Effect of laser power in laser powder bed fusion on Ni content and structure of Nitinol

Abstract

Laser powder bed fusion (LPBF) additive manufacturing is capable of producing Nitinol stents of complex shapes, but a slight change of Ni content during LPBF can result in the stents not being suitable. Thus, there is a need to predict how laser power (P) and thus energy (E) affects the two important outcomes of Nitinol LPBF: defect level and Ni content. In this study, how Ni content may change during LPBF has been studied under two conditions: a sufficiently low P (thus E) but close to fully dense condition and a sufficiently high P that has caused keyhole pore formation. The loss of Ni has been found to be 0.36at% and 0.56at% for the sufficiently low and sufficiently high P conditions. Thus, 0.4–0.5at%Ni loss is expected for defect-free Nitinol LPBF. The morphological features of the melt pool affecting defect formation and relating to how P affects Ni loss will be discussed. With the significant loss of Ni due to LPBF, for the starting powder, which contains 50.5at%Ni and is fully austenitic, the LPBF as-built structure has remained mainly austenitic, but the samples also contain a small amount of martensite.