Method for preparing high-performance neodymium-iron-boron magnet in one-stop mode

Abstract

The invention belongs to the technical field of magnetic materials, and particularly relates to a method for preparing a high-performance neodymium-iron-boron magnet in a one-stop mode. The method for preparing the high-performance neodymium-iron-boron magnet in the one-stop mode comprises the following steps that rapid-quenching neodymium-iron-boron magnetic powder is subjected to cold pressing to obtain a cold-pressed blank, the surface of the cold-pressed blank is coated with a release agent, then preheating is conducted, and then the cold-pressed blank is put into a heat flow deformation mold to be subjected to pre-pressing, heat flow deformation and forming; the heat flow change mold sequentially comprises a pre-pressing section, a heat flow change section and a forming section from top to bottom. According to the method, pre-pressing, heat flow change and forming are carried out in a one-stop mode in a heat flow change mold, densification-grain flow change, magnetic domain preferred orientation-orientation uniformization and finished product forming of the magnet are completed, the traditional hot pressing process is avoided, the production procedure is greatly simplified, the production cost is reduced, and the production efficiency is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of magnetic materials, and in particular relates to a one-stop method for preparing high-performance NdFeB magnets. Background Art

[0002] With the rapid development of emerging fields such as new energy vehicles and intelligent manufacturing, more stringent requirements have been put forward for the magnetic properties, temperature resistance, corrosion resistance and other properties of rare earth permanent magnet materials. Compared with the traditional sintered NdFeB preparation technology, hot pressing rheological technology is a near-final forming technology. Through the hot rheological forming of isotropic magnetic materials pressed into billets in a high temperature and high pressure environment, the density, hardness and toughness of the magnetic materials are improved, and the material yield is high; anisotropy can be obtained without magnetic field orientation during the preparation process, and it has significant advantages in the manufacture of special-shaped magnets such as radiation-oriented ring magnets and thin-sheet magnets. At the same time, the forming temperature of thermorheological magnets is low, and they have a nanocrystalline structure (the grain size is only 1 / 10 of that of sintered magnets). Their temperature resistance and corrosion resistance are significantly better than those of sintered magnets, and it is expected to reduce the dependence of high-temperature-resistant magnets on expensive and scarce heavy rare earth elements (dysprosium and terbium).

[0003] The forming of existing hot-pressed rheological NdFeB magnets generally includes three steps: cold stamping (pressing the rapidly quenched powder into a blank), hot stamping (600-800℃, increasing the density of the blank to close to the theoretical density of the magnet), and hot extrusion (700-900℃, implementing hot rheology to obtain high orientation consistency, so that the grains inside the magnet flow along the direction of the extrusion stress and form a texture structure with consistent magnetic domain orientation). However, in order to refine the grains and improve the performance of the magnet, an ultra-rapid cooling process is usually used to manufacture quick-quenching powder. Due to the excessively rapid cooling speed, the surface of the quick-quenching powder is an amorphous phase and lacks the constraint of the grain boundary phase. When it is heated in the mold for a long time and undergoes two heating processes (hot stamping and thermorheology), the quick-quenching powder is prone to excessive growth (coarsening) of grains from the outside to the inside, which will cause the rheological ability and magnetic properties of the magnet to decrease. At the same time, hot forming molds (hot stamping and thermorheology) at high temperatures are prone to damage such as wear and thermal adhesion, resulting in shutdown. When there are many processes, it seriously affects production efficiency, is not conducive to large-scale automated production, and increases production costs. All of these limit the promotion and application of this technology. Summary of the invention

[0004] The purpose of the present invention is to provide a one-stop method for preparing high-performance NdFeB magnets in response to the above-mentioned technical problems, which integrates the traditional hot pressing and thermorheological processes into one process, and can complete the densification of the magnet - grain rheology, magnetic domain preferential orientation - orientation uniformity and finished product shaping in one stop, thereby reducing the abnormal growth of grains inside the magnet caused by repeated heating of hot pressing and thermorheology, resulting in a decrease in magnetic properties.

