Sintering of sintered NdFeB means that in order to further improve the performance and usability of the magnet, improve the contact properties between the powders, increase the strength, and make the magnet have high-performance microstructure characteristics, it is necessary to heat the green body below the melting point of the powder matrix phase. The temperature is kept for a period of time.
Sintering is an extremely important process for sintering NdFeB, which is highly valued by all manufacturers and researchers. The relative density of the NdFeB powder compact is generally 50% to 70%, and the porosity is generally 30% to 50%. The bonding between the particles is all mechanically combined, and the bonding strength is extremely low. If the molding pressure is very large, some of the particles that have been in contact with each other have been elastically or plastically deformed, and the sample is more likely to be cracked, and the microstructure thereof is insufficient to produce high magnetic properties.
During the sintering of the green body, a series of physicochemical changes will occur. First, the gas adsorbed on the surface of the powder particles is removed, the evaporation and volatilization of the organic matter, the elimination of the stress, the reduction of the oxide on the surface of the powder particles, and the recovery and recrystallization of the deformed powder particles. Secondly, atomic diffusion, material migration, and contact between particles change from mechanical contact to physical chemical contact, forming a combination of metal bonds and covalent bonds. Finally, the contact surface between the powders is enlarged, and the sintered neck appears, followed by the growth of the sintered neck, the increase in density, and the growth of crystal grains. The powder green body has a large porosity and a large surface area, so that the surface energy is also large, and at the same time, it has lattice distortion energy, so that the powder green body as a whole is in a high energy state. From an energy point of view, this is unstable, with a tendency and driving force to spontaneously sinter and bond into a dense body. Therefore, under certain temperature conditions, ie, where kinetics permit, the contact between the powder particles will be from point to face in order to reduce surface area and surface energy. As the contact surface between the particles expands, the green body begins to shrink and densify, eventually becoming a sintered body. In short, sintering is the process by which a powder compact is changed from a green body to a blank.
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