Texture and anisotropy in selective laser melting of NiTi alloy

Abstract

Selective laser melting (SLM) is used to manufacture dense NiTi parts. The microstructure and texture are assessed (before and after annealing followed by furnace cooling) and linked to the compression behaviour and shape memory response. It is shown that SLM strongly orients the fine austenite subgrains towards the building direction. This texture induces the highest spring back along the building (vertical) direction and the lowest along the horizontal direction after compression. The compressive stiffness, on the other hand, is the highest along horizontal direction and the lowest in vertical direction. The internal stresses due to SLM processing are another factor that may induce large martensite plates, decreasing the spring back. Although post-annealing (followed by furnace cooling) annihilates these large SLM stress-induced martensite plates, it is unsuccessful to achieve completely isotropic properties. The furnace cooling after annealing may even segregate austenite and martensite within SLM solidified tracks, causing a mixed shape memory response.