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 degrees C/4 h/air cooling (AC). The alpha laths were coarsened after heat treatment 850 degrees C/2 h/AC, and the higher annealing temperature contributed to the appearance of alpha(s). After solution and aging treatment, there were the discontinuous alpha(GB), coarsened alpha(p), and fine alpha(s). There was the small Widmanstatten structural alpha s sharing the uniform crystallographic orientation after heat treatment 930 degrees C/1 h/AC + 550 degrees C/4 h/AC. The alpha' martensite and extremely fine dispersed granular alpha(s) were obtained after heat treatment 930 degrees C/1 h/water quenching (WQ) + 550 degrees C/4 h/AC. The heat treatment 930 degrees C/1 h/WQ + 800 degrees C/2 h/AC was found to be the best heat treatment in this study. The discontinuous alpha(GB), dispersed alpha(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 alpha/beta interface phase distribution in the as-deposited and heat treated specimens was concerned. Two break-up mechanisms of alpha(p), including boundary splitting and termination migration, were observed and discussed.