Method for improving bending strength of neodymium-iron-boron magnet

Abstract

The invention discloses a method for improving the bending strength of a neodymium-iron-boron magnet, which comprises the following steps: (1) according to the proportion of Al < x > Cu < y > Ga < z > Re < (1-x-y-z) >, smelting metal raw materials into a repair alloy, and Re refers to a rare earth element; x ranges from 0.005 to 0.3, y ranges from 0.01 to 0.5, and z ranges from 0 to 0.1; the repair alloy is smashed to be made into alloy powder with the particle size being 3-50 micrometers; mixing the alloy powder with an organic solvent and glue to prepare alloy slurry; and (2) coating the surface of the magnet substrate or an easy-to-crack area with alloy slurry, drying a solvent, then carrying out vacuum high-temperature repair treatment at 500-700 DEG C, carrying out heat preservation for 0.5-10 hours, cooling, and then carrying out tempering treatment, so as to obtain the magnet with improved bending strength. The Al-Cu-Ga-Re alloy adopted by the invention has the characteristics of low melting point and good magnet wettability, cracks in the magnet can be repaired, the magnetic performance of the magnet is not influenced, the bending strength of the magnet is improved and reaches 400 MPa or above, the risk of fragmentation of the magnet in the use process is effectively reduced, and the service life of the magnet is prolonged.

Description

Technical Field

[0001] The invention belongs to the field of rare earth permanent magnetic materials and provides a method for improving the bending strength of a neodymium iron boron magnet. Background Art

[0002] NdFeB magnet is a high-performance permanent magnetic material widely used in various electronic devices, motors, generators and other fields. However, during the preparation and use of NdFeB magnets, due to the complexity of its crystal structure and the mechanical stress during the preparation process, it is easy to produce defects such as cracks. These cracks may cause the magnet to break during use, thus causing safety problems. Therefore, how to effectively repair the cracks of NdFeB magnets and improve their bending strength is a problem that needs to be solved in the current field of rare earth magnet preparation technology.

[0003] The existing solution is mainly to change the ratio of raw materials of magnets, such as increasing the ratio of rare earth elements to increase the proportion of grain boundary rare earth-rich phase with good plasticity, and finally achieve the purpose of improving the bending strength of magnets and reducing cracks. In addition, the preparation process can also be improved, such as optimizing sintering conditions, adjusting heat treatment parameters, etc., to reduce the occurrence of cracks.

[0004] However, there are still some problems with the existing solutions. First, increasing the proportion of rare earth raw materials will increase the volume ratio of the grain boundary rare earth-rich phase of the magnet as a whole, which will reduce the volume ratio of the main phase (Nd2Fe14B phase), which will inevitably reduce the magnetic properties of the magnet. In addition, rare earths are expensive, and increasing rare earth elements will inevitably increase costs. And the preparation process improvement plan can often only reduce the occurrence of cracks, but cannot completely eliminate cracks, especially those small cracks that already exist. In addition, most of the existing solutions cannot repair existing cracks, which will reduce the service life and safety of the magnet.

[0005] It is necessary to develop a process for repairing tiny cracks in NdFeB magnets to improve the bending strength of the magnets and extend their service life. Summary of the invention

[0006] The purpose of the present invention is to provide a method for repairing cracks in a NdFeB magnet, which can improve the bending strength and the safety of use without affecting the magnetic properties.

[0007] The technical solution adopted by the present invention is: A method for improving the bending strength of a neodymium iron boron magnet, the method comprising the following steps: (1) Smelting metal raw materials into a repair alloy according to the ratio of AlxCuyGazRe (1-xyz), wherein Re refers to a rare earth element; the value range of x is 0.005 to 0.3, the value range of y is 0.01 to 0.5, and the value range of z is 0 to 0.1; crushing the repair alloy to prepare alloy powder with a particle size of 3 to 50 microns; mixing the alloy powder with an organic solvent and glue to obtain an alloy slurry; (2) Coating the alloy slurry on the surface of the NdFeB magnet substrate or the crack-prone area, drying the solvent, and then performing vacuum high-temperature repair treatment at a temperature of 500-700°C, keeping the temperature for 0.5-10 hours, cooling and then tempering to obtain a magnet with improved bending strength.

[0008] In the step (1), z can be 0, that is, the ratio of the repair alloy is AlxCuyRe (1-xy).

[0009] When z is not 0, the preferred value range of z is 0.001 to 0.1, and more preferably 0.005 to 0.01.

