Sintered Re-Fe-B permanent magnet with grain size gradient as well as preparation method and application of sintered Re-Fe-B permanent magnet

Abstract

The invention discloses a sintered Re-Fe-B permanent magnet with grain size gradient as well as a preparation method and application of the sintered Re-Fe-B permanent magnet. The sintered Re-Fe-B permanent magnet has a first structure and a second structure, and the first structure and the second structure are symmetrical. The first structure is divided into n areas from the outer surface of the sintered Re-Fe-B permanent magnet to the core part in the pressing direction, the area close to the outer surface is marked as a first area, and n is larger than or equal to 2 and smaller than or equal to 5; wherein the average sizes D1 to Dn of the complete crystal grains from the first region to the nth region are gradually increased, namely D1 < D2 <... < Dn; the thicknesses of all the areas are independently selected from H * 10%-H * 40%, and the sum of the thicknesses of all the areas is H * 50%. The grain size of the sintered neodymium-iron-boron permanent magnet has a gradient structure, and gradient distribution of the performance of the sintered neodymium-iron-boron permanent magnet is achieved. The fine grain structure at the corner of the sintered neodymium-iron-boron permanent magnet is beneficial to the improvement of coercive force, has higher temperature resistance, and is more suitable for the phenomenon that the field loss of the corner of the sintered neodymium-iron-boron permanent magnet is more serious than that of a core part during the operation of the motor.

Description

Technical Field

[0001] The present invention belongs to the technical field of rare earth permanent magnet material preparation, and particularly relates to a sintered Re-Fe-B permanent magnet with a grain size gradient and a preparation method thereof. Background Art

[0002] As an important part of new energy technology, the core material of permanent magnet motors is permanent magnets. Neodymium iron boron is widely used in permanent magnet motors due to its high remanence density, high coercivity, high magnetic energy product and other characteristics. In recent years, due to the wide application of new energy technology, permanent magnet motors have put forward higher requirements for the working temperature of permanent magnets, which requires the magnets to have higher temperature resistance.

[0003] In order to ensure the normal operation of the magnet at high temperatures, during the manufacture of neodymium iron boron, on the one hand, more heavy rare earth elements such as Dy / Tb are added to the magnet by means of substrate addition or grain boundary diffusion, and on the other hand, the overall coercivity of neodymium iron boron is improved by grain refinement and optimized organizational structure. During the operation of the motor, the temperature distribution of the permanent magnet is uneven. Due to the influence of eddy current, the temperature at the corners of the magnet will be higher than that at the center. Therefore, under the condition of reducing costs, the solution of only strengthening the coercivity at the corners of the magnet rather than the overall improvement of coercivity has been recognized by more and more motor designers. At present, the high coercivity at the corners is mainly obtained by increasing the content of heavy rare earths at the corners. With the rising price of heavy rare earths, there is an urgent need for a lower-cost solution to reduce the use of heavy rare earths and improve the corner coercivity of the magnet. Summary of the Invention

[0004] To solve the above deficiencies of the prior art, the technical solution of the present invention is as follows:

[0005] A sintered Re-Fe-B permanent magnet, the sintered Re-Fe-B permanent magnet has Structure One and Structure Two, and Structure One and Structure Two are symmetrical;

[0006] Structure One refers to the regions from the outer surface to the core of the sintered Re-Fe-B permanent magnet in the pressing direction, which are divided into n regions, and the region close to the outer surface is denoted as the first region, 2≤n≤5;

[0007] Wherein, the average size D 1 to D n of the complete grains from the first region to the nth region gradually increases, that is, D 1 <D 2 <……<D n ; preferably D n ≥1.1*D 1 ;

[0008] The thicknesses of the respective regions are independently selected from H*10% to H*40%, and the sum of the thicknesses of the respective regions is H*50%;

[0009] H represents the length of the sintered Re-Fe-B permanent magnet along the pressing direction, and the total thickness of Structure 1 and Structure 2 is H. The thicknesses of Structure 1 and Structure 2 are equal, each being H*50%;

[0010] The core part refers to the position that is H*50% away from the outer surface of the sintered Re-Fe-B permanent magnet along the pressing direction.

[0011] According to an embodiment of the present invention, the sintered Re-Fe-B permanent magnet has a grain size gradient distribution substantially as Figure 1 shown.

[0012] According to an embodiment of the present invention, n is, for example, selected from 2, 3, 4, 5.

[0013] According to an embodiment of the present invention, the D n can be 1.1 to 2 times that of D 1 , for example, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 times. The inventors have found that when the D n particle size < 1.1D 1 , the difference in Hcj, δHcj, between the first region and the nth region of the sintered Re-Fe-B permanent magnet < 15 kA / m, and the magnetic properties of the first region, i.e., the edge region, of the sintered Re-Fe-B permanent magnet cannot be improved.

[0014] According to an embodiment of the present invention, in the sintered Re-Fe-B permanent magnet, the thicknesses of the respective regions are, for example, H*10%, H*20%, H*30%, H*40%.

