Heterogeneous microstructure evolution in Ti-6Al-4V alloy thin-wall components deposited by plasma arc additive manufacturing

Microstructural heterogeneity was observed in titanium alloys deposited by high-energy density beam additive manufacturing (AM) technologies in the as-built condition; these heterogeneities included the presence of coarse prior-beta columnar grains, non-equilibriumlayer bands(LBs), and mixed microstructures. This is particulary prominent with plasma arc AM(PAM), which has a higher heat input when compared to laser beam and electron beam AM technologies. In this study, several Ti-6Al-4V layers were deposited by PAM to investigate the characteristics of the generated microstructural heterogeneity. The results show that epitaxial growth of prior beta columnar grains in the same direction is inhibited by pulsed perturbation, which results in formation of columnar grains with near-equiaxed grains. Horizontal LBs could be observed after depositing six layers. The phase transformation (beta ->alpha) followed the Burgers orientation relationship, resulting in triangular stars or rhombic patterns. Precipitation of secondary alpha nano dispersoids occurred along{10-10} orientation. The LB region of heterogeneity was affected by thermal cycles in the beta-transus and the recrystallisation temperature ranges, which leads to alpha lamellae of the same orientation finally forming coarse alpha colonies, and dispersion strengthening of the alpha lamellae with alpha nano depersoids contributes to the microhardness. (C) 2018 Elsevier Ltd. All rights reserved.