Fig. 1. Schematic of FSW with ultrasonic [16].

Fig. 2. Schematic of the tensile specimen of the Al/Mg joint.

Fig. 3. Surface appearances and tool surfaces of the joints after the both welding processes: (a) coarse surface with the obvious shoulder marks and (b) severe tool adhesion for conventional FSW; (c) smooth surface and (d) no obvious tool adhesion for ultrasonic aided FSW.

Fig. 4. Macrostructures of the Al/Mg joints: (a) conventional FSW and (b) ultrasonic aided FSW.

Fig. 5. Microstructures at different positions in Fig. 4(b) at the SZ of joint with ultrasonic: (a) Position a, (b) Position b, (c) Position c, (d) Position d, (e) and (f) partial enlarged picture of Al/Mg interfaces.

Fig. 6. XRD pattern of the Al/Mg joint with ultrasonic.

Fig. 7. Engineering stress-strain curves of the typical joints.

Fig. 8. (a) Tensile properties of joints with and without ultrasonic and (b) comparison with the published papers [13], [14].

Fig. 9. Fracture locations of joints: (a) conventional FSW and (b) ultrasonic aided FSW; Fracture surface morphologies of joints: (c) macro fracture surface, (d) and (e) micro fracture surface marked by d and e in Fig. 9(c) of joint with ultrasonic; (f) macro fracture surface, (g) and (h) micro fracture surface marked by g and h in Fig. 9(f) of joint without ultrasonic.