[0015] According to a preferred embodiment of the present invention, when n = 3, Structure 1 of the sintered Re-Fe-B permanent magnet in the pressing direction includes 3 regions, which are, in sequence from the outer surface to the core part, the first region, the second region, and the third region. The thicknesses of the 3 regions are independently selected from H*10% to H*30%, and the sum of the thicknesses of the 3 regions is H*50%; the average size D 1 of all the complete grains in the first region, the average size D 2 of all the complete grains in the second region, the average size D 3 of all the complete grains in the third region, and D 1 < D 2 < D 3 ; exemplarily, the thickness of the first region is H*10%, the thickness of the second region is H*20%, and the thickness of the third region is H*20%.

[0016] According to a preferred embodiment of the present invention, when n = 2, in the pressing direction, the structure I of the sintered Re-Fe-B permanent magnet includes 2 regions, which are the first region and the second region in sequence from the outer surface to the core. The thicknesses of the 2 regions are independently selected from H*10% to H*40%, and the sum of the thicknesses of the 2 regions is H*50%; the average size D of all complete grains in the first region 1 , and the average size D of all complete grains in the second region 2 , and D 1 < D 2 ; Exemplarily, the thickness of the first region is H*20%, and the thickness of the second region is H*30%.

[0017] According to a preferred embodiment of the present invention, when n = 5, in the pressing direction, the structure I of the sintered Re-Fe-B permanent magnet includes 5 regions, which are the first region, the second region, the third region, the fourth region, and the fifth region in sequence from the outer surface to the core. The thicknesses of the 5 regions are independently selected from H*10%, and the sum of the thicknesses of the 5 regions is H*50%; the average size D of all complete grains in the first region 1 , the average size D of all complete grains in the second region 2 , the average size D of all complete grains in the third region 3 , the average size D of all complete grains in the fourth region 4 , the average size D of all complete grains in the fifth region 5 , and D 1 < D 2 < D 3 < D 4 < D 5 .

[0018] According to an embodiment of the present invention, the sintered Re-Fe-B permanent magnet is (Nd 1-x R x ) a Fe 1-a-b-c M b B c , wherein,

[0019] R is selected from one or more of Sm, La, Ce, Y, Pr, Ho, Gd, Dy, Tb, 0 ≤ x ≤ 1, 28 wt% ≤ a ≤ 35 wt%;

[0020] M is selected from one or more of Al, Ga, Zr, Ti, Cu, Co, 0 wt% ≤ b ≤ 5 wt%;

[0021] The content of B is 0.8 wt% ≤ c ≤ 1.2 wt%; the balance is Fe.

[0022] The present invention also provides a method for preparing the above-mentioned sintered Re-Fe-B permanent magnet, and the preparation method includes the following steps:

[0023] (1) Prepare a rapidly solidified thin sheet of Re-Fe-B alloy, and crush the rapidly solidified thin sheet of Re-Fe-B alloy to obtain magnetic powders with different particle sizes;

[0024] (2) Compression molding: Along the pressing direction, fill magnetic powders with different particle sizes in sequence, and after magnetic field orientation molding, obtain a green compact with different powder particle sizes in the pressing direction;

[0025] (3) Subject the green compact obtained in step (2) to two-step sintering densification and then tempering in a vacuum environment to obtain the sintered Re-Fe-B permanent magnet.

[0026] According to an embodiment of the present invention, in step (1), the raw materials for preparing the rapidly solidified thin sheet include:

[0027] Nd and R, where R is selected from one or more of Sm, La, Ce, Y, Pr, Ho, Gd, Dy, and Tb;

[0028] M is selected from one or more of Al, Ga, Zr, Ti, Cu, and Co;

[0029] B;

[0030] The balance is Fe.

[0031] According to an embodiment of the present invention, the usage amounts of the various elements in the raw materials for preparing the rapidly solidified thin sheet refer to the mass percentages described above.

[0032] According to an embodiment of the present invention, in step (1), the rapidly solidified thin sheet is prepared by a method known in the art, for example, by using rapid solidification strip casting technology. Exemplarily, the preparation method of the rapidly solidified thin sheet is as follows: Heat the preparation raw materials to melting by electromagnetic induction in a vacuum or inert gas, keep them warm for 10-60 min, then pour them on the surface of a chill roll, and then enter a cooling cylinder for further cooling to obtain a rapidly solidified thin sheet of Re-Fe-B alloy.

[0033] According to an embodiment of the present invention, in step (1), the specific steps of the crushing include: subject the rapidly solidified thin sheet of Re-Fe-B alloy to hydrogen explosion coarse crushing, then stir and mix the materials, and then obtain magnetic powders after ball milling and jet milling and then stir and mix the materials. Exemplarily, the steps of the crushing are as follows: respectively take the rapidly solidified thin sheet of Re-Fe-B alloy, first subject it to hydrogen explosion coarse crushing, then stir the powder for 1-5 h for mixing, then ball mill for 1-5 h, then grind and crush by jet milling, and stir again for 1-5 h to obtain magnetic powders with different particle size central values. Preferably, the particle size central value D50 of the magnetic powders is 1.5 μm to 5 μm.