[0005] The one-stop method for preparing high-performance NdFeB magnets in the technical solution of the present invention comprises the following steps:

[0006] The rapidly quenched NdFeB magnetic powder is cold pressed to obtain a cold pressed blank, a release agent is coated on the surface of the cold pressed blank, the blank is preheated, and then placed in a thermorheological mold for pre-pressing, thermorheology and forming.

[0007] Pre-pressing, thermorheology and molding are carried out in a one-stop manner in the thermorheological mold, completing the densification of the magnet - grain rheology, magnetic domain preferential orientation - orientation uniformity and finished product molding, avoiding the traditional hot pressing process, greatly simplifying the production process, reducing production costs and improving production efficiency.

[0008] Preferably, the particle size of the rapidly quenched NdFeB magnetic powder is 100-500 μm.

[0009] Furthermore, the cold pressing pressure is 200-600 MPa, and the time is 30-60 s.

[0010] Furthermore, the density of the cold pressed blank is 3.6-4.0 g / cm 3 .

[0011] Furthermore, the preparation of the release agent refers to patent CN117253688B.

[0012] Furthermore, during preheating, the heating rate is 2 to 10°C / min, the preheating temperature is 400 to 600°C, and the preheating time is 60 to 180s.

[0013] Furthermore, the thermorheological mold is composed of a pre-pressing section, a thermorheological section and a molding section from top to bottom.

[0014] Furthermore, the pre-pressing section and the forming section are both rectangular structures, and the long side of the pre-pressing section is perpendicular to the long side of the forming section. The pre-pressing section and the forming section are connected by a gradually deforming thermorheological section, and the length direction of the thermorheological section gradually narrows and the width direction gradually widens.

[0015] In the thermo-rheological section, the length direction of the mold becomes narrower, applying compressive stress to the blank, and the width direction becomes wider, applying tensile stress to the blank, so that the grain boundary phase in the magnet softens at high temperature, the grains deform, and grow from spherical to disc-shaped (the thickness direction is the easy magnetic axis direction of the grains). Under the action of compression and tensile stress of the inner wall of the mold, the grains flow and deflect along the softened grain boundary phase, and the thickness direction is perpendicular to the compressive stress direction, so that the magnet orientation (i.e., the easy magnetic axis direction of the grains) is transformed from the disordered orientation before hot-pressing rheology to pointing in the direction of compressive stress.

[0016] Furthermore, the height of the pre-pressing section is greater than or equal to 50% of the height of the cold-pressed blank. The higher the height of the pre-pressing section, the more efficient the densification process is, and the higher the density of the cold-pressed blank is.

[0017] Furthermore, the dimensional change rate of the thermorheological segment in the length direction and the width direction is ≥1:0.5.

[0018] Furthermore, the cross-sectional area of ​​the forming section is 50-80% of the cross-sectional area of ​​the pre-pressing section, further improving the compactness of the NdFeB magnet. If the cross-sectional area of ​​the forming section is too small, the compression during forming will be relatively large, which will easily increase the probability of the blank sticking to the mold and cause performance degradation.

[0019] Furthermore, the temperature during thermorheology and molding is 500 to 1000°C, preferably 700 to 900°C.

[0020] Furthermore, the extrusion stress of the pre-pressing section is 300-1000 MPa, and the extrusion speed is 0.5-3 mm / s. The extrusion stress is preferably 400-800 MPa, and the extrusion speed is preferably 1.0-2.0 mm / s.

[0021] Furthermore, the density of the NdFeB blank after the pre-pressing stage is 6.0-6.5 g / cm 3 , greater than or equal to 80% of the theoretical density, the structure is relatively dense, which can reduce the probability of cracks or voids in subsequent processing.

[0022] Furthermore, the extrusion stress during thermorheology and molding is 200-1000 MPa, and the extrusion speed is 2-10 mm / s. The extrusion stress is preferably 300-800 MPa, and the extrusion speed is preferably 3-5 mm / s.

[0023] Furthermore, the forming section in the above-mentioned thermo-rheological mold can also be tile-shaped or moon-shaped; finally, a tile-shaped or moon-shaped NdFeB magnet is obtained.

[0024] The present invention also provides a high-performance NdFeB magnet, which is prepared by the one-stop method for preparing a high-performance NdFeB magnet.

