Effect of laser energy density on the phase transformation behavior and functional properties of NiTi shape memory alloy by selective laser melting

NiTi shape memory alloy (SMA) was prepared via selective laser melting (SLM). Orthogonal tests were conducted with varying laser power and scanning speeds to explore the influence of laser energy density on microstructure, phase transformation behavior, mechanical properties and functional properties (superelasticity and shape memory effect). As energy density increases from 24.3 to 106.3 J/mm3, microstructure transforms from fine to columnar grains. Ni consumption intensifies, boosting transformation temperature and B2 content at room temperature. Post cyclic stretching, B2 content in alloy escalates further at room temperature. Tensile strength peaked at 53.6 J/mm3, while the elongation rate significantly increases with increasing energy density. During cyclic stretching, low or high energy densities in SLM NiTi induce high-density dislocations and stable residual martensite, respectively, significantly inhibiting martensitic transformation and resulting in extremely poor functional properties. The alloy produced at 53.6 J/mm3 boasts exceptional superelasticity due to stress-induced martensitic transformation and phase inversion post-unloading. Following heating past the critical transition, it nearly fully recovers (97.78 % recovery rate). Despite multiple shape memory fatigue tests, it maintains a recovery rate above 90 %. This study provides crucial theoretical insights for optimizing NiTi SMA phase transformation behavior and enhancing the performance.