A method for preparing a high coercivity sintered Nd-Fe-B magnet by grain boundary diffusion and the magnet prepared thereby

Abstract

The present invention relates to the technical field of sintered NdFeB production, and particularly relates to a method for preparing a high coercivity sintered NdFeB magnet by grain boundary diffusion; in the present invention, a combination of Cu and Ga with a ratio of (1:1 to 2:1) can form an R-Cu-Ga phase at a temperature of 430°C to 540°C during diffusion annealing heat treatment, which can achieve the effect of improving coercivity with less reduction in remanence. At the same time, it has excellent diffusivity and reduces the usage amount of heavy rare earths. The diffusion depth improvement effect of the present invention is obvious, the process is simple, and the equipment used is conventional equipment, which can realize batch production and promotion.

Description

Technical Field

[0001] The present invention relates to the technical field of sintered NdFeB production, and particularly relates to a method for preparing a high coercivity sintered NdFeB magnet by grain boundary diffusion and the magnet itself. Background Art

[0002] Sintered NdFeB permanent magnets are widely used in high-tech fields such as electronic information, medical equipment, new energy vehicles, household appliances, and robots. During the development process in the past few decades, NdFeB permanent magnets have developed rapidly, and the remanence performance has basically reached the theoretical limit. However, there is still a large gap between the coercivity and the theoretical value. Therefore, improving the coercivity of the magnet is a major research hotspot.

[0003] Since the traditional manufacturing process consumes a large amount of heavy rare earth metals such as Tb or Dy, the cost increases. Although the grain boundary diffusion technology can greatly reduce the content of heavy rare earths, with the soaring price of the current heavy rare earth Tb, the cost is still very high. Therefore, continuously reducing the content of heavy rare earths is still important. By using a diffusion hardening Nd2Fe 14 B matrix containing heavy rare earth elements to form a large number of core-shell structures can increase the coercivity. However, the diffusion depth of the grain boundary diffusion technology is limited, and improving the grain boundary diffusivity of the magnet is particularly crucial.

[0004] To solve the problem of insufficient diffusion layer depth in the traditional vacuum grain boundary diffusion technology, the Chinese invention patent (ZL201410682495.0) discloses a grain boundary diffusion method for preparing a high coercivity magnet by pre-thermally pressing and diffusing a diffusion substrate and a diffusion alloy sheet and then annealing. However, it has the following deficiencies: The preparation process and hot pressing process of the low melting point diffusion alloy sheet are additionally increased, resulting in cumbersome steps, low production efficiency, high cost, difficulty in mass production, and large equipment investment; the bonding between the diffusion alloy sheet and the diffusion substrate is poor, there is a solid diffusion phenomenon in element diffusion, the diffusion depth is limited, and the improvement effect of the coercivity is poor. Summary of the Invention

[0005] To overcome the above deficiencies, the purpose of the present invention is to provide a method for preparing a high coercivity sintered NdFeB magnet by grain boundary diffusion, which is combined with Cu and Ga to form an R-Cu-Ga phase, achieving the effect of improving the coercivity with less reduction in remanence, and having excellent diffusivity at the same time. The present invention also provides a method for preparing a high coercivity sintered NdFeB magnet by grain boundary diffusion.

[0006] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0007] A method for preparing a high coercivity sintered NdFeB magnet by grain boundary diffusion, wherein its general formula is R x T 100-x-y-a-b-c B y Cu a Gab A c , wherein x, y, a, b, and c are mass percentages, and 29% ≤ x ≤ 32.5%, 0.88% ≤ y ≤ 0.96%, 0.3% ≤ a ≤ 1%, 0.15% ≤ b ≤ 0.5%, 0% ≤ c ≤ 1.5%;

[0008] R is at least one of rare earth elements, and the rare earth elements include Pr, Nd, Dy, Tb, Ho, Gd, and Ce;

[0009] T includes Fe and Co; B is boron element; Cu is copper element; Ga is gallium element;

[0010] A includes at least one of Al, Nb, Zr, and Ti.

