Influence of processing variables and aluminum content on the microstructure and mechanical properties of cast Mg-Al alloys.

Abstract

Using statistical design of experiments, a variety of industrially relevant Mg-Al microstructures were developed and characterized by investigating casting variables such as; Al content, solidification rate, grain refinement, gas content, HIPing, and post-casting thermal treatment. The microstructures were microscopically characterized and the statistical results not only exemplified the power of experimental design but also generated a wealth of knowledge concerning individual factors and their interactions. A significant contribution towards this goal was the measurement of cast Mg-Al solidification parameters via thermal analysis which were then related to the resultant microstructure. A statistical model was developed for the liquidus and solidus temperatures as a function of the solidification rate and Al content. The thermal analysis data was used in an existing solidification modeling code. Statistical analysis of the experiments led to the determination of the variables that are significant with respect to tensile and impact testing. These included Al content, HIPing, solidification rate, grain refinement and hydrogen gas level in descending significance. Furthermore, differences in the test response as a result of testing conditions such as strain rate were highlighted. Additional experimentation was used to construct an outline for the development of strength and ductility in Mg-Al alloys. The single most significant factor contributing to the variability in the tensile and impact data was the Al content. Increasing the Al content increased the strength while reducing the total amount of deformation possible, and reduced the amount of planar cleavage fracture. HIPing and solid solution heat treatment were shown to reduce the strength although a significant increase of elongation to failure is achievable after only 2.2 hours whereas further heat treatment failed to produce any significant additional increase. This exhibits a window of opportunity to reduce solution heat treat times while maintaining optimal properties. The effect of strain rate appears to be secondary compared to the effect of porosity level on the die cast mechanical properties. This fact indicates that development of a reduced-porosity high volume die casting process would greatly expand the envelope within which to design Mg-Al components.