Development of high-performance NdFeB Sintered Magnets
The means for improving the properties of NdFeB sintered magnets are: (1) increasing the saturation magnetic polarization of the matrix phase; (2) increasing the volume fraction of the matrix phase in the magnet; (3) increasing the degree of orientation; (4) increasing the density of the magnet; ) Control the die. The following is a summary of several new process technologies for the production of high performance Nd-Fe-B sintered magnets:
1. Wet press forming technology (HILOP)
In order to reduce the oxidation of the powder during the milling and molding process and to improve the orientation of the powder, Hitachi Metals Co., Ltd. proposed a wet pressing process called HILOP (Hitachi Low Oxygen Process). The process flow is shown in Figure 1. The mineral oil is used as a solvent, and the powder obtained by the gas flow under anaerobic conditions is mixed into a slurry, and the slurry is pressed and formed under a magnetic field of 1120 kA/m (14 kOe). The mixture was treated under vacuum (13 Pa) at 100 ° C ≤ T ≤ 300 ° C for 1 hour, and the oil in the pressed compact was filtered off, followed by vacuum sintering. The advantages of wet press forming are: (1) Since the powder is in oil before and after magnetic field forming, it is not in contact with air until sintering, so the oxygen content in the magnet is greatly reduced, from 0.58% to 0.16% in the conventional process. (2) During the magnetic field forming process, the magnetic powder is oriented in a wet state, which reduces the frictional force and cohesive force between the powder particles, and thus the orientation of the magnetic powder is greatly improved. (3) Since the wet pressing process is not easily oxidized, the particle size of the magnetic powder can be controlled to be finer and more uniform, so that the average grain size of the sintered magnet is also finer and more uniform, and as the crystal grain size of the magnet is reduced and uniformly distributed, The corrosion resistance and mechanical strength of the magnet are also controlled.
M. Takaheshi et al. obtained a magnet having a magnetic property of (BH)max = 408 kJ/m3 and Hci = 1048 kA/m by a wet press molding process. Its composition is Nd28.86Dy0.75Fe remaining Nb0.34Al0.08Cu0.05B1.01, its oxygen content is less than 0.2%, and the magnet density is 7.7g/cm3.
2, rubber mold pressure (RIP) technology
Rubber Isostatic Pressing is to place a rubber mold filled with alloy powder in a metal, magnetically oriented with a pulsed magnetic field, placed on a press, and a static magnetic field, so that the upper and lower indenters will be rubber mold and alloy powder. Compressed together, the alloy powder in the rubber mold is subjected to isostatic compression (see Figure 2 for the process principle). The conventional magnetic field forming process is carried out in a metal mold, and the powder particles move in one direction whether pressed in parallel with the direction of the magnetic field or perpendicular to the direction of the magnetic field. Due to the obstruction of the friction between the mold wall and the magnetic powder, and the magnetic repulsive force between the magnetic powder, some of the particles are easily magnetized away from the orientation magnetic field. Compared with general metal molding, RIP has the following characteristics:
(1) RIP equipment has a simple structure and magnetic properties are better than metal molding (see Table 1).
(2) RIP is isostatic compression, and the c-axis orientation of the magnetic powder is not easily disturbed during the compression process.
(3) RIP automation devices are simpler and more compact than metal molding.
Table 1 Effect of two pressing methods on magnetic properties of Nd-Fe-B sintered magnets
|Rubber mold pressure||1.31||1122||322|
3. Homogenization annealing technique of Nd-Fe-B alloy ingot
In order to improve the magnetic properties, it is necessary to increase the volume fraction of the matrix phase (Nd2Fe14B) in the magnet, and the content of Nd in the alloy is reduced to be close to the stoichiometric composition. When the content of Nd is low, a large amount of α-Fe is precipitated in the ingot after smelting, and is sintered. The combination of α-Fe and Nd-rich Nd2Fe14B is difficult, so α-Fe should be avoided in the ingot. The results show that  Nd-Fe-B alloy ingots are homogenized and annealed, which reduces the amount of α-Fe and reduces the number of second phases associated with α-Fe. Since α-Fe reacts with the Nd-rich boundary phase and Nd1+εFe4B4 during annealing, more Nd2Fe14B phase is formed. When the amount of α-Fe precipitated in the alloy ingot is less than 2% by mass, the crushing and milling are not difficult, and the reduction of Nd causes a significant increase in the corrosion resistance of the magnet.
