A circumferential multi-axis oriented anisotropic permanent magnet and a method of manufacturing the same

Abstract

The application relates to the technical field of permanent magnet materials, and discloses a circumferential multi-axis orientation anisotropic permanent magnet and a preparation method thereof, which comprises the following steps: (1) preparing a magnet blank by hot pressing amorphous or nanocrystalline magnetic powder; and (2) positively extruding the magnet blank through an extrusion die to make the hot-pressed magnet undergo thermal deformation and obtain a circumferential multi-axis vertical orientation permanent magnet; the die cavity of the extrusion die comprises an extrusion zone, a transition zone and a forming zone which are sequentially communicated; the cross section of the transition zone is gradually changed from a circular shape of the extrusion zone into a target cross section same with that of the forming zone, and the cross sectional area of the transition zone is gradually reduced along the extrusion direction; and the cross section of the forming zone is a polygon, a hollow polygon or a closed figure surrounded by straight line segments and curve segments. The preparation method has the advantages that a special multi-axis orientation anisotropic magnet can be prepared at one time, and the manufacturing efficiency is high. The prepared magnet has the characteristics of high magnetic energy product and high coercivity.

Description

Technical Field

[0001] This invention relates to the field of magnetic materials technology, and in particular to a circumferentially multiaxially oriented anisotropic permanent magnet and its preparation method. Background Technology

[0002] With the development of new energy vehicles, national defense, robotics, and high-end sensors, the demand for high-performance rare-earth permanent magnet materials is constantly increasing. The shape and internal grain orientation structure of rare-earth permanent magnets directly affect the magnetic pole distribution and performance of the magnets. Therefore, anisotropic rare-earth permanent magnets that combine high magnetic properties, special shapes, and specific orientation structures have significant application value.

[0003] Existing anisotropic hot-deformable rare-earth permanent magnets are mainly prepared using hot pressing and upsetting deformation processes, or by first preparing block or columnar magnets and then machining them into magnetic rings or other target shapes through wire cutting, grinding, etc. During upsetting, differences in material flow and deformation in different regions can easily lead to uneven grain orientation and magnetic property distribution. Cracks are also prone to form at the magnet edges, requiring the removal of defective areas. Subsequent machining increases material loss and production steps, and may cause edge chipping or microcracks. More importantly, machining can only change the external shape of the magnet and cannot alter the grain orientation already formed during hot deformation. Therefore, it is difficult to obtain multi-axis orientation structures corresponding to different circumferential sides through machining.

[0004] To directly prepare radially oriented toroidal magnets, existing technologies also employ a reverse extrusion method. During reverse extrusion, the magnet blank flows along the annular gap between the punch or mandrel and the outer die in the direction opposite to the punch's movement, forming a radially oriented magnetic ring under the constraint of the inner and outer die walls and radial compressive stress. While this method can directly form the inner hole of the magnetic ring, the material flow path, stress state, and effective deformation degree differ at the top, middle, and bottom of the ring, easily leading to inconsistent grain deformation and orientation in different axial regions. Typically, the middle region, in a relatively stable flow stage, achieves better radial orientation, while the initial extrusion region and the region near the punch end are prone to insufficient deformation or complex flow states, resulting in uneven distribution of microstructure and magnetic properties at different locations of the magnet.

[0005] The inner and outer surfaces of the reverse-extruded magnetic ring are constrained by the mandrel and the outer mold, respectively. During cooling and demolding, significant friction and additional stress are easily generated, leading to tangential cracks, spalling, or demolding difficulties. For small-sized, thin-walled, or high aspect ratio magnetic rings, the slender mandrel used to form the inner hole is also prone to displacement, bending, or breakage under high-temperature extrusion pressure and mold wall friction, resulting in misalignment of the inner and outer walls, uneven wall thickness, and fluctuations in orientation and magnetic properties. Furthermore, the material flow channel formed by reverse extrusion is typically annular, and the resulting magnet mainly exhibits a radial orientation that continuously varies around the center. The product's cross-sectional shape and orientation structure are limited by the annular mold cavity, making it difficult to form a multi-axial orientation structure corresponding to different circumferential forming surfaces.

