Mg alloys friction stir lap welding with Zn foil assisted by ultrasonic

Mg alloys friction stir lap welding with Zn foil assisted by ultrasonic

Journal of Materials Science & Technology 35 (2019) 1712–1718 Contents lists available at ScienceDirect Journal of Materials Science & Technology jo...

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Journal of Materials Science & Technology 35 (2019) 1712–1718

Contents lists available at ScienceDirect

Journal of Materials Science & Technology journal homepage: www.jmst.org

Research Article

Dissimilar Al/Mg alloys friction stir lap welding with Zn foil assisted by ultrasonic Shude Ji a,∗ , Shiyu Niu a , Jianguang Liu b,∗ a

School of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136, China Beijing Key Laboratory of Civil Aircraft Structures and Composite Materials, Beijing Aeronautical Science & Technology Research Institute of COMAC, Beijing 102211, China b

a r t i c l e

i n f o

Article history: Received 26 September 2018 Received in revised form 11 October 2018 Accepted 16 October 2018 Available online 19 March 2019 Keywords: Dissimilar Al/Mg alloys Friction stir lap welding Zn foil Ultrasonic power Microstructures Tensile shear load

a b s t r a c t The Zn-added ultrasonic assisted friction stir lap welding (UaFSLW) was carried out to improve the quality of dissimilar Al/Mg alloys joint. The effects of ultrasonic power on the joint quality were also investigated. The results indicated that the larger effective lap width and mixing region between Mg and Al (Mg/Al MR) were attained by Zn foil addition and external ultrasonic assistance. Compared with the conventional joint, the finer and better-distributed Mg-Zn IMCs placing the continuous Al-Mg IMCs were formed in the Mg/Al MR of the Zn-added UaFSLW joint. The Zn foil addition and external ultrasonic assistance significantly improved the tensile shear load of the joint, and the load was increased with the increase of the ultrasonic power. The maximum tensile shear load of 7.95 kN was attained, which was 52.6% larger than that of the conventional joint. © 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

1. Introduction Mg alloys, as the lightest metal materials in engineering applications, are widely used in automotive, aerospace and other manufacturing fields [1]. In order to heighten the utilization ratio of Mg alloy and make up its shortcomings such as large brittleness and poor corrosion resistance, the Al/Mg dissimilar alloys joining has become a hot spot of research in recent years [2]. For traditional fusion welding of Al/Mg dissimilar alloys, pore, slag inclusion, poor weld crystallization condition and large local stress concentration are inevitably due to the fusion-resolidification [3]. In fact, the largest harm to the joint strength is produced by a large amount of brittle and hard Al-Mg intermetallic compounds (IMCs) in the weld [4,5]. Friction stir welding (FSW), as a relatively new solidstate welding process, can realize the joining of metal, metal matrix composite and plastic-ceramic composite by the plastic deformation heat of materials and the friction heat between the rotating tool and base material (BM) [6–10]. The heat input during FSW is smaller than that of traditional fusion welding, avoiding the fusionresolidification [11]. Therefore, the sound dissimilar joint of Al/Mg alloys with fewer Al-Mg IMCs can be attained via FSW.

∗ Corresponding authors. E-mail addresses: [email protected] (S. Ji), [email protected] (J. Liu).

Although the formation of Al-Mg IMCs during FSW is inevitable, assisted process such as external ultrasonic assistance can be used to lighten the damage of the Al-Mg IMCs to the joint quality [12]. The mechanical effect of ultrasonic is positive for the improvement of joint formation. Ji et al. [13] studied the dissimilar AZ31B/6061T6 alloys ultrasonic assisted FSW (UaFSW) process and found that ultrasonic was beneficial to forming the longer and more complicated interface between Al and Mg substrates, and the tensile strength was increased by 56 MPa compared to that of the FSW joint without the ultrasonic assistance. Besides the mechanical effect of ultrasonic, the cavitation of ultrasonic significantly affects the morphology and distribution of the IMCs in the dissimilar Al/Mg FSW joint. Lv et al. [14] friction stir welded the dissimilar 6061T4 Al/AZ31B Mg alloys assisted by ultrasonic. They found that the thickness of the IMCs layer at the joint interface and the amount of the IMCs block at the boundary between the stir zone (SZ) and the thermo-mechanically affected zone (TMAZ) were reduced due to the external ultrasonic assistance. Meng et al. [12] investigated UaFSW process of dissimilar 6061-T6 Al/AZ31B Mg alloys and pointed out that the continuous IMCs were broken into pieces or particles, improving the joint tensile strength. Besides the external ultrasonic assistance, the addition of other metal is also an effective assisted process which can restrain the formation of the Al-Mg IMCs. This added metal can react with Al or Mg BM during FSW, forming new structures replacing the Al-Mg

https://doi.org/10.1016/j.jmst.2019.03.033 1005-0302/© 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

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3. Results and discussion 3.1. Joint formations

Fig. 1. Schematic of Zn-added UaFSLW process.

