[0001] This invention relates to bonded particle permanent magnets and to a method of making
them. In accordance with the invention, such magnets are readily fabricated into desired
shapes from melt-spun rare earth-iron alloy ribbons. These magnets have intrinsic
coercivities and energy products of the same order as samarium-cobalt magnets but
are much less costly. The bonded magnet compacts are magnetically isotropic. They
may be readily magnetized in any preferred direction in a suitable magnetic field.
Background
[0002] There has long been a need for relatively inexpensive but very strong permanent magnets.
Therefore, considerable work has been done on the development of alloys and processes
for making magnets of exceptional strength.
[0003] Before this invention, sintered or bonded samarium-cobalt (Sm-Co) powder magnets
have been used in applications where high magnetic remanence and coercivity are needed
in a shaped permanent magnet. However, such Sm-Co powder magnets are very expensive.
The high price is a function of both the cost of the metals and the cost of their
manufacture into magnets. Samarium is one of the least abundant rare earth elements,
while cobalt is a critical metal with unreliable worldwide availability.
[0004] Processing Sm-Co powder magnets involves many critical steps. One such step is grinding
alloy ingot into very fine powder. Ideally, each powder particle is a single crystal
that is inherently magnetically anisotropic. To obtain an oriented permanent magnet,
the anisotropic powder particles must be oriented in a magnetic field before the position
of each particle is fixed by sintering or bonding. After sintering or bonding, the
magnet must be finally magnetically aligned in the same direction in which the particles
were initially oriented to obtain optimum magnetic properties, That is, the magnets
are anisotropic. Sintered Sm-Co magnets may approach densities nearing 100% of alloy
density. For bonded Sm-Co magnets, however, it is difficult to obtain densities much
greater than about 75%. Conventional powder metal compaction equipment is not capable
of achieving higher packing densities because of the shape and hardness of the powder
particles.
[0005] This invention relates to high density, bonded, rare earth-transition metal magnets
with properties nearly rivalling bonded samarium cobalt magnets. However, these novel
magnets are based on the relatively common and inexpensive light rare earth elements,
neodymium and praseodymium; the transition metal element, iron; and boron. These alloys
and the method by which they are processed to achieve superior hard magnetic properties
are described in detail in co-pending European application number 83304909.1.
[0006] For use in this invention, the magnetic alloys are made by melt-spinning. Melt-spinning
is a process by which a molten stream of alloy is impinged on the perimeter of a rotating
quench wheel to produce rapidly quenched alloy ribbons. These ribbons are relatively
brittle and have a very finely crystalline microstructure. They-may be compacted and
bonded as will be described hereafter to create - novel, isotropic, high density,
high performance permanent magnets.
Brief Summary
[0007] In accordance with a preferred practice of the subject invention,isotropic, bonded
particle magnets are produced with compact densities of at least about 75% of the
constituent RE-Fe alloy density. Unexpectedly, the constituent alloy does not have
to be ground into a fine powder in order to obtain a magnet with high magnetic remanence.
Rather,melt-spun rare earth-iron ribbon is simply compacted in a powder metal die
in a suitable press.
[0008] At compaction pressures of about 1,103,162 kPa (160,000 psi), a compact with a density
of about .80% is achieved. The melt-spun ribbons fracture during compaction into brick-like
segments, each containing many randomly oriented crystallites. These segments. -pack
together very closely, promoting both high compact density and green strength. The
green compacts can be easily handled without damage. On the other hand, it has been
found that compacting spherical powder particles of like alloy will not yield a green
compact with any appreciable green strength. The compacts are so weak they cannot
be removed from a die without fracture.
[0009] A preferred alloy for use herein would be a melt-spun form of Nd
0.15(Fe
0.95B
0.05)
0.85 alloy having a suitable finely crystalline microstructure. The ribbon itself is magnetically
isotropic. It need not be magnetized before or during compaction.
[0010] After pressing, the ribbon particles of the green compact are coated with a binder
agent which may be later hardened to form a self-supporting, unmagnetized but magnetizable,
magnetically isotropic, composite body. The binder agent may be a hardenable resinous
substance such asian epoxy; a lower melting metal such as lead-tin solder; or any
other suitable organic or inorganic binder.
[0011] By practicing this invention, one can now make a magnetizable body of bonded melt-spun
alloy ribbons in almost any desired shape. The ribbon segments may be compacted to
high density in almost any conventional die press. Furthermore, the compacts are magnetically
isotropic. That is, they may be magnetized in any desired direction to achieve optimum
properties for a particular application.
[0012] For example, arcuate shaped field magnets for direct current motors could be formed
by compacting melt-spun rare earth-iron ribbon in a punch and die set. These arcuate
shaped bodies would first be magnetized after compaction in an applied magnetic field
in which the field lines radially intersect the compact to induce radially oriented,
remanent magnetization. In like manner, a bonded magnet of any other shape could be
magnetized in a magnetic field having field lines oriented in any desired direction.
