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

Abstract

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-β columnar grains, non-equilibrium layer 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 β 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 (β → α) followed the Burgers orientation relationship, resulting in triangular stars or rhombic patterns. Precipitation of secondary α nano dispersoids occurred along{10-10} orientation. The LB region of heterogeneity was affected by thermal cycles in the β-transus and the recrystallisation temperature ranges, which leads to α lamellae of the same orientation finally forming coarse α colonies, and dispersion strengthening of the α lamellae with α nano depersoids contributes to the microhardness.