[0011] As an improvement of the present invention, R x T 100-x-y-a-b-c B y Cu a Ga b A c contains an R-Cu-Ga phase, wherein R is at least one of rare earth elements Pr and Nd, and in the sintered Nd-Fe-B magnet, the ratio of the percentage contents of Cu and Ga is Cu:Ga = 1:1 to 2:1.

[0012] As a further improvement of the present invention, it includes 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti by mass percentage, and the balance is Fe.

[0013] As a further improvement of the present invention, it includes 31.5% PrNd, 0.94% B, 0.75% Cu, 0.5% Ga, 1% Co, 0.2% Al, 0.2% Ti by mass percentage, and the balance is Fe.

[0014] As a further improvement of the present invention, it includes 30.5% PrNd, 0.5% Dy, 0.92% B, 0.5% Cu, 0.3% Ga, 0.5% Co, 0.15% Ti by mass percentage, and the balance is Fe.

[0015] A method for preparing a high coercivity sintered Nd-Fe-B magnet by grain boundary diffusion, which includes the following steps:

[0016] Step S1: Mix according to the component ratio of the Nd-Fe-B base material elements to form a mixed powder, then melt it into a sheet alloy, and then obtain a block-shaped Nd-Fe-B permanent magnet after crushing and magnetic field forming, and then cut the block-shaped Nd-Fe-B permanent magnet into sheet-shaped base materials;

[0017] Step S2: Apply a heavy rare earth Dy and / or Tb diffusion source of 0.3 wt% - 1.2 wt% on the sheet substrate for grain boundary diffusion treatment, and then obtain the NdFeB magnet by heat treatment. Among them, the high-temperature treatment temperature and time of the heat treatment are 800°C - 1000°C and 2 h - 8 h respectively, and the tempering temperature and time of the heat treatment are 430°C - 540°C and 4 h - 10 h respectively.

[0018] As an improvement of the present invention, in step S2, the grain boundary diffusion heat treatment is at 900°C for 2 h, and the tempering treatment of the heat treatment is at 480°C for 5 h.

[0019] As a further improvement of the present invention, in step S1, the mixed powder is melted at a high temperature of 1500°C ± 20°C in an induction furnace, and then hydrogenated and pulverized by a jet mill to fine powder with an average particle size of 3 μm - 5 μm.

[0020] As a further improvement of the present invention, in step S1, the powder is formed under a magnetic field condition with a magnetic field strength above 1.6 T, and then sintered at 1080°C for 10 h to obtain a block-shaped NdFeB permanent magnet.

[0021] As a further improvement of the present invention, in step S1, the block-shaped NdFeB permanent magnet is cut into sheet substrates with a thickness of 2 mm - 8 mm.

[0022] In the present invention, the high content of low-melting-point elements Cu and Ga can greatly improve the fluidity of the rare earth-rich phase during the grain boundary diffusion heat treatment, which is beneficial for the heavy rare earth elements in the diffusion source to diffuse deeper into the NdFeB substrate. At the same time, in combination with the tempering temperature of 430°C - 540°C, an R-Cu-Ga phase is formed at the grain boundary. The R-Cu-Ga phase can improve the boundary structure distribution during the tempering process, which is beneficial for the formation of a continuous, straight and regular thin-layer rare earth-rich phase, playing a good role in demagnetizing coupling, thereby improving the coercivity. Moreover, the tempering temperature range is 430°C - 540°C. When the tempering temperature is too low (lower than 430°C), the formation temperature of the R-Cu-Ga phase cannot be reached; when the tempering temperature is too high (higher than 540°C), the already formed R-Cu-Ga phase will be damaged.