[0013] The invention will be better understood in view of the Figures and detailed description
which follow.
Figures
[0014]
Figures l(a) to l(d) are schematic illustrations of the manufacture of a right circular
cylindrical shaped magnet in accordance with the invention.
Figure 2 is a second quadrant demagnetization plot for a bonded magnet made in accordance
with the invention compared to the demagnetization of an unbonded sample of melt-spun
ribbons of the same rare earth-iron alloy normalized to 100% density.
Figure 3 is a plot of compact density as a function of uniaxial compaction pressure
for a right circular cylindrical magnet body formed of melt-spun rare earth-iron ribbon.
Figure 4 is a plot comparing second quadrant demagnetization for oriented Sm2Co17 and SmCo5 bonded powder magnets and melt-spun bonded Nd-Fe-B powder magnets.
Figures 5 and 6 are scanning electron micrographs of cut and polished sections of
compacted and epoxy bonded magnets of melt-spun Nd-Fe-B alloy ribbon.
Detailed Description and Examples
[0015] In accordance with a preferred embodiment of the invention, iron, rare earth elements
and a small amount of boron are melted and rapidly quenched by the melt spinning process
to create relatively brittle alloy ribbons. These alloys have high inherent intrinsic
coercivities of the order of a kiloOersted or more, some higher than twenty kiloOersteds
and remanent magnetization of the order of 8 kiloGauss. Such high coercivities and
high remanent magnetism are believed to be due to the presence of a very finely crystalline
phase (atomic ordering less than about 500 nanometers) composed of iron and low atomic
weight rare earth elements (atomic No. less than or equal to 62) that do not have
full or exactly half full f-orbitals. The phase is stabilized by the presence of a
small amount of boron. European application No. 83304909.1 describes suitable compositions
and methods of making such and is incorporated herein by reference.
[0016] Preferred alloys contain from about 10 to 50 atomic percent neodymium, praseodymium;
or mischmetal comprised principally of these rare earth elements; a small amount of
boron (generally less than about 10 atomic percent); and the balance iron. Other rare
earth elements such as samarium and transition metal elements such as cobalt may be
incorporated in amounts that do not severely degrade the magnetic properties of the
melt-spun alloys. Other metals may be incorporated in small amounts which tend to
dilute but not destroy the magnetic properties of the preferred melt-spun RE-Fe alloys.
[0017] A preferred method of making the high coercivity alloys is to melt suitable amounts
of the elements together and then quench a stream of the alloy on the perimeter of
a spinning quench wheel to create a friable alloy ribbon with a very finely crystalline
microstructure. This process is referred to herein as melt-spinning.
[0018] Figure 1 is a schematic representation of a method for making bonded permanent magnets
in accordance with the invention. Referring to Figure l(a), the alloy 2 is melted
in a crucible 4 and ejected through a small orifice 6. The ejected stream of alloy
impinges on a rotating quench wheel 8 to form a ribbon 10 of solidified alloy with
a very.finely crystalline phase. Ribbon 10 is generally quite thin and very brittle.
It can be broken into pieces small enough to fit into a die cavity by almost any crushing
means. Melt-spun ribbons have been placed, for example, between two clean sheets of
paper and an ordinary wooden writing pencil has been rolled over the sandwiched material.
The resultant ribbon segments can be-poured directly into a die cavity. Ball-milling
or otherwise milling the ribbon in air creates smaller ribbon sections but does not
cause any detectable loss of magnetic properties or compactability in conventional
tooling. It has been noted, however, that some deterioration of magnetic properties
occurs when ribbons are ground for excessively long periods of time.
[0019] Figure l(b) shows a die for making a cylindrical compact 12. The compact is formed
between a pair of opposing punches 14 and 16 in tool 18. This process is referred
to herein as uniaxial compaction, the axis being parallel to the travel of the compaction
punches. It has been found that under ordinary conditions for making conventional
powder metal compacts of iron or other such metal powders, rare earth-iron compacts
of eighty percent density or greater can be made.-The compacting process apparently
tends to fracture the subject RE-Fe ribbon segments and to pack them together in a
manner such that the ribbon sections lie parallel and directly adjacent to each other
almost as the bricks in a brick wall are oriented with respect to one another. Each
ribbon segment is much larger than a single magnetic domain. It is magnetically isotropic
and is readily magnetized to a strong permanent magnet in an applied magnetic field.
[0020] As shown at Figure l(c), once a desired compact density is achieved, compact 12 is
removed from the press and placed in side-arm tube 20. A hardenable liquid resin 22
is retained in a syringe 24. Syringe needle 26 is inserted through stopper 28 and
a vacuum is drawn through the side arm of tube 20. Once tube 20 is evacuated, enough
resin 22 is dripped onto compact 12 to saturate the pores between particles. The resin
is then cured and any excess is machined away.
