The Build Orientation Dependency of NiTi Shape Memory Alloy Processed by Laser Powder Bed Fusion

This study investigated the orientation dependency effect on the thermomechanical behavior of NiTi shape memory alloys fabricated by laser powder bed fusion (LPBF). The LPBF method, with high flexibility of process control, has shown great potential in making dense components with enhanced thermomechanical properties through in situ microstructure control. In this study, the LPBF method was employed to alter the crystallographic texture of a NiTi shape memory alloy by manipulating the build orientation. The microstructural and phase analyses showed the alteration of NiTi texture from < 001 > preferred orientation to < 011 > direction via changing the build orientation. The test specimens were then mechanically tested under tension, compression, and torsion, while relating back to the observed textures. Comparing the theoretical transformation strains calculated from the lattice deformation theory with the recovery strains obtained from the experiments, the experimental data show a similar trend, but with lower recovery strain magnitudes. In addition to the polycrystalline aggregate, the presence of defects, slip, and retained martensite phases plays key roles in this discrepancy between experiments and theory. The transformation strains showed the highest orientation dependency in tension as compared with compression, whereas this effect was less significant in torsion.