*4.3. Weld Cross-Section*

The mechanical behavior was also affected by materials penetration. Figure 8a,b show the profiles of the weld cross-section, for the hybrid welded joint at *tw* = 0.2 s and for conventional HPUSW joint at *tw* = 0.4 s, under clamping force of 1975 N. The joint cross-section after hybrid welding has no obvious defects, whereas, in a conventional HPUSW joint, cracks appear on the copper, just under the tip of the sonotrode. This happens because the material is under high-stress concentration below the edge of the sonotrode, due to the action of long-term ultrasonic waves. In addition, in the conventional HPUSW joint, cracks propagate from the base metal to the welding interface under high-pressure action, which reduces the area of the welding and allows the aluminum alloy to leak from the crack and get attached to the sonotrode tip. This effect decreases the welding quality and may even cause damage to the sonotrode [25].

**Figure 8.** Profile of weld cross-section (**a**) RUSW at 0.2 s, and (**b**) HPUSW at 0.4 s.
