Evaluation method and manufacturing method of RTB sintered magnets using the evaluation method

Abstract

To provide an evaluation method capable of more appropriately evaluating the state of cracks in a molded body, and a manufacturing method of an R-T-B system sintered magnet to which the evaluation method is applied.SOLUTION: An evaluation method for evaluating the state of cracks on the basis of the state of bubbles generated from a molded body 2 formed by molding powder by immersing the molded body 2 in a solution L in a container 3, decompressing the inside of the container 3 by 10 kPa or more and less than 50 kPa. It is preferable to reduce the pressure in the container 3 from 20 kPa to 40 kPa, more preferably from 20 kPa to 30 kPa.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an evaluation method and a method for producing an RTB based sintered magnet to which the evaluation method is applied. [Background technology]

[0002] RTB sintered magnets (R is at least one rare earth element and always includes at least one of Nd and Pr. T is at least one transition metal element and always includes Fe) are known as the most high-performance magnets among permanent magnets. RTB sintered magnets contribute to energy conservation and reduced environmental impact by making various motors smaller and lighter. RTB sintered magnets are generally manufactured by powder metallurgy, using raw alloys and going through processes such as crushing, molding, and sintering. During the molding process, cracks can occur in the green body due to fluctuations in the molding conditions, so it is necessary to understand the state of cracks in the green body.

[0003] In Patent Document 1, in order to easily evaluate the state of cracks in a green body of an RTB sintered magnet, a green body of raw material powder is immersed in a solution at atmospheric pressure, and the state of cracks in the green body is evaluated based on the state of bubble generation. Also, the green body is immersed in a paint at atmospheric pressure, and the state of cracks in the green body is evaluated based on the state of penetration of the paint. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-31500 A Summary of the Invention

[0005] However, in the evaluation method of Patent Document 1, if the cracks on the surface of the molded body are such that the entire crack is exposed, gas can easily escape from the cracks and paint can easily penetrate, so that it is possible to appropriately evaluate based on the state of bubble generation and the state of paint penetration. However, when the molded body is removed from the die, the cracks on the surface side of the molded body may be covered with powder when removed, and in such a case where the cracks are covered with powder and are in the vicinity of the surface (including cracks where most of the cracks are in the vicinity of the surface and some of them are connected to the surface), bubbles are difficult to generate and the solution is difficult to penetrate because they are covered with powder even when immersed in a solution, so that it is difficult to appropriately evaluate. An RTB-based sintered magnet obtained from a molded body having cracks in the vicinity of the surface may become defective due to chipping or cracking of the magnet caused by the cracks, and as a result, mass productivity is deteriorated. Therefore, it is necessary to appropriately evaluate not only cracks that expose the entire cracks on the surface of the molded body, but also cracks in the vicinity of the surface. [Problem to be solved by the invention]

[0006] Accordingly, an object of the present disclosure is to provide an evaluation method capable of more appropriately evaluating the state of cracks in a compact, and a method for producing an RTB based sintered magnet to which this evaluation method is applied. [Means for solving the problem]

[0007] In consideration of the above points, the evaluation method of the present disclosure, in exemplary embodiment 1, is an evaluation method in which a molded body made from a powder is immersed in a solution in a container, the pressure inside the container is reduced to 10 kPa or more but less than 50 kPa, and the state of cracks is evaluated based on the state of bubbles generated from the molded body.

[0008] In a second aspect, the evaluation method is as described in the first aspect, wherein the pressure inside the container is reduced to 20 kPa or more and 40 kPa or less.

[0009] Aspect 3 is the evaluation method according to aspect 2, wherein the pressure inside the container is reduced to 20 kPa or more and 30 kPa or less.

[0010] In view of the above, in exemplary aspect 4, the method for producing an RTB based sintered magnet of the present disclosure is a method for producing an RTB based sintered magnet (R is at least one rare earth element and necessarily includes at least one of Nd and Pr; T is at least one transition metal element and necessarily includes Fe), which comprises a first molding step of molding an RTB based alloy powder to obtain a first molded body, an evaluation step of immersing the first molded body in a solution in a container and reducing the pressure inside the container to at least 10 kPa but less than 50 kPa to evaluate the state of cracks based on the state of bubbles generated from the first molded body, a second molding step of feeding back the results of the evaluation step to the first molding step and molding the RTB based alloy powder under molding conditions that do not generate cracks to obtain a second molded body, and a sintering step of heating the second molded body to obtain a sintered body.

