Effect of volume energy density on selective laser melting NiTi shape memory alloys: microstructural evolution, mechanical and functional properties

Abstract

Equi-atomic NiTi shape memory alloy (SMA) samples were manufactured by selective laser melting (SLM) with different laser volume energy densities via synchronously varying laser power and scanning speed. The processing quality, phase transformation, and microstructural evolution were investigated to explore the mechanisms that are responsible for mechanical and functional properties. The results showed scattered micro-sized spherical gaseous defects inside the deposit. One-stage phase transformation occurred without intermediate R-phase transition, and the temperature of both endothermic and exothermic peaks was lower than that of the ingot. The microstructure initially grew along the building direction, but displayed different crystal orientations between coarse columnar grains and refined irregular sub-grains. The micro-hardness mapping revealed that homogenous anti-indentation properties were acquired independent of location variations. While the anisotropic behavior in tensile properties occurred due to the passive effect of columnar B2 austenitic grains, a synergistic effect of strength and plasticity was acquired primarily due to the combined effect of sub-grain refinement strengthening and transformation-induced plasticity. The pre-deformed NiTi samples recovered to their original shape when heated above its critical phase transition point. This work demonstrated that the crack-free NiTi SMA with exceptional comprehensive performances could be fabricated by SLM through the control of energy density.