An Improved Upcycling Approach For Producing Bimetallic Tube Via Friction Stir Extrusion of Aluminium Chips

An Improved Upcycling Approach For Producing Bimetallic Tube Via Friction Stir Extrusion of Aluminium Chips | 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 An Improved Upcycling Approach For Producing Bimetallic Tube Via Friction Stir Extrusion of Aluminium Chips Riccardo Puleo, Muhammad Adnan, Giuseppe Ingarao, Livan Fratini This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8205051/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 21 Apr, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted 5 You are reading this latest preprint version Abstract Bimetallic tubes are employed in applications where a single component must satisfy multiple performance requirements, for instance, combining high strength with corrosion resistance. In literature, manufacturing processes like rotary piercing, forward/backward extrusions, and tube cladding have been commonly used to produce high-performance bimetallic tubes, typically starting from bulk materials. Moreover, recently, new sustainable processes belonging to the solid-state recycling (SSR) category, namely friction stir extrusion (FSE), have also been adopted for tube manufacturing. However, both conventional and SSR-based approaches generally rely on multi-step routes involving pre-heating, homogenization, or pre-consolidation to obtain workable billets, which increases energy consumption. This study goes beyond the issues of the existing extrusion-based recycling processes, proposing a single-step FSE approach able to directly convert AA7075 and AA2024 aluminum chips into bimetallic tubes, offering a sustainable upcycling pathway that does not involve any pre-heating or pre-consolidation stages. Three combinations of rotational speed and axial load were investigated to assess their influence on tube quality. The resulting bimetallic tubes were characterized through microstructural and macrostructural analyses, which revealed the absence of voids and inclusions at the bonding interface, the material composition, a grain refinement (avg. 4.6 µm), and an enhanced hardness (up to 175 HV) under optimal processing conditions. As a matter of fact, this process opens new opportunities for the fabrication of bimetallic tubular components, which can be used in electrical, structural, lightweight, and corrosion-resistant applications. bi-metallic friction stir extrusion FEM recycling 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 1. Introduction In many industrial sectors, the mechanical and thermal properties of a single material are often insufficient to meet the diverse property requirements of a component. Nowadays, an increasing demand for bimetallic tubes is visible due to the need to address the mechanical properties requirement issues, also aiming to offer enhanced and complementary properties [ 1 ]. An example is the automotive sector, where bimetallic tubes are widely employed in exhaust systems to withstand high temperatures while maintaining corrosion resistance and structural rigidity. One of the most common materials often used across various industries, particularly in aerospace and aeronautics, is aluminium and its alloys. This metal is usually preferred, among others, due to its favorable strength-to-weight ratio. Specifically, the 2xxx and 7xxx series aluminium alloys are notable for their superior mechanical properties and corrosion resistance [ 2 ]. In this regard, developing efficient and cost-effective manufacturing techniques for bi-metallic aluminium tubes has become increasingly important to achieve industry needs and promote material efficiency. Currently, various techniques have been explored for the fabrication of bimetallic tubes, including conventional extrusion [ 3 ], the hydroforming technique [ 4 ], and magnetic pulse cladding [ 5 ], etc. These methods primarily rely on mechanical bonding, where interfacial adhesion is achieved through plastic deformation induced by applied pressure. However, such bonds often exhibit low strength and are prone to failure at elevated temperatures. To address these limitations and promote atomic-level bonding, advanced techniques such as spin bonding[ 6 ] and shear-assisted extrusion with localized heating have been investigated. Although spin bonding can achieve strong metallurgical interface bonding, the experimental setup is highly complex, resulting in increased manufacturing costs. Explosive cladding has also been employed to fabricate bimetallic tubes [ 7 , 8 ], demonstrating excellent metallurgical bonding at the interface. Nonetheless, this method often leads to undesirable effects such as non-uniform outer diameters, high surface roughness, and significant safety concerns due to the use of explosives. Therefore, there is a clear need to develop a cost-effective, safe, and simplified manufacturing process capable of producing high-quality bimetallic tubes with reliable interfacial strength. Additionally, Ce Ji et al.[ 9 ] developed composite tubes using 45-grade carbon steel and 316L stainless steel via a three-roll skew rolling process. Yuling Chang et al.[ 10 ] fabricated thin-walled Cu/Al composite tubes through a spinning process that effectively converted plate materials into tubular forms. Bao Wang et al.[ 11 ] successfully produced Ni/Al composite tubes by layering Ni and Al foils, followed by gas expansion and forming. Wei Zhang et al.[ 12 ] introduced Mg/Al composite tubes using a modified hot extrusion method, which involved adjusting the die geometry to enhance shear forces and achieve a flat bonding interface. However, the extrusion ratio was limited to around 7 due to constraints associated with the billet and die design. Recently, due to the growing demand for more sustainable approaches, new environmentally friendly techniques are gaining interest from most researchers. SSR processes have gained significant attention, as they aim to reduce the environmental impact of conventional manufacturing by focusing on the direct recycling of materials. Among the SSR processes, FSE has emerged as an innovative technology in metal recycling, offering a sustainable and energy-efficient method for converting scrap or waste metal into valuable components [ 13 , 14 ]. While FSE is widely recognized for its application in wire production, its potential for manufacturing tubes is increasingly being explored. Literature indicates that most existing tube fabrication methods require multiple processing steps, typically involving at least one high-temperature operation; nevertheless, extrusion remains the most frequently used technique [ 15 ]. In contrast, the FSE process offers a more energy-efficient alternative, requiring fewer steps and lower processing temperatures [ 16 ]. Despite its advantages, no prior research has been reported on the fabrication of bimetallic tubes combining aluminium 7xxx and 2xxx series alloys using the FSE method. Moreover, the friction stir extrusion of tubes has been commonly performed starting from a bulk component[ 17 – 19 ] or a pre-consolidation step that primarily turns recycling chips into a billet [ 20 ], which increases the energy demand of the process. Therefore, a more sustainable approach that aims to reduce the impact on chips recycling and the steps needed for tube production is still missing. For this reason, this study aims to demonstrate the capability of FSE for directly turning AA2024 and AA7075 recycling chips into high-quality aluminium bimetallic tubes by skipping any preliminary pre-consolidation or pre-heating step. Specifically, the FSE technique has been adopted to fabricate bimetallic tubes consisting of AA7075 as the outer tube and AA2024 as the inner tube. The process is structured in two stages: outer tube production (AA7075) and inner tube production (AA2024). The experimental campaign was performed at 15 kN vertical load and three different rotational speeds equal to 500, 700, and 1000 rpm. The microstructural evolution at the bonding interface, along with the mechanical properties of the extruded bimetallic tubes, was comprehensively characterized using optical microscopy (OM), scanning electron microscopy (SEM), and microhardness testing. The results confirmed the feasibility and effectiveness of the FSE process in producing high-performance, multilayered aluminium components with refined microstructures and robust interfacial bonding. 2 Experimental Procedures For the outer tube, AA7075 aluminium alloy chips were used, whereas AA2024 aluminium alloy chips were considered for the inner one because of the different material ductility. Table 1 shows the chemical compositions of the Starting material, i.e., AA7075-T6 and AA2024-T3 aluminium alloy, used in this work. The recycling chips were produced from the milling operation of AA7075-T6 round bar and the turning operation of AA2024-T3 bar. Additionally, the chips were submerged in the acetone for effective cleaning before processing. Table 1 Chemical composition analysis of AA2024-T3[ 21 ] and AA7075-T6[ 22 ] aluminum alloys. Material Al Cu Mn Mg Si Fe Cr Zn 2024-T3 Bal. 4.91 0.54 1.31 0.145 0.31 0.09 0.10 7075-T6 Bal. 1.5 0.3 2.6 0.4 0.5 0.22 5.4 2.1 Experimental campaign The experimental tests were performed on an ESAB LEGIO Friction Stir Welding machine, specifically adapted for producing aluminium tubes from recycled chips. The FSE process was structured in four different steps: AA7075 chips loading, AA7075 tube extrusion, AA2024 chips loading, and AA2024 tube extrusion. A schematic diagram of the FSE-based bimetallic tube extrusion process is presented in Fig. 1 . In the first step, 20 grams of AA7075 aluminium alloy chips were placed into a custom-designed split die (Fig. 2 a), which had an internal diameter of 25 mm and a height of 76 mm. The die was manufactured and securely assembled to a backing plate using bolts and nuts. The chips were compacted using a 25 mm diameter flat-head H13 steel tool (Fig. 2 b) under an axial force of 5 kN. This compaction is part of the chips' preparation and aims to ensure uniform density, die filling, and to avoid scattering during extrusion. The outer tube was then extruded using a 23 mm diameter tool with a 20° conical taper profile (Fig. 2 c). It is worth mentioning that the conical profile was used instead of a flat one to promote smooth material flow and easy tool insertion [ 19 ]. After the outer tube was extruded, 20 grams of AA2024 aluminium alloy chips were poured into the internal cavity of the previously extruded tube. These chips were compacted using a flat tool of 23 mm diameter, again under a force of 5 kN. The inner tube was then extruded