US3771221A

US3771221A - Method and apparatus for producing fine-particle permanent magnets

Info

Publication number: US3771221A
Application number: US00204581A
Authority: US
Inventors: K Bachmann; F Hofer; C Schuler; Ag Widen
Original assignee: BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1971-12-03
Filing date: 1971-12-03
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

Fine-particle permanent magnets are prepared by hydrostatically driving a pre-pressed member composed of powder of magnetically aligned particles through a die opening of diminishing crosssection, so as to cause compression of the pre-pressed member into a permanent magnet form wherein the hydrostatic force is supplied by a surrounding liquid medium under a compressive force.

Description

United States Patent Bachmann et al.

METHOD AND APPARATUS FOR PRODUCING FINE-PARTICLE PERMANENT MAGNETS Inventors: Kurt Bachmann, Nussbaumen,

Switzerland; Franz Hofer, Veldhoven, Netherlands; Claus Schuler, Widen AG, Switzerland Brown, Boveri & Company, Limited, Baden, Switzerland Filed: Dec. 3, 1971 Appl. No.: 204,581

Assignee:

US. Cl 29/608, 29/420.5, 72/60,

72/258, 264/24, 264/108, 264/111 Int. Cl H01f 7/06, HOlf 3/08 Field of Search 29/608, 420, 420.5;

References Cited UNITED STATES PATENTS 5/1972 Westendorp et a1 29/608 X Primary ExaminerCharles W. Lanham Assistant ExaminerCarl E. Hall Attorney-Norman F. Oblon et al..

[57] ABSTRACT Fine-particle permanent magnets are prepared by hydrostatically driving a pre-pressed member composed of powder of magnetically aligned particles through a die opening of diminishing cross-section, so as to cause compression of the pre-pressed member into a permanent magnet form wherein the hydrostatic force is supplied by a surrounding liquid medium under a compressive force.

15 Claims, 2 Drawing; Figures l 111 'll/111

SHEET

1 or 2 PATENTEDHBV 13 1975 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a method and apparatus for producing fine-particle permanent magnets, and more particularly to a method and apparatus for producing fine-particle permanent magnets from an intermetallic compound of a 3d transition element and a rare earth metal, without sintering.

2. Description of the Prior Art It is known to prepare fine-particle permanent magnets from intermetallic compounds of a 3d transition metal, such as the Group VIII metals including cobalt,

iron'or nickel, in combination with a rare earth metal of the Lanthanide series. By the term 3d transition element" is intended to mean a member of the first transition metal series. For instance, in US. Pat. 3,424,578, there is disclosed a fine-particle permanent magnet prepared from an intermetallic compound of the form RA wherein R is a rare earth of the Lanthanide series, such as Y, La, Ce, Pr, Nd or Sm, or mixtures thereof, and A is Co, Fe, Mn of mixtures thereof. Several different rare earth metals, for instance, may be incorporated into the same crystal lattice. Permanent magnets prepared from these types of ferro or ferrimagnetic compounds are known to possess extremely high remanence, high magnetic crystal anisotropy and very high coercive force, and hence, are considered to be quite desirable for many specialized purposes.

In the usual method for preparing fine-particle permanent magnets, an intermetallic compound is first produced by conventional foundry metallurgical techniques. This compound is then ground into a powder having a grain size of about several microns. The powder is aligned in a magnetic field and pressed into a unitary structure. This structure is then sintered at a temperature of from about l,O50 to l,l C. for a short period of time, in an inert atmosphere.

, One difficulty with this prior art technique, however, is the finding that sintering tends to deteriorate the magnetic properties of the intermetallic compound. It is nowbelieved that this deterioration is caused by the presence of a relatively large quantity of oxygen which has been adsorbed into the powder during the earlier processing procedures. Even when sintering is conducted in an inert atmosphere, therefore, the adsorbed oxygen is frequently present in sufficient quantities to effect reaction with the rare earth metal in the intermetalliccompound and thereby cause disintegration of the crystal structure of the powder and inactivation of the rare earth metal.

