Effect of laser emission mode on processability map, microstructure and martensitic transformation of Shape Memory NiTi alloy produced by laser powder bed fusion

Abstract

In this work, the process-ability, microstructure, and martensitic transformation of NiTi shape memory alloy samples built through Laser Powder Bed Fusion (LPBF) were correlated to different thermal cycles induced using continuous wave (CW) and pulsed wave (PW) laser emission modes. The effect of the temporal and spatial profile of the laser on the microstructure and martensitic transformation is investigated. In detail, relative density versus laser volumetric energy density curves were obtained using the two laser emission modes; the corresponding optimal conditions were determined for two values of hatch distance (80 and120 μm). Microstructural investigation of the samples was carried out for analyzing the liquid pool shape and the texture orientation. The effect of the laser emission mode on the transformation temperatures of the built samples was analyzed by differential scanning calorimetry (DSC). Finally, thermal modeling of the LPBF process was considered for obtaining the temperature field associated with the investigated process conditions. It was found that the feasibility maps don't overlap when the laser emission mode changes. However, using CW mode led to a small shift when the laser volumetric energy density was high. In contrast, PW mode needed lower volumetric energy density values to get the most relative density. Moreover, the different temporal profiles of the laser power emission induce different sizes and shapes of the melt pools, and it was found that they also induce a different texture of the microstructure. The numerical results showed that the PW emission mode induced higher temperatures than the CW one, promoting faster cooling rates. Finally, the laser emission mode also affects the martensitic transformation of the NiTi parts.