Effect of active forced cooling on microstructure and corrosion properties of friction stir welded Al/Cu dissimilar joints: Quasi-in-situ study

Effect of active forced cooling on microstructure and corrosion properties of friction stir welded Al/Cu dissimilar joints: Quasi-in-situ study | 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 Effect of active forced cooling on microstructure and corrosion properties of friction stir welded Al/Cu dissimilar joints: Quasi-in-situ study Zhaoyi Pan, Yue Mao, Qiang Chu, Qinlian Zhang, Zhenzhong Wu, Weiqi Qiao, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7373892/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Corrosion resistance is a critical service performance of the Al/Cu dissimilar joints. Using the effect of forced cooling of flow water, a submerged friction stir welding (SFSW) technology was developed to enhance the corrosion resistance of the Al/Cu joints. The weld formation of the joints was not only improved by active forced cooling, but also it could refine the grains of the WNZ, and effectively reduce the interfacial IMCs layer thickness. The potential difference between the Al and Cu gives rise to the formation of a macroscopic galvanic effect, which accelerates the corrosion of the joints. Nevertheless, the corrosion resistance of SFSW joints was higher than that of FSW joints. During the 240 h corrosion period, no significant corrosion occurred on the Cu side due to cathodic protection, while the heat affected zone (HAZ) of Al side suffered the most severe corrosion, forming distinct corrosion grooves. Compared with the FSW joint, the SFSW joint had smaller width and depth of corrosion grooves on the HAZ of Al side, with a significantly reduced overall corrosion degree. It demonstrates that the active forced cooling contributes to improve the microstructure and corrosion resistance of Al/Cu joints. Friction stir welding Al/Cu dissimilar joint microstructure corrosion properties forced cooling Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 1. Introduction Al/Cu dissimilar composite components can give full play to the performance advantages of Al and Cu, which has broad application potential in new energy vehicles, electric power, refrigeration and other fields [ 1 – 2 ]. However, due to the significant differences in physical and chemical properties of Al and Cu, achieving high-performance Al/Cu joints faces numerous challenges [ 3 – 5 ]. The joints are prone to porosity and excessive intermetallic compounds(IMCs) when using the traditional fusion welding process[ 6 – 7 ]. In recent years, friction stir welding(FSW) technology has been widely applied to the joining of dissimilar materials, which can effectively overcome welding difficulties arising from differences in physical and chemical properties of dissimilar materials [ 8 – 9 ]. Zhou et al.[ 10 ] conducted the friction stir spot welding of Al to Cu and claimed that the three IMCs layers (Al 2 Cu-AlCu-Al 4 Cu 9 ) could be formed at the interface under the high heat input, the continuous IMCs layer with an appropriate thickness contributed to higher tensile properties. The effect of tool offset on microstructure and tensile properties of the dissimilar joint of 6061 Al alloy and pure Cu by Hou et al.[ 11 ]. They pointed out that the low tool offset(offset toward Al side) resulted in the formation of excessive IMCs and voids, which decreased the tensile strength of the joints. Shojaeefard et al.[ 12 ] revealed that a greater amount of IMCs was generated at higher rotational speed, resulting in a reduction in the joint's tensile strength. From the above, it can be concluded that the hard and brittle IMCs can be still inevitably produced at Al/Cu interface, and their thickness is directly related to the mechanical properties of the FSWed joint. Recently, several effective strategies have been applied to reduce the thickness of IMCs layer, with the application of forced cooling during the FSW process being particularly prominent. Tang et al. [ 13 ] carried out the splat cooling assisted FSW of 6061-T6 aluminium and pure copper and compared it with normal FSW joints, which indicated that the splat cooling significantly reduced the heat input and the thickness of interfacial IMCs layer, leading to a 42% improvement in bonding strength. Lader et al. [ 14 ] performed the FSW of dissimilar joints of AA1100-O and brass under both air and water environments, the results showed that the finer grain and the thinner IMCs layer were produced at UFSW joint, thereby enhancing the overall quality of the welded joint. Sundar et al. [ 15 ] carried out the research on the dissimilar joining of 6061 aluminum alloy and TC4 titanium alloy using underwater friction stir welding technology (UFSW), they observed that the UFSW joint exhibited a notably thinner IMCs layer of 3.731 µm in comparison to the normal FSW joint. In conclusion, the forced cooling can refine the grains, reduce the the thickness of IMCs layer, and improve the mechanical properties of the joints. In addition to mechanical properties, corrosion resistance is another critical indicator that needs to be considered during the service life of components. For the Al/Cu dissimilar joints, the significant potential difference between Al and Cu induces galvanic corrosion, which further accelerates the corrosion of the welded joints. Tang et al. [ 16 ] studied the corrosion behavior of the Al/Cu welded joints under different welding parameters by polarization curve and claimed that the corrosion resistance of the Al/Cu joint is lower than that of Al alloy BM and pure Cu BM, which is attributed to the formation of a corrosive primary battery between Al and Cu by joining, leading to a reduction in the whole corrosion resistance of the welded joint, while a thicker IMCs layer further accelerated corrosion. However, the research on the corrosion behavior of the Al/Cu joints remains insufficient, the corrosion evolution rule of the Al/Cu joints is still not fully understood, and the corrosion mechanism of Al/Cu joints remains to be revealed. Furthermore, it is worth noting that the effect of forced cooling on the corrosion resistance of the joint is also unclear and needs to be systematically studied. In this study, submerged friction stir welding (SFSW) was employed to fabricate the Al/Cu dissimilar joints, the microstructure and corrosion properties of the Al/Cu SFSW joints were systematically investigated, and a comparative study was performed between the SFSW and normal FSW joints to elucidate the influence of forced cooling on the corrosion behavior of the Al/Cu joints. 2. Experimental procedures 3 mm-thick sheet of 6061-T6 aluminum alloy and T2 copper were selected as base materials (BMs). Prior to welding, using different types of sandpaper to polish the welding surface and mating surface of the test plate, and clean them with acetone. SFSW process was carried out in a dedicated water tank, which was made of stainless steel and equipped with an inlet and outlet. During the welding process, flowing water was used to cool the weld, with the water layer thickness maintained at approximately 25 mm, as illustrated in Fig. 1 (a). Through process optimization, the welding parameters are set to a rotational speed of 800 rpm, a welding speed of 40 mm/min. For Al/Cu dissimilar joining, defects are prone to occur in the joint during center welding, whereas offsetting the tool pin toward the Al side was conducive to improve defect-free joints[ 17 ], thus, the offset was 1.2 mm (see Fig. 1 (b)). The 6061-T6 aluminum alloy plate was positioned on the advancing side, so the T2 pure copper plate was on the retreating side. The tool dimensions were as follows: shoulder diameter of 16 mm; pin root diameter of 5 mm, pin bottom diameter of 4 mm, and pin length of 2.7 mm. The tilt angle of the tool was 2.5°. Metallographic, TEM and EBSD samples were prepared from the welded joints using electrical discharge wire cutting. Then, the samples were ground and polished to achieve a scratch-free mirror finish. The EBSD testing was employed to analyze the grain structure and texture component. The interfacial characteristics and composition was characterized using SEM and EDS. A TEM was applied to investigate the crystal structure and orientation relationships of IMCs layer. For electrochemical testing, samples were cut perpendicular to the welding direction, with a copper wire welded to their backside. All surfaces except the test surface were encapsulated in epoxy resin, The surface to be tested was polished and cleaned with alcohol. The potentiodynamic polarization curve (PDP) and electrochemical impedance spectroscopy (EIS) of the BMs and Al/Cu joint were measured by using VersaSTAT 4 electrochemical workstation. The electrolyte used was a 3.5 wt% NaCl solution, and a three-electrode testing system was adopted, as illustrated in Fig. 2 , where the BM and welded joints served as working electrodes, the saturated calomel electrode as the reference electrode, and the platinum plate was the counter electrode. Before the PDP and EIS test, the sample was immersed in the electrolyte to measure the open-circuit potential(OCP). Subsequent tests were performed only after the OCP had stabilized. For EIS, the frequency range was 10 5 ~ 10 − 2 Hz with an amplitude of 10 mV. The immersion corrosion tests were conducted using a neutral 3.5 wt% NaCl solution at room temperature, with immersion durations set at 6 h, 12 h, 24 h, 48 h, 96 h, and 240 h, respectively. According to the GB/T 19746 − 2018, the immersion solution was replaced every two days. The macro-and micro-scopic morphology of corroded samples were observed by OM and SEM. The element content of the corrosion products was analyzed via EDS. 3. Results 3.1 Surface morphology of the joints The surface morphologies of the Al/Cu joints manufactured by normal FSW and SFSW are presented in Fig. 3 . There are no obvious defects in both FSW and SFSW joints, but the surface of the FSW weld is relatively rough with a certain amount of burrs and flash. The surface of the SFSW joint exhibits extremely smooth and its formation quality is high. This indicates that the SFSW process can effectively improve the welding quality of the Al/Cu joints. This is because the heat input of the normal FSW process is large, the mixing and reaction of the both metals are enhanced, leading to the accumulate of a large amount of metals and IMCs at the shoulder, and resulting in serious adhesion of the tool, which greatly reduces the smoothing effect of the shoulder on the weld surface and the containment effect on the metals, ultimately leading to a rough weld surface and increased flash. 3.2 Microstructure evolution Figure 4 depicts the welding nugget zone (WNZ) of the Al/Cu FSW and SFSW joints. In Al/Cu FSW, Cu exhibits higher flow stress, and its fluidity is not as good as Al at the same welding temperature. Under the action of mechanical stirring, a number of Cu particles are stripped off from the matrix and flows and mixes with the Al in the WNZ, forming a composite-like structure that strengthens the WNZ. It is worth noting that the size of Cu particles in the WNZ of SFSW joints is relatively smaller compared to the FSW joints. This can be due to the higher heat input in normal FSW, which causes significant material softening, making it easier for more larger Cu blocks to peel off from the Cu matrix. In contrast, under the forced cooling effect of flowing water, the welding temperature is significantly decreased, in turn ,the degree of softening and plastic deformation of Cu are also reduced. As a result, the Cu particles stripped from the Cu matrix are smaller in size. Additionally, the mechanical stirring action of the tool causes Cu particles to peel off the matrix and be stirred into the Al alloy, and plastic deformation occurs and gradually broken. The IMCs are formed by the elements interdiffusion between Cu particles and the surrounding Al alloy matrix under the action of thermal-mechanical, forming a multiphase particle composite structure. Figure 5 presents the microstructure and corresponding EDS results of a typical particle composite structure in both FSW and SFSW joints. From Fig. 5 (b-c), the interface between Cu particles and Al matrix is relatively blurry, indicating that Cu particles and Al matrix have undergone sufficient diffusion at the interface, which forms an IMCs layer surrounding the Cu particles. The Al element was also observed inside the particles, indicating that under the action of welding thermal cycling, the Al element of the matrix diffused into the interior of the Cu particles and reacted, thereby