Preparation method of grain boundary diffusion sintered neodymium-iron-boron magnet

Abstract

The invention discloses a preparation method of a grain boundary diffusion sintered neodymium-iron-boron magnet, and belongs to the field of rare earth permanent magnet materials. The preparation method comprises the following steps: respectively preparing the alloy I and the alloy II into casting pieces by adopting rapid hardening smelting equipment, sequentially carrying out hydrogen decrepitation and jet milling on the casting pieces to obtain alloy powder I and alloy powder II, sequentially carrying out orientation molding, isostatic cool pressing and high-temperature sintering on the alloy powder I to obtain a sintered neodymium-iron-boron magnet, preparing the alloy powder II into slurry as a diffusion source, and carrying out sintering to obtain the neodymium-iron-boron magnet. And coating the slurry on a sintered neodymium-iron-boron magnet and carrying out tempering heat treatment to finally prepare the grain boundary diffusion sintered neodymium-iron-boron magnet. In the preparation method, alloy elements diffused in a grain boundary are mainly distributed in a grain boundary phase, the alloy elements enter an Nd-rich phase to form an intermetallic compound, the ductility of the intermetallic compound is superior to that of a pure rare earth phase, so that crack propagation is inhibited, diffusion is low-melting-point liquid phase diffusion, pores and microcracks among main phase grains can be wetted and filled, stress concentration is reduced, and the mechanical property of the product is improved. The mechanical property of the magnet is improved.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet materials, specifically relating to a method for preparing a grain boundary diffusion sintered NdFeB magnet. Background Technology

[0002] Sintered NdFeB magnets possess excellent magnetic properties and are hailed as the "King of Permanent Magnets" in the industry. They are widely used in electronic products, new energy vehicles, and medical equipment, and are currently the most widely used permanent magnet material. [ However, with the rapid popularization of consumer electronics products, such as smartphones, the demand for sintered NdFeB magnets is increasing, and these components are becoming smaller and lighter. Consequently, the processing requirements for sintered NdFeB magnets are becoming increasingly stringent, with some components as thin as 0.5 mm after processing. Material failures such as fracture and breakage are becoming increasingly common in sintered NdFeB magnets during processing, magnetization, assembly, and service. Research shows that the fracture mode of sintered NdFeB magnets is mostly intergranular fracture, and their grain boundary bonding strength is lower than that within the grains. Therefore, strengthening the grain boundary phase is key to improving the mechanical properties of sintered NdFeB magnets.

[0003] Studies have shown that doping with alloying elements such as copper, cobalt, titanium, and aluminum can strengthen the grain boundary phase and improve the mechanical properties of magnets to some extent. However, the doping of these alloying elements also affects the magnetic properties of the magnets to a certain degree. Therefore, there is an urgent need for a method to improve the mechanical properties of NdFeB magnets without affecting or even improving their magnetic properties. Summary of the Invention

[0004] This invention addresses the technical problem that existing technologies cannot improve the magnetic and mechanical properties of magnets by doping with grain boundary strengthening elements. The aim is to provide a method for preparing grain boundary diffusion sintered NdFeB magnets, which simultaneously optimizes the magnetic properties of permanent magnets while ensuring their mechanical properties, and ensures that the mechanical and magnetic properties of the diffused magnets have good consistency and stability.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing grain boundary diffusion sintered NdFeB magnets involves using a rapid solidification melting device to prepare alloys I and II into castings. The castings are then subjected to hydrogen crushing and air jet milling to obtain alloy powders I and II. Subsequently, alloy powder I is subjected to orientation molding, cold isostatic pressing, and high-temperature sintering to obtain sintered NdFeB magnets. Alloy powder II is then prepared into a slurry as a diffusion source. The slurry is coated onto the sintered NdFeB magnets and subjected to tempering heat treatment to finally obtain grain boundary diffusion sintered NdFeB magnets.

[0007] Furthermore, the chemical composition of alloy I is RE x By M b Fe bal In this composition, RE is rare earth element Pr and / or Nd, with a content x of 29 wt.% to 31 wt.%; B content y is 0.8 wt.% to 1.0 wt.%; M is one or any combination of Ga, Ti, Zr and Ni, with a total content b not exceeding 2 wt.%; the balance is Fe and unavoidable impurities.

