MIM-based large-size neodymium iron boron permanent magnet and preparation method thereof

Abstract

The invention discloses a large-size neodymium-iron-boron permanent magnet based on MIM and a preparation method thereof, and the method comprises the following steps: mixing neodymium-iron-boron powder with a binder to obtain slurry; the slurry is injected into a mold cavity in an MIM injection mode to be cured and formed, and a first-stage blank is obtained; the first-stage blank is subjected to degreasing treatment and magnetizing, then cold isostatic pressing is conducted, and a second-stage blank is obtained; and sintering the secondary blank, and naturally cooling to obtain the large-size neodymium-iron-boron permanent magnet. According to the preparation method, high-quality preparation of the large-size neodymium-iron-boron permanent magnet is realized by adopting a large-size natural rubber mold, MIM technology forming, high-pulse magnetic field magnetizing and a special sintering process.

Description

Technical Field

[0001] The invention relates to the technical field of rare earth permanent magnet material preparation, in particular to a large-size NdFeB permanent magnet based on MIM and a preparation method thereof. Background Art

[0002] NdFeB permanent magnet materials play a key role in many fields such as modern industry, electronic equipment, and new energy due to their excellent magnetic properties. In the field of wind power generation, NdFeB permanent magnets are used to manufacture the core components of generators to improve power generation efficiency; in the drive motors of new energy vehicles, they can enhance motor performance and reduce energy loss.

[0003] However, with the increasing demand for large-sized NdFeB permanent magnets in various industries, the disadvantages of traditional preparation processes have become increasingly prominent. Taking powder metallurgy as an example, when pressing cylindrical products with a diameter of more than 80 mm or square products with a side length of more than 120 mm, the uneven distribution of powder will lead to inconsistent product density. This density difference not only affects the mechanical properties of the product, but also causes fluctuations in magnetic properties, which cannot meet the requirements of high-end application scenarios. Moreover, during the pressing process, the internal stress of large-sized products is concentrated, which is very easy to crack, seriously reducing the product yield.

[0004] The sintering method also faces challenges when preparing large-sized NdFeB permanent magnets. During the sintering process, it is difficult to control the temperature and time, and it is difficult to ensure the uniformity of the internal structure of the product. This will result in insufficient residual magnetic properties and fail to provide a stable and strong magnetic field for related equipment. In addition, the production capacity of traditional sintering equipment is limited and it is difficult to meet the needs of large-scale production.

[0005] Metal injection molding (MIM) technology, as an advanced powder metallurgy molding technology, has significant advantages. Just like plastic injection molding, it can mix metal powder with a binder and inject it into the mold through an injection machine to achieve high-precision molding of complex-shaped products. It has high production efficiency and is suitable for mass production. MIM technology has been successfully applied in the preparation of small precision parts, but it has not been fully explored and applied in the preparation of large-sized NdFeB permanent magnets. Introducing MIM technology into the preparation of large-sized NdFeB permanent magnets is expected to overcome the defects of traditional processes, solve problems such as pressing difficulties, crack generation, and insufficient residual magnetic properties, and provide strong support for the development of related industries. Summary of the invention

[0006] In view of this, the present invention provides a large-size NdFeB permanent magnet based on MIM and a preparation method thereof to solve the problems raised in the above-mentioned background technology. By adopting a large-size natural rubber mold, MIM technology molding, high pulse magnetic field magnetization and special sintering process, high-quality preparation of large-size NdFeB permanent magnets is achieved.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] In a first aspect, the present invention discloses a method for preparing a large-size NdFeB permanent magnet based on MIM, comprising the following steps:

[0009] S1, mixing NdFeB powder with a binder to obtain a slurry;

[0010] S2, injecting the slurry into the mold cavity by MIM injection method to solidify and shape, so as to obtain a primary blank;

[0011] S3, degreasing and magnetizing the primary blank, and then performing cold isostatic pressing to obtain a secondary blank;

[0012] S4, sintering the secondary blank and naturally cooling it to obtain a large-sized NdFeB permanent magnet.

[0013] As a further solution of the present invention: the pressure of the cold isostatic pressing is 200MPa-300MPa, and the pressing time is 20-60 seconds.

[0014] As a further solution of the present invention: the mold cavity is formed by enclosing a soft mold.

[0015] As a further solution of the present invention: the magnetic field strength of the magnetization is 4-6T, and the pulse frequency is 50-100HZ.