forming a particle composite structure. In Fig. 5 (a)and(d), the Cu particles are surrounded by two layers of gray Al/Cu IMCs. According to the point scanning results, the Al 2 Cu layer is located near the Al side, while the Al 4 Cu 9 layer is located near the Cu particles. Furthermore, within the particle composite structure, there is a gray layered phase region that can be identified as the Al 4 Cu 9 phase, while the white structure corresponds to the Cu(Al) solid solution. Thus, the larger particle composite structures are composed of three components: the Cu(Al) solid solution, the Al 2 Cu phase, and the Al 4 Cu 9 phase. Some of the particle composite structures consist of an outer Al 2 Cu phase and an inner island-like Al 4 Cu 9 phase, with the original Cu matrix having completely vanished. It can be also observed that a number of smaller particles have completely transformed into a single IMCs of Al 2 Cu. Through the above analysis, it can be concluded that the particle composite structures in the WNZ have different microstructural characteristics. The analysis of the reasons shows that the material mixing degree, element content and heat distribution in the WNZ of the joint are obviously uneven, which leads to the obvious difference in the atomic diffusion rate in different areas, and thus the evolution process of the particle composite structure is different. The microstructure in middle of the WNZ of the Al side for the Al/Cu joints are illustrated in Fig. 6 . Both FSW and SFSW joints exhibit finer equiaxed grains in the WNZ of Al side (see Fig. 6 (a-b)). This phenomenon arises from the combined action of welding thermal cycling and severe plastic deformation experienced by the WNZ during welding, which induces dynamic recovery and dynamic recrystallization [ 18 – 20 ]. The grain size distributions are displayed in Fig. 6 (c-d). For the FSW joint, the average grain size of the Al-side WNZ is 6.43 µm, whereas that of the SFSW joint is 1.64 µm. Evidently, the average grain size of the SFSW joint is smaller than that of the FSW joint, which suggests that the SFSW process with additional forced cooling can significantly refine the grains. The lower the temperature and the larger the strain rate, the smaller the grain size in the WNZ [ 21 ]. In the SFSW process, the forced cooling effect of flowing water reduces the temperature of the welding process, and the strain rate is mainly determined by the rotational speed and welding speed [ 22 ]. These factors collectively contribute to the grain refinement observed in the SFSW joint. On the Cu side, the WNZ of both FSW and SFSW joints also present equiaxed grains morphology (see Fig. 7 ). The average grain size of the WNZ in the FSW joint is 2.36 µm, while that in the SFSW joint is smaller, at 1.49 µm. This indicates that forced cooling also contributes to grain refinement on the Cu side. Figure 8 illustrates the (111) pole diagram of the WNZ in the Al side and Cu side for the Al/Cu FSW and SFSW joints, where Fig. 8 (e) shows the pole diagram of the face-centered cubic (FCC) metal simple shear standard (111). Figure 8 presents the (111) pole figures of the WNZ on both the Al and Cu sides in Al/Cu FSW and SFSW joints, where Fig. 8 (e) displays the standard (111) pole figure of the simple shear for face-centered cubic (FCC) metals. During welding, the metal within the WNZ undergoes intense friction and shear from the tool, leading to significant shear deformation. Thus, the analysis focuses primarily on the shear texture in the WNZ. The texture type of the WNZ in Al side of the FSW and SFSW joints is C-type shear texture components (see Fig. 8 (a) and (c)). Similarly, the WNZ on Cu side of both joints are provided with C-type texture components. However, the texture intensity in the Al side and Cu side of SFSW joint are decreased compared to FSW joint, which may be attributed to the weakening of materials flow caused by the active forced cooling[ 23 ]. The evolution of the interfacial IMCs layer is crucial to joint performance, especially closely related to the thickness of the IMCs layer. During the welding process, Al and Cu atoms interdiffuse across the interface and react to form the IMCs layer. As displayed in Fig. 9 (a), the entire interfacial IMCs layer of the FSW joint is distinctly divided into three sub-layers. Based on point scanning results(Fig. 9 (c)), the phase adjacent to the Al side is identified as Al 2 Cu, while that adjacent to the Cu side is Al 4 Cu 9 , with the AlCu phase present in the middle. For SFSW joint, the thickness of the IMCs layer is significantly thinner than that of the FSW joint, decreasing from 1.62 µm in FSW joint to 0.56 µm in SFSW joint, as depicted in Fig. 9 (b). In addition, the number of IMCs sub-layers has also been reduced from 3 layers for FSW joints to 2 layers for SFSW joints, which are identified as Al 2 Cu and Al 4 Cu 9 phase, respectively. The active forced cooling in SFSW process reduces the peak temperature and shortens the high-temperature dwell time[ 24 – 25 ], and thereby inhibits atomic diffusion and reduces diffusion rate. In addition, the diffusion reaction time decreases, and the growth rate of interfacial IMCs layer decreases, ultimately leading to a reduced IMCs layer thickness. To accurately determine the phase compositions of the interfacial reaction layer in the SFSW joint and analyze the microscopic interface types, TEM tests were performed on the interface zone, as shown in Fig. 10 . The reaction layer is composed of two sub-layers with relatively uniform thicknesses, exhibiting nano-scale equiaxed grains, as depicted in Fig. 10 (a). Based on the high-resolution images and selected area electron diffraction (SAED) analysis results (see Fig. 10 (b-e)), the interface from right to left (A-D) correspond to the Cu matrix, Al 4 Cu 9 layer, Al 2 Cu layer, and Al matrix, respectively. Figure 10 (f-h) present high-resolution images of three phase interfaces: Al 4 Cu 9 /Cu, Al 2 Cu/Al 4 Cu 9 , and Al/Al 2 Cu. It can be seen that all three phase interfaces are non-coherent interfaces, and there is no strong orientation relationship between the phases. This phenomenon may be due to the shear force applied by the tool, which destroys the potential parallel relationship between the phases. 3.3 Electrochemical corrosion behavior The open circuit potential (OCP) refers to the potential difference between the working electrode and the reference electrode in the absence of external current. Generally, the OCP value is indicative of the corrosion tendency of the material. Figure 11 shows the OCP test results of 6061-T6 Al alloy and T2 pure Cu in a 3.5 wt% NaCl solution. Within the 1800 s test duration, the OCP of both base metals (Al alloy and pure Cu) remains stable, the OCP of the pure Cu is approximate − 0.23V, while that of 6061-T6 Al alloy is around − 0.76 V, resulting in a potential difference of 0.53 V between the two materials. According to corrosion thermodynamics, the higher OCP value, the lower the tendency of the material to be corroded. Therefore, when Al alloy and pure Cu are fully joined via FSW, the potential difference between them induces a macroscopic galvanic effect in the Al/Cu joint, forming a macroscopic galvanic corrosion primary battery. In this battery, Cu acts as the cathode and is protected, whereas Al serves as the anode and is accelerated corrosion. Figure 12 shows the potentiodynamic polarization curves of the BMs and Al/Cu joints. The potentiodynamic polarization curve represents the relationship between the electrode polarization potential and polarization current. The electrochemical parameters of the BMs and welded joint can be obtained by Tafel extrapolation method, as listed in Table 1 . The self-corrosion potentials E corr of Al alloy and pure Cu are − 0.716 V and − 0.268 V, respectively, which shows a slight difference from the measured OCP values. This is owing to the polarization of the electrode caused by additional current during the testing process, leading to a certain drift in the corrosion potential. The self-corrosion potential E corr reflects both the difficulty of metal corrosion and the thermodynamic stability of the metal under electrochemical corrosion conditions. The corrosion current density i corr indicates the corrosion rate of the metal and characterizes the kinetic process of the corrosion reaction [ 26 – 27 ]. The self-corrosion potential of FSW and SFSW joints are − 0.719V and − 0.665V, respectively, and thus the SFSW joint has a higher corrosion potential. The corrosion current densities of the two joints are 1.29 × 10 − 5 A/cm 2 and 8.27 × 10 − 6 A/cm 2 , respectively, both of which are higher than those of the 6061-T6 Al alloy and the pure Cu, suggesting that the corrosion resistance of the Al/Cu welded joints are worse than that of the BMs, and the macroscopic galvanic effect of the Al/Cu joints accelerates the corrosion of the joints. In addition, the SFSW joint shows a higher self-corrosion potential and a lower corrosion current density compared to the FSW joint, demonstrating better corrosion resistance, which indicates that forced cooling contributes to the improvement of corrosion resistance of Al/Cu joints. The underlying mechanism can be analyzed as follows: due to the potential difference between IMCs and Al, low potential Al undergoes galvanic corrosion. In the FSW joint, a thick IMCs layer is formed at interface, which further accelerates the corrosion of Al, resulting in the decrease of the corrosion resistance of the whole Al/Cu joints. Table 1 electrochemical parameters of the BMs and welded joints Specimen Corrosion potential E corr (V) Current density i corr (A/cm 2 ) 6061-T6 Al alloy -0.716 4.48 × 10 − 6 T2 pure Cu -0.268 1.88 × 10 − 6 FSW joint -0.719 1.29 × 10 − 5 SFSW joint -0.665 8.27 × 10 − 6 To further analyze the electrochemical behavior of the BMs and the Al/Cu joints, the electrochemical impedance spectra (EIS) of BMs and the Al/Cu joints were measured in a 3.5 wt% NaCl solution, the results are shown in Fig. 13 . The larger the diameter of the capacitive loop in the Nyquist diagram, the greater the impedance modulus of the material, and the better the corrosion resistance [ 28 ]. As shown in Fig. 13 (a), the capacitive loop diameter of the pure Cu is the largest, indicating that the pure Cu has the best corrosion resistance. The capacitive loop diameter of the Al/Cu FSW and SFSW joints are smaller than that of the BMs, implying that the corrosion resistance of the FSW and SFSW joints is lower than that of the BMs, which is consistent with the test results of the potentiodynamic polarization curve. From the EIS, there is a capacitive loop in the medium-frequency and high-frequency regions of the BMs and the welded joint, and there is also a low-frequency inductive loop in the 6061-T6 Al alloy, FSW joint, and SFSW joint. The presence of inductive loop indicates that the samples undergo dissolution during the testing process of EIS. The inductive behavior is caused by the absorption/desorption of intermediate substances formed during the charge transfer [ 29 ]. The equivalent circuits shown in Fig. 14 are used to fit the EIS data, where the 6061-T6 Al alloy, FSW joint, and SFSW joint employ the circuit shown in Fig. 14 (a), and the T2 pure Cu uses the circuit shown in Fig. 14 (b). R s is the resistance of the corrosive electrolyte, R ct is the charge transfer resistance, CPE dl is the double layer capacitance, R f is the resistance of the corrosion products, CPE f is the capacitance of the corrosion products, and CPE is the constant phase element. The electrochemical parameters obtained by fitting are shown in Table 2 . It can be seen from the table that the pure Cu has the largest charge transfer resistance R ct value (2812 Ω·cm 2 ) and corrosion product resistance R f value (2243 Ω·cm 2 ), suggesting that the pure Cu has the best corrosion resistance. The ( R ct + R f ) value can reflect the overall corrosion resistance of the material, therefore, the corrosion resistance of the BMs and welded joints is ordered as follows: T2 pure Cu > 6061-T6 Al alloy > SFSW joint > FSW joint. The corrosion resistance of welded joints is worse than that of the BMs, but forced cooling contributes to improve the corrosion resistance of the Al/Cu joints. Table 2 Fitting results of EIS data of BMs and welded joint Parameter 6061-T6 Al Pure Cu FSW joint SFSW joint R s (Ω·cm 2 ) 5.24 5.471 5.84 4.283 CPE f (µΩ −1 cm −2 s n ) 57.43 52.09 303.7 211.05 n f 0.82 0.87 0.81 0.79 R f (Ω·cm 2 ) 1160 2243 387.5 1984 CPE dl (µΩ −1 cm −2 s n ) 46.18 64.42 513.31 275.88 n dl 0.87 0.93 0.91 0.79 R ct (Ω·cm 2 ) 2460 2812 1486 1248 R L (Ω·cm 2 ) 1481 / 