Understanding the role of magnetic fields on freckle formation during solidification through in situ imaging

During directional solidification, segregation of alloying components may lead to channels of segregate forming, called ‘freckle’. This work aims to understand how the application of a magnetic field effects freckle formation. Using synchrotron X-ray imaging we quantify the influence of magnetic field strength and orientation on the formation, growth and motion of freckle channels and the resulting suppression of the solidification temperature.By increasing the Bz magnetic component (perpendicular to the sample surface, freckle channels can be driven to one side and suppressed elsewhere, giving a freckle free region in otherwise difficult to cast alloys. The freckle motion can be attributed to the thermoelectric Lorentz force acting on the liquid, changing inter-dendritic liquid flow by causing the solute-enriched liquid to accumulate at one side, biasing channel motion. In the bulk liquid electromagnetic damping force prevails, decreasing the solidification temperature.