Microstructure and mechanical properties of Ti-6.5Al-2Zr-Mo-V alloy processed by Laser Powder Bed Fusion and subsequent heat treatments

Abstract

In this work, near-α Ti-6.5Al-2Zr-Mo-V alloys were successfully fabricated through Laser Powder Bed Fusion (LPBF) and subjected to post-heat treatments. A comparative study on the microstructure and mechanical properties of different samples was performed to reveal the effects of LPBF process parameters and post-heat treatments. The optimal LPBF process window for fabricating relative density > 99.9% samples is located in the volumetric energy density (VED) range of 125–167 J/mm3. The extremely rapid cooling rate in LPBF induced a full martensite α′ structure. Substructures characterized by low angle grain boundaries (LAGBs), tangled high angle grain boundaries (HAGBs), and several inter-variants were found in the as-built samples. In the optimal VED range, no significant microstructural difference among the samples was found. This resulted in a stable level of mechanical properties for the optimal as-built samples: 1172–1227 MPa in ultimate tensile strength (UTS), 7.9–8.8% in elongation, ~ 425 Hv in microhardness. After sub-β heat treatments, the LPBF-induced substructures of LAGBs and HAGBs were gradually eliminated, resulting in a decrease in UTS but an increase in elongation. An increase of ~ 57° boundaries was found in the HT700 and HT800 samples, suggesting a boundary transformation during the recrystallization. The transformation is beneficial to the mechanical property. It contributed to a small drop of UTS (~20 MPa) for the HT700 and HT800 samples and significantly improved ductility compared with the as-built samples. The HT900 sample underwent complete recrystallization with an α-lath dominated structure. Thereby it exhibits a better ductility but a lower UTS compared with the HT700 and HT800 samples. After the super-β heat treatments, a large amount of martensite reappeared. Due to the large size of prior β grains, the ductility was significantly decreased. The results of this study can be used to guide the adjustment of the microstructure of LPBF-processed Ti-6.5Al-2Zr-Mo-V alloy through post-heat treatments to obtain a good combination of tensile properties.