The effects of refining time on the microstructure and mechanical properties of A356 alloy were investigated by using the vacuum negative pressure density instrument

OM

XRD

SEM

EDS and mechanical properties test.The results show that the melt density equivalent is decreased from 11.4% to 1.6%firstly and then increased to 3.4% with the increase of refining time.The as-cast microstructure of A356 alloy is mainly composed of primaryα-Al matrix

eutectic Si phase and Mg

2

Si phase

and the eutectic Si phase is distributed along the grain boundary.The Si phase morphology is changed from irregular plate to spherical

together with the fracture mechanism transformed from intergranular to dimple after T5 heat treatment

meanwhile the Si phase length is reduced from 184μm to 28μm.The internal defect is changed from shrinkage cavity to micro-porosity with the increase of refining time and decrease of thickness.Under the same refining time

the mechanical properties is decreased continuously with the increase of thickness

whereas the mechanical properties are improved firstly and then decreased with the increase of refining time at the same thickness state.The average tensile strength

yield strength

elongation and section shrinkage of the as-cast and T5 states in 12 mm thickness condition are 206.3 MPa

106.0 MPa

6.8%

11.2%and 281.3 MPa

231.6 MPa

10.8%

18.9%

respectively.The Si phase particle nearby the grain boundary is about 8μm.