Towards understanding the role of heat treatment on the deformation micro-mechanisms in wire arc directed energy deposited Ti6Al4V

Cold metal transfer-based wire-arc directed energy deposition presents a promising route for manufacturing large-scale Ti6Al4V engineering components, combining high deposition rates and low costs. While cold metal transfer has been applied for high-deposition-rate additive manufacturing of Ti-alloys, the as-deposited conditions are known to exhibit coarse columnar β-grains, residual stresses, and anisotropic mechanical properties, which limit their structural performance. This study aims to address these limitations through investigating the effects of post-deposition heat treatments on the microstructural evolution and mechanical behaviour of the Ti6Al4V deposits. Three distinct routes were explored, including stress relief at 650 ◦C for 5 h, near-β annealing at 950 ◦C for 4 h, and solution treatment plus ageing at 950 ◦C/1 h followed by 600 ◦C/4 h ageing. Microstructural characterization reveals that near-β annealing disrupts continuous grain boundary α networks, refinesα-laths into globular morphologies, and enhances ductility. In contrast, the solution-treated and aged condition introduces refined martensitic α′ that decomposes into fine secondary α-laths contributing to yield strength by additional α/β interfaces and dislocation barriers. Despite refinement, ductility is retained in solution treated and aged condition due to more homogeneous stress distribution and strain partitioning. Detailed electron backscatter diffraction including Kernel average misorientation and Schmid factor analyses, highlights the role of the α/β morphology, grain boundary α continuity, and variant reorientation in controlling the fracture and deformation behaviour. This work demonstrates that tailored heat treatments can manipulate the microstructure across multiple length scales, enabling the design of wire-arc directed energy deposited Ti6Al4V components with superior and balanced strength-ductility profiles for aerospace and high-performance applications.