Within the framework of classical solidification theories, the effect of a low-frequency electromagnetic vibrating field on the as-cast microstructures of direct chilling (DC) casting AI-Zn-Mg-Cu-Zr ingots was discussed. In comparison with the conventional DC ingots, the microstructures of the low-frequency electromagnetic vibrating casting (LFEVC) ingots are gradually refined with increasing electromagnetic intensity. The increased number of nuclei is likely to be as a result of electromagnetic undercooling and forced convection. Grains were assumed to grow first into a global morphology and then into a dendritic one after exceeding a critical size. The unstable wavelength of a growing global grain was deduced to evaluate this transition from a global grain to a dendritic grain. Decreasing the electromagnetic frequency and/or increasing of electromagnetic intensity lead to a longer wavelength and therefore are suitable for less dendritic or net-global grains. (c) 2006 Elsevier B.V. All rights reserved.