A method for reducing the deformation of thin-film sintered NdFeB magnet blanks

Abstract

The invention discloses a method for reducing the deformation of thin-sheet sintered NdFeB magnet blanks. The method comprises the following steps: placing green blanks in a graphite box in a layer, pressing a graphite pressing plate with a thickness of 1.5 cm to 2 cm on the green blanks, placing counterweights with a weight of 1 kg to 1.5 kg at four corners of the graphite pressing plate, covering the graphite box with a graphite box cover, and then transporting the box to a vacuum sintering furnace. Natural cooling is adopted during the sintering process. The method has the advantages that the good thermal conductivity of the graphite pressing plate improves the heating uniformity of a layer of green blanks, thereby improving the shrinkage consistency thereof and reducing the deformation. The four counterweights exert pressure on the graphite pressing plate to prevent the warping deformation of the thin-sheet sintered NdFeB magnet blanks. The natural cooling reduces the internal stress of the thin-sheet sintered NdFeB magnet blanks and avoids the warping deformation of the thin-sheet sintered NdFeB magnet blanks due to rapid shrinkage. While significantly reducing the deformation, the appearance of the product will not be damaged, and the method has high versatility.

Description

Technical Field

[0001] The invention relates to a technology for reducing the deformation of a thin-sheet sintered NdFeB magnet blank, in particular to a method for reducing the deformation of a thin-sheet sintered NdFeB magnet blank. Background Art

[0002] Thin-sheet sintered NdFeB products are characterized by a length or width that is 5-20 times the length of the magnetizing direction. Typically, the length is greater than 100mm, the width is 30-80mm, and the magnetizing dimension is less than 20mm. The finished product weight of a single thin-sheet sintered NdFeB magnet is over 1000g. With the rapid development of the NdFeB industry, the new energy market is increasingly demanding thin-sheet sintered NdFeB magnets, and the requirements for product appearance are becoming increasingly stringent. Forming and sintering are crucial processes for controlling the appearance of thin-sheet sintered NdFeB magnets.

[0003] Domestic NdFeB manufacturers have made extensive improvements and optimizations to the forming presses and processes used to produce thin-sheet sintered NdFeB magnets, significantly enhancing the appearance of these products. The current sintering process primarily involves a slow, step-wise temperature increase during the sintering ramp-up phase and a slow contraction during the sintering degassing phase. However, as is well known, vacuum sintering furnaces primarily transfer heat through radiation and conduction. Consequently, temperature gradients inevitably occur within the furnace at the same time, leading to uneven heating of the sintered NdFeB magnet green compacts. This uneven heating during the sintering ramp-up phase leads to deformation during the sintering degassing and contraction phase. Consequently, one of the primary causes of deformation in sintered NdFeB magnets during the sintering process is the uneven heating of the green compacts, which results in inconsistent deformation. Therefore, the slow step-by-step temperature increase during the sintering heating stage obviously still results in uneven heating, and the product deformation during the sintering process is not effectively controlled. The deformation of the thin-film sintered NdFeB magnet blank is still large, which is not conducive to the mass production of thin-film sintered NdFeB magnets.

[0004] In recent years, researchers have proposed the use of sintering equipment to reduce the deformation of sintered NdFeB magnet blanks during the sintering process, and some equipment has also emerged. For example, the Chinese patent with publication number CN204686013U discloses a "sintering boat for improving sintering deformation and cracking of ring-shaped NdFeB magnets." The patent provides a hollow sintering boat with one end closed. The main body of the sintering boat is made of graphite or molybdenum. Before sintering, a thin layer of quartz sand or / and corundum is laid inside the main body of the sintering boat to reduce friction between the magnet and the sintering boat. Then, the ring magnet is placed in the hollow sintering boat main body, and the cylindrical core rod is placed in the inner hole of the ring magnet, and then sintering is carried out. The above patent can limit the external and internal dimensions of the ring magnet through the hollow sintering boat main body and the cylindrical core rod, thereby effectively improving the deformation caused by uneven heating during the sintering of the magnet green body and reducing the deformation of the ring magnet. However, inserting a cylindrical core rod into the inner hole of an annular NdFeB magnet can easily lead to damage of the inner wall of the annular NdFeB magnet, thereby increasing the number of defective products. In addition, the above method can only be applied to annular magnets and is not very versatile. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for reducing the deformation of thin-sheet sintered NdFeB magnet blanks, which can reduce the deformation without damaging the product appearance and has high versatility.