950.7 286.62 L (H·cm − 2 ) 27925 / 15799 23007 3.4 Quasi-in-situ immersion corrosion Figure 15 presents the macroscopic corrosion morphologies of the FSW joints after immersion in a 3.5 wt% NaCl solution for different times, specifically 6 h, 12 h, 24 h, 48 h, 96 h, and 240 h. For convenience, the heat affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ) are combined into one area for observation and analysis, which is collectively known as the HAZ. Therefore, the areas observed in this study are the WNZ, HAZ and BMs. After immersing for 6 h (see Fig. 15 (b)), the WNZ of Al side for the FSW joint lost its metallic luster, and severe pitting corrosion occurs in the HAZ of Al side, with large corrosion pits observed, while slight pitting corrosion occurs in the Al BM with small pits. The whole Cu side (including the WNZ of Cu side, the HAZ of Cu side, Cu BM) has not undergone corrosion, and metallic luster can still be observed. As the corrosion going to 12 h, slight pitting corrosion begins to occur in the WNZ of Al side of the FSW joint. This is due to the uneven distribution of the WNZ, which contains a large number of Cu particles and Al/Cu IMCs. There is a potential difference between these Cu particles/IMCs and the Al matrix, forming a micro-zone galvanic corrosion primary battery, leading to the initiation of pitting corrosion. The number of corrosion pits in the HAZ of Al side and the Al BM increases significantly, and the size of the pits has increased. A large amount of white corrosion products accumulates around the pits, as shown in Fig. 15 (c). When the corrosion reaches to 48 h, some corrosion pits of the HAZ on the Al side begin to converge, forming corrosion groove, and meantime the corrosion continues to expand into the interior of the matrix. There is no significant change in the corrosion degree of the WNZ of Al side, and the number of pits of Al BM further increases. A small amount of brown oxide appears on the surface of the Cu side, but there are no obvious corrosion traces, as shown in Fig. 15 (e). At 240 h, the deep corrosion groove was already formed in the HAZ of Al side, penetrating the entire area. The WNZ of Al side and Al BM are still dominated by pitting corrosion, and the surface of Cu side is completely covered by brown oxides, but no obvious corrosion traces are observed, as shown in Fig. 15 (g). Figure 16 presents the macroscopic corrosion morphologies of the SFSW joints after immersion in a 3.5 wt% NaCl solution for different times. The corrosion evolution of the SFSW joints is similar to that of FSW joints. During the initial 6 h of immersion, the Cu side (including the WNZ of Cu side, the HAZ of Cu side, Cu BM) still maintains its metallic luster, and the WNZ of Al side becomes dark, but no obvious corrosion occurs. The HAZ of Al side exhibits more severe pitting corrosion, with larger pit sizes. The Al BM shows obvious pitting corrosion, but compared to the HAZ, the degree of corrosion is lighter, as shown in Fig. 16 (b). As the corrosion time increases, the corrosion development rate of the HAZ of Al side is faster, and the corrosion continues to expand to the surface and interior of the matrix. The area of the pits continues to expand, and they are interconnected, starting to undergo large-scale corrosion, ultimately forming a corrosion trace that runs through the entire HAZ. The Al BM and the WNZ of Al side are still mainly pitting corrosion, and there is obvious oxidation phenomenon on the Cu side, but there are no obvious corrosion traces, as shown in Fig. 16 (c-g). By comparing the macroscopic corrosion morphology of FSW and SFSW joints, it can be seen that the width of the corrosion groove in the HAZ of Al side of the SFSW joint is significantly smaller than that of the FSW joint, and the degree of corrosion is lower than that of the FSW joint. This is because forced cooling reduces the width of the HAZ of Al side and refines the grain size of the HAZ at the same time, thus narrowing the corrosion width and reducing the degree of corrosion in the HAZ of Al side for the SFSW joint. According to the macroscopic corrosion morphology results of the two types joints, there is almost no corrosion phenomenon on the Cu side of the Al/Cu joint, and the main corrosion occurs on the Al side. which is due to the large potential difference between Al and Cu. After FSW, Al and Cu come into full contact. When the joint is immersed in the corrosive medium, a macroscopic galvanic effect will occur between Al/Cu, forming a macroscopic galvanic corrosion primary battery. The Cu with high potential is protected as a cathode, the corrosion rate is greatly reduced, while Al with low potential is accelerated as an anode, and the dissolution rate is significantly increased. The corrosion degree of different areas on the Al side is also different, the HAZ experiences the most severe corrosion and being the weak area of corrosion in the entire joint, not only the corrosion initiation is earlier, but it also develops at a faster rate. The coarse grains and precipitates in the HAZ accelerate the development of pitting corrosion, leading to a deterioration of corrosion resistance. In addition, the corrosion degree of the Al BM is higher than that of the WNZ, because the WNZ contains a large number of Cu particles which belong to high potential elements. Under the the action of diffusion, the Cu content in the WNZ increases, resulting in an overall increase in the corrosion potential of the WNZ. A higher corrosion potential means better corrosion resistance. In summary, the corrosion resistance of different areas of the joint is ranked as follows: Cu side > WNZ of Al side > Al BM > HAZ of Al side. Figure 17 and Fig. 18 display the cross-sectional corrosion morphologies of the FSW and SFSW joints after immersion for 240 h in different areas, respectively. The maximum corrosion depth of the FSW and SFSW joints is in the HAZ of Al side. The maximum corrosion depth of the HAZ of Al side of the FSW joint is 208 µm (see Fig. 17 (c)), and the maximum corrosion depth of the HAZ on the Al side of the SFSW joint is 200 µm (see Fig. 18 (c)). At the same time, it can be seen that the corrosion groove width in the HAZ of Al side of the FSW joint reaches 1226 µm, which is significantly larger than that of the SFSW joint (260 µm), and the corrosion degree in the HAZ of Al side of the SFSW joint is significantly reduced compared to the FSW joint. Futhermore, the corrosion depth of the Al BM is greater than that of the WNZ of Al side, indicating that the corrosion resistance of the Al BM is lower than that of the WNZ. No obvious corrosion pits are observed in the HAZ of Cu side of both types of joints. The micro-corrosion morphology of FSW joint after immersion for 96 h is shown in Fig. 19 , and the corresponding composition analysis results are shown in Table 3 . According to Fig. 19 (a) and Table 3 , the oxide film formed on the surface of the Cu side is mainly CuO/Cu 2 O. Cracked corrosion products are observed in the WNZ (see Fig. 19 (b)) and HAZ of Al side (see Fig. 19 (c)). Quantitative composition analysis results show that the cracked corrosion products mainly contain Al and O elements, suggesting that this corrosion products are mainly Al(OH) 3 /Al 2 O 3 . The block corrosion products in the WNZ of Al side and the corrosion product film covering the surface are mainly composed of Al(OH) 3 /Al 2 O 3 . In addition, there are flocculent corrosion products on the surface of the Al BM, which are also Al(OH) 3 /Al 2 O 3 . Table 3 Component analysis results of corresponding points in Fig. 21 (at.%) Spots Al Mg Si O Cu A - - - 22.11 77.89 B 31.42 0.1 1.38 67.05 0.05 C 26.51 0.24 0.48 72.77 - D 33.81 0.11 0.68 65.40 - E 78.91 0.5 0.72 19.78 0.09 F 26.54 0.15 2.01 71.30 - According to the electrochemical corrosion and immersion corrosion test results, the corrosion mainly occurs on the Al side, while the Cu side has almost no obvious corrosion. As shown in Fig. 20 (a), when the Al/Cu joints are immersed in 3.5wt % NaCl solution, the potential of Cu is higher than Al, the potential difference between the both sides can cause the macroscopic galvanic effect of the joint, forming a macroscopic galvanic corrosion primary battery. The Cu side is protected as the cathode, and the corrosion process is significantly inhibited, while the Al side acts as the anode and is accelerated to dissolve. Therefore, the macroscopic galvanic effect in the joint will significantly promote the corrosion of the Al side, and the driving force for corrosion is the potential difference between the both sides. There are a large number of Cu particles and Al/Cu IMCs in the the WNZ of Al side, and the precipitates with different types and sizes are contained in the HAZ of Al side and Al BM. There is a potential difference between these Cu particles, IMCs and precipitates and Al matrix, which leads to the generation of micro-scale galvanic effects, forming a micro-scale galvanic corrosion primary battery, and further promoting the corrosion on Al side. Therefore, the galvanic corrosion of the joint is determined by both macroscopic and microscopic galvanic effects. Taking the HAZ of Al side as an example, the mechanism of microscopic galvanic corrosion of the joint is analyzed, as illustrated in Fig. 20 (b). There are two kinds of coarse precipitates in the HAZ, namely, Al(Fe,Mn)Si phase and Mg 2 Si phase. The electrode potential of Al(Fe,Mn)Si phase is higher than that of Al matrix, and Mg 2 Si phase is lower than that of Al matrix. Therefore, compared to the Al matrix, the Al(Fe,Mn)Si phase is the cathode and the Mg 2 Si phase is the anode. When the joint is immersed in 3.5wt % NaCl solution, galvanic corrosion occurs on the Al matrix around the Al(Fe,Mn)Si phase, and loose corrosion products gradually cover the surface of corrosion pits. In the early stage of corrosion, Mg 2 Si phase acts as the anode, Mg element will be preferentially dissolved out before Si, and then Si continues to remain to form a Si-rich cathode phase, which also promotes the dissolution of Al matrix. The anodic reaction for corrosion of Al matrix is as follows: In the neutral corrosive medium, the cathode reaction is mainly oxygen absorption corrosion, and the reaction equation is as follows: The reaction equation for generating corrosion products is as follows: As the corrosion continues, the precipitates gradually separate from the matrix due to the lack of joining with the matrix, and the corrosion products cover the surface of the corrosion pits and matrix, providing a certain degree of protection for the matrix. However, the adhesion of these corrosion products to the substrate is not tight, and it is prone to fall off from the surface, and hydrolysis will occur. The peeling and hydrolysis of the corrosion products cause part of the matrix to be exposed again. Under the action of the corrosive medium (Clˉ), the exposed Al matrix is further dissolved, resulting in continuous expansion of pitting corrosion on the surface and inside of the matrix, the size of corrosion pits continues to expand, and some pits gradually converge, forming obvious corrosion grooves until the joint completely fails. 4. Conclusion The Al/Cu dissimilar metals were joined using submerged friction stir welding technology, and the influences of active forced cooling on the microstructure and corrosion properties of the Al/Cu joints were systematically investigated. The following conclusions can be drawn from the study and analysis of the experimental results. The active forced cooling could effectively improve the weld formation of the Al/Cu joints. In addition, it could refine the grains in the WNZ of the Al and Cu side, and effectively reduce the IMCs layer thickness at the interface. The potential difference between the 6061-T6 Al alloy and T2 pure Cu led to the generation of macroscopic galvanic effect in the Al/Cu joints, that is, a macroscopic galvanic corrosion primary battery was formed, which accelerated the corrosion of the joint and results in lower corrosion resistance of the FSW and SFSW joints than the BMs. However, the corrosion resistance of SFSW joints was higher than that of FSW joint. The forced cooling contributes to improve the corrosion resistance of Al/Cu joints. During the immersion corrosion period of 240 h, no significant corrosion occurred on the Cu side due to cathodic protection. The HAZ of Al side was the most seriously corroded area of the joints, which formed obvious corrosion grooves. Compared with the FSW joint, the SFSW joint had smaller width and depth of corrosion grooves on the HAZ of Al side, and the corrosion degree was significantly reduced. The corrosion on the Al side is synergistic promoted by both macroscopic and microscopic galvanic effects. Declarations Funding The authors gratefully acknowledge the financial support from Key Research and Development Projects of Shaanxi Province (GrantNo.2020ZDLGY13-04). Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Statements The data that has been used is confidential. Author contributions Yue Mao: Methodology, Resources, Writing - review & editing. Zhaoyi Pan: Conceptualization, review & editing, Supervision. Qiang Chu: Investigation, Formal analysis. Qinlian Zhang: Data Curation. Zhenzhong Wu: Visualization. Rui Xu: Validation. Wenbo Zhang: Visualization. Wei Liu: Investigation. Weiqi Qiao: Formal analysis. Linchuan Liu: Resources. Xingyu Huang: review & editing. Linduo Liang: Investigation. 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2","display":"","copyAsset":false,"role":"figure","size":39703,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of electrochemical corrosion for samples.\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/9dafd62e092977c7e9c75682.jpeg"},{"id":93119148,"identity":"e0e3f06a-4276-438a-82fd-de46bf16571e","added_by":"auto","created_at":"2025-10-09 09:16:10","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":83712,"visible":true,"origin":"","legend":"\u003cp\u003eThe surface morphology of the Al/Cu joints: (a) FSW, (b) SFSW.\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/4bb9fc8456657431f0540625.jpeg"},{"id":93117639,"identity":"abf71d2b-9aec-4860-ad09-db50def13950","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":73524,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure of WNZ of the Al/Cu joints: (a) FSW, (b) SFSW.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/bb77aaa64a088e13a5218b08.jpeg"},{"id":93117633,"identity":"1e118380-83fa-4aa7-a70a-36892485a20e","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":393149,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure and EDS analysis results in WNZ: (a) particle composite structure in FSW joints, (b-c) EDS mapping of the FSW joint, (d) scanning results of spots in Fig.5(a), (e) particle composite structure in SFSW joints and corresponding EDS results.\u003c/p\u003e","description":"","filename":"image5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/57bba0fddd90674389a17ee5.jpeg"},{"id":93117656,"identity":"0cfccd93-728e-476d-a341-5f9b3936d5fa","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":395067,"visible":true,"origin":"","legend":"\u003cp\u003eGrain morphology and grain size statistics in middle of the WNZ of Al side for Al/Cu joint: (a) IPF image of the grains for the FSW joint, (b) IPF image of the grains for the SFSW joint, (c) the distribution of grain size of the FSW joint, (d) the distribution of grain size of the SFSW joint\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/153eda340d6b863a4ee72dec.jpeg"},{"id":93117642,"identity":"4c682714-cc07-42c3-a90e-db96a6892bfa","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":515133,"visible":true,"origin":"","legend":"\u003cp\u003eGrain morphology and grain size statistics in middle of the WNZ of Cu side for Al/Cu joint: (a) IPF image of the grains for the FSW joint, (b) IPF image of the grains for the SFSW joint, (c) grain size distribution of FSW joint, (d) grain size distribution of SFSW joint\u003c/p\u003e","description":"","filename":"image7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/503772b4775a08d7e88607f5.jpeg"},{"id":93117647,"identity":"bae90ace-7384-401d-939d-3ac08ef3e4bb","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":143121,"visible":true,"origin":"","legend":"\u003cp\u003e(111) poles figures at middle of the WNZ in Al side and Cu side for FSW and SFSW joints: (a) Al side for FSW joint, (b) Al side for SFSW joint, (c) Cu side for FSW joint, (d) Cu side for SFSW joint, (e) the standard (111) pole figure for face-centered cubic metals under simple shear\u003c/p\u003e","description":"","filename":"image8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/7be519b68b9485cda9135ef8.jpeg"},{"id":93117645,"identity":"e8015000-563e-4683-b180-6a5b7697b79d","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":93423,"visible":true,"origin":"","legend":"\u003cp\u003eMorphology of interfacial IMCs layer on the cross-sections of Al/Cu joints, along with corresponding EDS spot scanning results: (a) FSW joint, (b) SFSW joint, (c) EDS results of marked points shown in Fig. 9(a), (d) EDS results of marked points shown in Fig. 9(b).\u003c/p\u003e","description":"","filename":"image9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/e8127b41b3a202a0f00676fe.jpeg"},{"id":93119149,"identity":"a2d66658-0ff9-4645-9115-92900e9a1dce","added_by":"auto","created_at":"2025-10-09 09:16:10","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":887329,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of the interfacial microstructure of the SFSW joint: (a) TEM bright-field image of IMCs layer morphology, (b) HRTEM with IFFT image and SAED pattern of the point A in Fig. 10(a), (c) HRTEM with IFFT image and SAED pattern of the point D in Fig. 10(a), (d) HRTEM and SAED pattern of the point B in Fig. 10(a), (e) HRTEM and SAED pattern of the point C in Fig. 10(a), (f) HRTEM image of Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e/Cu interface, (g) HRTEM image of Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eCu interface, (f) HRTEM image of Al\u003csub\u003e2\u003c/sub\u003eCu/Al interface.\u003c/p\u003e","description":"","filename":"image10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/eccd5c8c3e362ef95f152896.jpeg"},{"id":93118088,"identity":"1af70f85-43fe-4712-bd55-5b1def0a24a1","added_by":"auto","created_at":"2025-10-09 09:08:10","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":727250,"visible":true,"origin":"","legend":"\u003cp\u003eOpen circuit potential of 6061-T6 Al alloy and T2 pure Cu in 3.5 wt% NaCl solution.\u003c/p\u003e","description":"","filename":"image11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/23bb8908673a899b3ffdc934.jpeg"},{"id":93118092,"identity":"b3bb8c35-afa1-400e-9442-8b06ee77bb18","added_by":"auto","created_at":"2025-10-09 09:08:10","extension":"jpeg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":1167226,"visible":true,"origin":"","legend":"\u003cp\u003ePotentiodynamic polarization curves of BMs and welded joints in 3.5 wt% NaCl solution\u003c/p\u003e","description":"","filename":"image12.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/bd3aa47be479f2a7c74af7f9.jpeg"},{"id":93118087,"identity":"207d68ca-ace3-4d0b-bebb-7ad0b030ad7f","added_by":"auto","created_at":"2025-10-09 09:08:10","extension":"jpeg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":109792,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical impedance spectra of BMs and welded joints in 3.5wt % NaCl solution: (a) Nyquist diagram, (b-c) Bode diagram\u003c/p\u003e","description":"","filename":"image13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/68a3181701825b7efb269496.jpeg"},{"id":93117653,"identity":"2ba32b89-accf-493b-874a-0d6478624f69","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":194435,"visible":true,"origin":"","legend":"\u003cp\u003eEquivalent circuit for fitting electrochemical impedance spectra data: (a) 6061-T6 Al alloy, FSW joint, and SFSW joints, (b) T2 pure Cu\u003c/p\u003e","description":"","filename":"image14.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/18d733c0b89250dac2931185.jpeg"},{"id":93117677,"identity":"8663c9ee-adf3-481e-b567-756c141bf5c0","added_by":"auto","created_at":"2025-10-09 09:00:11","extension":"jpeg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":270858,"visible":true,"origin":"","legend":"\u003cp\u003eCorrosion morphology of Al/Cu FSW joint immersed in 3.5 wt% NaCl solution for different times: (a) immersed for 0 h, (b) immersed for 6 h, (c) immersed for 12 h, (d) immersed for 24 h, (e) immersed for 48 h, (f) immersed for 96 h, (g) immersed for 240 h.\u003c/p\u003e","description":"","filename":"image15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/374ed90cd056910cd66b98b9.jpeg"},{"id":93117671,"identity":"2af388ee-0385-4c25-8735-43a87b371134","added_by":"auto","created_at":"2025-10-09 09:00:11","extension":"jpeg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":186932,"visible":true,"origin":"","legend":"\u003cp\u003eCorrosion morphology of Al/Cu SFSW joint immersed in 3.5 wt% NaCl solution for different times: (a) immersed for 0 h, (b) immersed for 6 h, (c) immersed for 12 h, (d) immersed for 24 h, (e) immersed for 48 h, (f) immersed for 96 h, (g) immersed for 240 h.\u003c/p\u003e","description":"","filename":"image16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/6e39566e56c6255e451038fb.jpeg"},{"id":93117655,"identity":"42c3c6bf-12a6-4aa8-aa0b-c934d217c967","added_by":"auto","created_at":"2025-10-09 09:00:10","extension":"jpeg","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":77116,"visible":true,"origin":"","legend":"\u003cp\u003eCorrosion depth of the FSW joint after immersion for 240 h in different areas: (a) HAZ in Cu side, (b) WNZ in Al side, (c) HAZ in Al side, and (d) Al BM\u003c/p\u003e","description":"","filename":"image17.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/37098f6b8d6b987be40844ed.jpeg"},{"id":93118094,"identity":"6adf0c26-104c-446a-8d05-77ae22b9a901","added_by":"auto","created_at":"2025-10-09 09:08:10","extension":"jpeg","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":125143,"visible":true,"origin":"","legend":"\u003cp\u003eCorrosion depth of the SFSW joint after immersion for 240 h in different areas: (a) HAZ in Cu side, (b) WNZ in Al side, (c) HAZ in Al side, and (d) Al BM\u003c/p\u003e","description":"","filename":"image18.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/85f17287cb9121a51a639837.jpeg"},{"id":93118122,"identity":"4066d030-ec56-4a86-8ef8-6e01fa817519","added_by":"auto","created_at":"2025-10-09 09:08:12","extension":"jpeg","order_by":19,"title":"Figure 19","display":"","copyAsset":false,"role":"figure","size":239388,"visible":true,"origin":"","legend":"\u003cp\u003eCorrosion morphology of FSW joint after immersion for 96 h: (a) HAZ in Cu side, (b) WNZ in Al side, (c) HAZ in Al side, and (d) Al BM\u003c/p\u003e","description":"","filename":"image19.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/d64815bcdc3aa1680f7a5d8b.jpeg"},{"id":93117691,"identity":"317d292f-59fd-4c22-bfaf-10218ebf20f2","added_by":"auto","created_at":"2025-10-09 09:00:12","extension":"jpeg","order_by":20,"title":"Figure 20","display":"","copyAsset":false,"role":"figure","size":264102,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of corrosion mechanism of the Al/Cu joint: (a) macroscopic galvanic effect, (b) microscopic corrosion mechanism\u003c/p\u003e","description":"","filename":"image20.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/10c84e8583e7a61c7cca52c0.jpeg"},{"id":93120149,"identity":"3f2a69a4-2139-45f9-b669-a64d819d58f3","added_by":"auto","created_at":"2025-10-09 09:24:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6909763,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7373892/v1/a200a99e-cc51-41a5-a26e-fe0988b99997.pdf"}],"financialInterests":"","formattedTitle":"Effect of active forced cooling on microstructure and corrosion properties of friction stir welded Al/Cu dissimilar joints: Quasi-in-situ study","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAl/Cu dissimilar composite components can give full play to the performance advantages of Al and Cu, which has broad application potential in new energy vehicles, electric power, refrigeration and other fields [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e–\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, due to the significant differences in physical and chemical properties of Al and Cu, achieving high-performance Al/Cu joints faces numerous challenges [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e–\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The joints are prone to porosity and excessive intermetallic compounds(IMCs) when using the traditional fusion welding process[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e–\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In recent years, friction stir welding(FSW) technology has been widely applied to the joining of dissimilar materials, which can effectively overcome welding difficulties arising from differences in physical and chemical properties of dissimilar materials [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e–\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Zhou et al.