[0023] The present invention has the following advantages:

[0024] 1. By combining Cu and Ga, the ratio of the percentage content of Cu and Ga is Cu:Ga = 1:1 - 2:1, thereby improving the coercivity of the sintered NdFeB magnet;

[0025] 2. An R-Cu-Ga phase can be formed at the tempering temperature of 430°C - 540°C. While improving the coercivity of the sintered NdFeB magnet, its remanence remains unchanged or decreases slightly;

[0026] 3. The bulk neodymium iron boron permanent magnet is cut into sheet substrates with a thickness of 5 mm to 8 mm, and then a diffusion source of 0.3 wt% to 1.2 wt% of Dy and / or Tb is coated on the sheet substrates for grain boundary diffusion treatment, so that it has excellent diffusivity. Embodiment

[0027] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] A method for preparing a high coercivity sintered neodymium iron boron magnet by grain boundary diffusion according to the present invention has a general formula of R x T 100-x-y-a-b-c B y Cu a Ga b A c , where x, y, a, b, and c are mass percentages, and 29% ≤ x ≤ 32.5%, 0.88% ≤ y ≤ 0.96%, 0.3% ≤ a ≤ 1%, 0.15% ≤ b ≤ 0.5%, 0% ≤ c ≤ 1.5%;

[0029] R is at least one of rare earth elements, and the rare earth elements include Pr, Nd, Dy, Tb, Ho, Gd, and Ce;

[0030] T includes Fe and Co;

[0031] B is boron element;

[0032] Cu is copper element;

[0033] Ga is gallium element;

[0034] A includes at least one of Al, Nb, Zr, and Ti.

[0035] The present invention provides an embodiment of a method for preparing a high coercivity sintered neodymium iron boron magnet by grain boundary diffusion. This embodiment includes 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti by mass percentage, and the balance is Fe.

[0036] The present invention provides another embodiment of a method for preparing a high coercivity sintered neodymium iron boron magnet by grain boundary diffusion. This embodiment includes 31.5% PrNd, 0.94% B, 0.75% Cu, 0.5% Ga, 1% Co, 0.2% Al, 0.2% Ti by mass percentage, and the balance is Fe.

[0037] The present invention provides another embodiment of preparing a high coercive force sintered NdFeB magnet by grain boundary diffusion, which comprises 30.5% PrNd, 0.5% Dy, 0.92% B, 0.5% Cu, 0.3% Ga, 0.5% Co, 0.15% Ti, and the balance Fe in mass percentage.

[0038] The present invention provides a method for preparing a high coercive force sintered NdFeB magnet by grain boundary diffusion, comprising the following steps:

[0039] Step S1, mixing the NdFeB substrate elements according to the composition ratio to form a mixed powder, then smelting to form a sheet alloy, and then crushing and magnetic field forming to obtain a block NdFeB permanent magnet, and then cutting the block NdFeB permanent magnet into a sheet substrate;

[0040] Step S2, coating 0.3wt%~1.2wt% of Dy and / or Tb diffusion source on the sheet substrate for grain boundary diffusion treatment, and then obtaining a NdFeB magnet by heat treatment, wherein the heating temperature and time of the heat treatment are 900°C and 2h respectively, and the tempering temperature and time of the heat treatment are 430°C~540°C and 5h respectively.

[0041] Wherein, in step S1, the mixed powder is subjected to high-temperature melting at 1500°C ± 20°C in an induction furnace, and then crushed into 3μm to 5μm powder by hydrogen crushing and air flow grinding. The powder is formed under a magnetic field condition with a magnetic field strength of more than 1.6T, and then sintered at 1080°C for 10 hours to obtain a block NdFeB permanent magnet, and the block NdFeB permanent magnet is cut into 8mm sheet substrates.

[0042] The present invention provides Examples 1-3 and Comparative Examples 1-2, as follows: Embodiment 1:

[0043] 1. Preparation of diffusion substrate:

[0044] The raw materials of each component are mixed according to the composition formula of 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti and the balance Fe in mass percentage, and are successively smelted in an induction furnace at a temperature condition of 1500°C, and are rapidly quenched and strip-spun to form a sheet alloy with a thickness of 0.3±0.05mm, which is then crushed into a powder of 3-5μm by hydrogen crushing and air flow grinding, and is formed under a magnetic field condition of a magnetic field intensity of more than 1.6T, and then sintered at 1080°C for 10h to obtain a block NdFeB permanent magnet, which is cut into a 5mm sheet substrate for grain boundary diffusion.