[0006] Therefore, existing technologies still require a permanent magnet that can avoid the forming and demolding problems caused by slender mandrels, reduce subsequent internal hole and outer contour processing, and achieve uniform circumferential multiaxial grain orientation and magnetic property distribution during the same hot processing process. Summary of the Invention

[0007] The purpose of this invention is to provide a method and mold for efficiently preparing high-performance circumferentially multiaxially oriented anisotropic hot-deformed long strip magnets. The problem to be solved is to provide a high-performance circumferentially multiaxially oriented anisotropic rare-earth long strip magnet. This long strip magnet is made by crushing amorphous or nanocrystalline NdFeB rapidly quenched strips into micron-sized magnetic powder, hot-pressing it into a full-density isotropic magnet, and then performing forward extrusion through an extrusion die with a square, polygonal, hollow polygonal, or tile-shaped extrusion port. This causes thermorheology in the hot-pressed magnet, and the easy magnetization axes of the principal phase grains in the corresponding regions of different circumferential forming surfaces are preferentially oriented along the normal direction of the corresponding circumferential forming surface. Thus, the target cross-section and circumferentially multiaxially oriented structure of the magnet are simultaneously formed in a single forward extrusion process, directly obtaining a near-net-shape anisotropic rare-earth long strip magnet.

[0008] The length of this high-performance rare earth long magnet can be customized as needed, and it has a near-net-shape long strip circumferential multi-axis orientation structure. Under optimized process conditions, its maximum magnetic energy product along the circumferential orientation direction can reach more than 200 kJ / m³, and the non-uniformity of the maximum magnetic energy product in each part of the stable extrusion section is less than 5%.

[0009] A method for fabricating a circumferentially multiaxially oriented anisotropic permanent magnet includes the following steps:

[0010] (1) Amorphous or nanocrystalline magnetic powder is hot-pressed into a magnet blank;

[0011] (2) The magnet blank is extruded through an extrusion die to cause thermorheology in the hot-pressed magnet, thereby obtaining a permanent magnet with a multi-axis vertical orientation along the circumferential direction; the cavity of the extrusion die includes an extrusion zone, a transition zone and a forming zone connected in sequence; the cross-section of the extrusion zone is circular; the cross-section of the transition zone gradually transitions from a circle to the same target cross-section as the forming zone, and the cross-sectional area of ​​the transition zone gradually decreases along the extrusion direction; the cross-section of the forming zone is a polygon, a hollow polygon, or a closed shape enclosed by straight line segments and curved lines.

[0012] Step (1) hot pressing employs a vertically loaded extrusion device to first hot press amorphous or nanocrystalline NdFeB rapidly quenched magnetic powder into a fully density isotropic magnet. Then, through a specific extrusion die, the hot-pressed magnet undergoes thermorheology, forming a circumferentially multiaxial orientation, resulting in a high-performance circumferentially multiaxially oriented rare-earth elongated permanent magnet. Both processes are completed in a high vacuum or protective atmosphere environment.

[0013] Preferably, the cross-section of the forming area is an equilateral triangle, square, rectangle, regular pentagon, regular hexagon, or tile shape; the tile shape is formed by an outer arc edge and an inner arc edge arranged opposite each other, and two side edges connecting the outer arc edge and the inner arc edge.

[0014] Preferably, the longitudinal section of the transition zone is trapezoidal, and the angle α between the inclined surface of the trapezoid and the central axis of the mold cavity is 10° to 80°. More preferably, the angle α is 25° to 55°.

[0015] Preferably, the hot pressing temperature in step (1) is 600~900 ℃, the holding time is 15~40 min, and the hot pressing pressure is 100~200 MPa; the forward extrusion in step (2) is carried out at 600~900 ℃. When the hot pressing time is within the above range, the grain size inside the precursor can be effectively controlled, and the magnetic properties of the magnet after forward extrusion can be further improved.