[0021] This bonded body 30 need not be magnetized when it is formed. Permanent magnetism
is induced in the bonded compact body 30 by exposing it to a magnetic field of suitable
direction and field strength. The field may be created by suitable magnetizing means
such as a magnetic induction coil 32. Coil 32 is activated to create a field represented
by flux lines 34. The flux lines 34 run parallel to the axis of the cylindrical bonded
body 30.
[0022] Clearly, in accordance with this invention, magnets can be formed in almost any shape
that is adaptable to formation by powder metal pressing techniques such as uniaxial
compaction in a rigid die or isostatic compaction in a flexible sleeve. A key advantage
of this method over the conventional methods of making particulate Sm-Co magnets is
that the compaction need not take place concurrently with magnetization. Nor do the
ribbons have to be ground to a size commensurate with single domain size. The rare
earth-iron alloy ribbon of this invention is isotropic and need not be magnetized
until after the bonded magnet is fully formed. This simplifies the magnet making process
and eliminates all the problems associated with grinding fine powders and handling
magnetized green compacts. Unexpectedly high remanent magnetizations of 7 kiloGauss
(at least 6 kiloGauss being desired) and energy products of 9 megaGauss Oersted or
more have been achieved.
[0023] How the quenched alloy particles are coated or impregnated to effect binding is not
critical to this invention. While the preferred practice, to date, employs hardenable
liquid epoxy binder resin, any other type of polymeric resin that does not interfere
with the magnetic properties of the rare earth-iron alloys would be suitable. In fact,
almost any type of organic or inorganic binder:may be used so long as it does not
adversely effect the magnetic properties of the alloys.
[0024] For example, a very thin layer of lead or other low melting metal could be sputtered
or sprayed on to melt-spun alloy ribbon before compacting. The compact could then
be heated to melt the lead and bond the particles. Another practice would be to blend
melt-spun RE-Fe ribbon fragments with a dry resin powder. After compaction, the resin
would be cured or melted at a suitable elevated temperature to bond the alloy particles.
[0025] It is only necessary to achieve adequate bonding strength to stabilize the motion
of the constituent alloy particles for whatever application in which the magnet body
is to be used. In some cases, a wax binder would be sufficient; in others, a relatively
rugged and highly adhesive binder such as an epoxy would be more advantageous.
[0026] Another clear advantage of the invention is that the direction of magnetization of
the bonded rare earth-iron body can be tailored to a desired application. The body
is first magnetized after it is shaped and the alloy particles are mechanically bonded
together. Thus, the unmagnetized body is simply placed in a magnetic field of desired
direction and adequate strength to establish its remanent magnetic direction and energy
product. The magnet bodies can be made and stored in an unmagnetized state and be
magnetized immediately before use. A preferred practice would be to install a bonded
compact in the device in which it will be used and only then magnetize it in situ.
[0027] The neodymium-iron alloys of the following examples were all made by melt spinning.
The melt spinning tube was made of quartz and measured about 102 mm (4 inches) long
and 12.7 nm (1/2 inth) in diameter. About 5 grams of premelted and solidified mixtures
of pure neodymium, iron and boron-metals were melt-spun during each run. The mixtures
were remelted in the quartz tube by means of an induction coil surrounding it. An
ejection pressure of about 34.47 kPa( 5 psi) was generated in the tube with argon
gas. The ejection orifice was round and about 500 microns in diameter. The orifice
was located about 3.18 mm to 6.35 mm(1/8 to 1/4 inches) from the chill surface of
the cooling,. disc. The disc was rotated at a constant revolution rate such that the-velocity
of a point on the perimeter of the disc was about 15 meters per second. The chill
disc was originally at room temperature and was not externally cooled. The resultant
melt spun ribbons were about 30-50 microns thick and about 1.5 millimeters wide. They
were brittle and, easily broken into small pieces. Melt spun ribbons processed in
this manner exhibited optimum magnetic properties for a given RE-Fe-B composition.
Example 1
[0028] A 15 gram sample of melt-spun Nd
0.2(Fe
0.95B
0.05)
0.8 ribbon was ground in an argon atmosphere in a vibrating mill (Shatterbox, Spex Industries).
The resultant powder was sieved to a particle size less than about 45 microns.
[0029] The powder was then placed in a rubber tube with an internal diameter of 8 mm. Rubber
plugs sized to be slidable within the tube were inserted in either end. Steel rams
were then inserted in either end of the tube. This assembly was placed in a pulsed
magnetizing coil having a field strength of 40 kOe. The field was pulsed, drawing
the rams together and causing the plugs to compress and lightly compact the powder
between them. If the powder particles were magnetically anisotropic, this pulsed pressing
step would physically orient them along their individual preferred magnetic axes.