[0045] 2. Grain boundary diffusion treatment

[0046] The NdFeB magnet is obtained by grain boundary diffusion treatment, using the method of coating with Dy diffusion source followed by heat treatment. Among them, the content of coated Dy is 1.0 wt% (this weight is the total mass of Dy in the diffusion source), the temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is carried out at 430 °C, the time is 5 h, and the magnetic properties are measured (see Table 1). Example 2:

[0047] 1. Preparation of the diffusion substrate:

[0048] Mix the raw materials of each component according to the formula of 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti, and the rest Fe by mass percentage. First, melt them in an induction furnace under the temperature condition of 1500 °C, then quickly quench and spin-ribbon to make a sheet alloy with a thickness of 0.3 ± 0.05 mm, crush it into powder with a particle size of 3 - 5 μm by hydrogen crushing and jet milling, form it under a magnetic field condition of more than 1.6 T magnetic field intensity, and then sinter it at 1080 °C for 10 h to obtain a bulk NdFeB permanent magnet. Cut the bulk NdFeB permanent magnet into 5 mm sheet substrates for grain boundary diffusion preparation.

[0049] 2. Grain boundary diffusion treatment

[0050] The NdFeB magnet is obtained by grain boundary diffusion treatment, using the method of coating with Dy diffusion source followed by heat treatment. Among them, the content of coated Dy is 1.0 wt% (this weight is the total mass of Dy in the diffusion source), the temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is carried out at 480 °C, the time is 5 h, and the magnetic properties are measured (see Table 1). Example 3:

[0051] 1. Preparation of the diffusion substrate:

[0052] Mix the raw materials of each component according to the formula of 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti, and the balance is Fe by mass percentage. First, melt them in an induction furnace under the temperature condition of 1500 °C, then quickly quench and spin-ribbon to make a sheet alloy with a thickness of 0.3 ± 0.05 mm, crush it into powder with a particle size of 3 - 5 μm by hydrogen crushing and jet milling, form it under a magnetic field condition of more than 1.6 T magnetic field intensity, and then sinter it at 1080 °C for 10 h to obtain a bulk NdFeB permanent magnet. Cut the bulk NdFeB permanent magnet into 5 mm sheet substrates for grain boundary diffusion preparation.

[0053] 2. Grain boundary diffusion treatment

[0054] The NdFeB magnet is obtained by grain boundary diffusion treatment, using the method of coating with Dy diffusion source followed by heat treatment. Among them, the content of coated Dy is 1.0 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is carried out at 540 °C, and the time is 5 h. The magnetic properties are measured (see Table 1).

[0055] Comparative Example 1:

[0056] 1. Preparation of the diffusion substrate:

[0057] Mix the raw materials of each component according to the composition formula of 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti, and the balance is Fe by mass percentage. First, melt in an induction furnace under the temperature condition of 1500 °C, then quickly quench and spin-ribbon to form a sheet alloy with a thickness of 0.3 ± 0.05 mm, crush it into powder with a particle size of 3 - 5 μm by hydrogenation and jet milling, form it under a magnetic field condition of more than 1.6 T magnetic field strength, and then sinter at 1080 °C for 10 h to obtain a massive NdFeB permanent magnet. Cut the massive NdFeB permanent magnet into 5 mm sheet substrates for grain boundary diffusion.

[0058] 2. Grain boundary diffusion treatment

[0059] The NdFeB magnet is obtained by grain boundary diffusion treatment, using the method of coating with Dy diffusion source followed by heat treatment. Among them, the content of coated Dy is 1.0 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is carried out at 410 °C, and the time is 5 h. The magnetic properties are measured (see Table 1).

