Thermoelectric Magnetohydrodynamic Control in Alloy Solidification

Magnetic fields have been shown to have a significant effect during solidification in a wide range of conditions from the slow growth of traditional casting to the more rapid growth of Additive Manufacturing. An underlying phenomenon is Thermoelectric Magnetohydrodynamics (TEMHD), which, due to inherent thermal gradients, generate thermoelectric currents and ultimately a Lorentz force through interaction with the magnetic field. In casting this leads to inter-dendritic convective solute transport. This can be used to control freckle defect formation in the GaIn system, where the magnetic field can be used to reposition channel formation, introduce preferential growth of secondary arms, plume migration and complex grain boundary interactions. These mechanisms have been observed by X-ray synchrotron experiments and predicted by TESA (ThermoElectric Solidification Algorithm), a parallel Cellular Automata Lattice Boltzmann based numerical model.
In laser AM, melt pools are subject to large thermal gradients and consequently form relatively large thermoelectric currents. The system is highly dependent on the orientation and strength of the magnetic field with competition between Marangoni flow and TEMHD resulting in control of the depth, width and potential deflections of the melt pool. This leads to significant changes in the microstructure including modification to the melt pool boundary layer and epitaxial growth. The numerical predictions also compare favourably to X-ray synchrotron experiments.