Sintered sintered NdFeB means that in order to further improve the performance and usability of magnets, to improve the contact between the powder properties and improve the strength of the magnet with high-performance microstructure features, the green body needs to be heated to below the melting point of the powder matrix The temperature and insulation for a period of time process.
Sintering is extremely important process, all manufacturers and the majority of researchers attach great importance. The relative density of NdFeB powder compacts is generally 50% to 70%, the porosity is generally 30% to 50%, the combination of particles are all mechanical bonding, bond strength is extremely low. If the molding pressure is very high, some of the particles that have come into contact with each other already have elastic or plastic deformation. In this case, the sample is more likely to crack and its microstructure is insufficient to produce high magnetic properties.
During the sintering of the green body, a series of physicochemical changes will take place. First of all, the gas (including water vapor) adsorbed on the surface of the powder particles is excluded, the evaporation and volatilization of organic substances (such as oil that can be contained in the isostatic pressure or added antioxidants and lubricants, etc.), the elimination of stress, Reduction of oxides, recovery and deformation of deformed powder particles. Second, atomic diffusion, material migration, and particle-to-particle contact are changed from mechanical to physical to chemical to form a combination of metal and covalent bonds. Finally, the contact surface between the powder expands, there sintering neck, followed by the sintering neck grows, the density increases, such as grain growth. Powder green body porosity, surface area is large, so the surface energy is also large, but also has a lattice distortion energy, the powder green as a whole in a state of high energy. From an energy point of view, this is not stable, with the propensity and driving force to spontaneously sinter and bond into a dense body. Therefore, under certain temperature conditions, kinetic conditions, the contact between the powder particles will be from point to surface in order to reduce the surface area and surface energy. As the interface between particles expands, the green body begins to shrink and densify, eventually becoming a sintered body. In short, sintering is the process of converting a powder combination from a green body to a green body.
Sintering is divided into liquid phase sintering and solid phase sintering, the two sintering methods have much in common.
1, liquid sintering
NdFeB sintered permanent magnet consists of the main phase (Nd2Fe14B), Nd-rich phase and B (Nd1.1Fe4B4) phase. If the content of B in the magnet is less than 6.8%, the content of the B-rich phase is small, so that the magnet can be considered to be composed of the main phase and the Nd-rich phase. As can be seen from the ternary phase diagram of NdFeB, the main phase has a melting point of about 1185 ° C and the Nd-rich phase has a melting point of about 655 ° C (equilibrium state). NdFeB permanent magnet sintering temperature 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, first, the formation 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 fine particles and Large particles of the protuberances and edges will be dissolved in the liquid phase, when the solid phase in the liquid phase solubility exceeds its saturation, it is necessary to precipitate on the surface of large particles; Third, solid phase sintering, that is, if the liquid phase sintering When the liquid phase is insufficient, part of the particles will be in direct contact with each other to become solid-phase sintered. Therefore, it can be said that solid phase sintering is the latter stage of liquid phase sintering.
2, solid-phase sintering
Before sintering powder is a mechanical contact, at the sintering temperature, in order to reduce the surface energy, the contact area gradually expanded to form a sintered neck. The broadening of the sintering neck is achieved by atomic diffusion and material migration, with the result that the powder particles are brought closer together, resulting in the densification of the sintered body.
Densification of liquid phase sintering
The various stages of liquid phase sintering are intertwined, that is, the first stage has not yet ended, the second stage has begun, the same, the second stage is not over, the third stage has already begun . The first stage due to the formation of liquid phase, the rapid increase in shrinkage, shrinkage at this time depends mainly on the number of liquid phase. As the sintering temperature increases, the amount of liquid phase increases and the amount of shrinkage increases. The second stage is mainly the solid phase precipitation and dissolution to densification, and slow down. The third stage is mainly densified by the diffusion of solid phase or material migration, and the densification rate is further slowed down.
