Heat transfer mode shift for regulating microstructure of NiTi alloys in laser powder bed fusion

The function-graded NiTi part is one of the greatest potential smart materials due to its superelasticity, shape memory, biocompatibility and the ability of coupling complex functionality. Laser powder bed fusion (LPBF) is a promising method for the preparation of microstructure-gradient NiTi parts. The microstructure of the NiTi alloy prepared by LPBF was tailored by switching the heat transfer modes through the variation of the process parameters. The LPBF-fabricated NiTi alloy with fine grains is obtained in the conduction mode because the laser energy is enough to melt the powder while, the growth of the grains is restricted without enough power. In comparison, the coarse grains penetrating through two or more layers is obtained in the keyhole mode, due to the appearance of the keyholes reheating the previous layers. This work proved the feasibility for the preparation of microstructure-gradient NiTi alloys by LPBF.