Based on the magnetic-fluid-thermal multiphysics transient coupling numerical simulation results of the magnesium alloy direct-chill (DC) casting, the effects of conventional vibration electromagnetic field (VMF), differential phase vibration electromagnetic field (DP-VMF), conventional low-frequency electromagnetic field (LFMF), and differential phase low-frequency electromagnetic field (DP-LFMF) on melt flow were systematically investigated from the perspective of impulse. Based on thermal behavior and crystal growth theory, the relationships between the velocity field, temperature field, and the morphology of the solidification structure were discussed, and the effect and mechanism of different electromagnetic fields in improving the solidification structure were revealed. Simultaneously, the effects of different electromagnetic fields on AZ31B and AZ80 alloys were investigated. The DC casting experiment verified the theoretical results. Results show that applying low-frequency electromagnetic fields (LFMF and DP-LFMF) can effectively inhibit the formation of columnar grain, but the effect of microstructure refinement is weak; the impact of vibration electromagnetic fields (VMF and DP-VMF) is precisely the opposite. The structure refinement effect of DP-VMF and the inhibition effect of DP-LFMF on columnar grains are better than those of their conventional electromagnetic fields. In the presence of DP-VMF, the average grain size of the center, 1/2 radius, and the edge of the ingot decrease by about 42%, 49%, and 77%, respectively, compared with no electromagnetic field.

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