[0008] Furthermore, the chemical composition of alloy II is RE z H 100-z In this alloy, RE is the rare earth element Pr and / or Nd, and the RE content z is 60 wt.% ~ 100 wt.%, more preferably 70 wt.% ~ 85 wt.%. H is at least one of Cu, Al, and Co. Excessive alloy element content will form non-ferromagnetic grain boundary phases, occupying grain boundary space, reducing the effective volume of the main phase, weakening the magnetic domain pinning effect, and leading to a decrease in magnetic properties. Excessive RE content will cause a decrease in the volume fraction of the main phase, with more rare earth elements entering the grain boundaries, forming rare earth-rich phases that make the grain boundary phase too thick or locally accumulated, disrupting grain boundary homogeneity, causing stress concentration, and reducing mechanical properties.

[0009] Furthermore, the alloy castings I and II are crushed into micron-sized alloy powders I and II using a hydrogen crushing + air jet milling process; the average particle size of alloy powder I is 2.5 µm ~ 5.5 µm, and the average particle size of alloy powder II is 1 µm ~ 3 µm.

[0010] Further, powder alloy I is pressed into a compact under a magnetic field of not less than 2T, and then subjected to cold isostatic pressing at 150 MPa ~ 250 MPa for 30 s ~ 60 s; then the compact is subjected to high-temperature vacuum sintering under vacuum conditions at a temperature of 1000℃ ~ 1100℃ for 3 h ~ 9 h to obtain sintered NdFeB magnets. The processed magnets are then ultrasonically cleaned in dilute nitric acid and pure water to remove surface oil and rust.

[0011] Further, powder alloy II is mixed with propylene glycol at a ratio of 5 to 6:1 and stirred for 30 to 60 minutes. Then, at least one of polyvinyl alcohol and polyethylene glycol is added as a binder and stirred for 30 to 60 minutes to obtain a slurry as a diffusion source.

[0012] Furthermore, the diffusion source slurry is coated onto the magnet surface using a screen printing machine. The weight of the coated diffusion source is controlled to be 1% to 5% of the magnet's mass, preferably 3% of the magnet's mass. After coating with the diffusion source, the magnet undergoes a diffusion heat treatment at 850℃ to 950℃ for 4 to 8 hours, followed by a low-temperature heat treatment at 400℃ to 600℃ for 3 to 5 hours to obtain a sintered NdFeB magnet with grain boundary diffusion.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] (1) The present invention coats a diffusion source on a sintered NdFeB magnet and controls the chemical composition ratio of the diffusion source within a suitable range by strictly screening it, so as to improve the mechanical properties of the permanent magnet and the magnetic properties of the permanent magnet, and have good consistency and stability.

[0015] (2) In the preparation method of the present invention, the alloying elements diffused at the grain boundaries are mainly distributed in the grain boundary phase. The alloying elements enter the Nd-rich phase to form intermetallic compounds, which have better ductility than pure rare earth phases, thereby inhibiting crack propagation. Moreover, the diffusion is a low-melting-point liquid phase diffusion, which can wet and fill the pores and microcracks between the main phase grains, reduce stress concentration, and improve the mechanical properties of the magnet. The rare earth elements are uniformly distributed along the grain boundaries, the continuity of the grain boundary phase is improved, and a thinner rare earth-rich phase layer is formed, which reduces the magnetic coupling effect between grains, reduces the influence of the demagnetizing field, and improves the magnetic properties of the permanent magnet.

[0016] (3) The alloy powder used in this invention does not involve the addition of heavy rare earth elements, which can effectively reduce the preparation cost of magnets and achieve the purpose of cost reduction and efficiency improvement; and the process method is simple and easy to implement, improves production efficiency and shortens the production cycle, which is of great significance to the practical application of sintered NdFeB magnets. Attached Figure Description

[0017] Figure 1 (a) is the EPMA and mapping diagram of the grain boundary diffusion sintered NdFeB magnet prepared in Example 3. Figure 1 (b) shows the EPMA and mapping diagrams of the doped sintered NdFeB magnet prepared in Comparative Example 3. Detailed Implementation

[0018] The preparation method of the grain boundary diffusion sintered NdFeB magnet provided by the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0019] Example 1

[0020] A method for preparing a grain boundary diffusion sintered NdFeB magnet includes the following steps:

[0021] Smelting Nd separately 29.8 Zr0.2 Ni 0.3 Ga 0.2 B 0.9 Fe bal and Pr 80 Co 20 Alloy I and Alloy II (the subscripts for each element above indicate their mass fraction, the same below); Alloy I and Alloy II are made into alloy castings I and alloy castings II using a rapid solidification melting equipment; Castings I and castings II are crushed into powder I with an average particle size of 3.12 µm and powder II with an average particle size of 1.79 µm using a hydrogen crushing and air jet milling process.

