Effects of Thermoelectric Magnetic Convection on the Solidification Structure During Directional Solidification under Lower Transverse Magnetic Field

Abstract

This work investigated the thermoelectric magnetic convection (TEMC) during directional solidification under a transverse magnetic field numerically and experimentally. Numerical results show that the TEMC will form in liquid near the liquid/solid interface and in the dendritic network. The value of the TEMC mainly depends on the crucible diameter, the temperature gradient, and the magnetic field intensity. The value of the TEMC increases as the crucible diameter and the temperature gradient are increased. The value of the TEMC on the sample scale increases to a maximum when the magnetic field is of the order of 0.1 T, and then decreases as the magnetic field still increases. However, the value of the TEMC on the cell/ dendrite scale continues to increase with the increase of the magnetic field intensity when the applied magnetic field is less then 1 T. Two alloys are solidified directionally in the vertical configuration under a transverse magnetic field, and results show that the application of a lower transverse magnetic field (B < 1 T) modified the liquid/solid interface shape and the cellular/ dendritic array significantly. Indeed, it was observed that, along with the refinement of the cell/ dendrite, the magnetic field caused the deformation of the liquid/solid interfaces and the extensive segregations (i.e., channel and freckle) in the mushy zone. Comparison of the numerical and experimental results shows that the modification amplitude of the liquid/solid interface and the cellular/dendritic morphology is in good agreement with the value of the TEMC at the liquid/solid interface and in the dendritic network. This implies that changes of the interface shape and the cellular/dendritic morphology should be attributed, respectively, to the TEMC on the sample and the cell/dendrite scales.

The effects of Thermoelectric Magnetohydrodynamics in directional solidification under a transverse magnetic field

“…Macrosegregation occurs normal to the magnetic field orientation and to the thermal gradient. Descriptions of the mechanism have been given by authors of these experimental findings [4,6], supported with idealised steady state 2-dimensional models of TEMHD. They suggest that this behaviour is caused by interdendritic flow transporting high concentration of solute out of the mushy zone.…”

“…In the presence of a transverse magnetic field, macrosegregation on the sample scale has been observed for both in situ thin sample experiments [4] and post-mortem analysis of rod shaped samples [4,5,6]. Macrosegregation occurs normal to the magnetic field orientation and to the thermal gradient.…”

“…The application of an external magnetic field to a solidifying alloy has been experimentally observed to have a significant effect on microstructural evolution [1, 2,3,4,5,6,7,8,9,10,11]. These changes can be attributed to convective transport of solute by Thermoelectric Magnetohydrodynamics (TEMHD).…”

Analytic Solutions to Determine Critical Magnetic Fields for Thermoelectric Magnetohydrodynamics in Alloy Solidification

“…Also, the magnitude of the changes had a material dependency which could be attributed to the variation in thermoelectric power of the various alloys examined. More recently, microstructural changes have been observed over a wide range of magnetic field intensities from low-moderate (O(1T)) field strengths [19][20][21][22] to higher (O(10T)) strengths [23][24][25]. Furthermore, direct in situ observations of solidification under a relatively low magnetic field have shown indications of TEMHD flow at the meso scale [26].…”

Influence of a Slow Rotating Magnetic Field in Thermoelectric Magnetohydrodynamic Processing of Alloys

“…This phenomenon is known as Thermoelectric Magnetohydrodynamics (TEMHD) and experimental results for both high 1) and low 2) magnetic fields have shown that the application of a constant external magnetic field can cause fragmentation, acting to produce a finer grained casting, with a concomitant increase in yield strength. However, the undesirable effect of macro-segregation [3][4][5] and freckling 6) have been observed. Both of these effects can be attributed to the convective transport of heat and mass, altering the solidification process.…”