Influence of magnesium content on the mechanical properties of cast hypereutectic aluminum-silicon alloys

The current study focuses on the influence of the Mg content on the mechanical properties of cast hypereutectic Al-Si alloys. With increasing the Mg content, the formation of the primary Mg2Si tends to substitute for the primary Si progressively in the modified castings with the properties close to the Si crystals, both of which are believed to be important in Al alloys. Other than the Al-Si alloy system has been fully developed, the Al-Mg 2Si composites however just start to be concerned recently. A new alloy system is thus established based on the conventional cast hypereutectic Al-Si A390 alloy with increasing Mg content. The formation of the primary Mg 2Si is the main characteristic of adding Mg into the alloy A390, tending to substitute for the primary Si progressively. Equilibrium calculations and binary/ ternary/ quaternary phase diagram plots were carried out using the FactSage(TM) software, predicting the Al-Si alloy system alternatives with the Mg additions. The 6 and 10 wt% Mg alloys were found to be representative of the different decomposition behaviours in two transition regions where both primary phases can be precipitated in a particular order of Si to Mg 2Si for the alloy with lower magnesium content and Mg2Si to Si for the second composition. For both alloys, the Mg2Si phase is significant in the aluminum matrix.The T6 heat treatments of these new alloys increased the mechanical properties with respect to their as cast condition but smaller than the increment measured from the alloy A390. One of reasons might be relates to the heat treatments while another could be associated with the high level casting flaws, including the porosities and oxidation products that were encountered when casting these alloys. Further investigation of the microstructures showed that increased Mg additions favour the defect formation and reduce the castability. The tensile fracture analysis indicates most cracks were initiated by these porosities or oxidation products in the castings resulting in brittle fracture. This limits the application of the permanent mould castings for these new alloys even though weight reductions can be achieved 2.6% for the 6 wt% alloy and 4.9% for the 10 wt% alloy when compared to the conventional A390 alloy. However, this further can be improved by introducing die casting or semi-solid forming.Permanent mould castings were produced of these two alloy compositions, and further tested for their hardness and tensile properties in comparison with the conventional A390 alloy. T6 heat treatment and Bi modifications of these two alloy compositions were also tested. The preliminary results denote that their hardness was increased somewhat with Mg additions due to the solid solution strengthening mechanism, while the yield strength and elastic modulus exhibited only a slight difference with respect to A390, whereas ultimate tensile strength was lower for the new alloys. The structure modifications due to Bi additions (0.6~0.8 wt%) to the 6 wt% Mg alloy show an overall improvement of the mechanical properties. It was observed that the snowflake-shaped Bi-bearing phases segregating at the primary Mg2Si solidifying fronts, thus refining Mg2Si crystals.