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] conducted the friction stir spot welding of Al to Cu and claimed that the three IMCs layers (Al\u003csub\u003e2\u003c/sub\u003eCu-AlCu-Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e) could be formed at the interface under the high heat input, the continuous IMCs layer with an appropriate thickness contributed to higher tensile properties. The effect of tool offset on microstructure and tensile properties of the dissimilar joint of 6061 Al alloy and pure Cu by Hou et al.[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. They pointed out that the low tool offset(offset toward Al side) resulted in the formation of excessive IMCs and voids, which decreased the tensile strength of the joints. Shojaeefard et al.[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] revealed that a greater amount of IMCs was generated at higher rotational speed, resulting in a reduction in the joint's tensile strength. From the above, it can be concluded that the hard and brittle IMCs can be still inevitably produced at Al/Cu interface, and their thickness is directly related to the mechanical properties of the FSWed joint.\u003c/p\u003e\u003cp\u003eRecently, several effective strategies have been applied to reduce the thickness of IMCs layer, with the application of forced cooling during the FSW process being particularly prominent. Tang et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] carried out the splat cooling assisted FSW of 6061-T6 aluminium and pure copper and compared it with normal FSW joints, which indicated that the splat cooling significantly reduced the heat input and the thickness of interfacial IMCs layer, leading to a 42% improvement in bonding strength. Lader et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] performed the FSW of dissimilar joints of AA1100-O and brass under both air and water environments, the results showed that the finer grain and the thinner IMCs layer were produced at UFSW joint, thereby enhancing the overall quality of the welded joint. Sundar et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] carried out the research on the dissimilar joining of 6061 aluminum alloy and TC4 titanium alloy using underwater friction stir welding technology (UFSW), they observed that the UFSW joint exhibited a notably thinner IMCs layer of 3.731 µm in comparison to the normal FSW joint. In conclusion, the forced cooling can refine the grains, reduce the the thickness of IMCs layer, and improve the mechanical properties of the joints.\u003c/p\u003e\u003cp\u003eIn addition to mechanical properties, corrosion resistance is another critical indicator that needs to be considered during the service life of components. For the Al/Cu dissimilar joints, the significant potential difference between Al and Cu induces galvanic corrosion, which further accelerates the corrosion of the welded joints. Tang et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] studied the corrosion behavior of the Al/Cu welded joints under different welding parameters by polarization curve and claimed that the corrosion resistance of the Al/Cu joint is lower than that of Al alloy BM and pure Cu BM, which is attributed to the formation of a corrosive primary battery between Al and Cu by joining, leading to a reduction in the whole corrosion resistance of the welded joint, while a thicker IMCs layer further accelerated corrosion. However, the research on the corrosion behavior of the Al/Cu joints remains insufficient, the corrosion evolution rule of the Al/Cu joints is still not fully understood, and the corrosion mechanism of Al/Cu joints remains to be revealed. Furthermore, it is worth noting that the effect of forced cooling on the corrosion resistance of the joint is also unclear and needs to be systematically studied. In this study, submerged friction stir welding (SFSW) was employed to fabricate the Al/Cu dissimilar joints, the microstructure and corrosion properties of the Al/Cu SFSW joints were systematically investigated, and a comparative study was performed between the SFSW and normal FSW joints to elucidate the influence of forced cooling on the corrosion behavior of the Al/Cu joints.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Experimental procedures","content":"\u003cp\u003e\u003c/p\u003e\u003cp\u003e3 mm-thick sheet of 6061-T6 aluminum alloy and T2 copper were selected as base materials (BMs). Prior to welding, using different types of sandpaper to polish the welding surface and mating surface of the test plate, and clean them with acetone. SFSW process was carried out in a dedicated water tank, which was made of stainless steel and equipped with an inlet and outlet. During the welding process, flowing water was used to cool the weld, with the water layer thickness maintained at approximately 25 mm, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Through process optimization, the welding parameters are set to a rotational speed of 800 rpm, a welding speed of 40 mm/min. For Al/Cu dissimilar joining, defects are prone to occur in the joint during center welding, whereas offsetting the tool pin toward the Al side was conducive to improve defect-free joints[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], thus, the offset was 1.2 mm (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b)). The 6061-T6 aluminum alloy plate was positioned on the advancing side, so the T2 pure copper plate was on the retreating side. The tool dimensions were as follows: shoulder diameter of 16 mm; pin root diameter of 5 mm, pin bottom diameter of 4 mm, and pin length of 2.7 mm. The tilt angle of the tool was 2.5°.\u003c/p\u003e\u003cp\u003eMetallographic, TEM and EBSD samples were prepared from the welded joints using electrical discharge wire cutting. Then, the samples were ground and polished to achieve a scratch-free mirror finish. The EBSD testing was employed to analyze the grain structure and texture component. The interfacial characteristics and composition was characterized using SEM and EDS. A TEM was applied to investigate the crystal structure and orientation relationships of IMCs layer. For electrochemical testing, samples were cut perpendicular to the welding direction, with a copper wire welded to their backside. All surfaces except the test surface were encapsulated in epoxy resin, The surface to be tested was polished and cleaned with alcohol. The potentiodynamic polarization curve (PDP) and electrochemical impedance spectroscopy (EIS) of the BMs and Al/Cu joint were measured by using VersaSTAT 4 electrochemical workstation. The electrolyte used was a 3.5 wt% NaCl solution, and a three-electrode testing system was adopted, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, where the BM and welded joints served as working electrodes, the saturated calomel electrode as the reference electrode, and the platinum plate was the counter electrode. Before the PDP and EIS test, the sample was immersed in the electrolyte to measure the open-circuit potential(OCP). Subsequent tests were performed only after the OCP had stabilized. For EIS, the frequency range was 10\u003csup\u003e5\u003c/sup\u003e ~ 10\u003csup\u003e− 2\u003c/sup\u003e Hz with an amplitude of 10 mV.\u003c/p\u003e\u003cp\u003eThe immersion corrosion tests were conducted using a neutral 3.5 wt% NaCl solution at room temperature, with immersion durations set at 6 h, 12 h, 24 h, 48 h, 96 h, and 240 h, respectively. According to the GB/T 19746 − 2018, the immersion solution was replaced every two days. The macro-and micro-scopic morphology of corroded samples were observed by OM and SEM. The element content of the corrosion products was analyzed via EDS.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Surface morphology of the joints\u003c/h2\u003e\u003cp\u003eThe surface morphologies of the Al/Cu joints manufactured by normal FSW and SFSW are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. There are no obvious defects in both FSW and SFSW joints, but the surface of the FSW weld is relatively rough with a certain amount of burrs and flash. The surface of the SFSW joint exhibits extremely smooth and its formation quality is high. This indicates that the SFSW process can effectively improve the welding quality of the Al/Cu joints. This is because the heat input of the normal FSW process is large, the mixing and reaction of the both metals are enhanced, leading to the accumulate of a large amount of metals and IMCs at the shoulder, and resulting in serious adhesion of the tool, which greatly reduces the smoothing effect of the shoulder on the weld surface and the containment effect on the metals, ultimately leading to a rough weld surface and increased flash.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Microstructure evolution\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e depicts the welding nugget zone (WNZ) of the Al/Cu FSW and SFSW joints. In Al/Cu FSW, Cu exhibits higher flow stress, and its fluidity is not as good as Al at the same welding temperature. Under the action of mechanical stirring, a number of Cu particles are stripped off from the matrix and flows and mixes with the Al in the WNZ, forming a composite-like structure that strengthens the WNZ. It is worth noting that the size of Cu particles in the WNZ of SFSW joints is relatively smaller compared to the FSW joints. This can be due to the higher heat input in normal FSW, which causes significant material softening, making it easier for more larger Cu blocks to peel off from the Cu matrix. In contrast, under the forced cooling effect of flowing water, the welding temperature is significantly decreased, in turn ,the degree of softening and plastic deformation of Cu are also reduced. As a result, the Cu particles stripped from the Cu matrix are smaller in size.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAdditionally, the mechanical stirring action of the tool causes Cu particles to peel off the matrix and be stirred into the Al alloy, and plastic deformation occurs and gradually broken. The IMCs are formed by the elements interdiffusion between Cu particles and the surrounding Al alloy matrix under the action of thermal-mechanical, forming a multiphase particle composite structure. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents the microstructure and corresponding EDS results of a typical particle composite structure in both FSW and SFSW joints. From Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b-c), the interface between Cu particles and Al matrix is relatively blurry, indicating that Cu particles and Al matrix have undergone sufficient diffusion at the interface, which forms an IMCs layer surrounding the Cu particles. The Al element was also observed inside the particles, indicating that under the action of welding thermal cycling, the Al element of the matrix diffused into the interior of the Cu particles and reacted, thereby forming a particle composite structure. In Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)and(d), the Cu particles are surrounded by two layers of gray Al/Cu IMCs. According to the point scanning results, the Al\u003csub\u003e2\u003c/sub\u003eCu layer is located near the Al side, while the Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e layer is located near the Cu particles. Furthermore, within the particle composite structure, there is a gray layered phase region that can be identified as the Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e phase, while the white structure corresponds to the Cu(Al) solid solution. Thus, the larger particle composite structures are composed of three components: the Cu(Al) solid solution, the Al\u003csub\u003e2\u003c/sub\u003eCu phase, and the Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e phase.\u003c/p\u003e\u003cp\u003eSome of the particle composite structures consist of an outer Al\u003csub\u003e2\u003c/sub\u003eCu phase and an inner island-like Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e phase, with the original Cu matrix having completely vanished. It can be also observed that a number of smaller particles have completely transformed into a single IMCs of Al\u003csub\u003e2\u003c/sub\u003eCu. Through the above analysis, it can be concluded that the particle composite structures in the WNZ have different microstructural characteristics. The analysis of the reasons shows that the material mixing degree, element content and heat distribution in the WNZ of the joint are obviously uneven, which leads to the obvious difference in the atomic diffusion rate in different areas, and thus the evolution process of the particle composite structure is different.