Effect of subtransus heat treatment on the microstructure and mechanical properties of additively manufactured Ti-6Al-4V alloy

Abstract

As a powder-bed-based additive manufacturing technology, selective laser melting (SLM) offers high-level flexibility and enables efficient fabrication of complex parts. In connection with complex thermal events occurring during dynamic sequential layer-by-layer deposition, the as-built material is usually hierarchical at different length scales and possesses anisotropy at each level. As a result of a moderate heating temperature of the baseplate and high cooling rates involved in the process, the as-built Ti-6Al-4V alloy has an α′ martensite microstructure. Microstructure evolution occurring during post-SLM heat treatment is strongly affected by the stability of the initial acicular martensite. The present study was aimed at developing an optimum post-SLM heat treatment scheme at a temperature below the β transus temperature, based on the understanding of microstructure evolution occurring during subtransus treatment and the resultant mechanical properties of the alloy. It was observed that the growth of the α and β phases during the heat treatment was inhibited by the initial α′ phase. A higher heating temperature could effectively improve microstructure homogeneity on a micrometer-scale to some extent. Heating temperature affected the strength and fracture strain of the alloy far more than cooling rate. A post-SLM heat treatment at a temperature of 850 °C or higher could lead to an improvement of fracture strain to the level of the forged counterpart, accompanied by the losses in yield strength and ultimate compressive strength from the as-built values. Full annealing (i.e., subtransus treatment at a high temperature) was thus recommended to be an appropriate post-SLM heat treatment for Ti-6Al-4V.