[0016] Preferably, the hot pressing in step (1) is carried out under vacuum conditions, with a vacuum pressure of less than 0.01 Pa.

[0017] Preferably, the magnetic powder is RE-Fe-B rapid quenching magnetic powder, where RE is a rare earth element; the relative density of the magnet blank is more than 95% of the theoretical density of the magnetic powder material.

[0018] Preferably, the inner walls of the mold used for hot pressing in step (1) and the extrusion mold in step (2) are lined with graphite paper or coated with molybdenum disulfide and / or boron nitride as release agents.

[0019] Preferably, the compression strain rate in step (2) of the positive extrusion process is 1×10⁻⁶. -2 s -1The indentation strain rate is the ratio of the reduction in thickness of the hot-pressed magnet to its original thickness. The indentation strain rate is controlled at 1 × 10⁻⁶. -2 s -1 It is possible to obtain thermally deformable magnets with excellent magnetic properties.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] (1) This invention utilizes the transition zone of the positive extrusion die and the different circumferential forming surfaces to constrain the flow and force direction of the magnet material, so that the easy magnetization axes of the principal phase grains in the corresponding regions of the different circumferential forming surfaces are preferentially arranged along the normal direction of the corresponding surface. For square or polygonal cross sections, the orientation directions correspond to the normal directions of the side surfaces of each plane; for tile-shaped cross sections, the orientation directions in the corresponding regions of the arc-shaped surface change along the local normal direction of the arc-shaped surface, thereby forming a circumferential multi-axis orientation structure that matches the cross-sectional shape in the overall elongated magnet.

[0022] (2) The cross-section of the forming area of ​​the positive extrusion die of the present invention matches the cross-section of the target magnet, so that the target shape of the magnet and the circumferential multi-axis orientation structure are formed simultaneously in the same positive extrusion process, and a near-net-shape elongated magnet can be directly obtained. The magnetization axes of the principal phase grains in the regions corresponding to different sides of the permanent magnet obtained by the present invention are preferentially arranged along the normal direction of the corresponding side, thereby forming a circumferential multi-axis orientation structure corresponding to each planar side, and the distribution is uniform, such as... Figure 4 As shown in (b), this structure can greatly improve the magnetic properties of permanent magnets. Figure 4 In the radially oriented magnetic core with a circular cross-section shown in (a), the easy magnetization axis of the principal phase grains changes continuously with the circumferential position, and the orientation direction is radial. Even if it is cut into a square, the orientation around its perimeter remains unchanged, still radial, resulting in a significant decrease in its magnetic properties in all four directions. Therefore, the near-net-shape elongated magnet of the present invention does not simply change the shape of the magnet, but rather adjusts the grain arrangement and orientation by changing the shape of the magnet, thereby obtaining a permanent magnet with excellent magnetic properties.

[0023] (3) The present invention makes it easy to control the cross-sectional shape and size of the magnet, and can continuously produce long magnets, which is beneficial to improving the output and production efficiency of a single forming. The resulting magnet has a circumferential multi-axis orientation structure and has high magnetic energy product and coercivity. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the cross-sectional structure of the die cavity after the positive extrusion die is fitted into the outer die sleeve in this invention;

[0025] Figure 2 These are vertical and horizontal sectional views of the positive extrusion die with different cross-sectional shapes of the forming zone in this invention;

[0026] Figure 3 yes Figure 2 The diagram shows the cross-sectional grain arrangement and orientation direction of the magnet corresponding to different forming regions. The arrows indicate the orientation direction of the easy magnetization axis of the main phase grains.

[0027] Figure 4 A schematic diagram comparing the grain arrangement and orientation direction of a radially oriented magnet with a circular cross-section (a) and a circumferentially oriented magnet with a square cross-section (b) of the present invention;

[0028] Figure 5 yes Figure 2 The diagram shows the three-dimensional structure of the magnet corresponding to the different forming zones.

