Microstructure-gradient approach for effective determination of post-heat treatment temperature of an additive manufactured Ti-6Al-4V sample

In this paper, a Ti-6Al-4V wall was fabricated using wire and arc additive manufacturing to be as-deposited (AD) coupons for investigating the effect of post-heat treatments on microstructure and mechanical properties. A Gleeble thermal-mechanical simulator was used to create a microstructure gradient in the sample with different heat treatment temperatures to efficiently determine the optimal post-treatment temperature, which is followed by furnace heat treatments to find proper aging (Age) conditions. Solid solution at 830 ℃ for 2 h followed by water cooling + aging at 500 ℃ for 4 h followed by furnace cooling was considered to be the optimal post-heat treatment candidate because it gained a 12.85% improvement of yield strength (YS) and 3.33% improvement of the ultimate tensile strength (UTS), though tensile fracture elongation (EL) decreased 3.37% compared to the as-deposited samples. With this condition, microstructure evolution including β grains, α widmanstätten, grain boundary α (GB α), and α’ martensite was characterized at the same position of the sample experienced deposition (AD), solid-solution treatment (AD-ST), and aging (AD-ST-Age) respectively. The relationship between such step-by-step microstructure evolution with tensile strength as well as hardness change was analyzed and established. In the end, brittle fracture surfaces of AD, AD-ST, and AD-ST-Age samples were observed to find the factors that contribute to failure.