The effects of different Si contents and ultrasonic treatment on microstructure and mechanical properties of Al-5 Mg-xSi alloy were investigated by metal mold gravity casting. The results indicate that the shape and distribution of the primary α-Al phase and the reinforced phase Mg

2

Si in the alloy are also varied with the increase of Si content. After ultrasonic treatment

the main phase composition in microstructure of the alloy with different Si contents has little change

where the α-Al grains become rounder and the distribution of β-Mg

2

Si is also more uniform. However

the β-Mg

2

Si agglomeration occurs when the Si content is 2%. The tensile strength and elongation of the alloy are firstly increased and then decreased with the increase of Si content. The as-cast mechanical properties of the alloy presents desirable when the Si content is 1.6%

where the tensile strength and elongation is 141.2 MPa

and 1.5%

respectively. After ultrasonic treatment

the mechanical properties of the alloy are desirable with the same Si content

where the tensile strength and elongation is 168.0 MPa and 2.6%

which are 19% and 73% higher than the as-cast ones

respectively.