[0006] The technical solution adopted by the present invention to solve the above technical problems is: a method for reducing the deformation of thin-sheet sintered NdFeB magnet blanks, comprising the following steps:

[0007] (1) The materials are prepared according to the grade of thin-sheet sintered NdFeB magnets, and the casting sheets are prepared by conventional rapid solidification strip spinning process, in which the refining temperature is controlled at 1430℃~1520℃;

[0008] (2) placing the cast sheet prepared in step (1) into a hydrogen crushing furnace for crushing to obtain coarse powder, controlling the hydrogen content to be ≤500 ppm during the crushing process, and the dehydrogenation temperature to be 550°C to 600°C. The coarse powder is placed in a coarse powder cylinder and filled with inert gas for protection;

[0009] (3) adding the coarse powder obtained in step (2) to a jet mill for grinding, and adopting an oxygen supplementation process during the grinding process, and controlling the oxygen supplementation amount at 10-20 ppm to obtain a fine powder with a particle size of 3.6 μm to 4.0 μm, and placing the fine powder with a particle size of 3.6 μm to 4.0 μm into a fine powder cylinder and filling it with inert gas for protection, wherein the grinding particle size is measured using a Malvern 3000 particle size analyzer as a standard;

[0010] (4) placing the fine powder cylinder containing fine powder in step (3) on a three-dimensional mixer stand and stirring for 80 min to 150 min to ensure that the fine powder in the fine powder cylinder is evenly distributed;

[0011] (5) Select a corresponding forming mold according to the shape and size of the thin sintered NdFeB magnet, install the forming mold in a forming press, and fill the forming press with inert gas to make the oxygen content in the forming press ≤500ppm, pour the fine powder stirred in step (4) into the forming mold, first orient it in a magnetic field of ≥1.6T, and then press it to obtain a green body, wherein the forming press pressure is controlled at 1MPa~5MPa, and the green body density is controlled at 3.9g / cm 3 ~4.1g / cm 3 , then the green body is subjected to a film-wrapped vacuum plastic sealing process to obtain a vacuum-packed green body;

[0012] (6) cold isostatic pressing the vacuum-packed green body obtained in step (5) to obtain a cold isostatically pressed green body, wherein the pressure during the cold isostatic pressing process is 150 MPa to 200 MPa, and the green body density is increased to 4.5 g / cm 3 ~4.9g / cm 3 ;

[0013] (7) The cold isostatically pressed green body is placed in a glove box for film stripping. The oxygen content in the glove box is controlled to be below 500ppm. The green body after film stripping is placed in a pre-prepared graphite box. The graphite box has a rectangular accommodating cavity with an upper opening. The green body is placed at the bottom of the rectangular accommodating cavity and arranged in a single layer. After the green body is arranged, a rectangular graphite pressing plate is pressed on a layer of green body to completely cover the layer of green body. The thickness of the graphite pressing plate in the vertical direction is 1.5cm to 2cm. The graphite pressing plate is provided with a plurality of through holes distributed at intervals and passing through the upper and lower parts. Then, a counterweight block weighing 1kg to 1.5kg is placed at each of the four corners of the graphite pressing plate to press the graphite pressing plate. Finally, the upper opening of the graphite box is covered with a graphite box cover and then the box is transported to a vacuum sintering furnace, wherein the graphite box cover does not contact the counterweight block.

[0014] (8) Start the vacuum sintering furnace for sintering. The specific sintering process is as follows: first, at 5.0×10 -2 Pa~1.0×10 -3 The temperature is raised to 950℃~1020℃ under Pa vacuum conditions, kept warm for 4h~6h, then naturally cooled to 300℃~500℃, then raised to 1030℃~1100℃, kept warm for 4h~8h, then naturally cooled to below 800℃, then air-cooled to below 120℃, then subjected to secondary aging treatment and air-cooled to below 60℃ before being taken out of the furnace to obtain a thin-sheet sintered NdFeB magnet blank. During the secondary aging treatment, the first aging treatment temperature is 880℃~920℃, the time is 2h~3h, and the second aging treatment temperature is 480℃~540℃, and the time is 4h~5h.