[0060] Comparative Example 2:

[0061] 1. Preparation of the diffusion substrate:

[0062] Mix the raw materials of each component according to the composition formula of 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti, and the balance is Fe by mass percentage. First, melt in an induction furnace under the temperature condition of 1500 °C, then quickly quench and spin-ribbon to form a sheet alloy with a thickness of 0.3 ± 0.05 mm, crush it into powder with a particle size of 3 - 5 μm by hydrogenation and jet milling, form it under a magnetic field condition of more than 1.6 T magnetic field strength, and then sinter at 1080 °C for 10 h to obtain a massive NdFeB permanent magnet. Cut the massive NdFeB permanent magnet into 5 mm sheet substrates for grain boundary diffusion.

[0063] 2. Grain boundary diffusion treatment

[0064] The NdFeB magnet is obtained by grain boundary diffusion treatment, using the method of coating a Dy diffusion source followed by heat treatment. Among them, the content of coated Dy is 1.0 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is carried out at 560 °C, the time is 5 h, and the magnetic properties are measured (see Table 1).

[0065] Table 1:

[0066]

[0067] According to the data in Table 1, it can be seen that the coercivity of the NdFeB magnet within the reasonable tempering temperature range (430 °C - 540 °C) is significantly higher, and the coercivity of the NdFeB magnet beyond the reasonable tempering temperature range is lower. Among them, the tempering temperature of 480 °C is the best. It can be seen that 430 °C - 540 °C is the reasonable tempering temperature of the present invention. Exceeding the reasonable tempering temperature range will have an adverse effect on the formation of the R-Cu-Ga phase, thereby reducing the coercivity of the NdFeB magnet.

[0068] The present invention provides Example 4 and Comparative Examples 3 - 5, as follows: Example 4:

[0069] 1. Mix the raw materials of each component according to the composition formula of 31.5% PrNd, 0.94% B, 0.75% Cu, 0.5% Ga, 1% Co, 0.2% Al, 0.2% Ti, and the balance being Fe by mass percentage (Cu:Ga = 1.5:1). First, melt in an induction furnace under the temperature condition of 1500 °C, and then make a sheet alloy with a thickness of 0.3 ± 0.05 mm by rapid quenching and strip casting, pulverize it into a powder with a particle size of 3 - 5 μm by hydrogen decrepitation and jet milling, form it under a magnetic field condition with a magnetic field strength above 1.6 T, and then sinter at 1080 °C for 10 h to obtain a massive NdFeB permanent magnet. Cut the massive NdFeB permanent magnet into a 5 mm sheet substrate;

[0070] 2. The grain boundary diffusion treatment is carried out by coating a Dy diffusion source followed by heat treatment to obtain the NdFeB magnet. Among them, the content of coated Dy is 0.8 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is 480 °C, the time is 5 h, and the magnetic properties are measured (see Table 2).

[0071] Comparative Example 3:

[0072] 1. Mix the raw materials of each component according to the composition formula of 31.5% PrNd, 0.94% B, 0.3% Cu, 0.45% Ga, 1% Co, 0.2% Al, 0.2% Ti, and the balance being Fe by mass percentage (Cu:Ga = 1:1.5). Then, successively melt the mixture in an induction furnace under the temperature condition of 1500 °C, and make a flaky alloy with a thickness of 0.3 ± 0.05 mm by rapid quenching and strip casting. After hydrogen embrittlement and jet milling, the powder is ground to 3 - 5 μm, formed under the magnetic field condition of a magnetic field strength above 1.6 T, and then sintered at 1080 °C for 10 h to obtain a bulk neodymium iron boron permanent magnet. Cut the bulk neodymium iron boron permanent magnet into a 5 mm flaky substrate;

[0073] 2. The grain boundary diffusion treatment is carried out by coating a Dy diffusion source and then heat treatment to obtain a neodymium iron boron magnet. The content of the coated Dy is 0.8 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C and the time is 2 h, and the tempering temperature is 480 °C and the time is 5 h. The magnetic properties are measured (see Table 2).