using a 21 mm diameter tool with the same 20° conical taper profile. The tests were performed at 15 kN vertical load and three rotational speeds equal to 500 (low), 750 (medium), and 1000 (high) rpm. In the current study, the outer layer of the bimetallic tube (AA7075) had an average thickness of approximately 1.0 mm, as well as the inner layer (AA2024). Details of the tool geometries are shown in Table 2 , while process parameters, including rotational speed and axial extrusion force, are summarized in Table 3 . It is worth mentioning that for monitoring temperature during the extrusion process and for tuning the numerical simulations, a K-type thermocouple was inserted into a hole drilled at 1/3-height of the die, positioned 1 mm from the inner die wall (Fig. 2 a, A-A section). Table 2 Geometry characterization of tapered tools. Tool Material Uniform shank (Ø) Tool tip Holding shank Tapered length Uniform shank For AA7075 H-13 tool steel 23 mm 22 mm Ø 30 mm 10 mm 70 mm For AA2024 H-13 tool steel 21 mm 20 mm Ø 30 mm 10 mm 70 mm Table 3 FSE process parameters and tube material layers disposition. ID Rotational speed [rpm] Force [kN] Total tube thickness [mm] Outer tube Inner tube ID1 500 15 2 7075 2024 ID2 750 15 2 7075 2024 ID3 1000 15 2 7075 2024 Consequently, the bimetallic tubes were cut longitudinally to the extrusion direction to prepare samples for mechanical and microstructural evaluation. The cross-sections were mounted, ground, and polished, then etched with Keller’s reagent (consisting of 2 mL HF, 3 mL HCl, 5 mL HNO₃, and 190 mL H₂O) to expose their microstructural details. Optical microscopy (OM), along with SEM and EDS, was utilized to study the internal structure and elemental distribution, particularly at the bonding interface. The grain size was quantified using the mean linear intercept technique. For mechanical assessment, Vickers microhardness tests were conducted using a 0.5 kg load and a 15-second dwell time. Hardness measurements were taken across the wall thickness of the tube, starting from the outer diameter toward the inner surface, at intervals of approximately 0.2 mm. Each point was tested three times, and the average of the results was used to ensure consistency and reliability. 2.3 Numerical campaign A numerical campaign was performed using the commercial finite element software SFTC DEFORM 3D. The numerical setup involves five components: a die, a backing plate, two tools, and material chips (Fig. 3 ). The modelling of the chips batch is complex, and for this reason, the aluminium chips were considered as a single block porous material, following the material formulation of Shima-Oyane [ 23 ]. Specifically, an initial relative density value of 0.7 was assigned to the porous billet, calculated experimentally considering the mass of the loaded chips and the geometry of the die’s chamber. The porous billet was characterized by a mesh size of 35000 elements with a refining mesh window close to the tool-material contact (Fig. 3 , A-A section). The other components were considered as rigid material made of H-13 steel. The two tools were defined by a mesh size of 60000 elements, while for both the die and backing plate, the mesh size was 30000 elements. A tuning approach was performed for calculating the proper thermal and frictional coefficients. In this regard, the experimental data of the thermocouple were tuned to the numerical one, and the following values were obtained: shear factor of 0.2 and interface heat transfer coefficients (IHTC) of 11, 45, and 45 W/mm²/K for the tool-material, die-material, and backing plate-material contacts, respectively. The methodology of this numerical calibration has already been applied and assessed in the literature [ 24 ]. 3. Results and discussion 3.1. Macroscopical analysis Figure 4 depicts the bimetallic tubes fabricated from AA7075 and AA2024 aluminum alloy chips using the FSE process under various process parameters. All tubes were successfully extruded, demonstrating the viability of the FSE technique. However, surface quality and material consolidation varied with processing conditions. At the beginning of the process, the material is compressed and heated up thanks to the initial stirring and frictional actions of the tool. In these early stages, proper process parameters are crucial for obtaining optimal chips bonding and high surface quality. In this regard, when the heat generated is too low (ID1, Fig. 4 a), a lack of consolidation of the first extruded layers is visible near the top surface of the tube. On the other hand, at a high rotational speed (ID3, Fig. 4 c), excessive heat led to early softening of the material, which resulted in the surface deteriorating and uneven surface roughness in large parts of the extruded tube. At a medium rotational speed (ID2, Fig. 4 b), optimal surface conditions were achieved. At process parameters ID2, the tube exhibited a uniform and quite smooth outer surface with no visible cracks at the top. Additionally, the bottom surface showed no signs of adhesion between the backing plate and the workpiece material, indicating effective material flow. It is worth remarking that the material behavior of AA7075 recycling chips plays an important role in material bonding; high mechanical properties may hinder proper consolidation. Difficulty in chips bonding for AA7075 recycling chips was also experienced by Puleo et al. [ 24 ] in the friction stir consolidation process. The final length of the bimetallic tube, after trimming both top unconsolidated chips and bottom consolidated billet, was approximately 40 mm. 3.2 Microstructural investigations Figure 5 illustrates the cross-sectional microstructure of the AA7075/AA2024 bimetallic tube fabricated under different process parameters. In Fig. 5 a, corresponding to the low rotational speed condition (ID1), insufficient bonding is observed at the interface of the two materials. A distinct separation indicates that inadequate heat was generated during the process, preventing effective metal-to-metal bonding. High-resolution imaging of the interface further confirms the presence of unbonded regions. In contrast, Figs. 5 b and 5 c, corresponding to medium (ID2) and high (ID3) rotational speeds, show no visible surface defects in the cross-section. The interface appears continuous and free from voids or delamination, indicating successful consolidation and a structurally sound bimetallic tube. Looking at the third column of Fig. 5 (higher magnification), a distinct interfacial region, commonly referred to as the diffusion layer, is clearly visible and outlined by two yellow dashed lines. This zone represents a transition area where the AA2024 and AA7075 interface bonding occurs. The presence of this diffusion layer confirms atomic interdiffusion between the two alloys, which contributes to the formation of a strong metallurgical bond. The gradual transition in composition within this layer (i.e., EDS images) further supports the effectiveness of the FSE process in achieving solid-state bonding without the formation of defects or interfacial discontinuities. For investigating the microstructure of the bimetallic tubes at the interface, SEM analysis was performed. Figure 6 presents SEM micrographs along with corresponding EDS results, focusing on the transition zone between AA2024 and AA7075, marked by a red dashed line. Figure 6 a (ID1), at low rotational speed, displays discontinuity and areas of insufficient bonding, indicating that the heat generated during the process was not adequate to achieve full metallurgical joining. It is important to note that the darker areas near the interface are not voids or porosity. In contrast, the samples extruded at medium and high rotational speeds (ID2 and ID3) haven’t shown any signs of discontinuity in the transition region. Additionally, the EDS mapping presented in Fig. 6 further validates and supports the quality of the bond. In this regard, the images of ID1 clearly delineate the transition zone between the AA7075 and AA2024 alloys, with an elemental distribution that matches the expected chemical composition of each material. Specifically, zinc (Zn) was predominantly detected in the AA7075 region, while copper (Cu) was more concentrated in the AA2024 region. On the contrary, ID2 and ID3 show more elemental diffusion. This elemental distribution provides strong evidence of alloying elements’ diffusion across the interface and metallurgical bonding between the two aluminium alloys, processed via the FSE process. Further grain size analysis was performed on the cross-section of the bimetallic tube to enhance microstructural characterization. Specifically, the grain dimension was measured along the cross section of the tube, from the outer (AA7075) to the inner tube (AA2024), and images of the microstructure were acquired in three specific observation zones: the outer surface of the AA7075 tube (Fig. 7 a), the AA7075/AA2024 interface (Fig. 7 b), and the inner surface of the AA2024 tube (Fig. 7 c). A better refiguration of the selected surfaces and observation zones is given in Fig. 7 . Results of tube micrography are presented in Fig. 7 for the ID1 sample. In particular, fine grains (avg. 4.64 µm) were observed near the outer surface of the AA7075 tube (Fig. 7 a), with a gradual increase toward the interface (avg. 5.60 µm). At the interface (Fig. 7 b), grain size slightly increased from the AA7075 to the AA2024 side (avg. 5.65 µm). Lastly, close to the inner surface of the AA2024 tube (Fig. 7 c), a fine and equiaxed structure was again observed (avg. 4.60 µm). These variations in grain size are mainly attributed to the stirring action of the tool. As already experienced by Baffari et al. [ 25 ] during wire production through the FSE process, the material directly in contact with the tool, which is forced through the extrusion reduction zone, undergoes high pressure and temperature, promoting dynamic recrystallization and grain refinement. For this reason, the grain dimension of the inner surface of both AA7075 and AA2024 tubes is smaller than the outer surface (Fig. 8 ). Similar behavior was observed in samples ID2 and ID3 (Fig. 8 ), with an overall decrease in average grain size at higher rotational speeds. 