Ni Loss and Phase Structure Formed During Pulse Laser Powder Bed Fusion of Nitinol

Abstract

In recent years, there has been a significant research effort in the laser powder bed fusion (LPBF) processing of Nitinol aiming potentially for stenting applications. To achieve superelasticity that Nitinol stents require, Ni content needs to be within ±0.1at% of the optimal Ni content of 50.8at%, but the high thermal intensity of the laser in LPBF results in a Ni-loss. How to achieve the targeted Ni content in LPBF processed Nitinol alloys, whilst producing defect-free parts is not certain, requiring further evaluation. In the LPBF processing of a Nitinol powder with 51.2at% Ni herein, we exhibit defect-free processing obtaining the idealised Ni content with the use of a constant scan velocity and narrow power and thus energy (E) window. An exponential decay in Ni content with increasing E was observed and is correlated to the aspect ratio of the melt pool shape. Under optimum, defect free conditions, a primarily austenite structure with a tiny amount of twinned martensite is reported by room temperature. Sequential transmission electron micrographs taken between −80 °C to 0 °C on a defect-free sample have illustrated the transformation of twinned martensite to austenite with the major portion of the transformation taking place well below 0 °C.