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.