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

Abstract

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 α′ 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 α′ 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 α′ phase are transformed into β 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 α′ phase is supersaturated and experiences pseudospinodal decomposition 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.