The sintered magnet usually uses pure metal or intermediate alloy as raw material, and uses the electromagnetic induction heating principle of generating eddy current in the raw material by alternating magnetic field. In the vacuum or inert gas environment, the raw material is heated and melted by medium-low frequency induction melting, and the melt is homogenized by stirring. The melting point of rare earth metals is between 800~1500℃, Fe and Co are 1536℃ and 1495℃ respectively, and pure B is as high as 2077℃. Some high melting point metals as additives, such as Ti, Cr, Mo or Nb, have melting points between 1600~3400℃. Considering the inhibition of the volatilization of rare earth elements, the melting temperature is usually controlled within 1000~1600℃. The high melting point elements are fused by the alloying of the rare earth metal molten liquid, or the alloys of high melting point elements (usually iron alloys) are directly used as raw materials, such as B-Fe (melting point ~1500℃) and Nb-Fe (melting point ~1600℃) alloys. In order to ensure the low oxygen environment of melting and casting, it is necessary to vacuum the melting and casting furnace body, and make the parts and raw materials in the furnace fully deflate, the vacuum level is usually 10-2~10-3, the pressure boost rate of the furnace body before heating (internal deflate and external leakage) also need to be controlled at a low level, such as the capacity of 1t melting furnace, The boost rate should be lower than 5×10-4~1×10-3 L/s. Vacuum smelting can fully vent the molten liquid, remove impurities and harmful gas elements with low boiling point, and improve the purity of the alloy. However, since the vapor pressure of rare earth metals is very low (less than 1Pa), the volatilization loss is very considerable, so the furnace is usually filled with inert gas in the smelting process to increase the ambient pressure to suppress the volatilization of rare earth. It is more convenient to use high purity argon. In general, fill to a level of 50kPa. After homogenization of molten alloy, exhaust and slagging are fully completed, casting can be carried out. Alloy casting is a very critical process, because the composition, crystal state and spatial distribution of the phase are crucial to the performance of sintered magnets. Alloy ingot has experienced thick "cannonball", 20mm thick "book", 5mm "pancake", and now it has developed to only 0.3mm thickness of fast setting sheet. Many efforts have been made to avoid component segregation and heterogenic phase formation and distribute neodymium-rich phase distribution reasonably.
