Effect of laser hatch spacing on the pore defects, phase transformation and properties of selective laser melting fabricated NiTi shape memory alloys

This study investigated the effect of hatch spacing on the pore defects, phase transformation, mechanical properties and shape memory effect of selective laser melting fabricated NiTi (SLM-NiTi) alloys. As the hatch spacing increases, the main pore defects inside samples evolve from keyholes in the overlapped zone to evenly distributed gas pores, and then to un-fusion pores between the molten pools or layers. The SLM-NiTi sample with 110 μm hatch spacing exhibits the most excellent tensile properties and shape recovery rate due to its lowest porosity and the smallest size of spherical pores. High-energy X-ray diffraction result reveals that obvious residual internal stress retained in the large hatch spacing samples and its distribution shows strong directionality along B2-(100). The dislocation density has very limited inhibitory influence on B2→B19' martensite transformation temperature (1–5 °C from small to large hatch spacing) because it is greatly offset by the promotion effect of internal stress, while the Ni evaporation is the major facilitating factor to the increase of transformation temperature (contribution 92%–62% from small to large hatch spacing), the rest is from the contribution of Ni₄Ti₃ precipitates.