Effect of heat treatment on the phase transformation and mechanical properties of Ti6Al4V fabricated by selective laser melting

Abstract

In this work, Ti6Al4V ELI samples were fabricated with previously optimized parameters by selective laser melting (SLM) and then heat-treated under vacuum (HT) or hot isostatic pressed (HIP). Optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure. As-built microstructure was found to be dominated by columnar grains and acicular α′ martensite. Thermo-mechanical treatments allowed to modify the microstructure in the size and shape of the grains, as well in the volume fraction and chemical composition of the phases. Apart from the phase transformation, HIP treatment also allowed to significantly reduce the interior defects, such as pores and internal cracks. Tensile and fatigue properties of the Ti6Al4V ELI samples corresponding to the different microstructures were subsequently investigated. As-built samples exhibited a high tensile strength but a poor ductility with an elongation generally smaller than 6% and low fatigue resistance. HT samples displayed a reduced tensile strength but improved elongation behavior and relatively enhanced fatigue resistance versus as-built samples. The HIP treatment also resulted in a reduction in strength but improved furthermore the ductility and the fatigue behavior due to the elimination of internal defects.