Anisotropic Fatigue Performance of Directed Energy Deposited Ti-6al-4v: Effects of Build Orientation

Directed energy deposition (DED), an important branch of additive manufacturing technology, has unique processes that attribute to the fatigue performance anisotropy associated with the build orientation of DED materials. However, the mechanism of fatigue performance anisotropy still remains unclear due to multiple factors of the defect and microstructure, which has limited further application of DED materials. This paper investigates the effects of build orientation on the anisotropic fatigue performance of DED Ti-6Al-4V treated with stress relief annealing, specifically the impact of defects and microstructures on fatigue performance anisotropy. The defects in the fully dense DED Ti-6Al-4V are non-directional porosity defects with smooth morphology and high sphericity; the microstructures are mainly characterized by prior-β columnar grains, α colonies and basketweave structures. To represent fatigue performance anisotropy of the material, standard round bar specimens were sampled from three build orientations, i.e., X, Y and Z directions, where Z-direction represents the build direction. These specimens were subsequently subjected to constant amplitude high cycle fatigue tests at the same loading condition and room temperature. The fatigue test results showed that the logarithmic mean fatigue lives of the X- and Y-direction specimens were close to each other, but significantly longer than those of the Z-direction specimens. Fractography and EBSD tests were conducted to explain the causes of fatigue performance anisotropy, suggesting that microstructures are more likely to explain the anisotropy caused by the build orientation than the porosity defects, with crystallographic orientation and α colony being two critical factors.