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe microstructure in middle of the WNZ of the Al side for the Al/Cu joints are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Both FSW and SFSW joints exhibit finer equiaxed grains in the WNZ of Al side (see Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a-b)). This phenomenon arises from the combined action of welding thermal cycling and severe plastic deformation experienced by the WNZ during welding, which induces dynamic recovery and dynamic recrystallization [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The grain size distributions are displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c-d). For the FSW joint, the average grain size of the Al-side WNZ is 6.43 \u0026micro;m, whereas that of the SFSW joint is 1.64 \u0026micro;m. Evidently, the average grain size of the SFSW joint is smaller than that of the FSW joint, which suggests that the SFSW process with additional forced cooling can significantly refine the grains. The lower the temperature and the larger the strain rate, the smaller the grain size in the WNZ [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In the SFSW process, the forced cooling effect of flowing water reduces the temperature of the welding process, and the strain rate is mainly determined by the rotational speed and welding speed [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. These factors collectively contribute to the grain refinement observed in the SFSW joint.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eOn the Cu side, the WNZ of both FSW and SFSW joints also present equiaxed grains morphology (see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The average grain size of the WNZ in the FSW joint is 2.36 \u0026micro;m, while that in the SFSW joint is smaller, at 1.49 \u0026micro;m. This indicates that forced cooling also contributes to grain refinement on the Cu side.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates the (111) pole diagram of the WNZ in the Al side and Cu side for the Al/Cu FSW and SFSW joints, where Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(e) shows the pole diagram of the face-centered cubic (FCC) metal simple shear standard (111). Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents the (111) pole figures of the WNZ on both the Al and Cu sides in Al/Cu FSW and SFSW joints, where Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(e) displays the standard (111) pole figure of the simple shear for face-centered cubic (FCC) metals. During welding, the metal within the WNZ undergoes intense friction and shear from the tool, leading to significant shear deformation. Thus, the analysis focuses primarily on the shear texture in the WNZ.\u003c/p\u003e\u003cp\u003eThe texture type of the WNZ in Al side of the FSW and SFSW joints is C-type shear texture components (see Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (a) and (c)). Similarly, the WNZ on Cu side of both joints are provided with C-type texture components. However, the texture intensity in the Al side and Cu side of SFSW joint are decreased compared to FSW joint, which may be attributed to the weakening of materials flow caused by the active forced cooling[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe evolution of the interfacial IMCs layer is crucial to joint performance, especially closely related to the thickness of the IMCs layer. During the welding process, Al and Cu atoms interdiffuse across the interface and react to form the IMCs layer. As displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a), the entire interfacial IMCs layer of the FSW joint is distinctly divided into three sub-layers. Based on point scanning results(Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(c)), the phase adjacent to the Al side is identified as Al\u003csub\u003e2\u003c/sub\u003eCu, while that adjacent to the Cu side is Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e, with the AlCu phase present in the middle.\u003c/p\u003e\u003cp\u003eFor SFSW joint, the thickness of the IMCs layer is significantly thinner than that of the FSW joint, decreasing from 1.62 \u0026micro;m in FSW joint to 0.56 \u0026micro;m in SFSW joint, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(b). In addition, the number of IMCs sub-layers has also been reduced from 3 layers for FSW joints to 2 layers for SFSW joints, which are identified as Al\u003csub\u003e2\u003c/sub\u003eCu and Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e phase, respectively. The active forced cooling in SFSW process reduces the peak temperature and shortens the high-temperature dwell time[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], and thereby inhibits atomic diffusion and reduces diffusion rate. In addition, the diffusion reaction time decreases, and the growth rate of interfacial IMCs layer decreases, ultimately leading to a reduced IMCs layer thickness.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo accurately determine the phase compositions of the interfacial reaction layer in the SFSW joint and analyze the microscopic interface types, TEM tests were performed on the interface zone, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eThe reaction layer is composed of two sub-layers with relatively uniform thicknesses, exhibiting nano-scale equiaxed grains, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(a). Based on the high-resolution images and selected area electron diffraction (SAED) analysis results (see Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(b-e)), the interface from right to left (A-D) correspond to the Cu matrix, Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e layer, Al\u003csub\u003e2\u003c/sub\u003eCu layer, and Al matrix, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(f-h) present high-resolution images of three phase interfaces: Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e/Cu, Al\u003csub\u003e2\u003c/sub\u003eCu/Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e, and Al/Al\u003csub\u003e2\u003c/sub\u003eCu. It can be seen that all three phase interfaces are non-coherent interfaces, and there is no strong orientation relationship between the phases. This phenomenon may be due to the shear force applied by the tool, which destroys the potential parallel relationship between the phases.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Electrochemical corrosion behavior\u003c/h2\u003e\u003cp\u003eThe open circuit potential (OCP) refers to the potential difference between the working electrode and the reference electrode in the absence of external current. Generally, the OCP value is indicative of the corrosion tendency of the material. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows the OCP test results of 6061-T6 Al alloy and T2 pure Cu in a 3.5 wt% NaCl solution. Within the 1800 s test duration, the OCP of both base metals (Al alloy and pure Cu) remains stable, the OCP of the pure Cu is approximate \u0026minus;\u0026thinsp;0.23V, while that of 6061-T6 Al alloy is around \u0026minus;\u0026thinsp;0.76 V, resulting in a potential difference of 0.53 V between the two materials. According to corrosion thermodynamics, the higher OCP value, the lower the tendency of the material to be corroded. Therefore, when Al alloy and pure Cu are fully joined via FSW, the potential difference between them induces a macroscopic galvanic effect in the Al/Cu joint, forming a macroscopic galvanic corrosion primary battery. In this battery, Cu acts as the cathode and is protected, whereas Al serves as the anode and is accelerated corrosion.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e shows the potentiodynamic polarization curves of the BMs and Al/Cu joints. The potentiodynamic polarization curve represents the relationship between the electrode polarization potential and polarization current. The electrochemical parameters of the BMs and welded joint can be obtained by Tafel extrapolation method, as listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The self-corrosion potentials \u003cem\u003eE\u003c/em\u003e\u003csub\u003ecorr\u003c/sub\u003e of Al alloy and pure Cu are \u0026minus;\u0026thinsp;0.716 V and \u0026minus;\u0026thinsp;0.268 V, respectively, which shows a slight difference from the measured OCP values. This is owing to the polarization of the electrode caused by additional current during the testing process, leading to a certain drift in the corrosion potential. The self-corrosion potential \u003cem\u003eE\u003c/em\u003e\u003csub\u003ecorr\u003c/sub\u003e reflects both the difficulty of metal corrosion and the thermodynamic stability of the metal under electrochemical corrosion conditions. The corrosion current density \u003cem\u003ei\u003c/em\u003e\u003csub\u003ecorr\u003c/sub\u003e indicates the corrosion rate of the metal and characterizes the kinetic process of the corrosion reaction [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe self-corrosion potential of FSW and SFSW joints are \u0026minus;\u0026thinsp;0.719V and \u0026minus;\u0026thinsp;0.665V, respectively, and thus the SFSW joint has a higher corrosion potential. The corrosion current densities of the two joints are 1.29 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e A/cm\u003csup\u003e2\u003c/sup\u003e and 8.27 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e A/cm\u003csup\u003e2\u003c/sup\u003e, respectively, both of which are higher than those of the 6061-T6 Al alloy and the pure Cu, suggesting that the corrosion resistance of the Al/Cu welded joints are worse than that of the BMs, and the macroscopic galvanic effect of the Al/Cu joints accelerates the corrosion of the joints. In addition, the SFSW joint shows a higher self-corrosion potential and a lower corrosion current density compared to the FSW joint, demonstrating better corrosion resistance, which indicates that forced cooling contributes to the improvement of corrosion resistance of Al/Cu joints. The underlying mechanism can be analyzed as follows: due to the potential difference between IMCs and Al, low potential Al undergoes galvanic corrosion. In the FSW joint, a thick IMCs layer is formed at interface, which further accelerates the corrosion of Al, resulting in the decrease of the corrosion resistance of the whole Al/Cu joints.\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\u003eelectrochemical parameters of the BMs and welded joints\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpecimen\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCorrosion potential \u003cem\u003eE\u003c/em\u003e\u003csub\u003ecorr\u003c/sub\u003e (V)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCurrent density \u003cem\u003ei\u003c/em\u003e\u003csub\u003ecorr\u003c/sub\u003e (A/cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6061-T6 Al alloy\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e-0.716\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e4.48 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eT2 pure Cu\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e-0.268\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e1.88 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFSW joint\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e-0.719\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e1.29 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSFSW joint\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e-0.665\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e8.27 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\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\u003eTo further analyze the electrochemical behavior of the BMs and the Al/Cu joints, the electrochemical impedance spectra (EIS) of BMs and the Al/Cu joints were measured in a 3.5 wt% NaCl solution, the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e. The larger the diameter of the capacitive loop in the Nyquist diagram, the greater the impedance modulus of the material, and the better the corrosion resistance [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e(a), the capacitive loop diameter of the pure Cu is the largest, indicating that the pure Cu has the best corrosion resistance. The capacitive loop diameter of the Al/Cu FSW and SFSW joints are smaller than that of the BMs, implying that the corrosion resistance of the FSW and SFSW joints is lower than that of the BMs, which is consistent with the test results of the potentiodynamic polarization curve. From the EIS, there is a capacitive loop in the medium-frequency and high-frequency regions of the BMs and the welded joint, and there is also a low-frequency inductive loop in the 6061-T6 Al alloy, FSW joint, and SFSW