Quantitative analysis on the anisotropic behaviour of superplastic deformation in laser melted (LM) Ti–6Al–4V alloy

In this study, the effects of anisotropy on the superplastic deformation mechanisms in direct metal laser melted (LM) Ti–6Al–4V alloys were investigated. An analysis is presented based on inelastic deformation theory, which consists of grain matrix deformation (GMD) and phase/grain boundary sliding (P/GBS). Flow stress–strain rate curves of the alloy were obtained by the load relaxation test and the step strain rate test at 775, 825, 875 and 925 °C. The results showed that GMD and P/GBS in tensile tests, with either transverse, parallel or 45° inclined microstructures with respect to the tensile axis and the additive layer build (ALM) direction, were well described at 875 °C by the plastic deformation and viscous flow equation of the theory. With change in the microstructural directionality, the flow stress curves for GMD significantly deviate in comparison to those for P/GBS. The mode of GMD, with respect to anisotropic microstructural build direction, agrees well with the extreme iso-stress model. Excellent elongations, over 1000%, were achieved in spite of relatively modest measured strain rate sensitivity values, m, in the range 0.35–0.45, due to excellent deformation accommodation in β phase.