Casting process technology
The casting belt process is similar to the rapid quenching process, and the thickness of the ingot is further reduced to 250 to 350 μm and the width is several centimeters. The cooling wheel rotates at a linear speed of lm/s, which is much slower than the wheel speed at the time of rapid quenching. Therefore, the resulting cast piece is crystalline. This new technology enables the mass production of thin strips with fine and uniform grain structure and no α-Fe precipitation. At present, it has been used for mass production of high magnetic energy product Nd-Fe-B magnets. It can produce magnets with a maximum magnetic energy product (BH)max higher than 400kJ/m3, and can also obtain high Hci magnets. The casting belt process features are:
(1) A large amount of α-Fe formation can be avoided when the rare earth content is low, which creates conditions for producing high (BH)max magnets.
(2) Uniform microstructure, a sufficiently large distance (about 3 μm) between the rare earth-rich photo layers, so that the columnar Nd2Fe14B grains can form single crystal powder particles therebetween after the powdering.
(3) The diffuse distribution of the Nd-rich phase results in an optimal distribution of the liquid phase during sintering, which is very advantageous for increasing the density.
4, duplex alloy technology
The conventional single-phase process has one drawback, namely, the non-uniformity of the Nd-rich phase distribution in the alloy, which does not guarantee the formation of a thin and uniform liquid-phase barrier around all of the Nd2Fe14B phase grains. The result affects both the magnetic orientation of the grains and the adverse effects on the densification and coercivity of the sintered magnet.
To this end, a duplex alloy method is proposed in which a matrix phase alloy powder having a composition very close to a stoichiometric composition is mixed with a liquid phase alloy powder in a certain ratio, and then subjected to magnetic field forming and sintering to form a magnet. The matrix phase is vacuum smelted and crushed, and the liquid phase alloy is prepared by rapid quenching or HDDR process. The advantages of the two-phase method are:
(1) The quenched or HDDR magnetic powder crystal grains are very fine and can be uniformly dispersed around the Nd2Fe14B grains during sintering to form a uniform liquid phase separation layer, which can reduce the excess liquid phase and increase the matrix phase. The volume fraction, in turn, increases the sintered density and coercivity of the magnet.
(2) The quenched powder or HDDR powder has fine grain size, strong anti-oxidation ability, and is not easy to be oxidized during milling, so the magnet has good oxidation resistance. It was reported that in 1990, Japan produced a magnet with a maximum energy product of 416 kJ/m3 (52.3 MGOe) by a two-phase method. In 1996, Germany also reported the production of NdDyFeB magnets with Cu and Co added by biphasic method, Br=1.41T, Hci=l080kA/m, (BH)max=385kJ/m3 (48MGOe).
In addition to vacuum smelting in the production of matrix phase alloys in the two-phase process, in 1995 Sumitomo Metal Mining Co., Ltd. proposed to use the reduction diffusion (R/D) process to obtain  better because of the reduction and diffusion of matrix phase alloys. The reaction is carried out below the temperature of the coating reaction, and it is easier to form single-phase Nd2Fe14B crystal grains without precipitation of α-Fe. The company uses the R/D method to prepare the matrix phase Nd2Fe14B powder, and uses the rapid quenching method to prepare the liquid phase alloy powder. The sintered magnets obtained by mixing in a certain ratio have the following properties:
(1) Br=1.375T, Hci=1037kA/m, (BH)max=358kJ/m3;
(2) Br = 1.325T, Hci = 1680 kA/m, and (BH)max = 326 kJ/m3.