oa Modeling and characterization of deformation in magnesium alloys for energy efficiency

The need to produce vehicles with improved fuel efficiency and reduced emissions has led the automotive industry to consider the use of "lightweighting" materials in the construction of automotive body and chassis systems. Towards this end, interest has been increasingly focusing on the use of sheet magnesium in the manufacture of panels and structural components by utilizing recent advances in twin-roll casting technology of magnesium. However, challenges in the areas of manufacturing, material processing and modeling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to its hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying the microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming and warm forming along with sheet manufacturing techniques such as Twin Roll Casting (TRC). In this work we investigate the superplastic behavior of twin-roll cast AZ31B through mechanical testing, microstructure characterization and modeling. The experimental results show that the as-received AZ31B sheet consists of relatively large gains, and while it is brittle at low temperature it is ductile at high temperature with activation energies of the deformation close to that of lattice diffusion. The material analyzed possesses medium strain-rate sensitivity of 0.27 with a relatively modest ductility of about 74%. Based on the experimental results, we use a physically-based constitutive model for deformation. The model integrates the main microstructural features, grain shape and grain orientations, within a self-consistent viscoplasticity theory with internal variables. Simulation of the deformation process at room temperature shows activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning. The predicted texture from the model is consistent with the experimental results. With the use of the appropriate model parameters, the stress strain relationship can be described accurately over the range of low strain rates.