[0034] According to an embodiment of the present invention, in step (1), the magnetic powder is prepared into n groups of powders with different particle sizes according to the size of the median particle size, and are respectively denoted as powder A, powder B, powder C... powder N. The median particle sizes of powder A, powder B, powder C... powder N are Ra, Rb, Rc... Rn respectively, and satisfy 5 μm ≥ Rn >... > Rc > Rb > Ra ≥ 1.5 μm.

[0035] According to an embodiment of the present invention, during the jet milling process, by adding 0 - 800 ppm O 2 , the oxygen content in n groups of powders with different particle sizes is respectively regulated. Moreover, the smaller the particle size, the higher the oxygen content. Preferably, the oxygen contents of the first group of powder to the nth group of powder are O(A), O(B), O(C)... O(N) respectively, and 1500 ppm ≥ O(A) > O(B) > O(C) >... > O(N) ≥ 50 ppm. The inventors found that the smaller the particle size of the powder, the higher its sintering activity. Therefore, during the sintering process, fine-grained powders are more likely to grow. Thus, in order to coordinate the grain growth process in different regions and inhibit the grain growth tendency in the fine-grained powder region, O 2 is added during jet milling to regulate the oxygen content in powders with different particle sizes, especially to increase the oxygen content of powders with smaller particle sizes (such as the oxygen content of powder A).

[0036] According to an embodiment of the present invention, in step (2), sequentially filling in magnetic powders with different particle sizes specifically means: along the pressing direction, filling in the above n groups of powders with different particle sizes in the order of from small to large and then from large to small, to obtain a green compact with different powder particle sizes in the pressing direction. Preferably, along the pressing direction, powder A to powder N are sequentially added from bottom to top. When adding to half of the thickness of the green compact in the pressing direction, powder N to powder A are then sequentially added. The addition amounts of different powders respectively correspond to the thicknesses of n regions of the above sintered Re-Fe-B permanent magnet.

[0037] According to a preferred embodiment of the present invention, when the above sintered Re-Fe-B permanent magnet has 3 regions from the outer surface to the core part in the pressing direction, powder A is selected for the first region, powder B is selected for the second region, and powder C is selected for the third region; the median particle size of powder A is Ra', the median particle size of powder B is Rb', and the median particle size of powder C is Rc', 5 μm ≥ Rc' > Rb' > Ra' ≥ 1.5 μm; in step (2), sequentially filling in magnetic powders with different particle sizes includes: sequentially filling in powder A, powder B, powder C, powder C, powder B, and powder A, and the powder filling amounts are 10%, 20%, 20%, 20%, 20%, and 10% of the volume of the green compact respectively.

[0038] According to an embodiment of the present invention, in step (2), the magnetic field orientation forming specifically includes: performing orientation pressing in a magnetic field strength of 2T, and then obtaining the green body through cold isostatic pressing.

[0039] According to an embodiment of the present invention, in step (3), in order to suppress the grain growth tendency in the fine-grained region, two-step sintering is used for sintering densification. First, it is heated to a high temperature section for short-time sintering and then quickly cooled to a low temperature section for long-time low-temperature sintering. During the low-temperature sintering process, the grain growth is suppressed. The obtained green body is heated to T1 in a vacuum environment, held for 0-10 min and then quickly cooled to T2, sintered at T2 for 2-10 h for densification, and after cooling, two-stage aging heat treatment is carried out. The first stage is aged at 750-950 °C for 2-6 h, and the second stage is aged at 450-650 °C for 2-6 h to obtain the target product or the target diffusion substrate. Wherein, T1 = T2 + △T, T2 = 900-1050 °C, and △T = 20-100 °C.

[0040] According to an embodiment of the present invention, for the sintered Re-Fe-B permanent magnet prepared in steps (1)-(3), the difference in Br between the first region and the nth region, δBr ≤ 0.01 T (for example, 0.005 T), and the difference in Hcj, δHcj ≥ 15 kA / m (for example, 20 kA / m, 30 kA / m, 40 kA / m).

[0041] According to an embodiment of the present invention, the preparation method further includes: (4) performing grain boundary diffusion treatment on the sintered Re-Fe-B permanent magnet.

[0042] According to an embodiment of the present invention, the grain boundary diffusion treatment includes: uniformly arranging the prepared diffusion source on the outer surface of the sintered Re-Fe-B permanent magnet, infiltrating in a vacuum environment at 720-900 °C for 5-30 h, and performing vacuum aging at 400-600 °C for 2-10 h.

[0043] Preferably, the diffusion source includes at least one of rare earth elements such as Tb, Dy, Ho, Pr, Nd, La, Ce, etc.

