Advanced finite element modeling and stability analysis of the Superplastic Forming process.

摘要

Metal forming industries are constantly looking for ways to increase their productivity and competitiveness by advancing innovative, economical, energy-efficient, and environmentally friendly metal forming techniques. Superplastic Forming (SPF) has a great potential to he one of these exciting advanced forming techniques. SPF is a near net shape forming process used with superplastic materials, a unique class of metals that has the ability to undergo extraordinary large tensile ductility. SPF offers many advantages over conventional forming operations including weight reduction, greater design flexibility, and the ability to shape hard metals and form complex shapes. However, low production rate, non-uniformity of the formed part and limited predictive capabilities of deformation due to lack of accurate constitutive and failure models are among the main obstacles hindering the widespread use of SPF.; Although the available analytical and numerical studies on SPF employ different optimization schemes, however, they do not account for a number of important features, leading to the current limited predictive capabilities. These features include anisotropy, microstructural evolution, multiaxiality and multiscale failure mechanisms. These issues need be addressed in a comprehensive approach which integrates the mechanics, materials and manufacturing aspects of superplastic forming process.; This research aims to advance the state of the art of SPF by proposing new and creative techniques that help in optimizing the process and overcoming its disadvantages. To study the effectiveness of the new proposed techniques in a cost-effective manner, the Finite Element method (FE) is used. User defined subroutines are compiled to customize the code and implement the material behavior through microstructure-based constitutive laws that are based on large viscoplastic deformation into a FE code. A nonlinear multiscale failure criterion that accounts for geometrical necking and microstructural aspects is utilized to devise variable strain rate forming paths that are able to reduce the forming time without sacrificing the integrity of the formed part. In addition, the effects of back pressure and initial grain size gradient in the formed sheet on the SPF process are studied in this work.; Keywords: Superplastic Forming, Finite Element Modeling, Stability Analysis, Back Pressure, Grain size.