Isotropic rapidly quenched rare earth permanent magnet powder
The preparation of bonded magnetic powder is quite different from that of sintered NdFeB because sintered NdFeB alloy ingot or sintered body does not have practical coercivity when broken to the particle size used to bond the magnet. Mass production is prepared in an inert gas environment. The molten alloy is cooled at 105~106℃/s and condensed into microcrystalline or even amorphous structures. After crystallization and heat treatment, the grains grow to tens or hundreds of nanometres, and high coercivity is obtained in submicron grains smaller than the critical size of Nd2Fe14B single-domain.
Usually, it is difficult for magnetic particles to be broken into such fine single crystal particles, and the technology of directional growth of submicron grains by fast quenching is not mature. Therefore, the polycrystalline powder is made by melting fast quenching method, and the easy magnetization axis of each grain has no strong disposition tendency, and the magnetic particles are isotropic. Such a high cooling rate is achieved by dumping or spraying the hot molten alloy liquid onto the water-cooled rotating copper wheel with a linear velocity of 16 30m/s. The liquid alloy is thrown out along the tangent direction under the acceleration of the rotating copper wheel and is condensed into a thin strip with a thickness of ~100μm. The cooling rate sensitively determines the grain size of the thin strip. It can sensitively affect the shape of demagnetization curve and intrinsic coercivity of magnetic powder.
As the cooling rate depends on many factors such as alloy liquid temperature, flow velocity, copper wheel speed and temperature, argon atmosphere, etc., it is difficult to strictly control and synchronize optimization. If the optimal quenchable nanocrystalline microstructure is taken as the mass production target, it is very easy to lead to too wide grain size distribution, and the corresponding inherent coercivity distribution of grains is also wide, resulting in poor square degree of demagnetization curve of magnetic powder.
