Sintered NdFeB permanent magnets are produced by powder metallurgy. There are sixteen process links in total, including several monitoring and analysis at different stages. Strictly speaking, these sixteen links are indispensable. The production of sintered NdFeB is a systematic project with interlocking links. Each link must lay a solid foundation for the next link. Once a process link fails to meet the requirements, the permanent magnet produced may not meet the target performance. Requirements and become waste products, or the yield is not high.
Raw Material
As the saying goes, "the medicine is good, the medicine is good". This sentence is very suitable for the production of sintered NdFeB magnets. Good raw materials are the basis for the production of high-quality magnetic materials. Raw materials are generally purchased according to the requirements of high-grade, medium-grade or low-grade sintered NdFeB magnets produced by magnetic material manufacturers, and according to the corresponding national standards. Before smelting, raw materials should be cut and surface treated.
Component Design
The composition design of sintered NdFeB is very important. It involves whether the quality of the product and the magnetic performance index can meet the customer's requirements, because many intrinsic magnetic properties of the material, such as magnetic polarization and Curie temperature, are determined by the composition of the material. decided. The basic principle of composition design is to ensure a sufficiently high intrinsic performance while taking material cost into consideration. (The cost of raw materials accounts for about 65%-90% of the total cost of sintered NdFeB materials. Under the premise of meeting the magnetic performance requirements of users, cheap components should be used as much as possible, and rare earth metals and other precious metal materials should be used less)
Melting Ingot/slab
Smelting is the first process for sintered NdFeB strong magnets to enter the production process. The melting furnace produces alloy strips. This process requires the furnace temperature to reach about 1300 degrees and lasts for four hours to complete. After this process, the raw materials are processed into alloy sheets through hot melting and cooling, and then proceed to the next process.
Milling
The purpose of pulverization is to crush large alloy ingots into powders of a certain size. The latest pulverization process is to pulverize NdFeB flakes (SC flakes) through hydrogen breaking and jet milling. In order to obtain a well-oriented magnet, the powder particle size is required to be small (3-4μm) and the size distribution is concentrated, and the powder particles are spherical or nearly spherical.
Orientation and Profiling
In the last issue, Magneto has introduced the magnetic orientation of sintered NdFeB to everyone. Powder magnetic field orientation is one of the key technologies for producing high-performance sintered NdFeB. After the crushed magnetic powder is loaded into the mold, an external magnetic field is applied for orientation, and after orientation, the powder is pressed. At present, there are three commonly used pressing methods: membrane pressing, mold pressing and cooling isostatic pressing, and rubber mold isostatic pressing. Under the same neodymium content, rubber mold isostatic pressing can obtain a larger magnetic energy product.
Sintering and Tempering
The relative density of sintered NdFeB powder compact is relatively high, the contact between particles is mechanical contact, and the bonding strength is low. In order to further increase the density, improve the contact properties between powder particles, improve the strength, and make the magnet have high permanent magnetic properties Microstructural characteristics, 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 called sintering.
After the sintered magnet is quenched at high temperature, the grain boundary phase distribution is uneven and the grain boundary is not clear, so it 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 up again. The tempering temperature needs to be determined through experiments or thermal difference analysis.
Machining and Surface Treatment
The shapes of sintered NdFeB magnets in practical applications are various, such as discs, cylinders, rings, squares, tiles, sectors and various irregular shapes. Due to the different shapes and sizes of permanent magnet components, in the production process, except for large-size regular permanent magnet components, other magnets are difficult to form at one time. Therefore, generally in the process of powder metallurgy, large blanks are produced first, and after sintering and tempering treatment, mechanical processing (including cutting, drilling, etc.) Magnets of various shapes and sizes. There are 3 types of machining, including
1. Cutting cylindrical and square columnar magnets into disc-shaped and square-shaped components is called cutting processing
2. Processing circular and square magnets into fan-shaped, tile-shaped, or magnets with grooves or other complex shapes is called shape processing
3. Processing round rods and square rod magnets into cylindrical or square cylindrical components is called punching
Machining methods include grinding and slicing, EDM cutting and laser processing.
Quality Inspection
The quality monitoring during the production process of sintered NdFeB permanent magnets and the quality inspection of the final product should include the items listed in the table below, but not every item needs to be tested, which needs to be determined by the requirements of the product order contract.