Large-size neodymium-iron-boron magnet and method for producing same

Abstract

This application provides a large-size NdFeB magnet and its preparation method. The preparation method includes: mixing a main alloy powder and an auxiliary alloy powder in a ratio of 10:(1-3) to obtain a mixed alloy powder; molding the mixed alloy powder and then isostatically pressing it to obtain a green blank; sintering the green blank using a multi-stage heating method to obtain a sintered magnet; wherein the multi-stage heating includes: heating to 1000-1100℃ at at least three gradually decreasing heating rates and holding at that temperature for 1-6 hours; and cooling the sintered magnet to obtain a NdFeB magnet. This application, through a dual alloying process and gradually reducing the heating rate during sintering, can effectively suppress cracking of the magnet during sintering and obtain a large-size magnet with excellent magnetic properties.

Description

Technical Field

[0001] This invention belongs to the field of magnetic materials technology, and in particular, relates to a neodymium iron boron magnet and its preparation method. Background Technology

[0002] Sintered NdFeB magnets are widely used in aerospace, electronics and communications, clean energy, transportation, medical devices, and home appliances due to their excellent magnetic properties. With the continuous expansion of applications, the need for ultra-large, high-performance magnets is becoming increasingly urgent.

[0003] In existing technologies, the production of large magnets typically involves first processing them into normal-sized magnets, and then bonding multiple magnets together to form a larger magnet. While this method reduces the eddy current effect and increases the magnet volume, making the fabrication of large magnets feasible, the addition of binders reduces the magnet's saturation magnetization, severely impacting its performance and failing to meet the high-performance requirements of ultra-large magnets. Directly fabricating large magnets, due to their large size, is prone to cracking during the fabrication process, leading to product scrap and significant economic losses. Therefore, finding a new method to fabricate high-performance large magnets is an urgent problem to be solved.

[0004] The content of the background section is merely the technology known to the inventor and does not necessarily represent the prior art in this field. Summary of the Invention

[0005] To address the problem of existing large-size magnets being prone to cracking, this application provides a large-size neodymium iron boron magnet and its preparation method.

[0006] The method for preparing large-size neodymium iron boron magnets provided in this application includes:

[0007] The main alloy powder and the auxiliary alloy powder are mixed in a ratio of 10:(1~3) to obtain a mixed alloy powder;

[0008] The mixed alloy powder is shaped and then isostatically pressed to obtain a green blank;

[0009] The green blank is sintered using a multi-stage heating method to obtain a sintered magnet; wherein the multi-stage heating includes: heating to 1000~1100℃ at at least three gradually decreasing heating rates and holding at that temperature for 1~6 hours; and

[0010] The sintered magnet is cooled to obtain the neodymium iron boron magnet;

[0011] The main alloy powder contains light rare earth element R. L1 Fe and B, the light rare earth element R L1 Selected from one or more of Y, La, Ce, Pr and Nd,

[0012] The auxiliary alloy powder contains light rare earth element R. L2 R, heavy rare earth element H Fe, B, Co, Al, Ga, and the light rare earth element R L2 The heavy rare earth element R is selected from one or more of Y, La, Ce, Pr and Nd. H Including Dy and / or Tb, based on the total weight of the auxiliary alloy powder, the light rare earth element R in the auxiliary alloy powder L2 The content of the heavy rare earth element R is 20~25wt%, and the content of the heavy rare earth element R is 20~25wt%. H The content of Mg is 10~15 wt%, the content of B is 0.9~0.95 wt%, the content of Co is 1~3 wt%, the content of Al is 0.1~1 wt%, and the content of Ga is 0.2~0.5 wt%.

[0013] In some embodiments of this application, the auxiliary alloy powder further comprises M1 and Cu, wherein M1 is selected from at least one of Ti and Zr, the content of M1 is 0.2~0.5 wt%, and the content of Cu is 0.1~1 wt%.

[0014] Optionally, the D50 of the main alloy powder is 3.8~4.3μm, and the D50 of the auxiliary alloy powder is 3.4~3.8μm.

