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.