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Modification of temperature and solute distribution during directional solidification caused by electromagnetically-driven convectionWillers, B.; Eckert, S.; Nikrityuk, P. A.; Eckert, K.
The application of time varying magnetic fields can be considered as an effective tool to organize a well-defined flow structure in the liquid phase affecting the nucleation and solidification parameters. Once a flow occurs in the liquid melt during solidification, nucleation and grain growth are mainly governed by the convective transport of heat and solute. The consequences on the structure of solidified ingots are widely discussed in the literature. So it is known, that the application of mechanical or electromagnetic stirring promotes the formation of fine, equiaxed grains [1-4].
Solidification experiments as well as numerical simulations were carried out considering the directional solidification of Pb Sn alloys from a water cooled copper chill. A rotating magnetic field (RMF) was applied for melt agitation. Thermocouples were used to measure the temperature field during solidification. Profiles of the velocity in the liquid phase were determined by means of Ultrasound Doppler velocimetry (UDV) .
Numerical simulations are based on the classical mixture formulation . To calculate the viscosity in the mushy region the model proposed by Roplekar and Dantzig  was implemented into the code. The amplitude of the electric currents induced by the RMF are altered if the electrical conductivity of the growing solid phase deviates from that of the liquid phase. Previous investigations  showed that for such a situation numerical predictions of the velocity field using a stationary Lorentz force term become questionable. For our calculations presented here the Lorentz force term has been recalculated for each time step.
The comparison between numerical simulations and solidification experiments delivered a good agreement. Due to the permanent changes both of the aspect ratio of the fluid volume and the material properties during solidification the flow field shows a transient behaviour being more complex as known from the stationary, isothermal case. Our results disclose that the forced convection causes distinct modifications of the temperature and concentration field. The electromagnetic stirring promotes the columnar-to-equiaxed transition and causes a considerable grain refinement. Modifications of the grain structure and effects of macrosegregation are discussed with respect to the details of the flow field.
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Keywords: solidification, PbSn alloys, forced convection, rotating magnetic field
Contribution to proceedings
MCWASP Conference, Modeling of Casting, Welding and Advanced Solidification Processes XI, 28.05.-02.06.06, Opio, France
MCWASP Conference, Modeling of Casting, Welding and Advanced Solidification Processes XI, 28.05.-02.06.2006, Opio, France