Balance of strength and plasticity of additive manufactured Ti-6Al-4V alloy by forming TiB whiskers with cyclic gradient distribution

Abstract

Elemental alloying has been demonstrated to be an effective method to improve the strength of additively manufactured Ti-6Al-4V alloys by inhibiting the formation of coarse columnar primary β grains. However, it is still a challenge to obtain a high-strength titanium alloy with high plasticity. To balance the strength and plasticity, TiB whiskers with cyclic gradient distribution were synthesized in situ as the reinforcement phase in Ti-6Al-4V alloy fabricated by wire arc additive manufacturing (WAAM) based on cold metal transfer (CMT) technology. The ultimate tensile strength of the deposited Ti-6Al-4V alloy with 0.05 wt% boron addition reached 1089 MPa, which was 17% higher than that of directly deposited Ti-6Al-4V alloy, and the elongation remained at 8% without significant reduction. The synergistic balance of strength and ductility was attributed to the grain refinement and cyclic gradient distribution of TiB whiskers. When the boron content was 0.15 wt%, the elongation of the deposited alloy was reduced to 5.2% though tensile strength reached 1178 MPa due to the formation of quasi-continuous TiB whiskers distributed along the prior β grain boundaries. Furthermore, the formation of heat-affected bands between the two adjacent layers of TiB-reinforced Ti-6Al-4V alloy was discussed based on the measurement of the thermal history during the additive manufacturing process and the part’s microstructure evolution.