Resistance spot welded NiTi shape memory alloy to Ti6Al4V: Correlation between joint microstructure, cracking and mechanical properties

Despite the popularity of joining NiTi and Ti6Al4V in aerospace and biomedical applications, effective solutions for their dissimilar joining are limited due to brittle intermetallic compounds. In this work, we successfully joined NiTi/Ti6Al4V using resistance spot welding. Results indicate that the number of cracks is the primary factor determining the lap-shear load. The extensive accumulation of brittle Ti2Ni at the bottom of the weld pool leads to stress concentration and is the main cause of crack initiation. X-ray diffraction and phase diagrams revealed the solidification sequence of liquid metal in the joint, including L→NiTi, L+NiTi→Ti2Ni, L→βTi+Ti2Ni. Electron backscatter diffraction analysis showed that weld nugget grains exhibited random orientation, with stress concentration mainly within the Ti2Ni phase on the Ti6Al4V side and at the boundary between the NiTi and Ti2Ni phases, contributing to high susceptibility to deformation and cracking in these regions. Nanoindentation analysis further demonstrated that the welding process diminished the superelastic performance of NiTi, attributable to Ti2Ni phase, grain coarsening and the orientation deviation of B2 NiTi.