Production process of sintered neodymium iron boron magnet-(2) powder making
We have learned about alloy smelting and casting NdFeB maget. After casting, the alloy needs to be made into powder through a powdering process before it can enter the subsequent processing process. Obtaining a suitable powder shape, average particle size and particle size distribution is the basic purpose of powder preparation. The above-mentioned characteristic difference of the powder is manifested in the macroscopically as the change of the powder's loose density, tap density, angle of repose, fluidity, compression ratio, internal friction and external friction coefficient, etc., which are directly related to the powder filling in the forming process. The magnetic field orientation, blank pressing and demoulding, as well as the microstructure of the magnet generated by the sintering and heat treatment process, which sensitively affect the permanent magnetic properties, mechanical properties, thermoelectric properties and chemical stability of the magnet.
The ideal microstructure of a sintered magnet is that the fine and uniform main phase crystal grains are surrounded by smooth and thin additional items, and the easy magnetization direction of the main phase crystal grains are arranged along the orientation direction as uniformly as possible. Voids, large crystal grains and larger-sized soft magnetic phases will all seriously reduce the intrinsic coercivity of the magnet, while crystal grains whose easy magnetization direction deviates from the orientation direction will reduce the magnet’s remanence and the squareness of the demagnetization curve at the same time. For this reason, it is necessary to make alloy ingots or rapid cooling sheets into single crystal particles with an average particle size of 3~5μm, a maximum particle size of less than 20μm, and a shape close to a spherical shape. At the same time, the proportion of excessively fine grains must be controlled to avoid the tendency of severe oxidation of the powder. If necessary, the powder surface treatment can be used to enhance the anti-oxidation ability of the powder and improve the filling and compressibility.
1. Conventional mechanical crushing method
Rare earth transition group intermetallic compounds have high hardness and brittleness. The alloy ingots can be easily broken into small pieces with a jaw crusher or similar machinery, and then mechanically pulverized step by step to an average particle size of 3~5μm, but equipment wear will bring in The impurities also inevitably affect the quality of the powder. Due to the serious oxidation tendency of rare earth metals and their intermetallic compounds, coarse crushing (~10mm level) and medium crushing (~100μm level) are usually carried out under a protective atmosphere such as nitrogen or argon, while fine grinding (average particle size 3~5μm) Choose liquid protection ball mill or nitrogen, inert gas jet mill.
The two-alloy or multi-alloy method of sintered neodymium iron boron is also widely used. Usually, the alloy with the normal component of Nd2Fe14B and the Nd-rich fast-cooling alloy are mixed and ground, and the small-volume Nd-rich powder is evenly distributed to Nearly divided into the main body of alloy powder.
2. Hydrogen Decrepitation (HD)
The research on the hydrogen absorption behavior of rare earth metals, alloys and intermetallic compounds and the physical and chemical properties of hydrides has always been a major topic in the application of rare earths. The most direct example is hydrogen batteries. The alloy ingots of rare earth permanent magnet materials also have a strong tendency to absorb hydrogen. Hydrogen atoms enter the interstitial sites in the main phase of the intermetallic compound and the rare earth-rich grain boundary phase to form interstitial atom compounds, which increase the distance between atoms and expand the volume of the crystal lattice. , The resulting internal stress causes the alloy's grain boundary cracking (intergranular fracture), grain fracture (transgranular fracture) or ductile fracture in the brittle alloy, because this cracking or accompanying crackling sound, so it is It is called "Hydrogen Fragmentation" or "Hydrogen Burst".
3. Ammonia jet milling method
In the laboratory or large-scale production process, the fluidized bed jet mill with high pressure (0.6MPa) and high purity (99.995%) nitrogen as the power source is usually used, and the median particle size D50 measured by the laser particle size analyzer is about 5μm . Considering that the gas pressure is proportional to the average kinetic energy of the gas molecules, under the same pressure, the gas with a small molecular weight has a greater flight speed, and the increase of the gas flow rate is beneficial to increase the natural collision frequency of the powder particles. Hydrogen molecules and helium molecules can be described as the best candidates, but because of the explosive nature of hydrogen, helium is the best choice. The flow rate of helium is 2.9 times that of nitrogen, which can remove Nd-Fe-B coarse powder in a short time Crush to D50=2μm or less.