Microstructure and properties of equiatomic Ti–Ni alloy fabricated by selective laser melting

Abstract

Selective Laser Melting (SLM) as one of the additive manufacturing technologies can be used to produce Ti–Ni shape memory alloys with complex shape. In this work, equiatomic Ti50Ni50 (at.%) samples were produced by SLM with different scanning speed, and near-fully dense (99.5% relative density) parts were obtained under a low input laser energy density (40 J/mm3) with the scanning speed of 1000 mm/s. The different scanning speeds had limited influence on the phase composition, transformation temperatures and Vickers hardness. Under low magnification, the typical molten pool morphology with inhomogenous microstructure was shown in the samples of SLM-produced Ti–Ni alloy, and self-accommodate martensite (B19’) twins with a few austenite (B2), nanoscale Ti2Ni and rhombohedral (R) phases were found at higher magnification. Due to the formation of nanoscale Ti2Ni phase and inhomogenous microstructure, the SLM-produced Ti–Ni alloy exhbited lower phase transformation temperatures and larger hysteresis temperatures between the start and finish point of the phase transformation compared to the Ti–Ni powder. The formation of the R phase was contributed to the special repeat heating process and stress field formed by Ti2Ni phase and dislocations in SLM equiatomic Ti–Ni alloy. The SLM-produced Ti–Ni alloy exhibits higher compressive and tensile fracture strength but lower compressive and tensile fracture strain compared to the conventional cast samples.