Enhanced Comprehensive Mechanical Properties of Laser Additive Manufactured Tc17 by Design of New Heat Treatment Based on Continuous Cooling Transition

Abstract

AbstractHeat 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 cooling rate of continuous cooling transition on microstructure evolution and mechanical behavior was investigated at TC17 titanium alloy fabricated by laser directed energy deposition (LDED) technology. It was found that the number density and orientation characteristics of grain boundary α phases (αGB) are jointly influenced by the cooling rate and the structure of β/β grain boundaries (GBs). The average number density (λavg) of αGB has a consistent trend with the degree of variant selection (DVS) for precipitated α clusters subsequently, which is attributed to the autocatalytic effect of the pre-existing α on the post-precipitated α phases. The largest λavg of αGB and the highest DVS of α clusters could be simultaneously obtained in a suitable cooling rate (4℃/min). In that case, plenty of α/β phase interfaces and dominant variants type ensure high strength, meanwhile, the combinations of activated multi-slip systems, weakened precipitate free zone (PFZ), and varied crack propagation ways extend longer work-hardening durations 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.

Keywords: Continuous cooling transition, Microstructure evolution, Strength-plasticity match, TC17 titanium alloy