3.3 Numerical results and microstructural comparison To support the findings of the experimental campaign, numerical simulations were carried out, aiming to give an overview of the temperature (Fig. 9 a) and strain (Fig. 9 b) behavior during FSE tube processing. In this regard, a plot of the temperature and strain profile, at the end of the AA7075 tube production, has been proposed in Fig. 9 for the case study ID3. The results reveal that high strain levels occurred both across the tube thickness and under the tool. Furthermore, the detailed cross-sectional view in Fig. 9 b indicates that the strain is not uniformly distributed through the tube’s thickness; instead, it visibly increases from the outer surface toward the inner one. This strain distribution demonstrates that the tool stirring action is more emphasized at the inner tube’s surfaces than the external one, corroborating the observation regarding the occurrence of recrystallization phenomena in the inner surface, previously discussed. Numerical simulations were also employed to further investigate and explain the grain growth phenomena observed at the interface of the bi-metallic tube (Fig. 8 ). A point-tracking approach was adopted to track the temperature evolution of a selected point over the entire process. In particular, a point, initially located at the interface of the AA7075 extruded tube, was tracked from the considered process time backward to the beginning of the process (Fig. 10 ). The results of the point tracking showed that, during the FSE process, the material in contact with the tool, particularly near the extrusion channel, was subjected to the highest temperature levels, ranging between 480 and 550°C (Fig. 11 ). During the AA7075 extrusion stage (Fig. 11 a), the tracked point moved from the initial position (under the tool) toward the tube wall, experiencing a peak temperature of approximately 400°C, which lies within the recrystallization temperature range for aluminium alloys. Under these conditions, the combined effect of high strain and elevated temperature promoted recrystallization, resulting in grain refinement of the AA7075 alloy. During the production of the AA2024 tube, point-tracking analysis revealed that the temperature at the monitored location increased again to approximately 350°C (Fig. 11 b), partly due to thermal exchange with the already processed AA7075 tube. This secondary temperature peak, located within the recrystallization range, acted as an additional thermal cycle, leading to grain coarsening at the interface in both materials. For this reason, the grain size in correspondence to the AA7075 inner zone resulted in being bigger than the outer (Fig. 8 ), despite the tool stirring refined the inner portion of the AA7075 tube. 3.4 Hardness characterization The mechanical properties of the obtained bimetallic tube were investigated through Vickers microhardness measurements by performing twelve indentation points (Fig. 13 b) along the cross-section of the tube. The results of these measurements are reported in Fig. 13 a. Typically, regions containing unbonded or poorly bonded chips exhibit lower hardness compared to areas where strong metallurgical bonding has occurred. Data shows noticeable variation in hardness along different paths across the thickness of the tube. Compared to the average Vickers hardness of the base material, all measured values were higher, indicating that the FSE process enhances the mechanical properties of the extruded material. A consistent trend was observed: microhardness values increased progressively from the outer surface toward the inner surface. For instance, considering ID1, the hardness at point 1 (outer surface) was 87 HV, which increased to 149 HV at point 12 (inner surface). Similar trends were noted for other process conditions. This improvement aligns with the observations made by Baffari et al. [ 25 ]; in his work, the authors found that after the FSE process, the hardness of the product was close to the T3 heat treatment hardness value. The AA7075 and AA2024 T3 hardness values are typically about 150 and 145 HV, respectively. Additionally, an overall increase in hardness has been reported with the increase in rotational speed; at the same vertical load, the increase in rotational speed results in an increase in frictional heating, which enhances chip consolidation, reduces internal voids, and promotes grain refinement, as already shown in Chap. 3.2. These observations are consistent with the microstructural analysis, which demonstrated grain size refinement from the AA7075 toward the AA2024 layers of the bimetallic tube. However, in the transition zone between AA7075 and AA2024, a localized decrease in microhardness was noted. This reduction resulted from variation in grain size and minor defects such as porosity or unbonded regions, which act as stress concentrators, reducing the overall hardness. Similarly, the external surface of the AA7075 tube exhibited low hardness values due to the roughness and poor quality of the surface [ 25 ], especially for ID1. 4 Conclusion In this study, it was proved that FSE can be successfully applied to produce AA7075/AA2024 bimetallic tubes in a single-step process directly from recycled aluminium chips, demonstrating the effectiveness of this upcycling route. Enhanced microstructural and hardness characteristics were observed across the thickness of the tube, along with a detailed SEM analysis of the bonding quality at the tube interface. The key findings are summarized as follows: The results demonstrated that the FSE process is a viable method for fabricating bimetallic tubes. Among the evaluated conditions, the tube produced at 750 rpm and 15 kN exhibited superior surface finish, refined microstructure, and enhanced mechanical properties. Optical microscopy and grain size analysis revealed uniformly distributed equiaxed grains, and an overall gradual decrease in grain size was observed toward the inner wall, resulting from the tool’s thermal and stirring actions. Moreover, microstructural analysis confirmed that adequate process conditions led to dynamic recrystallization, resulting in fine-grained structures (avg. 4.62 µm, at the outer and inner surface of the bi-metallic tube) and effective bonding between dissimilar aluminum alloys, thus contributing to improved mechanical performance. The hardness characterization revealed an overall increase from the outer layer (AA7075) to the inner layer (AA2024). The average hardness for the AA7075 layer was about 150 HV, while a value of nearly 160 HV was obtained for the AA2024 one. At the interface between the two layers, a drop in hardness was observed (around 140 Hv). Numerical simulations showed that the material located under the tool and near the extrusion channel experiences high temperatures and strains, conditions that promote recrystallization during FSE. Moreover, a point tracking approach for temperature monitoring at the interface showed that the inner surface of the AA7075 tube is subjected to a secondary heating effect, due to the extrusion of the second tube, which affected the grain size at the tube’s interface. SEM and EDS analyses verified the presence of a metal-to-metal bonded interface at proper process parameters (750 rpm − 15 kN and 1000 rpm – 15 kN). The thermal and mechanical input during FSE was sufficient to facilitate elemental diffusion across the interface, leading to strong interlayer bonding. Declarations Declaration of competing interest The Authors disclose no actual or potential conflict of interest that could inappropriately influence, or be perceived to influence, this work. Author contributions Riccardo Puleo: Numerical campaign development, Draft writing. Muhammad Adnan: Experimental campaign development, Draft writing. Giuseppe Ingarao: Research supervision, Draft, Final revision. Livan Fratini: Research supervision, Paper revision. Acknowledgement This study was funded by Italian MUR funds through European Union-NEXT Generation EU scheme; title of the project: “Finalizing processes for multi-material based Functionally Graded billets and wires obtained through solid state recycling operations of aluminum alloy chips - FULL RECYCLE”, PRIN 2022 (CUP: B53D23006550006). References Swarnkar R, Karmakar S, Pal SK (2023) An investigation of bimetallic tube fabrication through a novel friction stir extrusion based technology for automotive applications. Mater Today Commun 35:106363. https://doi.org/10.1016/j.mtcomm.2023.106363 Xie Z, Zhou L, Li J, et al (2025) Mechanical and Corrosion Properties of AA2024 Aluminum Alloy with Multimodal Gradient Structures. Metals (Basel) 15:177. https://doi.org/10.3390/met15020177 Kang CG, Jung YJ, Kwon HC (2002) Finite element simulation of die design for hot extrusion process of Al/Cu clad composite and its experimental investigation. J Mater Process Technol 124:49–56. https://doi.org/10.1016/S0924-0136(02)00106-1 Wang X, Li P, Wang R (2005) Study on hydro-forming technology of manufacturing bimetallic CRA-lined pipe. Int J Mach Tools Manuf 45:373–378. https://doi.org/10.1016/j.ijmachtools.2004.09.015 Fan Z, Yu H, Li C (2016) Plastic deformation behavior of bi-metal tubes during magnetic pulse cladding: FE analysis and experiments. J Mater Process Technol 229:230–243. https://doi.org/10.1016/j.jmatprotec.2015.09.021 Mohebbi MS, Akbarzadeh A (2011) Fabrication of copper/aluminum composite tubes by spin-bonding process: experiments and modeling. The International Journal of Advanced Manufacturing Technology 54:1043–1055. https://doi.org/10.1007/s00170-010-3016-5 Guo X, Tao J, Wang W, et al (2013) Effects of the inner mould material on the aluminium–316L stainless steel explosive clad pipe. Mater Des 49:116–122. https://doi.org/10.1016/j.matdes.2013.02.001 Gómez X, Echeberría J (2000) Microstructure and mechanical properties of low alloy steel T11–austenitic stainless steel 347H bimetallic tubes. Materials Science and Technology 16:187–193. https://doi.org/10.1179/026708300101507532 Ji C, Niu H, Li Z, et al (2024) Deformation law and bonding mechanism of 45 carbon steel/316L stainless steel cladding tubes fabricated by three-roll skew rolling bonding process. J Mater Process Technol 325:118277. https://doi.org/10.1016/j.jmatprotec.2023.118277 Chang Y, Chen H, Zhou J, et al (2023) Micro-nano interface structure and mechanical characteristics of thin wall Cu/Al composite tubes prepared by strong staggered spinning. Mater Charact 206:113405. https://doi.org/10.1016/j.matchar.2023.113405 Wang B, Wang D, Wang S, et al (2023) Fabrication of NiAl alloy hollow thin-walled component through hot gas forming of Ni/Al laminated tube and conversion process. Journal of Materials Research and Technology 26:7224–7236. https://doi.org/10.1016/j.jmrt.2023.09.057 Zhang W, Hu H jun, Hu G, et al (2023) A direct extrusion‐shear deformation composite process that significantly improved the metallurgical bonding and texture regulation grain refinement and mechanical properties of hot-extruded AZ31/AA6063 composite tubes. Materials Science and Engineering: A 880:145090. https://doi.org/10.1016/j.msea.2023.145090 Adnan