Fatigue crack growth behavior of an additively manufactured titanium alloy: Effects of spatial and crystallographic orientations of 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 alpha and beta laths with the majority of alpha laths showing distinct crystallographic and spatial orientations. They show distinct cracking behaviors: (i) Fatigue cracks with lengths lower than-400 mu m grow along alpha/beta 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 alpha lamellae associated with substantial plastic deformations; (ii) Fatigue cracks with lengths over-400 mu m are primarily deflected by alpha/beta boundaries in the as-deposited sample, but are retarded by severe plastic deformations in the heat-treated sample; (iii) Secondary cracks appear along the alpha/beta 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 alpha 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 alpha lamellae in the titanium samples. These findings highlight the significance of simultaneously tuning the spatial orientations of alpha lamellae and their crystallographic orientations to enhance the fatigue crack growth resistance of LDED-produced titanium alloys.