[0029] Figure 6 This is a schematic diagram of the hot extrusion process of the magnet by hot pressing combined with positive extrusion according to the present invention;

[0030] Figure 7 This is a physical image of the positively extruded elongated magnet prepared in Example 1 of the present invention;

[0031] Figure 8 The XRD patterns of the positively extruded square cross-section magnet prepared in Example 1 of this invention were obtained by testing along the top surface and two adjacent side surfaces respectively;

[0032] Figure 9 These are microstructure and grain orientation morphology images of different regions of the positively extruded magnet prepared in Example 1 of this invention;

[0033] Figure 10 These are the room temperature hysteresis loops measured along the x-axis, y-axis, and z-axis directions of the positive extrusion magnet prepared in Example 1 of this invention.

[0034] Figure 11 This describes the preparation results and magnetic properties of the positively extruded plate magnet in Embodiment 2 of the present invention, wherein... Figure 11 (a) shows the overall morphology of the die and magnet after positive extrusion. Figure 11 (b) shows the morphology of the obtained plate magnet. Figure 11 (c) shows the room temperature hysteresis loops of the plate magnet measured along the thickness direction and the plate surface direction, respectively.

[0035] Figure reference numerals: 1-Outer mold sleeve; 2-Upper punch; 3-Lower punch; 4-Vertical sectional view of the positive extrusion die; 5-Horizontal sectional view of the positive extrusion die; 6-Schematic diagram of the grain arrangement and orientation direction of the magnet cross-section; 7-Schematic diagram of the three-dimensional structure of the magnet; 8-Transition zone; 9-Extrusion zone; 10-Forming zone; 11-Magnetic powder to be processed; 12-Densed magnet blank; 13-Anisotropic permanent magnet blank. Detailed Implementation

[0036] The present invention will now be described in detail with reference to the accompanying drawings.

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0038] Example 1

[0039] A method for fabricating a circumferentially multiaxially oriented anisotropic permanent magnet includes the following steps:

[0040] 1) Sieve the RE-Fe-B rapid quenching magnetic powder and set it aside for later use.

[0041] 2) Graphite paper is laid on the inner wall of the outer mold sleeve, the upper punch and the lower punch, and other parts that may come into contact with magnetic powder, to lubricate and prevent the magnetic powder from sticking to the mold during the high temperature process.

[0042] 3) The RE-Fe-B rapid quenching magnetic powder obtained in step 1) is loaded into the outer mold sleeve, cold-pressed and compacted at a pressure of 150 MPa, and hot-pressed at 600~900 ℃ to form a fully density isotropic magnet precursor. After cooling to room temperature, it is demolded and taken out.

[0043] In step 3), during the hot pressing process, the upper punch moves downward under pressure to hot press the magnetic powder into a full-density isotropic RE-Fe-B magnet blank.

[0044] 4) Apply MoS2 to the inner wall of the positive extrusion die, the upper punch, and the parts where the positive extrusion die and the outer die sleeve may come into contact, to lubricate and prevent the magnet blank from sticking to the die during the high-temperature process.

[0045] 5) Place the positive extrusion die into the outer mold sleeve, and place the full-density isotropic magnet blank obtained in step 3) into the upper half of the outer mold sleeve, placing it above the positive extrusion die.

[0046] 6) Introduce protective gas such as argon or evacuate to 10°C. -3 Pa, after heating to 600~900 ℃, the upper punch moves downward, causing the magnet blank to flow downward in the cylindrical extrusion cavity formed between the upper punch, the outer die sleeve and the positive extrusion die, and undergo rheological orientation in the transition zone, forming a rare earth elongated permanent magnet with circumferential multiaxial orientation in the forming zone.

