Fatigue crack growth behavior of an additively manufactured titanium alloy: Effects of spatial and crystallographic orientations of α lamellae

Laser-based directed energy deposition (LDED) enables the rapid near-net-shape fabrication of large-scale titanium components for aerospace applications. However, the fatigue failure of the LDED-produced titanium alloys hinders their widespread use in critical load-bearing structures subjected to cyclic loading. Here, we investigate the fatigue crack growth behavior of LDED-produced Ti-6Al-2Zr-Mo-V alloy in both as-deposited and heat-treated states. The two states of samples consist of α and β laths with the majority of α laths showing distinct crystallographic and spatial orientations. They show distinct cracking behaviors: (i) Fatigue cracks with lengths lower than ∼400 µm grow along α/β boundaries and are surrounded by limited plastic deformations in the as-deposited sample, while in the heat-treated sample, they grow along basal or prismatic planes of the α lamellae associated with substantial plastic deformations; (ii) Fatigue cracks with lengths over ∼400 µm are primarily deflected by α/β boundaries in the as-deposited sample, but are retarded by severe plastic deformations in the heat-treated sample; (iii) Secondary cracks appear along the α/β boundaries in the as-deposited sample, whereas in the heat-treated sample they initiate in the interior and preferentially along the prismatic plane of the α lamellae due to localized shearing and plastic deformation. The fatigue crack growth behaviors are strongly correlated with orientations (both crystallographic and spatial ones) and the size of the α lamellae in the titanium samples. These findings highlight the significance of simultaneously tuning the spatial orientations of α lamellae and their crystallographic orientations to enhance the fatigue crack growth resistance of LDED-produced titanium alloys.