[0011] A fifth aspect is a method for producing an RTB based sintered magnet according to the fourth aspect, in which the pressure inside the container is reduced to 20 kPa or more and 40 kPa or less.

[0012] A sixth aspect is a method for producing an RTB based sintered magnet according to the fifth aspect, in which the pressure inside the container is reduced to 20 kPa or more and 30 kPa or less. Effect of the Invention

[0013] According to the present disclosure, it is possible to provide an evaluation method that can more appropriately evaluate the state of cracks in a compact, and a method for producing an RTB based sintered magnet that applies the evaluation method. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic cross-sectional view of an evaluation device according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The evaluation method of the present disclosure and a method for producing an RTB based sintered magnet to which the evaluation method is applied will be described in detail below with reference to the drawings.

[0016] Fig. 1 is a schematic cross-sectional view of an evaluation device 1 according to the present disclosure. In order to properly evaluate cracks that are covered with powder and exist in the vicinity of the surface (including cracks where most of the cracks exist in the vicinity of the surface and some of the cracks communicate with the surface), the evaluation method according to the present disclosure immerses a molded body 2 molded from the powder in a solution L in a container 3 as shown in Fig. 1, reduces the pressure in the container 3 to 10 kPa or more but less than 50 kPa, and observes the state of bubbles generated from the molded body 2 to evaluate the state of the cracks.

[0017] The molded body 2 (first molded body) is a powder molded body, for example, an RTB alloy powder molded body. The material of the molded body 2 is not limited to this, and any material that is used in the ceramics field including magnets or other fields where powder is molded and then sintered can be used.

[0018] The molded body 2 is molded, for example, in a rectangular parallelepiped shape, and is placed on a support member 4 and immersed in the solution L. The molded body 2 is not limited to a rectangular parallelepiped shape, and may be any shape, such as a semi-cylindrical, bow-shaped, cylindrical, or ring-shaped shape. Although only one molded body 2 is placed on the support member 4 in FIG. 1, the present invention is not limited to this, and multiple molded bodies may be placed on the support member 4.

[0019] The support member 4 on which the molded body 2 is placed is, for example, a mesh-like metal plate, and is provided so that the state of the bubbles can be observed from the surface side (back side) of the molded body 2 that comes into contact with the support member 4. Note that the support member 4 is not limited to this, and any material may be used as long as it can support the molded body 2 even under reduced pressure and does not react with the solution L to generate bubbles. Furthermore, when the back side of the molded body 2 is to be observed by turning it over, the hole does not have to be formed.

[0020] The container 3 is, for example, a colorless and transparent acrylic plate, and the generation of bubbles can be observed from the side and back side of the molded body 2. Note that the container 3 is not limited to this, and does not have to be colorless and transparent as long as the side can be confirmed and the back side can be confirmed by turning over the molded body 2. In addition, any material can be used as long as it can withstand reduced pressure and does not react with the solution L to generate bubbles.

[0021] The solution L is a solvent that does not react with the molded body 2, and is a liquid substance with a relatively low viscosity. For example, when the molded body 2 is a molded body 2 obtained by molding an RTB-based alloy powder, a hydrocarbon-based solvent, a fatty acid-based solvent, an alkaline cleaning agent, a terpineol solution, kerosene (light oil), etc. can be used, and the viscosity can be adjusted to be low. The material of the solution L is not limited to this, and any liquid substance with a low viscosity that does not react with the molded body 2 to generate bubbles can be used. In the case of the solution L that reacts with the molded body 2 to generate bubbles, it becomes impossible to evaluate whether the bubbles are caused by cracks. In addition, if the viscosity is high, the penetration speed into the cracks becomes slow, and the generation of bubbles is slow and the amount of bubbles generated per unit time is small, making it difficult to judge, which is not preferable.

[0022] The viscosity of solution L is, for example, 100×10 -3 It is preferable that the viscosity is 100×10 Pa·s or less. -3 Even if the molded body 2 is immersed in a solution having a viscosity exceeding 50×10 Pa·s, it is difficult to determine the presence or absence of cracks from the bubbles that are generated. -3 The viscosity is preferably 10×10 Pa s or less. -3 The viscosity of acetone, a typical organic solvent, is 0.3×10 -3 Since the practical lower limit is 0.1×10 -3 The viscosity of solution L is measured using a Brookfield viscometer.

