*4.1. Interface Temperature*

Figure 4 shows the measured temperature as the current changes from 0 to 3900 A. The results show that at the same welding time of 0.2 s, the measured peak temperature at the welding interface in RUSW is 445 ◦C, which is significantly higher than the interface temperature of 301 ◦C in conventional HPUSW. The peak temperatures with a current of 1300 A, 2600 A, and 3900 A at the welding time of 0.2 s are 328 ◦C, 380 ◦C, and 445 ◦C, respectively. This means that the temperature rises at a high gradient as the welding current increases, because the resistance heat *Q*RSW has a quadratic relationship with the electric current, according to Equation (3). The higher interface temperature benefits interfacial diffusion, resulting in higher welding strength.

**Figure 4.** Measured temperatures for various currents.

Figure 5 shows the values of the interface temperature during hybrid welding and conventional HPUSW, at different welding moments. It is evident that the peak interface temperature in hybrid welding is similar to that in conventional HPUSW under the same conditions (welding time of 0.4 s, clamping force of 1975 N, welding time of 0.5 s, and welding pressure of 1575 N). This demonstrates that the additional resistance heat can speed up the HPUSW process.

**Figure 5.** Measured temperature progress with welding time and pressure.
