Effect of heat input modes on microstructure, mechanical properties and porosity of laser welded NiTi-316L joints: A comparative study

Laser welding of NiTi to stainless steel (SS) wires is prospective in biomedical devices. In this work, the influence of different heat input modes, high power density with short welding time (HPDS) and low power density with long welding time (LPDL), on laser welded NiTi-316L wire joints was investigated. The HPDS mode resulted in non-uniform microstructure and hardness distribution in the fusion zone (FZ). On the contrary, a relatively uniform intermetallic compounds (IMCs) network of Fe2Ti + Ni3Ti formed in the FZ welded with LPDL mode. Finite volume modeling (FVM) was used to reveal the flow of molten metals between HPDS and LPDL modes. Different mechanisms for high porosity in the HPDS joint were proposed: processing-induced large pores were caused by the instability/collapse of laser keyhole, while diffusion-induced small pores initiated from the Ni vaporization. A low laser power density and long welding time in LPDL mode could significantly decrease the porosity by weakening laser keyhole, increasing time for pore escaping and uniformizing FZ microstructure. LPDL mode could result in an enhanced joint tensile strength to maximum 290.2 MPa because of the increased effective joining area (by decreasing porosity) and the reduced brittle Fe–Ti IMCs segregation in joints.