Study On Liquid Metal Flow Driven By A Modulated Helical Magnetic Field

The macro-segregation in the metal solidification process deteriorates the performance of materials in most cases, because the solute distribution is uneven. Channel segregation, for example, the A-&V-segregation in ingot and the freckle defects in the nickel-based alloys, is a serious macro-segregation occurring under slow solidification conditions. Liquid metal flow driven by a helical magnetic field constructed on permanent magnets, which flow behaviors and influence on the solidification of tin-lead (Sn-Pb) alloy have been investigated in this study.The helical magnetic field was constructed on several pieces of permanent magnets magnetizing in the radius directions. Liquid metal fluid was driven by Lorentz force exerted by the rotating magnetic stirrer. The axial stirring effect could be improved by change the arrangement of the magnetic poles of magnets. The spatial helical structure and the radial gradient of the magnetic field are conducive to enhance the stirring effects of the axial moving magnetic field.Moreover, the rotating direction of the helical magnetic field was periodically reversed to form a modulated helical magnetic field. The azimuthal and axial velocity distribution of liquid GaInSn alloy was quantitatively measured using ultrasonic Doppler velocimetry (UDV), which revealed the time-dependent flow structure and flow pattern varying with the modulation parameters:the rotating speed of the magnetic stirrer and the modulation frequency. The main results of the velocity measurement of the liquid metal were the follows:the azimuthal velocity of the screw flow was periodically reversed with the same modulated frequency, and their flow intensities gradually saturated when the modulation period Tm≥40s; The axial flow resulted from the competition of large vertical vortex driven by the traveling components of the helical magnetic field and the secondary flow driven by the rotating component of the helical magnetic field. There was an optimal modulated period Tm*with respect to the reversed characteristic and flow intensity. When Tm<2Tm*, the axial velocity exhibited typically reversed flow, when Tm>2Tm*, the secondary flow appeared and gradually dominated in the axial flow pattern.The channel segregation of Sn-Pb alloy occured by controlling the thermal conditions in directional solidification. Two types of magnetic field were applied in same thermal condition. The result demonstrated that the solidification structure was refined and the segregation in the upper part of ingot was eliminated in both magnetic field types. However, the channel segregation was observed in the region where the flow was spin-up caused by unidirectional magnetic field inevitably. The modulated magnetic field could significantly prevent the occurrence of the channel segregation, when its parameters were adjusted appropriately and the reversed flow hindered the channel segregation from generating.