*2.2. Ultrasonic Spot Welding*

USW was performed using a SONICS MSC4000-20. The schematic diagram of the USW system is shown in Figure 1a, which was comprised of ultrasonic generator, transducer, amplitude transformer, sonotrode tip and anvil. The size of the sonotrode tip was a square of 8 mm × 8 mm, consisting of 10 × 10 gridding knurls with a spacing and a depth of 0.8 mm and 0.4 mm, respectively. Figure 1b depicts the relative welding positions of both the NiTi and the Cu interlayer. The ultrasonic vibration direction during the USW process was perpendicular to the longitudinal direction of the NiTi sheets. In this study, the energy control mode was used, during which process the welding time and power input were adjusted automatically. The main process parameters selected were a frequency of 20 kHz, a welding amplitude of 55 μm and a constant clamping pressure of 0.38 MPa, with different energies of 500, 700 and 1000 J, respectively. The USW process was performed at the center of the overlapped position, as shown in Figure 1b.

**Figure 1.** (**a**) Schematic illustration of USW process; (**b**) Schematic diagram showing the welding position of NiTi with Cu interlayer.
