sintered NdFeB refers to the process of heating the green body to the temperature below the melting point of the powder matrix phase and holding it for a period of time in order to further improve the performance and usability of the magnet, improve the contact properties between the powders, improve the strength, and make the magnet have high-performance microstructure characteristics.

sintering is an important process in the sintering of NdFeB, all the manufacturers and the majority of researchers attach importance to it. The relative density of NdFeB powder compacts 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 low.
If the molding pressure is very large, some of the particles that have been in contact with each other have already produced elastic or plastic deformation, then the sample is easier to crack, and its microstructure is not enough to produce high magnetic properties.

During the sintering process of the 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 excluded, the evaporation and volatilization of organic matter (such as oil or added antioxidants and lubricants, etc.), the elimination of stress, the reduction of oxides on the surface of the powder particles, the recovery and recrystallization of the deformed powder particles.

Secondly, atomic diffusion, material migration, and the contact between particles are changed from mechanical contact to physical and chemical contact, forming a combination of metal bonds and covalent bonds. Finally, the contact surface between the powders expands, and a sintering neck appears, followed by sintering neck growth, density increase, grain growth, etc. The porosity of the powder green body is large, the surface area is also large, so the surface energy is also large, and it also has the lattice distortion energy, so that the powder green body is in a high energy state as a whole. This is unstable from an energy 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, when the dynamics allow, the contact between the powder particles will be from point to surface, so as to reduce the surface area and surface energy. As the contact surface between the particles expands, the green body begins to shrink and densialize, and finally becomes a sintered body. In short, sintering is the process by which a powder bond is changed from a green body to a blank.

sintering is divided into liquid phase sintering and solid phase sintering. These two sintering methods have a lot in common.

1, liquid phase sintering

NdFeB sintered permanent magnet consists of a main phase (Nd2Fe14B), a Nd-rich phase and a B- rich (Nd1.1Fe4B4) phase. If the B content in the magnet is less than 6.8%, the component of the B- rich phase is small, and the magnet is considered to be composed of the main phase and the Nd-rich phase.

From the ternary phase diagram of NdFeB, it can be seen that the melting point of the main phase is about 1185°C, and the melting point of the Nd-rich phase is about 655°C (equilibrium state). The sintering temperature of the NdFeB permanent magnet is generally 1080°C. At this temperature, it is generally composed of a solid main phase and a liquid Nd-rich phase, which is called liquid phase sintering. The basic process of

liquid phase sintering

liquid phase sintering can be roughly divided into three stages. First, the generation and flow of liquid phase; Second, dissolution and precipitation, that is, if the solid phase can be dissolved in the liquid phase, when the liquid phase appears, the protrusions and corners of fine particles and large particles will be dissolved in the liquid phase. When the solubility of the solid phase in the liquid phase exceeds its saturation, it is necessary to precipitate on the surface of large particles; third, solid-phase sintering, that is, if the liquid phase during liquid-phase sintering is insufficient, some particles will be in direct contact with each other, thus becoming solid-phase sintering. Therefore, it can be said that solid-phase sintering is the latter stage of liquid-phase sintering.

2, solid-phase sintering

The powder before sintering is in mechanical contact. At the sintering temperature, in order to reduce the surface energy, its contact area gradually expands to form a sintering neck. The enlargement of the sintering neck is achieved by atomic diffusion and mass migration, as a result of which the powder particles are brought closer together, leading to densification of the sintered body.

Densification of liquid phase sintering

The various stages in liquid phase sintering are merged with each other, that is, before the first stage is over, the second stage has already started, and likewise, before the second stage is over, the third stage has already started.

In the first stage, due to the formation of the liquid phase, the shrinkage rate increases rapidly. At this time, the shrinkage rate mainly depends on the amount of the liquid phase. With the increase of sintering temperature, the amount of liquid phase increases, and the amount of shrinkage also increases. The second stage of

is mainly the precipitation and dissolution of solid phase to densification, and the rate is slowed down. The third stage mainly depends on the diffusion of solid phase or material migration to densify, and the rate of densification process is further slowed down.