining Ti/Al dissimilar metals. However, due to the significant differences in the thermophysical properties and crystal microstructure of titanium and aluminum, it is highly probable that large residual stresses will be generated after welding. This can facilitate the formation and expansion of cracks. Meanwhile, titanium and aluminum are metallurgically incompatible systems due to their minimal mutual solubility at room temperature. The formation of brittle intermetallic compounds during the welding results in an increase in the brittleness of the joint, rendering it highly susceptible to fracture under stress. Consequently, the modulation of the formation of Ti/Al intermetallic compounds represents a pivotal approach to improving the quality of Ti/Al laser welded joints. In this paper, the current research status on the modulation of interfacial intermetallic compounds in Ti/Al laser welded joints is reviewed from three aspects: laser welding heat input, pre-weld pretreatment and post-weld heat treatment, and interlayer. Finally, on the basis of analyzing the existing problems in the current intermetallic compound modulation research, the future research and development direction of modulating intermetallic compounds in Ti/Al welded joints is proposed.\u003c/p\u003e","manuscriptTitle":"A review on controlling the formation of intermetallic compounds in Ti/Al dissimilar metal laser weld seams","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-07 21:27:34","doi":"10.21203/rs.3.rs-6054738/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revisions Needed","date":"2025-07-18T07:31:26+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-07-03T13:05:29+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-03T12:49:17+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-05-12T05:20:30+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-05-09T14:17:19+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-05-07T05:15:41+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f2ad47a1-5c59-4fed-8e82-a19e45827038","owner":[],"postedDate":"July 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-10-06T13:56:48+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-07 21:27:34","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6054738","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6054738","identity":"rs-6054738","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}