Effects of solidification structure and Mg content on fast heat treatment of A356 were investiagted. The results reveal that eutectic Si morphology in solidification structure of A356 alloy is converted from fibre to spherical structure after modified by Sr

and the grain size of primary α phase is decreased. After 540 ℃×20 min+170 ℃×90 min fast heat treatment

the Mg

2

Si strengthening phase in microstructure is fully dissolved into matrix

and tensile strength is basically the same as that of the T6 heat treated alloy. With the increase of Mg content

the solid time required for Mg

2

Si phase to fully dissolve into matrix is increased during solution treatment. When the Mg content of the alloy is increased from 0.3% to 0.9%

540 ℃×40 min+170 ℃×90 min fast heat treatment is required

and the mechanical properties of alloy can be the same as that of the T6 heat treated ones