Forging specimen design for Mg alloys

Abstract

The automotive industry is seeking every possible way to reduce the vehicle weight in order to achieve better fuel efficiency. Forged magnesium (Mg) alloys, due to low density and high stiffness-to-weight ratio, have potential to replace the current structural materials. Forging of Mg alloys is not new, however, forging process finite element simulation and design guidelines are needed to help design the final product. Three materials in two conditions were under investigation – extruded/cast AZ31, extruded/cast AZ80, and extruded/cast ZK60. Simple uniaxial compression tests were conducted in longitudinal and transverse directions of the cylindrical billets to examine their forging behaviour. By comparing the experimental results to an isotropic simulation result, it was observed that anisotropy was pronounced at forging temperature and could not be ignored. An anisotropic material model in finite element simulation was necessary to capture material’s real-life behavior. Hill’s anisotropic material model in DEFORM 3D was therefore selected for this design project. Hill’s material model in DEFORM 3D not only required material’s rate-sensitive flow stress data, but also 6 anisotropic coefficients that need to be generated from compressive yield stresses and shear yield stresses. Shear hat tests were conducted for extruded materials in order to produce these anisotropic coefficients. Complete material models were created for all extruded materials at their own optimal forging conditions. A small-scale forging sample designed for the 110-ton press was necessary to validate the material model and simulation results. The first design iteration was based on existing axisymmetric design in literature. The second attempt was proposed based on a rib-web geometry forging specimen. The final design improved the second one to accommodate for more test samples. Both the geometric and load comparisons between simulation and experiment were promising. The model was then used to design a simplified forging sample to test material forgeability and access forging process parameters. The simplified forging sample was intended to fit in between lab-scale specimens and the final component. A few trial geometries were proposed and evaluated based on material flow and die complexity. A symmetric I-beam geometry with distinct rib features was selected as the final design. Simulations of all extruded materials were carried out to compare with the experiments. The material model served its purpose but can still be improved based on the geometric comparison.