[0015] In some embodiments of this application, the multi-stage heating includes: first heating to 550~650℃ at a first heating rate, then heating to 750~850℃ at a second heating rate, then heating to 1000~1100℃ at a third heating rate, and holding at that temperature for 1~6 hours.

[0016] In some embodiments of this application, the first heating rate is 5~10℃ / min, the second heating rate is 3~4℃ / min, and the third heating rate is 1~2℃ / min.

[0017] In some embodiments of this application, the composition of the main alloy powder further includes Cu, Al, Co, Ga, and M2, wherein M2 is selected from at least one of Ti and Zr. Based on the total weight of the main alloy powder, the light rare earth element R... L1 The content of Mg2+ is 28~29.8 wt%, B is 0.88~0.98 wt%, Cu is 0.1~0.5 wt%, Al is 0.1~0.3 wt%, Co is 0.5~1 wt%, Ga is 0.1~0.2 wt%, and M2 is 0.1~0.3 wt%.

[0018] Optionally, the main alloy powder does not include heavy rare earth elements.

[0019] In some embodiments of this application, the process of isostatically pressing the mixed alloy powder after molding to obtain a green blank includes:

[0020] The mixed alloy powder is added to the molding equipment in N stages, where N≥2. During the first to the (N-1)th addition, the mixed alloy powder is evenly distributed in the mold cavity of the molding equipment. Then, the mixed powder in the mold cavity is magnetized and demagnetized. During the Nth addition, the mixed alloy powder is concentrated in the center of the mold cavity and subjected to magnetization and molding. After demagnetization, a pressed blank is obtained.

[0021] The green billet is obtained by isostatic pressing of the billet.

[0022] Optionally, the magnetic field strength for magnetization and demagnetization is 1.8~3.2T;

[0023] Optionally, the sintering of the green blank is carried out in a vacuum induction furnace with a vacuum level of 10. -2 ~10 2 Pa.

[0024] In some embodiments of this application, cooling the sintered magnet to obtain the NdFeB magnet includes:

[0025] The sintered magnet is first cooled to a predetermined temperature in a vacuum at a predetermined cooling rate and held for 2-8 hours. Then it is cooled to room temperature at a cooling rate of 4-8℃ / min to obtain the neodymium iron boron magnet.

[0026] Optionally, the predetermined temperature is 450~550℃, and the predetermined cooling rate is 0.5~2℃ / min.

[0027] This application further provides a large-size neodymium iron boron magnet, which is prepared by any of the preparation methods described above.

[0028] In some embodiments of this application, the mass of the neodymium iron boron magnet is greater than 4 kg, and the density of the neodymium iron boron magnet is 7.55~7.65 g / cm³. 3 .

[0029] In some embodiments of this application, the remanence of the neodymium iron boron magnet is greater than 14 KGs and the coercivity is greater than 23 KOe.

[0030] Through the above technical solution, this application provides a method for preparing large-size NdFeB magnets and the magnets thereof. A dual-alloy process is used, employing a main alloy powder and an auxiliary alloy powder, both containing a main phase, to form a dual-main-phase alloy powder combination. The elemental composition of the main alloy powder is controlled to ensure magnet performance. Appropriate proportions of Al, Ga, Ti, and Cu are added to the auxiliary alloy powder to optimize the grain boundary phase, improve magnet toughness, and suppress cracking. Simultaneously, a special heating rate process is used during sintering to ensure a more uniform distribution of the rare-earth-rich phase. The cooling rate is controlled during cooling to regulate the shrinkage rate at various locations of the large-size magnet, effectively suppressing cracking during the preparation process. This method yields large-size magnets with a remanence greater than 14 kgs and a coercivity higher than 23 koe.

[0031] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0032] The accompanying drawings, which form part of this disclosure, are used to provide a further understanding of this disclosure. The illustrative embodiments of this disclosure and their descriptions are used to explain this disclosure and do not constitute an undue limitation of this disclosure.

