Improvement of damage-tolerant properties of Ti-6Al-4V manufactured by DED-LB/W through beta-annealing

Damage tolerance is a key design approach in safety-critical sectors, where structural reliability is ensured by assuming the presence of flaws and enabling preventive inspection strategies. Additive Manufacturing (AM) technologies such as Direct Energy Deposition with Laser Beam and Wire (DED-LB/W) offer promising capabilities for producing Ti-6Al-4V components with damage-tolerant properties, although defect population and microstructural characteristics remain limiting factors. In this work, Ti6Al4V specimens manufactured by DED-LB/W were subjected to beta-annealing and compared with as-built counterparts to assess the influence of heat treatment on microstructure and mechanical behaviour. All batches were produced using optimized parameters to minimize defects, as verified by X-ray Computed Tomography. Mechanical performance was evaluated through hardness, tensile, high cycle fatigue, fatigue crack growth and fracture toughness tests. The beta-annealed condition exhibited improved crack propagation resistance, shifting da/dN curves toward higher ΔK values with higher ΔKth and KIc, consistent with a fully lamellar α+β microstructure characterized by coarsened α-colonies and increased β content. However, this condition led to reduced quasi-static strength and accelerated fatigue crack initiation, highlighting the trade-off between crack initiation and propagation resistance. In contrast, the as-built condition showed better performance for small defect sizes, associated with a finer martensitic α′ microstructure. Directional effects were also observed, indicating anisotropic behaviour governed by microstructure. Overall, the results suggest that, under low defect populations, microstructure plays a predominant role in governing mechanical behaviour. Tailored post-processing is therefore essential to optimize the balance between safe-life and damage-tolerant design approaches, supporting the suitability of DED-LB/W/Ti-6Al-4V for safety-critical applications.