Modeling and optimization of superplastic forming processes

Formability and optimization of superplastic forming (SPF) is investigated. Finite element modeling and simulation of some SPF processes is conducted.; In Chapter one, a general introduction and literature review are presented about biaxial instability analysis, the idea of optimum deformation paths, simulation of SPF, and some implementation issues of finite element modeling of SPF. The objective of this research is summarized at the end of this chapter.; In Chapter two, the superplastic blow forming process of thin sheets is analyzed and an optimal stable deformation path which reduces production time is obtained. The analysis is based on an analytical model for the superplastic forming (SPF) of a long rectangular box made of Ti-6Al-4V alloy at 900{dollar}spcirc{dollar}C, the use of a microstructure-based constitutive equation for the strain rate and grain growth, a stability criterion and a variable strain rate control. It is shown that by imposing a variable strain rate control scheme derived from the stability analysis, an optimal forming time can be developed while maintaining a stable deformation path. Some other control schemes also show effectiveness in either reducing the localized thinning in the formed sheet or reducing the required forming time. Effects of friction and initial grain sizes on the forming pressure profile and the thickness distribution of the formed sheet are also investigated. In Chapter three, the instability of biaxial stretching of thin sheets of viscoplastic metals under plane stress conditions is investigated using a linear stability analysis. An instability criterion for biaxial stretching is developed based on the assumption that localized necking initiates along the direction perpendicular to the major principal stress direction. Various 'optimum' variable strain rate paths, which ensure a stable deformation of the sheet without neck formation, are computed for different strain ratios based on the instability analysis. The variable strain rate paths are applied in the finite element modeling of the superplastic uniaxial extension of s tabular specimen and superplastic blow-forming of a hemisphere. A reduction of forming time is achieved compared with the established constant strain rate forming method, while uniformities in the thickness distribution of the formed parts are maintained.; In Chapter four, finite element simulation of superplastic blow forming processes is conducted using MARC/MENTAT and Pro/ENGINEER. The optimum deformation path is applied to the SPF of a rectangular box die. The predicted pressure-time cycles of different pressurization schemes are compared. The geometric effect on the predicted pressure-time profiles is investigated.