Abstract: A theoretical model on the grain size prediction was established according to the thermodynamics of the alloy solidification and consideration on the factors of the nucleation and grain growth. The grain size of the near liquidus semicontinuous casting 6061 aluminum alloy was calculated by using the model. The calculated results are well consistent with the experimental ones with the maximum error less than 7μm. Through the analysis of the growing disturbance along the normal direction to the liquidsolid interface during the solidification， a equation of the critical grain diameter corresponding to a certain wave number and the dominant wavelength surround the spherical grains were obtained. The spherical grain growth and its transformation conditions and mechanism of dendritic grains with specific type were well explained by these expressions， which is of great significance to the study on the formation and evolution of the grains with different morphologies， and to the prediction of microstructures and properties of semisolid alloy as well as the processing optimization.

Key words: semisolid forming, near liquidus semicontinuous casting, microstructural prediction, grain size, grain morphology