Effect of annealing on microstructure and mechanical properties of Ti17 titanium alloy repaired by wire laser direct energy deposition

Ti17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) titanium alloy, renowned for its outstanding high-temperature strength and fatigue resistance, has become a staple in the production of aero-engine blisks. The repair of damaged Ti17 blisk using direct energy deposition (DED) offers considerable potential for extending service life. In this study, wire laser direct energy deposition (W-LDED) was employed to fabricate a Ti17 repaired specimen with a modified Ti17 wire. The effect of annealing heat treatment on microstructure and tensile properties was investigated. With an increase in annealing temperature from 600 °C to 660 °C, the primary α_P phase underwent coarsening and dissolution, leading to a steady decline in its volume fraction. The secondary α_S phase showed precipitation, followed by coarsening. The grain boundary α_GB phase underwent continuous coarsening over the entire temperature range. These microstructural transformations had a profound effect on mechanical behavior. Tensile stress steadily declined as the annealing temperature increased. In contrast, tensile strain exhibited a complex, non-monotonic pattern, peaking at 8.25% at 615 °C. This behavior is attributed to a fracture mechanism transition governed by microstructural evolution. Specifically, α_GB coarsening weakened boundary strength, while the density and morphology of intragranular α phases controlled interior strengthening through interface hardening. This delicate balance between boundary and interior strength dictated strain distribution homogeneity and crack propagation under tensile load. At lower temperatures, strong interiors and weakened boundaries favored intergranular cracking. Near 615 °C, a transient equilibrium enabled uniform deformation and peak ductility. At higher temperatures, excessively weakened boundaries triggered premature intergranular failure and a concurrent decline in both stress and strain. Higher temperatures excessively weakened boundaries, triggering premature intergranular failure and a decline in both strength and ductility. Thus, annealing temperature plays a pivotal role in guiding crack propagation paths through microstructural manipulation, ultimately governing the material's performance. This study provides valuable insights into the intricate relationship between processing parameters, microstructure, and mechanical properties, offering a roadmap for optimizing heat treatment protocols in blisk repair applications using W-LDED.