Effects of printing volumetric energy densities and post-processing treatments on the microstructural properties, phase transformation temperatures and hardness of near-equiatomic NiTinol parts fabricated by a laser powder bed fusion technique

Equiatomic and near-equiatomic NiTinol alloys, mainly due to the exhibition of adjustable mechanical and thermal shape memory properties, are widely exploited to produce different smart and functional devices. Laser powder bed fusion technique was used in this research to process dense NiTinol parts from a prealloyed near-equiatomic NiTi powder, using three different volumetric energy densities. In the next step, samples, processed using a specific set of printing parameters, were annealed using two different procedures. Effects of different used processing volumetric energy densities and post-processing heat treatments on the microstructural properties, phase composition, crystallographic information, phase transformation behavior and hardness of the processed samples were investigated. As-printed Parts with Ni contents ranging from 55.12 to 55.27 wt%, density higher than at least 96% of theoretical and a similar phase structure as that of used powder, dominated by austenitic NiTi, were obtained. Applying a hot isostatic processing treatment at 1100 °C for 4 h or annealing the samples in a pressureless sintering furnace, working under a vacuum environment at the same temperature and holding time, had significant effects on the properties of the printed parts. These treatments, in addition to increasing the density of the printed samples, their average grain size and reducing the residual stresses, shifted the phase transformations to lower temperatures. These results suggest that post-printing heat treatments affect and, as similar to NiTinol alloys having Ni contents above 55.7 wt%, can be used as a strategy to alter the phase transformation temperatures of printed less Ni-rich near-equiatomic NiTinol alloys and consequently obtain NiTinol parts suitable for different applications.