[0044] Preferably, the sintered Re-Fe-B permanent magnet is optionally processed into a thin sheet and then subjected to grain boundary diffusion treatment.

[0045] The present invention also provides a sintered Re-Fe-B permanent magnet prepared by the above preparation method, and the sintered Re-Fe-B permanent magnet has the meaning as described above.

[0046] The present invention also provides an application of the above sintered Re-Fe-B permanent magnet, preferably used in an electric motor.

[0047] Advantages of the present invention:

[0048] The grain size of the sintered Nd-Fe-B permanent magnet of the present invention has a gradient structure, realizing a gradient distribution of the performance of the sintered Nd-Fe-B permanent magnet. The fine-grained structure at the corners of the sintered Nd-Fe-B permanent magnet helps to improve the coercivity, has higher temperature resistance, and is more in line with the phenomenon that the corner demagnetization of the sintered Nd-Fe-B permanent magnet is more serious than that of the core during the operation of the motor. To coordinate the growth process of powders with different particle sizes, the present invention controls the oxygen content of magnetic powders with different particle sizes during the jet milling process, increases the oxygen content of fine-grained powders, and at the same time adopts a two-step sintering method during the sintering process. First, it is heated to a higher temperature to activate the grain boundaries, and then cooled to a lower temperature for long-time low-temperature sintering densification, so as to inhibit the grain growth of fine-grained powders in the edge region during the sintering process. Compared with the prior art, the use of rare earths is saved and the material cost is reduced. Description of the Drawings

[0049] Figure 1 It is a schematic structural diagram of a sintered Re-Fe-B permanent magnet with a gradient distribution of grain size.

[0050] Figure 2 It is a schematic diagram of the sintering regime of the two-step sintering method.

[0051] Figure 3 It is an EPMA image of different regions of the permanent magnet after grain boundary diffusion in Example 7.

[0052] Figure 4 It is a grain size distribution diagram of different regions of the product in Example 7.

[0053] Figure 5 It is a schematic diagram of the partition of the blank product in Example 1. Detailed Embodiments

[0054] The technical solutions of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to illustrate and explain the present invention exemplarily, and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection of the present invention.

[0055] Unless otherwise specified, the raw materials and reagents used in the following embodiments are all commercially available products, or can be prepared by known methods.

[0056] In the following embodiments, the test method for the average grain size of the grains of the sintered Re-Fe-B permanent magnet is as follows: take the cross section of the nth area of ​​the sintered Re-Fe-B permanent magnet as the observation surface, polish the observation surface and place it under a scanning electron microscope (SEM) to magnify it 10,000 times, and randomly select 10 observation positions on each observation surface for sampling; perform image analysis on the sampled images at different observation positions, measure the cross-sectional area of ​​each grain and convert it into the equivalent diameter of each grain, then draw the grain size distribution diagram at different observation positions, and calculate the median diameter (i.e., the center value of the grain diameter) D50 of the grains at different observation positions; then take the average value of the median diameter D50 of the grains at the 10 observation positions in the observation surface, that is, the average grain size of the grains in the observation surface.

[0057] Example 1

[0058] A method for preparing a sintered NdFeB magnet comprises the following steps:

[0059] (1) Mix the raw materials according to the following ratio: Pr 20 Nd 80 The proportion is 31.0wt%, Ga accounts for 0.2wt%, Al accounts for 0.3wt%, Co accounts for 1.0wt%, B accounts for 1.0wt%, and the rest is Fe and impurities. The prepared materials are vacuum melted at 1420℃ and kept warm for 15min, then poured on the surface of the quenching roller, and then dropped into a water cooling barrel for cooling to obtain NdFeB flakes;

[0060] (2) The NdFeB flakes obtained in step (1) were divided into three parts, and were subjected to hydrogen explosion and ball milling respectively, followed by jet milling. During the jet milling process, the speed of the classifying wheel was adjusted to obtain powder A with a particle size center value D50 of 3 μm, powder B with a particle size of 3.5 μm, and powder C with a particle size of 4 μm. During the grinding process of powder A, 350 ppm of O was introduced into the grinding chamber. 2 , the oxygen content of powder A is O(A) = 800-1000ppm; during the grinding process of powder B, 150ppm of O is introduced into the grinding chamber 2 , oxygen content of powder B O(B) = 600-800ppm; during the grinding process of powder C, no additional O 2 , C powder oxygen content O(C)≤600ppm;

[0061] (3) The magnetic powder of step (2) is sequentially filled into a vacuum press mold in the order and proportion of A powder / 10%-B powder / 20%-C powder / 40%-B powder / 20%-A powder / 10%, and oriented and pressed under a magnetic field strength of 2T, and a green body is obtained after cold isostatic pressing;

[0062] (4) Place the green body from step (3) into a vacuum sintering furnace, heat it from room temperature to 250 °C at a rate of 5 °C / min, hold at 250 °C for 30 min; heat it from 250 - 400 °C at a rate of 3 °C / min, hold at 400 °C for 30 min; heat it from 400 - 700 °C at a rate of 5 °C / min, hold at 700 °C for 90 min, and then heat it to 1050 °C at a heating rate of 8 °C / min and sinter for 4 h. Then, perform the first-stage aging treatment at 900 °C for 4 h and the second-stage aging treatment at 550 °C for 4 h to obtain the Nd-Fe-B blank. The schematic structural diagram is as Figure 5 shown, where the A particle size region is the first region, the B particle size region is the second region, and the C particle size region is the third region; the size of the Nd-Fe-B blank is 45 mm - 60 mm - 35 mm, where the 45 mm direction is the pressing direction and the 35 mm direction is the orientation direction.

