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, refines α-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 back-scatter 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. • Smart heat treatments enable isotropy and enhanced properties in waDED Ti6Al4V. • Near-β annealing breaks GBα, limits variant selection and enhances ductility. • Triggering of reorientation-induced plasticity by local compositional variation. • Solution and ageing treatment engineer α′→α s promoting significant work hardening.