IMCs in the joint. These new structures are expected to have smaller harmful effects on joint quality. Pure Zn foil is a good choice. Gan et al. [15] selected a pure Zn foil acting as a barrier layer and carried out the friction stir-induced diffusion bonding of dissimilar Al and Mg alloys. The results stated that the Zn interlayer impeded the reaction between the upper Al and lower Mg, and no Al-Mg IMCs were formed in the joint. Niu et al. [16] studied the effects of the Zn interlayer on the dissimilar AZ31B Mg/7075-T6 Al friction stir lap welding (FSLW) joint, and found that the fine and discontinuous Mg-Zn eutectic structures replaced the continuous IMCs of Al12 Mg17 and Al3 Mg2 and dispersed in the SZ, which was beneficial for the joint to bear a higher external load. In fact, the hybrid process formed by two assisted processes has a more significant improvement on the joint quality [17]. Therefore, based on previous literature [12,16], the Zn-added FSLW assisted by ultrasonic was carried out in this study to obtain a high quality joint of dissimilar Al/Mg alloys. The effects of the ultrasonic power on the joint formation and tensile property were also investigated.

2. Experimental procedure 7075-T6 Al and AZ31B Mg alloys with the dimensions of 200 mm × 150 mm × 3 mm were selected as the BMs. The thickness of the pure Zn foil was 0.1 mm [15,16]. The schematic of Zn-added ultrasonic assisted FSLW (UaFSLW) process is displayed in Fig. 1. AZ31B Mg alloy was placed as the upper plate in this study. All FSLW experiments were carried out by a FSW machine (FSW-3LM-4012). A fixed welding process parameters combination (a rotating velocity of 1000 rpm, a welding speed of 50 mm/min and a plunge depth of 0.15 mm) was used during all welding experiments according to the previous work [16]. The rotating tool used in this study consisted of a concentric-circle-flute shoulder and a right-screwed pin. The diameters of the shoulder, the pin tip and bottom were 13 mm, 4 mm and 6 mm, respectively. The pin length of the rotating tool was 3.5 mm. A tool tilting angle of 2.5◦ with respect to Z-axis was kept during FSLW process. The ultrasonic vibration frequency of 20 kHz was selected and three output powers of 800, 1200 and 1600 W were used. The ultrasonic probe was attached to the bottom surface of the lower plate and 20 mm distance away from the weld center line. The specimens of metallographic analysis and tensile shear test were prepared using an electrical discharge cutting machine. The width of the tensile shear specimen was 30 mm. The metallographic specimen was etched for 15 s after being manually and mechanically polished according to the metallographic standard. The etching solution was made up of 5 ml acetic acid, 4.2 g picric acid, 10 ml distilled water and 100 ml ethanol. The treated metallographic specimen was analyzed using an optical microscope (OLYMPUS GX71) and a scanning electron microscope (SEM, VEGATE-Scan) equipped with an energy dispersive X-ray spectrometer (EDX). The tensile shear test was performed at the room temperature using a universal testing machine with a fixed tensile rate of 3 mm/min. The fracture morphologies were observed by a stereoscopic microscope (ZSA403) and the SEM.

