Controlling Freckle Defect Formation with Magnetic Fields During Directional Solidification of GaIn Alloy

Segregation of alloying components during solidification leads to stable solute channels, that solidify into defects called freckles. Freckles are caused by buoyancy driving lighter liquid elements, forming a macroscale channel that is fed by inter-dendritic flow. When fully solidified this channel represents a discontinuity in material properties and can lead to the failure of components. The application of a magnetic field, B, adds two physical phenomena to the process: the first is Electromagnetic Damping (EMD) of the liquid metal motion, the second is interstitial flow due to Thermoelectric (TE) Magnetohydrodynamics (MHD). TE effects translate temperature variations at the junction of two conductive materials into electric current, in this case between the solid and liquid.
The current, j¸ interacts with the magnetic field producing a Lorentz force F=j×B. Both the orientation and magnitude of the magnetic field determine the direction and strength of EMD and TEMHD effects.
Consider directional solidification of a solutal unstable buoyant alloy, namely Ga-25wt. %In. Both high velocity plumes of solute and the TE currents will be predominantly aligned to the thermal gradient (∇T), while the feeding inter-dendritic flow is primarily perpendicular to ∇T. A magnetic field orientated perpendicular to ∇T introduces EMD effects on the channel and also interacts with TE currents driving TEMHD flow. To capture these phenomena a parallel Cellular Automata Lattice Boltzmann Method that solves for microstructure solidification, fluid dynamics and electromagnetism using 100s millions of computational cells is used to simulate the freckle formation process at the sample scale. The results indicate that the channel formation can be significantly altered, showing the magnetic field as a potential technique for defect mitigation.