Another undesirable aspect of the prior art technique is that sintering is economically and technically an undesirable procedure, which has severely hindered the successful development of a continuous method for producing such permanent magnets.

Accordingly, a need exists for a method whereby fine-particle permanent magnets may be produced without the undesirable effects caused by conventional sintering steps.

SUMMARY OF THE INVENTION Accordingly, it is one object of this invention to form fine-particle permanent magnets whereby crystal structure disintegration of the powder and inactivation of the rare earth metal may be avoided.

It is another object of this invention to provide a method and apparatus for producing fine-particle permanent magnets from intermetallic compounds without deteriorating the magnetic properties of the intermetallic compound powder.

It is still a further object of this invention to provide fine-particle permanent magnets whereby the sintering procedure, required of prior art methods, can be avoided.

These and other objects, which will hereinafter become more apparent, have now been attained by hydrostatically driving a pre-pressed member composed of a powder of magnetically aligned particles through a die opening of diminishing cross-section, so as to cause compression of the pre-pressed member into a permanent magnet form, wherein the hydrostatic force is supplied by a surrounding liquid medium under a compressive force.

BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features and attendant advantages of this invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying Drawings, in which:

FIG. 1 is a diagrammatic cross-section of a device especially adapted for carrying out the method of this invention which is characterized by a large hydrostatic pressure-applying means for surrounding and driving a powder member through die opening of diminishing cross-section, and another hydrostatic means for surrounding and applying a counteracting pressure of lesser proportion to the further compacted powder form exiting from the die; and

FIG. 2 is a diagrammatic cross-section view of a container die positioned in a magnetic field and illustrates a magnetic powder being drawn therethrough for providing the compacted member of aligned magnetic particles shown in FIG. ll.

DETAILED DESCRIPTION OF THE INVENTION According to the invention, an intermetallic compound powder member is pre-pressed and the particles thereof are magnetically aligned and thereafter the prepressed, magnetically aligned powder member is cold worked by driving the same through a die opening of diminishing cross-section by a liquid, under pressure, acting on all sides of the powder member as the propelling force for driving the member through the die. This technique is somewaht analogous to hydrostatic extrusion, which has been used in the prior art for compacting various other types of materials. Heretofore, con ventional extrusion techniques, however, have never been considered for powders of intermetallic compounds, largely because of the known high brittleness of such compounds. To this effect, one of the important attributes of the present invention is the realization that when a highly pressurized liquid is caused to surround the compacted powder and is utilized as the propelling force for driving a powder member through a die, it acts with the deformation forces of the die to enable the brittle powder member to become ductile and deformable.

Any liquid which is inert in the system may be used herein, such as, for instance, any of a variety of mineral or petroleum oils.

The propelling pressure may advantageously be maintained in the system at between about 12 to kbar.

It is also desirable to provide a counteracting liquid pressure at the small diameter cross-section, or outlet of the die, which will completely surround and exert a lesser pressure on the compacted powder emerging from the die, in order to reduce still further the risk of cracking of the brittle powder during deformation. Good results are obtainable when the counteracting liquid is under a pressure of approximately one-half of the value of that of the first liquid.

It is desirable to use a conical die having an angle of inclination of less than, or equal to, approximately 60, so as to prevent the build-up of excessive deformation forces there-in.

In preparing the pre-pressed powder member used in this invention, an intermetallic compound, as described above, is produced by ordinary foundry metallurgy techniques. This compound is ground into a powder of several microns in size and is introduced into a highly ductile container. The container is completely filled with the powder and the entire container with its powder content is drawn through a first conical die. This expedient tends to lower the overall production costs by protecting the magnetic material from corrosive and mechanical attack, and by forming a very smooth solid surface which requires no subsequent machining.