Densification of liquid phase sintering and the number of liquid phase, liquid phase infiltration of the solid phase, interfacial energy, powder particle size, solid phase and liquid phase solubility and other factors. The densification process can be expressed as the densification coefficient α, as shown in the following equation.
α = (ρ burning - p pressure / ρ management - p pressure) × 100%
Where, α-densification coefficient;
ρ burning - sintered density;
ρ pressure - compact density;
ρ 理 - density of dense theory
Liquid phase sintering three stages of the change of densification coefficient process shown in the figure, the first densification process from the beginning to the end of about 9min, that is, the liquid phase and the flow of the process, this time due to the formation of the liquid phase, shrink The rate of rapid increase, the shrinkage rate depends mainly on the number of liquid phase. The second densification process starts from about 5min to about the end of the first 80min, which is the dissolution and precipitation process of the powder particles. At this time, the shrinkage rate is reduced. The third densification process starts from about 50 min to the end of sintering, which is extremely solid phase sintering time. The densification process mainly depends on solid phase diffusion or substance migration, and the shrinkage rate is slower than the second one.
3, sintering method classification
As far as the sintering method is concerned, in addition to the traditional sintering methods, some relatively new sintering technologies have been developed one after another.
(1) EDM sintering
Spark Sintering is a method in which a powder of metal or the like is charged into a mold made of graphite or the like and a specific sintering power source and a pressing pressure are applied to the sintered powder by means of upper and lower molds as energization electrodes, Deformation and cooling phase to complete the preparation of high-performance materials or parts of a method.
(2) discharge plasma sintering
The SPS technique is a pressure sintering method using DC pulse current energization sintering. The basic principle of the SPS technique is to generate Joule heat uniformly by each particle inside the sintered body through the discharge plasma instantaneously generated by the direct current pulse current applied to the electrode, Activation, sintering at the same time under pressure. This technique has the following sintering characteristics: (1 sintering temperature is low, generally lower than the normal sintering temperature of 200 ~ 300 ℃; (2 sintering time is short, just 3 ~ 5min can; (3 can be fine, uniform tissue and Can maintain the natural state of the original material; (4 can obtain high density materials; (5 can be shaped, thin-walled rings and other shaped and large-size workpieces.At present, SPS technology has become a research hotspot in the field of material preparation in foreign countries, Used in ceramics, metals, intermetallic compounds and other materials and the preparation of dissimilar materials such as connection.
(3) microwave sintering
Microwave is a very high frequency of electromagnetic waves. Usually 300MHz ~ 300GHz electromagnetic wave is zoned microwave band, the corresponding wavelength range of 1mm ~ 1m. Microwave sintering is the use of microwave dielectric materials in the magnetic field loss of materials so that the overall heating to the sintering temperature and densification of the rapid sintering of new technologies. In the microwave sintering apparatus, only the sample is at a high temperature, while the rest is still at a normal temperature. Therefore, the entire apparatus has the advantages of compact structure, extremely fast heating and sintering speed, and ultra-high temperatures above 2000 DEG C can be obtained easily and economically.
(4) electric field rapid reaction sintering method
Electric field rapid reaction sintering method that is electric field self-propagating high temperature synthesis, referred to as the electric field SHS (Self-propagating High-temperature Synthesis) method. At present, this method mainly uses the electric field as a means of combustion reaction of the system. Professor Z.A.Munir of the United States first synthesized TiC from Ti powder and C powder under the electric field. The principle is that the first use of specialized ignition to stimulate the chemical reaction between Ti and C system, the reaction occurs, the immediate use of an electric field to provide heat to maintain the reaction continues.
There are two kinds of sintering methods: sintering under argon and sintering in vacuum. The magnets prepared by these two sintering methods differ in their appearance, magnetic properties and microstructure. In order to determine which sintering method is more advantageous, it is necessary to carry out the corresponding test to determine.
In exploring the impact of other processes on the performance of the magnet, use this sintering process to do conventional sintering. But the process is not necessarily the best process. The sintering process suitable for this magnet needs to be determined experimentally.
Article from NdFeB Industry Network