Designed experimentation: microstructural optimization of Al AA 512 for the PM process and its possible use as a structural diecasting

摘要

Primary and secondary variants of an Al-Mg{sub}2Si casting alloy were studied to learn how best to control the microstructure in this alloy system to achieve improved mechanical properties under permanent mold (PM) casting conditions. The microstructure formed in this casting alloy system was analyzed using a statistically designed experiment in order to optimize the alloy composition. The microstructure of this alloy was found to consist of primary aluminum dendrites plus a complex interdendritic eutectic that can appear in either of two forms. The eutectic may appear either as a double binary of Al plus Mg{sub}2Si and Al plus Al{sub}12(Fe,Mn){sub}2Si as separate complex regular cells, or as a fine ternary structure of the three phases, depending primarily on the cooling rate and the balance between Fe and Mn in the alloy composition. The ternary form yields much better mechanical properties, particularly as regards ductility. This paper shows that the ternary formation can be controlled through optimization of alloy chemistry rather than extensive chlorine fluxing of the melt. The composition that was found to most reproducibly yield the ternary microstructure was a secondary composition. So this yields an economic and an environmental advantage.