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

摘要

The solution treatment stage of the T6 heat-treatment of Al-7 percent Si-Mg foundry alloys influences microstructural features such as Mg_2Si dissolution, and eutectic silicon spheroidisation and coarsening. Microstructural and microanalytical studies have been conducted across a range of Sr-modified Al-7 percent Si alloys, with an Fe content of 0.12 percent and Mg contents ranging from 0.3-0.7wt percent. 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 pi phase to beta phase in low Mg alloys (0.3-0.4 percent), this change is not readily apparent at higher Mg levels (0.6-0.7 percent). The pi to beta transformation is accompanied by a release of Mg into the aluminum matrix over and above that which arises from the rapid dissolution of Mg_2Si. 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.