[0047] The mold cavity includes, in sequence, an extrusion zone, a transition zone, and a forming zone. Figure 1This is a schematic diagram of the structure of the positive extrusion die after the outer mold sleeve is fitted. The extrusion zone has a circular cross-section; the top cross-section of the transition zone is circular, and the bottom cross-section has the same shape as the forming zone. The cross-sectional area of ​​the transition zone decreases from the extrusion zone to the forming zone. Taking a square cross-section as an example, the vertical and horizontal sectional views of the positive extrusion die are as follows. Figure 2 As shown in (a), the grain arrangement and orientation of the cross-section of the obtained magnet are as follows: Figure 3 As shown in (a), the three-dimensional structure is as follows: Figure 5 As shown in (a); the mold structure, cross-sectional grain arrangement and orientation direction, and three-dimensional structure corresponding to other cross-sectional forms are respectively as shown in (a). Figure 2 (b)~ Figure 2 (f) Figure 3 (b)~ Figure 3 (f) and Figure 5 (b)~ Figure 5 As shown in (f).

[0048] When the cross-section of the forming zone is tile-shaped, the tile-shaped cross-section is formed by the oppositely arranged outer arc edge and inner arc edge, as well as the two side edges connecting the outer arc edge and inner arc edge. The transition zone gradually transitions from a circular cross-section to the tile-shaped cross-section, so that the magnet blank is simultaneously subjected to axial extrusion force and constraint force applied by the inner and outer arc-shaped mold walls and the side mold walls during the process of entering the forming zone, ultimately forming a long strip magnet with a tile-shaped cross-section.

[0049] In a preferred embodiment of the present invention, the longitudinal sectional view of the transition zone has a trapezoidal outline.

[0050] In a preferred embodiment of the present invention, an upper punch that can move along the axial direction of the mold cavity is provided in the mold cavity. The upper punch has a cylindrical structure and its diameter is adapted to the diameter of the extrusion area of ​​the mold cavity. The upper punch extrudes from the extrusion area of ​​the mold cavity toward the forming area.

[0051] In a preferred embodiment of the present invention, the end of the upper punch contacting the magnet is a cylindrical structure, and the extrusion endpoint of the upper punch is the starting position of the transition zone in the mold cavity, that is, the position with the largest cross-sectional area of ​​the transition zone.

[0052] Example 2

[0053] A method for preparing a high-performance tetrahedral oriented rare-earth elongated permanent magnet includes the following steps:

[0054] Step 1) Amorphous magnetic powder with uniform composition, consisting of Nd 28.47 wt.%, Pr 0.56 wt.%, B 0.97 wt.%, Co 3.63 wt.%, Al 0.13 wt.%, Nb 0.56 wt.%, Zr 0.44 wt.%, with the balance being Fe, was prepared by rapid quenching.

[0055] Step 2) Place the uniform magnetic powder described in Step 1) into a hot press mold, heat it to 700 °C in an environment with a vacuum pressure of less than 0.01 Pa, keep the lower punch stationary, and move the upper punch downward under pressure. Load the pressure to 150 MPa, hold the pressure for 30 min, and densify the magnetic powder to obtain a fully density isotropic hot press magnet.

[0056] Step 3) The densified hot-pressed magnet from Step 2) is subjected to positive extrusion to obtain an anisotropic permanent magnet blank; the cavity of the positive extrusion die includes an extrusion zone, a transition zone and a forming zone in sequence along the axial direction; Figure 1 This is a schematic cross-sectional view of the die cavity after the extrusion die is fitted with the outer die sleeve. In the die structure used in this embodiment, the cross-section of the extrusion zone is circular, with a cross-sectional area of ​​314 mm. 2 The cross-section of the forming area is square, with a cross-sectional area of ​​25 mm. 2 The axial length of the transition zone is 13 mm, and the longitudinal sectional view of the transition zone is trapezoidal, with an angle α of 30° between the inclined surface of the trapezoid and the axis of the mold cavity. The densified hot-pressed magnet is placed in a positive extrusion mold and heated to 750°C at a heating rate of 10 °C / min, followed by a heating rate of 1 × 10⁻⁶. -2 s -1 The compression strain rate is used for positive extrusion, and after positive extrusion is completed, the material is cooled in the furnace to obtain a long strip magnet with dimensions of 5 mm × 5 mm × 12 mm.