Prior to entrance through the die, or at the deforming portion of the die, the powder within the container is subjected to a magnetic field of sufficient magnitude to cause alignment of the powder. The aligning field may extend parallel to the direction of the container movement. Compaction of the magnetic powder in the die, to a first degree, while simultaneously subjecting the powder to magnetic alignment in the drawing direction, will result in a well developed homogeneous alignment of the particles within the compacted powder.

The pre-pressed powder member is then subjected to the hydrostatic extrusion process of this invention. Using this technique, extremely high degrees of deformation are possible, even approaching 100 percent of the theoretical density for the particular material being treated. This degree of intensive deformation is quite advantageous in that it provides a high degree of homogenity throughout the product permanent magnet, so that applied mechanical stresses are distributed homogeneously throughout, and the very high resulting magnetic values are distributed homogeneously throughout.

The methods of this invention can be used to allow continuous production of fine-particle permanent magnets at a low cost, or can be used for quasi-continuous production of diverse dimension bar magnets, or the like, which can be provided to very close tolerances.

What is especially quite interesting is the fact that when using the methods of this invention, there is mini.- mal destruction of the magnetic orientation of the powder particles.

Referring now to the drawings, and more particularly to FIG. 1, wherein is shown a die member 4 having a continuous opening composed of a conical, tapered inlet portion 2 of decreasing cross-section, and an outlet portion 3, being substantially tubular in configuration. A pre-pressed powder member 1 is shown being driven through the die 4, whereby it is compressed to about percent of its theoretical density. The compacted powder member then emerges through the die outlet section 3.

The die 4 is disposed in one end of a high-pressure cylindrical vessel 9 which also contains a liquid medium 5, such as an oil, which surrounds the inlet portion 2 of the die 4. The liquid 5 may be maintained under a fairly high pressure of from about 12 to 20 kbar by means of a plunger 1 1 movably disposed in the other end of the high pressure vessel 9, being spaced at a distance in advance of the inlet portion 2 of the die 4, and thereby is effective to propel the powder member 1 through the die. A pressure equalizing valve 10 may be provided, if desired, in the liquid medium 5 in the cylindrical vessel 9 intermediate the plunger 11 and the die member 4.

A second substantially cylindrical vessel 12 is secured to the high pressure vessel 9, having a portion thereof disposed within the end of the high pressure vessel 9 opposite the plunger 11 for closing the vessel 9 and supporting the die 4 therein with the outlet portion 3 being open into the vessel 12. The vessel 12 is filled with a liquid medium 6, which may also be an oil, for receiving and completely enveloping the compacted powder member 1 exiting from the outlet 3 of the die 4. Preferably, the liquid 6 in the vessel 12 is maintained under a pressure substantially less than that applied to the liquid medium 5 in the vessel 9, for example, about one-half thereof, or approximately between 6 and 10 kbars, by a plunger 13 movably disposed in an opposite portion thereof, for providing a counter-propelling force on the powder member 1.

In the illustrated embodiment, the angle of inclination, designated by the reference numeral 8, of the conical inlet portion 2 of the die 4, preferably, is not more than 60.

Using the above-discussed technique, it has been possible to successfully produce fine-particle permanent magnets of the intermetallic compound SmCO Referring now to FIG. 2, first the powder member 1, shown being formed into a permanent magnet in FIG. 1 above, may first be precompressed and have the particles thereof magnetically aligned by placing the powder in a highly ductile container 14, and drawing the same through a die 15 disposed in a magnetic field. In one embodiment, the container 14 consists of a low carbon content stainless steel, such as 304 8.8., 304 L or 305 steel (AISI-SAE standard 18/8 steels of very low carbon content). The thickness of the container wall in this instance was 0.1 mm.

The entire container with the powder contents is drawn through the die, or drawing block 15. This die was suitably prepared from M3 or M4 grade tool steel (AISI-SAE standard). In this example, the container 14 is held and pulled through the die by a clamp 19.

