Microstructural optimization through heat treatment for enhancing the fracture toughness and fatigue crack growth resistance of selective laser melted Ti-6Al-4V alloy

The yield strength (sigma(y)) of Ti-6Al-4V alloy, additively manufactured via selective laser melting (SLM) of powder beds, can exceed 1000 MPa while possessing a mode I fracture toughness (K-Ic) of similar to 50 MPa root m The possibility of enhancing K-Ic as well as fatigue crack growth resistance, without a significant penalty on sigma(y), via a judicious heat treatment process that transforms martensitic alpha', which is present in the as-SLM microstructure due to rapid cooling of the molten metal, into an alpha/beta phase mixture is examined. It was demonstrated that duplex annealing at temperatures below the beta transus temperature of the alloy would lead to such a microstructure while retaining the mesostructure, whose nature depends on the process parameter combinations utilized. Near-doubling of the fracture toughness with only a similar to 20% reduction in sigma(y) was noted upon heat treatment. While the strength becomes isotropic after heat treatment, significant anisotropy in the fracture toughness of the heat-treated alloy with columnar prior beta structure was noted. While the steady state fatigue crack growth (FCG) rates are comparable to corresponding values of the same alloy, but manufactured using conventional means, the threshold for fatigue crack initiation (Delta K-0) increases by 34%-56%. The enhancement in Delta K-0 was found to be a result of the transition in the near-threshold crack growth, from trans-to inter-granular and caused by the alpha/beta basket weave microstructure, which imparts a high crack path tortuosity. Overall, this study demonstrates that post-processing heat-treatment can improve the damage tolerance of SLM Ti64 by increasing both K-Ic and Delta K-0. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.