Influence of Mg Content on the Microstructure and Solid Solution Chemistry of Al-7%Si-Mg Casting Alloys During Solution Treatment

The solution treatment stage of the T6 heat-treatment of Al-7%Si-Mg foundry alloys influences microstructural features such as MgSi dissolution, and eutectic silicon spheroidisation and coarsening. Microstructural and microanalytical studies have been conducted across a range of Sr-modified Al-7%Si alloys, with an Fe content of 0.12% and Mg contents ranging from 0.3-0.7wt%. Qualitative and quantitative metallography have shown that, in addition to the above changes, solution treatment also results in changes to the relative proportions of iron-containing intermetallic particles and that these changes are composition-dependent. While solution treatment causes a substantial transformation of π phase to β phase in low Mg alloys (0.3-0.4%), this change is not readily apparent at higher Mg levels (0.6-0.7%). The π to β transformation is accompanied by a release of Mg into the aluminum matrix over and above that which arises from the rapid dissolution of MgSi. Since the level of matrix Mg retained after quenching controls an alloy's subsequent precipitation hardening response, a proper understanding of this phase transformation is crucial if tensile properties are to be maximised.