Study On The Flow Of Molten Metal Driven By Permanent Helical Magnetic Field

Segregation and other solidification defects usually happen during solidification of metallic materials. Segregation include microsegregation and macrosegregation. The former could be reduced or eliminated by such as diffusion annealing for a long time, but the latter can’t be eliminated during subsequent heat treatment process, and which should be controlled during solidification process. At present, interdendritic liquid flow enriched in solute is considered as the reason of formation of macrosegregation. Therefore, it’s of great importance to control its flow during metal’s solidification process. The relative motion between molten metal and magnetic field will generate induced current, the interaction between induced current and magnetic field will generate electromagnetic force, and controlling segregation by controllable electromagnetic force becomes an important means to reduce segregation.The permanent magnetic field has been constructed by small permanent magnet units, and the three dimensional flow of GaInSn alloy driven by which has also been researched in this paper.The numerical simulations and experimental measurements have been performed on the helical magnetic field constructed upon small permanent magnet units and the ideal helical magnetic field, and conclude that helical magnetic field can be considered as superposition of rotating component and travelling component. What’s more, the results between the two types helical magnetic field by comparison are approximate show that it’s reasonable to instead the ideal magnetic field by helical magnetic field constructed by small permanent magnet units in the experiment. In addition, the proper magnetic pole configuration has been optimized by experiments.The flow of molten metal has been researched by Ultrasonic Doppler Velocimetry (UDV). The azimuthal velocity as well as the axial velocity of GaInSn alloy have been measured quantitatively in the magnetic field, and the results show that the alloy can be driven to form three dimensional flow. The permanent magnetic helix drive alloy to form swirling flow in azimuthal direction, and the flow intensity increases with the increasement of the rotating speed of permanent magnetic helix. However, the azimuthal flow intensity increases till10s when the rotating speed stay the same, after which, flow intensity is saturated and flow is tending towards stability. In the axial direction, the flow pattern depends on the experimental parameters so it’s more complex, existing secondary flow and meridional flow two different flow patterns. When the diameter of alloy liquid column is smaller(40mm), secondary flow will be observed in axial direction, and meridional flow will appear when the diameter of alloy liquid column is bigger(70mm).When magnetic field is modulated, the fluid can flow back and forth periodically in azimuthal direction in any modulation period, and the flow period is consistent with the modulation period of magnetic field. With the increasement of the modulation period, the azimuthal velocity increases and flow strengthens, but when Tm≥20s, flow is saturated and the azimuthal velocity doesn’t increase. The axial flow relate to the diameter of alloy liquid column and modulation period of magnetic field, the controllable meridional flow can be appeared by controlling appropriate diameter of alloy liquid column and modulation period of magnetic field.Due to the direction of the secondary flow does not change according to the change of a rotating magnetic field, but the direction of the meridional flow can response to change in the direction of the traveling magnetic field. If the axial flow is meridional flow during the solidification of metal, and we can set appropriate modulation period, periodically changing the direction of magnetic field, and thus produce periodic intervention solute precipitation on solidification front, so as to achieve the purpose of reducing segregation, and when the axial flow pattern is secondary flow, it can’t reduce the segregation because its direction does not change according to the change in the direction of magnetic field. The results in this paper are expected to control the macrosegregation of large castings and forgings, nickel-based alloy and high alloy steel in solidification process.