A permanent magnet tube and its preparation method

Abstract

This invention relates to a permanent magnet tube and its preparation method, belonging to the field of permanent magnet material preparation technology. It addresses the problems of high residual stress, uneven grain boundary phase distribution, and grain boundary defects in permanent magnet tubes prepared by existing methods, which lead to reduced magnetic properties. The preparation method of the permanent magnet tube includes cold pressing, hot pressing, thermorheology, and pressure-assisted heat treatment; wherein the thermorheology temperature is 700~1000℃; the pressure-assisted heat treatment includes the following steps: cooling the permanent magnet tube blank obtained by thermorheology to 640~660℃, applying a first auxiliary pressure, and holding at that temperature; further cooling to 590~610℃, applying a second auxiliary pressure, and holding at that temperature; further cooling to 490~510℃, applying a third auxiliary pressure, and holding at that temperature to obtain the permanent magnet tube. The permanent magnet tube prepared by the method of this invention exhibits excellent magnetic properties.

Description

Technical Field

[0001] This invention relates to the field of permanent magnet material preparation technology, and in particular to a permanent magnet tube and its preparation method. Background Technology

[0002] Nd-Fe-B based permanent magnets are widely used in electric motors, generators, sensors, and other fields due to their excellent magnetic properties. Radially oriented permanent magnet tubes (rings) are a type of rare-earth permanent magnet with a special orientation structure, where the easy magnetization direction (c-axis) of their magnetocrystalline anisotropy is radially distributed along the ring's radial direction. These magnets are widely used in high-end fields such as brushless DC motors, servo motors for high-end CNC machine tools, intelligent robots, new energy vehicles, wind turbines, aerospace navigation and control systems, magnetic sensors, and linear actuators.

[0003] Currently, methods for manufacturing permanent magnet tubes (rings) generally include sintering, bonding, and hot pressing / thermodynamics. Sintered magnetic rings are produced using powder metallurgy, which requires consideration of shrinkage during sintering, thus increasing manufacturing costs in later processing. Furthermore, their size is limited by the magnetic field, making it difficult to achieve certain specific sizes. Bonded permanent magnet tubes have magnetic properties limited by their density, resulting in lower energy products that cannot meet the requirements of high-performance equipment.

[0004] Existing hot-pressing / thermorheology processes for fabricating permanent magnet tubes present the following technical problems. First, due to intense plastic deformation during thermorheology, significant residual stress is generated within the magnet, and microcracks and defects easily form at grain boundaries. These residual stresses and defects significantly reduce the magnet's coercivity and maximum energy product. Second, the uneven distribution of grain boundary phases during thermorheology, with some grain boundaries lacking sufficient rare-earth-rich phases, results in poor demagnetization coupling between grain boundaries and the main phase grains, affecting the overall magnetic properties of the magnet. Third, the traditional direct cooling method after thermorheology cannot effectively repair the grain boundary defects generated during the process, limiting further improvements in magnet performance. Summary of the Invention

[0005] In view of the above analysis, the present invention aims to provide a permanent magnet tube and its preparation method to solve one of the following technical problems: the permanent magnet tube prepared by the existing preparation method has large residual stress, uneven distribution of grain boundary phase, and grain boundary defects, which leads to a decrease in magnetic properties.

[0006] On one hand, the present invention provides a method for preparing a permanent magnet tube, the method comprising cold pressing, hot pressing, thermorheology, and pressure-assisted heat treatment; wherein, the thermorheology temperature is 700~1000℃; the pressure-assisted heat treatment comprises the following steps: cooling the permanent magnet tube blank obtained by thermorheology to 640~660℃, applying a first auxiliary pressure, and holding at the temperature; continuing to cool to 590~610℃, applying a second auxiliary pressure, and holding at the temperature; continuing to cool to 490~510℃, applying a third auxiliary pressure, and holding at the temperature, thereby obtaining a permanent magnet tube.

[0007] Furthermore, the fabrication method of the permanent magnet tube employs a thermorheological permanent magnet tube fabrication device. This device includes a mold, which mainly comprises an outer mold, a heating device, a sample chamber, a positive pressure head, a negative pressure head, and an auxiliary pressure rod. The auxiliary pressure rod is sleeved on the outer wall of the negative pressure head, and the outer mold is sleeved on the outer walls of the positive pressure head and the auxiliary pressure rod. A sample chamber is provided between the positive pressure head, the negative pressure head, and the auxiliary pressure rod. The heating device is nested in the side wall of the outer mold. The positive pressure head, the negative pressure head, and the auxiliary pressure rod are coaxially arranged. Each of the positive pressure head, the negative pressure head, and the auxiliary pressure rod is connected to an independent hydraulic drive mechanism.

