Effect of Hatch Angle Rotation on the Thermal and Mechanical Properties of Micro Selective Laser Melted NiTi Shape Memory Alloy

NiTi shape memory alloy (SMA) has been widely used for bio-medical and aerospace applications due to its unique properties, i.e. shape memory effect and pseudoelasticity. However, the high ductility and work-hardening effect of NiTi lead to poor machinability. Additive manufacturing (AM), with excellent capability of fabricating complicated structures, has been used to fabricate NiTi components. To meet the increasing demand of product miniaturization, micro selective laser melting (μSLM) system equipped with finer laser beam has been developed to improve manufacturing resolution. This work studies the fabrication of NiTi SMA parts by μSLM for the first time. The effect of hatch angle rotation on the thermal and mechanical behaviors of μSLMed NiTi is analyzed. Columnar grains accompanied with equiaxed grains are observed in μSLMed NiTi. Laser rotation angles of 45/60/90° lead to weak crystallographic texture. Ti-rich secondary phases including Ti₂Ni/Ti₄Ni₂O_x and TiC_1-xN_x are detected in the raw NiTi powder and the as-printed NiTi parts, respectively. The as-printed parts under different hatch angles show similar phase constitution. The thermal-induced transformation behavior was depressed with absence of transformation peak. The variation of hatch angle cannot activate the transformation peak. Varying hatch angle from 45° to 90°, the compressive strength and ductility reduce, and the hardness increases. The depressed thermal-/stress-induced phase transformation of the μSLM NiTi can be attributed to the Ti-rich secondary phases, which cause variation of matrix Ni/Ti ratio and inhomogeneous microstructure.