Elevated-Temperature Tensile Behavior of Additive Friction Stir Deposition-Processed Ti-6Al-4V: Gradient Microstructure and Deformation Mechanisms

To overcome the limited elevated-temperature strength typical of fusion-based additive manufacturing, near-fully dense Ti-6Al-4V specimens were fabricated via solid-state additive friction stir deposition (AFSD). The process induced a pronounced microstructural gradient, with grain sizes ranging from 4.76 μm at the top (dominated by continuous dynamic recrystallization) to 7.62 μm at the bottom (driven by intrinsic thermal cycles). Crucially, the formation of a non-equilibrium substructure—comprising high-density dislocations, stable subgrain boundaries, and deformation twins—contributed to the deformation response through the combined effects of dislocation slip, substructural strengthening, and local strain accommodation. Benefiting from grain refinement and multiple AFSD-induced substructural features, including dislocation accumulation, stable subgrain boundaries, and interface-associated lattice distortion, the as-deposited alloy exhibited superior yield strength compared with the rolled base material at room and intermediate temperatures (400–600 °C). Notably, at 700 °C, the AFSD-processed alloy exhibited a pronounced increase in ductility, with elongation exceeding 140%. This superplastic-like high-temperature response is associated with the fine-grained α/β microstructure and the deformation-induced evolution of a continuous β-phase network, which may enhance interfacial strain accommodation and delay localized failure. Overall, these results suggest that AFSD-induced microstructural and substructural modification provides a promising route for improving the elevated-temperature performance of dual-phase titanium alloys.