[0057] During the forward extrusion process, the hot-pressed magnet billet flows along the extrusion direction under upward thrust, passing through the extrusion zone, transition zone, and forming zone sequentially. At the extrusion temperature, the rare-earth-rich grain boundary phase in the magnet softens or forms a liquid phase, lubricating the grain boundaries and providing conditions for atomic diffusion, grain boundary sliding, and grain rotation. When the magnet billet moves from the extrusion zone (circular cross-section) into the transition zone (gradually transitioning from circular to square), it is subjected to axial extrusion force as well as transverse compressive stress applied along the normal direction by different sidewalls of the forward extrusion die. Under these stresses and thermorheological effects, the originally approximately equiaxed and disordered principal phase grains undergo anisotropic growth and rotation, gradually forming lath-like grains. The easy magnetization axes of the principal phase grains in the regions corresponding to the sides of the square cross-section are preferentially aligned along directions perpendicular to the corresponding sides, thus forming a four-sided orientation structure. The magnet then enters the forming zone, where the cross-sectional shape and size remain unchanged. The forming zone further constrains the magnet's shape and material flow, which helps maintain the already formed orientation structure and obtain a long, strip-shaped magnet with stable cross-sectional dimensions. Therefore, the target cross-section and circumferential multi-axis orientation structure of the magnet can be formed simultaneously in the same positive extrusion process, without the need to first prepare a solid cylindrical magnet and then obtain a square cross-section magnet by cutting the outer contour.

[0058] The microstructure and magnetic properties of the heat-deformed magnets prepared in the above embodiments were tested. The resulting elongated magnet specimen is shown in the image. Figure 7 As shown. According to Figure 9 The left side shows the observation of different regions of the magnet. After grinding and polishing the sample surface, scanning electron microscopy was used for observation. The microstructure and orientation morphology of different regions are shown below. Figure 9 As shown. After positive extrusion, a rectangular magnet with a square cross-section of 5 mm × 5 mm × 12 mm was obtained. For magnetic performance testing, a cuboid test specimen with dimensions of 2 mm × 1.5 mm × 2 mm was cut from the outer bottom region of this rectangular magnet. The x-axis and y-axis were perpendicular to two adjacent sides of the square cross-section, respectively, and the z-axis was parallel to the positive extrusion direction. After magnetization, the room temperature hysteresis loop was measured along the x-axis, y-axis, and z-axis, respectively. The results are shown below. Figure 10 As shown, the hysteresis loops measured along the x-axis and y-axis are basically coincident, both exhibiting high remanent magnetic polarization and good squareness. The remanence of the hysteresis loop measured along the z-axis is significantly reduced, showing a significant difference from the x-axis and y-axis directions, indicating that the magnet has a distinct dominant orientation in two mutually perpendicular directions perpendicular to the extrusion axis. Specific magnetic properties are shown in Table 1.

[0059] Table 1. Magnetic properties of positively compressed magnets in different directions

[0060]

[0061] Table 1 shows that the remanent magnetic polarization (J) of the magnet along the x-axis and y-axis directions is...r ) and maximum energy product (BH) max The results were significantly higher than those along the z-axis, and the magnetic properties along the x-axis and y-axis were similar. This indicates that the positively extruded magnet exhibits significant magnetic anisotropy in two mutually perpendicular directions to the extrusion axis, suggesting that the obtained square-section magnet forms a circumferentially oriented four-sided structure along its four sides. Furthermore, to further verify the grain orientation of the square-section positively extruded magnet, a 5 mm × 5 mm × 5 mm cube sample was cut from the top of the obtained magnetic strip, and XRD tests were performed on the top surface and two adjacent sides of the sample. The test results are as follows: Figure 8 As shown in the figure, the positively compressed magnet is mainly composed of RE2Fe. 14 The main phase is B. In the XRD patterns of the two sides, the diffraction intensity of orientation characteristic peaks such as (004), (105), (006), and (008) is significantly enhanced, while the non-orientation characteristic peaks such as (214) and (115) are relatively weak, indicating that the easy magnetization c-axis of the main phase grains has a significant preferred orientation along the normal direction of the corresponding side. In contrast, in the top surface test, the diffraction peaks such as (311), (410), (411), and (330) are more obvious, while the above-mentioned orientation characteristic peaks are weaker, indicating that the c-axis orientation along the length of the magnet is less pronounced. Therefore, the above XRD and magnetic property results show that the regions corresponding to the four sides of the square cross-section magnet all have a preferred orientation along the normal direction of the corresponding side. At the same time, Figure 9 The microstructure shows that after positive extrusion, lath-shaped main phase grains with preferred arrangement characteristics are formed in the side region of the square magnet. Combined with the symmetry of the square cross section and the constraint of the material by the four side walls of the mold, it can be determined that the magnet forms preferred orientations along the normal of the corresponding side in the regions corresponding to the four sides.