[0063] Performance test:

[0064] After uniformly removing the skin, take 10 samples of 1 mm - 1 mm - 1 mm each from the Nd-Fe-B blank in the A particle size region, B particle size region, and C particle size region respectively for magnetic property testing, and take the average value of the results at different positions. The test results are shown in Table 1.

[0065] Example 2

[0066] The preparation method of Example 2 is the same as that of Example 1, except that the material ratio in Example 2 is 32.0 wt% Nd, 0.3 wt% Zr, 0.15 wt% Al, 1.0 wt% Co, 0.96 wt% B, and the rest is Fe and impurities, and the Nd-Fe-B blank is prepared.

[0067] The performance test of the Nd-Fe-B blank is the same as that of Example 1, and the test results are shown in Table 1.

[0068] Example 3

[0069] The preparation method of Example 3 is the same as that of Example 1, except that in step (2), A powder with a median particle size D50 of 2.5 μm, B powder with a median particle size of 3 μm, and C powder with a median particle size of 4 μm are obtained. During the pressing process in step (3), the powders filled in are A powder / 10% - B powder / 20% - C powder / 40% - B powder / 20% - A powder / 10% in sequence, and the Nd-Fe-B blank is prepared.

[0070] The performance test of the Nd-Fe-B blank is the same as that of Example 1, and the test results are shown in Table 1.

[0071] Example 4

[0072] Example 4 has the same preparation method as Example 1, except that in Example 4, the Nd-Fe-B magnet prepared in Example 1 is processed into an Nd-Fe-B blank with an orientation direction of 8 mm, that is, the size of the Nd-Fe-B blank is 45 mm - 60 mm - 8 mm.

[0073] Diffusion: A slurry with 50 wt% Dy is evenly coated on the 45 mm - 60 mm large surfaces on both sides of the blank prepared above. After drying, the coating amount is 0.7% of the mass of the thin sheet. The coated Nd-Fe-B thin sheet is first heat-treated at 850 °C for 4 h, and then at 550 °C for 3 h to obtain a permanent magnet after Dy diffusion.

[0074] The performance test of the permanent magnet after Dy diffusion is the same as that of Example 1, and the test results are shown in Table 1.

[0075] Example 5

[0076] Example 5 has the same preparation method as Example 1. The difference is that in step (2), A powder with a median particle size D50 of 3 μm and B powder with a median particle size D50 of 4 μm are obtained. In the powder filling process in step (3), the powders filled in turn are A powder / 20% - B powder / 60% - A powder / 20% to prepare an Nd-Fe-B blank; the obtained blank includes an A particle size area (i.e., the first area) and a B particle size area (i.e., the second area).

[0077] The performance test of the Nd-Fe-B blank is the same as that of Example 1.

[0078] Example 6

[0079] Example 6 has the same preparation method as Example 1, except that during the jet milling process in Example 6, the rotational speed of the classification wheel is adjusted to obtain A powder with a median particle size D50 of 2.5 μm, B powder with a median particle size D50 of 3 μm, C powder with a median particle size D50 of 3.5 μm, D powder with a median particle size D50 of 4 μm, and E powder with a median particle size D50 of 4.5 μm. It is measured that the oxygen content of each group of powders is: oxygen content O(A) = 1000 - 1200 ppm, O(B) = 800 - 1000 ppm, O(C)

[0080] = 600 - 800 ppm, O(D) ≤ 600 ppm, O(E) ≤ 600 ppm.

[0081] In the powder filling process, the powders filled in turn are A powder / 10% - B powder / 10% - C powder / 10% - D powder / 10% - E powder / 20% - D powder / 10% - C powder / 10% - B powder / 10% - A powder / 10%; the obtained blank product includes an A particle size area (i.e., the first area), a B particle size area (i.e., the second area), a C particle size area (i.e., the third area), a D particle size area (i.e., the fourth area), and an E particle size area (i.e., the fifth area) to prepare an Nd-Fe-B blank.

[0082] Samples were taken from the A to E grain size regions of the Nd-Fe-B blanks and processed into 10 sample pieces each with dimensions of 1mm - 1mm - 1mm for magnetic property testing. The results at different positions were averaged, and the test results are shown in Table 1.