[0023] Moreover, it is preferable that the solution L is colorless and transparent, which allows easy confirmation of the location of cracks and enables feedback to the molding process, such as changing the mold or molding conditions.

[0024] The pressure in the container 3 after the molded body 2 is immersed in the solution L is reduced by a vacuum pump P connected to the container 3. When reducing the pressure, the pressure in the container 3 is reduced to 10 kPa or more and less than 50 kPa, based on atmospheric pressure. As a result of investigation, the inventors have found that by reducing the pressure in the container 3 to a specific range of 10 kPa or more and less than 50 kPa, cracks in the vicinity of the surface can be appropriately evaluated. This makes it possible to reliably check not only cracks that expose the entire surface of the molded body 2, but also cracks in the vicinity of the surface, thereby reducing defects in the magnets finally obtained and suppressing deterioration of mass productivity. When the pressure is reduced to less than 10 kPa, the solution L is less likely to penetrate, so gas is less likely to escape from cracks covered with powder, and bubbles are not generated. When the pressure is reduced to 50 kPa or more, the solution L is more likely to penetrate through fine pores present on the surface of the molded body 2 other than the cracks covered with powder, and the solution penetrates from the entire molded body 2, resulting in bubbles being generated throughout the entire molded body 2. Therefore, it is not possible to confirm the location of the cracks, and therefore it is not possible to perform an appropriate evaluation. Therefore, to confirm the presence of cracks in the vicinity of the surface covered with powder, evaluation is performed based on the state of bubbles that are generated when the pressure is reduced to 10 kPa or more and less than 50 kPa. It is preferable to reduce the pressure inside the container 3 to 20 kPa or more and 40 kPa or less, and more preferably to reduce the pressure to 20 kPa or more and 30 kPa or less.

[0025] Next, a method for producing an RTB sintered magnet (R is at least one rare earth element and always includes at least one of Nd and Pr. T is at least one transition metal element and always includes Fe) will be described. The method for producing an RTB sintered magnet includes a first molding step, an evaluation step, a second molding step, and a sintering step.

[0026] In the first compacting step, an RTB alloy powder is prepared and compacted to obtain a first compact 2. The compacting is performed under predetermined compacting conditions.

[0027] In the evaluation step, the first molded body 2 is immersed in the solution in the container 3, and the pressure inside the container 3 is reduced to 10 kPa or more but less than 50 kPa, and the state of cracks is evaluated based on the state of bubbles generated from the first molded body 2.

[0028] In the second compacting step, if bubbles are generated in the evaluation step, the location of the crack is checked, and feedback is given to the compacting step, such as changing the mold or the compacting conditions. The RTB alloy powder is then compacted under appropriate compacting conditions that do not generate cracks, to obtain a second compact. If no cracks are detected in the evaluation step, the result that no cracks were detected is fed back, and the second compacting step is carried out without changing the compacting conditions. This makes it possible to obtain a second compact that does not generate cracks.

[0029] In the sintering step, the first compact 2 obtained in the first compacting step and the second compact obtained in the second compacting step, or the second compact obtained in the second compacting step, are heated and sintered. Of the first compacts 2 that have been checked for cracks, those in which no cracks were found may be sintered with the second compact, or may be discarded together with those in which cracks were found. In this way, an RTB sintered magnet is obtained. EXAMPLES

[0030] The present disclosure will be described in more detail with reference to examples, but is not limited thereto.

[0031] Example 1 The RTB alloy was coarsely pulverized by hydrogen pulverization to obtain a coarsely pulverized powder. Next, zinc stearate was added as a lubricant to the obtained coarsely pulverized powder in an amount of 0.04% by mass relative to 100% by mass of the coarsely pulverized powder, and mixed. The mixture was then dry-pulverized in a nitrogen gas stream using an airflow pulverizer (jet mill device) to obtain a particle size D 50 (Volume center value obtained by measurement using airflow dispersion laser diffraction method = D 50 ) was obtained as a finely pulverized powder (RTB alloy powder) with a particle size of 4 μm.

