Optimization of superplastic forming processes using the finite element method

Superplastic forming is an emerging technology involving the elevated temperature forming of sheet materials capable of achieving forming strains of 200-1000 percent. The exceptional formability afforded by the process permits the manufacture of complex-shaped parts in fewer stages with minimum waste. The design of a superplastic forming process is more difficult that conventional manufacturing operations. From the mechanical point of view, superplastic material is characterized by high strain-rate sensitivity of the flow stress. Successful production of components by superplastic forming, requires a process design that guarantees optimal superplastic conditions. For example, it is essential to control the strain-rate occurring during the forming process. Strain-rates deviating from the superplastic regime can result in necking and rupture. Different studies dealing with superplastic forming, have shown that the numerical prediction of the optimal pressure cycle history needs a long time of computation. In this paper, the analysis of the superplastic sheet-forming process is studied by the use of the finite element code ABAQUS. The pressure controlling algorithm used in this investigation is aimed to obtain in a low computation cost, a practical pressure time history. Two different approaches have been used to simulate the conical-bulging process. They correspond to a 2D model with two dimensional continuum axisymmetric elements, and a 3D model with three dimensional shell elements.