To initiate the drawing operation, the container 14 is filled with the powder and then pulled at one end to form a point or tip, which is passed throughthe die block 15 to be engaged by clamp 19 and the remainder of the container is drawn through the die block 15 by moving clamp 19 away from block 15 as indicated by the arrow in FIG. 2.

In this example, the die entrance had a crosssectional diameter of mm and the die outlet had a cross-sectional diameter of 6-7 mm. The container, of course, had the same outer diameter after being drawn through the die block 15, as the outlet diameter of the die.

If desired, the container can be drawn repeatedly through a series of blocks of decreasing cross-sections to achieve very high degrees of compaction.

It is preferable to use a slow drawing speed, and particularly between 3 to 30 meters pre minute. For economical considerations, the draw length should be at least I meter, since the beginning portions and end portions are generally unuseable.

The drawing angle 16 of the conical portion of the die should not exceed about 7-9. Of course, the particular degree of the angle will depend upon the desired degree of deformation. The smaller the angle, the smaller will be the degree of reduction.

The die block 15 in FIG. 2 is shown as being surrounded by a magnetic coil 17 to which a current may be applied for inducing a magnetic flux which produces a magnetic field parallel to the direction of the draw and the movement of the container 14. The field extends from a point prior to the entrance of the container 14 into the die block 15 such that the particles of the powder are still in a relatively loosely packed state, and extends across the die block adjacent that portion of the die at which the reduction is discontinued. A magnetic shield 18 surrounds the coil 17 to define the extent of the container 14 exposed to the magnetic field. It has been found that when the powder particles are magnetically aligned during this period, they may be compacted by being drawn through the die block 15 while strictly maintaining their magnetic orientation.

Having now fully described the invention, it will be apparent to one or ordinary skill in the art, that many changes and modifications can be made thereto without departing from the spirit and scope of the invention.

What is claimed and desired to be secured by letters patent of the United States is:

1. A method of producing fine-particle permanent magnets from intermetallic compounds of a 3d transi- :Efiiiih'iifiififi eatiireietfient; which camprises the steps of:

pre-compressing and magnetically aligning the particles of a powder of said intermetallic compound; enveloping said pre-compressed and magnetically aligned particles at the inlet of a die of diminishing cross-section with an inert liquid, and

driving said aligned powder through said die of diminishing cross-section by means of said inert liquid so as to further compress said powder into a compacted form.

2. The method as set forth in claim 1 further comprising the step of maintaining said liquid at said inlet under a pressure of from about 10 to 20 kbar.

3. The method as set forth in claim 1 further comprising the step of providing a counter-acting pressure on the emerging compacted form at the outlet of said die by means of a surrounding inert liquid maintained under pressure.

4. The method as set forth in claim 3 further comprising the step of maintaining said inert liquid at said outlet of said die at a pressure of approximately one half of said pressure of said liquid at said inlet.

5. The method as set forth in claim 4 further comprising the step of reducing the diameter of said powder by substantially fifty percent as said powder is driven through said die.

6. A method of producing fine-particle permanent magnets from intermetallic compounds of a 3d transitional element and a rare earth element, which comprises the steps of:

encapsulating a powder of said intermetallic compound in a ductile container;

precompressing and magnetically aligning the particles of said powder within said container; enveloping said container at the inlet of a die of diminishing cross-section with an inert liquid, and

driving said container through said die of diminishing cross-section by means of said inert liquid so as to further compress said powder into a compacted form.

7. The method as set forth in claim 6 wherein said step of pre-compression is accomplished by drawing said ductile container through a die at the rate of about 3 to 30 meters per minute, and wherein s aid draw 8. An apparatus for producing fine-particle permanggt magnets from intermetallic compounds of a 3d 7 transitional element and a rare earth element, comprismg:

means for precompressing and magnetically aligning the particles of a powder of said intermetallic compound;

a die of diminishing cross-section having an inlet portion and an outlet portion;

means for surrounding said inlet portion of said die with an inert liquid; and

means for applying pressure to said liquid for driving said magnetically aligned powder through said die to further compress said powder into a compacted form.