[0008] Furthermore, the fabrication method of the permanent magnet tube includes the following steps: Step 1: Apply a release agent to the inner wall of the sample cavity, then put the amorphous or nanocrystalline RE-TM-B magnetic powder into the sample cavity, evacuate to below 0.01MPa, and repeatedly purge with argon gas to finally maintain a vacuum state or argon atmosphere below 0.01MPa. Step 2: Apply pressure to the magnetic powder in both directions using a positive pressure head and a negative pressure head, with an auxiliary pressure rod moving accordingly, to cold press the magnetic powder into a cold-pressed blank; Step 3: The cold-pressed billet is heated to the hot-pressing temperature by the heating device. The positive and negative pressure heads apply pressure to the magnetic powder in both directions. The auxiliary pressure rod moves accordingly to hot-press the cold-pressed billet into a dense hot-pressed billet. Step 4: The auxiliary pressure rod moves away from the hot-pressed blank, and at the same time, a release agent is sprayed onto the inner wall of the cavity formed between the hot-pressed blank and the auxiliary pressure rod. Step 5: Continue heating the hot-pressed billet to the thermorheological temperature, and apply pressure to the hot-pressed billet with the counter-pressure head to extrude the hot-pressed billet into a permanent magnet tube blank, thus completing the thermorheological process; wherein, the thermorheological temperature is 700~1000℃. Step 6: Perform pressure-assisted heat treatment. S601. Cool the permanent magnet tube blank to 640~660℃, apply the first auxiliary pressure, and keep it warm; S602, continue cooling to 590~610℃, apply second auxiliary pressure, and maintain the temperature; S603, continue cooling to 490~510℃, apply a third auxiliary pressure, keep warm, and obtain a permanent magnet tube; wherein the first auxiliary pressure < the second auxiliary pressure < the third auxiliary pressure.

[0009] Furthermore, in step 1, RE is one or more of Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Lu, and TM is one or more of Fe, Co, Ni, Cu, Mn, Cr, Al, Ga, Ti, Zr, and Nb; in the RE-TM-B magnetic powder, the mass percentage of RE is 25-35%, the mass percentage of TM is 64-72%, the mass percentage of B is 0.7-1.3%, and the remainder is trace elements.

[0010] Furthermore, in step 6, the cooling rates v1 in S601, v2 in S602, and v3 in S603 satisfy the following relationship: v1>v2>v3.

[0011] Furthermore, v1 is 80~90℃ / s, v2 is 50~60℃ / s, and v3 is 20~30℃ / s.

[0012] Furthermore, the first auxiliary pressure is 10~15MPa, the second auxiliary pressure is 20~25MPa, and the third auxiliary pressure is 30~35MPa.

[0013] Furthermore, in step 5, the rheological rate is controlled to be 0.5~8% / s.

[0014] Furthermore, in step 3, the pressure of hot pressing is controlled to be 100~600MPa.

[0015] The present invention also provides a permanent magnet tube, which is prepared by the above-described preparation method.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: a) The preparation method of the present invention employs a pressure-assisted heat treatment method after thermorheology, which can utilize the thermo-mechanical coupling effect to promote the uniform flow and wetting of the grain boundary phase of the permanent magnet tube, eliminate residual stress and repair grain boundary defects, and avoid magnetic performance attenuation caused by residual stress.

[0017] (b) In the preparation method of the present invention, during the pressure-assisted heat treatment process, the rare earth-rich grain boundary phase flows uniformly under the action of temperature and pressure, fills the microcracks and defects at the grain boundary, repairs the grain boundary damage generated during the thermorheological process, and improves the demagnetizing coupling effect of the grain boundary on the main phase grain.

[0018] c) In the preparation method of the present invention, the step cooling-heat holding heat treatment method, combined with precise pressure control, promotes the multiple uniform wetting and redistribution of the grain boundary phase, which significantly increases the number of continuous thin-layer grain boundaries and improves the continuity of the grain boundary phase, thereby enhancing the demagnetizing coupling effect of the grain boundaries and improving the magnetic performance of the permanent magnet tube.

[0019] d) In the preparation method of the present invention, cold pressing, hot pressing, thermorheology and pressure-assisted heat treatment are completed sequentially and continuously in the same mold, avoiding the coarsening of the structure and loss of performance caused by multiple heating-cooling cycles, and improving process efficiency and product consistency.

[0020] e) The permanent magnet tube of the present invention has excellent magnetic properties, such as remanence B. r For example, 12.8~15kGs or higher; coercivity H cj It is above 10.2 kOe, for example, 10.2~19.2 kOe; the maximum magnetic energy product is above 35 MGOe, for example, 35 MGOe~55 MGOe.

[0021] Other features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0022] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same content.

[0023] Figure 1 This is a schematic diagram of the structure of the thermo-pressed thermorheological permanent magnet tube of the present invention; Figure 2 This is a schematic diagram of the cold pressing process in the preparation method of the present invention.