[0016] As a further solution of the present invention: the sintering treatment is specifically: heating the temperature to 1000-1200° C. at a heating rate not exceeding 5° C. / min, and keeping the temperature for 2-6 hours.

[0017] As a further solution of the present invention: the degreasing treatment adopts solvent degreasing or heat treatment.

[0018] As a further solution of the present invention: the specific parameters of the MIM injection are: the temperature of the front section of the barrel of the injection machine is 140-180°C, the temperature of the middle section of the barrel is 160-200°C, the temperature of the rear section of the barrel is 120-160°C, the nozzle temperature is 150-170°C, the screw speed is 50-200rpm; the mold cavity temperature is 30-80°C; the injection pressure is 70-150MPa; the holding pressure is 30-80MPa; the injection rate is 50-200mm / s.

[0019] As a further solution of the present invention: the mass percentages of neodymium powder, boron powder and iron powder in the NdFeB powder are 2%, 14% and 93.5% respectively; the NdFeB powder is prepared by smelting, crushing and air flow grinding.

[0020] As a further solution of the present invention: the adhesive is a thermoplastic resin, and the mass ratio of the adhesive to the NdFeB powder is 90-95:5-10.

[0021] In a second aspect, the present invention discloses a large-sized NdFeB permanent magnet prepared by the above-mentioned preparation method. The size of the NdFeB magnet is not less than 120mm*120mm*80mm, or a cylinder with a diameter of more than D150, or an irregular product.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention combines metal injection molding (MIM) technology with a large-size natural rubber mold to ensure the uniformity and integrity of the product during the pressing process. MIM technology can accurately inject feed with good fluidity into the rubber mold, accurately control the molding temperature with the help of the mold cooling system, and achieve high-precision molding of large-size products, greatly expanding the application range of NdFeB permanent magnets and meeting the needs of large-size permanent magnets in large motors, wind power generation and other fields.

[0024] Cold isostatic pressing and MIM injection technology have a synergistic effect in reducing cracks in large-sized NdFeB permanent magnets: When the MIM injection technology is used to inject the feed into the rubber mold, high-precision molding of complex shapes can be achieved, but during the flow and filling process of the feed in the mold, internal stress concentration points, pores and other defects may be formed. These defects are potential hazards for cracks in the subsequent sintering process. During cold isostatic pressing, by applying pressure evenly in all directions, the tiny pores inside the MIM injection molded product can be effectively closed, making the internal structure of the product more dense and uniform. At the same time, isostatic pressing can release the internal stress generated by MIM injection molding, reduce stress concentration points, and reduce the risk of cracks caused by stress concentration.

[0025] After cold isostatic pressing, the density differences of various parts of the blank are greatly reduced, which effectively reduces cracks caused by inconsistent shrinkage during sintering and improves product quality.

[0026] By introducing high pulse magnetic field magnetization technology during the preparation process, and the magnetic field strength reaches 3.5-5T, the residual magnetic properties of the product are greatly improved. At the same time, in conjunction with the isostatic pressing process, the internal structure of the product is further optimized to make the product density more uniform. This not only enhances the magnetism of the product, but also ensures the stability and consistency of the magnetic properties throughout the product range. Compared with products produced by traditional preparation processes, the large-size NdFeB permanent magnets prepared by the present invention can provide a stronger and more stable magnetic field for the equipment in practical applications, effectively improving the performance and efficiency of related equipment, such as in magnetic resonance imaging (MRI) equipment, which can improve the clarity and accuracy of imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a high-magnification effect diagram of the magnet prepared in Comparative Example 1 of the present invention;

[0028] Figure 2 This is a high-magnification effect diagram of the magnet prepared in Comparative Example 2 of the present invention;

[0029] Figure 3 This is a high-magnification effect diagram of the magnet prepared in Comparative Example 3 of the present invention;

[0030] Figure 4 This is a high-magnification effect diagram of the magnet made in No. 14 of the present invention; DETAILED DESCRIPTION

[0031] For ease of understanding of the present invention, the present invention will be described more fully below in conjunction with specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thoroughly understood.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0033] Any range described in the present invention includes the end value and any numerical value between the end values ​​and any sub-range formed by the end value or any numerical value between the end values.

[0034] Example 1

[0035] (1) A conventional casting-hydrogen crushing-airflow grinding process is used to prepare a powder with a composition of Nd 2.0 Fe 14 B 93.5 Pr 2.5. Dy 3.0 Al 0.5 Cu 0.5 (wt.%), NdFeB powder with an average particle size D50 = 3 μm.