M, Buffa G, Baghdadchi A, et al (2024) Unveiling the mechanical and microstructural properties of SiC reinforced aluminum wires recycled from scraps by friction stir extrusion. Materials Science and Engineering: A 916:. https://doi.org/10.1016/j.msea.2024.147333 Buffa G, Campanella D, Adnan M, et al (2024) Improving the Industrial Efficiency of Recycling Aluminum Alloy Chips Using Friction Stir Extrusion: Thin Wires Production Process. International Journal of Precision Engineering and Manufacturing - Green Technology. https://doi.org/10.1007/s40684-023-00573-w Behnagh RA, Fathi F, Yeganeh M, et al (2019) Production of seamless tube from aluminum machining chips via double-step friction stir consolidation. International Journal of Advanced Manufacturing Technology 104:4769–4777. https://doi.org/10.1007/s00170-019-04326-5 Baffari D, Reynolds AP, Masnata A, et al (2019) Friction stir extrusion to recycle aluminum alloys scraps: Energy efficiency characterization. J Manuf Process 43:63–69. https://doi.org/10.1016/j.jmapro.2019.03.049 Abu-Farha F (2012) A preliminary study on the feasibility of friction stir back extrusion. Scr Mater 66:615–618. https://doi.org/10.1016/j.scriptamat.2012.01.059 Swarnkar R, Karmakar S, Pal SK (2023) An investigation of bimetallic tube fabrication through a novel friction stir extrusion based technology for automotive applications. Mater Today Commun 35:. https://doi.org/10.1016/j.mtcomm.2023.106363 Asadi P, Akbari M, Sadowski T, et al (2024) Examining the impact of tool taper angle in Al-Si tube manufacturing by friction stir extrusion. J Manuf Process 131:532–544. https://doi.org/10.1016/j.jmapro.2024.09.047 Behnagh RA, Fathi F, Yeganeh M, et al (2019) Production of seamless tube from aluminum machining chips via double-step friction stir consolidation. International Journal of Advanced Manufacturing Technology 104:4769–4777. https://doi.org/10.1007/s00170-019-04326-5 Ahn J, Chen L, He E, et al (2017) Effect of filler metal feed rate and composition on microstructure and mechanical properties of fibre laser welded AA 2024-T3. J Manuf Process 25:26–36. https://doi.org/10.1016/j.jmapro.2016.10.006 Saravanan V, Banerjee N, Amuthakkannan R, Rajakumar S (2015) Microstructural Evolution and Mechanical Properties of Friction Stir Welded Dissimilar AA2014-T6 and AA7075-T6 Aluminum Alloy Joints. Metallography, Microstructure, and Analysis 4:178–187. https://doi.org/10.1007/s13632-015-0199-z Shima S OM (1976) Plasticity theory for porous metals. Int J Mech Sci 18:286–291 Puleo R, Latif A, Ingarao G, et al (2023) Solid bonding criteria design for aluminum chips recycling through Friction Stir Consolidation. J Mater Process Technol 319:. https://doi.org/10.1016/j.jmatprotec.2023.118080 Baffari D, Reynolds AP, Li X, Fratini L (2017) Influence of processing parameters and initial temper on Friction Stir Extrusion of 2050 aluminum alloy. J Manuf Process 28:319–325. https://doi.org/10.1016/j.jmapro.2017.06.013 Cite Share Download PDF Status: Published Journal Publication published 21 Apr, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted Editorial decision: Major Revisions Needed 04 Jan, 2026 Reviewers agreed at journal 03 Dec, 2025 Reviewers invited by journal 03 Dec, 2025 Editor assigned by journal 30 Nov, 2025 First submitted to journal 27 Nov, 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. 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1","display":"","copyAsset":false,"role":"figure","size":105169,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of FSE bimetallic tube extrusion process: AA7075 chips compaction (Step 1), AA7075 tube extrusion (Step 2), AA2024 chips loading and compaction (Step 3) and AA2024 tube extrusion (Step 4).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/c2792e200fbae2c29c95e0b5.png"},{"id":97542649,"identity":"258fa009-194c-4a5f-9ed7-1318835a9b06","added_by":"auto","created_at":"2025-12-05 15:25:11","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":242967,"visible":true,"origin":"","legend":"\u003cp\u003eSketches of the (a) designed die with section, (b) compacting tool, and (c) 20° tapered 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for (a) ID1, (b) ID2, and (c) ID3.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/76b06fcc6b08dada6d6b7e60.png"},{"id":97542654,"identity":"0a308c27-c2d1-4d28-b9d6-1b5aec215856","added_by":"auto","created_at":"2025-12-05 15:25:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":779625,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure of the cross-section of the bonding layer in AA7075/2024 bimetallic tube prepared by different process methods (a) ID1, (b) ID2, and (c) ID3.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/48771f1f62ce7d8e68d2d926.png"},{"id":97672169,"identity":"0ac30fb3-885a-4f56-8173-c9e85b5862e3","added_by":"auto","created_at":"2025-12-08 09:34:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":795875,"visible":true,"origin":"","legend":"\u003cp\u003eSEM microstructure of the transition zone of the bimetallic tube, along with EDS mapping for (a) ID1, (b) ID2, and (c) ID3.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/15fb8800ce409f29a30d2bff.png"},{"id":97542655,"identity":"0a00c778-7ec6-414d-a69c-1b2f2e3d3ec1","added_by":"auto","created_at":"2025-12-05 15:25:12","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":548763,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure of bimetallic tube representing grain size variation in the (a) outer surface of AA7075 tube, (b) AA7075/AA2024 tube interface, and (c) inner surface of AA2024 tube, ID1.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/77b0e848061f89fd88016ff8.png"},{"id":97671826,"identity":"81d297f0-15ab-4893-8ced-952f4f8b08aa","added_by":"auto","created_at":"2025-12-08 09:33:09","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":18545,"visible":true,"origin":"","legend":"\u003cp\u003eThe average grain size of the bimetallic tube from the outer to the inner surface, for each ID.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/1e741719342ddf76be98a68e.png"},{"id":97671211,"identity":"3a0eb5b2-e2d4-4a50-877f-c86f8203d6d2","added_by":"auto","created_at":"2025-12-08 09:32:08","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":141377,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Temperature and (b) effective strain results (with a zoomed view of the cross-section) at the end of AA7075 extrusion for AA7075 (ID3).\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/70244f5a9b1e828294f8fb10.png"},{"id":97542659,"identity":"5eb7508c-1e1b-47f7-b595-4183b59e23c9","added_by":"auto","created_at":"2025-12-05 15:25:12","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":36612,"visible":true,"origin":"","legend":"\u003cp\u003ePoint tracking approach from the (a) final position (tube extrusion), backward to the (b) initial position.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/a37498469317916f8cc60564.png"},{"id":97672236,"identity":"87608ec1-c7af-4085-8f22-59bc6b8fade0","added_by":"auto","created_at":"2025-12-08 09:34:55","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":99958,"visible":true,"origin":"","legend":"\u003cp\u003eNumerical simulation results: (a) recording point location for temperature acquisition, (b) strain distribution of the cross section of the numerical AA7075 tube, and (c) temperature evolution of the extruded tubes, for ID3.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/0adb559373429565f93a5c54.png"},{"id":97542661,"identity":"313b4b4a-24ed-4e3d-abd5-257b45f61350","added_by":"auto","created_at":"2025-12-05 15:25:12","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":114142,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 13.\u003c/strong\u003e(a) Average hardness of bimetallic tube for ID1, ID2, and ID3, and (b) indentation points considered for the hardness measurement.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/e721b6bda1581cbf88e0ac77.png"},{"id":107928055,"identity":"48dc0308-389c-4ef2-ac3f-8b1e5cdfbd71","added_by":"auto","created_at":"2026-04-27 16:06:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3406506,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8205051/v1/36067fe2-d3e5-4024-874f-b057c04afea5.pdf"}],"financialInterests":"","formattedTitle":"An Improved Upcycling Approach For Producing Bimetallic Tube Via Friction Stir Extrusion of Aluminium Chips","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn many industrial sectors, the mechanical and thermal properties of a single material are often insufficient to meet the diverse property requirements of a component. Nowadays, an increasing demand for bimetallic tubes is visible due to the need to address the mechanical properties requirement issues, also aiming to offer enhanced and complementary properties [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. An example is the automotive sector, where bimetallic tubes are widely employed in exhaust systems to withstand high temperatures while maintaining corrosion resistance and structural rigidity.\u003c/p\u003e\u003cp\u003eOne of the most common materials often used across various industries, particularly in aerospace and aeronautics, is aluminium and its alloys. This metal is usually preferred, among others, due to its favorable strength-to-weight ratio. Specifically, the 2xxx and 7xxx series aluminium alloys are notable for their superior mechanical properties and corrosion resistance [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In this regard, developing efficient and cost-effective manufacturing techniques for bi-metallic aluminium tubes has become increasingly important to achieve industry needs and promote material efficiency.\u003c/p\u003e\u003cp\u003eCurrently, various techniques have been explored for the fabrication of bimetallic tubes, including conventional extrusion [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], the hydroforming technique [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], and magnetic pulse cladding [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], etc. These methods primarily rely on mechanical bonding, where interfacial adhesion is achieved through plastic deformation induced by applied pressure. However, such bonds often exhibit low strength and are prone to failure at elevated temperatures. To address these limitations and promote atomic-level bonding, advanced techniques such as spin bonding[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] and shear-assisted extrusion with localized heating have been investigated. Although spin bonding can achieve strong metallurgical interface bonding, the experimental setup is highly complex, resulting in increased manufacturing costs. Explosive cladding has also been employed to fabricate bimetallic tubes [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], demonstrating excellent metallurgical bonding at the interface. Nonetheless, this method often leads to undesirable effects such as non-uniform outer diameters, high surface roughness, and significant safety concerns due to the use of explosives. Therefore, there is a clear need to develop a cost-effective, safe, and simplified manufacturing process capable of producing high-quality bimetallic tubes with reliable interfacial strength. Additionally, Ce Ji et al.