[0024] Figure 3 This is a schematic diagram of the hot pressing process in the preparation method of the present invention.

[0025] Figure 4 This is a schematic diagram of the thermal deformation process in the preparation method of the present invention.

[0026] Figure 5 This is a schematic diagram of the pressure-assisted heat treatment process in the preparation method of the present invention.

[0027] Figure label: 1-Outer mold, 2-Heating device, 3-Sample chamber, 4-Positive pressure head, 5-Reverse pressure head, 6-Auxiliary pressure rod. Detailed Implementation

[0028] The preferred embodiments of the present invention are described in detail below. These embodiments are used to illustrate the principles of the present invention and are not intended to limit the scope of the present invention.

[0029] This invention provides a method for preparing a permanent magnet tube, which includes cold pressing, hot pressing, thermorheology, and pressure-assisted heat treatment.

[0030] The preparation method of the present invention employs pressure-assisted heat treatment after thermorheology, which can utilize the thermo-mechanical coupling effect to promote the uniform flow and wetting of the grain boundary phase in the permanent magnet tube, eliminate residual stress and repair grain boundary defects, and avoid magnetic performance attenuation caused by residual stress.

[0031] Specifically, the preparation method of this invention uses a device for hot-pressing thermorheological permanent magnet tubes. The device mainly includes a mold, such as... Figure 1 As shown, the mold has an overall axisymmetric structure. The mold mainly includes an outer mold 1, a heating device 2, a sample cavity 3, a positive pressure head 4, a negative pressure head 5, and an auxiliary pressure rod 6. The auxiliary pressure rod 6 is sleeved on the outer wall of the negative pressure head 5, and the outer mold 1 is sleeved on the outer walls of the positive pressure head 4 and the auxiliary pressure rod 6. The sample cavity 3 is provided between the positive pressure head 4, the negative pressure head 5, and the auxiliary pressure rod 6. The heating device 2 is nested in the side wall of the outer mold 1. The positive pressure head 4, the negative pressure head 5, and the auxiliary pressure rod 6 are coaxially arranged. The positive pressure head 4, the negative pressure head 5, and the auxiliary pressure rod 6 are each connected to an independent hydraulic drive mechanism.

[0032] Specifically, the preparation method of the aforementioned permanent magnet tube includes the following steps: Step 1: Apply a release agent to the inner wall of sample cavity 3, then put amorphous or nanocrystalline RE-TM-B magnetic powder into sample cavity 3, evacuate to below 0.01MPa, repeatedly purge with argon gas, and finally maintain a vacuum state or argon atmosphere below 0.01MPa. Step 2: Apply pressure to the magnetic powder in both directions through the positive pressure head 4 and the negative pressure head 5, and the auxiliary pressure rod 6 moves accordingly to cold press the magnetic powder into a cold-pressed blank; Step 3: The cold-pressed billet is heated to the hot-pressing temperature by the heating device 2. The positive pressure head 4 and the reverse pressure head 5 apply pressure to the magnetic powder in both directions. The auxiliary pressure rod 6 moves accordingly to hot-press the cold-pressed billet into a dense hot-pressed billet. Step 4: The auxiliary pressure rod 6 moves away from the hot-pressed blank, and simultaneously sprays a release agent onto the inner wall of the cavity formed between the hot-pressed blank and the auxiliary pressure rod; Step 5: Continue heating the hot-pressed billet to the thermorheological temperature. The counter-pressure head 5 applies pressure to the hot-pressed billet, extruding it into a permanent magnet tube blank to complete the thermorheological process. The thermorheological temperature is 700~1000℃. Step 6: Perform pressure-assisted heat treatment. S601. Cool the permanent magnet tube blank to 640~660℃, apply the first auxiliary pressure, and keep it warm; S602, continue cooling to 590~610℃, apply second auxiliary pressure, and maintain the temperature; S603, continue cooling to 490~510℃, apply a third auxiliary pressure, keep warm, and obtain a permanent magnet tube; wherein the first auxiliary pressure < the second auxiliary pressure < the third auxiliary pressure.

[0033] Specifically, in step 1 above, the release agent is either molybdenum disulfide or graphite.

[0034] Specifically, in step 1 above, RE is one or more of Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Lu, and TM is one or more of Fe, Co, Ni, Cu, Mn, Cr, Al, Ga, Ti, Zr, and Nb; in RE-TM-B magnetic powder, the mass percentage of RE is 25-35%, the mass percentage of TM is 64-72%, the mass percentage of B is 0.7-1.3%, and the remainder is trace elements.

[0035] Specifically, the composition of TM is as follows: Co: 0~15%, Ni: 0~1%, Cu: 0~1.5%, Mn: 0~1%, Cr: 0~1%, Al: 0~1%, Ga: 0~1%, Ti: 0~1%, Zr: 0~1%, Nb: 0~1%, Fe: 39.5~72%; where the mass percentage of Fe, Co, Ni, Cu, Mn, Cr, Al, Ga, Ti, Zr and Nb refers to the mass percentage of the corresponding element in RE-TM-B magnetic powder.

