Effects of post-processing on cyclic fatigue response of a titanium alloy additively manufactured by electron beam melting

Abstract

To establish the benefits of electron beam melting (EBM) for the fabrication of Ti–6Al–4V alloy components, it is necessary to properly understand the fatigue performance of the alloy. In the present study, we investigated the high-cycle fatigue behaviors of EBM-fabricated Ti–6Al–4V alloy component samples and systematically assessed their correlation with the microstructure, porosity, and (quasistatic) tensile properties of the material. The associated texture evolution and elemental distributions of the components were also examined. The employed samples were post-processed by hot isostatic pressing (HIP) and heat treatment (HT) at 920°C for 2h with and without the application of 100-MPa gas pressure, respectively. The as-built samples consisted of fine acicular α (hcp) microstructures, attributable to the β (bcc) → α′ (hcp) martensitic transformation during the EBM process, and these microstructures were coarsened by the HIP and HT processes. However, the fatigue properties of the HIP samples were found to be superior to those of the other samples despite the coarsened microstructure and reduced tensile strength. Notably, a small amount of porosity was observed in the as-built and HT samples, although all the samples were almost fully dense, having relative densities of ~100%. The pores were found to act as crack initiation sites, with the defects having a greater effect on the fatigue properties of the samples than the microstructures. Optimization of the parameters of both the EBM process and the raw powder production process is required to enhance the performance of EBM-fabricated Ti–6Al–4V alloy components.