[0022] Powder I was placed in a magnetic field of 2.0 T and pressed into a compact. Then, it was subjected to cold isostatic pressing at 200 MPa for 60 s. The compact was then subjected to high-temperature vacuum sintering. The compact was sintered under vacuum conditions at a temperature of 1050℃ for 6 h to obtain a sintered NdFeB magnet.

[0023] Powder II was mixed with propylene glycol at a ratio of 5.8:1 and stirred for 60 min. Polyvinyl alcohol was then added and stirred for another 30 min to obtain a slurry, which was used as a diffusion source. The sintered NdFeB magnet was processed into a 10 mm × 10 mm × 3 mm cuboid (the 3 mm direction was the magnet orientation direction). The cut magnet was cleaned in a 3% dilute nitric acid solution, followed by ultrasonic cleaning with pure water for 1 min to remove oil and other impurities adhering to the surface. The magnet was then dehydrated in anhydrous ethanol and dried with a hair dryer. The diffusion source slurry was then applied to two 10 mm × 10 mm surfaces of the magnet using a screen printing machine. The weight of the applied diffusion source was controlled to be 3% of the magnet's mass.

[0024] Finally, heat treatment was carried out, first at 900℃ for 6 hours, and then at 500℃ for 3 hours, to obtain grain boundary diffusion sintered NdFeB magnets.

[0025] In this example, to illustrate the effect of the present invention in preparing grain boundary diffused NdFeB magnets, Nd... 29.8 Zr 0.2 Ni 0.3 Ga 0.2 B 0.9 Fe bal Alloy I was used to prepare the original magnet 1 as a control by the same melting, hydrogen crushing, air jet milling, isostatic pressing, molding, sintering and heat treatment process.

[0026] Comparative Example 1

[0027] Powder I (Nd) was prepared using the same method as in Example 1. 29.8 Zr 0.2 Ni0.3 Ga 0.2 B 0.9 Fe bal ) and Powder II (Pr 80 Co 20 The powder was uniformly mixed at a mass ratio of 97:3 to obtain a mixed powder. The mixed powder was pressed into shape in a magnetic field of 2.0T and isostatically pressed at 200 MPa for 60 s. Then the pressed blank was sintered under vacuum at a temperature of 1050℃ for 6 h. Then it was subjected to heat treatment, which included two processes: a first-stage heat treatment at a temperature of 900℃ for 6 h and a second-stage heat treatment at a temperature of 500℃ for 3 h.

[0028] Comparative Example 2

[0029] In this comparative example, the process of preparing grain boundary diffusion sintered NdFeB magnets differs from that of Example 1 in that the Co element in the diffusion source is removed, while the other preparation processes are the same as in Example 1.

[0030] Table 1

[0031] Magnet categories <![CDATA[Coercive force H cj / KOe]]> <![CDATA[Residual magnetic flux density B r / KGs]]> <![CDATA[Flexural strength σ b / MPa]]> Original permanent magnet 1 10.71 14.48 254.4 Example 1 15.92 14.44 366.3 Comparative Example 1 14.43 12.36 371.9 Comparative Example 2 15.51 14.38 294.3

[0032] As can be seen from Table 1, compared with the doped magnet of Comparative Example 1, the magnetic properties of the grain boundary diffusion magnet prepared in this invention are improved while the mechanical properties are almost unaffected, exhibiting good consistency and stability; compared with the original magnet 1, both the magnetic properties and mechanical properties are significantly improved; compared with Comparative Example 2, the magnetic properties of the grain boundary diffusion magnet remain almost unchanged, while the mechanical properties are improved by 72 MPa.