[0074] Comparative Example 4:

[0075] 1. Mix the raw materials of each component according to the composition formula of 31.5% PrNd, 0.94% B, 1% Cu, 0.4% Ga, 1% Co, 0.2% Al, 0.2% Ti, and the balance being Fe by mass percentage (Cu:Ga = 2.5:1). Then, successively melt the mixture in an induction furnace under the temperature condition of 1500 °C, and make a flaky alloy with a thickness of 0.3 ± 0.05 mm by rapid quenching and strip casting. After hydrogen embrittlement and jet milling, the powder is ground to 3 - 5 μm, formed under the magnetic field condition of a magnetic field strength above 1.6 T, and then sintered at 1080 °C for 10 h to obtain a bulk neodymium iron boron permanent magnet. Cut the bulk neodymium iron boron permanent magnet into a 5 mm flaky substrate;

[0076] 2. The grain boundary diffusion treatment is carried out by coating a Dy diffusion source and then heat treatment to obtain a neodymium iron boron magnet. The content of the coated Dy is 0.8 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C and the time is 2 h, and the tempering temperature is 480 °C and the time is 5 h. The magnetic properties are measured (see Table 2).

[0077] Comparative Example 5:

[0078] 1. Mix the raw materials of each component according to the composition formula of 31.5% PrNd, 0.94% B, 0.15% Cu, 0.1% Ga, 1% Co, 0.2% Al, 0.2% Ti, and the balance being Fe by mass percentage (Cu:Ga = 1.5:1). Then, successively melt the mixture in an induction furnace under the temperature condition of 1500 °C, and make a flaky alloy with a thickness of 0.3 ± 0.05 mm by rapid quenching and strip casting. After hydrogen crushing and jet milling, the powder is ground to 3 - 5 μm, formed under the magnetic field condition of a magnetic field strength above 1.6 T, and then sintered at 1080 °C for 10 h to obtain a massive neodymium iron boron permanent magnet. Cut the massive neodymium iron boron permanent magnet into 5 mm flaky substrates;

[0079] 2. The grain boundary diffusion treatment is carried out by coating a Dy diffusion source and then performing heat treatment to obtain a neodymium iron boron magnet. The content of the coated Dy is 0.8 wt% (this weight is the total mass of Dy in the diffusion source). The temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is 480 °C, and the time is 5 h. The magnetic properties are measured (see Table 2).

[0080] Table 2 is as follows:

[0081]

[0082] According to the data in Table 2, when the content or ratio of Cu and Ga elements is not within a reasonable range, the coercivity of the neodymium iron boron substrate is lower, indicating that the formation of the R - Cu - Ga phase is inhibited, and the coercivity is thus reduced; when exceeding the content range of Cu and Ga elements (see Comparative Example 5), the coercivity increment of the neodymium iron boron magnet is lower, indicating poorer diffusivity.

[0083] The present invention provides Example 5 and Comparative Example 6: Example 5:

[0084] 1. Preparation of the diffusion substrate:

[0085] Mix the raw materials of each component according to the composition formula of 30.5% PrNd, 0.5% Dy, 0.92% B, 0.5% Cu, 0.3% Ga, 0.5% Co, 0.15% Ti, and the balance being Fe by mass percentage. Then, successively melt the mixture in an induction furnace under the temperature condition of 1500 °C, and make a flaky alloy with a thickness of 0.3 ± 0.05 mm by rapid quenching and strip casting. After hydrogen crushing and jet milling, the powder is ground to 3 - 5 μm, formed under the magnetic field condition of a magnetic field strength above 1.6 T, and then sintered at 1080 °C for 10 h to obtain a massive neodymium iron boron permanent magnet. Cut the massive neodymium iron boron permanent magnet into one flaky substrate of 2 mm, 5 mm, and 8 mm each for grain boundary diffusion.

[0086] 2. Grain boundary diffusion treatment

[0087] The grain boundary diffusion treatment is carried out by coating a Tb diffusion source and then performing heat treatment to obtain a neodymium-iron-boron magnet. The content of the coated Tb is 0.7 wt% (this weight is the total mass of Tb in the diffusion source). Among them, the temperature of the grain boundary diffusion heat treatment of the 3 sheet-shaped substrates is 900 °C and the time is 2 h. The optimal tempering temperature is 480 °C and the time is 5 h, and the magnetic properties are measured (see Table 3).

