Thin ring magnet and method of manufacturing thin ring magnet

Abstract

Thin ring magnets with a height / diameter aspect ratio of approximately 0.1 and a large hole diameter of 30% to 50% of the diameter are prone to cracking during manufacturing, resulting in a significant decline in magnetic properties, and a solution to this problem is needed. [Solution] This method involves forming a preliminary hole by electrical discharge machining, and then drilling away any structurally altered layers around the outer periphery of the preliminary hole caused by the heat generated by the electrical discharge machining. The distortion-affected areas caused by this drilling are then heat-treated to restore their magnetic properties. This manufacturing method can produce thin ring magnets with a diameter of 1mm to 3mm, a height of 0.1mm to 0.35mm, a height / diameter aspect ratio of 0.08 to 0.33, and a hole diameter of 0.5mm to 1.5mm, which accounts for 30% to 50% of the diameter.

Description

[Technical Field]

[0001] The present invention relates to a thin ring magnet made of a rare earth sintered magnet and a method for manufacturing the thin ring magnet. [Background technology]

[0002] A rare earth sintered magnet such as an NdFeB sintered magnet is manufactured through, for example, a step of preparing an alloy powder, a step of press-molding the alloy powder to form a compact, and a step of sintering the compact. The sintered body obtained by the process of sintering the green body cannot be divided into individual pieces and used as parts as it is because uneven shrinkage is unavoidable during sintering. Therefore, machining such as grinding, cutting, and polishing is essential.

[0003] In particular, in the case of ring-shaped magnets, the complexity of the outer and inner diameter side surfaces in addition to the flat top and bottom surfaces further exacerbates the unevenness of shrinkage during the sintering process. Furthermore, in the case of ring magnets that are thin and have a small aspect ratio of height to diameter, although a compact can be formed, when the compact is sintered the unevenness of shrinkage becomes greater than the height, making it unsuitable for grinding and polishing.

[0004] Regarding the processing methods for NdFeB sintered magnets, it has been disclosed that wire cutting and electrical discharge machining result in less stress concentration, as mechanical processing can cause degradation of magnetic properties (Patent Document 1). On the other hand, increasing the electrical discharge machining speed can cause a degradation layer due to sludge, and as a countermeasure, processing at a speed of 2.0 mm / min or more has been disclosed (Patent Document 2). Both of these are related to processing the material.

[0005] On the other hand, regarding the processing of minute holes in sintered magnets, the processing of magnets for watches has been reported, but degradation due to the processing has also been reported (Non-Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 10-12399 [Patent Document 2] Japanese Patent Publication No. 2023-141524 [Non-patent literature]

[0007] [Non-Patent Document 1] Journal of the Horological Society of Japan 150(1994)p118 Summary of the Invention [Problem to be solved by the invention]

[0008] Generally, ring magnets are manufactured by drilling a hole in the center of a disk magnet. The hole is drilled using either a drill or electric discharge machining, but if the volume ratio of the hole is large compared to the size of the magnet, or if the aspect ratio (height / diameter (outer diameter)) is small, around 0.1, drilling will cause the magnet to crack, so a preliminary hole is drilled using electric discharge machining and then finished using a drill. With current processing methods, distortion and heat generated during processing result in a deterioration of the magnetic properties, so there is a need to develop a processing method that prevents this deterioration. In particular, thin magnets with diameters of 1 to 3 mm and heights of 0.1 to 0.35 mm, i.e., a height / diameter aspect ratio of approximately 0.1, and relatively large hole diameters of 0.5 to 1.5 mm, with hole diameters that are approximately 30 to 50% of the diameter, are prone to cracking and suffer from a significant decline in magnetic properties, and a solution to this problem is needed. [Means for solving the problem]

[0009] The inventors first investigated the metallurgical causes of the deterioration of magnetic properties. Figure 1 compares a ring magnet after drilling using electrical discharge machining with the magnetic material before machining. The knick phenomenon appears on the demagnetization curve, and the squareness (the magnetic field at which magnetization decreases by 10%), which is also an indicator of a magnet's high-temperature durability, deteriorates. Figures 2 and 3 show the results of observing the cross section of a ring machined by electrical discharge machining. First, in the enlarged view of the machined surface (b), no abnormalities such as grain boundary cracks were observed in the interior 11, but irregularities of 100 μm to 200 μm were present on the machined surface 12. From the observation of the structure, the interior 11 exhibited a good NdFeB structure (c), but the structure of the electrical discharge machined surface 12 (d and enlarged view e of d) revealed the presence of a surface molten zone 121, where Fe particles (122, 122Fe) were observed to have precipitated. The resulting altered structure layer was 10 μm to 150 μm thick.

[0010] Until now, it has been understood that deterioration occurs due to stress and strain caused by cutting processing, as reported in literature, but this is the first time that a magnetically altered layer has been found due to the precipitation of iron particles in the surface melted area caused by electrical discharge processing. Based on the above results, it was determined that the main causes of the deterioration in properties were the presence of iron particles (structurally altered layer) and the area affected by distortion during drilling (magnetic property deterioration layer). As a countermeasure, it was decided to remove the magnetically altered layer and restore the lost magnetism of the magnetic property deterioration layer.

