*4.5. Fracture Morphology*

Figure 10a,b show the macroscopic fracture morphology of the Cu/Al joint made by RUSW at 0.2 s and by conventional HPUSW at 0.4 s, under a clamping force of 1975 N on the Al side. Figure 10c is an enlarged view of Figure 10a, showing the microscopic topography of the fracture. It is observed that there are differences in fracture behavior. For the joint fabricated with RUSW, only welded regions are observed in the fracture surface. This demonstrates that the fracture of the hybrid welding joint occurs at the IMC of the Cu/Al interface, indicating high welding strength. In the case of a joint fabricated with HPUSW, some welded regions and scratched regions exist in the fracture surface (Figure 10b). There are two fracture models demonstrated in Figure 10c. From the enlarged view of the two regions (Figure 10e,f) from Figure 10c, it is obvious that a large number of dimples of different sizes are present in the c region, while there are some cleavage planes in the f region. These features indicate that the fracture of the RUSW is ductile–brittle type.

**Figure 10.** Fracture surface (**a**) RUSW at 0.2 s; (**b**) HPUSW at 0.4 s; (**c**) enlarge view of (**a**); (**d**) region d in (**c**); (**e**) enlarge view of (**d**); (**f**) region f in (**c**).
