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Thin sample alloy solidification in electromagnetic driven convection
During the directional solidification of Ga-In25%wt., density variations in the liquid cause plumes of solute to be ejected from the interface through natural convection. This can lead to the formation of chimneys during solidification and ultimately freckles. The application of external magnetic fields can be used to suppress these plumes. Two magnetic systems are considered. The first is a rotating magnetic wheel, which provides conditions analogous to forced convection at the solidification interface. The forced convection causes preferential growth of secondary branches and causes the plumes to be transported downstream and back into the bulk. The second is through the application of a static magnetic field that interacts with inherent thermoelectric currents, generating a Lorentz force that drives fluid flow within the inter-dendritic region. However, in the bulk where there are no thermoelectric currents, electromagnetic damping dominates and plumes are stunted. Using a fully coupled transient numerical model each of these systems has been analysed. Comparisons to experiments are given for the cases of natural and forced convection. The experimental setup uses a Hele-Shaw cell with an electric heater and Peltier cooler, allowing for control over the thermal gradient and cooling rate
The effects of natural, forced and thermoelectric magnetohydrodynamic convection during the solidification of thin sample alloys
Using a fully coupled transient 3-dimensional numerical model, the effects of convection on the microstructural evolution of a thin sample of Ga-In25%wt. was predicted. The effects of natural convection, forced convection and thermoelectric magnetohydrodynamics were investigated numerically. A comparison of the numerical results is made to experimental results for natural convection and forced convection. In the case of natural convection, density variations within the liquid cause plumes of solute to be ejected into the bulk. When forced convection is applied observed effects include the suppression of solute plumes, preferential secondary arm growth and an increase in primary arm spacing. These effects were observed both numerically and experimentally. By applying an external magnetic field inter-dendritic flow is generated by thermoelectrically induced Lorentz forces, while bulk flow experiences an electromagnetic damping force. The former causes preferential secondary growth, while the latter slows the formation of solute plumes. This work highlights that the application of external forces can be a valuable tool for tailoring the microstructure and ultimately the macroscopic material properties