Fig. 2 displays the cross-section morphologies of the joints. Some Al substrates with various sizes exist in a mixing region between Mg and Al (Mg/Al MR) of all joints. This result indicates that the effective mechanical interlocking is formed in the joint under each process. The effective mechanical interlocking is beneficial for the joint to bear external load [18]. For the conventional joint in Fig. 2(a), the Mg/Al MR size is the smallest, and the Al substrates with a large size are continuously distributed in the Mg/Al MR. The hook and cold lap are respectively formed at the advancing side (AS) and the retreating side (RS). The formations of the hook and the cold lap are related to the material flow behaviors in the SZ and the TMAZ. The materials in the SZ flow downwards and accumulate near the pin tip when the right-screwed rotating pin rotates anticlockwise. The accumulated materials force the materials in the TMAZ to flow upwards, leading to the up-bending morphology of the lap interface [19]. The hook which just extends upwards is formed at the AS. The lap interface at the RS firstly bends upwards, and then extends into the SZ due to the combined action of the rotating pin and the shoulder, forming the cold lap (Fig. 2(a)). Therefore, a small effective lap width (ELW) is formed in the conventional joint due to the shape feature of the cold lap (Figs. 2(a)). The ELW means the horizontal distance from the hook tip to the cold lap tip, which is one of the important factors affecting the joint performance [20]. For the Zn-added UaFSLW joints under three ultrasonic powers in Fig. 2(b), (c) and (d), the morphology of the cold lap at the RS is changed significantly compared to that in the conventional joint, and presents a similar appearance to the hook at the AS. During the welding process, the Zn with a liquid state exists in the Mg/Al MR because the melting point of Zn (419.5 ◦ C) is lower than the peak temperature during FSLW process [21]. Niu et al. [16] reported that the molten Zn as a lubricant reduced the flow stress of the material in the SZ during the Zn-added FSLW process. During the Zn-added UaFSLW, the external ultrasonic assistance is conducive to increasing the heat input, thereby further decreasing the viscosity and flow stress of the material in the SZ compared with that under the only Zn-added FSLW process [22]. The effect of ultrasonic high-frequency vibration is also beneficial to improving the materials flow behavior [23]. Therefore, a part of cold lap and the continuous Al substrates in the SZ are broken into small pieces and dispersed into the Mg/Al MR due to the large flow rate of the material. Moreover, the Mg/Al MR by the external ultrasonic assistance is enlarged compared with that by conventional FSLW process. The above-mentioned results make that the ELW and the length of the boundary between Mg/Al MR and TMAZ in lower plate are increased by the Zn foil addition and the external ultrasonic assistance, as shown in Fig. 3. The maximum ELW of 3.92 mm is attained under 1600 W ultrasonic power, increasing by 78.2% compared to that under the conventional process. The length of the boundary between the Mg/Al MR and the TMAZ in lower plate of the Zn-added UaFSLW joint at 1600 W is 6.27 mm, which is 0.47 mm larger than that of the conventional joint. It is noteworthy that the ELW and the Mg/Al MR are increased with the increase of the ultrasonic power (Figs. 2 and 3). The ELW value is obviously increased when the power increases from 800 W to 1200 W. However, the ELW at 1600 W is only slightly larger than that of the joint at 1200 W, as shown in Fig. 3(a). This is probably because the materials in the Mg/Al MR is close to the liquid state due to the combined effect of the molten Zn and the ultrasonic at 1200 W. The flow rate is close to the limiting value. Therefore, the ELW cannot further increase significantly when the ultrasonic power increases form 1200 W to

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Fig. 2. Cross-sections of the joints: (a) the conventional joint; Zn-added UaFSLW joints under the output powers of (b) 800 W, (c) 1200 W and (d) 1600 W.

Fig. 3. Comparative analyses of the joints under different processes: (a) ELW and EST; (b) the length of the boundary between Mg/Al MR and TMAZ in lower plate.

1600 W. The area of the Mg/Al MR always significantly increases when the ultrasonic power increases form 800 W to 1600 W (Fig. 2), which is related to the changes of materials flow behavior and the atom diffusion. The stirring, dispersing and impact crushing effects of ultrasonic improve the kinetic energy levels and diffusivity of metal atom [24]. The diffusion degree of Zn reaches the biggest value at 1600 W ultrasonic power in this study, thereby increasing the Mg/Al MR area. Moreover, the cold lap height of the Zn-added UaFSLW joint is reduced when the ultrasonic power is increased, forming a large effective sheet thickness (EST). The EST means the vertical distance from the highest point of hook or cold lap to the top surface of upper sheet, and it is also increased with increasing the ultrasonic power (Figs. 2 and 3). The maximum value of 2.33 mm is attained at 1600 W in this study. This is because the plasticized degree of the materials in the SZ is increased with the increase of the ultrasonic power, decreasing the pushing force of the materials in the Mg/Al MR to those in the TMAZ.

