Patent Application
20220199322
This application is based upon and claims the benefit of priority from Japanese Patent Applications No. 2020-212651, filed on 22 Dec. 2020 and No. 2021-171026, filed on 19 Oct. 2021, the entire contents of which are incorporated herein by reference.
The present disclosure relates to an extrusion die for a hot-deformed magnet and a method for manufacturing a hot-deformed magnet using the same.
Conventionally, an R-T-B based permanent magnet having excellent magnetic properties is known as a kind of permanent magnet, and is widely used. The R-T-B-based permanent magnets are roughly classified into two types. One is a sintered magnet produced by a powder metallurgy method, and the other is a hot-deformed magnet produced by a hot plastic deforming method.
Examples of the method for manufacturing the hot-deformed magnet include a die upset method, an upsetting forging method, a backward extrusion method, and a forward extrusion method. Among them, the forward extrusion method is suitable for manufacturing the hot-deformed magnet used for a high-efficiency motor such as an IPM. The properties of the hot-deformed magnet greatly respond to plastic deformation during hot deforming, and in the forward extrusion method, they can greatly respond to the shape of the extrusion die responsible for plastic deformation.
In the method for manufacturing the hot-deformed magnet according to the related art, cracks generated in the hot-deformed magnet have not been studied, and the occurrence of cracks have not been sufficiently suppressed. In the case that cracks are generated in the hot-deformed magnet, the residual magnetic flux density Br may decrease as the main phase volume fraction decreases. In addition, the local demagnetizing field increases from the crack as a starting point, and magnetic reversal nuclei are likely to be generated. As a result, the coercivity HcJ may decrease. In view of the above, the present inventors have repeatedly studied cracks generated in the hot-deformed magnet, and have newly found a technique capable of suppressing the occurrence of cracks.
According to various aspects of the present disclosure, there are provided an extrusion die for a hot-deformed magnet capable of suppressing the occurrence of cracks in the hot-deformed magnet, and a method of manufacturing a hot-deformed magnet using the same.
An extrusion die for a hot-deformed magnet according to one aspect of the present disclosure having a starting end-face and a terminal end-face facing each other, and including a plastic deforming portion extending from the starting end-face to the terminal end-face. The plastic deforming portion has a cross-sectional area in a cross section orthogonal to a facing direction of the starting end-face and the terminal end-face gradually decreasing from a starting end portion at the starting end-face toward a terminal end portion at the terminal end-face of the plastic deforming portion.
In the above extrusion die, since the cross-sectional area of the plastic deforming portion gradually decreases from the starting end portion toward the terminal end portion, when the extrusion die is used for manufacturing a hot-deformed magnet, the pressure applied to a molded body during hot-deforming gradually increases. That is, the pressure applied to the molded body is not loosened in hot-deforming, hence, the occurrence of cracks due to loosening of the pressure is effectively suppressed.
In the extrusion die for the hot-deformed magnet according to another aspect, a ratio of a cross-sectional area of the terminal end portion to a cross-sectional area of the starting end portion of the plastic deforming portion is 60 to 90%. When the extrusion die is used for manufacturing the hot-deformed magnet, the hot-deformed magnet having high magnetic properties can be obtained. In addition, the occurrence of cracks is further suppressed, and thus the hot-deformed magnet having higher magnetic properties is obtained.
In the extrusion die for the hot-deformed magnet according to another aspect, the starting end portion of the plastic deforming portion has an end-face shape extending in one direction, and the terminal end portion of the plastic deforming portion also has an end-face shape extending in one direction.
In the extrusion die for the hot-deformed magnet according to another aspect, a first direction in which the end-face shape of the starting end portion of the plastic deforming portion extends and a second direction in which the end-face shape of the terminal end portion of the plastic deforming portion extends intersect with each other when viewed from a facing direction of the starting end-face and the terminal end-face of the extrusion die. In this case, large plastic deformation in the molded body can cause.
In the extrusion die for the hot-deformed magnet according to another embodiment, the end-face shape of the starting end portion and the end-face shape of the terminal end portion of the plastic deforming portion are rectangular.
