Effect of P and Sr additions on the microstructure of hypereutectic Al-15Si-14Mg-4Cu alloy

The effects of P and Sr additions on the primary and eutectic phases in Al-15Si-14Mg-4Cu alloy were examined using thermal analysis and microstructural examination. With the addition of P, the primary Mg2Si particles were refined considerably, accompanied by a change in morphology from a coarse dendritic to a fine polygonal shape. Through the addition of Sr, eutectic Mg2Si and Si particle morphologies were also altered from coarse script-like and coarse acicular-type morphologies, respectively, to a fine fibrous form. Iron impurities in the alloy resulted in the formation of the π-Fe phase (Al8Mg3FeSi6) in the structure, and the addition of Sr transformed the morphology of the π-Fe from a script-like to a fine twin platelet form. The results show that addition of Sr or Sr + P reduces the eutectic temperature and reduces the size and therefore increases the number density of Mg2Si particles, Si and π-Fe phases in the alloy.