Effects of subtransus heat treatments on microstructure features and mechanical properties of wire and arc additive manufactured Ti–6Al–4V alloy

Post heat treatment is necessary to optimize the microstructure of additive manufactured Ti–6Al–4V to satisfy the aeronautical criterion. However, the relationship between the unique heat treated microstructure features and corresponding mechanical properties of wire and arc additive manufactured (WAAMed) Ti–6Al–4V has not been completely understood so far. In this study, five subtransus heat treatment regimes were used to the WAAMed Ti–6Al–4V alloy, and the different heat treated microstructure and the resultant mechanical properties were investigated. The microstructure was not substantially changed after heat treatment 600 °C/4 h/air cooling (AC). The α laths were coarsened after heat treatment 850 °C/2 h/AC, and the higher annealing temperature contributed to the appearance of α_s. After solution and aging treatment, there were the discontinuous α_GB, coarsened α_p, and fine α_s. There was the small Widmanstätten structural α_s sharing the uniform crystallographic orientation after heat treatment 930 °C/1 h/AC + 550 °C/4 h/AC. The α′ martensite and extremely fine dispersed granular α_s were obtained after heat treatment 930 °C/1 h/water quenching (WQ) + 550 °C/4 h/AC. The heat treatment 930 °C/1 h/WQ + 800 °C/2 h/AC was found to be the best heat treatment in this study. The discontinuous α_GB, dispersed α_s with various crystallographic orientations were obtained, which simultaneously increased the ultimate tensile strength (UTS) and elongation (EL) to 886 ± 8 MPa and 16.6 ± 1.6%, comparing to 847 ± 12 MPa and 12.2 ± 2.8% for the as-deposited specimen. Besides, the α/β interface phase distribution in the as-deposited and heat treated specimens was concerned. Two break-up mechanisms of α_p, including boundary splitting and termination migration, were observed and discussed.