[0036] Specifically, in step 2 above, considering that excessive cold pressing pressure can lead to mold cracking and internal cracks or delamination in the pressed blank, while insufficient pressure results in insufficient strength of the green blank, making it prone to edge and corner chipping, excessive holding time can reduce production efficiency, and excessive holding time can lead to insufficient pressure transmission and uneven density of the pressed blank, the cold pressing pressure is controlled at 10~50MPa, for example 10MPa, 20MPa, 30MPa, 40MPa, 50MPa; and the holding time is controlled at 20~200s, for example 20s, 50s, 70s, 100s, 120s, 150s, 170s, 200s.

[0037] Specifically, in step 3 above, considering that excessive hot pressing pressure can easily lead to mold damage and flash in the hot-pressed billet, while insufficient pressure cannot achieve full density, affecting the final product performance, excessively high hot pressing temperature and excessively long holding time can easily lead to abnormal grain growth, resulting in a significant reduction in the final coercivity. Conversely, excessively low temperature and excessively short holding time can easily lead to low product density and uneven distribution of grain boundary phases. Therefore, the hot pressing pressure is controlled at 100~600MPa, for example, 100MPa, 200MPa, 300MPa, 400MPa, 500MPa, 600MPa; the hot pressing temperature is controlled at 550~650℃, for example, 550℃, 570℃, 600℃, 620℃, 650℃; and the holding time is controlled at 20~200s, for example, 20s, 50s, 70s, 100s, 120s, 150s, 170s, 200s.

[0038] Specifically, in step 3 above, considering that an excessively high heating rate can easily lead to uneven temperature field in the product, affecting consistency, and can also cause excessive internal thermal stress, resulting in cracking, while an excessively low heating rate can easily lead to grain coarsening and reduce production efficiency, the heating rate is controlled at 10~50℃ / s, for example, 10℃ / s, 20℃ / s, 30℃ / s, 40℃ / s, and 50℃ / s.

[0039] Specifically, in step 4 above, the release agent is either molybdenum disulfide or graphite.

[0040] Specifically, in step 5 above, considering that excessive thermorheological pressure can lead to difficulties in material flow and increase the risk of mold damage, while insufficient pressure can result in inadequate grain orientation, excessively high temperatures can easily induce abnormal grain growth, forming coarse equiaxed crystals and affecting grain orientation. Insufficiently low temperatures can prevent the effective liquefaction of rare-earth-rich grain boundary phases, preventing grains from achieving preferred orientation and directional growth, thus affecting grain orientation. Therefore, the thermorheological pressure is controlled at 50~600MPa, for example, 50 MPa, 100MPa, 120MPa, 300MPa, 400MPa, 500MPa, 600MPa; and the thermorheological temperature is controlled at 700~1000℃, for example, 700℃, 750℃, 800℃, 850℃, 900℃, 950℃, 1000℃.

[0041] Specifically, in step 5 above, considering that an excessively high heating rate would lead to an uneven temperature field in the product, while an excessively low rate would easily cause abnormal grain growth, the heating rate is controlled to be 10~50℃ / s, for example, 10℃ / s, 20℃ / s, 30℃ / s, 40℃ / s, or 50℃ / s.

[0042] Specifically, in step 5 above, considering that an excessively high rheological rate can easily lead to material cracking or stress concentration, while an excessively low rate results in insufficient grain orientation driving force and low efficiency, and that excessive deformation can easily damage the texture or generate cracks, while insufficient deformation leads to inadequate orientation and difficulty in obtaining high remanence, the rheological rate is controlled at 0.5~8% / s, for example, 0.5% / s, 1% / s, 1.5% / s, 2% / s, 3% / s, 4% / s, 5% / s, 6% / s, 7% / s, and 8% / s; and the deformation amount is controlled at 50~80%, for example, 50%, 60%, 70%, and 80%.

[0043] Specifically, in step 6 above, by cooling in stages, sufficient kinetic time windows are provided for impurity phase decomposition, stress relief and grain boundary phase optimization, so that the microstructure control targets corresponding to each temperature range can be completed sequentially and orderly.

[0044] Specifically, in step 6 above, the cooling rates v1 in S601, v2 in S602, and v3 in S603 conform to the following relationship: v1>v2>v3. This is because at high temperatures, atomic diffusion is rapid and the driving force for phase transformation is large, allowing for a faster cooling rate to suppress grain growth and shorten the cycle. As the temperature decreases, atomic migration ability declines, necessitating a slower cooling rate to provide sufficient time for uniform spread of the grain boundary phase and interface densification, while simultaneously avoiding thermal mismatch stress and the generation of microcracks.

