Nb addition and post-processing effects on microstructure and mechanical behavior of L-DED Ti–6Al–4V

• Minor niobium addition during laser deposition enables microstructural control for improved strength–ductility balance in titanium alloy components. • Post-processing routes selectively transform martensite and defect populations, producing distinct strength and ductility trade-offs. • Aging treatments promote nanoscale precipitation for strengthening, while high-pressure heat treatment enhances ductility through pore elimination. • The transition from pore-driven fracture to uniform microvoid coalescence reveals a clear processing–microstructure–property relationship. The increasing demand for additively manufactured Ti–6Al–4V components with an improved strength–ductility balance necessitates effective alloying and post-processing strategies. This study investigates in-situ alloying with 1.0 wt% Nb in laser-directed energy deposition (L-DED) Ti–6Al–4V and evaluates the effects of direct aging, hot isostatic pressing (HIP), and solution treatment followed by aging on microstructure, fracture behavior, and tensile properties. The as-deposited alloy exhibits a fine acicular α′ martensitic structure with processing-induced defects, resulting in mixed brittle–ductile fracture and moderate mechanical performance. Direct aging promotes α′ tempering and nanoscale precipitation, leading to increased strength with retained ductility, while HIP effectively eliminates porosity and enhances elongation at the expense of strength. Solution treatment with aging produces the highest strength with moderate ductility. Fractographic analysis indicates a transition from pore-initiated fracture to homogeneous microvoid coalescence after post-processing. These findings demonstrate that minor Nb addition combined with tailored post-processing provides an effective approach for controlling microstructure–property relationships in L-DED Ti–6Al–4V for structural applications.