[0062] Therefore, by combining XRD tests in different directions, magnetic property tests along the x, y, and z axes, microstructural observations, and the symmetrical constraint relationship of the four sidewalls of the square mold, it can be determined that the obtained square cross-section magnet forms preferred orientations along the corresponding side normals in the regions corresponding to the four sides. Based on this, the following diagrams are drawn: Figure 4 (b) shows a schematic diagram of the four-sided oriented grain arrangement.

[0063] To investigate the magnetic property distribution of the magnet along the extrusion direction, samples were cut from the top, upper middle, middle, lower middle and bottom of the outer side of the magnetic strip, and the room temperature magnetic properties were tested along the x-axis direction perpendicular to the outer surface of the magnet. The results are shown in Table 2.

[0064] Table 2 Magnetic properties of different regions of the positive extrusion magnet along the extrusion direction

[0065]

[0066] Table 2 shows that the intrinsic coercivity (H) of the outer side of the magnetic strip from top to bottom is... cj The remanent magnetic polarization (J) increased from 883 kA / m to 1073 kA / m. r The maximum energy product (BH) increased from 1.09 T to 1.13 T. max From 203.7 kJ / m 3 Increased to 220.7 kJ / m 3 As the extrusion process proceeds, the actual deformation of the magnet gradually increases, and the lath-shaped main phase grains are more fully arranged, thus gradually improving the magnetic properties. The maximum magnetic energy product in the stable extrusion regions of the middle, lower middle, and bottom of the magnet is 211.1~220.7 kJ / m³, with a non-uniformity of approximately 4.35%, which is less than 5%.

[0067] Example 3

[0068] To further verify the applicability of the positive extrusion method of the present invention to magnets with different cross-sectional shapes, a plate-shaped anisotropic permanent magnet was prepared using a positive extrusion die with a rectangular cross-section of the forming area. The cross-section of the extrusion area of ​​the positive extrusion die is a circle with a diameter of 20 mm, and the cross-sectional dimensions of the forming area are 10 mm × 2.5 mm, with a cross-sectional area of ​​25 mm². Except for the cross-sectional shape and dimensions of the forming area of ​​the positive extrusion die, the raw materials and hot pressing and positive extrusion process conditions are the same as in Example 1. After hot pressing densification and positive extrusion, a plate-shaped anisotropic permanent magnet was obtained. The overall morphology of the die and the magnet after positive extrusion is as follows. Figure 11 As shown in (a), the morphology of the obtained plate magnet is as follows: Figure 11 As shown in (b).

[0069] Magnetic performance test samples were cut from the main body of the obtained plate-shaped magnet. Magnetization and room temperature magnetic performance tests were performed along the thickness direction and the plate surface direction of the magnet, respectively. The resulting hysteresis loop is shown below. Figure 11 As shown in (c). The thickness direction is the direction in which the magnet is subjected to the main thermal deformation during the positive extrusion process, and the plate surface direction is the test direction within the large plane of the plate magnet.