[0015] Compared with the prior art, the advantage of the present invention is that before sintering, the green body is placed in a graphite box, which has a rectangular accommodating cavity with an upper opening, and the green body is placed at the bottom of the rectangular accommodating cavity in a single layer. After the green body is arranged, a rectangular graphite pressing plate is pressed on a layer of green body to completely cover the layer of green body. The thickness of the graphite pressing plate in the up and down directions is 1.5 cm to 2 cm, and a plurality of through holes distributed at intervals and passing through the graphite pressing plate are provided. Then, a counterweight block weighing 1 kg to 1.5 kg is placed at each of the four corners of the graphite pressing plate to press the graphite pressing plate. Finally, a graphite box cover is used to cover the upper opening of the graphite box and then the box is transported to a vacuum sintering furnace, wherein the graphite box cover does not contact the counterweight block. When the vacuum sintering furnace is opened for sintering, the specific sintering process is as follows: first, at 5.0×10 -2 Pa~1.0×10 -3 Pa vacuum conditions are heated to 950 ℃ ~ 1020 ℃, kept warm for 4h ~ 6h, then naturally cooled to 300 ℃ ~ 500 ℃, then heated to 1030 ℃ ~ 1100 ℃, kept warm for 4h ~ 8h, then naturally cooled to below 800 ℃, then air-cooled to below 120 ℃, then subjected to secondary aging treatment and air-cooled to below 60 ℃ before being taken out of the furnace to obtain a thin-sheet sintered NdFeB magnet blank. During the secondary aging treatment, the first-level aging treatment temperature is 880 ℃ ~ 920 ℃, the time is 2h ~ 3h, the second-level aging treatment temperature is 480 ℃ ~ -540 ℃, the time is 4h ~ 5h. Since the graphite pressing plate is in full contact with a layer of green billet, the graphite pressing plate has good thermal conductivity. During the sintering process, heat conduction is carried out through the graphite pressing plate, which improves the thermal conductivity of a layer of green billet during high-temperature sintering and high-temperature shrinkage. The overall heating uniformity during the sintering process is improved, thereby improving its shrinkage consistency and reducing deformation. The through holes arranged on the graphite pressing plate provide channels for degassing during the green body sintering process, avoiding the graphite pressing plate from affecting the degassing of the green body. In addition, after four counterweights are placed on the graphite pressing plate, the four counterweights generate pressure on the graphite pressing plate, which can prevent the thin-sheet sintered NdFeB magnet blank from warping and deformation during the high-temperature shrinkage stage of the sintering process. At the same time, in the sintering shrinkage stage, the natural cooling method is adopted to reduce the internal stress of the thin-sheet sintered NdFeB magnet blank, which can solve the problem of warping and deformation of the thin-sheet sintered NdFeB magnet blank due to rapid shrinkage. Therefore, the present invention can significantly reduce the deformation without damaging the appearance of the product, and is applicable to sintered NdFeB magnets with flat upper and lower surfaces, and has high versatility. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The present invention is a schematic diagram of the position of the sintered NdFeB magnet blank in the graphite basin in the method for reducing the deformation of the thin-sheet sintered NdFeB magnet blank. DETAILED DESCRIPTION

[0017] The present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

[0018] Embodiment: A method for reducing the deformation of a thin-film sintered NdFeB magnet blank comprises the following steps:

[0019] (1) The materials are prepared according to the grade of thin-sheet sintered NdFeB magnets, and the casting sheets are prepared by conventional rapid-setting strip-spinning process, in which the refining temperature is controlled at 1500°C;

[0020] (2) placing the cast sheet prepared in step (1) into a hydrogen crushing furnace for crushing to obtain coarse powder, controlling the hydrogen content to be ≤500ppm during the crushing process, and the dehydrogenation temperature to be 600°C. The coarse powder is placed in a coarse powder cylinder and filled with inert gas for protection;

[0021] (3) adding the coarse powder obtained in step (2) to a jet mill for grinding, and adopting an oxygen supplementation process during the grinding process, and controlling the oxygen supplementation amount at 10-20 ppm to obtain a fine powder with a particle size of 3.8 μm to 4.0 μm, and placing the fine powder with a particle size of 3.8 μm to 4.0 μm into a fine powder cylinder and filling it with inert gas for protection, wherein the grinding particle size is measured using a Malvern 3000 particle size analyzer as a standard;