[0011] The layer of altered structure containing iron particles causes fatal damage to the magnetic properties, so it was decided to completely remove it by drilling. Next, it was decided to restore the magnetic properties of the layer that had deteriorated due to drilling by heat treatment. As a result of the test, as shown in Figure 4, it was confirmed that the magnetic properties could be improved by drilling to remove the surface molten areas and precipitated Fe particles that deteriorated due to electrical discharge machining.

[0012] Next, we found that the magnetic properties could be further restored by subjecting the drilled product to heat treatment. Figure 5 shows the effect of heat treatment temperature. We found that when heat treatment was performed at temperatures between 400°C and 600°C, the magnetic properties recovered by 20%, from 36 MGOe to 43 MGOe at 550°C. As a result, we devised a method of optimizing the diameter of the preliminary hole relative to the final hole diameter to improve the magnetic properties, and completely removing the surface melted area caused by electrical discharge machining by drilling. As for rare earth magnets, Commercially available Nd magnets,The height was examined in the range of 0.1mm to 3mm and the hole diameter in the range of 0.5mm to 5mm. [Effects of the Invention]

[0013] The present invention relates to a small, thin ring magnet suitable for use in small, thin products such as watches and magnetic denture attachments. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 10 is a diagram showing the change in magnetic properties due to the formation of a ring by electrical discharge machining on a magnetic material. [Figure 2] (a) Ring cross section produced by electrical discharge machining, (b) enlarged view of the ring cross section, (c) internal structure of the sintered magnet, and (d) structure of the electrical discharge machined surface. [Figure 3] (e) is an enlarged view of the structure of the electrical discharge machined surface, and (f) is a diagram showing the detected peak of Fe particles on the electrical discharge machined surface. [Figure 4] FIG. 10 is a diagram showing the influence of electrical discharge machining, electrical discharge machining and drilling, and electrical discharge machining, drilling and heat treatment on magnetic properties. [Figure 5] FIG. 1 is a diagram showing the influence of heat treatment temperature on the magnetic properties ((BH)max) of drilled materials (electric discharge machining + drilling). BEST MODE FOR CARRYING OUT THE INVENTION

[0015] The method for manufacturing the thin ring magnet of the first embodiment is as follows. NdFeB sintered magnet In the method for manufacturing a thin ring magnet, Process 1) Diameter is 1mm~3mm, Height: 0.1mm~0.35mm ,and Height / diameter aspect ratio is 0.08~0.15 A thin, flat plate consisting of NdFeB sintered magnet Prepare the base magnet as follows: Step 2) A preliminary hole having a diameter of 0.35 mm to 1.35 mm is formed in the material magnet in a direction perpendicular to the flat surface by electrical discharge machining using a current of 10 A to 30 A and a voltage of 30 V to 80 V. Step 3) The preliminary hole is subjected to the electric discharge machining. Iron particles In order to remove the structurally altered phase, the diameter of the preliminary hole is enlarged by 0.1 mm to 0.4 mm by drilling to form a hole with a diameter of 0.5 mm to 1.5 mm; Step 4) During the drilling process The effect of distortion To restore the magnetism lost due to 450℃~550℃ , heat treating for 0.5hr~1.5hr, The present invention is characterized in that it comprises:

[0016] The second embodiment is The thin ring magnet of the NdFeB sintered magnet manufactured by the manufacturing method of the first embodiment is, The diameter is 1mm to 3mm, the height is 0.1mm to 0.35mm, the height / diameter aspect ratio is 0.08 to 0.15, and the hole diameter is 0.5mm to 1.5mm, which is 30% to 50% of the diameter. The thin ring magnet is made of sintered NdFeB magnet, and is characterized in that the inner structure of the ring is free of a layer of altered structure containing an Fe phase and a layer of deteriorated magnetic properties due to distortion during drilling.

[0017] This will be explained in detail below. <Magnet material> The material of the thin ring magnet is Commercially available NdFeB sintered disc magnets The diameter is 1mm to 3mm. height The diameter is 0.1 mm to 1.0 mm, and the aspect ratio of height / diameter is 0.08 to 0.33. Preferably, the diameter is 1 mm to 3 mm, height The diameter is 0.1 mm to 0.35 mm, and the height / diameter aspect ratio is 0.08 to 0.15. These are the size and aspect ratio required for a thin ring magnet used as a small precision part.

[0018] <Thin ring magnet> The hole diameter required for thin ring magnets is 0.5mm to 1.5mm, which is 30% to 50% of the 1mm to 3mm diameter of the magnetic material. If the hole diameter is small, the magnetic force of the magnet will be high, but machining the hole diameter will be difficult, and new issues will arise, such as assembling it as a precision small part and the strength of the parts inserted into the hole. Conversely, if the hole diameter is large, it will be easier to machine the hole diameter, but the magnetic portion will become smaller and the magnetic force will decrease.

