The sintering of sintered ndFeb refers to the process of heating the raw billet to the temperature below the melting point of the powder matrix and holding the heat for a period of time in order to further improve the performance and usability of the magnet, improve the contact properties between powders, improve the strength, so that the magnet has high performance microstructure characteristics.

Sintering is the more important process of sintered NdFeB powder billet, the relative density is generally 50% ~ 70%, porosity is generally 30% ~ 50%, the combination of particles is mechanical combination, low strength.

If the molding pressure is high, some particles that have been in contact with each other have produced elastic or plastic deformation, then the sample is easier to crack, and its microstructure is not enough to produce high magnetic properties.

A series of physical and chemical changes will occur in the sintering process of green billet. Firstly, the removal of gas (including water vapor) adsorbed on the surface of powder particles, the evaporation and volatilization of organic matter (such as oil or added antioxidants and lubricants in isostatic pressure), the elimination of stress, the reduction of oxide on the surface of powder particles, the recovery and recrystallization of deformed powder particles.

Secondly, atomic diffusion, material migration, the contact between particles from mechanical contact to physical and chemical contact, forming the combination of metallic bonds and covalent bonds. The contact surface between powders expands, and sintering neck appears, followed by sintering neck growth, density increase, grain growth, etc.

The porosity and surface area of the powder green billet are large, so the surface energy is also large. At the same time, it also has lattice distortion energy, so that the powder green billet is in a high-energy state as a whole. This is unstable from an energy point of view, with a tendency and drive to spontaneously sinter and bond into a dense body.

Thus, under certain temperature conditions, that is, if the dynamics permit, the contact between powder particles will be from the point to the surface in order to reduce the surface area and surface energy. As the contact surface between particles expands, the green body begins to shrink and densify, becoming a sintered body. In short, sintering is the process of powder combination from green to blank.

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

1. Liquid phase sintering

Ndfeb sintered permanent magnet is composed of main phase (Nd2Fe14B), Nd rich phase and B rich phase (Nd1.1Fe4B4). If the content of B in the magnet is less than 6.8%, the composition of B-rich phase is less, so it can be considered that the magnet is composed of main phase and Nd-rich phase.

As can be seen from the ternary phase diagram of NdFeB, the melting point of the main phase is about 1185℃ and that of the Nd-rich phase is about 655℃ (equilibrium state). The sintering temperature of NdFeB permanent magnets is generally 1080℃. 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, one, the formation and flow of liquid phase; Two, dissolution and precipitation, that is, if the solid phase can be dissolved in the liquid phase, when the liquid phase appears, small particles and large particles of the protrusions and corners 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 the large particles; Three, solid phase sintering, that is, if the liquid phase of liquid phase sintering is insufficient, some particles will be in direct contact with each other, thus becoming solid phase sintering. So it can be said that solid phase sintering is the latter stage of liquid phase sintering.

2. Solid phase sintering

Before sintering, the powder is in mechanical contact. At sintering temperature, in order to reduce the surface energy, its contact area gradually expands to form a sintering neck. The expansion of the sintered neck is achieved through atomic diffusion and material migration, resulting in closer proximity of the powder particles, resulting in densification of the sintered body.

Densification of liquid phase sintering

The phases in liquid phase sintering are fused with each other, that is, before the end of the first stage, the second stage has begun, similarly, before the end of the second stage, the third stage has begun.

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 number of liquid phase. With the increase of sintering temperature, the amount of liquid phase increases and the amount of shrinkage also increases.

In the second stage, solid phase is mainly precipitated and dissolved to densify, and the rate slows down. In the third stage, the densification process is further slowed down by diffusion or migration of solid phase.