3.2. Microstructures The enlarged views of the microstructures in the conventional joint are displayed in Fig. 4 (a) and (b). The regions 1 and 2 are located at the bottom and the top of the Mg/Al MR, respectively. There are two representative microstructures in these regions: the lamellar structures (Fig. 4(c)) and the flocculent structures (Fig. 4(d)). The EDS results of the points 1 and 2 state that the lamellar and flocculent structures are Al3 Mg2 +Al and Al12 Mg17 , respectively. According to the Al-Mg binary phase diagram, two eutectic reactions occur during the solidification process. One is L → Al12 Mg17 + Mg at the eutectic temperature of 437 ◦ C, the other is L → Al3 Mg2 +Al at the eutectic temperature of 450 ◦ C [25]. The eutectic tissues of Al12 Mg17 and Al3 Mg2 are the hard and brittle IMCs, and the fracture crack easily initiates and spreads along the Al-Mg IMCs during the tensile shear test. The crack propagation rate is very fast due to the continuous morphology of the Al-Mg IMCs

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Fig. 4. Enlarged views of the conventional joint: (a) and (b) regions 1 and 2 marked in Fig. 2(a); (c) and (d) SEM views of typical structures; (e) scanning results of points 1 and 2; (f) scanning results of region A marked in Fig. 2(a).

Fig. 5. Enlarged SEM views of the regions in the UaFSLW joints marked in Fig. 2: (a) and (b) regions 3 and 4 at 800 W; (c) and (d) regions 5 and 6 at 1200 W; (e) and (f) regions 7 and 8 at 1600 W.

(Fig. 4(c) and (d)). The map scanning results in Fig. 4(f) indicate that the Al and Mg substrates are interlaced in the bottom of the Mg/Al MR, forming the mechanical interlocking, and Al and Mg elements are mixed at the boundary between the Mg/Al MR and the TMAZ. Fig. 5 displays the enlarged views of the typical regions at the boundary between the Mg/Al MR and the TMAZ in the lower plate of the Zn-added UaFSLW joints. The continuous Al-Mg IMCs with the lamellar or flocculent morphology disappear, and new fine particles are formed, as shown in Fig. 5. These particles are uniformly distributed in the Mg/Al MR. Fig. 6 displays the enlarged SEM views of the fine particles in the regions marked in Fig. 5. The EDS results of points 3 and 4 demonstrate that the white particles are main MgZn2 and a little Al-Mg-Zn phase. Very few Al elements exist in the Mg/Al MR, and the Mg and Zn elements are uniformly distributed, as displayed by the map scanning results in Fig. 6(f). During the FSLW, the molten Zn is transferred vertically and horizontally with the rotation of the rotating pin, and finally dispersed in the Mg/Al MR,

as shown in Fig. 6(f). The nucleation sites of the Mg-Zn IMCs are more dispersed compared to those of the Al-Mg IMCs in the conventional joint when the eutectic reactions occur. The amount of the dispersed Zn around the nucleated Mg-Zn IMCs is not enough to make the IMCs grow into a continuous morphology during FSLW. Therefore, the fine Mg-Zn IMCs particles distribute discontinuously in the Mg/Al MR of the Zn-added UaFSLW joint. The similar distribution of Mg-Zn IMCs was found in the only Zn-added joint, as reported by Niu et al. [16]. For the Zn-added UaFSLW joint, the types of IMCs in the Mg/Al MR are changed compared to the conventional joint. This change is mainly attributed to three aspects as following. Firstly, there are two main eutectic reactions during the solidification process according to Al-Mg-Zn ternary phase diagram. One is L → MgZn2 at the eutectic temperature of 480 ◦ C, the other is L → MgZn2 + Al6 Mg11 Zn11 at the eutectic temperature of 340 ◦ C [14,26]. Secondly, the Mg-Zn IMCs are preferentially formed before the Al-Mg

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Fig. 6. SEM and EDS results of the joints: (a–c) regions 1, 2 and 3 marked in Fig. 5; (d) and (e) point scanning results of points 3 and 4; (f) map scanning results of region B marked in Fig. 2.

Fig. 7. Schematics of IMCs formation in the UaFSLW joint.

