CA1195814A - Method of improving magnets - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for improving the remanence of magnets products. The method comprises consolidating a particle charge of a magnet alloy to form a magnet product. The method includes applying a magnetic field to the particle charge to magnetically align the particles and then hot isostatically pressing the particles to consolidate them to full density.

Description

1 This ~pplication is a division~l of application serial number 39~,598 filed January 21, 1982.
It is conventional practice to produce magnets from powdered magnetic alloys, including rare earth cobalt magnets, by compacting as by die pressing a charge of aliyned or oriented fine powder of a magnetic alloy of the desired magnet composition. Thereafter, the compacted charge i5 heat treated at temperatures on the order of 2000 to 2090F. It i5 known that by increasing the density in .
the production of magnets of this type from particle charges of the magnetic material that remanence can be improved. Conventionally, density is increased by raising the sintering temperature after die pressing; howe-ver, this results in a corresponding lowering of coercive force.
It is accordingly an object of the present invention to provide a method for producing from powdered magnetic alloy magnets with increased density, and thus improved rP~-n~nce, without resorting to higher sintering temperatures that serve to lower coercive force.

Another ob~ect of the invention is in the production of magnets to provide for improved a-lignment or orientation to achieve higher remanence values.
These and other objects of the invention will be apparent from the following description and specific examples.
It has been determined broadly in accordance with the in-vention that improved remanence is a function of both the degree D5~
1 of alignmen-t of the individual magnetic dipoles (powder particles) and density (the number of dipoles that are present in a given volume of the body of the magnet material).
Accordingly, in the broader aspects of the invention it has been discovered that i~ one subjects a particle charge of magnet alloy, which may be one or more transition elements, e.g. nickel, cobalt, iron, chromium, manganese, copper, zirconium and titanium, in combination with a-t least one rare earth element, e.y. samarium, to a temperature that is below the full density sintering temperature but above the temperature necessary to produce a close-pore structure and then sub]ects the material while at this temperature to isostatic compacting, increased density and thus improved remanence is achieved while maintaining good coercive force.
Coercive force is maintained by mainkaining the temperature below the full density sintering temperature. Additionally, remanence is improved by aligning or orienting the material - by khe use of a pulsating magnetic field within a container.
~The container may be collapsible within which the material can thereafter be isostatically compacted. The pulsating ` magnetic field should have a pulse duration not exceeding one second per pulse and each pulse typically will be on the order of 15 millisecond. At least one pulse and pre-ferably two pulses at a power level of at least 50,000 Oe is suitable ~or the purpos~e. Conventionally, highly oxiented SmCo5 magnets have been produced by the use of superconducting solenoids to generate the high-intensity magnetic fields. These superconducting solenoids must be operated at cryogenic tem~
peratures ~-4500F~ to pass the high-density current necessary to generate these high-intensi~ magnetic fields. In the pxactice of the invention, however, the required high-in-density maynetic fields are produced by discharging an i~s~

1 assemblage oE capaci-tors, e.g. Eour hundred to one thousand capacitors, thexeby eliminating the need for superconducting solenoids. The container may be a rubber bag and pre-ferably after alignment the bag is evacuated in the presence of a constant DC field which serves to maintain alignment.
Alternately, the particles of magnet material may be aligned within a preformed containex, which will be collapsible and of a material such as stainless steel. The step of sub-jec~ing the aligned material to a steady DC field in an evacuated container h~s been found to "lock in" the alignment and thus insure improved remanence. The following constitutes specific examples with respect to the practice of the invention as described above and demonstrate its u~ility:

Example l: SmCo5 powder was oriented in a die cavity with an applied magnetic field and pressed. The applied field and the pressing direction being normal to each other. The pressed powder after sintexing and post sintering had the properties as set forth in Table I.
The sintered m~gnet was loosely wrapped with stainless steel foil (not pressure tight; for handling convenience only~ and as-hot-isostatically préssed (~IPed~ at 1750F.
The as-XIPed magnet had the properties as set forth in Table I.
The HIPed magnet was reheat treated at 1670 F for three hours and quenched. The magnetic properties after HIPing and heat treatment are set forth in Table I.

1 T~BI.E I
(r) (l~c) (Oe) BHmax Oe Pgm/Cm Die Pressed and Sintered 8,900 8,600 15,100 19.8 12,400 8~07 After HIP 9,400 9,100 11,500 22.1 10,000 8.53 Heat Treatment after HIP 9,400 9,200 >20,000 22.1 15,800 Example 2- Another magnet prepared according to the same procedures prescribed as in Example 1 had the properties set forth on Table II.
TABLE II
10Br Hc Hci BHmax Hk. p 3 G Oe Oe MGOe Oe gm/Cm Die Pressed and Sintered 9,000 8,400 14,000 19~0 9,400 8.07 After HIP 9/400 8,800 10,600 21.1 9,200 8.53 Example 3: Another magnet of SmCo5 from a batch other than in Examples 1 and 2 was prepared as described in Example 1. The properties are recorded in Table III.

TARLE III
Br Hc Hci BHmax Hk G Oe Oe MgOe Oe Die Pressed and Sintered 9,200 8,800 >20,000 21.0 9,400 After HIP 9,600 4,600 7,60013.2 2,200 .Heat Treatment after HIP 9,600 9,400 >20,00022.5 9,600 It may be seen from the magnetic property data that remanence is improved by hot isostatic pressing after con-ventional aligning and cold pressing. Further improvement is achieved with respect to coercive force when a~ter hot-isostatic pressing the magnet is subjected to post sintering heat treatment. The deterioration of the coercive force after ~IIPing is believed to be due to phase separation~

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1 Example 4: Usiny the powder from the same ba-tch as in Example 3, a magnet w~s made by sintering SmCo5 powder that was pre-viously oriented and cold isostatically pressed. The magnet had the properties set forth in Table IV.