[0019] <Electric discharge machining> When manufacturing thin ring magnets, electrical discharge machining (EDM), which allows for drilling by melting, is used because drilling a thin disk can easily crack the magnet material. However, as shown in Figure 3(e), this melting drilling process leaves a surface molten zone on the machined surface around the hole, forming a structurally altered layer that precipitates Fe particles and causes fatal damage to the magnetic properties, so it must be removed. Therefore, the holes drilled by electrical discharge machining are positioned as preliminary holes for drilling holes to form the ring magnet, and their diameters are 0.35 mm to 1.35 mm. Removal of 0.1mm to 0.3mm in the radial direction, 0.05mm to 0.15mm including the altered layer This is expected to be the case. This removal layer also reduces the unevenness of the processed surface.

[0020] The conditions for electrical discharge machining are a current of 10A to 30A in the direction perpendicular to the magnetic material, and a voltage of 30V to 80V, taking into consideration the thickness of the raw magnet, the diameter of the preliminary hole, and the unevenness of the machined surface formed by electrical discharge machining and the thickness of the altered layer. If the energy input (current x voltage) is low, it will take a long time, but if the energy input is high, the hole can be drilled in a short time.

[0021] <Drilling> Drilling is a mechanical process that removes the layer of altered structure on the machining surface of a pre-hole and reduces unevenness to make it smooth. The thickness is 0.1mm to 0.3mm in the radial direction, and 0.05mm to 0.15mm including the altered structure layer. However, the method is not limited to drilling as long as removal and smoothing are possible.

[0022] <Heat treatment> When the structurally altered layer created by electrical discharge machining is removed by drilling, the magnetic properties improve, but do not recover to the level of the magnetic material, as shown in Figure 4. This is because the magnetic properties are degraded by the mechanical processing such as drilling. As shown in Figure 5, when the drilled product (hole-machined material) is heat-treated at a temperature of 400°C to 500°C, preferably 450°C to 550°C, magnetic properties almost equivalent to those of the magnetic material are obtained and distortion disappears. [Example]

[0023] Example 1 Diameter 3.0mm, height 0.9mm, height / diameter A thin, disk-shaped NdFeB sintered magnet with an aspect ratio of 0.3 was used as the base magnet, and a 1.5 mm hole (pre-hole) was drilled using electrical discharge machining at 20 A and 50 V. The magnetic properties at this point were measured with VSM after magnetization in a 3 T magnetic field, and the (BH)max was found to be 34 MGOe. Next, to remove the altered layer, holes were drilled using a 1.7 mm diameter diamond drill, followed by heat treatment at 500°C for 1 hour in a vacuum atmosphere. As a result, (BH)max was improved to 44.3 MGOe. The (BH)max of the 3 mm diameter sintered magnet used was 44.2 MGOe, so the (BH)max was recovered by removing the structurally altered layer and removing the distortion degradation caused by the drilling process through heat treatment.

[0024] <Example 2> Diameter 2.0mm, height 0.2mm, height / diameter A thin, disk-shaped NdFeB sintered magnet with an aspect ratio of 0.1 was used as the base magnet, and a 0.8 mm hole (pre-hole), which is 40% of the diameter, was drilled using electrical discharge machining at 15 A and 40 V. The magnetic properties at this point were measured with VSM after magnetization in a 3 T magnetic field, and the (BH)max was 30 MGOe. Next, to remove the altered layer, holes were drilled using a 1.0 mm diameter diamond drill, followed by heat treatment at 500°C for 1 hour in a vacuum atmosphere. As a result, (BH)max was improved to 40.0 MGOe. The (BH)max of the 2mm diameter sintered magnet used was 40.2 MGOe, so the (BH)max was recovered by removing the structurally altered layer and removing the layer of magnetic properties that had been degraded by drilling through heat treatment. [Industrial Applicability]

[0025] Thin sintered ring magnets are expected to be used in precision components such as thin magnetic attachments and watch parts. [Explanation of symbols]

[0026] 10: Machining surface 11: Inside 12: Electric discharge machined surface (enlarged view) 121: Surface melted part 122:Fe particles 122Fe:Fe particle detection peak

Claims

[Claim 1] In a method for manufacturing a thin ring magnet of NdFeB sintered magnet, Step 1) preparing a thin, flat, sintered NdFeB magnet material having a diameter of 1 mm to 3 mm, a height of 0.1 mm to 0.35 mm, and a height / diameter aspect ratio of 0.08 to 0.15; Step 2) A preliminary hole having a diameter of 0.35 mm to 1.35 mm is formed in the material magnet in a direction perpendicular to the flat surface by electrical discharge machining using a current of 10 A to 30 A and a voltage of 30 V to 80 V. Step 3) enlarging the diameter of the preliminary hole by 0.1 mm to 0.4 mm by drilling to remove a structurally altered phase due to iron particles generated by the electrical discharge machining, thereby forming a hole with a diameter of 0.5 mm to 1.5 mm; Step 4) heat treatment at 450°C to 550°C for 0.5 hours to 1.5 hours to recover the magnetism lost due to the distortion caused by the drilling process; A method for manufacturing a thin ring magnet of NdFeB sintered magnet, comprising:

Images