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An experimental apparatus that uses a superconducting magnet and enables the simultaneous application of an alternating electric field with a frequency of up to 50 kHz and a magnetic field of up to 10 T was designed and assembled. Electromagnetic vibrations were induced in Al-7 wt pct Si alloy during solidification by simultaneous application of the two fields. The thorough investigation, which was carried out over wide ranges of intensity (an electromagnetic pressure range of 0 to 2.25 ϫ 10 5 Pa) and frequency (0 to 50 kHz), clarified the effects of the two main parameters on the microstructural refinement brought about by electromagnetic vibrations. Low-intensity vibrations changed the highly columnar dendritic structure into one composed of large, equiaxed dendrites. As the intensity, and consequently, the magnetic pressure were increased, at about 0.93 ϫ 10 5 Pa, fine isolated grains started to appear and dominated the structure as the pressure was increased further. At low frequencies, the structure was one with large, equiaxed dendrites, which disintegrated to form a fine and homogeneous structure as the frequency was increased. At about 1.5 kHz, the trend reversed and the structure gradually became a completely columnar dendritic one at frequencies higher than 10 kHz. Metallographic observations showed that the cavitation phenomenon has been a main factor behind the observed microstructural refinement. The effects of mechanical vibrations of the experimental apparatus were also investigated and found to have no contribution to the observed effects.

ALIREZA RADJAI and KENJI MIWASimultaneous imposition of alternating electric and stationary magnetic fields on a molten metal will induce a vibrating motion in the liquid, which can lead to the formation and collapse of cavities and affect the solidification structure. Following earlier works on Al-Si alloys, the process is used to refine the microstructure of gray iron. It is found that in addition to the refinement of columnardendritic structure of primary austenite into a fine and homogeneous one, the eutectic cell structure is also extensively refined. The effects of the two main parameters involved in the process, that is, the frequency and the intensity of vibrations are, for the first time, quantitatively presented. The refinement of the cells proceeds as the frequency is increased up to about 500 Hz, where a reverse trend starts and results in a complete termination of the effects at about 10 kHz. The increase in the number of cells because of the increase in the intensity of vibrations shows a sharp jump at an electromagnetic pressure of about 10 5 Pa, where the cavitation phenomenon is more likely to occur by overcoming the static pressure. However, increasing the electromagnetic pressure to higher values does not essentially result in a considerable further refinement, implicating the existence of a limit in the process of structural refinement by the cavitation phenomenon.

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