Effects of low superheat pouring with a shearing field（LSPSF） on the grain size and morphology of primary Si in hypereutectic Al-20Si alloy have been observed.The results show that semi-solid Al-20Si alloy slurry and billets with excellent quality can be prepared by LSPSF.Primary Si with uniform distribution is converted from plate-like structure into polygonal bulk structure

obtaining obvious refinement.The LSPSF refining effects can be enhanced by P modification

however

the segregation of primary Si phase can be observed with lower pouring temperature.The refinement and spheroidization of primary Si in the Al-20Si alloy is attributed to enhancing heterogeneous nucleation and controlling the anisotropic growth of primary Si phase by LSPSF.