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

Highlights: • The effect of the grain boundary discontinuity on tensile properties in wire arc additive manufactured titanium alloys was investigated. • The improvement mechanism of tensile properties was elucidated. • A quantitative model has been successfully applied to investigate the anisotropic ductility. -- Abstract: 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 (α_GB). 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 α_GB. 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 α_GB was higher than 0.6 because the discontinuous α_GB 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.