On the nature of variant selection along build direction in additively manufactured Ti-6Al-4V walls

Microstructures in additively manufactured (AM) Ti-6Al-4V components result from a complex interplay between several competing factors such as thermal gradients, cooling rates, and repeated thermal cycles. As a result, microstructures vary significantly along the build direction. While several studies reported the variations in microstructure along the build direction in terms of grain size, morphology, and crystallographic texture, the changes in variant selection (VS), resulting from the phase transformation, along the build direction have not been reported. In this work, we report on the VS along the build direction in two slender walls of Ti-6Al-4V produced using Laser-Powder Bed Fusion (L-PBF) and Wire Laser-Direct Energy Deposition (L-DED) processes. We show that while both L-DED and L-PBF samples exhibited a decline in VS throughout the build, the underlying mechanisms responsible are different. We analyzed the variant clusters in detail and observed that the type and resulting intervariant boundaries depend on the process. L-DED predominantly favored Type II (60°/⟨11¯20⟩) intervariant boundaries, along with Cluster 1 variants, whereas L-PBF samples exhibited a preference for Type IV (63.26°/⟨10 5 5 ¯3⟩) boundaries and Cluster 2 variants. Furthermore, it is shown that the presence of primarily facilitated massive transformation (α_m) in L-DED samples has a significant role in variant selection.