Microstructure and mechanical properties of resistance-welded NiTi/stainless steel joints

Resistance welding was employed to join NiTi and stainless steel (SS), with the aim to fabricate a NiTi/stainless steel joint for biomedical applications. The effects of welding current and post-welding cold drawing with 30% area reduction on the joint microstructures and mechanical properties were investigated. At a welding current of 40 A, a NiTi/SS joint with a reaction layer of several microns in size was obtained. The tensile strength of this joint reached 440 MPa, with 7.9% rupture elongation and a fracture on the NiTi side that occurred via micro-void coalescence mechanism. Good plasticity and bending performance of the joint were verified by the bending test. After cold drawing, the microstructures in the heat-affected zones (HAZs) were refined and the microhardness in the HAZs increased. The tensile strength of the joint increased to 830 MPa, with 6.2% elongation. The 40-μm-thick weld obtained at the welding current of 45 A consisted of a reaction layer and a NiTi molten zone. Local embrittlement occurred near the NiTi fusion line owing to grain coarsening, the existence of re-solidified grain boundaries in the HAZ and eutectics in the molten zone. The tensile strength of the joint was established as 340 MPa, with 5.8% rupture elongation and a cleavage-mediated fracture on the NiTi side. This type of joint could not be drawn as a result of its reduced ductility.