[0083] Example 7

[0084] Example 7 has the same preparation method as Example 1, except that in the airflow milling process of step (2) in Example 7, the rotational speed of the classification wheel was adjusted to obtain A powder with a median particle size D50 of 3.0 μm, B powder with 4.1 μm, and C powder with 4.8 μm, where O(A) = 800 - 1000 ppm, O(B) ≤ 600 ppm, and O(C) ≤ 600 ppm. In step (3), the magnetic powders from step (2) were filled into the vacuum press mold in the order and proportion of A powder / 20% - B powder / 20% - C powder / 20% - B powder / 20% - A powder / 20% and oriented and pressed into shape under a magnetic field strength of 2T. After cold isostatic pressing, a green compact was obtained. The green compact was sintered to obtain an Nd-Fe-B blank with dimensions of 10mm - 40mm - 20mm, where the 10mm direction is the pressing direction and the 20mm direction is the orientation direction. The blank was processed into a product with dimensions of 10mm - 20mm - 4mm, where the 4mm direction is the orientation direction, and an Nd-Fe-B blank was prepared. The grain sizes in each region are as Figure 4 shown, and the grain sizes of the powders in the above different regions increased slightly after sintering.

[0085] Diffusion: A slurry containing 50 wt% Tb was evenly coated on the 20mm - 4mm side surfaces of both sides of the Nd-Fe-B blank. After drying, the coating amount was 0.5% of the mass of the thin sheet. The coated neodymium iron boron thin sheet was first heat-treated at 850°C for 4 h, and then at 550°C for 3 h to obtain a permanent magnet after Tb diffusion. The EPMA images of different regions after Tb diffusion are as Figure 3 shown.

[0086] The performance testing of the permanent magnet after Tb diffusion was the same as that of Example 1, and the test results are shown in Table 1.

[0087] Table 1 Magnetic properties at different positions of different products in the examples

[0088]

[0089] It can be analyzed from Table 1 that the gradient change of the grain size leads to the gradient change of the coercivity. Grain refinement improves the Hcj in the edge region of the magnet (the Br difference between regions D 1 and D n while ensuring the stability of Br (the Br difference between regions D 1 and D nThe Hcj difference in the region is greater than 15 kA / m), and the magnet edge region D 1 The finer the grains in 1 , the higher the Hcj. Without adding rare earth elements additionally, the temperature resistance of the product corners is improved. Moreover, Examples 4 and 7 also verified that the magnet with refined grains in the edge region can also be used as a diffusion substrate to further increase the coercivity through the grain boundary diffusion technology, and the coercivities of the fine-grained positions in regions A and B at the corners are still greater than that of the central region C.

[0090] Comparative Example 1

[0091] The preparation method of Comparative Example 1 is the same as that of Example 1, except that in step (2), A powder with a median particle size D50 of 3.0 μm and B powder with a median particle size D50 of 3.2 μm are obtained, and in the powder filling process of step (3), the powders filled in sequence are A powder / 20% - B powder / 60% - A powder / 20% to obtain an Nd-Fe-B blank.

[0092] The performance test of the Nd-Fe-B blank is the same as that of Example 1, and the test results are shown in Table 2.

[0093] Comparative Example 2

[0094] The preparation method of Comparative Example 2 is the same as that of Example 4, except that in step (2), A powder with a median particle size D50 of 4.0 μm is obtained, and in the powder filling process of step (3), the powder filled is A powder / 100% to obtain an Nd-Fe-B blank.

[0095] The performance test of the Nd-Fe-B blank is the same as that of Example 1, and the test results are shown in Table 2.

[0096] Comparative Example 3

[0097] The preparation method of Comparative Example 3 is the same as that of Example 4, except that in step (2), A powder with a median particle size D50 of 4.0 μm is obtained, and in the powder filling process of step (3), the powder filled is A powder / 100%. An Nd-Fe-B blank with an orientation direction of 8 mm is prepared;

[0098] Diffusion: A slurry with a Dy content of 50 wt% is evenly coated on both large surfaces of the above Nd-Fe-B blank. After drying, the coating amount is 0.9% of the mass of the thin sheet. The remaining steps are the same as those in Example 4 to obtain a diffused permanent magnet.

[0099] The performance test of the diffused permanent magnet is the same as that of Example 1, and the test results are shown in Table 2.

[0100] Table 2 Magnetic properties of different positions of different products in the comparative examples

[0101]

[0102]

[0103] As can be seen from Table 1 and Table 2:

[0104] Since the particle size D of the B powder 2 is slightly larger than the particle size of the B powder, that is, D 2 <D 1 *1.1, so the particle size difference in different regions of the magnet is small, and the magnetic property difference in different regions of the magnet of Comparative Example 1 is small (the Hcj difference between the A particle size region and the B particle size region is only 5 kA / m), and the grain gradient scheme does not obtain an obvious improvement in performance.