IMCs because Zn and Mg have the same crystal lattice [27]. Lastly, Zn and Al can easily form a solid solution rather than IMCs [28]. Therefore, the conclusion that the addition of the Zn foil impedes the formation of the Al-Mg IMCs can be drawn according to the results in Fig. 6. Liu et al. has also attained the similar conclusion [29]. The morphology of IMCs in the Mg/Al MR of the joint is further changed by the external ultrasonic assistance. This change is related to the mechanical and acoustic cavitation effects of ultrasonic. The materials containing molten Zn in the Mg/Al MR are in a solid-liquid mixing state, thereby producing the ultrasonic cavitation. A large number of cavitation bubbles constantly generate and collapse, forming sharp shock waves [30]. The Mg-Zn IMCs in the Mg/Al MR, which has no sufficient growth time, are broken into pieces with a smaller size by the continuously produced sharp shock waves. Meantime, these smaller pieces are transferred

by the mechanical effect of the ultrasonic and the rotation of the rotating tool, and eventually dispersed throughout the Mg/Al MR (Fig. 6). Besides, the high frequency vibration of ultrasonic is beneficial to improving the kinetic energy levels and diffusivity of metal atom. The diffusion height of Zn in the Zn-added UaFSLW joints are larger than that of the conventional joint in this study, reducing the amount of Zn accumulating at the bottom of the Mg/Al MR. Therefore, the quantity of IMCs at the boundary between the Mg/Al MR and the TMAZ in lower plate is reduced, as shown in Fig. 5(d), (e) and (f). It is noteworthy that the size of the Mg-Zn IMCs is decreased with the increase of the ultrasonic power (Fig. 6). The even sizes of the Mg-Zn IMCs in the UaFSLW joints at 800, 1200 and 1600 W were 1.63, 0.96 and 0.65 ␮m, respectively. The schematics of the effects of ultrasonic power on the formation of the Mg-Zn IMCs are presented in Fig. 7. For the Zn-added UaFSLW joint at 800 W, the

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Fig. 8. Fracture loads of the joints under different processes.

Mg-Zn IMCs are mainly concentrated at the SZ bottom, forming a relatively small Mg/Al MR. The amount of the cavitation bubbles is small due to the lower ultrasonic power, so the generated shock waves is not enough to completely break the IMCs. Moreover, a relatively large amount of Zn accumulates at the SZ bottom, providing a favorable condition for the growth of the IMCs. When the ultrasonic power increases from 800 W to 1200 W, the diffusion degree of Zn is increased due to the increase of the kinetic energy levels and diffusivity of metal atoms, reducing the amount of Zn at the SZ bottom. More cavitation bubbles are generated and more quickly collapsed because of the larger ultrasonic power, producing the sharper shock waves. These are why the Mg/Al MR height and the IMCs size of the joint at 1200 W are respectively increased and decreased compared to the joint at 800 W. The degrees of the Zn diffusion and the shock waves both reach the maximum when the ultrasonic power of 1600 W is used. Therefore, the Mg/Al MR is the largest and the Mg-Zn IMCs are the smallest in the Zn-added UaFSLW joint (Figs. 2(d) and 6 (c)). 3.3. Tensile shear loads Fig. 8 exhibits the tensile shear fracture loads of the joints under different processes. The smallest value of 5.21 kN is attained under the conventional FSLW process. The tensile shear loads of the Znadded UaFSLW joints at 800, 1200 and 1600 W are respectively 6.72, 7.82 and 7.95 kN, which are 30.0%, 50.1% and 52.6% larger than that of the conventional joint. The Zn foil addition and the external ultrasonic assistance significantly improve the tensile shear load of the dissimilar Al/Mg FSLW joint. This result is mainly attributed to the improvements of the joint formations and microstructures. The fracture locations of the joints under different processes are presented in Fig. 9. The Mg/Al MR of all joints are completely separated from the lower plate. This result indicates that the main fracture path is along the boundary between the Mg/Al MR and the TMAZ in the lower plate.