[0025] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0026] (1) The present invention optimizes the mold and forming process, integrates the hot stamping and thermorheological processes into one process, and can complete the densification of the magnet - grain rheology, magnetic domain preferential orientation - orientation uniformity and finished product shaping in one stop, thereby achieving stable production of high-performance hot-pressed NdFeB magnets;

[0027] (2) The height of the pre-pressing section in the thermo-rheological die is greater than or equal to 50% of the height of the cold-pressed blank, which helps to efficiently complete the densification process and improve the density of the cold-pressed blank;

[0028] (3) The cross-sectional area of ​​the forming section is 50-80% of the cross-sectional area of ​​the pre-pressing section, which can improve the compactness of the NdFeB magnet while ensuring a suitable compression ratio;

[0029] (4) The present invention does not require secondary heating, which can reduce the abnormal growth of grains inside the magnet caused by repeated heating due to hot pressing and thermorheology, resulting in a decrease in magnetic properties;

[0030] (5) The method of hot pressing NdFeB magnets of the present invention reduces a process step, shortens the process flow, reduces production costs, and improves production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a front cross-sectional view of the thermorheological mold of the present invention;

[0032] In the figure: 1-pre-pressing section, 2-thermal rheological section, 3-forming section;

[0033] Figure 2 is a side cross-sectional view of the thermorheological mold of the present invention;

[0034] In the figure: 1-pre-pressing section, 2-thermal rheological section, 3-forming section;

[0035] Figure 3 A top view of the thermorheological mold of the present invention;

[0036] Figure 4 A schematic diagram of the process of preparing high-performance NdFeB magnets in one stop according to the present invention;

[0037] Figure 5 The theoretical mechanism diagram of the one-stop preparation of high-performance NdFeB magnets of the present invention;

[0038] Figure 6 This is a SEM image of the high performance NdFeB magnet obtained in Example 1;

[0039] Figure 7 This is a SEM image of the high performance NdFeB magnet obtained in Comparative Example 1;

[0040] Figure 8 This is the SEM image of the high performance NdFeB magnet obtained in Comparative Example 1. DETAILED DESCRIPTION

[0041] The technical scheme of the present invention is further described below by specific examples and accompanying drawings. It should be understood that the specific examples described herein are only used to help understand the present invention and are not intended for specific limitations of the present invention. The accompanying drawings used herein are only for better illustrating the disclosed content of the present invention and do not have a limiting effect on the scope of protection. If not otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used raw materials in the art, and the methods used in the embodiments are all conventional methods in the art.

[0042] The rapidly quenched NdFeB magnetic powder used in the following examples and comparative examples is from MQ Company of Canada. The release agent refers to Example 1 in patent CN117253688B. The first release agent is obtained by dissolving 88 parts of boron nitride, 10 parts of glass powder, and 2 parts of titanium dioxide in ethanol. The second release agent is obtained by mixing graphite powder in ethanol at a mass ratio of 92wt%.

[0043] The thermorheological mold in the embodiment of the present invention is as follows Figure 1-3 As shown, from top to bottom, there are pre-pressing section 1, thermorheological section 2 and forming section 3. Both pre-pressing section 1 and forming section 3 are rectangular structures, and the long side of pre-pressing section 1 is perpendicular to the long side of forming section 3. Pre-pressing section 1 and forming section 3 are connected by gradually deforming thermorheological section 2. The length direction of thermorheological section 2 gradually narrows and the width direction gradually widens. The height of pre-pressing section 1 is 50% of the height of cold-pressed blank. The dimensional change rate of thermorheological section 2 in length direction and width direction is 1:0.5, so that the compressive stress and tensile stress in the thermorheological process are 1:0.5 (as shown in FIG. Figure 4 As shown in FIG. 1 , the cross-sectional area of ​​the forming section 3 is 60% of the cross-sectional area of ​​the pre-pressing section, so that the cross-sectional area of ​​the cold-pressed blank is compressed to 60% of the original after passing through the thermorheological die. Figure 5 As shown, after the NdFeB blank is extruded into the thermorheological die, it is closely arranged after extrusion in the pre-pressing section, and then the grains are deflected in the thermorheological section, and preferentially grow in the compression and tension directions. Finally, it reaches consistent orientation and anisotropy after passing through the forming section, and finally exhibits excellent magnetic properties.

