Impact butt welding of NiTi and stainless steel- An examination of impact speed effect

Decreasing the impact speed can extend the effective heating time and escalate the heating rate. Additional flash is generated near the NiTi/SS interface due to the lengthening of the effective heating time. With decreasing impact speed, the plastic deformation zone of the SS is enlarged, and the microstructure in the heat-affected zone (HAZ) is increasingly coarsened, but those of NiTi demonstrate the opposite trends. In all of the NiTi/SS joints, the weld consists of a diffusion layer with a thickness that increases slightly from 1 μm to 1.7 μm as the impact speed decreases from 40 mm/s to 27.5 mm/s. The mechanical properties of the joint deteriorate with decreasing impact speed due to the increased remnant of semi-molten NiTi at the interface. The joint welded at an impact speed of 40 mm/s has the highest strength of 522 ± 41MPa with (7 ± 2)% rupture elongation, and it fractures via micro-void coalescence.