Abstract
NiTi shape memory alloys were successfully joined by ultrasonic spot welding (USW) using an aluminum interlayer. The microstructural phase transition and the mechanical properties of the joints were investigated. The results have shown that the NiTi/Al/NiTi (with thicknesses of 0.5 mm/0.1 mm/0.5 mm, respectively) interfaces were evenly connected, and there were no unconnected areas nor welding defects when a welding energy of 1000 J, welding pressure of 0.4 MPa, and welding amplitude of 55 μm were used. As a solid-state joining technique, no intermetallic compounds (IMCs) formed at the interface of the USWed joints. The joining mechanisms of NiTi to the Al interlayer were based on atomic diffusion and metallic bonding. At room temperature, the NiTi USWed joint exhibits a fully austenitic structure. Meanwhile, the welded area has a broader transition temperature range and hysteresis compared to the non-affected NiTi base material. Digital image correlation (DIC) was used during mechanical testing to accurately determine the strain fields during deformation. The fracture morphology was a mixture of cleavage (brittle) fracture and ductile with dimples throughout the fracture surface with an interfacial fracture mode.
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