9. An apparatus as set forth in claim 8, further comprising:

means exposed to said outlet portion of said die for providing liquid in surrounding relation with said compacted powder form exiting from said outlet portion; and

means for applying pressure to said means exposed to said outlet portion of said die.

10. An apparatus as set forth in claim 8, wherein the angle of inclination of said inlet portion of said die is not more than 60.

11. An apparatus as set forth in claim 8, wherein said means for pre-compressing said powder comprises:

a second die having an inlet portion of diminishing cross-section and an outlet portion;

means for drawing said powder through said die; and

means for establishing a magnetic field about at least said inlet portion of said die for aligning the particles of said powder in the direction through which said powder is drawn.

12. An apparatus as set forth in claim 11, wherein said magnetic field extends substantially from a position immediately prior to said inlet portion of said sec and die through said inlet portion.

13. An apparatus as set forth in claim 12, further comprising means for shielding said means for establishing a magnetic field.

14. An apparatus as set forth in claim 1 1, wherein the drawing angle of said inlet portion of said second die is less than approximately 9.

15. An apparatus as set forth in claim 8 further comprising a very low carbon content stainless steel container for encapsulating said powder.

Claims

1. A method of producing fine-particle permanent magnets from intermetallic compounds of a 3d transitional element and a rare earth element, which comprises the steps of: pre-compressing and magnetically aligning the particles of a powder of said intermetallic compound; enveloping said pre-compressed and magnetically aligned particles at the inlet of a die of diminishing cross-section with an inert liquid, and driving said aligned powder through said die of diminishing cross-section by means of said inert liquid so as to further compress said powder into a compacted form.

6. A method of producing fine-particle permanent magnets from intermetallic compounds of a 3d transitional element and a rare earth element, which comprises the steps of: encapsulating a powder of said intermetallic compound in a ductile container; precompressing and magnetically aligning the particles of said powder within said container; enveloping said container at the inlet of a die of diminishing cross-section with an inert liquid, and driving said container through said die of diminishing cross-section by means of said inert liquid so as to further compress said powder into a compacted form.

7. The method as set forth in claim 6 wherein said step of pre-compression is accomplished by drawing said ductile container through a die at the rate of about 3 to 30 meters per minute, and wherein said draw length is at least one meter.

8. An apparatus for producing fine-particle permanent magnets from intermetallic compounds of a 3d transitional element and a rare earth element, comprising: means for precompressing and magnetically aligning the particles of a powder of said intermetallic compound; a die of diminishing cross-section having an inlet portion and an outlet portion; means for surrounding said inlet portion of said die with an inert liquid; and means for applying pressure to said liquid for driving said magnetically aligned powder through said die to further compress said powder into a compacted form.

9. An apparatus as set forth in claim 8, further comprising: means exposed to said outlet portion of said die for providing liquid in surrounding relation with said compacted powder form exiting from said outlet portion; and means for applying pressure to said means exposed to said outlet portion of said die.

10. An apparatus as set forth in claim 8, wherein the angle of inclination of said inlet portion of said die is not more than 60*.

11. An apparatus as set forth in claim 8, wherein said means for pre-compressing said powder comprises: a second die having an inlet portion of diminishing cross-section and an outlet portion; means for drawing said powder through said die; and Means for establishing a magnetic field about at least said inlet portion of said die for aligning the particles of said powder in the direction through which said powder is drawn.

12. An apparatus as set forth in claim 11, wherein said magnetic field extends substantially from a position immediately prior to said inlet portion of said second die through said inlet portion.

14. An apparatus as set forth in claim 11, wherein the drawing angle of said inlet portion of said second die is less than approximately 9*.