[0033] Figure 1 A flowchart illustrating the fabrication process of a neodymium iron boron magnet according to an embodiment of this application is shown. Detailed Implementation

[0034] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0035] The following disclosure provides many different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and installations are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or installations discussed. In addition, examples of various specific processes and materials are provided in this invention, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0036] In this invention, the terms "first," "second," and other ordinal numbers are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0037] Furthermore, unless otherwise specified, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It will also be understood that terms, such as those defined in common dictionaries, shall be interpreted as having the same meaning as they have in the context of the relevant technology and the invention, and shall not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0038] The specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings and examples, so as to better understand the solution of the present invention and its advantages in various aspects. However, the specific embodiments and examples described below are for illustrative purposes only and are not intended to limit the present invention.

[0039] Figure 1 This application illustrates a method for preparing a neodymium iron boron magnet according to an embodiment of the present application, comprising the following steps S1 to S4.

[0040] S1: Mix the main alloy powder and the auxiliary alloy powder in a ratio of 10:(1~3) to obtain a mixed alloy powder.

[0041] The main alloy powder used in this application includes the light rare earth element R. L1 Fe and B, light rare earth element R L1 The main alloy powder is selected from one or more of Y, La, Ce, Pr, and Nd. Optionally, the main alloy powder may also include Cu, Al, Co, Ga, and M2, where M2 is selected from at least one of Ti and Zr. In this application, the main alloy powder may or may not include heavy rare earth elements, depending on the required performance of the magnet.

[0042] Optionally, in this application, the total weight of the main alloy powder is used as the basis for the light rare earth element R. L1 The content of Mg is 28~29.8 wt%, B is 0.88~0.98 wt%, Cu is 0.1~0.5 wt%, Al is 0.1~0.3 wt%, Co is 0.5~1 wt%, Ga is 0.1~0.2 wt%, and M2 is 0.1~0.3 wt%. Under the above ratio, the remainder is Fe and unavoidable impurities.

[0043] In this application, the auxiliary alloy powder comprises the light rare earth element R. L2 R, heavy rare earth element H Fe, B, Co, Al, Ga, and light rare earth elements R L2 R is selected from one or more of Y, La, Ce, Pr and Nd, and is a heavy rare earth element. H Includes Dy and / or Tb. Optionally, the auxiliary alloy powder may also include M1 and Cu, wherein M1 is selected from at least one of Ti and Zr.

[0044] Optionally, based on the total weight of the auxiliary alloy powder, the light rare earth element R in the auxiliary alloy powder is... L2 The content is 20~25wt%, heavy rare earth element R H The content of M1 is 10-15 wt%, B is 0.9-0.95 wt%, Co is 1-3 wt%, Al is 0.1-1 wt%, and Ga is 0.2-0.5 wt%. When M1 and Cu are present, the content of M1 can be 0.2-0.5 wt%, and the content of Cu can be 0.1-1 wt%. The remainder is Fe and unavoidable impurities.

[0045] This application improves the impact fracture toughness of a magnet by adding auxiliary alloys, resulting in sufficient deposition of R-rich phase particles in the grain boundary phase. The auxiliary alloy powders used in this application include elements such as Co, Al, and Ga, which can precipitate phases in the grain boundary phase. These precipitates have lower hardness than the main phase, thus improving the toughness of the magnet. Furthermore, when the auxiliary alloy powder contains elements M1 and Cu, they can also form precipitates in the grain boundary phase. These precipitates also have lower hardness than the main phase, thus improving the toughness of the magnet.

[0046] In this application, the predetermined ratio of the main alloy powder to the auxiliary alloy powder is 10:(1~3), at which the auxiliary alloy can exert a better effect. In some embodiments of this application, the predetermined ratio of the main alloy powder to the auxiliary alloy powder may be 10:1, 10:1.5, 10:2, 10:2.5, or 10:3. Optionally, the D50 of the main alloy powder is 3.8~4.3 μm, and the D50 of the auxiliary alloy powder is 3.4~3.8 μm. Within these particle size ranges, the main and auxiliary alloys can better synergize, thereby improving the performance of the obtained magnet. In some embodiments of this application, the D50 of the main alloy powder may be 3.8 μm, 3.9 μm, 4.0 μm, 4.1 μm, 4.2 μm, or 4.3 μm. In some embodiments of this application, the D50 of the auxiliary alloy powder may be 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, or 3.8 μm.