[0105] Comparing the magnetic properties of Comparative Example 2 and Example 4, it can be seen that Comparative Example 2 only uses one kind of particle size of magnetic powder, and the magnetic properties of different regions of its magnet are basically the same as those of the C particle size region (core region) of Example 4; however, since Example 4 uses the grain gradient of the present invention to improve the coercivity of the edge region (A particle size region) of the magnet, the demagnetization resistance of the edge region of the magnet is further improved, which is more in line with the situation that the edge region of the magnet is more likely to be demagnetized than the core region when the magnet works in the motor.

[0106] Comparing the magnetic properties of Comparative Example 3 and Example 4, it can be seen that Comparative Example 3 only uses one kind of particle size of magnetic powder, and the magnetic properties of different regions of its magnet are basically the same as those of the edge region A particle size region of Example 4. However, 0.9% of Dy needs to be added in Comparative Example 3, while only 0.7% is needed in Example 4. The grain gradient scheme in Example 4 can save 0.2% of Dy, greatly reducing the use of heavy rare earth elements and significantly reducing the cost of using rare earths.

[0107] The above has described the exemplary embodiments of the present invention. However, the protection scope of this application is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made by those skilled in the art within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A sintered Re-Fe-B permanent magnet, characterized in that, the sintered Re-Fe-B permanent magnet has Structure 1 and Structure 2, and Structure 1 and Structure 2 are symmetrical; Structure 1 means from the outer surface to the core of the sintered Re-Fe-B permanent magnet in the pressing direction, which is divided into n regions. The region close to the outer surface is denoted as the first region, and 2 ≤ n ≤ 5; Among them, the average sizes D1 to D of the complete grains from the first region to the nth region n gradually increase, that is, D1 < D2 <... < D n ; the thicknesses of each region are independently selected from H*10% to H*40%, and the sum of the thicknesses of each region is H*50%; H represents the length of the sintered Re-Fe-B permanent magnet along the pressing direction, and the total thickness of Structure 1 and Structure 2 is H. The thicknesses of Structure 1 and Structure 2 are equal, each being H*50%; the core refers to the position at a distance of H*50% from the outer surface of the sintered Re-Fe-B permanent magnet along the pressing direction.

2. The sintered Re-Fe-B permanent magnet according to claim 1, characterized in that, D n ≥ 1.1 * D1. Preferably, the D n is 1.1 to 2 times that of D 1 .

3. The sintered Re-Fe-B permanent magnet according to claim 1 or 2, characterized in that, When n = 3, the structure I of the sintered Re-Fe-B permanent magnet in the pressing direction includes three regions, which are the first region, the second region, and the third region in sequence from the outer surface to the core. The thicknesses of the three regions are independently selected from H*10% to H*30%, and the sum of the thicknesses of the three regions is H*50%; the average size D of all complete grains in the first region 1 , the average size D of all complete grains in the second region 2 , the average size D of all complete grains in the third region 3 , and D 1 < D 2 < D 3 . Preferably, when n = 2, in the pressing direction, Structure 1 of the sintered Re-Fe-B permanent magnet includes two regions, which are the first region and the second region in sequence from the outer surface to the core. The thicknesses of the two regions are independently selected from H*10% to H*40%, and the sum of the thicknesses of the two regions is H*50%. The average size D of all complete grains in the first region 1 , and the average size D of all complete grains in the second region 2 , and D 1 < D 2 . Preferably, when n = 5, in the pressing direction, Structure 1 of the sintered Re-Fe-B permanent magnet includes 5 regions, which are, in sequence from the outer surface to the core, the first region, the second region, the third region, the fourth region, and the fifth region. The thicknesses of the 5 regions are independently selected from H*10%, and the sum of the thicknesses of the 5 regions is H*50%; the average size D of all complete grains in the first region 1 , the average size D of all complete grains in the second region 2 , the average size D of all complete grains in the third region 3 , the average size D of all complete grains in the fourth region 4 , the average size D of all complete grains in the fifth region 5 , and D 1 < D 2 < D 3 < D 4 < D 5 .

4. The sintered Re-Fe-B permanent magnet according to any one of claims 1-3, characterized in that, The sintered Re-Fe-B permanent magnet is (Nd 1-x R x ) a Fe 1-a-b-c M b B c , where R is selected from one or more of Sm, La, Ce, Y, Pr, Ho, Gd, Dy, Tb, 0 ≤ x ≤ 1, 28wt% ≤ a ≤ 35wt%; M is selected from one or more of Al, Ga, Zr, Ti, Cu, Co, 0wt% ≤ b ≤ 5wt%; the content of B is 0.8wt% ≤ c ≤ 1.2wt%; the balance is Fe.

5. The method for preparing the sintered Re-Fe-B permanent magnet according to any one of claims 1-4, characterized in that, the preparation method includes the following steps: (1) Prepare a rapidly solidified thin sheet of Re-Fe-B alloy, and crush the rapidly solidified thin sheet of Re-Fe-B alloy to obtain magnetic powders with different particle sizes; (2) Compression molding: Along the pressing direction, fill in magnetic powders with different particle sizes in sequence, and after magnetic field orientation molding, obtain a green compact with different powder particle sizes in the pressing direction; (3) Subject the green compact in step (2) to two-step sintering densification and then tempering in a vacuum environment to obtain the sintered Re-Fe-B permanent magnet.