[0032] Then, zinc stearate was added as a lubricant to the RTB alloy powder at 0.05 mass% relative to 100 mass% of the RTB alloy powder, and mixed. The compact was then covered with the powder and molded under molding conditions that generated internal cracks near the surface to obtain a compact. The molding device used was a so-called perpendicular magnetic field molding device (horizontal magnetic field molding device) in which the magnetic field application direction and the pressure direction are perpendicular to each other. The compact was molded to have a rectangular parallelepiped shape. The obtained compact was observed in detail, and cracks that exposed the entire cracks on the surface of the compact and cracks that were internal near the surface were confirmed.

[0033] Next, a solution of a mixture of a hydrocarbon solvent and a fatty acid solvent (viscosity: 1.4 × 10 ―3 An evaluation device containing a pressure of 100 kPa (Pa·s) was prepared, and the molded body was immersed in the solution. The pressure inside the container was then gradually reduced from 0 kPa to 50 kPa as shown in Table 1, and the state of bubble generation was evaluated. Cases where no bubbles were generated in the cracks located near the surface, or where bubbles were generated to the extent that the location of the cracks could not be seen, were rated as ×, and cases where bubbles were generated such that the location of the cracks located near the surface was visible were rated as 〇.

[0034] [Table 1]

[0035] When the pressure inside the container was not reduced (0 kPa), air bubbles were found in the cracks that exposed the entire surface of the molded body, but no air bubbles were generated from the cracks that were internal to the surface. When the pressure was reduced to 50 kPa, air bubbles were generated throughout the molded body, so the location of the cracks was not visible. On the other hand, when the pressure was reduced to 20 kPa and 30 kPa, air bubbles were generated in such a way that the location of the cracks that were internal to the surface could be seen. Therefore, it was found that the pressure inside the container needs to be reduced to between 10 kPa and 50 kPa. [Industrial Applicability]

[0036] INDUSTRIAL APPLICABILITY The present disclosure has industrial applicability in that it provides an evaluation method that can more appropriately evaluate the state of cracks in a compact, and a manufacturing method for an RTB based sintered magnet to which the evaluation method is applied. [Explanation of symbols]

[0037] 1...Evaluation device 2...Molded body 3…Container 4...Support member L…Solution P…Decompression pump

Claims

1. An evaluation method in which a compact formed from R-T-B alloy (R is at least one rare earth element and must include at least one of Nd and Pr; T is at least one transition metal element and must include Fe) powder is immersed in a solution having a viscosity of 100 x 10-3 Pa·s or less, which is a mixture of a hydrocarbon solvent and a fatty acid solvent in a container, and the pressure inside the container is reduced to 10 kPa or more but less than 50 kPa, and the state of cracks is evaluated based on the state of bubbles that arise from the compact.

2. The evaluation method according to claim 1 , wherein the pressure inside the container is reduced to 20 kPa or more and 40 kPa or less.

3. The evaluation method according to claim 2 , wherein the pressure inside the container is reduced to 20 kPa or more and 30 kPa or less.

4. A method for producing an R-T-B based sintered magnet (R is at least one rare earth element and necessarily contains at least one of Nd and Pr, and T is at least one transition metal element and necessarily contains Fe), comprising the steps of: a first compacting step of compacting the R-T-B based alloy powder to obtain a first compact; an evaluation step of immersing the first molded body in a solution having a viscosity of 100×10 −3 Pa·s or less, which is a mixture of a hydrocarbon solvent and a fatty acid solvent in a container, reducing the pressure in the container to 10 kPa or more but less than 50 kPa, and evaluating the state of cracks based on the state of bubbles generated from the first molded body; a second compacting step in which the result of the evaluation step is fed back to the first compacting step, and the R-T-B based alloy powder is compacted under compacting conditions in which no cracks are generated, to obtain a second compact; a sintering step of heating the first compact and the second compact, or the second compact, to obtain a sintered body; The present invention relates to a method for producing an RTB based sintered magnet having the above structure.

5. The method for producing a sintered RTB based magnet according to claim 4, wherein the pressure inside the vessel is reduced to 20 kPa or more and 40 kPa or less.

6. The method for producing a sintered RTB based magnet according to claim 5, wherein the pressure inside the vessel is reduced to 20 kPa or more and 30 kPa or less.

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