Investigation of thermoelectric magnetic convection and its effect on solidification structure during directional solidification under a low axial magnetic field

Thermoelectric magnetic convection (TEMC) at the scale of both the sample (L = 3 mm) and the cell/dendrite (L - 100 mu m) was numeerically and experimentally examined during the directional solidification of Al-Cu alloy under an axial magnetic field (B <= IT). Numerical results show that TEMC on the sample scale increases to a maximum when B is of the order of 0.1 T, and then decreases as B increases further. Howwever, at the cellular/dendritic scale, TEMC continues to increase with increasing magnetic field intensity up to a field of 1 T. Experimental results show that application of the magnetic field caused changes in the macroscopic interface shape and the cellular/dendritic morphology (i.e. formation of a protruding interface, decreases in the cellular spacing, and a cellular dendritic transition). Changes in the macroscopic interface shape and the cellular/dentritic morphology under the magnetic field are in good agreement with the computed velocities of TEMC at the scales of the macroscopic interface and cell/dendrite, respectively. This means that changes in the interface shape and the cellular morphology under a lower magnetic field should be attributed respectively to TEMC on the sample scale and the cell/dendrite scale. Further, by investigating the effect of TEMC on the cellular morphology, it has been proved experimentally that the convection will reduce the cellular spacing and cause a cellular-dendritic transition. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.