Preparation Process of Sintered NdFeB Magnets - Raw Material Preparation(2)

1.Smelting

Rare earth raw materials are usually in the form of pure metals, and rare earth alloys are often selected for cost reasons, such as praseodymium and neodymium metals, lanthanum and cerium metals, mixed rare earth and dysprosium and iron alloys, etc. High melting point elements (such as B, Mo, Nb, etc.) are added in the form of ferroalloys. Nd-Fe-B magnets have the characteristics of polymetallic phase, Nd-rich phase is a necessary condition for high orthotropy, and B-rich phase is bound to symbiosis. Therefore, it is usually required that rare earth and B in the original formula are higher than the positive component of R2Fe14B, but sometimes in order to adjust the composition of grain boundary phase (especially when Cu, Al and Ga are added), the content of B will be slightly lower than the positive component. Due to the reaction of rare earth metals and crucible materials, as well as melting and sintering volatilization, a certain loss of rare earth metals should be considered in the formulation. In order to reduce the impurity content in the alloy, the purity of the raw material should be strictly controlled, and the surface oxidation layer and attachments should be fully removed. The heat source of medium and low frequency induction melting is the induced eddy current formed by alternating magnetic field in the raw material. The skin effect of the eddy current makes the current concentrate on the surface of the raw material. If the size of the raw material block is too large, the eddy current cannot penetrate to the center of the material block, the core can only be melted by heat conduction, which is not practical in the actual production. Limit it to three to six times the skin depth. The following figure shows the relationship between power frequency - skin depth - raw material size. It can be seen that the higher the frequency, the more significant the skin effect, and the smaller the raw material size is required.

The selection of melting frequency is subject to another important role of induction melting -- electromagnetic stirring, that is, the interaction of force between molten metal liquid and alternating magnetic field is used to promote the melting of unmelted solid and the homogenization of molten metal liquid. The magnitude of electromagnetic force is inversely proportional to the square root of current frequency, too high frequency will weaken the electromagnetic stirring effect of alternating power supply. The frequency band used in actual production is around 1000~2500Hz, and the size of raw materials should be controlled below 100mm.