Department of Materials Engineering, Shanghai University
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DOI:
10.7502/j.issn.1674-3962.2014.06.05
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Abstract:
The thermo-electric currents are due to the Seebeck effect when temperature gradients exist in the material. Thermo-electric currents in the presence of static magnetic fields generate significant electromagnetic forces (TEM forces). Those forces may produce various phenomena like pumping, stirring in liquid metals as well as solid motions, stresses in the solid metal. Those effects may be encountered especially during the solidification of metallic materials because of the existence of significant temperature gradients. In liquid metals the application of a static magnetic field enhances TEM convection at moderate intensity but also damps it when it is strong enough. Moreover, it has been found that the thermoelectric magnetic effect own muti-scales effect, the smaller the length scales are (for example when primary or secondary dendrite arm spacings are considered), the higher the magnetic field strength which is needed to damp the TEM convection. So far, many solidification experiments on various types of alloys have been carried out. The experiments have shown that TEM convection occurs both in the liquid bulk but also in the deep mushy zone. TEM convection may strongly influence the meso-macrosegregation patterns, the solidification structures and the grain boundaries leading to the striking grain boundary structure of the mushy zone. The flow pattern and accordingly the segregations may be controlled by changing the orientation of the applied magnetic field, i.e., axial or transverse. Furthermore, the TEM forces on the solid promote the appearance of direct Columnar-to-Equiaxed Transitions thanks to the possible enhanced fragmentation of the dendrites. Such mechanisms have been recently partly observed by X-ray in situ observations, where channel formation effects as well as detachment of grains along with horizontal motions were observed. This further confirms that there exists a significant influence of the thermoelectric magnetic effects on the solidification structure during directional solidification under the magnetic field.