[0036] (2) NdFeB powder and a binder are mixed and kneaded in a mass ratio of 90:10 at a temperature of 160° C. for 60 minutes to obtain a slurry; the binder is obtained by mixing polypropylene, paraffin, stearic acid, LDPE, and BHT in a mass ratio of 40:35:5:8:4.

[0037] (3) The slurry was injected into a mold cavity (the mold cavity was made of rubber, with dimensions of 120*120*80, in mm) by MIM injection and cured to obtain a blank; the MIM injection parameters were as follows: injection machine barrel temperature: front section 140°C / middle section 160°C / rear section 120°C, nozzle temperature 150°C, screw speed 50 rpm; mold cavity temperature 30°C; injection pressure 70 MPa; holding pressure 30 MPa; injection rate 50 mm / s.

[0038] (4) The blank is degreased using trichloroethylene solvent at a degreasing temperature of 180° C. for 0.5 hours; the degreased blank is magnetized at a magnetic field strength of 4 T and a pulse frequency of 50 Hz.

[0039] (5) The magnetized blank is subjected to cold isostatic pressing at a pressure of 200 MPa for 15 min, and the medium is hydraulic oil.

[0040] (6) Sintering the cold isostatically pressed blank by heating it to 1000° C. at a rate of 1° C. / min, keeping it at that temperature for 2 hours, and naturally cooling it to room temperature to obtain a large-sized NdFeB permanent magnet.

[0041] Example 2

[0042] (1) A conventional casting-hydrogen crushing-airflow grinding process is used to prepare a powder with a composition of Nd 2.0 Fe 14 B 93.5 Pr 2.5. Dy 3.0 Al 0.5 Cu 0.5 (wt.%), NdFeB powder with an average particle size D50 = 3 μm.

[0043] (2) NdFeB powder and a binder are mixed and kneaded in a mass ratio of 95:5 at a kneading temperature of 160° C. for 60 minutes to obtain a slurry; wherein the binder is a mixture of polypropylene, paraffin, stearic acid, LDPE, and BHT in a mass ratio of 45:30:10:8:4.

[0044] (3) The slurry is injected into the mold cavity by MIM injection method and solidified to obtain a blank; the MIM injection parameters are: barrel temperature: front section 180°C / middle section 200°C / rear section 160°C; nozzle temperature 170°C; screw speed 200rpm; mold cavity temperature 80°C; injection pressure 150MPa; holding pressure 80MPa; injection rate 200mm / s.

[0045] (4) The blank is subjected to thermal degreasing treatment under nitrogen protection, the degreasing temperature is 300°C, and the degreasing time is 1.5 hours; the degreasing blank is magnetized, and the magnetic field strength is 6T and the pulse frequency is 100Hz.

[0046] (5) The magnetized blank is subjected to cold isostatic pressing at a pressure of 300 MPa for 30 min, and the medium is hydraulic oil.

[0047] (6) The cold isostatically pressed blank is sintered by heating the blank to 1200° C. at a rate of 5° C. / min, keeping the temperature for 6 hours, and then cooling the blank to room temperature at a rate of 3° C. / min to obtain a large-sized NdFeB permanent magnet.

[0048] Example 3

[0049] (1) A conventional casting-hydrogen crushing-airflow grinding process is used to prepare a powder with a composition of Nd 2.0 Fe 14 B 93.5 Pr 2.5. Dy 3.0 Al 0.5 Cu 0.5 (wt.%), NdFeB powder with an average particle size D50 = 3 μm.

[0050] (2) NdFeB powder and a binder are mixed and kneaded in a mass ratio of 92.5:7.5 at a kneading temperature of 160°C for 60 minutes to obtain a slurry; the binder is prepared by mixing polypropylene, paraffin, stearic acid, LDPE, and BHT in a ratio of 42.5:32.5:7.5:6:3.

[0051] (3) The slurry is injected into the mold cavity by MIM injection method for curing and molding to obtain a blank; the MIM injection parameters are: barrel temperature: front section 160°C / middle section 180°C / rear section 140°C; nozzle temperature 160°C; screw speed 125rpm; mold cavity temperature 55°C; injection pressure 110MPa; holding pressure 55MPa; injection rate 125mm / s.

[0052] (4) The blank is degreased with acetone at a temperature of 240° C. for 1 hour. The degreased blank is magnetized at a magnetic field strength of 5 T and a pulse frequency of 75 Hz.

[0053] (5) The magnetized blank is subjected to cold isostatic pressing at a pressure of 250 MPa for 22.5 min, using hydraulic oil as the medium.

