Variant-orientation-dependent slip activation and strain localization in electron beam wire-fed welded Ti-6Al-4V alloy

Electron beam wire-fed welding (EBW-WF) efficiently joins thick Ti-6Al-4V, but the fusion zone (FZ) exhibits pronounced plastic strain heterogeneity and premature damage, whose crystallographic origins remain insufficiently understood. Although α variants are common, the role of variant orientation anisotropy in slip activation and strain localization remains unclear. This work systematically investigates the FZ microstructure and plastic deformation, emphasizing α variant selection, slip activation, and damage evolution. The FZ consists predominantly of α laths with minor retained β phase. Rapid solidification and the subsequent β→α transformation lead to pronounced variant selection dominated by Type-II (60^∘/[112¯0]) and Type-IV (63.26^∘/[10‾553¯]) α/α boundaries, together with triangular variant clusters that minimize transformation-induced shear and dilatational strains. The angle θ between the c-axis of α variants and the loading direction governs slip activation. Variants of the same type exhibit similar θ values and consistent slip behavior: basal, prismatic, and pyramidal 〈a〉 slip systems exhibit high activation tendencies at θ = 22–68°, 50–90°, and 44–90°, respectively, with co-activation at θ = 50–68°; basal slip dominates at θ < 58°, while prismatic and pyramidal slip become more active at higher θ . Additionally, lattice rotation and slip transfer also serve as complementary mechanisms to accommodate strain heterogeneity. Strain localization originates from micro-shear bands (MSBs) growth in α variants with large θ values, particularly under strong orientation mismatch with neighbors and with lath long axes aligned with the maximum shear direction. Prismatic < a > slip dominates, promoting slip localization and MSBs formation. Orientation-dependent slip, elastic modulus mismatch, and interfacial constraints further intensify local strain heterogeneity, making MSB-rich regions preferential sites for crack initiation. These findings establish a variant-scale mechanistic framework for EBW-WF Ti-6Al-4V joints, highlighting a systematic correlation between α-variant orientation ( θ ), slip activation, and MSB formation in the FZ, therby providing guidance for damage control and plasticity optimization.