**1. Introduction**

Near equiatomic NiTi is one of the most important shape memory alloys (SMAs) due to its excellent functional properties, namely shape memory effect and superelasticity, combined with high corrosion resistance, as well as, biocompatibility [1,2]. The functional properties originate from a reversible phase transformation between austenite with a B2 cubic structure and a B19' monoclinic martensite [3,4]. When the material is deformed in the austenitic phase it can exhibit superelastic properties, that is, it can undergo a significant deformation during loading with full recovery to its original shape upon unloading [5]. The excellent mechanical and functional properties exhibited by these alloys make this material widely desired in both medical and engineering fields [6–9]. To achieve successful fabrication of complex parts of NiTi, it is necessary to develop effective and efficient processing technologies due to the poor machinability of these alloys. In recent years, various welding technologies have been used to join NiTi SMAs both to themselves and to other conventional engineering alloys such as stainless steels [10,11], Ti6Al4V alloys [12–14] and Cu-based alloys [15]. Joining techniques such as resistance spot welding [11,16], arc welding [17] and laser welding [10,12] are some examples which are capable to produce defect-free joints. However, a major drawback of the NiTi joints produced by traditional fusion welding techniques is the formation of brittle intermetallic compounds (IMCs) in the weld region, such as Ti2Ni, which tend to reduce the mechanical strength of joints [17–20]. In addition, fusion welding methods can also contribute to significant changes in the transformation temperatures which can impair the potential applications of the joints [19,21].

Considering the possible formation of IMCs in the weld metal, the addition of an interlayer is suggested as a potential solution to adjust the chemical compositions in the weld region and improve the mechanical properties of NiTi joints [12,22]. Cu is a soft metal with a melting point lower than NiTi, and it shows not only high thermal and electrical conductivity, good corrosion resistance and ductility but also a good metallurgical compatibility with NiTi [23–25]. For this reason, Cu interlayers have been used in dissimilar laser welding of NiTi to titanium alloys or stainless steel [14,26,27] to limit the mixing of the base material (BM) and increase the mechanical properties of the joints. It has been found that proper selection of the thickness of the Cu interlayer can enhance the mechanical properties of joints by reducing the amount of brittle Ni-Ti-based IMCs [14].

The thermal history experienced by NiTi during welding can significantly affect its shape memory and superelastic properties [28]. Thus, it is necessary to reduce the heat input of the welding process to restrict the thermophysical deterioration in the weld zone [26]. Solid state joining techniques are known for their low heat input and possibility to avoid solidification defects. For example, friction welding has been carried out on NiTi SMAs in recent years [29–31]. However, it has been reported for dissimilar joints of NiTi to stainless steel obtained by friction welding that high welding times can promote the formation of brittle phases at the weld interface [30]. Currently, there is a need to develop other solid-state techniques that can successfully join NiTi to itself and to other relevant engineering materials.

Ultrasonic spot welding (USW) is a rapidly developing non-melting joining method which is widely used in plastic forming, electronics and automotive fields [32,33]. As compared to the fusion welding processes, such as resistance and laser welding, USW can produce high strength joints without metal depletion or reduced extension of the heat affected zone. Such translates into almost no detrimental effects produced on the BM. USW is especially suitable for achieving effective joints of miniature components [34,35], such as metallic foils, wires and plates, which make this process particularly interesting to weld materials with lower weldability. Traditionally, studies have been mainly focused on the joining of light materials for weight reduction in industrial products [32–35]. However, knowledge on the use of USW in NiTi SMAs is currently extremely limited. Thus, carrying out USW on NiTi is a very worthwhile investigation since this technique has great potential for the fabrication of variable electromagnetic switches, radiator fins and other components based on NiTi SMAs.

In the current study, the operating procedures, weld morphology, chemical compositions, interface characteristics and the tensile shear properties of the ultrasonic spot welded NiTi joints with Cu interlayer were analyzed and discussed. Additionally, the effects of welding energy on microstructural characteristics and failure behavior were investigated in detail.

## **2. Materials and Methods**

### *2.1. Materials*

Ni-Ti shape memory alloy sheets (50.8 at.%), 150 μm thick, were used as the BM. Copper (99.9% purity) foils with a thickness of 20 μm were chosen as the interlayer material. The as-received NiTi alloy sheets were subjected to cold rolling and subsequently stress relieving annealing by heat treatment at 400 ◦C for 45 min in Ar atmosphere to stabilize the phase transformation temperatures. The DSC result of the BM showed that the transformation start and finish temperatures of austenite and martensite are −45.43, −12.5, 22.91 and −58.75 ◦C, respectively, which suggests that

the NiTi alloy was fully austenitic at room temperature, presenting superelastic behavior. The NiTi sheets were machined into rectangular specimens of 60 mm length and 15 mm width (8 mm width for tensile test of NiTi BM), and the Cu foil was processed into a square shape of 15 mm × 15 mm, which is equal to the overlapped region of the BM specimens. Prior to welding, the oxide layer on the NiTi surface was removed by using a mixed solution of 7.5% HF, 20% HNO3 and 72.5% H2O for 40–50 s.