[0010] The preferred value range of x is 0.1-0.3, and the preferred value range of y is 0.2-0.4.

[0011] In step (1), the rare earth element includes one or more of Nd, Pr, Dy, Tb, Ho, Gd, La, Ce, Lu, Y, and Sc, preferably one or more of Nd, Pr, Dy, Tb, Ho, and Gd.

[0012] In the step (1), the pulverization can be carried out by mechanical crushing and / or hydrogen crushing process.

[0013] In the step (1), the mass ratio of alloy powder, organic solvent and glue is 1:1-3:0.01-0.05, more preferably 1:2:0.02-0.03.

[0014] The organic solvent is an alcohol solvent, preferably anhydrous ethanol.

[0015] The glue is an organic adhesive such as PVB, PVC, etc.

[0016] In the step (2), the substrate surface usually refers to the entire surface of the two largest surfaces of the magnet (generally length × width), because large surfaces are more likely to crack during the grinding process; the crack-prone area is generally the corner area of ​​the largest surface, because cracks usually originate from the edges of the magnet and are distributed on two surfaces.

[0017] Generally speaking, for a magnet substrate with an aspect ratio less than 3:1, the corner area is generally the surface area of ​​the magnet substrate within a width range of 0~Kmm from the magnet edge line, K is the width of the corner area, and the value range of K is 20~40% of the width of the magnet substrate.

[0018] For long strip magnet substrates with an aspect ratio of 3:1 or more, the entire surface of the two largest surfaces of the magnet (generally the length × width) is generally directly coated. Long strip magnet substrates are more prone to breakage, so it is preferred to coat the entire surface.

[0019] In the step (2), after the solvent is dried, the weight gain of the magnet is 0.01-0.2%, preferably 0.05-0.15%.

[0020] The solvent can be dried in a high temperature drying tunnel or oven, and the temperature is generally 70~130℃.

[0021] The vacuum degree of the vacuum high temperature repair treatment is generally 0.001~0.5Pa.

[0022] The temperature of the vacuum high temperature repair treatment is preferably 600-700°C.

[0023] The tempering treatment is generally carried out at 800-900°C for 2-4 hours, and then at 440-650°C for 3-4 hours.

[0024] The annealing process is carried out in a vacuum environment.

[0025] In the present invention, the alloy slurry can be applied by spraying or printing.

[0026] The NdFeB magnet substrate is obtained by processing a sintered large NdFeB magnet into a desired shape and size, cleaning the magnet and drying it after surface treatment.

[0027] The surface treatment of the magnet is to remove oil stains and rust spots on the surface of the magnet by grinding, pickling or sandblasting, and then clean it with clean water and dry it.

[0028] The Al-Cu-Ga-Re alloy used in the present invention has the characteristics of low melting point and good magnet wettability. During the repair process, the metal alloy melts, penetrates into the cracks, and combines with the magnet as a whole, playing a role similar to welding, repairing the cracks in the magnet. Since the coating amount is small (there is no need to coat the entire magnet surface, only the crack-prone area needs to be coated, and even point coating is also possible), and it is coated on the magnet surface, it will basically not affect the original component ratio of the magnet, and the content in the magnet as a whole is also very low, but in the magnet surface layer, elements such as Re, Al, and Cu will be significantly higher than the inside of the magnet, and there are enriched areas of Al, Cu, and Re elements. In the repair process of the present invention, the temperature used is below 700°C and the time is also short. Compared with the conventional grain boundary diffusion process after coating the diffusion source (800°C~1000°C, more than 10 hours), the temperature and time of the repair process are significantly reduced, and no adverse effects such as grain growth will be caused. The bending strength of the magnet repaired by the present invention is significantly improved, reaching more than 400MPa. If a higher temperature and longer treatment time are used, the repair effect will be reduced and the improvement in bending strength will not be obvious.

[0029] The method of the present invention uses a very small amount of Re element, which hardly increases the cost of magnet preparation. However, through the magnet repair treatment, the bending strength of the magnet is improved, the risk of the magnet breaking during use is effectively reduced, and the service life of the magnet is extended.

[0030] The beneficial effects of the present invention are: 1) effectively repairing the tiny cracks that already exist in the NdFeB magnet during the preparation process to improve its bending strength and safety of use and extend its service life; 2) only increasing the rare earth content on the surface of the magnet or filling the gaps and cracks in the magnet will basically not affect the volume ratio of the main phase of the magnet, so it will not affect the magnetic properties and the cost is relatively low. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below in conjunction with specific embodiments. The embodiments described below are exemplary and are only used to better understand the present invention, and should not be construed as limiting the present invention.