[0045] Specifically, v1 is 80~90℃ / s, for example 80℃ / s, 85℃ / s, 90℃ / s; v2 is 50~60℃ / s, for example 50℃ / s, 55℃ / s, 60℃ / s; v3 is 20~30℃ / s, for example 20℃ / s, 25℃ / s, 30℃ / s.

[0046] Specifically, considering that excessively high holding temperatures in S601 lead to abnormal grain growth and deterioration of magnetic properties, while excessively low temperatures result in insufficient phase decomposition kinetics, and excessively high first auxiliary pressures cause grain boundary phases to be squeezed out of the grain boundary region, disrupting grain boundary continuity and uniformity; while insufficient pressures cannot provide enough driving force to promote atomic diffusion, the holding temperature in S601 is controlled at 640~660℃, for example, 640℃, 650℃, or 660℃; and the first auxiliary pressure is controlled at 10~15MPa, for example, 10MPa, 11MPa, 12MPa, 13MPa, 14MPa, or 15MPa.

[0047] Specifically, considering that excessively high holding temperatures in S602 would cause the decomposed rare-earth-rich phase to flow excessively, accumulating and thickening at grain boundaries, thus weakening the demagnetizing coupling effect; while excessively low temperatures would result in excessively high viscosity and poor fluidity of the grain boundary phase, failing to fully wet the grain boundaries for uniform spreading. Excessive second auxiliary pressure would squeeze the liquid grain boundary phase out of the grain boundary region, causing local segregation; while insufficient pressure would result in insufficient driving force, making it difficult to drive the rare-earth-rich phase to migrate and repair grain boundary defects. Therefore, the holding temperature in S602 is controlled at 590~610℃, for example, 590℃, 600℃, 610℃; and the second auxiliary pressure is controlled at 20~25MPa, for example, 20MPa, 21MPa, 22MPa, 23MPa, 24MPa, 25MPa.

[0048] Specifically, considering that excessively high holding temperatures in S603 can lead to the re-aggregation and coarsening of grain boundary phases, disrupting the already formed thin-layer continuous grain boundary structure; while excessively low temperatures result in insufficient atomic diffusion, making it impossible to effectively eliminate interfacial microcracks and thermal mismatch stress. Excessive third auxiliary pressure can cause stress concentration at grain boundaries, inducing microcrack propagation; while insufficient pressure cannot promote the densification of the grain boundary phase at the nanoscale, leading to inadequate interfacial repair. Therefore, the holding temperature in S603 is controlled at 490~510℃, for example, 490℃, 500℃, 510℃; and the third auxiliary pressure is controlled at 30~35MPa, for example, 30MPa, 31MPa, 32MPa, 33MPa, 34MPa, 35MPa.

[0049] Specifically, in S601, the heat preservation time is 3~10 minutes; in S602, the heat preservation time is 13~20 minutes; and in S603, the heat preservation time is 20~30 minutes.

[0050] Specifically, in the preparation method of the present invention, cold pressing, hot pressing, thermorheology and pressure-assisted heat treatment are completed sequentially and continuously in the same mold, and there is no cooling process between the cold pressing, hot pressing and thermorheology processes.

[0051] Specifically, the present invention also provides a permanent magnet tube, which is prepared by the above-described preparation method.

[0052] Specifically, the permanent magnet tube of the present invention has a uniform distribution of grain boundary phases, resulting in excellent magnetic properties, such as remanence B. r For example, 12.8 kGs or higher, such as 12.8~15 kGs; coercivity H cj It is above 10.2 kOe, for example, 10.2~19.2 kOe; the maximum magnetic energy product is above 35 MGOe, for example, 35 MGOe~55 MGOe.

[0053] The preparation method of the present invention employs pressure-assisted heat treatment after thermorheology, which can utilize the thermo-mechanical coupling effect to promote the uniform flow and wetting of the grain boundary phase in the permanent magnet tube, eliminate residual stress and repair grain boundary defects, and avoid magnetic performance attenuation caused by residual stress.

[0054] In the preparation method of the present invention, during the pressure-assisted heat treatment process, the rare earth-rich grain boundary phase undergoes uniform flow under the action of temperature and pressure, filling the microcracks and defects at the grain boundary, repairing the grain boundary damage generated during the thermorheological process, and improving the demagnetizing coupling effect of the grain boundary on the main phase grain.

[0055] In the preparation method of the present invention, the step cooling-heat holding heat treatment method, combined with precise pressure control, promotes the multiple uniform wetting and redistribution of the grain boundary phase, which significantly increases the number of continuous thin-layer grain boundaries and improves the continuity of the grain boundary phase, thereby enhancing the demagnetizing coupling effect of the grain boundaries and improving the magnetic performance of the permanent magnet tube.

