Patent Grant
7730755
The present invention relates to a process of producing a permanent magnet having excellent magnetic properties by extrusion molding.
Permanent magnets constituted of a rare earth element, a metal of the iron group and boron in the shape of a plate, such as plane, arcuate, semi-circular or crescent, and having magnetic anisotropy imparted by hot (or warm) plastic working have been industrially and domestically used. These permanent magnets are manufactured as will now be described below.
A raw material prepared by mixing a rare earth, a metal of the iron group and boron is melted and the molten magnet alloy thus obtained is jetted out onto a rotating roll of e.g. copper to form thereon a rapid-quenched flaky ribbon composed of nano-sized crystal grains. The magnet alloy powder obtained by rapid-quenching as described above is crushed into an appropriate particle diameter and cold pressed into a compact. The compact is hot or warm pressed into a body having higher density, and is then subjected to hot or warm plastic working to form a plate sized as desired and having magnetic anisotropy. Examples of the method for plastic working to impart magnetic anisotropy to the plate include (1) upsetting, (2) extrusion and (3) rolling. The magnet material subjected to plastic working is magnetized in the later step, whereby a practically useful permanent magnet having magnetic anisotropy is provided.
JP-A-9-129463, for example, generally describes the manufacture of a ring-shaped permanent magnet and the like by extrusion.
Upsetting (1) can realize high magnetic properties, but is inferior to both extrusion (2) and rolling (3) in productivity, material yield, acceptable product ratio, and cost of manufacture. On the other hand, although both extrusion (2) and rolling (3) are superior in productivity, material yield, acceptable product ratio, and cost of manufacture, they have the drawback of being unable to realize high magnetic properties. In addition, extrusion (2) is excellent in material yield and acceptable product ratio in comparison with rolling (3). While each method has its own characteristics as described above, there is an industrial demand for the manufacture of a plate-shaped permanent magnet by extrusion, since extrusion (2) is excellent in a good balance between material yield, acceptable product ratio and productivity.
The disclosure of JP-A-9-129463 relates to the manufacture of a ring-shaped permanent magnet and the manufacture of any permanent magnet in the shape of a plate, such as plane, arcuate, semi-circular or crescent is not considered. Therefore, there is a demand for a method which can manufacture a plate-shaped permanent magnet having improved magnetic properties by extrusion.
In view of the problems in the conventional art as pointed out above, it is an object of the present invention to provide a process capable of producing a permanent magnet having high magnetic properties by extrusion, which is superior in terms of material yield and acceptable product ratio; and a permanent magnet produced by extrusion.
18: Preform
20: permanent magnet
Namely, the present invention relates to the following (1).
(1) A process of producing a permanent magnet, which comprises extruding a preform to form a plate-shaped permanent magnet, wherein the preform is extruded in such a way that a dimension of a cross section of the preform is reduced in an X-direction and enlarged in a Y-direction perpendicular to the X-direction.
According to the process of (1) above, by extruding the preform in such a way that the dimension of the cross section of the preform is reduced in an X-direction and enlarged in a Y-direction perpendicular to the X-direction, a permanent magnet having magnetic properties equal to or higher than those of the permanent magnet produced by upsetting can be produced.
Furthermore, the present invention relates to the following (2).
(2) A plate-shaped permanent magnet formed by extruding a preform, wherein the preform is extruded in such a way that a dimension of a cross section of the preform is reduced in an X-direction and enlarged in a Y-direction perpendicular to the X-direction, whereby said permanent magnet has a strain ratio ε2/ε1 with respect to the preform in a range of 0.2 to 3.5, wherein ε1 is a strain in the direction of extrusion of the preform and ε2 is a strain in the Y-direction.
The permanent magnet of (2) above is subjected to a plastic working to have a strain ratio with respect to the preform in the range of 0.2 to 3.5, whereby the permanent magnet has magnetic properties equal to or higher than those of the permanent magnet produced by upsetting.
According to the production process of the present invention, a permanent magnet having high magnetic properties can be produced at low cost.
Furthermore, the permanent magnet of the present invention is excellent in magnetic properties.
The process of producing a permanent magnet and the permanent magnet according to the present invention will now be described by way of preferred embodiments thereof with reference to the accompanying drawings.
