Semisolid metal forming requires feedstock material with a fine-grained, non-dendritic structure to achieve thixotropic properties. In this study, a Controlled Nucleation method without stirring was used for the formation of thixotropic structures. Hypoeutectic Al-Si alloy was cast into a permanent mould using different pouring temperatures, with and without gauze. As the pouring temperature increased, the grain size increased and the tendency to form columnar grains also increased. Gauze experiments show that the mechanism of the "wall crystals" plays a very important role at low pouring temperatures. The survival rate of the wall crystals determines the final thixotropic structure including both grain size and morphology up to 675DGC pouring temperature, while other nucleation mechanisms might operate at higher temperatures