Resistance spot welding of NiTi shape memory alloy sheets: Microstructural evolution and mechanical properties

Resistance spot welding was used for welding superelastic NiTi shape memory alloy sheets. The microstructure of the welded sample showed a fully austenitic NiTi nugget without any visible weld defect due to the insignificant change in the transformation temperature during the welding process. Additionally, the fusion zone has a homogenous microhardness distribution with an average hardness value of ~340 HV, which is below the average hardness value for NiTi base material (~410 HV) because of the large grain size in the weld nugget. The perseverance of the superelastic response as a main functional property of NiTi alloy after the RSW procedure is significant. The welded sample failed via pullout failure mode during tensile shear loading conditions. Face digital image correlation imaging confirmed that strain is localized in the area around the nugget. The dominant fracture mode of the sample during tension-shear loading was ductile, and the EBSD results of the fracture area revealed the excessive deformation around the HAZ resulted in a transgranular fracture. Therefore, resistance spot welding is highly capable of providing a sound, straightforward and reliable joint between NiTi sheets.