The influence of w（Mg）/w（Si）mass ratio on the microstructure and mechanical properties of Al-4 Mg-xSi-0.6 Cr die casting alloys was investigated.The results reveal that Mg is mainly soluted inα-Al matrix and a small amount of Al

3

Mg

2

particle phase can be generated

which results in the high comprehensive mechanical properties withoutSi added in the alloy.With the addition ofSi elements

the strength of the alloy is increased

while ductility is decreased sharply due to Mg andSi formed Al-Mg

2

Si eutectic microstructure.Al-Mg

2

Si eutectic lamellar spacing is increased and it’s morphology is transferred into concave polygon with the decrease of Mg/Si mass ratio

which can accelerate fracture during tensile process as a result of stress concentration.When the Mg/Si mass ratio is decrease to 1.6

with the increase ofSi

the amount of Mg

2

Si phase keeps constant value.Meanwhile

the strength is increased and ductility is decreased due to long stripSi eutectic phase formed in the die casting alloy.The category and morphology of Cr phase are related with Mg/Si mass ratio.With excess of Mg

the Cr exists in form of large block Al13 Cr2 phase

while with excessSi

the Cr is distributed evenly with irregular Al5（Cr

Fe）Si.