Enhanced comprehensive mechanical properties of laser additive manufactured TC17 by design of new heat treatment based on continuous cooling transition

Heat treatment is critical for enhancing the mechanical properties of high strength titanium alloys, especially for exploiting the potential of laser additive manufactured titanium alloys. In this work, the influence of the cooling rate of continuous cooling transition on microstructure evolution and mechanical behavior was investigated in TC17 titanium alloy fabricated by laser directed energy deposition (LDED) technology. It was found that the number density and orientation characteristics of grain boundary alpha phases (alpha(GB)) are jointly influenced by the cooling rate and the structure of beta/beta grain boundaries (GBs). The average number density (lambda(avg)) of alpha(GB) has a consistent trend with the degree of variant selection (DVS) for precipitated alpha clusters subsequently, which is attributed to the autocatalytic effect of the pre-existing alpha on the post-precipitated alpha phases. The largest lambda(avg) of alpha(GB) and the highest DVS of alpha clusters could be simultaneously obtained at a suitable cooling rate (4 degrees C/min). In that case, plenty of alpha/beta phase interfaces and dominant variant type ensure high strength, meanwhile, the combinations of activated multi-slip systems and varied crack propagation paths extend work-hardening to maintain greater plastic deformation. This paper provides a novel thought for designing customized heat treatments of LDEDed high strength titanium alloys, and more importantly, promotes the engineering applications of large and complex components prepared by additive manufacturing technology.