In the extrusion die for the hot-deformed magnet according to another aspect, in the plastic deforming portion, from the end-face shape of the starting end portion to the end-face shape of the terminal end portion, the length of each side of the rectangle of the end-face shape changes exponentially. In this case, the cross-sectional area of the plastic deforming portion can be linearly reduced from the starting end portion at the starting end-face toward the terminal end portion at the terminal end-face of the plastic deforming portion. Therefore, the pressure applied to the molded body increases at a constant rate from the starting end portion toward the terminal end portion of the plastic deforming portion, hence, the occurrence of cracks can be further suppressed.
In the extrusion die for the hot-deformed magnet according to another aspect, the end-face shape of the terminal end portion of the plastic deforming portion is a partial annular shape.
A method for manufacturing a hot-deformed magnet according to one aspect of the present disclosure uses the above extrusion die and includes a step of hot-deforming a molded body obtained by molding magnetic powder with the extrusion die to obtain a hot-deformed magnet.
In the method for manufacturing the hot-deformed magnet, the pressure applied to the molded body is not loosened in the hot deforming step, hence, the occurrence of cracks due to loosening of the pressure is effectively suppressed.
Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, the same element or the element having the same function is denoted by the same reference numeral, and redundant description is omitted.
A extrusion die 10 for a hot-deformed magnet according to the first embodiment will be described with reference to
The extrusion die 10 includes a plastic deforming portion 12 extending from the starting end-face 10a to the terminal end-face 10b. The plastic deforming portion 12 has a starting end portion 12a at the starting end-face 10a and a terminal end portion 12b at the terminal end-face 10b.
The starting end portion 12a of the plastic deforming portion 12 has an end-face shape extending in one direction when viewed from a direction in which the starting end-face 10a and the terminal end-face 10b face each other. The end-face shape of the starting end portion 12a in the present embodiment is rectangular.
Hereinafter, for convenience of description, a direction in which the starting end-face 10a and the terminal end-face 10b face each other is referred to as a Z direction, a direction in which the end-face shape of the starting end portion 12a of the plastic deforming portion 12 extends is referred to as an X direction, and a direction orthogonal to the Z direction and the X direction is referred to as a Y direction.
In the extrusion die 10, the cross-sectional area in the X-Y cross-section of the plastic deforming portion 12 gradually decreases substantially linearly from the starting end portion 12a toward the terminal end portion 12b.
The terminal end portion 12b of the plastic deforming portion 12 has an end-face shape extending in one direction when viewed from a direction in which the starting end-face 10a and the terminal end-face 10b face each other. The end-face shape of the terminal end portion 12b in the present embodiment is rectangular. The end-face shape of the starting end portion 12a extends in the X direction (that is, the long side extends along the X axis), whereas the end-face shape of the terminal end portion 12b extends in the Y direction (that is, the long side extends along the Y axis). When viewed from the facing direction of the starting end-face 10a and the terminal end-face 10b, the X direction (first direction) in which the end-face shape of the starting end portion 12a extends and the Y direction (second direction) in which the end-face shape of the terminal end portion 12b extends intersect with each other, more specifically, are orthogonal to each other. In the plastic deforming portion 12, the long side (or the long axis) and the short side (or the short axis) are interchanged between the rectangular end-face of the starting end portion 12a and the rectangular end-face of the terminal end portion 12b. The end-face of the starting end portion 12a and the end-face of the terminal end portion 12b are in a relationship of skew lines.
As shown in
Inside the extrusion die 10, the contour of the plastic deforming portion 12 is formed by a curve as shown in
In the Y-Z cross section shown in
In the X-Z cross section shown in
In the hot-deformed magnet, when a crack is generated in a portion, the magnetization in the portion is reduced, and the magnetization per unit volume decreases. As a result, the residual magnetic flux density decreases. By using the extrusion die for manufacturing a hot-deformed magnet, the occurrence of cracks in the hot-deformed magnet can be suppressed, and thus a decrease in residual magnetic flux density can be suppressed.