[0047] This application employs a dual-alloy process to ensure the performance of large-size magnets. Under the premise of achieving high performance, it further suppresses cracking problems by controlling the grain boundary phase through composition.

[0048] This application may further include steps for preparing primary and secondary alloy powders. In some embodiments of this application, the process for preparing the primary alloy powder may be: preparing neodymium iron boron strips using raw materials, with a vacuum degree of 10 in the vacuum induction furnace. -2 ~10 2 Pa, melting temperature 1300~1500℃, casting temperature 1400~1500℃, and strip thickness 0.2~0.5mm; the strip is added to a hydrogen crusher and hydrogen is absorbed under a hydrogen pressure of about 0.3~0.7MPa; then, it is pulverized by an air jet mill to obtain fine powder with a particle size of 3.8~4.3μm. The pressure in the air jet mill grinding chamber is about 0.68MPa. After pulverization, a lubricant is added at a weight of about 0.12% of the powder, preferably zinc stearate.

[0049] In some embodiments of this application, the process for preparing auxiliary alloy powder may be as follows: neodymium iron boron strips are prepared using raw materials, and the vacuum degree of the vacuum induction furnace is 10... -2 ~10 2 Pa, melting temperature 1300~1500℃, casting temperature 1400~1500℃, and strip thickness 0.2~0.5mm; the strip is added to a hydrogen crusher, and hydrogen absorption is carried out under a hydrogen pressure of about 0.3MPa; then, it is pulverized by an air jet mill to obtain fine powder with a particle size of 3.4~4.8μm. The pressure of the air jet mill grinding chamber is about 0.68MPa. After pulverization, a lubricant is added at a weight of about 0.12% of the powder, preferably zinc stearate.

[0050] S2: After the mixed alloy powder is formed, it isostatically pressed to obtain a green blank.

[0051] Optionally, this step may specifically include:

[0052] The mixed alloy powder is added to the molding equipment in N stages, where N≥2. During the first to the (N-1)th addition, the mixed alloy powder is evenly distributed within the mold cavity of the molding equipment. The powder is then magnetized and demagnetized within the mold cavity. During the Nth addition, the mixed alloy powder is concentrated in the center of the mold cavity, where it undergoes magnetization and pressing. After demagnetization, a pressed blank is obtained.

[0053] The green billet is obtained by isostatic pressing of the billet.

[0054] This step, through multiple feedings and charging / demagnetizing processes, makes the compacted blank more compact, reducing its susceptibility to cracking during subsequent sintering. During the Nth feeding, the mixed alloy powder is concentrated in the center of the mold cavity, resulting in a higher density at the center of the blank (e.g., 1-5% higher than at the edges). This minimizes the difference in deformation between the center and edges during sintering, thus suppressing cracking.

[0055] S3: The green blank is sintered by multi-stage heating to obtain sintered magnets.

[0056] The inventors discovered that for large-sized magnets, after pressing and sintering in a sintering furnace, the resulting sintered magnets are prone to cracking. The inventors first precisely traced the crack formation mechanism and stages. Through multi-dimensional testing and verification, they found that risk factors inducing cracking existed in the pressing, sintering, and aging stages, and that the stress distribution in each process stage had a cumulative effect, ultimately leading to the destruction of the magnet's structural integrity. However, further analysis of the crack locations revealed that cracking at the corners of large-sized magnets was not the primary problem, while cracking at the center was particularly likely to render the magnet unusable. After pinpointing the crack location, the inventors confirmed that thermal stress concentration in the sintering process was the key cause of cracks in the central region, where the probability of cracking was approximately 60-75% higher than at the corners, especially during the sintering process at approximately 600-1100℃, where the probability of cracking was highest. Based on this, this application employs a multi-stage heating process.

[0057] In this application, the multi-stage heating includes heating to 1000-1100°C at at least three gradually decreasing heating rates and holding at that temperature for 1-6 hours. That is, heating from room temperature to 1000-1100°C includes at least three heating intervals, with the heating rate gradually decreasing in each of these intervals. Through multi-stage heating with decreasing heating rates, cracking of the pressed blank can be effectively prevented.

