Morphology of Si Phase in Al-Mg-Si-Cu Alloys with Excess Si Addition

The morphology of Si phase and its growth manner in the Al-Mg-Si-Cu alloys with amounts of excess silicon were investigated using by a combination of the higher magnification microstructure and DSC measurements. Solidification characteristics of the alloys were predicted by thermodynamic calculation and compared to the experimental results. It was found that addition of higher amount of excess silicon led to the formation of the evidently morphological Si phase, especially when the silicon content was beyond 1.35 wt.%. The Si phase was one of the dominant phases in the alloys and its reaction peak was identified with the onset temperature of 550.43oC in the DSC curves. These experimental results were in good agreement with the thermodynamic calculations by the Gulliver-Scheil model. Keywords: Al-Mg-Si-Cu alloy; morphology; thermodynamic calculation; excess Si