Research On Methods Of Manufacturing Sintered NdFeB Permanent Magnets

This paper summarizes the latest development on methods of manufacturing sintered NdFeB permanent magnets. On the basis of that, systematic research is done on effect of NdFeB ingots on microstructure characteristics and magnetic properties, effect of pressure and temperature and surface status on hydrogen-absorbing of NdFeB alloys, effect of control and calculation of needed hydrogen and optional hydrogen decrepitation on HD process, effect of powder-making process and sintering process on magnetic properties. Moreover, microcosmic mechanism is analyzed and summarized.It is shown that the grain of traditional metallic ingot is coarse, and that the distribution of Nd-rich phase is obviously conglomerated and there exists partial precipitation of -Fe, and that the grain of sheet ingot is smaller, pillar-like grain is clear and the Nd-rich phase is thinner and distributed equally. Compared with traditional ingot, microstructure of the sheet ingot is obviously optimized. However, there is certain difference between outside and inside of the sheet ingot. Meanwhile, it is found that NdFeB alloy cast strip acquired by strip-casting process has excellent pillar-like grain and smaller gratitude, and its Nd-rich phase is uniformly and thinly distributed around the main phase. Therefore, NdFeB cast strip has ideal microstructure to produce magnets with high magnetic properties.During the process of hydrogen absorption, the higher the temperature is, the quicker hydrogen is absorbed, and the less needed time is. When the temperature goes up to a certain value, the incubation period disappears. The higher hydrogen pressure is, the shorter the incubation period is and the quicker hydrogen is absorbed. Ingots with fresh surface have no incubation period, and the longer the ingots are stored, the longer the incubation period is and the more slowly hydrogen is absorbed.It is found that the fracture of selective hydrogen decrepitation is almost all intergranular fracture. There is no crack inside most Nd2FeuB particles in the powder of selective hydrogen decrepitation. Formed Nd2Fei4B particles are fewer. There are transgranular cracks in some Nd2Fei4B particles, which are formed by press strains in two sides of Nd2Fei4B phase because of hydrogen absorption of Nd-rich phase.Research shows that when the sintering temperature is lower than the proper one, remanence and maximum energy product increase with the increase of temperature, and when the temperature is higher than the proper one, remanence and maximum energy product decrease with the increase of temperature. The proper temperature is 1120 C.Compared with vacuum sintering, the magnet made by Argon sintering has lower density, maximum energy product and remanence, but higher coercivity. Accordingly, vacuum sintering can be used to make magnet with better magnetic properties and it is proper for NdFeB magnets.After magnet is annealed, Nd-rich phase becomes smaller and is distributed more uniformly, so its coercivity is greatly improved. Magnet of high vacuum annealing has more and bigger voids, and there are more linear crystal interface defects produced by the volatilization of sheet-like Nd-rich phase, So density, maximum energy product and remanence become smaller. Since the voids in magnet of low vacuum annealing are less and smaller, and the linear crystal interface defects are obviously fewer, density, maximum energy product and remanence of the magnet become bigger. Accordingly, low vacuum annealing can be used to make magnet with better magnetic properties, and it is proper for sintered NdFeB magnet.