[0032] Embodiment 1: Step 1: Nd by weight percentage 23 Pr 7.5 Dy 0.5 Zr 0.2 B 0.95 Fe bal The raw materials were proportioned and the NdFeB alloy was prepared by SC rapid solidification process, HD+JM powder making, orientation molding, and sintering at 1080℃ for 6 hours to obtain NdFeB blanks, which were processed into several magnetic sheets with a size of 20mm*10mm*5mm. Five magnetic sheets were randomly selected, cleaned, and numbered 1-1, 1-2, 1-3, 1-4, and 1-5.

[0033] Step 2: Mix the raw materials Al, Cu, Ga and Pr according to Al 0.2 Cu 0.3 Ga 0.01 Nd 0.49 The mixture is melted into repair alloy and made into 5-micron particles by hydrogen crushing + air flow grinding process.

[0034] Step 3: Mix the alloy powder with anhydrous ethanol and PVC in a mass ratio of 1:2:0.02 and stir evenly to obtain a slurry.

[0035] Step 4: Apply the slurry to the two largest surfaces of the magnet (20mm*10mm surface). After drying at 80℃, the weight gain of the magnet is 0.1%.

[0036] Step 5: Place the dried magnet in a vacuum sintering furnace for repair. The repair process is to keep the temperature at 600°C for 5 hours. During the repair process, the furnace is kept in a vacuum state of 0.005Pa to prevent air from oxidizing the magnet.

[0037] Step 6: Temper the magnet at 900℃ for 3 hours and 510℃ for 4 hours. During the tempering process, the furnace is kept in a vacuum state to prevent air from oxidizing the magnet.

[0038] Step 7: Test the bending strength and magnetic properties of the magnet, and the test results are shown in Table 1. The test method for the irreversible flux loss at 120°C is as follows: saturate the magnet, use a fluxmeter to detect the magnetic flux at room temperature, marked as Φ0; then put the magnet into a high-temperature oven, heat it to 120°C and keep it warm for 2 hours; take out the magnet and cool it to room temperature and test the magnetic flux at room temperature again, marked as Φ1, (Φ0-Φ1) ÷ Φ0 × 100% is the irreversible flux loss of the magnet at 120°C.

[0039] Comparative Example 1: The blank preparation, processing, tempering and magnetic property testing are the same as in Example 1, except that no slurry coating and repair treatment are performed. The samples are numbered D1-1, D1-2, D1-3, D1-4, and D1-5. The test results are shown in Table 1.

[0040] Embodiment 2: Alloy powder composition is Al 0.2 Cu 0.3 Ga 0.01 Dy 0.49 , other processes are the same as in Example 1, the samples are numbered 2-1, 2-2, 2-3, 2-4, and 2-5, and the test results are shown in Table 1.

[0041] Embodiment 3: Alloy powder composition is Al 0.2 Cu0.3 Dy 0.5 , other processes are the same as in Example 1, the samples are numbered 3-1, 3-2, 3-3, 3-4, and 3-5, and the test results are shown in Table 1.

[0042] Example 4 The other processes are the same as those in Example 1. The difference from Example 1 is that in step 4, the slurry is applied to the corner areas of the two largest surfaces of the magnet. The corner area refers to the surface area within 2mm from the edge line of the 20mm*10mm surface. The weight gain of the magnet after drying at 80℃ is 0.052%. The samples are numbered 4-1, 4-2, 4-3, 4-4, and 4-5. The test results are shown in Table 1.

[0043] Comparative Example 2: The other processes are the same as those in Example 1. The difference from Example 1 is that the repair process in step five is changed to 900°C×20 hours. The samples are numbered D2-1, D2-2, D2-3, D2-4, and D2-5. The test results are shown in Table 1.