TABI.E IV

Br Hc Hci BHmax Hk p 3 G Oe Oe MgOeOe gm/Cm Cold Isostatically Pressed and Sintered 9,700 9,00017,700 23.59,600 8.31 After HIP 9,800 6,000~,000 22.05,200 8.49 Heat Treatment Afte~ HIP 9/800 9,200>20,000 24.012,400 Wi~h the magnet alloy of Example 4 the theoretical r~ m density is 8.6 gm/Cm3. The specific magnet had a density of 8.31 gm/Cm before hot isostatic pressing and the aensity increase after hot isostatic pressing was only about 2%, which accoun-ts for small improvement in remanence xeported in the example. It is anticipated that if the theoretical r~; ml7m density had been achieved during hot lsostatic pressing about a 3% increase in remanence would result.

Example 5. SmCo5 alloy was loaded into a stainless container and hydrogen admitted into the container. The pressure was built up to 30 atmospheres; hydrogen absorption by the alloy results in a disintegration of the alloy to about -60 mesh powder. The dehydrided powder was jet milled to about 4 p particle size.
The fine powder was loaded into a rubber bag o~ 3/4"
diameter ana the bag was contained :in a stainless or plastic sheath. The bag was then pressurized and the powder oriented by 3~ placing the rubber bag along with the sheath inside a coil, and pulsing the coil, at least three times, with enough power to generate 60,000 Oe within the coil.

1 The oriented powder was then placed in a steady DC
field of ~10 kOe and the bag evacuat~d to lock the alignment.
The evacuated bag contain.ing the powder was then placed in an isostatic press chamber and compressed with a pressure up to 100,000 psi. The green compact was subsequently sintered between 1000 - 1200C and post sinter aged between 870 - 930C.
The magnets prepared from these four batches of powder in the manner described above had the properties set forth in Table V, which Table also shows magnetic properties of conventional commercial magnets.

Br Hc Hci BHm~xHk G Oe Oe MGOe Oe Batch #I10,000 9,600 11,800 24 8,600 Batch #II10,200 10,20018,000 26 14,8~0 Batch #III10~600 . 9,80017,000 28 10,400 Batch #IV10,200 9,600 15,300 25 10,800 Commercial) 8,800 8,600 15,000 19 10,000 Magnets )9,200 8,800 20,000 21 10,000

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Example 6: Powder.of SmCo5 was loaded in a rubber bag and orient~d in the poles of an electromagnet in a field of 25 kOe. The orientea powder was then evacuated maintaining the steady DC field. The evacuated bag containing the oriented powder was isostatically pressed followed by sintering and heat treatment. The magnet had the following properties shown in Table VI.

1 T~BI.E VI

Br Hc Hci Bllmax G Oe Oe MGOe Maynet Prepared Without a Pulsating d.c. Field 8,900 6,400 7,200 19.0 Example 7: A fourth batch of SmCo5 was processed into magnets by procedures as described in Example 1 except for a change in the compaction method. The powder contained in -the bag after ; alignment was initially compacted inside the bag by placing the ~ag towards the end of the coil and employing the field gradient present in the coil during pulsing to bring forth an initial compaction to an intermediate density by additional pulsing.
The oriented compacted powder placed in a steady DC field was evacuated, isostatically pressed and sintered. The sintered sample was of uniform diameter and had a 1at top and bottom contrary to the samples prepared without the field gradient packing which had a pyramidal top. The magnetic properties of the sintered magnet prepared as per this example are shown in Table VII.
; TABLE VII

Br Hc Hci ~Hmax Hk G Oe Oe MGOe Oe 10 000 9,800 ~20,000 25.0 13,000 Example 8: A rectangular preform which has the ~;m~n~ions of a die cavity was loaded with powder and the powder was oriented in a pulse coil. The oriented powder in the preform was transferred to a die press and placed between the uppex and lower punches. After all the powder has transferred into the die cavity the powder was pressed between the upper and lower punches under the application of a DC field. The die pressed part was sintered and post sintered~ The magnet prepared in this manner had the following properties set forth in Table VII~.

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1 Example 9: From the same batch of powder one magnet was pressed by dlrectly feediny the powder into the die cavity, applyiny the DC field, and pressing. The properties of these two magnets are set forth in Table VIII.
TABLE VIII

Br Hc Hc~ max Hk G Oe Oe MGOe Oe Premagnetized powder in a preform before : transferring into the die cavity 9,7508,700 ~>10,000 ~3.7 8,800 Dixect location of powder in the die cavity 8,8007,400 ~10,000 : 19.0 6,000 It may be seen from the data reported in Examples 5 through 9 that aligning by the use of a pulsating magnetic field in accordance with the p~actice of the in~ention, as opposed to the conventional practice of aligning by the use of a steady-state magnetic field, resulted in improvement in r~m~n~nce and energy product.

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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for improving the remanence of magnets products by consolidating a particle charge of a transition metal-rare earth magnet alloy to form a magnet particle, said method comprising applying a magnetic field to said particle charge to magnetically align said particles, and thereafter hot isostatically pressing said particles to consolidate the same to full density.

2. The method of claim 1 wherein prior to hot isostatic pressing said particle charge is heated to a temperature below the full density sintering temperature thereof but above the temperature necessary to render the particle charge substantially gas-impervious.

3. The method of claim 1 wherein said alloy contains cobalt as at least one transition element.

4. The method of claim 1 wherein said alloy contains as at least one rare earth element samarium.

5. The method of claim 1 wherein the aligned charge is given preliminary room temperature densification by die pressing or cold isostatic compaction before heating or final densification.