Effect of heat input on microstructure and mechanical property of wire-arc additive manufactured Ti-6Al-4V alloy

Wire-arc additive manufacturing (WAAM) combines an electric arc as a heat source and a wire as feedstock to build a layer-by-layer component. In arc welding, heat input is an important characteristic because it influences the cooling rate, which can affect the mechanical properties and microstructure of the deposited material. We investigated the effect of heat input on the microstructure and mechanical properties of WAAM deposited Ti-6Al-4V alloys. A high heat input (106 J/m, specimen H) produced a columnar grain exhibiting a large anisotropic tensile strength. However, a low heat input (5 × 105 J/m, specimen L) transformed the columnar grains to equiaxed grains owing to the accelerated solidification rates. The thermal history of the WAAM deposits was simulated using the finite-element method. The faster cooling rates in the solidification range (1600–1660 °C) in specimen L resulted in a larger fraction of the equiaxed grains and significant reduction of tensile strength anisotropy. Meanwhile, due to more thermal cycles and rapid cooling rates during the secondary-α transformation below the β-transus temperature (700–1006 °C), specimen H produced larger amounts of nitrogen, α′ martensite and fine-secondary α with a tangled dislocation, thereby exhibiting a larger tensile strength and hardness than specimen L.