**3. Experimental Details**

The RUSW system included a lateral-driven, high-power ultrasonic welder (4.0 kW) and an inverter resistance power supply (Figure 3a). The high-power ultrasonic welder used a lateral spot welder (Telsonic M5000) with a vibration frequency of 20 kHz. A rectangular sonotrode tip with dimensions of 7 mm × 5 mm was used in this study (Figure 3b). To avoid the occurrence of arcs and marks on the Cu/Al interface, the shape of the sonotrode tooth was trapezoidal, instead of triangular. A resistance spot welder with a maximum electrical current supply of 4000 A was used. A zero-to-peak amplitude of 24 μm, current of 3900 A, clamping force of 1975 N and welding time of 0.2 s were selected for the hybrid welding. The interval between each sonotrode tooth was 0.9 mm. In order to avoid high current density occurring at the anvil/Al interface, the interval of the anvil tooth was 0.1 mm larger than that of the sonotrode tooth. For the conventional HPUSW, the welding time was 0.5 s while the clamping force was set to 1575–1975 N. The specimens were 6061-T6 aluminum alloy and pure copper, cut into 100 <sup>×</sup> 25 <sup>×</sup> 0.8 mm3 pieces. The overlapping area of the specimens was 25 <sup>×</sup> 25 mm2. In this work, prior to welding, the samples were ultrasonically cleaned with acetone to remove surface contaminants. The ultrasonic vibration direction (VD) was perpendicular to the length direction of the workpiece (Figure 3d).

The fracture surface morphology was observed by a JEOL JSM-7001F field emission gun scanning electron microscope (FEG SEM) (Jeol, Tokyo, Japan), equipped with an energy-dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) analysis was carried out using a PANalytical Empyrean (Malvern PANalytical, Almelo, The Netherlands) diffractometer to identify the phases of IMC at the welding interface. The XRD spectra were measured from 20◦ to 90◦, with a step size of 0.02◦ and a scanning speed of 0.02 ◦/s. The tensile tests were performed using a Shimadzu AGS-X electronic testing machine (SHIMADZU, Kyoto, Japan). Tensile-shear tests were carried out at a tensile speed of 1 mm/min. At least three samples were tested for each process condition. The mechanical strength of

the joints was evaluated according to tensile-shear strength, which was calculated by dividing the maximum tensile-shear force by the sonotrode area of 7 mm × 5 mm.

The temperature was measured using 0.1 mm K-type thermocouples, inserted through a semicircular groove of radius 0.5 mm on the aluminum sheet surface. Thermocouple tip was placed at 1.5 mm from the center of the Cu/Al interface. Figure 3c,d show a schematic of the tensile-shear and thermocouple temperature measurement tests, respectively.

**Figure 3.** (**a**,**b**) The RUSW machine setup; (**c**) configuration of the tensile-shear, and (**d**) temperature measurement tests setup.