[0033] Example 2

[0034] A method for preparing a grain boundary diffusion sintered NdFeB magnet includes the following steps:

[0035] Smelting Nd separately 30 Zr 0.2 Ni 0.1 Ga 0.2 B 0.9 Fe bal and Pr 84 Al 16 Alloy I and Alloy II (the subscripts for each element above indicate their mass fraction, the same below); Alloy I and Alloy II are made into alloy castings I and alloy castings II using a rapid solidification melting equipment; Castings I and castings II are crushed into powder I with an average particle size of 3.43 µm and powder II with an average particle size of 1.31 µm using a hydrogen crushing and air jet milling process.

[0036] Powder I was placed in a magnetic field of 2.2 T and pressed into a compact. Then, it was subjected to cold isostatic pressing at 200 MPa for 60 s. The compact was then subjected to high-temperature vacuum sintering. The compact was sintered under vacuum conditions at a temperature of 1030℃ for 6 h to obtain a sintered NdFeB magnet.

[0037] Powder II was mixed with propylene glycol at a ratio of 5.8:1 and stirred for 60 min. Polyethylene glycol was then added and stirred for another 30 min to obtain a slurry, which was used as a diffusion source. The sintered NdFeB magnet was processed into a 10 mm × 10 mm × 3 mm cuboid (the 3 mm direction was the magnet orientation direction). The cut magnet was cleaned in a 3% dilute nitric acid solution, followed by ultrasonic cleaning with pure water for 1 min to remove oil and other impurities adhering to the surface. The magnet was then dehydrated in anhydrous ethanol and dried with a hair dryer. The diffusion source slurry was then applied to two 10 mm × 10 mm surfaces of the magnet using a screen printing machine. The weight of the applied diffusion source was controlled to be 3% of the magnet's mass.

[0038] Finally, heat treatment was carried out, first at 900℃ for 5 hours, and then at 480℃ for 3 hours, to obtain grain boundary diffusion sintered NdFeB magnets.

[0039] In this example, to illustrate the effect of the present invention in preparing grain boundary diffused NdFeB magnets, Nd... 30 Zr 0.2 Ni 0.1 Ga 0.2 B 0.9 Fe bal Alloy I was used to prepare the original magnet 2 as a control by the same melting, hydrogen crushing, air jet milling, isostatic pressing, molding, sintering and heat treatment process.

[0040] Comparative Example 3

[0041] Powder I (Nd) was prepared using the same method as in Example 2. 30 Zr 0.2 Ni 0.1 Ga 0.2 B 0.9 Fe bal ) and Powder II (Pr 84 Al 16The powder was uniformly mixed at a mass ratio of 97:3 to obtain a mixed powder. The mixed powder was pressed into shape in a magnetic field of 2.2T and isostatically pressed at 200 MPa for 60 s. Then, the pressed blank was sintered under vacuum at a temperature of 1030℃ for 6 h. Then, heat treatment was carried out, which included two processes: a first-stage heat treatment at a temperature of 900℃ for 5 h and a second-stage heat treatment at a temperature of 480℃ for 3 h.

[0042] Comparative Example 4

[0043] In this comparative example, the process for preparing grain boundary diffusion sintered NdFeB magnets differs from that in Example 2 in that the diffusion source Pr... 84 Al 16 Replace with Pr 96 Al4, the other preparation steps are the same as in Example 2.

[0044] Table 2

[0045] Magnet categories <![CDATA[Coercive force H cj / KOe]]> <![CDATA[Residual magnetic flux density B r / KGs]]> <![CDATA[Flexural strength σ b / MPa]]> Original permanent magnet 2 11.23 14.67 234.6 Example 2 15.76 14.59 348.9 Comparative Example 3 14.72 13.32 353.2 Comparative Example 4 15.53 14.39 328.4

[0046] As can be seen from Table 2, compared with the doped magnet of Comparative Example 3, the magnetic properties of the grain boundary diffusion magnet prepared in this invention are improved while the mechanical properties are almost unaffected, exhibiting good consistency and stability; compared with the original magnet 2, both the magnetic properties and mechanical properties are significantly improved; compared with Comparative Example 4, the magnetic properties of the grain boundary diffusion magnet remain almost unchanged, while the mechanical properties are slightly improved.