[0044] Table 1 Sample No. Flexural strength (MPa) Br(T) Hcj (kA / m) 120℃ irreversible flux loss (%) 1-1 420 1.332 1540 5.5 1-2 413 1.333 1537 5.8 1-3 432 1.332 1542 5.1 1-4 409 1.334 1538 6.1 1-5 418 1.333 1540 5.2 D1-1 305 1.333 1432 8.8 D1-2 355 1.334 1451 8.7 D1-3 287 1.335 1438 9.8 D1-4 311 1.334 1444 9.2 D1-5 298 1.334 1437 9.3 2-1 423 1.331 1611 2.5 2-2 417 1.330 1625 2.3 2-3 408 1.331 1617 2.2 2-4 425 1.335 1608 2.2 2-5 412 1.332 1618 2.6 3-1 409 1.330 1621 3.0 3-2 411 1.329 1631 2.7 3-3 420 1.330 1627 2.8 3-4 417 1.331 1636 2.9 3-5 408 1.332 1628 2.9 4-1 407 1.331 1517 7.5 4-2 404 1.332 1491 8.8 4-3 402 1.329 1512 8.0 4-4 415 1.332 1488 7.9 4-5 418 1.332 1493 7.2 D2-1 287 1.331 1575 4.9 D2-2 305 1.330 1562 5.1 D2-3 321 1.329 1577 4.8 D2-4 212 1.327 1561 5.0 D2-5 311 1.330 1583 4.6 The test results show that the bending strength of the magnet treated by the crack repair process is improved from about 300 MPa in comparative example 1 to more than 400 MPa in embodiment 1, and the magnetic properties are also improved. This is because the repair process not only repairs the cracks but also repairs the processing stress layer and grain boundary structure.

[0045] The Hcj of Example 2 is higher than that of Example 1. This is because the repair alloy powder of Example 2 contains heavy rare earth Dy, which not only repairs the cracks but also has a certain effect of grain boundary diffusion hardening.

[0046] The irreversible magnetic flux loss of Example 2 is slightly lower than that of Example 3, which indicates that adding Ga element to the repair alloy powder is beneficial to reducing the irreversible magnetic flux loss.

[0047] The alloy slurry of Example 4 only coats the crack-prone areas at the edges and corners. Compared with coating the entire surface in Example 1, the improvement effect of bending strength is equivalent, indicating that coating only the crack-prone areas can have a repair effect similar to coating the entire surface, but the effect of improving the irreversible magnetic loss of the magnet is not obvious.

[0048] Comparative Example 2 increases the temperature and time of the repair process, which is similar to the conventional grain boundary diffusion process. However, as shown in Table 1, the bending strength of the magnet is significantly higher when the repair process of the present invention is carried out at a lower temperature and time, indicating that the repair process is more conducive to improving the bending strength of the magnet than the conventional diffusion process.

Claims

1. A method for improving the bending strength of NdFeB magnets, characterized in that The method comprises the following steps: (1) Smelting metal raw materials into a repair alloy according to the ratio of AlxCuyGazRe (1-xyz), wherein Re refers to a rare earth element; the value range of x is 0.005 to 0.3, the value range of y is 0.01 to 0.5, and the value range of z is 0 to 0.1; crushing the repair alloy to prepare alloy powder with a particle size of 3 to 50 microns; mixing the alloy powder with an organic solvent and glue to obtain an alloy slurry; (2) Coating the alloy slurry on the surface of the NdFeB magnet substrate or the crack-prone area, drying the solvent, and then performing vacuum high-temperature repair treatment at a temperature of 500-700°C, keeping the temperature for 0.5-10 hours, cooling and then tempering to obtain a magnet with improved bending strength.

2. The method according to claim 1, characterized in that In the step (1), when z is 0, the ratio of the repair alloy is AlxCuyRe (1-xy); When z is not 0, the value range of z is 0.001~0.

1.

3. The method according to claim 1, characterized in that In the step (1), the value range of x is 0.1-0.3, and the value range of y is 0.2-0.

4.

4. The method according to claim 1, characterized in that In step (1), the rare earth element includes one or more of Nd, Pr, Dy, Tb, Ho, Gd, La, Ce, Lu, Y, and Sc.

5. The method according to claim 1, characterized in that In the step (1), the mass ratio of alloy powder, organic solvent and glue is 1:1-3:0.01-0.

05.

6. The method according to claim 1, characterized in that In the step (1), the organic solvent is an alcohol solvent, and the glue is PVB or PVC.

7. The method according to claim 1, characterized in that In the step (2), the substrate surface refers to the entire surface of the two largest surfaces of the magnet substrate, and the crack-prone area is generally the corner area of ​​the largest surface.

8. The method according to claim 1, characterized in that In the step (2), after the solvent is dried, the weight gain of the magnet is 0.01-0.2%.

9. The method according to claim 1, characterized in that In the step (2), the vacuum degree of the vacuum high temperature repair treatment is 0.001~0.5Pa.

10. The method according to claim 1, characterized in that In the step (2), the tempering treatment is carried out at 800-900°C for 2-4 hours and then at 440-650°C for 3-4 hours.