*4.2. IMC Layer*

During HPUSW of dissimilar joints, the thickness of the IMC layer determines the welding quality [20,21]. Figure 6a,b show backscattered images of the RUSW joint at *tw* = 0.2 s, *FN* = 1975 N, and HPUSW joint at *tw* = 0.5 s, *FN* = 1575 N, respectively. The thickness of the IMC layer in the joints prepared by hybrid welding and single ultrasonic welding is 1.5 μm and 2.2 μm, respectively. The interface temperature can be measured with good repeatability due to the proximity of the thermocouple to the center of welding spot and stable vibration amplitudes obtained under the same process parameters [22], which produces good repeatability in material vacancy concentration [23]. This results in good repeatability of the IMC thickness measurements. Although the interface temperature of the hybrid welding is the same as that of the conventional HPUSW, the duration of the hybrid welding is shorter, leading to a thinner IMC layer compared to the conventional HPUSW result.

The chemical composition of IMC in joints prepared by RUSW and conventional HPUSW was analyzed using EDS point analysis, and the results are shown in Figure 6c,d, respectively. The chemical composition at points A and B is 54.82 Al-45.12 Cu and 55.02 Al-44.98 Cu (by weight percentage (wt%)), respectively. This result demonstrates that the chemical composition of the IMC layer, in both the hybrid and conventional HPUSW methods, is primarily brittle Al2Cu alloy, according to the binary-phase diagram of Cu-Al [24]. In conclusion, the composition of the IMC layer is quite similar for joints prepared by both hybrid and conventional welding methods. However, the IMC layer is thinner in the hybrid welding approach due to the shorter duration of the welding process, producing better quality results.

**Figure 6.** (**a**,**b**) Cu/Al interface in the RUSW and high-power ultrasonic welding (HPUSW) joints, respectively; (**c**,**d**) EDS results for the lines intersecting the points indicated in (**a**,**b**).

X-ray diffraction analysis was performed on the Al side of the fracture surface to further evaluate the phase composition of RUSW, as shown in Figure 7. The results show that the IMC formed during hybrid welding is primarily composed of Al2Cu. Therefore, the XRD results are in agreement with the EDS results.

**Figure 7.** XRD patterns of the fracture surface.
