*3.1. Surface Morphology of Welds*

Representative surface morphology at the sonotrode tip and anvil sides of different samples after the USW process are exhibited in Figure 2. Figure 3 shows the depths of surface indentations at the edge of samples as depicted with red rectangles.

**Figure 2.** Surface morphology of different samples obtained with different welding energies: (**a**) sonotrode tip and (**b**) anvil sides of 500 J; (**c**) sonotrode tip and (**d**) anvil sides of 700 J; (**e**) sonotrode tip and (**f**) anvil sides of 1000 J. The red squares indicate the regions analyzed to determine the surface indentation depth depicted in Figure 3.

**Figure 3.** Indentation depths of different samples.

From Figure 2, it can be observed that the indentations of sonotrode tip and anvil were distinct on the specimen surfaces, and distinct levels of oxidation were observed on the weld material surface during welding process, due to NiTi's temperature sensibility and the open structure of the ultrasonic welder. Higher welding energy translates into a higher temperature experienced by the material making it more prone to surface oxidation. With increasing welding energy, the indentation depth in both sides exhibited an increasing trend, as shown in Figure 3, and the insert image shows the three-dimensional morphology of the surface indentation. During the USW process, continuous shear vibrations were applied to the NiTi BM by the action of sonotrode, which quickly generated frictional heat at the weld interface, leading to the rising of temperature and the material was softened, thus shear plastic deformation took place on the material surface [32,36–38], resulting in the formation of indentations. Under the same welding parameters, the depth of indentations depends on the accessibility of the sonotrode knurl penetrating into the NiTi material surface, during which a tighter engagement was provided, leading to more relative slippages and friction at the weld interface [36]. Therefore, with the welding energy increasing from 500 J to 1000 J, more frictional heat was generated at the weld interface during the USW process, and then the plastic deformation of NiTi increased since the welding energy was dispersed by shear deformation, resulting in the increasing depth of indentations on the weld material surface aided by a softening behavior of the material caused by the temperature raise.
