Solidification Microstructure of Rheocast Hyper-Eutectic Al–18Si Alloy

The effect of semi‐solid processing using a cooling plate on the morphology of primary Si particles for hyper‐eutectic aluminum–silicon alloy was investigated. Semi‐solid slurry was prepared using the cooling plate technique and was cast in sand molding strips with constant width of 25mm and length of 150 mm with the thicknesses of 6 and 25 mm. The optimum conditions to obtain optimum primary Si particle globularity and distribution for present hyper‐eutectic Al‐Si alloy were found at low fraction of solid ranges (fs =0.01 to fs=0.05) using cooling plate technique. Increasing the fraction of solid in the slurry increases the chance of abnormal large primary Si particles, especially, for fraction of solid above 0.05. The presence of abnormal large Si particles is the result of the primary Si agglomeration and coalescence which appears by increasing fraction of solid above 0.05. The hardness value depends on the morphology of distributed primary Si.