Effects of extra Si addition on microstructure and mechanical properties of in-situ 15%Mg2Si/Al composites have been investigated.The results indicate that the size of primary Mg2 Si particles is firstly increased and then decreased with increasing in extra Si content.With extra Si content of 5%

the maximum size of Mg2 Si reaches 41μm

while with the extra Si content of 10%

size of primary Si phase is decreased to about 24μm.In addition

the amount of Al-Mg2 Si binary eutectic structure is gradually reduced to even be disappeared with the extra Si content increased from 0to 10%.Furthermore

with the extra Si content increase

the irregularα-A phase becomes spherical gradually

and sphericalα-Al phase can be formed when the content of extra Si reaches 8%.The tensile strength and elongation of the in-situ 15%Mg2Si/Al composites are improved from 172 MPa to 200 MPa and from 1.4%to 3.1%

respectively

with the extra Si content increased from 0%to 10%.