[0044] Example 1

[0045] The one-stop method for preparing high-performance NdFeB magnets in this embodiment includes the following steps:

[0046] (1) The rapidly quenched NdFeB magnetic powder was cold pressed for 45 seconds at a pressure of 400 MPa to obtain a cold pressed blank. The density of the cold pressed blank was 3.7 g / cm 3 ;

[0047] (2) firstly, a first mold release agent with a thickness of 0.1 mm was coated on the surface of the cold pressed blank, and then dried at 60°C, and then a second mold release agent with a thickness of 0.2 mm was coated, and then dried at 60°C, and then the temperature was raised to 500°C at a rate of 5°C / min and preheated for 70s;

[0048] (3) The preheated blank is placed in a thermorheological mold and heated to 750°C in a microwave oven for thermorheological and forming. The extrusion stress in the pre-pressing section is 450 MPa, the extrusion speed is 1.5 mm / s, and the density of the NdFeB blank after the pre-pressing section is 6.1 g / cm 3 The extrusion stress of the thermorheological section is 400MPa and the extrusion speed is 3.5mm / s. The microstructure of the obtained NdFeB magnet is as follows: Figure 6 As shown, the magnet receives reasonable heat, the growth rate of the nano-grains at the boundary of the original quenched powder is appropriate, and the nano-grains inside the quenched powder are also uniform in size and have good orientation consistency.

[0049] Example 2

[0050] The one-stop method for preparing high-performance NdFeB magnets in this embodiment includes the following steps:

[0051] (1) The rapidly quenched NdFeB magnetic powder was cold pressed for 45 seconds at a pressure of 450 MPa to obtain a cold pressed blank. The density of the cold pressed blank was 3.8 g / cm 3 ;

[0052] (2) firstly, a first mold release agent with a thickness of 0.1 mm was coated on the surface of the cold pressed blank, and then dried at 60°C, and then a second mold release agent with a thickness of 0.2 mm was coated, and then dried at 60°C, and then the temperature was raised to 500°C at a rate of 5°C / min and preheated for 80s;

[0053] (3) The preheated blank is placed in a thermorheological mold and heated to 800°C in a microwave oven for thermorheological and forming. The extrusion stress in the pre-pressing section is 500 MPa and the extrusion speed is 1.5 mm / s. The density of the NdFeB blank after the pre-pressing section is 6.2 g / cm 3 The extrusion stress of the thermorheological section is 450MPa and the extrusion speed is 4.0mm / s.

[0054] Example 3

[0055] The one-stop method for preparing high-performance NdFeB magnets in this embodiment includes the following steps:

[0056] (1) The rapidly quenched NdFeB magnetic powder was cold pressed for 45 seconds at a pressure of 400 MPa to obtain a cold pressed blank. The density of the cold pressed blank was 3.8 g / cm 3 ;

[0057] (2) firstly, a first mold release agent with a thickness of 0.1 mm was coated on the surface of the cold pressed blank, and then dried at 60°C, and then a second mold release agent with a thickness of 0.2 mm was coated, and then dried at 60°C, and then the temperature was raised to 550°C at a rate of 5°C / min and preheated for 70s;

[0058] (3) The preheated blank is placed in a thermorheological mold and heated to 850°C in a microwave oven for thermorheological and forming. The extrusion stress in the pre-pressing section is 550 MPa, the extrusion speed is 1.0 mm / s, and the density of the NdFeB blank after the pre-pressing section is 6.2 g / cm 3 The extrusion stress of the thermorheological section is 400MPa and the extrusion speed is 4.5mm / s.