[0047] Example 3

[0048] A method for preparing a grain boundary diffusion sintered NdFeB magnet includes the following steps:

[0049] Separately smelt (PrNd) 30.1 Zr 0.1 Ti 0.2 Ga 0.2 B 0.92 Fe bal and Pr 75 Cu 25 Alloy I and Alloy II (the subscripts for each element above indicate their mass fraction, the same below); Alloy I and Alloy II are made into alloy castings I and alloy castings II using a rapid solidification melting equipment; Castings I and castings II are crushed into powder I with an average particle size of 2.78 µm and powder II with an average particle size of 1.53 µm using a hydrogen crushing and air jet milling process.

[0050] Powder I was placed in a magnetic field of 2.2 T and pressed into a compact. Then, it was subjected to cold isostatic pressing at 220 MPa for 60 s. The compact was then subjected to high-temperature vacuum sintering. The compact was sintered under vacuum conditions at a temperature of 1060℃ for 5 h to obtain a sintered NdFeB magnet.

[0051] Powder II was mixed with propylene glycol at a ratio of 5.8:1 and stirred for 60 min. Polyvinyl alcohol was then added and stirred for another 30 min to obtain a slurry, which was used as a diffusion source. The sintered NdFeB magnet was processed into a 10 mm × 10 mm × 3 mm cuboid (the 3 mm direction was the magnet orientation direction). The cut magnet was cleaned in a 3% dilute nitric acid solution, followed by ultrasonic cleaning with pure water for 1 min to remove oil and other impurities adhering to the surface. The magnet was then dehydrated in anhydrous ethanol and dried with a hair dryer. The diffusion source slurry was then applied to two 10 mm × 10 mm surfaces of the magnet using a screen printing machine. The weight of the applied diffusion source was controlled to be 3% of the magnet's mass.

[0052] Finally, heat treatment was carried out, first at 880℃ for 6 hours, and then at 480℃ for 4 hours, to obtain grain boundary diffusion sintered NdFeB magnets.

[0053] In this example, to illustrate the effect of the present invention in preparing grain boundary diffused NdFeB magnets, a magnet with the composition (PrNd) is used. 30.1 Zr 0.1 Ti 0.2 Ga 0.2 B 0.92 Fe bal Alloy I was used to prepare the original magnet 3 as a control by the same melting, hydrogen crushing, air jet milling, isostatic pressing, molding, sintering and heat treatment process.

[0054] Comparative Example 5

[0055] Powder I ((PrNd)) was prepared using the same method as in Example 3. 30.1 Zr 0.1 Ti 0.2 Ga 0.2 B 0.92 Fe bal ) and Powder II (Pr 75 Cu 25The powder was uniformly mixed at a mass ratio of 97:3 to obtain a mixed powder. The mixed powder was pressed into shape in a magnetic field of 2.2T and isostatically pressed at 220 MPa for 60 s. Then, the pressed blank was sintered under vacuum at a temperature of 1060℃ for 5 h. Then, heat treatment was carried out, which included two processes: a first-stage heat treatment at a temperature of 880℃ for 6 h and a second-stage heat treatment at a temperature of 480℃ for 4 h.

[0056] Comparative Example 6

[0057] In this comparative example, the process of preparing grain boundary diffusion sintered NdFeB magnets differs from that of Example 3 in that powder alloy II is mixed and stirred with propylene glycol at a ratio of 6:1, while other preparation processes are the same as in Example 3.

[0058] Comparative Example 7

[0059] In this comparative example, the process of preparing grain boundary diffusion sintered NdFeB magnets differs from that of Example 3 in that powder alloy II is mixed and stirred with propylene glycol at a ratio of 5:1, while other preparation processes are the same as in Example 3.

[0060] Table 3

[0061] Magnet categories <![CDATA[Coercive force H cj / KOe]]> <![CDATA[Residual magnetic flux density B r / KGs]]> <![CDATA[Flexural strength σ b / MPa]]> Original permanent magnet 3 11.87 14.31 274.5 Example 3 16.73 14.20 364.4 Comparative Example 5 15.13 12.67 371.1 Comparative Example 6 15.89 14.17 359.2 Comparative Example 7 14.97 14.09 334.7

[0062] As can be seen from Table 3, compared with the doped magnet of Comparative Example 5, the magnetic properties of the grain boundary diffusion magnet prepared in this invention are improved while the mechanical properties are almost unaffected, exhibiting good consistency and stability; compared with the original magnet 3, both the magnetic properties and mechanical properties are significantly improved; compared with Comparative Examples 4-5, both the magnetic properties and mechanical properties of the grain boundary diffusion magnet are slightly improved.