The extrusion die 10 is designed for forming a plate-shaped permanent magnet 20 having a rectangular cross section in which a width W1 (as measured in the Y-direction) is larger than a thickness T1 (as measured in the X-direction) as shown in
The through hole 12 formed in the entry-side die 22 has such an oblong cross section that the dimensions thereof in the X-direction in its cross section perpendicular to the direction of extrusion and in the Y-direction perpendicular to the X-direction may be substantially identical to the thickness T and width W of the preform 18, respectively. The preform 18 is mounted in the through hole 12 along a length direction (Z-direction which is perpendicular to the X- and Y-directions) under the conditions with a thickness and width directions being positioned in the X- and Y-directions, respectively. The uniformly sized through hole 16 formed at the outlet of the forming die 24 has such a rectangular cross section that the dimensions thereof in the X-direction in its cross section perpendicular to the direction of extrusion and in the Y-direction perpendicular to the X-direction may be respectively identical to the thickness T1 and width W1 of the permanent magnet 20 to be manufactured in its cross section perpendicular to the direction of extrusion (extrusion cross section), as shown in
The tapered hole 14 is formed to have a smoothly curved surface contour to realize the smooth plastic working of the preform 18. Additionally, in this embodiment, the inlet 24a of the forming die 24 is formed to have the same dimensions as those of the corresponding through hole 12 and be successively present with a predetermined length in the axial direction, and the connected part of the inlet 24a and the tapered surface is formed to have a curved surface having an appropriate radius of curvature, in order to realize the smooth plastic working of the preform 18. The outlet 24b of the tapered hole 14 is also smoothly continuous to the uniformly sized through hole 16 in order to realize the smooth plastic working of the preform 18.
The respective dimensions of the preform 18 and the through hole 12, tapered hole 14 and uniformly sized through hole 16 of the extrusion die 10 in the X-, Y- and Z-directions are controlled so that the permanent magnet 20 produced by extrusion of the preform 18 have a strain ratio ε2/ε1 in the range of from 0.2 to 3.5, preferably from 0.4 to 1.6, in which ε1 is a strain of the permanent magnet 20 in the direction of the extrusion of the preform 18 and ε2 is a strain in the Y-direction. Namely, when the plate-shaped permanent magnet 20 having the thickness T1, width W1 and length L1 is formed from the preform 18 having an oblong cross section and having the thickness T, width W and length L as in embodiment 1, the respective dimensions of the preform 18 and the through hole 12, tapered hole 14 and uniformly sized through hole 16 in the X-, Y- and Z-directions are controlled so that the relationship as represented by the following formula (1) is satisfied.
ε2/ε1=ln(W1/W)/ln(L1/L)=0.2 to 3.5 (1)
(In the formula (1), ln stands for logarithm natural.)
When the strain ratio ε2/ε1 is within the range defined by the formula (1) above, the permanent magnet 20 produced by extrusion becomes equal to or even superior to the permanent magnet produced by upsetting in terms of magnetic properties such as the residual magnetic flux density (Br), intrinsic coercive force (iHc) and maximum energy product ((BH)max). When the strain ratio ε2/ε1 is within the range of 0.4 to 1.6, the permanent magnet 20 is further improved in magnetic properties. Namely, when the strain ε1 imparted to the permanent magnet 20 by plastic working is closer to the strain ε2 in the Y-direction, the permanent magnet has a higher degree of magnetic anisotropy in the X-direction and better magnetic properties. Accordingly, the magnetic properties becomes highest when the strain ratio ε2/ε1 is 1. In the case that the strain ratio ε2/ε1 fails to fall within the range defined above, the magnet has only a low degree of magnetic anisotropy in the X-direction and fails to exhibit high magnetic properties.