[0088] Comparative Example 6:

[0089] 1. Preparation of the diffusion substrate:

[0090] Mix the raw materials of each component according to the composition formula of 30.5% PrNd, 0.5% Dy, 0.92% B, 0.15% Cu, 0.1% Ga, 0.5% Co, 0.15% Ti, and the balance being Fe by mass percentage. First, melt in an induction furnace at a temperature of 1500 °C, and then rapidly quench and strip to form a sheet alloy with a thickness of 0.3 ± 0.05 mm. After hydrogen embrittlement and jet milling, it is pulverized into a powder with a particle size of 3 - 5 μm, formed under a magnetic field condition with a magnetic field strength above 1.6 T, and then sintered at 1080 °C for 10 h to obtain a block-shaped neodymium-iron-boron permanent magnet. The block-shaped neodymium-iron-boron permanent magnet is cut into one sheet-shaped substrate of 2 mm, 5 mm, and 8 mm respectively for grain boundary diffusion preparation.

[0091] 2. Grain boundary diffusion treatment

[0092] The grain boundary diffusion treatment is carried out by coating a Tb diffusion source and then performing heat treatment to obtain a neodymium-iron-boron magnet. The content of the coated Tb is 0.7 wt% (this weight is the total mass of Tb in the diffusion source). Among them, the temperature of the grain boundary diffusion heat treatment of the 3 sheet-shaped substrates is 900 °C and the time is 2 h. The tempering temperature is 520 °C and the time is 5 h, and the magnetic properties are measured (see Table 3).

[0093] Table 3 is as follows:

[0094]

[0095] According to the data in Table 3, it can be seen that the increment of the coercivity of the neodymium-iron-boron magnet of the present invention at 5 mm and 8 mm thicknesses is significantly greater than that of the neodymium-iron-boron magnet of the conventional formula process, indicating that the neodymium-iron-boron magnet of the present invention has better diffusivity and a deeper diffusion depth.

[0096] Within the scope of the present invention, the combination of Cu and Ga in a ratio of (1:1 - 2:1) can form an R-Cu-Ga phase at a tempering temperature of 430 °C - 540 °C, achieving the effect of improving the coercivity with less reduction in the remanence. At the same time, it has excellent diffusivity, reduces the usage amount of heavy rare earths, and has an obvious effect of improving the diffusion depth, a simple process, and uses conventional equipment, enabling batch production and popularization.

[0097] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a high coercivity sintered Nd-Fe-B magnet by grain boundary diffusion, characterized in that The steps include the following:

1. Preparation of the diffusion substrate: Mix the raw materials of each component according to the composition formula of 30.5% PrNd, 0.92% B, 0.8% Cu, 0.4% Ga, 0.5% Co, 0.1% Al, 0.2% Ti, and the remaining Fe by mass percentage. First, melt them in an induction furnace under the temperature condition of 1500 °C, then make a flaky alloy with a thickness of 0.3 ± 0.05 mm by rapid quenching and strip casting, crush it into a powder with a particle size of 3 - 5 μm by hydrogenation and jet milling, form it under a magnetic field condition with a magnetic field strength above 1.6 T, and then sinter it at 1080 °C for 10 h to obtain a massive NdFeB permanent magnet. Cut the massive NdFeB permanent magnet into 5 - mm flaky substrates for grain boundary diffusion. Among them, the flaky substrate contains PrNd - Cu - Ga phase, and the ratio of the percentage contents of Cu and Ga is Cu:Ga = 2:1; 2. Grain boundary diffusion treatment For grain boundary diffusion treatment, a NdFeB magnet is obtained by the method of coating a Dy diffusion source and then performing heat treatment. Among them, the content of the coated Dy is 1.0 wt%, the temperature of the grain boundary diffusion heat treatment is 900 °C, the time is 2 h, the tempering temperature is 480 °C, and the time is 5 h.

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