[0058] In some embodiments of this application, the multi-stage heating includes: first heating to 550-650°C at a first heating rate, then heating to 750-850°C at a second heating rate, followed by heating to 1000-1100°C at a third heating rate, and holding at that temperature for 1-6 hours. The second heating rate is higher than the third heating rate but lower than the first heating rate.

[0059] Optionally, the first heating rate is 5~10℃ / min, the second heating rate is 4~6℃ / min, and the third heating rate is 1~3℃ / min. In some embodiments of this application, the first heating rate may be 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min. In some embodiments of this application, the second heating rate may be 4℃ / min, 5℃ / min, or 6℃ / min. In some embodiments of this application, the third heating rate may be 1℃ / min, 2℃ / min, or 3℃ / min.

[0060] In this application, the holding time (i.e., the sintering time after the temperature stabilizes) is 1 to 6 hours, and the sintering at this stage can also be referred to as secondary sintering. In some embodiments of this application, the holding time can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 6 hours.

[0061] Optionally, the magnetic field strength for magnetization and demagnetization is 1.8~3.2T. In some embodiments of this application, the magnetic field strength may be 1.8T, 2T, 2.2T, 2.4T, 2.6T, 2.8T, 3T or 3.2T.

[0062] Optionally, sintering is carried out in a vacuum induction furnace with a vacuum level of 10. -2 ~10 2 Pa. Sintering using a vacuum induction furnace yields sintered magnets with better magnetic properties.

[0063] S4: Cool the sintered magnet to obtain a neodymium iron boron magnet.

[0064] Optionally, this step includes: first cooling the sintered magnet to a predetermined temperature in a vacuum at a predetermined cooling rate, and then cooling it to room temperature at a cooling rate of 4~8℃ / min to obtain a NdFeB magnet. The vacuum cooling in this step not only prevents the magnet from cracking, but also has an aging effect, which is beneficial to improving the magnet's performance.

[0065] Optionally, the predetermined temperature is 450~550℃, and the predetermined cooling rate is 0.5~2℃ / min. In some embodiments of this application, the predetermined temperature may be 450℃, 470℃, 490℃, 510℃, 530℃, or 550℃. In some embodiments of this application, the predetermined cooling rate may be 0.5℃ / min, 1℃ / min, 1.5℃ / min, or 2℃ / min.

[0066] This application utilizes a dual-alloy process and gradually reduces the heating rate during sintering to effectively suppress cracking of the magnet during sintering, and can obtain large-size magnets with excellent magnetic properties.

[0067] This application further provides a large-size neodymium iron boron magnet, which can be manufactured by any of the above-described preparation methods. The large-size magnet of this application refers to a magnet with a mass greater than 4 kg, and it can be of any shape, such as a cuboid, cube, cylinder, tile, etc. This application can prepare magnets with a mass greater than 4 kg, for example, large-size trapezoidal magnets with a mass between 4 and 10 kg. The density of the neodymium iron boron magnet provided in this application is 7.55~7.65 g / cm³. 3 In some embodiments of this application, the density may be 7.55 g / cm³. 3 7.57 g / cm 3 7.59g / cm 3 7.61 g / cm 3 7.63 g / cm 3 Or 7.65g / cm 3 The magnets obtained in this application have a remanence greater than 14 KGs and a coercivity greater than 23 Koe, exhibiting high performance.

