Material model calibration for superplastic forming

Abstract

Some metals, such as Ti-6Al-4V, have a high elongation to failure when strained at certain strain rates and temperatures. This superplastic property can be used to form thin, geometrically complex components. Superplastic forming is however a slow process. The forming time can be minimised by optimising the pressure profile applied to the forming sheet. The optimisation of the superplastic forming pressure is usually done such that a target strain rate at a high strain rate sensitivity is maintained. Careful consideration of the strain rate is required, since localised thinning can occur when the material is strained too quickly. This paper demonstrates that it is essential to explicitly include strain rate sensitivity data, obtained from strain rate jump tests, during the calibration of material model used for superplastic forming simulations. Conventional calibration methods only consider stress-strain data at different strain rates. Such an approach implicitly assumes that a material model that matches the stress-strain data at the different strain rates, will automatically match strain rate sensitivity data. However, by explicitly including the strain rate sensitivity data, the selected material model is more susceptible to localised thinning as the applied strain rate is increased. It is essential for the selected material model to exhibit this behaviour to prevent superplastic forming simulations at high strain rates from predicting stable deformation, when in fact localized thinning will occur.