[0056] In the preparation method of the present invention, cold pressing, hot pressing, thermorheology and pressure-assisted heat treatment are completed sequentially and continuously in the same mold, avoiding the coarsening of the structure and loss of performance caused by multiple heating-cooling cycles, and improving process efficiency and product consistency.

[0057] The advantages of precise control in the preparation method of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0058] Example 1 This embodiment provides a method for manufacturing permanent magnet tubes. The method employs a hot-pressing thermorheological permanent magnet tube apparatus, which mainly comprises a press and a mold. The press primarily consists of a machine body and a power unit. Figure 1 As shown, the mold has an overall axisymmetric structure. The mold mainly includes an outer mold 1, a heating device 2, a sample cavity 3, a positive pressure head 4, a negative pressure head 5, and an auxiliary pressure rod 6. The auxiliary pressure rod 6 is sleeved on the outer wall of the negative pressure head 5, and the outer mold 1 is sleeved on the outer walls of the positive pressure head 4 and the auxiliary pressure rod 6. The sample cavity 3 is provided between the positive pressure head 4, the negative pressure head 5, and the auxiliary pressure rod 6. The heating device 2 is nested in the side wall of the outer mold 1. The positive pressure head 4, the negative pressure head 5, and the auxiliary pressure rod 6 are coaxially arranged. The positive pressure head 4, the negative pressure head 5, and the auxiliary pressure rod 6 are each connected to an independent hydraulic drive mechanism.

[0059] The preparation method in this embodiment is as follows: Step 1: Apply a release agent to the inner wall of sample cavity 3, then place the nanocrystalline magnetic powder into sample cavity 3, evacuate to 0.005 MPa, repeatedly purge with argon gas 3 times, and finally fill with argon gas to 0.008 MPa; the mass percentage of each element in the nanocrystalline magnetic powder is: Nd: 32%, Fe: 62%, Co: 4%, Ga: 0.5%, B: 0.9%, with the remainder being trace elements; the average particle size of the nanocrystalline magnetic powder is 100 μm; molybdenum disulfide is used as the release agent; Step 2: Apply a pressure of 30 MPa to the magnetic powder in both directions through the positive pressure head 4 and the negative pressure head 5, with the auxiliary pressure rod 6 moving accordingly, and hold the pressure for 60 s to cold press the magnetic powder into a cold-pressed blank. Step 3: The cold-pressed blank is heated to 600°C by heating device 2 at a heating rate of 30°C / s. The positive pressure head 4 and the reverse pressure head 5 apply a bidirectional pressure of 300MPa to the magnetic powder. The auxiliary pressure rod moves accordingly and the pressure is maintained for 60 seconds to hot press the cold-pressed blank into a dense hot-pressed blank. Step 4: Move the auxiliary pressure rod 6 15 mm away from the hot-pressed blank, and simultaneously spray the release agent onto the inner wall of the cavity formed between the hot-pressed blank and the auxiliary pressure rod; Step 5: Continue heating the hot-pressed billet to 850°C at a heating rate of 30°C / s. The back pressure head 5 applies a pressure of 400MPa to the hot-pressed billet at a rheological rate of 1% / s, with a deformation of 70%, extruding the hot-pressed billet into a permanent magnet tube blank, thus completing the thermorheological process. Step 6: Perform pressure-assisted heat treatment. S601. Cool the permanent magnet tube blank to 650℃ at a cooling rate of 80℃ / s, apply an auxiliary pressure of 13 MPa, and hold for 5 minutes. S602, continue cooling at a rate of 50℃ / s to 600℃, apply an auxiliary pressure of 23 MPa, and hold for 20 min; S603, continue cooling at a rate of 20℃ / s to 500℃, apply an auxiliary pressure of 33 MPa, and hold for 30 min to obtain a permanent magnet tube.

[0060] Figure 2 This is a schematic diagram of the cold pressing process. Figure 3 This is a schematic diagram of the hot pressing process. Figure 4 This is a schematic diagram of a thermorheological process. Figure 5 This is a schematic diagram of a pressure-assisted heat treatment process.

[0061] The permanent magnet tube prepared in this embodiment has a continuous grain boundary phase distribution, and the magnetic properties of the permanent magnet tube are as follows: Remanence B r =14.5kGs, coercivity H cj =13.5kOe, maximum magnetic energy product (BH) max =50.3 MGOe.