[0054] (6) The cold isostatically pressed blank is sintered by heating the blank to 1100° C. at a rate of 3° C. / min, keeping the temperature for 4 hours, and then cooling the blank to room temperature at a rate of 1.5° C. / min to obtain a large-sized NdFeB permanent magnet.

[0055] Comparative Example 1

[0056] (2) A conventional casting-hydrogen crushing-airflow grinding process is used to prepare a powder with a composition of Nd 2.0 Fe 14 B 93.5 Pr 2.5. Dy 3.0 Al 0.5 Cu 0.5 (wt.%), NdFeB powder with an average particle size D50 = 3 μm.

[0057] (2) NdFeB powder and a binder are mixed and kneaded in a mass ratio of 95:5 at a kneading temperature of 160° C. for 60 minutes to obtain a slurry; wherein the binder is a mixture of polypropylene, paraffin, stearic acid, LDPE, and BHT in a mass ratio of 45:30:10:8:4.

[0058] (3) The slurry is injected into the mold cavity by MIM injection method and solidified to obtain a blank; the MIM injection parameters are: barrel temperature: front section 200°C / middle section 220°C / rear section 180°C; nozzle temperature 170°C; screw speed 200rpm; mold cavity temperature 80°C; injection pressure 150MPa; holding pressure 80MPa; injection rate 200mm / s.

[0059] (4) The blank is subjected to thermal degreasing treatment under nitrogen protection, the degreasing temperature is 300°C, and the degreasing time is 1.5 hours; the degreasing blank is magnetized, and the magnetic field strength is 6T and the pulse frequency is 100Hz.

[0060] (5) The magnetized blank is subjected to cold isostatic pressing at a pressure of 300 MPa for 30 min, and the medium is hydraulic oil.

[0061] (6) The cold isostatically pressed blank is sintered by heating the blank to 1200° C. at a rate of 5° C. / min, keeping the temperature for 6 hours, and then cooling the blank to room temperature at a rate of 3° C. / min to obtain a large-sized NdFeB permanent magnet.

[0062] Comparative Example 2

[0063] (3) Using the conventional casting-hydrogen crushing-air flow grinding process, a powder with a composition of Nd 2.0 Fe 14 B 93.5 Pr 2.5. Dy 3.0 Al 0.5 Cu 0.5 (wt.%), NdFeB powder with an average particle size D50 = 3 μm.

[0064] (2) NdFeB powder and a binder are mixed and kneaded in a mass ratio of 95:5 at a kneading temperature of 160° C. for 60 minutes to obtain a slurry; wherein the binder is a mixture of polypropylene, paraffin, stearic acid, LDPE, and BHT in a mass ratio of 45:30:10:8:4.

[0065] (3) The slurry is injected into the mold cavity by MIM injection method for curing and molding to obtain a blank; the MIM injection parameters are: barrel temperature: front section 120°C / middle section 140°C / rear section 100°C; nozzle temperature 130°C; screw speed 200rpm; mold cavity temperature 80°C; injection pressure 150MPa; holding pressure 80MPa; injection rate 200mm / s.

[0066] (4) The blank is subjected to thermal degreasing treatment under nitrogen protection, the degreasing temperature is 300°C, and the degreasing time is 1.5 hours; the degreasing blank is magnetized, and the magnetic field strength is 6T and the pulse frequency is 100Hz.

[0067] (5) The magnetized blank is subjected to cold isostatic pressing at a pressure of 300 MPa for 30 min, and the medium is hydraulic oil.

[0068] (6) The cold isostatically pressed blank is sintered by heating the blank to 1200° C. at a rate of 5° C. / min, keeping the temperature for 6 hours, and then cooling the blank to room temperature at a rate of 3° C. / min to obtain a large-sized NdFeB permanent magnet.

[0069] Comparative Example 3

[0070] (1) A conventional casting-hydrogen crushing-airflow grinding process is used to prepare a powder with a composition of Nd 2.0 Fe 14 B 93.5 Pr 2.5. Dy 3.0 Al 0.5 Cu 0.5 (wt.%), NdFeB powder with an average particle size D50 = 3 μm.

[0071] (2) pouring the above-mentioned NdFeB powder into a conventional metal mold (the molding metal mold has the same size and shape as all the MIM injection mold cavities in Examples 1-3), and using a molding press to perform orientation molding under a pressure of 20 MPa and a magnetic field strength of 1.8 T to obtain a molded blank;

[0072] (3) The formed blank is subjected to cold isostatic pressing treatment at a pressure of 200 MPa for 15 min, and the medium is hydraulic oil.