[0068] The present invention will now be described with reference to specific embodiments. The process conditions and values ​​used in the following embodiments and comparative examples are exemplary, and their possible ranges are as shown in the foregoing description of the invention. For process parameters not specifically noted, conventional techniques can be used. Unless otherwise specified, the reagents and instruments used in the technical solutions provided by the present invention can all be purchased from conventional channels or the market. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0069] Example 1

[0070] This embodiment prepares a neodymium iron boron magnet, and the specific steps are as follows:

[0071] The raw materials shown in Table 1 were sequentially processed through melting, casting, hydrogen breaking, and air jet milling to prepare the main alloy powder. The raw materials with the compositions shown in Table 1 were sequentially processed through melting, casting, hydrogen breaking, and air jet milling to prepare the auxiliary alloy powder. During the melting process, the vacuum degree of the vacuum induction furnace was 10. -1 The parameters are: Pa, melting temperature 1500℃, casting temperature 1450℃, strip thickness 0.3mm, hydrogen absorption pressure 0.3MPa during hydrogen crushing, and grinding chamber pressure 0.68MPa during air jet milling. The D50 of the main alloy powder is 4.1μm, and the D50 of the auxiliary alloy powder is 3.7μm. The two are mixed in a 10:2 ratio to obtain a mixed alloy powder.

[0072] The mixed alloy powder was divided into three portions, each weighing 2 kg. The first portion was added to the mold cavity of a forming press and then magnetized and demagnetized under a 2.0T magnetic field. The second portion was then added to the mold cavity, and the same magnetization and demagnetization process was performed. Finally, the third portion was added to the mold cavity, with the addition point at the center of the cavity. The pressing and magnetization process was then carried out to obtain a compact. The compact was then subjected to isostatic pressing at 150 MPa for 15 seconds to obtain a green billet.

[0073] The green blank is placed in a vacuum induction furnace, and the temperature is first increased to 600℃ at a rate of 6℃ / min, then to 800℃ at a rate of 4℃ / min, and finally to 1060℃ at a rate of 1.5℃ / min. The temperature is then held for 5 hours to obtain the sintered magnet. The vacuum degree of the vacuum induction furnace is 10... -1 Pa.

[0074] After the heat treatment was completed, the sintered magnet was vacuum cooled to 500℃ at a rate of 0.5℃ / min and held at that temperature for 6 hours. Then, it was cooled to room temperature at a rate of 6℃ / min to obtain the magnet. Samples were taken to test the magnetic properties and to observe the cracking situation. See Table 2 for details.

[0075] Example 2

[0076] The difference between this embodiment and Embodiment 1 is that the Cu content in the auxiliary alloy powder is 0 and the Ti content is 0.4 wt%, that is, the M1 content in the auxiliary alloy is 0.6 wt%.

[0077] Example 3

[0078] The difference between this embodiment and Embodiment 1 is that the Al content in the main alloy powder is 0.5 wt%.

[0079] Example 4

[0080] The difference between this embodiment and Embodiment 1 is that the Zr content in the main alloy powder is 0, the Ti content is 0.5wt% (i.e., the M2 content in the main alloy is 0.5wt%), and the Dy content in the main alloy powder is 2wt%.

[0081] Example 5

[0082] The difference between this embodiment and Embodiment 1 is that the single alloy method is used, and the alloy powder of this embodiment is prepared using the raw material composition of the mixed alloy powder of Embodiment 1.

[0083] Samples were taken to test magnetic properties and to observe cracking. See Table 2 for details.

[0084] Example 6

[0085] The difference between this embodiment and Embodiment 1 lies in the sintering process; all other aspects are the same.

[0086] The green blank is placed in a vacuum induction furnace, and the temperature is first increased to 700℃ at 7℃ / min, then to 800℃ at 5℃ / min, and then to 1060℃ at 3℃ / min. The temperature is then held for 5 hours to obtain the sintered magnet. The vacuum degree of the vacuum induction furnace is 10... -1 Pa.

[0087] Samples were taken to test magnetic properties and to observe cracking. See Table 2 for details.

[0088] Example 7

[0089] The difference between this embodiment and Embodiment 1 lies in the cooling process; all other aspects are the same.

[0090] After heat preservation, the sintered magnet was vacuum cooled to 700°C at a rate of 0.5°C / min, and then cooled to room temperature at a rate of 6°C / min to obtain the magnet. Samples were taken to test the magnetic properties, and the cracking was observed; details are shown in Table 2.

[0091] Example 8

[0092] The difference between this embodiment and Embodiment 1 lies in the cooling process; all other aspects are the same.

