Modelling the effects of grain-size gradients on necking in superplastic forming

A step-bar model is developed and used to investigate the effects of grain-size gradients, geometrical irregularities and deformation rates on the necking of Ti–6Al–4V at 927 °C. A necking map, which indicates the necking mode for a range of deformation rates under various initial cross-sectional area and grain-size combinations, is obtained. Finite-element simulations of a superplastic gas-blow forming process are carried out to investigate the effects of initial grain-size distributions on the non-uniform thinning of a formed structural component. The effects of grain sizes on the gas-pressure history required to maintain a forming rate is also studied.