[0062] Example 2 This embodiment provides a method for preparing a permanent magnet tube. The preparation method in this embodiment is generally the same as that in Embodiment 1, except that: The mass percentage of each element in the nanocrystalline magnetic powder in step 1 is as follows: Nd: 23%, Pr: 8%, Fe: 63%, Co: 2.5%, Ga: 0.4%, B: 0.9%, and the remainder are trace elements; In step 2, apply a pressure of 20 MPa and hold for 150 seconds; In step 3, the temperature is increased to 650°C at a heating rate of 50°C / s, and a pressure of 150 MPa is applied and held for 20 seconds. In step 5, the temperature is increased to 900℃ at a heating rate of 50℃ / s, the rheological rate is 5% / s, a pressure of 300 MPa is applied, and the deformation is 60%. In step 6: S601, cool down to 660℃ at a cooling rate of 90℃ / s, with an auxiliary pressure of 15 MPa, and hold for 3 min; S602, cool down to 590℃ at a cooling rate of 60℃ / s, with an auxiliary pressure of 20 MPa, and hold for 15 min; S603, cooled to 490℃ at a cooling rate of 30℃ / s, with an auxiliary pressure of 35 MPa, and held at that pressure for 30 min.

[0063] The permanent magnet tube prepared in this embodiment has a continuous grain boundary phase distribution, and the magnetic properties of the permanent magnet tube are as follows: B r =14.0kGs, H cj =19.2kOe, maximum magnetic energy product (BH)max=45.2MGOe.

[0064] Example 3 This embodiment provides a method for preparing a permanent magnet tube. The preparation method in this embodiment is generally the same as that in Embodiment 1, except that: The mass percentage of each element in the nanocrystalline magnetic powder in step 1 is as follows: Nd: 27%, Ce: 5%, Fe: 63%, Co: 2%, Ga: 0.3%, B: 1%, with the remainder being trace elements; In step 2, maintain the pressure for 150 seconds; In step 3, the temperature is increased to 570°C at a heating rate of 10°C / s, and a pressure of 550 MPa is applied and held for 150 s. In step 5, the temperature is increased to 750°C at a heating rate of 10°C / s, the rheological rate is 1% / s, a pressure of 600 MPa is applied, and the deformation is 80%. In step 6: S601, cool down to 640℃ at a cooling rate of 85℃ / s, with an auxiliary pressure of 10 MPa, and hold for 8 min; S602, cool down to 610℃ at a cooling rate of 55℃ / s, with an auxiliary pressure of 25 MPa, and hold for 18 min; S603, cooled to 510℃ at a cooling rate of 25℃ / s, with an auxiliary pressure of 30 MPa, and held at the pressure for 25 min.

[0065] The permanent magnet tube prepared in this embodiment has a continuous grain boundary phase distribution, and the magnetic properties of the permanent magnet tube are as follows: B r =13.0kGs, H cj =10.2kOe, maximum magnetic energy product (BH)max=38MGOe.

[0066] Extensive research was conducted during the study process, and some suboptimal solutions are presented here as comparative examples.

[0067] Comparative Example 1 This comparative example provides a method for preparing a permanent magnet tube. The preparation method of this comparative example is basically the same as that of Example 1, except that: After step 5, step 6 is not performed; instead, the mixture is directly cooled to room temperature. The remaining parameters are the same as in Example 1.

[0068] The magnetic properties of the permanent magnet tube in this comparative example: B r =14.3kGs, H cj =12.2kOe, maximum magnetic energy product (BH) max =48.8MGOe.

[0069] Compared with Example 1, it can be seen that the remanence, coercivity and maximum energy product of the permanent magnet tube without pressure-assisted heat treatment are reduced.

[0070] Comparative Example 2 This comparative example provides a method for preparing a permanent magnet tube. The preparation method of this comparative example is basically the same as that of Example 1, except that: In step 6, no auxiliary pressure is applied in S601, S602, and S603, and the other parameters are the same as in Example 1.

[0071] The magnetic properties of the permanent magnet tube in this comparative example: B r =14.4 kGs, H cj =12.8 kOe, maximum magnetic energy product (BH) max =49.1 MGOe.

[0072] Compared with Example 1, it can be seen that without the application of auxiliary pressure, simple heat treatment cannot effectively promote the flow and wetting of grain boundary phases, and the effects of residual stress elimination and grain boundary defect repair are limited. The magnetic properties are significantly lower than those of Example 1.

[0073] Comparative Example 3 This comparative example provides a method for preparing a permanent magnet tube. The preparation method of this comparative example is basically the same as that of Example 1, except that: In step 6, S601 and S602 are omitted; the permanent magnet tube blank is directly cooled to 500°C at a cooling rate of 80°C / s, an auxiliary pressure of 33 MPa is applied, and the temperature is maintained for 30 min; the remaining parameters are the same as in Example 1.

[0074] The magnetic properties of the permanent magnet tube in this comparative example: B r =14.3 kGs, H cj =13.2 kOe, maximum energy product (BH) max =49.2 MGOe.

[0075] Compared with Example 1, it can be seen that without step-down cooling, it is impossible to simultaneously achieve impurity phase decomposition, stress relief, and grain boundary phase optimization. The magnetic properties are lower than those of Example 1.

