Preparation process of sintered NdFeB magnets (2) – Milling powder
In the previous issue, we learned about alloy smelting and casting. After casting, the alloy needs to be made into powder through a pulverizing process before entering the subsequent processing process. Obtaining suitable powder shape, average particle size and particle size distribution is the basic purpose of powder preparation. The above-mentioned characteristic differences of the powder are manifested as changes in the bulk density, tap density, angle of repose, fluidity, compression ratio, internal friction and external friction coefficient of the powder on a macroscopic scale, which are directly related to the powder filling, The magnetic field orientation, blank pressing and demoulding, and the magnet microstructure generated by the sintering and heat treatment processes sensitively affect the permanent magnetic properties, mechanical properties, thermoelectric properties and chemical stability of the magnets.
The ideal microstructure of sintered magnets is that the fine and uniform main phase grains are surrounded by smooth and thin additional items, and the easy magnetization directions of the main phase grains are aligned along the orientation direction as consistently as possible. Voids, large grains, and larger-sized soft magnetic phases will seriously reduce the intrinsic coercive force of the magnet, while grains whose easy magnetization direction deviates from the orientation direction will simultaneously reduce the remanence and demagnetization curve squareness of the magnet. For this reason, it is necessary to make alloy ingots or quick-cooling sheets into single-crystal particles with an average particle size of 3-5 μm, a maximum particle size of less than 20 μm, and a shape close to spherical. At the same time, the proportion of too fine grains must be controlled to avoid the tendency of the powder to be severely oxidized. , if necessary, the powder surface treatment can be used to enhance the oxidation resistance of the powder, improve filling and compressibility. sintered NdFeB magnets Milling powder
1. Conventional mechanical crushing method
Rare earth transition group intermetallic compounds have high hardness and brittleness, alloy ingots are easily broken into small pieces by jaw crusher or similar machinery, and then mechanically crushed step by step to an average particle size of 3~5μm, but equipment wear 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) is Choose liquid protection ball mill or nitrogen, inert gas jet mill.
The double-alloy method or multi-alloy method of sintered NdFeB is also widely used. Usually, the alloy with a positive composition close to Nd2Fe14B and the Nd-rich fast cooling alloy are mixed and ground, and the Nd-rich powder with a small volume is evenly distributed to the Nearly positively 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. Alloy ingots of rare earth permanent magnet materials also have a strong tendency to absorb hydrogen. Hydrogen atoms enter the interstitial positions in the main phase of intermetallic compounds and rare earth-rich grain boundary phases to form interstitial atom compounds, which increase the interatomic distance and expand the lattice volume. , the resulting internal stress induces grain boundary cracking (intergranular fracture), grain fracture (transgranular fracture) or ductile fracture of the alloy in a very brittle alloy. Called “hydrogen fragmentation” or “hydrogen burst”. sintered NdFeB magnets Milling powder
3. Ammonia jet milling method
In the laboratory or large-scale production process, a 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 5 μm . Considering that the gas pressure is proportional to the average kinetic energy of gas molecules, under the same pressure, the gas with small molecular weight has a greater flight speed, and the increase of gas flow rate is beneficial to increase the natural collision frequency of powder particles. Hydrogen molecules and helium molecules are the best candidates, but due to the explosiveness of hydrogen, helium is the best choice, and the flow rate of helium is 2.9 times that of nitrogen, which can make Nd-Fe-B coarse powder Grinding to D50=2μm or less.