Migration and alignment of Fe-rich particles in Cu melt under high magnetic field

The interaction among particles in front of solid-liquid interface during solidification plays a role in determining the trajectories, distribution and sizes of particles, which eventually determines the properties of material. By using the interaction to control the migration of particles, impurity particles can be removed from the melt. A method of using an external high magnetic field to simulate the migration of Fe in Cu melt is proposed. Static high magnetic field (0.1 Tesla and 12 Tesla) and gradient high magnetic field (-92.1 T2/m) are subjected to the solid-liquid mushy zone of Cu-30 wt%Fe alloy. The case without high magnetic field is also investigated for comparison. Both macro- and microstructure of the samples are observed by optical microscope. The results indicate that primary Fe dendrites in Cu-Fe alloy are transformed into spherical Fe-rich particles after solidification in mushy zone, and high magnetic field is capable of changing the migration, distribution and arrangement of Fe-rich particles. In the absence of a static high magnetic field, Fe particles are distributed in Cu melt homogeneously. With increasing the magnetic flux density of imposed static high magnetic field, Fe-rich particles gradually migrate upwards. The migration direction is opposite to the direction of the gravity, and there are no Fe-rich particles kept on the bottom of the samples imposed by magnetic field. In the presence of negative high gradient magnetic field, however, the Fe-rich particles migrate downward and the direction is along the direction of the gravity. A model is built up to clarify the body force of Fe-rich particles and to analyze their movement while they are affected by high magnetic field. The results show that the migration behaviors of Fe-rich particles are related to the viscous dragging force, the interaction force between magnetic dipoles, and the magnetization force induced by gradient high magnetic field. The displacement of Fe particles is closely dependent on the body force. Through the analysis the experimental results are well explained. The diameters of Fe-rich particles are statically summarized under different high magnetic field conditions and in different zones. With increasing magnetic flux density of static high magnetic field, the aggregation of particles is increased. The magnetic field gradient, however, reduces the aggregation of particles. This might be as a result of the competitive coagulation between Stokes sedimentation and Marangoni migration in Cu melt. Microstructure of the samples indicates that Fe-rich particles tend to align along the direction of high magnetic field and the degree of alignment is likely to be related to external magnetic field strength, resistance force, effective time, and initial condition of particles, etc. As they are parallel to the direction of high magnetic field, the energy of the system is minimum, suggesting that the system is stable. The present study shed light on how to remove strong magnetic impurity from Cu melt.