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 micro hardness in the HAZs increased. The tensile strength of the joint increased to 830 MPa, with 6.2% elongation. The 40-mu m-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 resolidified 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.