Critical assessment of the defect tolerance of beta-metastable titanium alloys processed by additive manufacturing compared to cast alloys

Titanium alloys constitute a very attractive choice for aerospace, aeronautics and biomedical applications mainly due to their excellent strength-to-weight ratio. Classical alloys, such as Ti-6Al-4V, exhibit a high strength but a low hardening capacity, ductility and fracture toughness. Many efforts have been made to develop new beta-metastable titanium alloys, in which the simultaneous activation of Transformation-Induced Plasticity (TRIP) and Twinning-Induced Plasticity (TWIP) effects increases significantly the hardening capacity and the resistance to plastic localization. Several studies have demonstrated that Ti-12wt%Mo presents a uniform deformation 3 to 4 times larger than classical Ti-6Al-4V alloy and a true fracture strain twice higher than the one of other classical titanium alloys. Moreover, this alloy exhibits an extraordinary resistance to damage as well as a unique fracture mechanism through shear bands instead of the common ductile fracture, dictated by nucleation-growth-coalescence of cavities. On the other hand, additive manufacturing such as L-PBF is commonly used to elaborate complex geometries to reduce even more the weight of the designed parts. However, defects such as porosity are almost inevitable during these processes causing a considerable decrease in the mechanical properties. Recent work has demonstrated that 3D printed Ti-12wt%Mo maintains its improved mechanical properties even when defects are present from the beginning of straining due to a large resistance to coalescence. In this study, additively manufactured samples as well as cast-and-wrought samples were considered to assess the influence on the mechanical response of Ti-12wt%Mo. This study deals with the influence of the process and the resulting microstructure on the mechanism of localization of deformation and the subsequent fracture mechanisms. The L-PBF process parameters have been tuned to also assess the influence of density on the mechanical behavior, and determine the defect tolerance of Ti-12wt%Mo.