In addition, a demagnetizing field is generated in a portion in which a crack is generated in the hot-deformed magnet, similarly to the magnet surface, and the demagnetizing field becomes a starting point of magnetization reversal. As the number of cracks in the hot-deformed magnet increases, the number of starting points of magnetization reversal increases, so that the coercivity of the hot-deformed magnet decreases. According to the above method for manufacturing a hot-deformed magnet, the occurrence of cracks that are starting points of magnetization reversal can be suppressed, and thus a decrease in coercivity can be suppressed.
By setting the ratio (area reduction ratio) of the area of the terminal end portion 12b to the area of the starting end portion 12a of the plastic deforming portion 12 to 60 to 90% (for example, 86.8%), the hot-deformed magnet having high magnetic properties (for example, coercivity) can be obtained. In addition, the occurrence of cracks is further suppressed, whereby the hot-deformed magnet having high magnetic properties (for example, residual magnetic flux density Br and coercivity HcJ) can be obtained.
In addition, the end-face of the starting end portion 12a and the end-face of the terminal end portion 12b of the plastic deforming portion 12 may not be in a relationship of skew lines but may be in a parallel positional relationship (for example, both extend in the X direction). When the end-face of the starting end portion 12a and the end-face of the terminal end portion 12b of the plastic deforming portion 12 are in a relationship of skew lines, a relatively large plastic deformation can be generated when the molded body passes through the plastic deforming portion 12, and the hot-deformed magnet having high magnetic properties (for example, coercivity) can be obtained.
Further, when the length of each side of the rectangle of the end-face shape changes exponentially from the end-face shape of the starting end portion 12a toward the end-face shape of the terminal end portion 12b of the plastic deforming portion 12, the cross-sectional area of the plastic deforming portion 12 can be linearly reduced from the starting end portion 12a at the starting end-face 10a to the terminal end portion 12b at the terminal end-face 10b of the plastic deforming portion 12. Therefore, the pressure applied to the molded body increases at a constant rate from the starting end portion 12a toward the terminal end portion 12b of the plastic deforming portion 12, hence, the occurrence of cracks can be further suppressed.
An extrusion die 10A for the hot-deformed magnet according to a second embodiment will be described with reference to
The starting end portion 12a of the plastic deforming portion 12A has an end-face shape extending in one direction, and more specifically, has a rectangular end-face shape, when viewed from the direction in which the starting end-face 10a and the terminal end-face 10b of the extrusion die 10A face each other. As shown in
The terminal end portion 12b of the plastic deforming portion 12A has a partially annular end-face shape when viewed from the facing direction of the starting end-face 10a and the terminal end-face 10b of the extrusion die 10A. More specifically, the partial annular shape of the end-face shape of the terminal end portion 12b is a semi-annular shape in which the opening angle θ of the inner arc is 180 degrees. As shown in
In the plastic deforming portion 12A, the shape and size of the contour gradually change between the rectangular end-face of the starting end portion 12a and the semicircular end-face of the terminal end portion 12b. More specifically, one short side (length L1) of the starting end portion 12a gradually changes to the outer arc of the terminal end portion 12b, the pair of long sides of the starting end portion 12a gradually changes to the pair of edges of the terminal end portion 12b, and the other short side (length L3) of the starting end portion 12a gradually changes to the inner arc of the terminal end portion 12b.
As described above, in the extrusion die 10A, similarly to the above extrusion die 10, since the cross-sectional area of the plastic deforming portion 12A gradually decreases from the starting end portion 12a toward the terminal end portion 12b, the pressure applied to the molded body during hot deforming is not loosened, hence, the occurrence of cracks can be effectively suppressed.
In the extrusion die 10A, the end-face shape of the terminal end portion 12b of the plastic deforming portion 12A is a partial annular shape (that is, a semi-annular shape) in which the opening angle θ of the inner arc is 180 degrees, but may be a partial annular shape in which the opening angle θ is smaller than 180 degrees. The opening angle θ may be 120 degrees or less, or may be 90 degrees or less.
Although the embodiments of the present disclosure have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications can be made without departing from the gist thereof.
For example, the end-face shape of the starting end portion and the terminal end portion of the plastic deforming portion is not limited to a rectangular shape, and may be an elliptical shape extending in one direction, or may be a perfect circle shape, a U shape, or a V shape.
