Effects of selective laser melting build orientations on the microstructure and tensile performance of Ti-6Al-4V alloy

In this paper, Ti-6Al-4V alloy was fabricated by selective laser melting (SLM) with different build orientations, namely SLM-1 and SLM-2, respectively. Microstructures of SLM-1 and SLM-2 were characterized by X-ray diffraction, optical microscope, scanning electron microscope and transmission electron microscope. The results show that the alpha and alpha' phases in the hexagonal close-packed (hcp) structure, as well as the residual beta phase in the body centered cubic (bcc) structure, exist in both samples. However, different SLM build orientations led to varied cooling rates and alpha/alpha' ratios in the final materials. The SLM-1 alloy with build orientation along its thickness direction had higher cooling rate and thus lower alpha/alpha' ratio than the SLM-2 alloy with build orientation along the length direction. Besides, minor martensitic (alpha '') phase was observed in the SLM-1 alloy and acted as an intermediate phase between p and alpha' phases. The low alpha/alpha' ratio and existence of alpha '' phase in the SLM-1 alloy result in a relatively high tensile strength but deteriorated plasticity. Comparatively, the higher alpha/alpha' ratio in the SLM-2 alloy without alpha '' martensite leads to a much better plasticity regardless of the slightly reduced tensile strength.