The sintering of sintered NdFeB is one of the most important processes
Sintered NdFeB is an anisotropic magnet produced by powder sintering. Generally, only blanks can be produced after sintering, and then through machining (such as wire cutting, slicing, grinding, etc.), magnets of various shapes can be formed. The sintering of sintered NdFeB means that in order to further improve the performance and usability of the magnet, improve the contact properties between powders, improve the strength, and make the magnet have high-performance microstructure characteristics, the green body needs to be heated to below the melting point of the powder matrix phase. temperature and hold for a period of time.
Sintering is an extremely important process in sintered NdFeB, and all manufacturers and researchers attach great importance to it. The relative density of the NdFeB powder compact is generally 50% to 70%, and the porosity is generally 30% to 50%. The bonding between particles is all mechanical bonding, and the bonding strength is extremely low. If the molding pressure is very high, some of the particles that have been in contact with each other have been elastically or plastically deformed, and the sample is easier to crack, and its microstructure is not enough to produce high magnetic properties.
During the sintering process of NdFeB green body, a series of physical and chemical changes will occur. First of all, the gas (including water vapor) adsorbed on the surface of the powder particles is removed, the evaporation and volatilization of organic substances (such as oil that may be stained in isostatic pressing or added antioxidants and lubricants, etc.) Reduction of oxides, recovery and recrystallization of deformed powder particles.
Secondly, atomic diffusion, material migration, the contact between particles is changed from mechanical contact to physicochemical contact, forming a combination of metal bonds and covalent bonds. The contact surface between the powders expands, and a sintering neck appears, followed by the growth of the sintering neck, the increase in density, and the growth of grains.
The powder green body has a large porosity and a large surface area, so the surface energy is also large, and it also has lattice distortion energy, so that the powder green body is in a high energy state as a whole. This is unstable from an energetic point of view, with a tendency and driving force to spontaneously sinter and bond into a dense body.
Therefore, under certain temperature conditions, that is, the kinetics allow, the contact between powder particles will be from point to surface in order to reduce the surface area and surface energy. As the interface between the particles expands, the green body begins to shrink and densify, becoming a sintered body. In short, sintering is the process by which a powder combination changes from a green body to a blank.
Sintered NdFeB permanent magnet materials have excellent magnetic properties and are widely used in electronics, electrical machinery, medical equipment, toys, packaging, hardware machinery, aerospace and other fields. The more common ones are permanent magnet motors, speakers, magnetic separators, Computer disk drives, magnetic resonance imaging equipment meters, etc.