A method for manufacturing a hot-deformed magnet using the above extrusion dies 10 and 10A will be described with reference to the flowchart shown in
In the R-T-B based permanent magnet, R represents a rare earth element. The permanent magnet contains at least neodymium (Nd) as a rare earth element. The permanent magnet may contain other rare earth elements in addition to Nd. The other rare earth element may be at least one selected from the group consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sin), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tin), ytterbium (Yb), and lutetium (Lu). In the R-T-B based permanent magnet, T represents a transition metal element. The permanent magnet contains at least iron (Fe) as a transition metal element. The permanent magnet may contain only Fe as a transition metal element. The permanent magnet may contain both Fe and cobalt (Co) as transition metal elements. In the R-T-B based permanent magnet, B is boron.
When manufacturing a hot-deformed magnet, first, a magnet material as a raw material is pulverized into magnetic powder (step S1). The pulverization can be performed by, for example, a cutter mill or a propeller mill, and can be performed in, for example, an argon gas atmosphere (or a nitrogen gas atmosphere). The particle diameter of the magnetic powder obtained by pulverization is, for example, about 100 to 300 μm. The magnetic powder is not finely pulverized to the size level of neodymium magnet crystals (1 μm or less, for example, several 10 to several 100 nm), and has a polycrystalline structure composed of a plurality of neodymium magnet crystals.
The magnetic powder obtained in step S1 is molded by a compression molding machine to obtain a molded body (step S2). The molding is performed in a nitrogen gas atmosphere (or an argon gas atmosphere) at a high temperature of 800° C. or less (for example, 750° C.) and a pressure of 200 MPa or less for several 10 seconds. By molding, a dense molded body is obtained. However, in the state of this molded body, the magnet particles are randomly oriented, and the easy magnetization axis directions are not aligned.
The molded body obtained in step S2 is hot-deformed by a forward extrusion method to obtain a hot-deformed magnet (step S3). The hot deforming is performed in a nitrogen gas atmosphere (or in an argon gas atmosphere or in the air) at a high temperature of 800° C. or less (for example, 750° C.) and a pressure of 100 MPa or less for several 10 seconds. The above extrusion dies 10 and 10A can be used for this hot deforming.
Here, experiments conducted by the inventors on the cross-sectional areas of the plastic deforming portions 12 and 12A of the extrusion dies 10 and 10A will be described.
As Samples 1 and 2, extrusion dies were prepared in which the rectangular end-face of the starting end portion of the plastic deforming portion was 11 mm×22 mm, the rectangular end-face of the terminal end portion in which the long side and the short side were reversed was 30 mm×7 mm, and the thickness was 20 mm, as in the above extrusion die 10. In Sample 1, as shown in
In Sample 1, as shown in
Then, hot deforming of the molded body was performed using Sample 1 and Sample 2 to obtain a hot-deformed magnet. As a result, no crack was observed in the hot-deformed magnet obtained using Sample 1, but cracks were observed in the hot-deformed magnet obtained using Sample 2.
As Sample 3, an extrusion die having a rectangular end-face of 20 mm×10 mm at the starting end portion of the plastic deforming portion and a semi-annular end-face of 13 mm in inner diameter, 5 mm thick, and an opening angle of 180 degrees of the inner arc at the terminal end portion was prepared as in the extrusion die 10A described above. In Sample 3, as shown in
In Sample 3, as shown in
Then, hot deforming of the molded body was performed using Sample 3 to obtain a hot-deformed magnet. As a result, no crack was observed in the hot-deformed magnet obtained using Sample 3.
This is considered to be because when the cross-sectional area of the plastic deforming portion gradually decreases from the starting end portion toward the terminal end portion without increasing at all as in Samples 1 and 3, the pressure applied to the molded body is gradually increased during hot deforming, and therefore the pressure is not loosened during hot deforming, but when the cross-sectional area of the plastic deforming portion increases even slightly as in Sample 2, the pressure is loosened to cause cracks.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-212651 | Dec 2020 | JP | national |
| 2021-171026 | Oct 2021 | JP | national |