[0093] After heat preservation, the sintered magnet was vacuum cooled to 550°C at a rate of 4°C / min, and then cooled to room temperature at a rate of 6°C / min to obtain the magnet. Samples were taken to test the magnetic properties, and the cracking was observed; details are shown in Table 2.

[0094] Example 9

[0095] The difference between this embodiment and Embodiment 1 lies in the molding process; all other aspects are the same.

[0096] The mixed alloy powder was added to the molding equipment all at once, and then molded to obtain a compact. The compact was then subjected to isostatic pressing to obtain a green billet. Samples were taken to test the magnetic properties and observe the cracking situation, as detailed in Table 2.

[0097] Comparative Example 1

[0098] The difference between this embodiment and Embodiment 1 lies in the sintering process; all other aspects are the same.

[0099] The green blank was placed in a vacuum induction furnace and heated to 1060°C at a rate of 7°C / min, then held at that temperature for 5 hours to obtain the sintered magnet. The vacuum degree of the vacuum induction furnace was 10... -1 Pa.

[0100] Samples were taken to test magnetic properties and to observe cracking. See Table 2 for details.

[0101] Comparative Example 2

[0102] The difference between this embodiment and Embodiment 1 is that the content of B in the auxiliary alloy powder is 0 wt%.

[0103] Test case

[0104] The magnets prepared in the examples and comparative examples were subjected to magnetic performance tests. The magnetic performance test method was as follows: the remanence, coercivity and squareness were tested using the NIM-62000TB permanent magnet material precision measurement system.

[0105] Crack rate test: Randomly select 50 magnets for visual inspection, observe and record whether there is cracking, and count the number of cracked magnets. Calculate the crack rate by dividing the number of cracked magnets by the total number of magnets.

[0106] Table 1

[0107]

[0108] Table 2

[0109]

[0110] As can be seen from Table 2, the neodymium iron boron magnets prepared according to the method provided in this application do not crack or have a very low crack rate, and have very good magnetic properties, with a remanence greater than 14 KGs and a coercivity greater than 23 KOe.

[0111] In Example 1, the magnet did not crack at all. In Example 2, the auxiliary alloy powder did not contain Cu, and the magnetic properties were not significantly different, but the cracking rate was 4%. This is because Cu can form precipitates in the grain boundary phase, and these precipitates are less hard than the main phase, thus improving the toughness of the magnet. In Example 3, the Al content of the main alloy powder was higher than that in Example 1, which slightly affected the precipitation of Al in the auxiliary alloy powder in the grain boundary phase. In Example 4, Dy was added, thus improving the coercivity of the magnet. In Example 5, the alloy powder was prepared directly using the raw material composition of the mixed alloy powder in Example 1 without using the dual alloying process. The remanence and coercivity of the prepared magnet were lower than those in Example 1, which was not conducive to improving the magnet performance and could not produce high-performance large-size magnets. In Example 6, the heating rate only began to decrease at 700℃, resulting in a slightly higher magnet cracking rate. In Example 7, the second-stage aging temperature was relatively high, resulting in a slightly higher magnet cracking rate. In Example 8, the vacuum cooling rate was slightly faster, resulting in a slightly higher magnet cracking rate. In Example 9, the density of the center and corners of the green blank is the same, and deformation will occur during sintering, so the cracking rate of the magnet is slightly higher.

[0112] In Comparative Example 1, the heating rate remained constant during the sintering process, yet all samples cracked. This was because the high heating rate resulted in significant thermal stress in the magnets, which was not released in time, leading to cracking. Comparative Example 2, using auxiliary alloy powder that did not contain boron (B), had a lower cracking rate, but the magnets exhibited poorer performance. This was because the auxiliary alloy powder composition of Comparative Example 2 could not form a main phase. Therefore, the main and auxiliary alloy powders used in the dual-alloy process could not form a dual-main-phase structure, resulting in poor magnet performance.

