Globularization Mechanism and Near Isotropic Properties in Subcritical Heat-Treated Ti6Al4V Fabricated by Directed Energy Deposition

Microstructure with globular α phase is desirable as it contributes to preferable comprehensive mechanical properties for titanium alloys. However, titanium alloys fabricated by directed energy deposition (DED) are mainly characterized by the lamellar α phase within the basket-weave microstructure, which often leads to severe anisotropy and inferior low cycle fatigue (LCF) properties. To address this, the subcritical annealing and the cyclic annealing were applied to DED Ti–6Al–4V in order to achieve the transformation from the lamellar α phase to the globular α phase. The microstructural characteristics and the globularization behavior of α phase during heat treatment were investigated. The results show that the aspect ratio of α is significantly decreased with the subcritical annealing due to the coarsening of lamellar α. Furthermore, the globular α is obtained with the cyclic annealing as a combination result of the termination dissolution and the side surface growth of the lamellar α. These contribute to a pronounced reduction of 85.4% in the ductility anisotropy, compared with the as-built specimens, and superior comprehensive mechanical properties including LCF are achieved with the formation of globular α.