Sintered NdFeB permanent magnets are produced using the powder metallurgy method. From material preparation to finished product delivery, there are generally more than a dozen process steps, including several inspections and analyses at different stages.
The entire production process is a systematic project, interlocking with each other. Generally, we call the process of producing magnet blanks the front-end production link, and the process of processing the blanks into final products is called the back-end processing link. Magnetic material manufacturers are mainly divided into two categories, one is a manufacturer that has both front-end production and back-end processing, and the other is a processing manufacturer that focuses on back-end processing.
1. Preparing materials
When manufacturers select raw materials, they generally purchase them according to the magnet performance requirements and the corresponding national standards. Before smelting, the raw materials are cut and surface-treated.
2. Smelting
Smelting is the first process for sintered NdFeB permanent magnets to enter the production process. The raw materials are hot-melted in the smelting furnace and cooled to form alloy strips. This process requires the furnace temperature to reach about 1,300 degrees and lasts more than four hours to complete.
3. Hydrogen Breaking + 4. Jet Mill
The two process steps of hydrogen crushing and jet milling are collectively called powder making, which is the process of crushing the smelted alloy belt strips and making magnetic powder. To obtain a well-oriented magnet, the powder particles are required to be small in size (3-4 μm) and have a concentrated size distribution, and the powder particles are spherical or approximately spherical.
5. Pressing
The crushed magnetic powder is loaded into a mold, an external magnetic field is applied for orientation, and the powder is pressed after orientation. Powder magnetic field orientation is one of the key process technologies for producing high-performance sintered NdFeB. In the molding process, there are currently three methods commonly used in the industry: molding, molding plus cold isostatic pressing, and rubber mold isostatic pressing. Under the same neodymium content, rubber mold isostatic pressing can obtain greater magnetic energy. product.
6. Heat Treatment
After pressing, the relative density of the magnet compact is relatively high. In order to make the magnet have high permanent magnetic properties, the compact needs to be heated to a temperature below the melting point of the basic phase of the powder and heat treated for a period of time. This process is also called sintering. After high-temperature quenching, it also needs to be tempered at a certain temperature to optimize the structure and obtain the best magnetic properties. (Tempering refers to cooling the sintered magnetic powder blank to a certain temperature and then heating it again)
7. Machining
Due to the characteristics and technical limitations of the magnetic field orientation forming process, it is difficult for sintered magnets to directly achieve shape and size accuracy for practical applications at one time. Many finished magnets are small in size and complex in shape, and can only be processed from blank magnets of a certain shape. Sintered NdFeB material is hard and brittle, and general machining can only carry out cutting, drilling, grinding, and rolling.
8. Grain Boundary Diffusion
The rare earth elements dysprosium and terbium can greatly improve the coercivity and temperature stability of the material. For sintered NdFeB materials that require higher coercivity and operating temperature, dysprosium and terbium often need to be added, but the prices of these two elements are very high. Very high, which will cause the magnet production cost to increase sharply. Currently, grain boundary diffusion technology is commonly used in the industry to reduce the amount of heavy rare earth elements added.
9. Surface Treatment
Sintered NdFeB is a powder material with very strong chemical activity. Tiny pores and cavities are inside, easily corroded and oxidized in the air. Over time, the magnetic properties will be attenuated or even lost, so strict surface inspection must be carried out before use. Anti-corrosion treatment. At present, the anti-corrosion treatment of NdFeB generally adopts electroplating, chemical plating, electrophoresis, phosphating, and other methods. Electroplating, as a mature metal surface treatment method, is widely used.
10. Magnetization
Magnetization is a key step for sintered NdFeB permanent magnets to obtain magnetism. The magnetizer is a tool for magnetizing magnetic materials or magnetic devices. It applies a magnetic field to the magnetized NdFeB magnets. If the magnetized magnetic field cannot reach the technical saturation magnetic field, the residual magnetism Br and coercive force Hcj of the permanent magnet will not reach the expected values. In addition to ordinary single-pole magnetization, sintered NdFeB can also be multi-pole magnetized according to actual needs, that is, after magnetization, multiple N and S poles can appear on a plane.
11. Packing and Shipping
Packaging and shipping are the last step before the magnet leaves the factory. The magnet manufacturer can package according to the purchaser's requirements. If the purchaser has special packaging requirements, he or she can inform the supplier in advance. Sintered NdFeB permanent magnets have strong magnetism, so domestic transportation generally uses land transportation. For exports overseas, they can be transported by air after undergoing special magnetic isolation packaging and inspection and appraisal by professional institutions.