[0073] (4) Sintering the cold isostatically pressed blank by heating it to 1000° C. at a rate of 10° C. / min, keeping it at that temperature for 2 hours, and cooling it with a cooling fan to obtain a large-sized NdFeB permanent magnet.

[0074] Test Case

[0075] The magnets obtained in the above embodiments and comparative examples were subjected to performance tests, and the test items and results are shown in Table 1.

[0076] Table 1

[0077]

[0078] As can be seen from Table 1, the injection temperature of each section of the MIM in Comparative Example 1 is too high, resulting in a 15% decrease in the remanence of the magnet (compared with Example 2), a 20% decrease in coercivity, and cracks on the surface of the magnet; this is due to the carbonization of the adhesive caused by the high temperature, which increased the porosity inside the magnet. The injection temperature of each section of the MIM in Comparative Example 2 is too low, resulting in a 12% decrease in the remanence of the magnet (compared with Example 2), a 18% decrease in coercivity, and cracks on the surface of the magnet; this is due to insufficient fluidity of the adhesive and incomplete filling of the mold cavity with slurry, resulting in pores and cold shut defects that are not completely eliminated inside the magnet after sintering. Comparative Example 3 uses traditional technology, and its parameters such as magnetic energy product, remanence, and coercivity are all lower than those of Example 1. There are also slight cracks on the surface of the magnet, which fully illustrates the advantages of the embodiment using a combination of MIN injection molding and cold isostatic pressing technology. Figure 1-4 The following are photos of the magnets obtained in Example 1 and Comparative Examples 1-3 observed using a VMS-2010F precision image measuring instrument, with a magnification of 30 times. It can be seen from the figure that the magnets of Comparative Examples 1-3 have cracks of varying degrees on their surfaces.

[0079] Although this specification is described according to implementation modes, not every implementation mode includes only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0080] Therefore, the above description is only a preferred embodiment of the present application and is not intended to limit the scope of implementation of the present application; that is, all equivalent changes made according to the scope of the claims of the present application are within the protection scope of the claims of the present application.

Claims

1. A method for preparing large-size NdFeB permanent magnets based on MIM, characterized in that: The following steps are involved: S1, mixing NdFeB powder with a binder to obtain a slurry; S2, injecting the slurry into the mold cavity by MIM injection method to solidify and shape, so as to obtain a primary blank; S3, degreasing and magnetizing the primary blank, and then performing cold isostatic pressing to obtain a secondary blank; S4, sintering the secondary blank and naturally cooling it to obtain a large-sized NdFeB permanent magnet.

2. The preparation method according to claim 1, characterized in that: The cold isostatic pressing pressure is 200 MPa-300 MPa, and the pressing time is 20-60 seconds.

3. The preparation method according to claim 1, characterized in that: The mold cavity is formed by enclosing a soft mold.

4. The preparation method according to claim 1, characterized in that: The magnetic field intensity of the magnetization is 4-6T, and the pulse frequency is 50-100HZ.

5. The preparation method according to claim 1, characterized in that: The sintering treatment is specifically as follows: heating the temperature to 1000-1200° C. at a heating rate not exceeding 5° C. / min, and keeping the temperature for 2-6 hours.

6. The preparation method according to claim 1, characterized in that: The degreasing treatment adopts solvent degreasing or heat treatment.

7. The preparation method according to claim 1, characterized in that: The specific parameters of the MIM injection are: the temperature of the front section of the barrel of the injection machine is 140-180°C, the temperature of the middle section of the barrel is 160-200°C, the temperature of the rear section of the barrel is 120-160°C, the nozzle temperature is 150-170°C, the screw speed is 50-200rpm; the mold cavity temperature is 30-80°C; the injection pressure is 70-150MPa; the holding pressure is 30-80MPa; the injection rate is 50-200mm / s.

8. The preparation method according to claim 1, characterized in that: The mass percentages of neodymium powder, boron powder and iron powder in the neodymium iron boron powder are 2%, 14% and 93.5% respectively; the neodymium iron boron powder is prepared through smelting, crushing and air flow grinding processes.

9. The preparation method according to claim 1, characterized in that: The adhesive is a thermoplastic resin, and the mass ratio of the adhesive to the NdFeB powder is 90-95:5-10.

10. A large-sized NdFeB permanent magnet prepared by the preparation method according to any one of claims 1 to 9.

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