[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] developed composite tubes using 45-grade carbon steel and 316L stainless steel via a three-roll skew rolling process. Yuling Chang et al.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] fabricated thin-walled Cu/Al composite tubes through a spinning process that effectively converted plate materials into tubular forms. Bao Wang et al.[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] successfully produced Ni/Al composite tubes by layering Ni and Al foils, followed by gas expansion and forming. Wei Zhang et al.[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] introduced Mg/Al composite tubes using a modified hot extrusion method, which involved adjusting the die geometry to enhance shear forces and achieve a flat bonding interface. However, the extrusion ratio was limited to around 7 due to constraints associated with the billet and die design.\u003c/p\u003e\u003cp\u003eRecently, due to the growing demand for more sustainable approaches, new environmentally friendly techniques are gaining interest from most researchers. SSR processes have gained significant attention, as they aim to reduce the environmental impact of conventional manufacturing by focusing on the direct recycling of materials. Among the SSR processes, FSE has emerged as an innovative technology in metal recycling, offering a sustainable and energy-efficient method for converting scrap or waste metal into valuable components [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. While FSE is widely recognized for its application in wire production, its potential for manufacturing tubes is increasingly being explored. Literature indicates that most existing tube fabrication methods require multiple processing steps, typically involving at least one high-temperature operation; nevertheless, extrusion remains the most frequently used technique [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. In contrast, the FSE process offers a more energy-efficient alternative, requiring fewer steps and lower processing temperatures [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Despite its advantages, no prior research has been reported on the fabrication of bimetallic tubes combining aluminium 7xxx and 2xxx series alloys using the FSE method. Moreover, the friction stir extrusion of tubes has been commonly performed starting from a bulk component[\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] or a pre-consolidation step that primarily turns recycling chips into a billet [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], which increases the energy demand of the process. Therefore, a more sustainable approach that aims to reduce the impact on chips recycling and the steps needed for tube production is still missing.\u003c/p\u003e\u003cp\u003eFor this reason, this study aims to demonstrate the capability of FSE for directly turning AA2024 and AA7075 recycling chips into high-quality aluminium bimetallic tubes by skipping any preliminary pre-consolidation or pre-heating step. Specifically, the FSE technique has been adopted to fabricate bimetallic tubes consisting of AA7075 as the outer tube and AA2024 as the inner tube. The process is structured in two stages: outer tube production (AA7075) and inner tube production (AA2024).\u003c/p\u003e\u003cp\u003eThe experimental campaign was performed at 15 kN vertical load and three different rotational speeds equal to 500, 700, and 1000 rpm. The microstructural evolution at the bonding interface, along with the mechanical properties of the extruded bimetallic tubes, was comprehensively characterized using optical microscopy (OM), scanning electron microscopy (SEM), and microhardness testing. The results confirmed the feasibility and effectiveness of the FSE process in producing high-performance, multilayered aluminium components with refined microstructures and robust interfacial bonding.\u003c/p\u003e"},{"header":"2 Experimental Procedures","content":"\u003cp\u003eFor the outer tube, AA7075 aluminium alloy chips were used, whereas AA2024 aluminium alloy chips were considered for the inner one because of the different material ductility. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the chemical compositions of the Starting material, i.e., AA7075-T6 and AA2024-T3 aluminium alloy, used in this work. The recycling chips were produced from the milling operation of AA7075-T6 round bar and the turning operation of AA2024-T3 bar. Additionally, the chips were submerged in the acetone for effective cleaning before processing.\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\u003eChemical composition analysis of AA2024-T3[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] and AA7075-T6[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] aluminum alloys.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"9\"\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=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMaterial\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAl\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCu\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMn\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eMg\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eCr\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eZn\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2024-T3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBal.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e4.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.145\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.09\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7075-T6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBal.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e5.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Experimental campaign\u003c/h2\u003e\u003cp\u003eThe experimental tests were performed on an ESAB LEGIO Friction Stir Welding machine, specifically adapted for producing aluminium tubes from recycled chips. The FSE process was structured in four different steps: AA7075 chips loading, AA7075 tube extrusion, AA2024 chips loading, and AA2024 tube extrusion. A schematic diagram of the FSE-based bimetallic tube extrusion process is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eIn the first step, 20 grams of AA7075 aluminium alloy chips were placed into a custom-designed split die (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), which had an internal diameter of 25 mm and a height of 76 mm. The die was manufactured and securely assembled to a backing plate using bolts and nuts. The chips were compacted using a 25 mm diameter flat-head H13 steel tool (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) under an axial force of 5 kN. This compaction is part of the chips' preparation and aims to ensure uniform density, die filling, and to avoid scattering during extrusion. The outer tube was then extruded using a 23 mm diameter tool with a 20\u0026deg; conical taper profile (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). It is worth mentioning that the conical profile was used instead of a flat one to promote smooth material flow and easy tool insertion [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. After the outer tube was extruded, 20 grams of AA2024 aluminium alloy chips were poured into the internal cavity of the previously extruded tube. These chips were compacted using a flat tool of 23 mm diameter, again under a force of 5 kN. The inner tube was then extruded using a 21 mm diameter tool with the same 20\u0026deg; conical taper profile. The tests were performed at 15 kN vertical load and three rotational speeds equal to 500 (low), 750 (medium), and 1000 (high) rpm.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn the current study, the outer layer of the bimetallic tube (AA7075) had an average thickness of approximately 1.0 mm, as well as the inner layer (AA2024). Details of the tool geometries are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, while process parameters, including rotational speed and axial extrusion force, are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eIt is worth mentioning that for monitoring temperature during the extrusion process and for tuning the numerical simulations, a K-type thermocouple was inserted into a hole drilled at 1/3-height of the die, positioned 1 mm from the inner die wall (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, A-A section).\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\u003eGeometry characterization of tapered tools.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTool\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMaterial\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eUniform shank (\u0026Oslash;)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTool tip\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eHolding shank\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eTapered length\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eUniform shank\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFor AA7075\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eH-13 tool steel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e23 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e22 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026Oslash; 30 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e10 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e70 mm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFor AA2024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eH-13 tool steel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e21 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e20 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026Oslash; 30 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e10 mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e70 mm\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\u003eFSE process parameters and tube material layers disposition.