The refining behavior and heterogeneous nucleating behavior of Al-30 Si-5 C master alloy on primary Si in Al-18 Si alloys were described.The results show that the mechanical stirring can accelerate the refining effect of Al-30 Si-5 Cmaster alloy on primary Si and a shorter holding time is also a promotion factor.The average size of primary Si grains can be decreased from 108μm to 35μm with 1% Al-SiC master alloy addition by applying mechanical stirring and holding temperature for 15 min.The EDS and DSC analysis indicates that SiC particles can act as the heterogeneous nucleation substrate of primary Si.Besides

the refining mechanism was discussed from the following aspects:（1）the basically appropriate crystallographic orientation relationship between Si phase and SiC phase

（2）adsorption and segregation of Si element on SiC particles enabled the rich Si layer to satisfy the nucleation supercooling degree

（3）surface morphology of SiC particles favorable for nucleation of embryos.