A magnetic alloy containing 29.5% by mass of Nd, 5% by mass of Co, 0.9% by mass of B and 0.6% by mass of Ga, with the balance of being substantially Fe, was produced by melting and cooled rapidly by a single-roll method to produce a magnetic alloy strip having a thickness of 25 μm and an average crystal grain diameter of 0.1 μm or less. The strip was then crushed to prepare a magnetic powder having a particle length of 200 μm or less. The powder was cold compacted and the resultant compact was hot pressed at a temperature of 800° C. and a pressure of 200 MPa in an argon gas atmosphere to produce a preform 18 having a rectangular cross section with a thickness T of 36 mm, a width W of 19 mm and a length L of 25 mm. The preform 18 had an average crystal grain diameter of 0.1 μm. The ration of bulk density of the preform 18 to the real density ratio of the magnetic powder was 0.999. Experiment 1 was conducted to alter the strain ratio ε2/ε1 permanent magnet 20 produced by extruding the preform 18 having a fixed shape and thereby verify the effect of the strain ratio ε2/ε1.
Each preform 18 was extruded with an extrusion die 10 having a through hole 12, a tapered hole 14 and a uniformly sized through hole 16 designed to produce a permanent magnet 20 having a thickness T1 of 8 mm as extruded and having a strain ratio ε2/ε1 of 0.1 according to Comparative Example 1, a strain ratio ε2/ε1 of 0.2 according to Example 1 of the invention, a strain ratio ε2/ε1 of 0.4 according to Example 2 of the invention, a strain ratio ε2/ε1 of 0.8 according to Example 3 of the invention, a strain ratio ε2/ε1 of 1.0 according to Example 4 of the invention, a strain ratio ε2/ε1 l of 1.6 according to Example 5 of the invention, a strain ratio ε2/ε1 of 2.0 according to Example 6 of the invention, a strain ratio ε2/ε1 of 3.5 according to Example 7 of the invention, or a strain ratio ε2/ε1 of 4.0 according to Comparative Example 2. The permanent magnets were respectively magnetized under the same conditions and were each examined for the residual magnetic flux density (Br), intrinsic coercive force (iHc) and maximum energy product ((BH)max) in the X-direction. The results are shown in Table 1. Table 2 shows the dimensions of the preforms 18 and the permanent magnets 20 according to Examples 1 to 7 of the invention and Comparative Examples 1 and 2.
When each preform 18 was extruded, the preform and the extrusion die 10 had a temperature of 800° C. and the preform was extruded by employing an 80-ton hydraulic press. Referring more specifically to the examination of the magnetic properties of each of the permanent magnets 20 according to Examples 1 to 7 of the invention and Comparative Examples 1 and 2, a magnetic test specimen having a width of 8 mm, a length of 8 mm and a thickness of 8 mm was taken from the widthwise and lengthwise central portion of each magnet and magnetized in a magnetic field of 3.2 MA/m. Each test specimen brought to saturation magnetization was examined for the magnetic properties by a BH tracer. According to the measurement on the test specimen according to Example 4 of the invention, the crystal grains had a flat shape with the size of 0.1 μm on the average in the X-direction and 0.5 μm on the average in the Y-direction.
In Table 1, the magnetic properties of the permanent magnets 20 made as examples for reference by upsetting, rolling and forward extrusion and having the same maximum compression strain as that of the magnets according to Examples 1 to 7 of the invention (i.e. strain across their thickness) are also shown. The followings describe the conditions under which the magnets according to the examples for reference were produced and examined for their magnetic properties.
Referring to upsetting, a solid cylindrical preform 18 having a diameter D of 25 mm and a thickness T of 36 mm was compressed between two vertically spaced apart flat dies to form a permanent magnet 20 having a thickness T1 of 8 mm. When the preform 18 was subjected to upsetting, the preform and the two flat dies had a temperature of 800° C. and a 200-ton hydraulic press was employed. The permanent magnet 20 had a diameter D1 of 53 mm. However, since cracking in the free surface not contacting the dies was large, only about 50% of the entire permanent magnet was found to be sound. Accordingly, a magnetic test specimen having a width of 8 mm, a length of 8 mm and a thickness of 8 mm was taken from a sound central portion, magnetized in a magnetic field of 3.2 MA/m and examined for the magnetic properties by a BH tracer. The magnetic properties shown in Table 1 for the product produced by upsetting are those which were determined in the direction of the thickness in which the maximum compression strain had been produced, i.e. in the direction of the maximum magnetic anisotropy.
