Evolution of Intermetallic Phases in Multicomponent Al-Si Foundry Alloys Containing Different Cu, Mg and Fe Content

In this paper, the influence of Cu, Mg and Fe content on the microstructure evolution of Al-Si based alloys has been studied. Initially, the as-cast microstructure of four Al-Si alloys containing different Cu, Mg and Fe content was studied using differential scanning calorimetry (DSC), optical and electron microscopy. Subsequently, the effect of different solution treatments on the microstructure evolution of the alloys was evaluated. The microstructure evolutions after solution treatment were verified by thermodynamic calculations. The results demonstrated that the dissolution of Q-Al5Cu2Mg8Si6 phase was strongly dependent on Cu and Mg content. That is, in 356 Al alloys containing low Cu content (e.g. 1.5%), the DSC peak corresponding to Q-phase disappeared after a solution treatment of 5 hours at 502C (935.6F). However, in the alloys containing high Cu content (e.g., 3.4%), the peak was still remaining even after 20 hours of solution treatment. In addition, the study also illustrated that in 356 Al alloys containing high Cu and Fe content, longer solution treatment led the dissolved Cu to be gradually lost to the N-Al2Cu2Fe phase.