Metallographic microscope（OM）

scanning electron microscope（SEM）

hardness and tensile properties were used to investigated the microstructure and mechanical properties of Al-Si alloy by gravity casting and by squeeze casting with different pressure

and the microstructure evolution and its mechanism were described.The results show that

there exist bright white short rod like Al2 Cu phase

white fishbone AlSiCuMg phase

black massive primary silicon phase and gray coral shaped eutectic silicon phase in the gravity casting Al-Si alloy.The primary silicon phase in squeeze casting Al-Si alloy is decreased with the increase of the pressure ratio

while the size is refined gradually

and the Al2 Cu and AlSiCuMg phase are decreased with refinement in grain size.Brinell hardness of squeeze cast Al-Si alloy are the higher than that of gravity casting Al-Si alloy

and with the increase of the extrusion ratio

the hardness of the alloy is increased firstly and then decreased.Compared with those of gravity casting AlSi alloy

the tensile strength and elongation are increased by 25.4% and 202.1%

respectively for squeeze cast Al-Si alloy with 600 MPa pressure.