Referring now to rolling, a billet for rolling was prepared by placing a total of 100 pieces of preforms 18 in 10 lines widthwise and in 10 rows lengthwise, covering their whole surfaces with mild iron plates having a thickness of 10 mm and welding them together to enclose the preforms completely. The billet as described was employed to prevent any temperature drop at the time of rolling and any cracking of the free surfaces of products, while also realizing the simultaneous manufacture of a multiplicity of products. Each individual preform 18 had a thickness T of 36 mm, a width W of 19 mm and a length L of 25 mm. A 2000-ton reverse four-high mill was used to repeat 10 passes of rolling to obtain a permanent magnet thickness T1 of 8 mm excluding the mild iron portion. The billet had an initial temperature of 800° C., while the rolls were at the room temperature. The resulting 100 pieces of permanent magnets 20 showed different magnetic properties depending on their widthwise or lengthwise position and the best magnetic properties were of the permanent magnet 20 situated in the vicinity of the center widthwise and at the front end of the first pass lengthwise. The permanent magnet 20 in that position was examined for the magnetic properties. More specifically, a magnetic test specimen having a width of 8 mm, a length of 8 mm and a thickness of 8 mm was taken from the widthwise and lengthwise central portion of the permanent magnet 20, magnetized in a magnetic field of 3.2 MA/m and examined for the magnetic properties by a BH tracer. The magnetic properties shown in Table 1 for the product of rolling are also those which were determined in the direction of the thickness, i.e. in the direction of the maximum magnetic anisotropy.
Forward extrusion is a method commonly employed in the art of extrusion and usually featured by the same degree of size reduction both in the X- and Y-directions. A permanent magnet 20 having a thickness T1 of 8 mm, a width W1 of 8 mm and a length L1 of 506 mm was formed from a preform 18 having a thickness T of 36 mm, a width W of 36 mm and a length L of 25 mm. Details of the die except the dimensions thereof and the extrusion conditions were same as those employed in Experiment 1. A magnetic test specimen having a width of 8 mm, a length of 8 mm and a thickness of 8 mm was taken from the lengthwise central portion of the permanent magnet 20, magnetized in a magnetic field of 3.2 MA/m and examined for the magnetic properties by a BH tracer. The magnetic properties shown in Table 1 for the product of forward extrusion are those which were equally determined in the directions of the thickness and width in which the same maximum compression strain had been produced, i.e. in the direction of the maximum magnetic anisotropy.
A preform 18 having the same dimensions as in Experiment 1 was produced under the same conditions as in Experiment 1 by employing a magnetic alloy containing 26.8% by mass of Nd, 0.1% by mass of Pr, 3.6% by mass of Dy, 6% by mass of Co, 0.89% by mass of B and 0.57% by mass of Ga, with the balance of being substantially Fe. In Table 1, Example 8 of the invention shows the magnetic properties of a permanent magnet 20 which was produced by extruding the thus obtained preform 18 to have a thickness T1 of 8 mm as extruded and a strain ratio ε2/ε1 of 1.0 as those of Example 4. Table 2 shows the dimensions of the preform 18 and the permanent magnet 20 according to Example 8. The conditions for extrusion and the specific method employed for determining magnetic properties were the same as those employed in Experiment 1.
While Embodiment 1 has been described as the case in which a plate-shaped permanent magnet 20 is produced from a preform 18 having an oblong cross section, it is also possible to produce a plate-shaped permanent magnet 20 from a solid cylindrical preform 18 as shown in
A solid cylindrical preform 18 having a diameter D of 14.5 mm and a length L of 22.5 mm was produced under the same conditions as in Experiment 1 by employing a magnetic alloy of the same composition as that employed in Experiment 1. In Table 1, Example 9 of the invention shows the magnetic properties of a permanent magnet 20 which was produced by extruding the thus obtained solid cylindrical preform 18 to have a thickness T1 of 3 mm as extruded and a strain ratio ε2/ε1 of 1.0. Table 3 shows the dimensions of the preform 18 and the permanent magnet 20 according to Example 9. A magnetic test specimen having a width of 8 mm, a length of 8 mm and a thickness of 3 mm was taken from the widthwise and lengthwise central portion of the permanent magnet 20 according to Example 9 of the invention, magnetized in a magnetic field of 3.2 MA/m and examined for the magnetic properties by a BH tracer.
