Effect of laser energy density on the evolution of Ni4Ti3 precipitate and property of NiTi shape memory alloys prepared by selective laser melting

Abstract

This research investigated the influence of laser energy density on the formation and evolution of Ni4Ti3 precipitate, together with the property of NiTi shape memory alloys fabricated by selective laser melting (SLM). The variant and its evolution mechanism of Ni4Ti3 precipitate was qualitatively analyzed by Gibbs free energy and quantitatively determined by transmission electron microscopy (TEM). Besides, the evaluation of coherency relation between B2 and Ni4Ti3 phase was well discussed. Larger Ni4Ti3 precipitates due to higher laser energy density lost their coherent relation with matrix and was unable to strengthen the matrix. Therefore, the matrix could better combine with Ni4Ti3 precipitates at a lower energy density. Furthermore, different variants of Ni4Ti3 precipitates owing to laser energy density and stress assisted re-aging effect during SLM process affected the mechanical property of NiTi sample. Finally, SLM NiTi sample with an ultrahigh compressive strength of 3053.45 MPa, tensile strength of 801.7 MPa and elongation of 7.64% were achieved. This work provides a novel approach to optimize the performance of SLM NiTi sample by tailoring the Ni4Ti3 precipitates through the laser energy density.