*2.3. Microstructural Characterization*

The surface profiles of the NiTi welds obtained using different welding parameters were examined using a digital microscope (VHX-2000C, KEYENCE, Osaka, Japan). Additionally, the metallographic observations were performed on the transverse section of the weld, which was parallel to the ultrasonic vibration direction. After mounting the cross sections of the welded joints, mechanical polishing using sandpapers from 240 to 2000 grits, followed by polishing solution agent was performed. The etching process was conducted in an acid solution consisting of 3% HF, 14% HNO3 and 82% H2O (in volume) with an immersion time of 20–30 s. The microstructures and chemical compositions of the cross sections for different welds were characterized via an optical microscope (OM; GX71, OLYMPUS, Tokyo, Japan) and with a scanning electron microscope (SEM; SU1510, HITACHI, Tokyo, Japan) equipped with an energy dispersive spectroscopy (EDS) analysis system.