[0063] Example 4

[0064] A method for preparing a grain boundary diffusion sintered NdFeB magnet includes the following steps:

[0065] Smelting Nd separately 30.1 Zr 0.2 Ni 0.2 Ga 0.2 B 0.9 Fe bal and Nd 80 Co 20 Alloy I and Alloy II (the subscripts for each element above indicate their mass fraction, the same below); Alloy I and Alloy II are made into alloy castings I and alloy castings II using a rapid solidification melting equipment; Castings I and castings II are crushed into powder I with an average particle size of 3.01µm and powder II with an average particle size of 1.76µm using a hydrogen crushing and air jet milling process.

[0066] Powder I was placed in a magnetic field of 2.0 T and pressed into a compact. Then, it was subjected to cold isostatic pressing at 220 MPa for 50 s. The compact was then subjected to high-temperature vacuum sintering. The compact was sintered under vacuum conditions at a temperature of 1065℃ for 5 h to obtain a sintered NdFeB magnet.

[0067] Powder II and propylene glycol were mixed and stirred at a ratio of 5.8:1 for 60 min. Polyvinyl alcohol was then added and stirred for another 30 min to obtain a slurry, which was used as a diffusion source. The sintered NdFeB magnet was processed into a 10 mm × 10 mm × 3 mm cuboid (the 3 mm direction was the magnet orientation direction). The cut magnet was cleaned in a 3% dilute nitric acid solution, followed by ultrasonic cleaning with pure water for 1 min to remove oil and other impurities adhering to the surface. The magnet was then dehydrated in anhydrous ethanol and dried with a hair dryer. The diffusion source slurry was then applied to the two 10 mm × 10 mm surfaces of the magnet using a screen printing machine. The weight of the applied diffusion source was controlled to be 3% of the magnet's mass.

[0068] Finally, heat treatment was carried out, first at 920℃ for 6 hours, and then at 520℃ for 5 hours, to obtain grain boundary diffusion sintered NdFeB magnets.

[0069] In this example, to illustrate the effect of the present invention in preparing grain boundary diffused NdFeB magnets, Nd... 30.1 Zr 0.2 Ni 0.2 Ga 0.2 B 0.9 Fe bal Alloy I was used to prepare the original magnet 4 as a control by the same melting, hydrogen crushing, air jet milling, isostatic pressing, molding, sintering and heat treatment process.

[0070] Comparative Example 8

[0071] Powder I (Nd) was prepared using the same method as in Example 4. 30.1 Zr 0.2 Ni 0.2 Ga 0.2 B 0.9 Fe bal ) and powder II (Nd 80 Co 20The powder was uniformly mixed at a mass ratio of 97:3 to obtain a mixed powder. The mixed powder was pressed into shape in a magnetic field of 2.2T and isostatically pressed at 220 MPa for 50 s. Then, the pressed blank was sintered under vacuum at a temperature of 1065℃ for 5 h. Then, heat treatment was carried out, which included two processes: a first-stage heat treatment at a temperature of 920℃ for 6 h and a second-stage heat treatment at a temperature of 520℃ for 5 h.

[0072] Comparative Example 9

[0073] In this comparative example, the process of preparing a grain boundary diffusion sintered NdFeB magnet differs from that of Example 4 in that the weight of the diffusion source coating is changed from 3% to 5% of the magnet's mass, while the other preparation steps are the same as in Example 4.

[0074] Table 4

[0075] Magnet categories <![CDATA[Coercive force H cj / KOe]]> <![CDATA[Residual magnetic flux density B r / KGs]]> <![CDATA[Flexural strength σ b / MPa]]> Original permanent magnet 4 12.06 14.33 244.4 Example 4 16.41 14.31 357.8 Comparative Example 8 15.82 13.51 361.1 Comparative Example 9 15.94 14.24 341.2

[0076] As can be seen from Table 4, compared with the doped magnet of Comparative Example 8, the magnetic properties of the grain boundary diffusion magnet prepared in this invention are improved while the mechanical properties are almost unaffected, exhibiting good consistency and stability; compared with the original magnet 4, both the magnetic and mechanical properties are significantly improved; compared with Comparative Example 9, the magnetic and mechanical properties of the grain boundary diffusion magnet are slightly increased, the coercivity is increased by 0.47 KOe, the remanence remains almost unchanged, and the bending strength is increased by 16.6 MPa.