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\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=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eID\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eRotational speed [rpm]\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eForce [kN]\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTotal tube thickness [mm]\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eOuter tube\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eInner tube\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eID1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e500\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7075\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2024\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eID2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e750\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7075\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2024\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eID3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7075\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2024\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\u003eConsequently, the bimetallic tubes were cut longitudinally to the extrusion direction to prepare samples for mechanical and microstructural evaluation. The cross-sections were mounted, ground, and polished, then etched with Keller\u0026rsquo;s reagent (consisting of 2 mL HF, 3 mL HCl, 5 mL HNO₃, and 190 mL H₂O) to expose their microstructural details. Optical microscopy (OM), along with SEM and EDS, was utilized to study the internal structure and elemental distribution, particularly at the bonding interface. The grain size was quantified using the mean linear intercept technique. For mechanical assessment, Vickers microhardness tests were conducted using a 0.5 kg load and a 15-second dwell time. Hardness measurements were taken across the wall thickness of the tube, starting from the outer diameter toward the inner surface, at intervals of approximately 0.2 mm. Each point was tested three times, and the average of the results was used to ensure consistency and reliability.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Numerical campaign\u003c/h2\u003e\u003cp\u003eA numerical campaign was performed using the commercial finite element software SFTC DEFORM 3D. The numerical setup involves five components: a die, a backing plate, two tools, and material chips (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The modelling of the chips batch is complex, and for this reason, the aluminium chips were considered as a single block porous material, following the material formulation of Shima-Oyane [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Specifically, an initial relative density value of 0.7 was assigned to the porous billet, calculated experimentally considering the mass of the loaded chips and the geometry of the die\u0026rsquo;s chamber. The porous billet was characterized by a mesh size of 35000 elements with a refining mesh window close to the tool-material contact (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, A-A section).\u003c/p\u003e\u003cp\u003eThe other components were considered as rigid material made of H-13 steel. The two tools were defined by a mesh size of 60000 elements, while for both the die and backing plate, the mesh size was 30000 elements. A tuning approach was performed for calculating the proper thermal and frictional coefficients. In this regard, the experimental data of the thermocouple were tuned to the numerical one, and the following values were obtained: shear factor of 0.2 and interface heat transfer coefficients (IHTC) of 11, 45, and 45 W/mm\u0026sup2;/K for the tool-material, die-material, and backing plate-material contacts, respectively. The methodology of this numerical calibration has already been applied and assessed in the literature [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Macroscopical analysis\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e depicts the bimetallic tubes fabricated from AA7075 and AA2024 aluminum alloy chips using the FSE process under various process parameters. All tubes were successfully extruded, demonstrating the viability of the FSE technique. However, surface quality and material consolidation varied with processing conditions.\u003c/p\u003e\u003cp\u003eAt the beginning of the process, the material is compressed and heated up thanks to the initial stirring and frictional actions of the tool. In these early stages, proper process parameters are crucial for obtaining optimal chips bonding and high surface quality. In this regard, when the heat generated is too low (ID1, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), a lack of consolidation of the first extruded layers is visible near the top surface of the tube. On the other hand, at a high rotational speed (ID3, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), excessive heat led to early softening of the material, which resulted in the surface deteriorating and uneven surface roughness in large parts of the extruded tube. At a medium rotational speed (ID2, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), optimal surface conditions were achieved. At process parameters ID2, the tube exhibited a uniform and quite smooth outer surface with no visible cracks at the top. Additionally, the bottom surface showed no signs of adhesion between the backing plate and the workpiece material, indicating effective material flow.\u003c/p\u003e\u003cp\u003eIt is worth remarking that the material behavior of AA7075 recycling chips plays an important role in material bonding; high mechanical properties may hinder proper consolidation. Difficulty in chips bonding for AA7075 recycling chips was also experienced by Puleo et al. [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] in the friction stir consolidation process.\u003c/p\u003e\u003cp\u003eThe final length of the bimetallic tube, after trimming both top unconsolidated chips and bottom consolidated billet, was approximately 40 mm.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Microstructural investigations\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e illustrates the cross-sectional microstructure of the AA7075/AA2024 bimetallic tube fabricated under different process parameters. In Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, corresponding to the low rotational speed condition (ID1), insufficient bonding is observed at the interface of the two materials. A distinct separation indicates that inadequate heat was generated during the process, preventing effective metal-to-metal bonding. High-resolution imaging of the interface further confirms the presence of unbonded regions. In contrast, Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, corresponding to medium (ID2) and high (ID3) rotational speeds, show no visible surface defects in the cross-section. The interface appears continuous and free from voids or delamination, indicating successful consolidation and a structurally sound bimetallic tube.\u003c/p\u003e\u003cp\u003eLooking at the third column of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (higher magnification), a distinct interfacial region, commonly referred to as the diffusion layer, is clearly visible and outlined by two yellow dashed lines. This zone represents a transition area where the AA2024 and AA7075 interface bonding occurs. The presence of this diffusion layer confirms atomic interdiffusion between the two alloys, which contributes to the formation of a strong metallurgical bond. The gradual transition in composition within this layer (i.e., EDS images) further supports the effectiveness of the FSE process in achieving solid-state bonding without the formation of defects or interfacial discontinuities.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFor investigating the microstructure of the bimetallic tubes at the interface, SEM analysis was performed. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents SEM micrographs along with corresponding EDS results, focusing on the transition zone between AA2024 and AA7075, marked by a red dashed line. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea (ID1), at low rotational speed, displays discontinuity and areas of insufficient bonding, indicating that the heat generated during the process was not adequate to achieve full metallurgical joining. It is important to note that the darker areas near the interface are not voids or porosity. In contrast, the samples extruded at medium and high rotational speeds (ID2 and ID3) haven\u0026rsquo;t shown any signs of discontinuity in the transition region. Additionally, the EDS mapping presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e further validates and supports the quality of the bond. In this regard, the images of ID1 clearly delineate the transition zone between the AA7075 and AA2024 alloys, with an elemental distribution that matches the expected chemical composition of each material. Specifically, zinc (Zn) was predominantly detected in the AA7075 region, while copper (Cu) was more concentrated in the AA2024 region. On the contrary, ID2 and ID3 show more elemental diffusion. This elemental distribution provides strong evidence of alloying elements\u0026rsquo; diffusion across the interface and metallurgical bonding between the two aluminium alloys, processed via the FSE process.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFurther grain size analysis was performed on the cross-section of the bimetallic tube to enhance microstructural characterization. Specifically, the grain dimension was measured along the cross section of the tube, from the outer (AA7075) to the inner tube (AA2024), and images of the microstructure were acquired in three specific observation zones: the outer surface of the AA7075 tube (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), the AA7075/AA2024 interface (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), and the inner surface of the AA2024 tube (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). A better refiguration of the selected surfaces and observation zones is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eResults of tube micrography are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e for the ID1 sample. In particular, fine grains (avg. 4.64 \u0026micro;m) were observed near the outer surface of the AA7075 tube (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), with a gradual increase toward the interface (avg. 5.60 \u0026micro;m). At the interface (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), grain size slightly increased from the AA7075 to the AA2024 side (avg. 5.65 \u0026micro;m). Lastly, close to the inner surface of the AA2024 tube (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec), a fine and equiaxed structure was again observed (avg. 4.60 \u0026micro;m). These variations in grain size are mainly attributed to the stirring action of the tool. As already experienced by Baffari et al. [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] during wire production through the FSE process, the material directly in contact with the tool, which is forced through the extrusion reduction zone, undergoes high pressure and temperature, promoting dynamic recrystallization and grain refinement. For this reason, the grain dimension of the inner surface of both AA7075 and AA2024 tubes is smaller than the outer surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSimilar behavior was observed in samples ID2 and ID3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e), with an overall decrease in average grain size at higher rotational speeds.