joint. The presence of inductive loop indicates that the samples undergo dissolution during the testing process of EIS. The inductive behavior is caused by the absorption/desorption of intermediate substances formed during the charge transfer [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe equivalent circuits shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e are used to fit the EIS data, where the 6061-T6 Al alloy, FSW joint, and SFSW joint employ the circuit shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e(a), and the T2 pure Cu uses the circuit shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e(b). \u003cem\u003eR\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e is the resistance of the corrosive electrolyte, \u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e is the charge transfer resistance, \u003cem\u003eCPE\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e is the double layer capacitance, \u003cem\u003eR\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e is the resistance of the corrosion products, \u003cem\u003eCPE\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e is the capacitance of the corrosion products, and CPE is the constant phase element. The electrochemical parameters obtained by fitting are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. It can be seen from the table that the pure Cu has the largest charge transfer resistance \u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e value (2812 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e) and corrosion product resistance \u003cem\u003eR\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e value (2243 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e), suggesting that the pure Cu has the best corrosion resistance. The (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e + \u003cem\u003eR\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e) value can reflect the overall corrosion resistance of the material, therefore, the corrosion resistance of the BMs and welded joints is ordered as follows: T2 pure Cu\u0026thinsp;\u0026gt;\u0026thinsp;6061-T6 Al alloy\u0026thinsp;\u0026gt;\u0026thinsp;SFSW joint\u0026thinsp;\u0026gt;\u0026thinsp;FSW joint. The corrosion resistance of welded joints is worse than that of the BMs, but forced cooling contributes to improve the corrosion resistance of the Al/Cu joints.\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\u003eFitting results of EIS data of BMs and welded joint\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"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\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6061-T6 Al\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePure Cu\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eFSW joint\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eSFSW joint\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e(Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.471\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5.84\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.283\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eCPE\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e(\u0026micro;Ω\u003csup\u003e\u0026minus;1\u003c/sup\u003ecm\u003csup\u003e\u0026minus;2\u003c/sup\u003es\u003csup\u003en\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e57.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e52.09\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e303.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e211.05\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003en\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.79\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e(Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1160\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2243\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e387.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1984\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eCPE\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e(\u0026micro;Ω\u003csup\u003e\u0026minus;1\u003c/sup\u003ecm\u003csup\u003e\u0026minus;2\u003c/sup\u003es\u003csup\u003en\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e46.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e64.42\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e513.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e275.88\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003en\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.79\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e(Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2460\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2812\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1486\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1248\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e(Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1481\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e/\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e950.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e286.62\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eL\u003c/em\u003e(H\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e27925\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e/\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e15799\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e23007\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Quasi-in-situ immersion corrosion\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e presents the macroscopic corrosion morphologies of the FSW joints after immersion in a 3.5 wt% NaCl solution for different times, specifically 6 h, 12 h, 24 h, 48 h, 96 h, and 240 h. For convenience, the heat affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ) are combined into one area for observation and analysis, which is collectively known as the HAZ. Therefore, the areas observed in this study are the WNZ, HAZ and BMs. After immersing for 6 h (see Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e(b)), the WNZ of Al side for the FSW joint lost its metallic luster, and severe pitting corrosion occurs in the HAZ of Al side, with large corrosion pits observed, while slight pitting corrosion occurs in the Al BM with small pits. The whole Cu side (including the WNZ of Cu side, the HAZ of Cu side, Cu BM) has not undergone corrosion, and metallic luster can still be observed. As the corrosion going to 12 h, slight pitting corrosion begins to occur in the WNZ of Al side of the FSW joint. This is due to the uneven distribution of the WNZ, which contains a large number of Cu particles and Al/Cu IMCs. There is a potential difference between these Cu particles/IMCs and the Al matrix, forming a micro-zone galvanic corrosion primary battery, leading to the initiation of pitting corrosion. The number of corrosion pits in the HAZ of Al side and the Al BM increases significantly, and the size of the pits has increased. A large amount of white corrosion products accumulates around the pits, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e(c). When the corrosion reaches to 48 h, some corrosion pits of the HAZ on the Al side begin to converge, forming corrosion groove, and meantime the corrosion continues to expand into the interior of the matrix. There is no significant change in the corrosion degree of the WNZ of Al side, and the number of pits of Al BM further increases. A small amount of brown oxide appears on the surface of the Cu side, but there are no obvious corrosion traces, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e(e). At 240 h, the deep corrosion groove was already formed in the HAZ of Al side, penetrating the entire area. The WNZ of Al side and Al BM are still dominated by pitting corrosion, and the surface of Cu side is completely covered by brown oxides, but no obvious corrosion traces are observed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e(g).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e presents the macroscopic corrosion morphologies of the SFSW joints after immersion in a 3.5 wt% NaCl solution for different times. The corrosion evolution of the SFSW joints is similar to that of FSW joints. During the initial 6 h of immersion, the Cu side (including the WNZ of Cu side, the HAZ of Cu side, Cu BM) still maintains its metallic luster, and the WNZ of Al side becomes dark, but no obvious corrosion occurs. The HAZ of Al side exhibits more severe pitting corrosion, with larger pit sizes. The Al BM shows obvious pitting corrosion, but compared to the HAZ, the degree of corrosion is lighter, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e (b). As the corrosion time increases, the corrosion development rate of the HAZ of Al side is faster, and the corrosion continues to expand to the surface and interior of the matrix. The area of the pits continues to expand, and they are interconnected, starting to undergo large-scale corrosion, ultimately forming a corrosion trace that runs through the entire HAZ. The Al BM and the WNZ of Al side are still mainly pitting corrosion, and there is obvious oxidation phenomenon on the Cu side, but there are no obvious corrosion traces, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e (c-g).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBy comparing the macroscopic corrosion morphology of FSW and SFSW joints, it can be seen that the width of the corrosion groove in the HAZ of Al side of the SFSW joint is significantly smaller than that of the FSW joint, and the degree of corrosion is lower than that of the FSW joint. This is because forced cooling reduces the width of the HAZ of Al side and refines the grain size of the HAZ at the same time, thus narrowing the corrosion width and reducing the degree of corrosion in the HAZ of Al side for the SFSW joint.\u003c/p\u003e\u003cp\u003eAccording to the macroscopic corrosion morphology results of the two types joints, there is almost no corrosion phenomenon on the Cu side of the Al/Cu joint, and the main corrosion occurs on the Al side. which is due to the large potential difference between Al and Cu. After FSW, Al and Cu come into full contact. When the joint is immersed in the corrosive medium, a macroscopic galvanic effect will occur between Al/Cu, forming a macroscopic galvanic corrosion primary battery. The Cu with high potential is protected as a cathode, the corrosion rate is greatly reduced, while Al with low potential is accelerated as an anode, and the dissolution rate is significantly increased. The corrosion degree of different areas on the Al side is also different, the HAZ experiences the most severe corrosion and being the weak area of corrosion in the entire joint, not only the corrosion initiation is earlier, but it also develops at a faster rate. The coarse grains and precipitates in the HAZ accelerate the development of pitting corrosion, leading to a deterioration of corrosion resistance.\u003c/p\u003e\u003cp\u003eIn addition, the corrosion degree of the Al BM is higher than that of the WNZ, because the WNZ contains a large number of Cu particles which belong to high potential elements. Under the the action of diffusion, the Cu content in the WNZ increases, resulting in an overall increase in the corrosion potential of the WNZ. A higher corrosion potential means better corrosion resistance. In summary, the corrosion resistance of different areas of the joint is ranked as follows: Cu side\u0026thinsp;\u0026gt;\u0026thinsp;WNZ of Al side\u0026thinsp;\u0026gt;\u0026thinsp;Al BM\u0026thinsp;\u0026gt;\u0026thinsp;HAZ of Al side.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e17\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e18\u003c/span\u003e display the cross-sectional corrosion morphologies of the FSW and SFSW joints after immersion for 240 h in different areas, respectively. The maximum corrosion depth of the FSW and SFSW joints is in the HAZ of Al side. The maximum corrosion depth of the HAZ of Al side of the FSW joint is 208 \u0026micro;m (see Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e17\u003c/span\u003e(c)), and the maximum corrosion depth of the HAZ on the Al side of the SFSW joint is 200 \u0026micro;m (see Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e18\u003c/span\u003e(c)). At the same time, it can be seen that the corrosion groove width in the HAZ of Al side of the FSW joint reaches 1226 \u0026micro;m, which is significantly larger than that of the SFSW joint (260 \u0026micro;m), and the corrosion degree in the HAZ of Al side of the SFSW joint is significantly reduced compared to the FSW joint. Futhermore, the corrosion depth of the Al BM is greater than that of the WNZ of Al side, indicating that the corrosion resistance of the Al BM is lower than that of the WNZ. No obvious corrosion pits are observed in the HAZ of Cu side of both types of joints.