Solidification and precipitation behaviour of Al-Si-Mg casting alloys

The effect of Mg content on the solidification and precipitation behaviour of both unmodified and Sr-modified Al-7Si-Mg casting alloys has been investigated at various solidification rates using cooling curve analysis, differential scanning calorimetry (DSC) and optical and electron microscopy. The Mg concentrations covered the range from 0.3 wt% to 0.7 wt%. The results indicate that increasing Mg content or cooling rate lowers the liquidus and binary Al-Si eutectic transformation temperatures. The latent heat of fusion of these alloys is strongly dependent on the level of Si present, but there is no observed dependence on Mg content. The solidification reactions observed under DSC are identified and it is noticed that the ternary eutectic solidification reaction L Al + Si + Mg₂Si is only observed at Mg levels of 0.6% and higher. The minor phases formed on solidification are identified and their response to solution heat treatment is examined. Increasing Mg content usually enhances precipitate hardening. However when Mg levels are increased above 0.5wt%, no apparent increase of yield strength with Mg is observed. This is correlated with dissolved Mg levels and energy released during reprecipitation.