[0113] Obviously, the above embodiments are merely examples for clearly illustrating the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method of producing a large-size neodymium-iron-boron magnet, characterized by, The mass of the neodymium-iron-boron magnet is greater than 4 kg, the remanence of the neodymium-iron-boron magnet is greater than 14 KGs, and the coercive force is higher than 23 KOe, and the preparation method comprises: The main alloy powder and the auxiliary alloy powder are mixed in a ratio of 10:(1-3) to obtain a mixed alloy powder; After the mixed alloy powder is formed, isostatic pressing treatment is performed to obtain a green body; The green body is sintered in a multi-stage heating mode to obtain a sintered magnet; and The sintered magnet is first cooled at a predetermined cooling rate in a vacuum to a predetermined temperature, and then cooled to room temperature at a cooling rate of 4-8 ℃ / min, to obtain the neodymium-iron-boron magnet; the predetermined temperature is 450-550 ℃, and the predetermined cooling rate is 0.5-2 ℃ / min; The main alloy powder comprises light rare earth element R L1 , Fe and B, the light rare earth element R L1 is selected from one or more of Y, La, Ce, Pr and Nd, the main alloy powder further comprises Cu, Al, Co, Ga and M2, the M2 is selected from at least one of Ti and Zr, the content of the light rare earth element R L1 is 28-29.8 wt%, the content of B is 0.88-0.98 wt%, the content of Cu is 0.1-0.5 wt%, the content of Al is 0.1 wt%, the content of Co is 0.5-1 wt%, the content of Ga is 0.1-0.2 wt%, and the content of M2 is 0.1-0.3 wt%. The composition of the auxiliary alloy powder includes light rare earth elements R L2 , heavy rare earth elements R H , Fe, B, Co, Al, Ga, the light rare earth elements R L2 are selected from one or more of Y, La, Ce, Pr and Nd, the heavy rare earth elements R H include Dy and / or Tb, the content of the light rare earth elements R L2 in the auxiliary alloy powder is 20-25 wt%, the content of the heavy rare earth elements R H is 10-15 wt%, the content of B is 0.9-0.95 wt%, the content of Co is 1-3 wt%, the content of Al is 0.1-1 wt%, and the content of Ga is 0.2-0.5 wt%. The multi-stage heating comprises: first heating at a first heating rate to 550-650 ℃, then heating at a second heating rate to 750-850 ℃, and then heating at a third heating rate to 1000-1100 ℃, and holding for 1-6 h; The first heating rate is 5-10 ℃ / min, the second heating rate is 3-4 ℃ / min, and the third heating rate is 1-2 ℃ / min.

2. The production method according to claim 1, characterized by, The composition of the auxiliary alloy powder further comprises M1 and Cu, the M1 is selected from at least one of Ti and Zr, the content of the M1 is 0.2-0.5 wt%, and the content of the Cu is 0.1-1 wt%.

3. The preparation method according to claim 2, characterized in that, The D50 of the main alloy powder is 3.8-4.3 μm, and the D50 of the auxiliary alloy powder is 3.4-3.8 μm.

4. The preparation method according to claim 1, characterized in that, After the mixed alloy powder is formed, isostatic pressing treatment is performed to obtain a green body, which comprises: The mixed alloy powder is divided into N times of addition to a forming device, N≥2; wherein, during the first to the N-1th addition, the mixed alloy powder is uniformly distributed in a mold cavity of the forming device, and then the mixed powder in the mold cavity is magnetized and demagnetized, during the Nth addition, the mixed alloy powder is concentratedly distributed in a center part of the mold cavity, magnetization and compression molding treatment are performed, and a compression body is obtained after demagnetization; and The compression body is subjected to isostatic pressing treatment to obtain the green body.

5. The production method according to claim 4, characterized by, The magnetic field strength of the magnetization and demagnetization is 1.8-3.2 T.

6. The preparation method according to claim 4, characterized in that, The sintering of the green body is performed in a vacuum induction furnace, the vacuum degree of which is 10 -2 ~10 2 Pa.

7. A large size neodymium-iron-boron magnet, characterized by, Obtained by the preparation method in any one of claims 1-6.

8. Large-size neodymium-iron-boron magnet according to claim 7, characterized in that The density of the neodymium-iron-boron magnet is 7.55 to 7.65 g / cm 3 .

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