Abstract: Columnar grain structure caused anisotropy in mechanical properties, especially in elongation, is an important concern for Ti-6Al-4 V alloy fabricated by directed energy deposition (DED). Several strategies have been proposed to reduce anisotropy by globularizing the grains, but these conventional approaches are costly and inefficient due to challenges faced during producing the columnar β-grain structures. However, understanding the impact of columnar grain-related microstructures on the anisotropic deformation behavior is still necessary. Despite the recognition of the importance of grain boundary Widmannstätten α colony (α_WGB) as a grain-related microstructure, it has received limited attention in available literature on anisotropy in mechanical properties. This study employed in-situ induction heating during DED to control α_WGB formation, yielding three Ti-6Al-4 V samples with varying α_WGB sizes. Anisotropic deformation of α_WGB and its impact on elongation in build and transverse directions were analyzed. α_WGB width grew from 0.5 µm to 32.4 µm via diffusion-controlled growth due to reduced cooling rate. Transverse deformation led to dislocation movement and accumulation, causing early failure and worsened ductile anisotropy within α_WGB. Notably, larger α_WGB size significantly exacerbated anisotropy in ductility. This work underscores α_WGB's role in anisotropic deformation and offers insights for optimizing mechanical properties in DED-fabricated titanium alloys.

Key words: Additive manufacturing, Directed energy deposition, Anisotropic mechanical properties, Titanium alloy, Microstructure