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For the conventional joint in Fig. 2(a), besides the path at the boundary between the Mg/Al MR and the TMAZ, another fracture path exists in the SZ. This path is along the cold lap at the RS where the crack easily initiates when the joint is subjected to external load. This result indicates that the ELW is one of the factors influencing the tensile shear load of the lap joint. The joint with the larger ELW possesses the longer crack propagation path during the tensile shear test [16]. As mentioned above, the ELW of the conventional joint is the smallest due to the large cold lap, and the Zn foil addition and the external ultrasonic assistance enlarge the ELW of the dissimilar Al/Mg joint. This is one of the reasons why the tensile shear load of the Zn-added UaFSLW joint is significantly improved. Besides, the fine and well-distributed Mg-Zn IMCs are formed at the boundary between the Mg/Al MR and the TMAZ of the Zn-added UaFSLW joint, replacing the continuous Al-Mg IMCs in the conventional joint. Fig. 10(a) presents the fracture surface morphologies of the typical region marked in conventional joint in Fig. 9(a). The white Al-Mg IMCs with a large size are continuously distributed at the boundary. Niu et al. [16] have reported that the micro-hardness values of the Al-Mg IMCs are obviously higher than those of the MgZn IMCs. The Al-Mg IMCs are brittle and hard compared with the Mg-Zn IMCs, and the continuous Al-Mg IMCs are concentrated at the boundary between the Mg/Al MR and the TMAZ. Therefore, the propagation speed of the crack along the continuous Al-Mg IMCs is fast. However, the fine and discontinuous-distributed Mg-Zn IMCs at the boundary have a pinning effect, which has a certain effect on preventing the propagation of the crack. This is another reason why the tensile shear load of the Zn-added UaFSLW joint is distinctly larger than that of the conventional joint. Compared with the Zn-added FSLW joint without the ultrasonic in Ref. [16], the tensile shear loads of the Zn-added UaFSLW joints under the three ultrasonic powers in this study are all heightened. This result states that the external ultrasonic assistance possesses a significant advantageous impact on improving the tensile shear load of the joint. Moreover, the tensile shear load of the Zn-added UaFSLW joint is increased with the increase of the ultrasonic power, and the load is increased by 1.1 kN when the power increases from 800 W to 1200 W, which is 0.97 kN larger than that when the power increases from 1200 W to 1600 W (Fig. 8). As mentioned above, the ELW is increased with the increase of the ultrasonic power, which is beneficial to improving the tensile shear load of the joint. The fracture surface morphologies of typical regions marked in the Znadded UaFSLW joints are displayed in Fig. 10(b) and (c). The size of the Mg-Zn IMCs at the boundary between the Mg/Al MR and the TMAZ is decreased when the larger ultrasonic power is used, which corresponds to the results in Figs. 5 and 6. These are why the maximum tensile shear load of 7.95 kN is attained at the 1600 W ultrasonic power. Although the size of the Mg-Zn IMCs is significantly decreased when the ultrasonic power increases from 1200 W to 1600 W, the ELW and the length of the boundary in Fig. 3 are not significantly increased. Therefore, when the power increases from

Fig. 9. Fracture locations of the joints: (a) the conventional joint; Zn-added UaFSLW joints under the output powers of (b) 800 W, (c) 1200 W and (d) 1600 W.

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Fig. 10. Fracture morphologies of different regions marked in Fig. 9: (a) region A; (b) region B and (c) region C.

1200 W to 1600 W, the increasing degree of the tensile shear load is much smaller than that when the power increases from 800 W to 1200 W. 4. Conclusions (1) For the conventional FSLW process, the cold lap with a large size was formed at the RS, leading to a small ELW. The brittle and hard Al-Mg IMCs were continuously distributed at the boundary between the Mg/Al MR and TMAZ. (2) The size of the cold lap at the RS was obviously reduced due to the Zn foil addition and external ultrasonic assistance, enlarging the ELW of the dissimilar Al/Mg alloys FSLW joint. The fine and discontinuous Mg-Zn IMCs replaced the continuous Al-Mg IMCs due to the Zn foil addition, and uniformly dispersed in the SZ due to the external ultrasonic assistance. These results were contributed to improving the tensile shear load of the lap joint. (3) The mechanical and acoustic cavitation effects of ultrasonic were beneficial to enlarging the ELW and breaking the IMCs into pieces. The ELW was increased with the increase of ultrasonic power, and the size of the Mg-Zn IMCs was decreased with increasing the ultrasonic power. The maximum tensile shear load of 7.95 kN was attained under the 1600 W ultrasonic power, which was 52.6% larger than that under the conventional FSLW process. (4) The hybrid process of the Zn foil addition and the external ultrasonic assistance had the significant advantages for improving the quality of the dissimilar Al/Mg alloys FSLW joint. Increasing the ultrasonic power within a reasonable range could further improve the joint quality.

Acknowledgement This work is supported by the National Natural Science Foundation of China (No. 51874201).

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