Effect of heat treatment on adiabatic shear susceptibility of Ti–6Al–4V titanium alloy manufactured by selective electron beam melting

Abstract

Ti–6Al–4V alloy manufactured by selective electron beam melting (SEBM) was heat treated with solution treatment at 1020°C for 2 h and aging at 500°C for 8 h, and the as-printed and heat-treated samples were shock loaded with split Hopkinson pressure bar (SHPB) at different strain rate (1700 s-1and 2700 s-1) and loading directions. The effects of microstructure, phase composition and orientation on adiabatic shear behavior were discussed. The results showed that the adiabatic shear behavior of as-printed Ti–6Al–4V was anisotropic, the differences of critical strain for adiabatic shearing between as-printed longitudinal and radial simples are 0.025 at 1700 s-1 and 0.004 at 2700 s-1 respectively; however, the differences of critical strain decrease to 0.002 at 1700 s-1 and 0.001 at 2700 s-1 after heat treatment, which indicate the heat treatment significantly decrease the anisotropy. The as-printed longitudinal samples with the were more prone to adiabatic shearing than the radial samples, due to the great influence of the orientation of α phase cells on adiabatic shear susceptibility. For example, when the c-axis of α phase cells was parallel to the loading direction, i.e., the as-printed longitudinal sample, it was difficult to active the basal{0001} < 11 2‾ 0 > and prismatic{101‾ 0} < 112‾ 0 > slip systems during shearing and only the pyramidal{101‾ 1} < 112‾ 0 > slip system with higher critical resolved shear stress (CRSS) could be activated, which increased the flow stress and enhances the thermal softening effect, and resulting in a higher adiabatic shear susceptibility of the as-printed longitudinal samples. The adiabatic shear susceptibility of the heat-treated samples in the longitudinal and radial directions was similar, because the orientation of the microstructure became weaker after recrystallization. The heat-treated samples were more susceptible to adiabatic shearing, because the lamellar spacing was reduced and the aspect ratio of the lamellar α phase became larger, which resulted in a larger α-phase interface area compared to that of the as-printed sample, causing greater resistance for dislocation moving during deformation, so the flow stress was elevated and the corresponding thermal softening effect was enhanced, which eventually led to an increase in adiabatic shear susceptibility.