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Numerical results and microstructural comparison\u003c/h2\u003e\u003cp\u003eTo support the findings of the experimental campaign, numerical simulations were carried out, aiming to give an overview of the temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea) and strain (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb) behavior during FSE tube processing. In this regard, a plot of the temperature and strain profile, at the end of the AA7075 tube production, has been proposed in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e for the case study ID3. The results reveal that high strain levels occurred both across the tube thickness and under the tool. Furthermore, the detailed cross-sectional view in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb indicates that the strain is not uniformly distributed through the tube\u0026rsquo;s thickness; instead, it visibly increases from the outer surface toward the inner one. This strain distribution demonstrates that the tool stirring action is more emphasized at the inner tube\u0026rsquo;s surfaces than the external one, corroborating the observation regarding the occurrence of recrystallization phenomena in the inner surface, previously discussed.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eNumerical simulations were also employed to further investigate and explain the grain growth phenomena observed at the interface of the bi-metallic tube (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). A point-tracking approach was adopted to track the temperature evolution of a selected point over the entire process. In particular, a point, initially located at the interface of the AA7075 extruded tube, was tracked from the considered process time backward to the beginning of the process (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe results of the point tracking showed that, during the FSE process, the material in contact with the tool, particularly near the extrusion channel, was subjected to the highest temperature levels, ranging between 480 and 550\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). During the AA7075 extrusion stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea), the tracked point moved from the initial position (under the tool) toward the tube wall, experiencing a peak temperature of approximately 400\u0026deg;C, which lies within the recrystallization temperature range for aluminium alloys. Under these conditions, the combined effect of high strain and elevated temperature promoted recrystallization, resulting in grain refinement of the AA7075 alloy. During the production of the AA2024 tube, point-tracking analysis revealed that the temperature at the monitored location increased again to approximately 350\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb), partly due to thermal exchange with the already processed AA7075 tube. This secondary temperature peak, located within the recrystallization range, acted as an additional thermal cycle, leading to grain coarsening at the interface in both materials. For this reason, the grain size in correspondence to the AA7075 inner zone resulted in being bigger than the outer (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e), despite the tool stirring refined the inner portion of the AA7075 tube.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Hardness characterization\u003c/h2\u003e\u003cp\u003eThe mechanical properties of the obtained bimetallic tube were investigated through Vickers microhardness measurements by performing twelve indentation points (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003eb) along the cross-section of the tube. The results of these measurements are reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003ea.\u003c/p\u003e\u003cp\u003eTypically, regions containing unbonded or poorly bonded chips exhibit lower hardness compared to areas where strong metallurgical bonding has occurred. Data shows noticeable variation in hardness along different paths across the thickness of the tube. Compared to the average Vickers hardness of the base material, all measured values were higher, indicating that the FSE process enhances the mechanical properties of the extruded material. A consistent trend was observed: microhardness values increased progressively from the outer surface toward the inner surface. For instance, considering ID1, the hardness at point 1 (outer surface) was 87 HV, which increased to 149 HV at point 12 (inner surface). Similar trends were noted for other process conditions. This improvement aligns with the observations made by Baffari et al. [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]; in his work, the authors found that after the FSE process, the hardness of the product was close to the T3 heat treatment hardness value. The AA7075 and AA2024 T3 hardness values are typically about 150 and 145 HV, respectively.\u003c/p\u003e\u003cp\u003eAdditionally, an overall increase in hardness has been reported with the increase in rotational speed; at the same vertical load, the increase in rotational speed results in an increase in frictional heating, which enhances chip consolidation, reduces internal voids, and promotes grain refinement, as already shown in Chap.\u0026nbsp;3.2.\u003c/p\u003e\u003cp\u003eThese observations are consistent with the microstructural analysis, which demonstrated grain size refinement from the AA7075 toward the AA2024 layers of the bimetallic tube. However, in the transition zone between AA7075 and AA2024, a localized decrease in microhardness was noted. This reduction resulted from variation in grain size and minor defects such as porosity or unbonded regions, which act as stress concentrators, reducing the overall hardness. Similarly, the external surface of the AA7075 tube exhibited low hardness values due to the roughness and poor quality of the surface [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], especially for ID1.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn this study, it was proved that FSE can be successfully applied to produce AA7075/AA2024 bimetallic tubes in a single-step process directly from recycled aluminium chips, demonstrating the effectiveness of this upcycling route. Enhanced microstructural and hardness characteristics were observed across the thickness of the tube, along with a detailed SEM analysis of the bonding quality at the tube interface. The key findings are summarized as follows:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eThe results demonstrated that the FSE process is a viable method for fabricating bimetallic tubes. Among the evaluated conditions, the tube produced at 750 rpm and 15 kN exhibited superior surface finish, refined microstructure, and enhanced mechanical properties.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eOptical microscopy and grain size analysis revealed uniformly distributed equiaxed grains, and an overall gradual decrease in grain size was observed toward the inner wall, resulting from the tool\u0026rsquo;s thermal and stirring actions. Moreover, microstructural analysis confirmed that adequate process conditions led to dynamic recrystallization, resulting in fine-grained structures (avg. 4.62 \u0026micro;m, at the outer and inner surface of the bi-metallic tube) and effective bonding between dissimilar aluminum alloys, thus contributing to improved mechanical performance.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe hardness characterization revealed an overall increase from the outer layer (AA7075) to the inner layer (AA2024). The average hardness for the AA7075 layer was about 150 HV, while a value of nearly 160 HV was obtained for the AA2024 one. At the interface between the two layers, a drop in hardness was observed (around 140 Hv).\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eNumerical simulations showed that the material located under the tool and near the extrusion channel experiences high temperatures and strains, conditions that promote recrystallization during FSE. Moreover, a point tracking approach for temperature monitoring at the interface showed that the inner surface of the AA7075 tube is subjected to a secondary heating effect, due to the extrusion of the second tube, which affected the grain size at the tube\u0026rsquo;s interface.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eSEM and EDS analyses verified the presence of a metal-to-metal bonded interface at proper process parameters (750 rpm \u0026minus;\u0026thinsp;15 kN and 1000 rpm \u0026ndash; 15 kN). The thermal and mechanical input during FSE was sufficient to facilitate elemental diffusion across the interface, leading to strong interlayer bonding.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eDeclaration of competing interest\u003c/h2\u003e\u003cp\u003eThe Authors disclose no actual or potential conflict of interest that could inappropriately influence, or be perceived to influence, this work.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eRiccardo Puleo: Numerical campaign development, Draft writing. Muhammad Adnan: Experimental campaign development, Draft writing. Giuseppe Ingarao: Research supervision, Draft, Final revision. Livan Fratini: Research supervision, Paper revision.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis study was funded by Italian MUR funds through European Union-NEXT Generation EU scheme; title of the project: \u0026ldquo;Finalizing processes for multi-material based Functionally Graded billets and wires obtained through solid state recycling operations of aluminum alloy chips - FULL RECYCLE\u0026rdquo;, PRIN 2022 (CUP: B53D23006550006).