Multiscale - Based optimization of superplastic forming

The stability of superplastic deformation under biaxial plane stress condition is - investigated. A new multiscale stability criterion is developed that accounts for both geometrical instabilities and microstructural features including grain growth and cavitation. This is achieved by combining a modified microstructure-based constitutive equation for superplastic deformation (SPD) with grain and cavitation evolution equations. These equations are then incorporated into the framework of Hart's stability analysis. The proposed criterion is then used to design optimum forming profiles based on variable strain rate loading paths. The effects of cavitation and biaxiality ratio on the stability of deformation are also investigated. The results clearly show the importance of accounting for both geometrical and microstructural features into the stability analysis. At the early stages of deformation, stability is dominated by geometrical considerations and as deformation continues, the effects of cavitation become more dominant.