Summary

The principal goal of the metals thrust is to improve the formability and crash resistance of lightweight Al and Mg alloys. Two complementary approaches will be pursued.  First, we will use multi-scale simulations to seek alloy compositions which enhance ductility.  Secondly we will use simulations to guide the development of pre-form annealing processes that will improve formability of existing alloys.  In addition, we plan to expand the scope of the CRL to consider a wider range of metals of interest to GM. To this end, we will begin a project to model the influence of microstructure on ductility of ferritic stainless steels.

 The strength and ductility of lightweight Al and Mg alloys is controlled primarily by the interaction of solutes with dislocations.  Solutes strengthen materials by impeding dislocation motion, but also give rise to undesirable transient material behavior that can lead to instabilities and reduce ductility.  In principle, alloy compositions may be tuned to retain strength, while eliminating undesirable behavior.  During the first phase of the CRL, we have developed a multi-scale model that can predict the influence of composition on flow strength and ductility of Aluminum alloys.  This approach will now be used to search for Al alloy compositions with optimal properties.  In addition, we plan to extend this framework to the more challenging hexagonal Mg alloy system, focusing in particular on understanding the complex dislocation core structures in these materials and their influence on flow behavior.  Simulations will be complemented by TEM studies of the evolution of dislocation and twin structures in Mg.    Once a predictive model is available, it will be used to identify Mg alloy compositions that optimize their ductility.   

  Pre-form annealing was developed by General Motors to form stampings of complex geometry from existing lightweight materials.  In this process, an aluminum or magnesium sheet is formed to an initial, “perform” shape at ambient temperature.  The partially formed panel is then annealed to recover formability. Following this anneal, the panel is formed to a final shape.  This approach has enabled complex parts to be stamped in both Al and Mg alloys. To implement and optimize this process in production, it will be necessary to understand the influence of pre-strain and annealing conditions on the microstructure and constitutive behavior of the alloy after the annealing step. To this end, we plan to adapt microstructure-based simulations developed during the first phase of the CRL to predict the evolution of microstructure and constitutive behavior of Al and Mg alloy sheets during the pre-form annealing process. Predictions will be validated experimentally, using data from ongoing experimental studies at India Science Lab.  The simulations will be used to optimize the pre-form annealing process.

Ferritic stainless steels have the potential to reduce the manufacturing costs of bipolar plates in hydrogen fuel cells. Initial channel forming tests, however, indicate that the ferritic SS sheet is prone to develop localized thinning instabilities during the stamping process. Since the stainless steel sheet is thin, and contains of the order of ten grains through the sheet thickness, anisotropic plastic deformation within individual grains plays an important role in the initiation and growth of localized necking. Preliminary modeling results suggest that, at this scale, the deformation of the sheet is strongly influenced by the arrangement and orientation of the individual grains, which offers a potential approach to improving ductility. To this end, we plan use polycrystal finite element models to predict how sheet thickness and controllable microstructural features such as grain size, shape, and orientation affect the initiation and growth of thinning instabilities during forming of stainless steel sheets.  Following experimental validation, the models will be used to search for microstructures that delay or prevent thinning instabilities.