Grain boundary discontinuity and performance improvement mechanism of wire arc additive manufactured Ti–6Al–4V

Most recent studies for improving tensile properties of wire arc additive manufactured Ti–6Al–4V focused on layer-by-layer plastic deformation but with complicated operation. Previous study demonstrated that a sub-transus solution increased the strength and decrease the anisotropic ductility by generating a new micro-lamellae recrystallization to discontinue α grain boundaries (αGB). The current study investigated the effects of near β transus solution and aging treatment on the microstructure and tensile properties. Both α lamellae recrystallization and α' martensite decomposition were observed, which induced a fine lamellar (α + β) structure and a discontinuous αGB. These changes increased the yield strength (YS) up to 904 MPa while maintaining a high elongation above 15.4%. According to the data, the Hall-petch relationship between the YS and α lamellae width was established, where the quadratic relationship between the elongation and α lamellae width was found. The anisotropic ductility disappeared when the discontinuous ratio of αGB was higher than 0.6 because the discontinuous αGB changed the failure mode of horizontal sample from opening to sliding modes similarly with that of vertical sample. The results would provide a pathway to improve the tensile properties of additive manufactured Ti–6Al–4V without complicated layer-by-layer plastic deformation.