Improving the Prediction of Part Thickness in Superplastic Forming Process of Ti–6Al–4V Alloy Using an Appropriate Constitutive Equation

Abstract

Superplastic alloys, such as Ti–6Al–4V, are a group of polycrystalline materials which can undergo large elongation in special conditions. By this specific characteristic, superplastic forming can be carried out to manufacture products with complicated shapes. Superplastic forming, in comparison with other similar processes, has a good capability to produce parts with uniform thickness; however, in superplastic blow forming, the final part does not have a uniform thickness distribution, and the prediction of the part thickness has a great significance to the process designers. Numerical methods are used to predict thickness distribution in final part and to improve it by suggesting a precise pressure–time diagram. In this regard, the constitutive equation plays a significant role in predicting the process performance. In the present study, superplastic blow forming has been simulated for a cone-shaped part via using commercial finite element code Abaqus and implementation of a proper constitutive equation by user subroutine UMAT. The constitutive equation considers grain growth hardening effects in addition to strain rate effect. A new method is also proposed to predict pressure–time diagram when UMAT subroutine is used. The effects of the process parameters on this diagram have also been investigated. Comparison between the findings of this research and those conducted by other researchers revealed that considering grain growth hardening has a significant role in the improvement of the accuracy of thickness distribution predictions.