[0022] (4) placing the fine powder cylinder containing fine powder in step (3) on a three-dimensional mixer stand and stirring for 80 minutes to ensure that the fine powder in the fine powder cylinder is evenly distributed;

[0023] (5) Select a corresponding forming mold according to the shape and size of the thin sintered NdFeB magnet, install the forming mold in a forming press, and fill the forming press with inert gas to make the oxygen content in the forming press ≤500ppm, pour the fine powder stirred in step (4) into the forming mold, first orient it in a magnetic field ≥1.6T, and then press it to obtain a green body, wherein the forming press pressure is controlled at 4.8MPa and the green body density is controlled at 4.0g / cm 3 , then the green body is subjected to a film-wrapped vacuum plastic sealing process to obtain a vacuum-packed green body;

[0024] (6) cold isostatic pressing the vacuum-packed green body obtained in step (5) to obtain a cold isostatically pressed green body, wherein the pressure during the cold isostatic pressing process is 180 MPa and the green body density is increased to 4.6 g / cm 3 ~4.8g / cm 3 ;

[0025] (7) The cold isostatically pressed green body is placed in a glove box for film stripping. The oxygen content in the glove box is controlled below 500 ppm. The green body 1 after film stripping is placed in a pre-prepared graphite box 2. The graphite box 2 has a rectangular accommodating cavity 3 with an upper opening. The green body 1 is placed at the bottom of the rectangular accommodating cavity 3 and arranged in a single layer. After the green body 1 is arranged, a rectangular graphite pressing plate 4 is pressed on a layer of green body 1 to completely cover the layer of green body 1. The thickness of the graphite pressing plate 4 in the vertical direction is 1.5 cm. The graphite pressing plate 4 is provided with a plurality of through holes 5 that are spaced and pass through the upper and lower parts. Then, a counterweight block 6 with a weight of 1 kg is placed at each of the four corners of the graphite pressing plate 4 to press the graphite pressing plate 4. Finally, a graphite box cover 7 is used to cover the upper end opening of the graphite box 2 and then the graphite box 2 is transported to a vacuum sintering furnace, wherein the graphite box cover 7 does not contact the counterweight block 6.

[0026] (8) Start the vacuum sintering furnace for sintering. The specific sintering process is as follows: first, at 5.0×10 -2 Pa~1.0×10 -3 The temperature was raised to 1020°C under Pa vacuum conditions, kept warm for 5 hours, then naturally cooled to 400°C, then raised to 1090°C, kept warm for 6 hours, then naturally cooled to below 800°C, then air-cooled to below 120°C, and then subjected to secondary aging treatment and air-cooled to below 60°C before being taken out of the furnace to obtain a thin-sheet sintered NdFeB magnet blank. During the secondary aging treatment, the first aging treatment temperature was 900°C for 2.5 hours, and the second aging treatment temperature was 500°C for 5 hours.

[0027] The following experiments are conducted to verify the performance of the method for reducing the deformation of thin-sheet sintered NdFeB magnet blanks of the present invention.

[0028] Taking 50H sintered NdFeB magnets as an example, the deformation of 50H sintered NdFeB magnet blanks currently on the market is 0.6mm to 0.8mm. To ensure the size and appearance of the finished sintered NdFeB magnets after subsequent processing, the current method is to increase the design margin, which increases the weight of the blanks and ultimately leads to increased costs.

[0029] A batch of 50H sintered NdFeB magnets was produced using the method of reducing deformation of thin sintered NdFeB magnet blanks. The design parameters for the 50H sintered NdFeB magnets are shown in Table 1. During production, the graphite platen 4 had a vertical thickness of 2 cm, and the counterweight 6 weighed 1.25 kg.

[0030] Table 1 Product information

[0031]

[0032] 30 products were randomly selected from a batch of 50H sintered NdFeB magnet blanks and numbered from 1 to 30. The deformation of these 30 products was tested. The specific data are shown in Table 2:

[0033] Table 2 Test data

[0034]

[0035]

[0036] Analysis of the data in Table 2 shows that the method for reducing the deformation of thin sintered NdFeB magnet blanks of the present invention can reduce the deformation of sintered NdFeB magnet blanks from 0.6mm to 0.8mm to below 0.55mm, and most of them are below 0.5mm.