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe micro-corrosion morphology of FSW joint after immersion for 96 h is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e19\u003c/span\u003e, and the corresponding composition analysis results are shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e19\u003c/span\u003e (a) and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the oxide film formed on the surface of the Cu side is mainly CuO/Cu\u003csub\u003e2\u003c/sub\u003eO. Cracked corrosion products are observed in the WNZ (see Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e19\u003c/span\u003e(b)) and HAZ of Al side (see Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e19\u003c/span\u003e(c)). Quantitative composition analysis results show that the cracked corrosion products mainly contain Al and O elements, suggesting that this corrosion products are mainly Al(OH)\u003csub\u003e3\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The block corrosion products in the WNZ of Al side and the corrosion product film covering the surface are mainly composed of Al(OH)\u003csub\u003e3\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. In addition, there are flocculent corrosion products on the surface of the Al BM, which are also Al(OH)\u003csub\u003e3\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.\u003c/p\u003e\u003cp\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\u003eComponent analysis results of corresponding points in Fig.\u0026nbsp;21 (at.%)\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=\"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=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpots\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\u003eMg\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCu\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e22.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e77.89\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e31.42\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e67.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.05\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e26.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e72.77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e33.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e65.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eE\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e78.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e19.78\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.09\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e26.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e71.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e-\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\u003eAccording to the electrochemical corrosion and immersion corrosion test results, the corrosion mainly occurs on the Al side, while the Cu side has almost no obvious corrosion. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e20\u003c/span\u003e(a), when the Al/Cu joints are immersed in 3.5wt % NaCl solution, the potential of Cu is higher than Al, the potential difference between the both sides can cause the macroscopic galvanic effect of the joint, forming a macroscopic galvanic corrosion primary battery. The Cu side is protected as the cathode, and the corrosion process is significantly inhibited, while the Al side acts as the anode and is accelerated to dissolve. Therefore, the macroscopic galvanic effect in the joint will significantly promote the corrosion of the Al side, and the driving force for corrosion is the potential difference between the both sides.\u003c/p\u003e\u003cp\u003eThere are a large number of Cu particles and Al/Cu IMCs in the the WNZ of Al side, and the precipitates with different types and sizes are contained in the HAZ of Al side and Al BM. There is a potential difference between these Cu particles, IMCs and precipitates and Al matrix, which leads to the generation of micro-scale galvanic effects, forming a micro-scale galvanic corrosion primary battery, and further promoting the corrosion on Al side. Therefore, the galvanic corrosion of the joint is determined by both macroscopic and microscopic galvanic effects.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTaking the HAZ of Al side as an example, the mechanism of microscopic galvanic corrosion of the joint is analyzed, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e20\u003c/span\u003e(b). There are two kinds of coarse precipitates in the HAZ, namely, Al(Fe,Mn)Si phase and Mg\u003csub\u003e2\u003c/sub\u003eSi phase. The electrode potential of Al(Fe,Mn)Si phase is higher than that of Al matrix, and Mg\u003csub\u003e2\u003c/sub\u003eSi phase is lower than that of Al matrix. Therefore, compared to the Al matrix, the Al(Fe,Mn)Si phase is the cathode and the Mg\u003csub\u003e2\u003c/sub\u003eSi phase is the anode. When the joint is immersed in 3.5wt % NaCl solution, galvanic corrosion occurs on the Al matrix around the Al(Fe,Mn)Si phase, and loose corrosion products gradually cover the surface of corrosion pits. In the early stage of corrosion, Mg\u003csub\u003e2\u003c/sub\u003eSi phase acts as the anode, Mg element will be preferentially dissolved out before Si, and then Si continues to remain to form a Si-rich cathode phase, which also promotes the dissolution of Al matrix. The anodic reaction for corrosion of Al matrix is as follows:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eIn the neutral corrosive medium, the cathode reaction is mainly oxygen absorption corrosion, and the reaction equation is as follows:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\u003cp\u003eThe reaction equation for generating corrosion products is as follows:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\u003cp\u003eAs the corrosion continues, the precipitates gradually separate from the matrix due to the lack of joining with the matrix, and the corrosion products cover the surface of the corrosion pits and matrix, providing a certain degree of protection for the matrix. However, the adhesion of these corrosion products to the substrate is not tight, and it is prone to fall off from the surface, and hydrolysis will occur. The peeling and hydrolysis of the corrosion products cause part of the matrix to be exposed again. Under the action of the corrosive medium (Clˉ), the exposed Al matrix is further dissolved, resulting in continuous expansion of pitting corrosion on the surface and inside of the matrix, the size of corrosion pits continues to expand, and some pits gradually converge, forming obvious corrosion grooves until the joint completely fails.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThe Al/Cu dissimilar metals were joined using submerged friction stir welding technology, and the influences of active forced cooling on the microstructure and corrosion properties of the Al/Cu joints were systematically investigated. The following conclusions can be drawn from the study and analysis of the experimental results.\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe active forced cooling could effectively improve the weld formation of the Al/Cu joints. In addition, it could refine the grains in the WNZ of the Al and Cu side, and effectively reduce the IMCs layer thickness at the interface.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe potential difference between the 6061-T6 Al alloy and T2 pure Cu led to the generation of macroscopic galvanic effect in the Al/Cu joints, that is, a macroscopic galvanic corrosion primary battery was formed, which accelerated the corrosion of the joint and results in lower corrosion resistance of the FSW and SFSW joints than the BMs. However, the corrosion resistance of SFSW joints was higher than that of FSW joint. The forced cooling contributes to improve the corrosion resistance of Al/Cu joints.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eDuring the immersion corrosion period of 240 h, no significant corrosion occurred on the Cu side due to cathodic protection. The HAZ of Al side was the most seriously corroded area of the joints, which formed obvious corrosion grooves. Compared with the FSW joint, the SFSW joint had smaller width and depth of corrosion grooves on the HAZ of Al side, and the corrosion degree was significantly reduced. The corrosion on the Al side is synergistic promoted by both macroscopic and microscopic galvanic effects.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cb\u003eFunding\u003c/b\u003e\u003c/p\u003e\n\u003cp\u003eThe authors gratefully acknowledge the financial support from Key Research and Development Projects of Shaanxi Province (GrantNo.2020ZDLGY13-04).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability Statements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that has been used is confidential.\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eYue Mao: Methodology, Resources, Writing - review \u0026amp; editing. Zhaoyi Pan: Conceptualization, review \u0026amp; editing, Supervision. Qiang Chu: Investigation, Formal analysis. Qinlian Zhang: Data Curation. Zhenzhong Wu: Visualization. Rui Xu: Validation. Wenbo Zhang: Visualization. Wei Liu: Investigation. Weiqi Qiao: Formal analysis. Linchuan Liu: Resources. Xingyu Huang: review \u0026amp; editing. Linduo Liang: Investigation.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eXu R, Li F, Yuan C, Zhang Y, Yan W, Zhao X (2021) Microstructure and phase constitution at the interface of double-sided electron beam welded Cu/Al clad metal sheet. 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Corros Sci 180:109203. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.corsci.2020.109203\u003c/span\u003e\u003cspan address=\"10.1016/j.corsci.2020.109203\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\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":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Friction stir welding, Al/Cu dissimilar joint, microstructure, corrosion properties, forced cooling","lastPublishedDoi":"10.21203/rs.3.rs-7373892/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7373892/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCorrosion resistance is a critical service performance of the Al/Cu dissimilar joints. Using the effect of forced cooling of flow water, a submerged friction stir welding (SFSW) technology was developed to enhance the corrosion resistance of the Al/Cu joints. The weld formation of the joints was not only improved by active forced cooling, but also it could refine the grains of the WNZ, and effectively reduce the interfacial IMCs layer thickness. The potential difference between the Al and Cu gives rise to the formation of a macroscopic galvanic effect, which accelerates the corrosion of the joints. Nevertheless, the corrosion resistance of SFSW joints was higher than that of FSW joints. During the 240 h corrosion period, no significant corrosion occurred on the Cu side due to cathodic protection, while the heat affected zone (HAZ) of Al side suffered the most severe corrosion, forming distinct corrosion grooves. Compared with the FSW joint, the SFSW joint had smaller width and depth of corrosion grooves on the HAZ of Al side, with a significantly reduced overall corrosion degree. It demonstrates that the active forced cooling contributes to improve the microstructure and corrosion resistance of Al/Cu joints.\u003c/p\u003e","manuscriptTitle":"Effect of active forced cooling on microstructure and corrosion properties of friction stir welded Al/Cu dissimilar joints: Quasi-in-situ study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-09 09:00:04","doi":"10.21203/rs.3.rs-7373892/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-09-27T23:10:32+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-27T20:38:47+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-08-22T17:19:25+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-22T06:17:41+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-08-21T07:11:36+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4fcb5419-d6b3-4f03-ade6-ac2fb1d29378","owner":[],"postedDate":"October 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-05T17:05:50+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-09 09:00:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7373892","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7373892","identity":"rs-7373892","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}