The static magnetic field and the alternating electric field were simultaneously imposed on AZ91D magnesium alloy melt, and α-dendrite particles were refined by the electromagnetic vibrations. The effect of the frequency of electromagnetic vibrations on microstructural refinement was quantitatively investigated. In the frequency range from 60 to 1000 Hz, the vibration frequency near 200 Hz was the most effective for the refinement of α-dendrite particles, and α-dendrite particles were refined up to approximately 100 μm from 1800 μm at this frequency. However, the effect of refinement by the electromagnetic vibrations became weak at frequencies above 400 Hz. Although the degree of refinement of the primary particles differed with the frequency of the electromagnetic vibrations, the dendrite arm spacing was almost constant, 30–40 μm, in our experiment. Therefore, the refinement of primary α-dendrite particles is likely to be caused by collapse of dendrite arms due to the cavitation phenomenon and the stirring of the melt.
