Microstructure and porosity in thixomolded Mg alloys and minimizing adverse effects on formability

The purpose of this research is to investigate possible changes in warm formability of thixomolded Mg alloys by applying severe deformation processing. The microstructure of as-thixomolded magnesium alloys can be complex, as it can contain a variety of constituents such as Mg17Al12 intermetallic particles, shrinkage and gas porosity, as well as internal oxide films and entrapped residues. These inherent heterogeneities affect their fracture properties when these alloys are hot rolled to sheet or formed. It has been found in this research that dissolved gases and heterogeneities can adversely influence warm formability of the molded sheet, and contribute to variability in strength and ductility. Using thixomolded AM60 Mg (Mg-6%Al) plate from a commercial supplier, various heat treatments, hot rolling treatment and friction stir processing treatment were performed to introduce severe plastic deformation processing to alter microstructure. Macrostructure and microstructure resulting after these treatments demonstrated that for a given lot of thixomolded Mg plate, considerable changes in the microstructure, porosity and warm formability can be caused by stirring and severe deformation processing approach. The knowledge gained from this study could benefit thixomolding community, as well as Mg wrought materials applications in regard factors that affect formability of these materials.