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eSwarnkar R, Karmakar S, Pal SK (2023) An investigation of bimetallic tube fabrication through a novel friction stir extrusion based technology for automotive applications. Mater Today Commun 35:106363. https://doi.org/10.1016/j.mtcomm.2023.106363\u003c/li\u003e\n \u003cli\u003eXie Z, Zhou L, Li J, et al (2025) Mechanical and Corrosion Properties of AA2024 Aluminum Alloy with Multimodal Gradient Structures. Metals (Basel) 15:177. https://doi.org/10.3390/met15020177\u003c/li\u003e\n \u003cli\u003eKang CG, Jung YJ, Kwon HC (2002) Finite element simulation of die design for hot extrusion process of Al/Cu clad composite and its experimental investigation. J Mater Process Technol 124:49\u0026ndash;56. https://doi.org/10.1016/S0924-0136(02)00106-1\u003c/li\u003e\n \u003cli\u003eWang X, Li P, Wang R (2005) Study on hydro-forming technology of manufacturing bimetallic CRA-lined pipe. Int J Mach Tools Manuf 45:373\u0026ndash;378. https://doi.org/10.1016/j.ijmachtools.2004.09.015\u003c/li\u003e\n \u003cli\u003eFan Z, Yu H, Li C (2016) Plastic deformation behavior of bi-metal tubes during magnetic pulse cladding: FE analysis and experiments. J Mater Process Technol 229:230\u0026ndash;243. https://doi.org/10.1016/j.jmatprotec.2015.09.021\u003c/li\u003e\n \u003cli\u003eMohebbi MS, Akbarzadeh A (2011) Fabrication of copper/aluminum composite tubes by spin-bonding process: experiments and modeling. The International Journal of Advanced Manufacturing Technology 54:1043\u0026ndash;1055. https://doi.org/10.1007/s00170-010-3016-5\u003c/li\u003e\n \u003cli\u003eGuo X, Tao J, Wang W, et al (2013) Effects of the inner mould material on the aluminium\u0026ndash;316L stainless steel explosive clad pipe. Mater Des 49:116\u0026ndash;122. https://doi.org/10.1016/j.matdes.2013.02.001\u003c/li\u003e\n \u003cli\u003eG\u0026oacute;mez X, Echeberr\u0026iacute;a J (2000) Microstructure and mechanical properties of low alloy steel T11\u0026ndash;austenitic stainless steel 347H bimetallic tubes. Materials Science and Technology 16:187\u0026ndash;193. https://doi.org/10.1179/026708300101507532\u003c/li\u003e\n \u003cli\u003eJi C, Niu H, Li Z, et al (2024) Deformation law and bonding mechanism of 45 carbon steel/316L stainless steel cladding tubes fabricated by three-roll skew rolling bonding process. J Mater Process Technol 325:118277. https://doi.org/10.1016/j.jmatprotec.2023.118277\u003c/li\u003e\n \u003cli\u003eChang Y, Chen H, Zhou J, et al (2023) Micro-nano interface structure and mechanical characteristics of thin wall Cu/Al composite tubes prepared by strong staggered spinning. Mater Charact 206:113405. https://doi.org/10.1016/j.matchar.2023.113405\u003c/li\u003e\n \u003cli\u003eWang B, Wang D, Wang S, et al (2023) Fabrication of NiAl alloy hollow thin-walled component through hot gas forming of Ni/Al laminated tube and conversion process. Journal of Materials Research and Technology 26:7224\u0026ndash;7236. https://doi.org/10.1016/j.jmrt.2023.09.057\u003c/li\u003e\n \u003cli\u003eZhang W, Hu H jun, Hu G, et al (2023) A direct extrusion‐shear deformation composite process that significantly improved the metallurgical bonding and texture regulation grain refinement and mechanical properties of hot-extruded AZ31/AA6063 composite tubes. Materials Science and Engineering: A 880:145090. https://doi.org/10.1016/j.msea.2023.145090\u003c/li\u003e\n \u003cli\u003eAdnan M, Buffa G, Baghdadchi A, et al (2024) Unveiling the mechanical and microstructural properties of SiC reinforced aluminum wires recycled from scraps by friction stir extrusion. Materials Science and Engineering: A 916:. https://doi.org/10.1016/j.msea.2024.147333\u003c/li\u003e\n \u003cli\u003eBuffa G, Campanella D, Adnan M, et al (2024) Improving the Industrial Efficiency of Recycling Aluminum Alloy Chips Using Friction Stir Extrusion: Thin Wires Production Process. International Journal of Precision Engineering and Manufacturing - Green Technology. https://doi.org/10.1007/s40684-023-00573-w\u003c/li\u003e\n \u003cli\u003eBehnagh RA, Fathi F, Yeganeh M, et al (2019) Production of seamless tube from aluminum machining chips via double-step friction stir consolidation. International Journal of Advanced Manufacturing Technology 104:4769\u0026ndash;4777. https://doi.org/10.1007/s00170-019-04326-5\u003c/li\u003e\n \u003cli\u003eBaffari D, Reynolds AP, Masnata A, et al (2019) Friction stir extrusion to recycle aluminum alloys scraps: Energy efficiency characterization. J Manuf Process 43:63\u0026ndash;69. https://doi.org/10.1016/j.jmapro.2019.03.049\u003c/li\u003e\n \u003cli\u003eAbu-Farha F (2012) A preliminary study on the feasibility of friction stir back extrusion. Scr Mater 66:615\u0026ndash;618. https://doi.org/10.1016/j.scriptamat.2012.01.059\u003c/li\u003e\n \u003cli\u003eSwarnkar R, Karmakar S, Pal SK (2023) An investigation of bimetallic tube fabrication through a novel friction stir extrusion based technology for automotive applications. Mater Today Commun 35:. https://doi.org/10.1016/j.mtcomm.2023.106363\u003c/li\u003e\n \u003cli\u003eAsadi P, Akbari M, Sadowski T, et al (2024) Examining the impact of tool taper angle in Al-Si tube manufacturing by friction stir extrusion. J Manuf Process 131:532\u0026ndash;544. https://doi.org/10.1016/j.jmapro.2024.09.047\u003c/li\u003e\n \u003cli\u003eBehnagh RA, Fathi F, Yeganeh M, et al (2019) Production of seamless tube from aluminum machining chips via double-step friction stir consolidation. International Journal of Advanced Manufacturing Technology 104:4769\u0026ndash;4777. https://doi.org/10.1007/s00170-019-04326-5\u003c/li\u003e\n \u003cli\u003eAhn J, Chen L, He E, et al (2017) Effect of filler metal feed rate and composition on microstructure and mechanical properties of fibre laser welded AA 2024-T3. J Manuf Process 25:26\u0026ndash;36. https://doi.org/10.1016/j.jmapro.2016.10.006\u003c/li\u003e\n \u003cli\u003eSaravanan V, Banerjee N, Amuthakkannan R, Rajakumar S (2015) Microstructural Evolution and Mechanical Properties of Friction Stir Welded Dissimilar AA2014-T6 and AA7075-T6 Aluminum Alloy Joints. Metallography, Microstructure, and Analysis 4:178\u0026ndash;187. https://doi.org/10.1007/s13632-015-0199-z\u003c/li\u003e\n \u003cli\u003eShima S OM (1976) Plasticity theory for porous metals. Int J Mech Sci 18:286\u0026ndash;291\u003c/li\u003e\n \u003cli\u003ePuleo R, Latif A, Ingarao G, et al (2023) Solid bonding criteria design for aluminum chips recycling through Friction Stir Consolidation. J Mater Process Technol 319:. https://doi.org/10.1016/j.jmatprotec.2023.118080\u003c/li\u003e\n \u003cli\u003eBaffari D, Reynolds AP, Li X, Fratini L (2017) Influence of processing parameters and initial temper on Friction Stir Extrusion of 2050 aluminum alloy. J Manuf Process 28:319\u0026ndash;325. https://doi.org/10.1016/j.jmapro.2017.06.013\u003c/li\u003e\n\u003c/ol\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":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"bi-metallic, friction stir extrusion, FEM, recycling","lastPublishedDoi":"10.21203/rs.3.rs-8205051/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8205051/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBimetallic tubes are employed in applications where a single component must satisfy multiple performance requirements, for instance, combining high strength with corrosion resistance. In literature, manufacturing processes like rotary piercing, forward/backward extrusions, and tube cladding have been commonly used to produce high-performance bimetallic tubes, typically starting from bulk materials. Moreover, recently, new sustainable processes belonging to the solid-state recycling (SSR) category, namely friction stir extrusion (FSE), have also been adopted for tube manufacturing. However, both conventional and SSR-based approaches generally rely on multi-step routes involving pre-heating, homogenization, or pre-consolidation to obtain workable billets, which increases energy consumption. This study goes beyond the issues of the existing extrusion-based recycling processes, proposing a single-step FSE approach able to directly convert AA7075 and AA2024 aluminum chips into bimetallic tubes, offering a sustainable upcycling pathway that does not involve any pre-heating or pre-consolidation stages. Three combinations of rotational speed and axial load were investigated to assess their influence on tube quality. The resulting bimetallic tubes were characterized through microstructural and macrostructural analyses, which revealed the absence of voids and inclusions at the bonding interface, the material composition, a grain refinement (avg. 4.6 \u0026micro;m), and an enhanced hardness (up to 175 HV) under optimal processing conditions. As a matter of fact, this process opens new opportunities for the fabrication of bimetallic tubular components, which can be used in electrical, structural, lightweight, and corrosion-resistant applications.\u003c/p\u003e","manuscriptTitle":"An Improved Upcycling Approach For Producing Bimetallic Tube Via Friction Stir Extrusion of Aluminium Chips","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-05 15:25:07","doi":"10.21203/rs.3.rs-8205051/v1","editorialEvents":[{"type":"communityComments","content":1},{"type":"decision","content":"Major Revisions Needed","date":"2026-01-04T15:57:01+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-12-03T10:17:17+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-03T09:49:23+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-01T02:24:11+00:00","index":"","fulltext":""},{"type":"submitted","content":"The International Journal of Advanced Manufacturing Technology","date":"2025-11-27T06:16:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8ce40280-e479-4f41-8a1e-116319717ae9","owner":[],"postedDate":"December 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-27T16:04:50+00:00","versionOfRecord":{"articleIdentity":"rs-8205051","link":"https://doi.org/10.1007/s00170-026-18114-5","journal":{"identity":"the-international-journal-of-advanced-manufacturing-technology","isVorOnly":false,"title":"The International Journal of Advanced Manufacturing Technology"},"publishedOn":"2026-04-21 15:58:23","publishedOnDateReadable":"April 21st, 2026"},"versionCreatedAt":"2025-12-05 15:25:07","video":"","vorDoi":"10.1007/s00170-026-18114-5","vorDoiUrl":"https://doi.org/10.1007/s00170-026-18114-5","workflowStages":[]},"version":"v1","identity":"rs-8205051","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8205051","identity":"rs-8205051","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}