[0059] Comparative Example 1

[0060] The method for preparing a neodymium iron boron magnet in this comparative example comprises the following steps:

[0061] (1) The rapidly quenched NdFeB magnetic powder was cold pressed for 45 seconds at a pressure of 400 MPa to obtain a cold pressed blank. The density of the cold pressed blank was 3.7 g / cm 3 ;

[0062] (2) firstly, a first mold release agent with a thickness of 0.1 mm was coated on the surface of the cold pressed blank, and then dried at 60°C, and then a second mold release agent with a thickness of 0.2 mm was coated, and then dried at 60°C, and then the temperature was raised to 520°C at a rate of 5°C / min and preheated for 70s;

[0063] (3) The preheated cold-pressed blank is then placed in a molding device for hot pressing at a temperature of 650°C for 10 seconds, a heating method of microwave heating, and a hot pressing pressure of 400 MPa;

[0064] (4) Thermoregulation treatment was then performed using microwave heating at a temperature of 800°C for 35 seconds, a pressure of 400 MPa, and a compression ratio of 50%. The microstructure of the resulting NdFeB magnet is shown in FIG. Figure 7 As shown, due to the repeated heating of the magnet through the hot pressing and thermorheological processes, the magnet is subjected to too much heat, and the nano-grains at the boundaries of the original quenching powder grow too fast, resulting in coarsening. The nano-grains inside the quenching powder have poor uniformity in size, some grains coarsen, and the orientation consistency decreases.

[0065] The magnetic properties of the NdFeB magnets obtained in the above embodiments and comparative examples were tested at 20° C. The test results are shown in Table 1.

[0066] Table 1 Magnetic properties of NdFeB magnets

[0067]

[0068] In Example 1-3, by optimizing the mold and forming process, NdFeB magnets with excellent performance were finally obtained; in Comparative Example 1, two heating processes (hot pressing and thermorheology) were performed, and the grains grew too fast and became coarse, such as Figure 8 As shown, the rheological ability and magnetic properties of the magnet are reduced.

[0069] Finally, it should be noted that the specific embodiments described herein are merely examples of the spirit of the present invention, and are not intended to limit the implementation methods of the present invention. Those skilled in the art of the present invention may make various modifications or supplements to the described embodiments or replace them in similar ways. It is not necessary and impossible to provide all examples of all implementation methods here. However, these obvious changes or modifications derived from the essential spirit of the present invention still fall within the scope of protection of the present invention, and interpreting them as any additional limitation is contrary to the spirit of the present invention.

Claims

1. A one-stop method for preparing high-performance NdFeB magnets, characterized in that: The following steps are involved: The rapidly quenched NdFeB magnetic powder is cold-pressed and pre-pressed to obtain a cold-pressed blank. A release agent is coated on the surface of the cold-pressed blank, the blank is preheated, and then placed in a thermorheological mold for thermorheological flow and forming.

2. The one-stop method for preparing high-performance NdFeB magnets according to claim 1, characterized in that: The cold pressing pressure is 200-600 MPa and the time is 30-60 s.

3. The one-stop method for preparing high-performance NdFeB magnets according to claim 1, characterized in that: The thermorheological mold consists of a pre-pressing section, a thermorheological section and a molding section from top to bottom.

4. The one-stop method for preparing high-performance NdFeB magnets according to claim 3, characterized in that: The pre-pressing section and the forming section are both rectangular structures, and the long side of the pre-pressing section is perpendicular to the long side of the forming section. The pre-pressing section and the forming section are connected by a gradually deforming thermorheological section, and the length direction of the thermorheological section gradually narrows and the width direction gradually widens.

5. The one-stop method for preparing high-performance NdFeB magnets according to claim 3, characterized in that: The height of the pre-pressing section is greater than or equal to 50% of the height of the cold-pressed blank.

6. The one-stop method for preparing high-performance NdFeB magnets according to claim 4, characterized in that: The dimensional change rate of the thermorheological section in the length direction and the width direction is ≥1:0.

5.

7. The one-stop method for preparing high-performance NdFeB magnets according to claim 3 or 4, characterized in that: The cross-sectional area of ​​the forming section is 50 to 80% of the cross-sectional area of ​​the pre-pressing section.

8. The one-stop method for preparing high-performance NdFeB magnets according to claim 3 or 4, characterized in that: The extrusion stress in the pre-compression section is 300-1000 MPa, and the extrusion speed is 0.5-3 mm / s.

9. The one-stop method for preparing high-performance NdFeB magnets according to claim 1, characterized in that: The extrusion stress during thermorheology and molding is 200-1000 MPa, and the extrusion speed is 2-10 mm / s.

10. A high performance NdFeB magnet, characterized in that: The magnet is prepared by the one-stop method for preparing high-performance NdFeB magnets as described in claim 1.

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