Segregation Channel Dynamics in Directionally Solidifying Ga-In Alloy with and without a Magnetic Field

The formation of freckle defects in the presence of an external magnetic field is studied by combining *in situ* synchrotron imaging with numerical simulations. The formation, growth and motion of freckle channels during directional solidification are investigated in a Hele-Shaw cell for a low melting point Ga-In alloy. The solidification cell is placed in a permanent magnet system providing a flux density of about 120 mT within the cell. A parallel numerical study, using a microscopic parallelized Cellular Automata lattice Boltzmann method, is validated by these *in situ* experiments. An excellent match between the numerical model and experiments conducted on thin rectangle sample alloy is achieved. Evaluation of the *in situ* X-ray data and numerical analysis shows the role of thermoelectric magnetohydrodynamics (TEMHD) and electromagnetic damping (EMD) in the formation of channels and ultimately freckle defects. The channel motion can be attributed to the thermoelectric Lorentz force acting on the inter-dendritic liquid flow by causing the solute to accumulate at one side of the cell. *In situ* synchrotron experiments allow us to resolve the complex channel dynamics and simultaneously show how large-scale flow fields may alter it. Both temperature gradient and grain orientation can affect the dynamics of the segregation channels formation in the presence of the magnetic field. The effect of electromagnetic damping force to convective transport needs future investigations. The *in situ* synchrotron data and numerical modelling will provide further understanding of the underlying mechanisms and identifying further interesting phenomena.