Joining mechanism and element distribution in laser micro-welding of NiTi-Cu dissimilar alloys

To satisfy the thermo-driven requirements of NiTi shape memory alloys in different industrial applications, NiTi wire and Cu plate were welded by laser micro-welding technology. The microstructure and element distribution of the weld were experimentally studied using optical microscopy (OM) and energy dispersive spectrometer (EDS). Based on ANSYS Fluent software, a three-dimensional (3D) computational fluid dynamics (CFD) model for laser micro-welding of NiTi-Cu dissimilar alloys was established to analyze the evolutions of temperature and fluid flow fields and the element transport mechanism. The results indicate that the process of laser micro-welding can be mainly divided into 3 procedures, namely the "drilling" procedure of laser in NiTi wire, the preheating procedure of laser energy on Cu plate, and the "drilling" procedure of laser in Cu plate. The mixing of Ni, Ti, and Cu elements mainly occurs in the drilling process of laser in Cu plate, where the elements are mixing well under the driven of the metal vapor recoil pressure and Marangoni vortex. The Cu content in the molten pool is helpful to form low hardness Cu-Ti intermetallic compounds (IMCs), reducing the possibility of the formation of brittle Ni-Ti IMCs. The experimental and simulation results are in good agreement. The research provides important theoretical support for optimization of the laser micro-welding of NiTi-Cu dissimilar alloys.