[0037] It can be seen from this that the method of reducing the deformation of thin-sheet sintered NdFeB magnet blanks of the present invention can effectively reduce the deformation of thin-sheet sintered NdFeB magnet blanks, so that there is no need to reserve too much design margin, the unit weight of the green blank does not need to be increased too much, and the product material cost is reduced.

Claims

1. A method for reducing the deformation of thin-film sintered NdFeB magnet blanks, characterized in that The following steps are involved: (1) The materials are prepared according to the grade of thin-sheet sintered NdFeB magnets, and the casting sheets are prepared by conventional rapid solidification strip spinning process, in which the refining temperature is controlled at 1430℃~1520℃; (2) placing the cast sheet prepared in step (1) into a hydrogen crushing furnace for crushing to obtain coarse powder, controlling the hydrogen content to be ≤500 ppm during the crushing process, and the dehydrogenation temperature to be 550°C to 600°C. The coarse powder is placed in a coarse powder cylinder and filled with inert gas for protection; (3) adding the coarse powder obtained in step (2) to a jet mill for grinding, and adopting an oxygen supplementation process during the grinding process, and controlling the oxygen supplementation amount at 10-20 ppm to obtain a fine powder with a particle size of 3.6 μm to 4.0 μm, and placing the fine powder with a particle size of 3.6 μm to 4.0 μm into a fine powder cylinder and filling it with inert gas for protection, wherein the grinding particle size is measured using a Malvern 3000 particle size analyzer as a standard; (4) placing the fine powder cylinder containing fine powder in step (3) on a three-dimensional mixer stand and stirring for 80 min to 150 min to ensure that the fine powder in the fine powder cylinder is evenly distributed; (5) Select a corresponding forming mold according to the shape and size of the thin sintered NdFeB magnet, install the forming mold in a forming press, and fill the forming press with inert gas to make the oxygen content in the forming press ≤500ppm, pour the fine powder stirred in step (4) into the forming mold, first orient it in a magnetic field of ≥1.6T, and then press it to obtain a green body, wherein the forming press pressure is controlled at 1MPa~5MPa, and the green body density is controlled at 3.9g / cm 3 ~4.1g / cm 3 , then the green body is subjected to a film-wrapped vacuum plastic sealing process to obtain a vacuum-packed green body; (6) cold isostatic pressing the vacuum-packed green body obtained in step (5) to obtain a cold isostatically pressed green body, wherein the pressure during the cold isostatic pressing process is 150 MPa to 200 MPa, and the green body density is increased to 4.5 g / cm 3 ~4.9g / cm 3 ; (7) The cold isostatically pressed green body is placed in a glove box for film stripping. The oxygen content in the glove box is controlled to be below 500ppm. The green body after film stripping is placed in a pre-prepared graphite box. The graphite box has a rectangular accommodating cavity with an upper opening. The green body is placed at the bottom of the rectangular accommodating cavity and arranged in a single layer. After the green body is arranged, a rectangular graphite pressing plate is pressed on a layer of green body to completely cover the layer of green body. The thickness of the graphite pressing plate in the vertical direction is 1.5cm to 2cm. The graphite pressing plate is provided with a plurality of through holes distributed at intervals and passing through the upper and lower parts. Then, a counterweight block weighing 1kg to 1.5kg is placed at each of the four corners of the graphite pressing plate to press the graphite pressing plate. Finally, the upper opening of the graphite box is covered with a graphite box cover and then the box is transported to a vacuum sintering furnace, wherein the graphite box cover does not contact the counterweight block. (8) Start the vacuum sintering furnace for sintering. The specific sintering process is as follows: first, at 5.0×10 -2 Pa~1.0×10 -3 The temperature is raised to 950℃~1020℃ under Pa vacuum conditions, kept warm for 4h~6h, then naturally cooled to 300℃~500℃, then raised to 1030℃~1100℃, kept warm for 4h~8h, then naturally cooled to below 800℃, then air-cooled to below 120℃, then subjected to secondary aging treatment and air-cooled to below 60℃ before being taken out of the furnace to obtain a thin-sheet sintered NdFeB magnet blank. During the secondary aging treatment, the first aging treatment temperature is 880℃~920℃, the time is 2h~3h, and the second aging treatment temperature is 480℃~540℃, and the time is 4h~5h.

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