Optimisation of the superplastic forming of aluminium alloys

Superplastic forming involves the shaping of metal sheets by gas pressure at elevated temperatures. It relies on the fact that fine-grained metals can exhibit high sensitivities of flow stress to strain rate, but this usually only occurs at quite slow strain rates. As a result, the process is much slower than conventional pressing operations and this is a major factor in process costs. Although there have been several attempts at optimising the process to minimise the forming time, most do not achieve a true optimum. In the present work, the non-linear system is optimised within the constraints of product thickness and damage (cavitation) level by searching a control space of enclosed volume history. The finite element method is used in that process, and good constitutive models and damage evolution models are required. These models are described, along with the optimisation strategy, and results are given for the forming of commercial aluminium alloys.