[0070] Depend on Figure 11 (c) It can be seen that the hysteresis loop measured along the thickness direction has a high remanent magnetic polarization and good squareness, while the remanent magnetic polarization and squareness measured along the plate surface direction are significantly lower, indicating that the thickness direction is the main easy magnetization direction of the obtained positively extruded plate magnet, and the magnet has obvious magnetic anisotropy. The specific magnetic properties measured along the thickness direction are shown in Table 3.

[0071] Table 3. Room temperature magnetic properties of positively extruded plate magnets along the thickness direction

[0072]

[0073] Table 3 shows that the coercivity (H) of the obtained positively compressed plate magnet along the thickness direction is... cj ), remanent magnetic polarization (J) r ), Maximum magnetic energy product ((BH) max ) and saturation magnetic polarization (J s The remanence ratios are 900 kA / m, 1.02 T, 170.3 kJ / m³, and 1.32 T, respectively, and the remanence ratio J r / J s The value of 0.77 indicates that the obtained magnet has good hard magnetic properties and obvious grain orientation. The above results show that, by adjusting the cross-sectional shape of the forming area of ​​the positive extrusion die, the preparation method of the present invention can not only be used to prepare elongated magnets with square cross-sections, but also to prepare plate-shaped anisotropic magnets with rectangular cross-sections, and has good applicability to magnets with different polygonal cross-sections.

Claims

1. A method for preparing a circumferentially multiaxially oriented anisotropic permanent magnet, characterized in that, Includes the following steps: (1) Amorphous or nanocrystalline magnetic powder is hot-pressed into a magnet blank; (2) The magnet blank is extruded through an extrusion die to cause thermorheology in the hot-pressed magnet, thereby obtaining a permanent magnet with a multi-axis vertical orientation along the circumferential direction; the cavity of the extrusion die includes an extrusion zone, a transition zone and a forming zone connected in sequence; the cross-section of the extrusion zone is circular; the cross-section of the transition zone gradually transitions from a circle to the same target cross-section as the forming zone, and the cross-sectional area of ​​the transition zone gradually decreases along the extrusion direction; the cross-section of the forming zone is a polygon, a hollow polygon, or a closed shape enclosed by straight line segments and curved lines.

2. The preparation method according to claim 1, characterized in that, The cross-section of the forming area is an equilateral triangle, square, rectangle, regular pentagon, regular hexagon, or tile shape; the tile shape is formed by an outer arc edge and an inner arc edge arranged opposite each other, as well as two side edges connecting the outer arc edge and the inner arc edge.

3. The preparation method according to claim 1, characterized in that, The longitudinal section of the transition zone is trapezoidal, and the angle α between the inclined surface of the trapezoid and the central axis of the mold cavity is 10° to 80°.

4. The preparation method according to claim 3, characterized in that, The included angle α is 25° to 55°.

5. The preparation method according to claim 1, 2, 3, or 4, characterized in that, The hot pressing temperature in step (1) is 600~900 ℃, the holding time is 15~40 min, and the hot pressing pressure is 100~200 MPa; the forward extrusion in step (2) is carried out at 600~900 ℃.

6. The preparation method according to claim 5, characterized in that, The hot pressing in step (1) is carried out under vacuum conditions, with a vacuum pressure of less than 0.01 Pa.

7. The preparation method according to claim 1, 2, 3, or 4, characterized in that, The magnetic powder is RE-Fe-B rapid quenching magnetic powder, where RE is a rare earth element; the relative density of the magnet blank is more than 95% of the theoretical density of the magnetic powder material.

8. The preparation method according to claim 1, 2, 3, or 4, characterized in that, The mold used for hot pressing in step (1) and the inner wall of the extrusion mold in step (2) are both lined with graphite paper or coated with molybdenum disulfide and / or boron nitride as release agents.

9. The preparation method according to claim 1, 2, 3, or 4, characterized in that, The compression strain rate in step (2) of the positive extrusion process is 1×10⁻⁶. -2 s -1 .

10. A circumferentially multiaxially oriented anisotropic permanent magnet prepared by the method according to any one of claims 1 to 9.

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