Effect of laser energy density on transformation behavior and mechanical property of NiTi alloys fabricated by laser powder bed fusion

Abstract

This research investigated the influence of laser energy density on the phase transformation characteristics and structure, together with the mechanical property of NiTi shape memory alloys fabricated by laser powder bed fusion (LPBF). Results showed that randomly oriented needle-shaped martensite was distributed in the B2 austenite matrix in samples with the higher energy density. In contrast, typical columnar grains were observed throughout multiple melt pools, showing obvious epitaxial growth characteristics in the samples with lower energy density. Besides, the martensite transformation temperature varied approximately linearly with the increasing laser energy density, showing a positive correlation. It can be explained that more intense evaporation of Ni element in the melt pool under higher energy density leads to a reduction in the Ni content of the matrix and contributes to a reduction in the lattice distortion of the B2 matrix. Finally, the samples with the energy density of 138.89 J/mm3 showed optimal tensile and compression strength of 648 and 2970 MPa, respectively. This work provides a novel approach to optimize the performance of LPBF NiTi sample by tailoring the phase composition and structure through the laser energy density.