Resistance Microwelding of Ti-55.8 wt pct Ni Nitinol Wires and the Effects of Pseudoelasticity

Nitinol has revolutionized many traditional engineering designs with its unique properties that include pseudoelasticity and the shape-memory effect (SME). However, the general lack of understanding on welding Nitinol limits its full potential in practical applications. The current study examined the microstructure and mechanical properties of resistance microwelded (RMW) Ni-rich Nitinol wires at different applied currents. The results revealed a solid-state bonding mechanism that consisted of six main stages, including (1) cold collapse, (2) dynamic recrystallization, (3) interfacial melting, (4) squeeze out, (5) excessive flash, and (6) surface melting. The joint strength and fracture mechanism were linked closely to the metallurgical properties of the welds. Through differential scanning calorimetry (DSC) testing, it was found that the weld metal underwent phase transformation at lower temperatures compared with the base material. Also, the pseudoelastic property of Nitinol was found to have a large effect on the contact resistance during the onset of welding current. Compression tests with varying temperatures confirmed distinct differences in displacement (i.e., cold collapse) under the welding load of 5 kg-f caused by changes in the thermodynamic stability of the austenite phase. Both the dynamic resistance and displacement measurements were found to be significantly different during the RMW of Nitinol crossed wires compared with welding 316 low-vacuum melted (LVM) stainless steel (SS) crossed wires.