[0076] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a permanent magnet tube, characterized in that, The method for preparing the permanent magnet tube includes cold pressing, hot pressing, thermorheology, and pressure-assisted heat treatment; wherein the thermorheology temperature is 700~1000℃. The pressure-assisted heat treatment includes the following steps: cooling the permanent magnet tube blank obtained by thermorheology to 640~660℃, applying a first auxiliary pressure, and holding it at that temperature; continuing to cool to 590~610℃, applying a second auxiliary pressure, and holding it at that temperature; continuing to cool to 490~510℃, applying a third auxiliary pressure, and holding it at that temperature, to obtain the permanent magnet tube.

2. The method for preparing a permanent magnet tube according to claim 1, characterized in that, The method for preparing the permanent magnet tube employs a thermorheological permanent magnet tube fabrication device. This device includes a mold, which primarily comprises an outer mold, a heating device, a sample chamber, a positive pressure head, a negative pressure head, and an auxiliary pressure rod. The auxiliary pressure rod is sleeved on the outer wall of the negative pressure head, and the outer mold is sleeved on the outer walls of the positive pressure head and the auxiliary pressure rod. A sample chamber is provided between the positive pressure head, the negative pressure head, and the auxiliary pressure rod. The heating device is nested within the side wall of the outer mold. The positive pressure head, the negative pressure head, and the auxiliary pressure rod are coaxially arranged. Each of the positive pressure head, the negative pressure head, and the auxiliary pressure rod is connected to an independent hydraulic drive mechanism.

3. The method for preparing a permanent magnet tube according to claim 2, characterized in that, The method for preparing the permanent magnet tube includes the following steps: Step 1: Apply a release agent to the inner wall of the sample cavity, then put the amorphous or nanocrystalline RE-TM-B magnetic powder into the sample cavity, evacuate to below 0.01MPa, and repeatedly purge with argon gas to finally maintain a vacuum state or argon atmosphere below 0.01MPa. Step 2: Apply pressure to the magnetic powder in both directions using a positive pressure head and a negative pressure head, with an auxiliary pressure rod moving accordingly, to cold press the magnetic powder into a cold-pressed blank; Step 3: The cold-pressed billet is heated to the hot-pressing temperature by the heating device. The positive and negative pressure heads apply pressure to the magnetic powder in both directions. The auxiliary pressure rod moves accordingly to hot-press the cold-pressed billet into a dense hot-pressed billet. Step 4: The auxiliary pressure rod moves away from the hot-pressed blank, and at the same time, a release agent is sprayed onto the inner wall of the cavity formed between the hot-pressed blank and the auxiliary pressure rod. Step 5: Continue heating the hot-pressed billet to the thermorheological temperature, and apply pressure to the hot-pressed billet with the counter-pressure head to extrude the hot-pressed billet into a permanent magnet tube blank, thus completing the thermorheological process; wherein, the thermorheological temperature is 700~1000℃. Step 6: Perform pressure-assisted heat treatment. S601. Cool the permanent magnet tube blank to 640~660℃, apply the first auxiliary pressure, and keep it warm; S602, continue cooling to 590~610℃, apply the second auxiliary pressure, and maintain the temperature; S603, continue cooling to 490~510℃, apply a third auxiliary pressure, keep warm, and obtain a permanent magnet tube; wherein the first auxiliary pressure < the second auxiliary pressure < the third auxiliary pressure.

4. The method for preparing a permanent magnet tube according to claim 3, characterized in that, In step 1, RE is one or more of Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Lu, and TM is one or more of Fe, Co, Ni, Cu, Mn, Cr, Al, Ga, Ti, Zr, and Nb; in the RE-TM-B magnetic powder, the mass percentage of RE is 25-35%, the mass percentage of TM is 64-72%, the mass percentage of B is 0.7-1.3%, and the remainder is trace elements.

5. The method for preparing a permanent magnet tube according to claim 3, characterized in that, In step 6, the cooling rates v1 in S601, v2 in S602, and v3 in S603 conform to the following relationship: v1>v2>v3.

6. The method for preparing a permanent magnet tube according to claim 5, characterized in that, The v1 is 80~90℃ / s, the v2 is 50~60℃ / s, and the v3 is 20~30℃ / s.

7. The method for preparing a permanent magnet tube according to claim 3, characterized in that, The first auxiliary pressure is 10~15MPa, the second auxiliary pressure is 20~25MPa, and the third auxiliary pressure is 30~35MPa.

8. The method for preparing a permanent magnet tube according to claim 3, characterized in that, In step 5, the rheological rate is controlled to be 0.5~8% / s.

9. The method for preparing a permanent magnet tube according to claim 3, characterized in that, In step 3, the pressure of hot pressing is controlled to be 100~600MPa.

10. A permanent magnet tube, characterized in that, The permanent magnet tube is prepared by the preparation method described in any one of claims 1 to 9.

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