Influence of non-equilibrium solidification of melt pools and annealing on microstructure formation and mechanical properties of laser powder bed fusion-built Ti-6Al-4V alloys

Microstructure and mechanical properties of laser powder bed fusion (LPBF) built Ti-6Al-4V specimens were investigated. Microstructure of as-built specimens shows a mixture of V-rich laths and V-poor laths of a martensitic & phase. A modulated structure is also observed in microstructure of a specimen built using an energy density of 55 J/mm3, indicating pseudospinodal decomposition of & phase during the LPBF technology. Mechanical tests reveal that this specimen has a smaller elastic modulus and a larger tensile elongation than that of a specimen built using an energy density of 33 J/mm3. After an annealing of the specimens at 1073 K for 2 h, the V-rich laths of & phase are transformed into & beta; phase whereas the V-poor laths remain. The annealing also leads to the formation of a modulated structure in the microstructure of the specimen built using the energy density of 33 J/mm3. In addition, it makes the differences between the mechanical behavior of the specimens insignificant. These findings suggest that & phase is supersaturated and experiences pseudospinodal decompo-sition during the annealing. It is concluded that non-equilibrium solidification of melt pools plays a key role in microstructure formation and therefore the mechanical behavior of LPBF-built Ti-6Al-4V material.