6. The preparation method according to claim 5, characterized in that, in step (1), the raw materials for preparing the rapidly solidified thin sheet include: Nd and R, where R is selected from one or more of Sm, La, Ce, Y, Pr, Ho, Gd, Dy, Tb; M is selected from one or more of Al, Ga, Zr, Ti, Cu, Co; B； the balance is Fe. Preferably, in step (1), the crushing step specifically includes: subject the rapidly solidified thin sheet of Re-Fe-B alloy to hydrogen explosion coarse crushing, then stir and mix the materials, and then subject them to ball milling and jet milling and then stir and mix the materials to obtain magnetic powders. Preferably, in step (1), the magnetic powders are respectively prepared into n groups of magnetic powders with different particle sizes according to the size of the particle size center value, and are respectively denoted as A powder, B powder, C powder... N powder. The particle size center values of A powder, B powder, C powder... N powder are Ra, Rb, Rc... Rn respectively, and satisfy 5μm ≥ Rn >... > Rc > Rb > Ra ≥ 1.5μm. Preferably, during the jet milling process, by adding 0 - 800 ppm O 2 , the oxygen content in n groups of powders with different particle sizes is regulated respectively, and the smaller the particle size, the higher the oxygen content. Preferably, the oxygen contents of the first group of powder to the nth group of powder are O(A), O(B), O(C) …… O(N) respectively, where 1500 ppm ≥ O(A) > O(B) > O(C) > …… > O(N) ≥ 50 ppm.

7. The preparation method according to claim 5 or 6, characterized in that, in step (2), the specific filling of magnetic powders with different particle sizes in sequence means: along the pressing direction, the above-mentioned n groups of powders with different particle sizes are filled in the order of from small to large and then from large to small in particle size, so as to obtain a green body with different powder particle sizes in the pressing direction. Preferably, when the above-mentioned sintered Re-Fe-B permanent magnet is divided into 3 regions from the outer surface to the core part of the sintered Re-Fe-B permanent magnet in the pressing direction, powder A is selected for the first region, powder B is selected for the second region, and powder C is selected for the third region; wherein the central value of the particle size of powder A is Ra', the central value of the particle size of powder B is Rb', and the central value of the particle size of powder C is Rc', 5μm≥Rc’>Rb’>Ra’≥1.5μm; in step (2), the filling of magnetic powders with different particle sizes in sequence includes: filling powder A, powder B, powder C, powder C, powder B and powder A in sequence, and the powder filling amounts are 10%, 20%, 20%, 20%, 20% and 10% of the volume of the green body respectively. Preferably, in step (2), the magnetic field orientation forming specifically includes: performing orientation pressing in a magnetic field strength of 2T, and then obtaining the green body after cold isostatic pressing.

8. The preparation method according to any one of claims 5-7, characterized in that, in step (3), in order to suppress the grain growth tendency in the fine-grained region, a two-step sintering method is used for sintering densification. First, it is heated to a high temperature section for short-time sintering and then quickly cooled to a low temperature section for long-time low-temperature sintering. During the low-temperature sintering process, the grain growth is inhibited. The obtained green body is heated to T1 in a vacuum environment, held for 0-10 min and then quickly cooled to T2, sintered at T2 for 2-10 h for densification, and after cooling, two-stage aging heat treatment is carried out. The first stage is aged at 750-950°C for 2-6 h, and the second stage is aged at 450-650°C for 2-6 h to obtain the target product or the target diffusion substrate. Wherein, T1 = T2 + △T, T2 = 900-1050°C, △T = 20-100°C.

9. The preparation method according to any one of claims 5-8, characterized in that, for the sintered Re-Fe-B permanent magnet prepared in steps (1)-(3), the difference in Br between the first region and the nth region is δBr≤0.01T, and the difference in Hcj is δHcj≥15 kA / m. Preferably, the preparation method further includes: (4) performing grain boundary diffusion treatment on the sintered Re-Fe-B permanent magnet. Preferably, the grain boundary diffusion treatment includes: uniformly arranging the prepared diffusion source on the outer surface of the sintered Re-Fe-B permanent magnet, infiltrating in a vacuum environment at 720-900°C for 5-30 h, and performing vacuum aging at 400-600°C for 2-10 h. Preferably, the diffusion source includes at least one of rare earth elements such as Tb, Dy, Ho, Pr, Nd, La, and Ce. Preferably, the sintered Re-Fe-B permanent magnet is optionally processed into a thin sheet and then subjected to grain boundary diffusion treatment.

10. Application of the sintered Re-Fe-B permanent magnet according to any one of claims 1-4 in an electric motor.

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