Improving the compressive properties of laser powder bed fusion Ti-6Al-4V containing oxygen and nitrogen by adding trace TiC+TiB nanoparticles

Abstract

The oxygen and nitrogen interstitial solutes strongly resist the motion of dislocations in titanium alloys, which accounts for the improvement of strength and the loss of ductility. In this study, compressive properties of L-PBF Ti64 with high oxygen and nitrogen content are improved by adding trace nanoparticles (0.1 wt% TiC + TiB) and CALPHAD-based design of heat treatments for the first time. The strength and plasticity of Ti64 increase by 8.8 % and 29.2 % respectively with the addition of nanoparticles before heat treatment. A good combination between strength and plasticity is obtained by solution treatment at 1040 °C and subsequently air-cooling (1040-AC) with the addition of nanoparticles. Compared to the L-PBF Ti64, the plasticity of TiC + TiB/Ti64 increases by 100 % without losing compressive strength after 1040-AC. A bi-modal structure including hard primary α phase and ductile secondary fine α lamellae, was obtained after 1040-AC, which is beneficial for its ductility and work-hardening. The addition of nanoparticles promotes the refinement and uniform distribution of the secondary α lamellae during cooling, which results in high strength and the distribution of α/α boundary being close to the theoretical random frequencies. Therefore, the TiC + TiB/Ti64 after 1040-AC exhibits excellent compressive strength and plasticity.