According to Embodiment 3, a permanent magnet 20 having an arcuate cross section with a thickness T1 in the X-direction, an outer arc length W1 in the Y-direction and an inner arc length W2 in the Y-direction is formed by extruding a preform 18 having an oblong cross section with a thickness T in the X-direction, a width W in the Y-direction and a length L in the Z-direction, as shown in
A preform 18 having a rectangular cross section with a thickness T of 24 mm, a width W of 23 mm and a length L of 25 mm was produced under the same conditions as in Experiment 1 by employing a magnetic alloy of the same composition as that employed in Experiment 1. In Table 1, Example 10 of the invention shows the magnetic properties of a permanent magnet 20 which was produced by extruding the thus obtained preform to have an arcuate cross section with a thickness T1 of 8 mm, an arc length ((W1+W2)/2) of 40 mm and an arc radius R1 of 40 mm and a strain ratio ε2/ε1 of 1.0. Table 4 shows the dimensions of the preform 18 and the permanent magnet 20 according to Example 10. A magnetic test specimen having a width of 8 mm, a length of 8 mm and a thickness of 7 mm obtained by removing a thickness of about 0.5 mm from each of its opposite arcuate surfaces was taken from the widthwise and lengthwise central portion of the permanent magnet 20 according to Example 10 of the invention, magnetized in a magnetic field of 3.2 MA/m and examined for its magnetic properties by a BH tracer.
According to the experimental results shown in Table 1, it is confirmed that the magnetic properties can be improved by controlling a strain ratio ε2/ε1 in the range of 0.2≦ε2/ε≦3.5 and further improved by controlling a strain ratio ε2/ε1 in the range of 0.4≦ε2/ε1≦1.6. It is also confirmed that the largest improvement in magnetic properties can be achieved by controlling a strain ratio ε2/ε1 approaching 1. The permanent magnets 20 according to Examples 1 to 10 of the invention were all good in appearance and none of them had any portion to be cut away, except a thickness of about 2 mm at each of the front and rear ends as viewed in the direction of its length. Furthermore, according to the penetrant and eddy-current flaw detection tests on each permanent magnet of the present invention, no surface or internal cracking was observed. Thus, it is confirmed that, according to the present invention, it is possible to produce a permanent magnet having high magnetic properties by extrusion which is excellent in terms of productivity, material yield, acceptable product ratio and manufacturing cost.
Modifications
The present invention is not restricted by the embodiments described above, and may be carried out in any other way as described below by way of examples.
1. A preform 18 having an oval cross section with a minor axis diameter D1, a major axis diameter D2 and a length L in the Z-direction as shown in
2. The preform and permanent magnet may be of any other shape in cross section than those described above, or of any other cross-sectional combination than those described above.
3. Although the tapered hole of the forming die according to Embodiment 1 has been described as having at its entrance a portion having along a certain length a cross section equal to that of the through hole, it is also possible to form a tapered hole having its taper connected directly to the adjacent end of the through hole.
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
The present application is based on Japanese Patent Application No. 2006-242146 filed on Sep. 6, 2006 and Japanese Patent Application No. 2007-176579 filed on Jul. 4, 2007, and the contents thereof are incorporated herein by reference.
Furthermore, all the documents cited herein are incorporated by reference in their entireties.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-242146 | Sep 2006 | JP | national |
| 2007-176579 | Jul 2007 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4051706 | Sakamoto et al. | Oct 1977 | A |
| 4623404 | Ibata et al. | Nov 1986 | A |
| 4648915 | Ibata et al. | Mar 1987 | A |
| 4653305 | Kanamaru et al. | Mar 1987 | A |
| 4892596 | Chatterjee | Jan 1990 | A |
| 4915891 | Chandhok et al. | Apr 1990 | A |
| 4960469 | Tanigawa et al. | Oct 1990 | A |
| 4985085 | Chatterjee | Jan 1991 | A |
| 5000796 | Chatterjee | Mar 1991 | A |
| 5289152 | Oguriyama et al. | Feb 1994 | A |
| 6639500 | Makino et al. | Oct 2003 | B2 |
| 7325434 | Esaki et al. | Feb 2008 | B2 |
| Number | Date | Country |
|---|---|---|
| 09-129463 | May 1997 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20080055031 A1 | Mar 2008 | US |
