Anisotropy in microstructural features and tensile performance of laser powder bed fusion NiTi alloys

Abstract

From the viewpoint of the poor formability and mechanical anisotropy of laser powder bed fusion (LPBF)-processed NiTi alloys, three different energy densities, i.e., minimum value, median value, and maximum value, were adopted in this work. The anisotropic behavior in the microstructural distribution and tensile properties were explored to evaluate the performance stability of NiTi components within a wide processing window. The results showed that an excellent deposition quality was acquired when the energy density was within the range of 45–75 J/mm3. The phase constitution and microstructural features were similar even under different processing parameters. In a single melting pool, the columnar grains within the body zone exhibited {110}<001> and {100}<011> textures. However, no preferred orientation was observed for the fine dot-like sub-structures within the overlapping zone. The anisotropy in tensile strength and plasticity was mainly attributed to the columnar grains with preferred crystallographic orientation in the body zone. Only a dimple-like morphology was observed on the fracture surface, indicating a ductile tensile failure mechanism. The findings demonstrated that the stable microstructural features and tensile performance of LPBF-processed NiTi components were ensured within a wide processing window (45–75 J/mm3), despite the anisotropic behavior along the transverse and longitudinal tensile directions.