强磁场作用下Cu熔体中富Fe颗粒的迁移与排列

凝固界面前沿颗粒间的相互作用决定了颗粒的运动轨迹、分布和材料的性能,控制熔体中颗粒的迁移可用于材料的净化和提纯.在Cu-30%Fe合金液固两相区施加不同的强磁场条件,富Fe颗粒的分布和排列不尽相同.当无强磁场作用时,富Fe颗粒较均匀地分布在Cu熔体中;随着施加稳恒强磁场磁感应强度的增加,富Fe颗粒向远离重力方向的试样上端迁移,样品底部几乎无富Fe颗粒;而施加向下的梯度磁场作用后,富Fe颗粒沿重力方向向下迁移.结合强磁场作用下颗粒的受力情况,分析了Fe颗粒的迁移行为.不同磁场条件和不同区域的颗粒直径统计分析表明,随磁感应强度增加,Fe颗粒聚合增加,但施加梯度强磁场后颗粒的团聚又逐渐减弱,对此从影响颗粒运动的Stokes和Marangoni凝并速度进行了讨论.从能量最低的角度解释了富Fe相沿平行磁场方向的取向排列.

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.