Effect of microstructural heterogeneity and grain morphology on the annealing behavior of additively manufactured metastable β titanium alloy

Titanium alloys fabricated by additive manufacturing usually exhibit significant heterogeneity in grain size and morphology. Recrystallization treatment is an effective approach to alter the microstructure and achieve a more homogeneous structure. In this study, recrystallization and grain growth behaviours of a metastable β titanium alloy fabricated by laser powder bed fusion (LPBF) and direct energy deposition (DED) were investigated. The LPBF sample developed a chessboard-like microstructure, with considerable variations in grain size and morphology across molten pools. Recrystallization started from the boundary of these pools, where grains were finer and the stored energy was higher. The DED sample had a heterogeneous structure with alternating layers of equiaxed grains and columnar grains oriented with <100> along the building direction. Results obtained from in-situ EBSD demonstrated that grain growth occurred preferentially in regions with fine equiaxed grains, and regions with columnar grains showed limited grain growth due to the relatively lower driving force and mobility.