Jan 25 , 2025
Sintered NdFeB magnets are produced using powder metallurgy. From material preparation to final product shipment, the production process generally goes through more than a dozen technical steps and includes several tests and analyses at different stages. This article will detail several important processes in the production of sintered NdFeB magnets.
Good raw materials are the foundation for producing high-quality sintered NdFeB magnets. When selecting raw materials, manufacturers usually follow national standards according to the required magnet performance. Before smelting, the raw materials are cut and surface-treated.
Smelting is the first process in the production of sintered NdFeB magnets. Raw materials are thermally melted in a smelting furnace and cooled to form alloy ribbon sheets. This process requires the furnace temperature to reach around 1300 degrees and takes more than four hours to complete.
Hydrogen decrepitation and jet milling are collectively referred to as powder-making. This process involves crushing the alloy ribbon sheets made by smelting and turning them into magnetic powder. To obtain well-oriented sintered NdFeB magnets, the powder particles must be small (3-4μm) with a concentrated size distribution, and the particles should be spherical or nearly spherical.
After crushing, the magnetic powder is loaded into a mold and an external magnetic field is applied for orientation. The powder is then pressed after orientation. Magnetic field orientation of the powder is one of the key technologies for producing high-performance sintered NdFeB magnets. Currently, the industry generally uses three methods for pressing: mold pressing, mold pressing with cold isostatic pressing, and rubber mold isostatic pressing. With the same Nd content, rubber mold isostatic pressing can achieve a larger magnetic energy product.
After pressing, the relative density of the magnet blanks is high. To ensure high permanent magnetic performance, the blanks need to be heated to a temperature below the melting point of the powder for heat treatment, a process also known as sintering. After rapid cooling from high temperatures, tempering is required at a certain temperature to optimize the microstructure and obtain the best magnetic properties. (Tempering means reheating and raising the temperature of the sintered magnetic blank after it has cooled to a certain temperature.)
Due to the characteristics and technical limitations of the magnetic field orientation forming process, sintered NdFeB magnets find it difficult to directly achieve the shape and dimensional accuracy required for practical applications. Many finished magnets are small and have complex shapes, and can only be made by machining blanks of a certain shape. Sintered NdFeB is hard and brittle, and typical machining operations include cutting, drilling, grinding, and rolling.
Rare earth elements such as dysprosium and terbium can significantly enhance the coercive force and temperature stability of the material. For sintered NdFeB materials that require high coercivity and working temperature, dysprosium and terbium are often added. However, these elements are very expensive and can greatly increase magnet production costs. The industry currently widely uses grain boundary diffusion technology to reduce the amount of heavy rare earth elements added.
Sintered NdFeB is a highly chemically active powder material with tiny pores and voids inside. It is prone to corrosion and oxidation in the air, which can lead to a decline in magnetic performance or even total loss over time. Thus, strict surface anti-corrosion treatment is necessary before use. Common anti-corrosion treatments for NdFeB include electroplating, chemical plating, electrophoresis, and phosphating, with electroplating being a widely used and mature metal surface treatment technique.
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