[0077] In addition, through Figure 1 It can be seen that in grain boundary diffusion sintered NdFeB magnets, alloying elements are mainly distributed in the grain boundary phase, forming intermetallic compounds and enhancing the grain boundary bonding strength. Compared with doped magnets, rare earth elements are uniformly distributed along the grain boundaries in grain boundary diffusion magnets, improving the continuity of the grain boundary phase, forming a thinner rare earth-rich phase layer, reducing the magnetic coupling between grains, and improving the magnetic properties of NdFeB magnets.

[0078] The above description is merely an example and illustration of the concept of the present invention. Any modifications or additions made by those skilled in the art to the specific embodiments described, or any substitutions made in a similar manner, as long as they do not deviate from the concept of the invention or exceed the scope defined in the claims, shall fall within the protection scope of the present invention.

Claims

1. A method for preparing grain boundary diffusion sintered NdFeB, characterized in that, Alloys I and II were cast into sheets using a rapid solidification melting device. The cast sheets were then subjected to hydrogen crushing and air jet milling to obtain alloy powders I and II. Subsequently, alloy powder I was subjected to orientation molding, cold isostatic pressing, and high-temperature sintering to obtain sintered NdFeB magnets. Alloy powder II was then made into a slurry as a diffusion source. The slurry was coated onto the sintered NdFeB magnets and subjected to tempering heat treatment to finally obtain grain boundary diffusion sintered NdFeB magnets.

2. The preparation method according to claim 1, characterized in that, The chemical composition of alloy I is RE x B y M b Fe bal In this composition, RE is rare earth element Pr and / or Nd, with a content x of 29 wt.% to 31 wt.%; B content y is 0.8 wt.% to 1.0 wt.%; M is one or any combination of Ga, Ti, Zr and Ni, with a total content b not exceeding 2 wt.%; the balance is Fe and unavoidable impurities.

3. The preparation method according to claim 1, characterized in that, The chemical composition of alloy II is RE z H 100-z In this context, RE represents rare earth elements Pr and / or Nd, and the content of RE is 60 wt.% to 100 wt.%; H represents at least one of Cu, Al, and Co.

4. The preparation method according to claim 3, characterized in that, The RE content in Alloy II is 70 wt.% ~ 85 wt.%.

5. The preparation method according to claim 1, characterized in that, Alloy castings I and II were crushed into micron-sized alloy powders I and II using a hydrogen crushing and air jet milling process. The average particle size of alloy powder I was 2.5 µm to 5.5 µm, and the average particle size of alloy powder II was 1 µm to 3 µm.

6. The preparation method according to claim 1, characterized in that, Powdered alloy I is pressed into a compact in a magnetic field of not less than 2T, and then subjected to cold isostatic pressing at 150 MPa ~ 250 MPa for 30 s ~ 60 s; then the compact is subjected to high-temperature vacuum sintering, and the compact is sintered under vacuum conditions at a temperature of 1000℃ ~ 1100℃ for 3 h ~ 9 h to obtain sintered NdFeB magnets.

7. The preparation method according to claim 1, characterized in that, Powdered alloy II and propylene glycol are mixed and stirred at a ratio of 5 to 6:1 for 30 to 60 minutes. Then, at least one of polyvinyl alcohol and polyethylene glycol is added as a binder and stirred for 30 to 60 minutes to obtain a slurry as a diffusion source.

8. The preparation method according to claim 1, characterized in that, The diffusion source paste is applied to the surface of the magnet using a screen printing machine. The weight of the applied diffusion source is controlled to be 1% to 5% of the magnet's mass.

9. The preparation method according to claim 1, characterized in that, After applying the diffusion source, the magnet is first subjected to diffusion heat treatment at 850℃ ~ 950℃ for 4 h ~ 8 h; then subjected to low-temperature heat treatment at 400℃ ~ 600℃ for 3 h ~ 5 h to obtain sintered NdFeB magnets with grain boundary diffusion.

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