To obtain the billet with homogeneous and spheroidized primary grains is the key step in the semi- solid forming process. Among the semi-solid billet preparation methods, the process of low-superheat direct chill （DC） casting is simpler and more effective. In this study, the billets of AISi7Mg alloy were prepared by low- superheat DC casting at various withdrawal rates. The effect of withdrawal rate on the surface quality of the billets was investigated, and the evolution mechanism of the microstructural morphology in the billets was analyzed. The results show that the periodic corrugations and a large quantity of fine shrinkage pits appear in the billet surface when the withdrawal rate is 100 mm.min-1, and the above defects in the billet surface can be eliminated completely when the withdrawal rate is above 150 mm.min-1. But when the withdrawal rate is too fast （250 mm.min-1）, the primary a grains, except the ones in the billet center, have not enough time for ripening due to the high silidification rate, and will solidify as the dendrite structure. When the withdrawal rate is between 150 - 200 mm.min-1, the dendritic growth of the primary a grains is effectively inhibited, and a billet of AISi7Mg alloy with a smooth surface and homogeneous, fine, non-dendritic grains can be obtained.
