*4.4. The Average Shear Stress*

The experiments show that the average shear stress of the joint obtained with additional resistance heat reaches 97 MPa and elongation extends to about 1.2% (Figure 9a). This value is significantly higher compared to the joint prepared without resistance heat, under the same clamping force of 1975 N. In our previous work, the average shear stress of the RUSW joint was also slightly higher than the average shear stress of 90 MPa obtained in conventional HPUSW, at a pressure of 1575 N and welding time of 0.5 s, which are the welding conditions that produce the highest strength in HPUSW of Cu/Al joint [9]. This is attributed to the fact that the additional resistance heat can increase the interface temperature (Figure 4), reduce the thickness of the brittle IMC layer (Figure 6), and avoid cracks at the edges of the weld zone (Figure 8). The strength of the hybrid weld is also much higher than that of RSW, because of the lower interface temperature in RSW. As the resistance of the steel sonotrode is an order of magnitude higher than that of the workpieces, the resistance heat mainly occurs at the sonotrode/Cu interface, rather than at the Cu/Al interface.

It is well known that the repeatability in welding dissimilar materials is poor [6]. The poor repeatability could be related to the fact that IMC may not be uniformly distributed across the entire weld interface (Figure 6a,b) [26]. Moreover, differences in IMC layer thickness may exist in different regions of the weld. While there were issues with repeatability in this study, some general trends in weld strength were observed. Figure 9b plots the weld tensile strength against the current. Although there is a lot of scattering, the figure suggests that a high current can produce welds with increased tensile strength. The duration of hybrid welding is shorter than that of conventional HPUSW technique, and thus the period of ultrasonic excitation is shorter, reducing the risk of fatigue fracture [27].

**Figure 9.** (**a**) The relationship between lap shear stress and strain; (**b**) the relationship between lap shear stress and electric current.