[0030] The rams were removed from the tube and the excess rubber sleeve was trimmed away.
The plugged tube was then reinserted into a hydraulic press and compacted between
rams to a pressure of 1,1
03,16
2 k
Pa (160,000 pounds per square inch).
[0031] The resultant right circular cylindrical compact measured 8mm high and 8mm in diameter.
The compact could be handled without breaking. It was taken out of the rubber compaction
tube and placed in a side arm Pyrex test tube. The tube was evacuated with a mechanical
vacuum pump. A hypodermic needle attached to a syringe carrying liquid epoxy resin
was then inserted through the rubber stopper of the tube. The resin was dropped into
the tube to saturate the compact. The epoxy was a conventional commercially available
epoxy comprised of a diglycidyl ether of bisphenol-A diluted with butyl glycidyl ether
and cured with 2-ethyl-4-methyl-imidazole. The compact was removed and allowed to
cure overnight (approximately 16 hours) in air at 100°C.
[0032] It was magnetized in the direction of precompaction, i.e. parallel with the original
pulsed magnetic field, with a 40 kiloOersted pulsed magnetic field. This was the maximum
magnetic field available at the time. The field is believed to be too weak to reach
magnetic saturation of the RE-Fe-8 alleys. Therefore, stronger fields might produce
even stronger magnets. The room temperature demagnetization (second quadrant) plot
of the hysteresis curve of this bonded magnet composition is shown in Figure 2. Magnetic
measurements were made on a vibrating sample magnetometer, Princeton Applied Research
(PAR) Model 155, at a room temperature of about 25°C. The sample was a cube about
2 mm on a side machined from the cylindrical magnet to fix in the magnetometer sample
holder.
[0033] Figure 2 compares demagnetization curves for non-bonded powder of the same melt-spun
ribbon batch as those used for the compact, corrected to 100% density (i.e., density
of the alloy). The density of the alloy ribbon in the compact was 85% of the density
of the alloy itself as determined by standard density measurement in water. The bonded
magnet formed from' the 85% dense compact has a residual magnetic indication of 85%
of that of the unbonded melt-spun ribbon corrected to 100% density.
Example 2
[0034] An experiment was run to determine the difference between (1) a bonded magnet in
which the finely ground alloy (less than 45 micron) ribbon particles were concurrently
magnetically aligned and prepressed in a pulsed magnetic field, and (2) a bonded magnet
formed from unaligned ground alloy particles. Powder particles of the same size and
composition as the melt-spun ribbon of Example 1 were precompacted in a plugged rubber
sleeve in a hand press but without concurrent application of a magnetic field. The
excess rubber at the ends of the sleeves was trimmed away and reinserted in a tool
in the hydraulic press. The powder preform was finally compacted at a pressure of
about 1,103,162 kPa (160 kpsi). The resultant 8 mm thick compact was then fabricated
in every other respect identically to the pre-oriented magnet of Example 1. The demagnetization
curve for the unaligned bonded magnet was identical to that of the prealigned magnet
plotted in Figure 2.
[0035] This experiment illustrates the magnetically isotropic behavior of the melt-spun,
rapidly quenched alloy particles...The sieved powder included all particle fractions
smaller than 45 micron metres, with many particles smaller than one micrometer, to
align. If the smallest particles were near enough single domain size they would be
expected to align along the field lines during the alignment step of Example 1. When
so aligned and magnetized in the same direction, the resultant magnets should have
measurably higher residual induction and a more square hysteresis loop than unoriented
magnet counterparts if the method had achieved near domain size, magnetically anisotropic
alloy particles. Thus, while the very finely crystalline alloys may be made up of
very tiny crystallites which would be expected to have preferred axes of magnetic
alignment, apparently, they cannot be ground finely enough by ball milling to take
advantage of magnetic alignments during the pressing step. It is not believed that
using other state-of-the art milling techniques would provide different results so
far as the creation of near domain size, anisotropic particles from the subject melt-spun
alloys is concerned.
[0036] Another proof of the isotropic nature of the ribbon particles was made as follows.
The prepulsed and compacted bonded magnet sample (2 x 2 mm cube) of Example 1 was
demagnetized. The sample was then pulsed in a 40 kOe field in a direction transverse
to the original direction of magnetic alignment. The demagnetization curve for the
sample magnetized in the transverse direction was then taken. It was exactly the same
as the demagnetization curve taken for the original alignment direction (shown in
Figure 2). Because the demagnetization curves were the same for magnetization in the
direction of alignment during compaction and for demagnetization transverse thereto,
it must be concluded that there was no magnetic alignment of particles in the pulsed
precompaction. That is, the ground powders and bonded compacts are both magnetically
isotropic.
Example 3
[0037] A comparison was made between isostatically and uniaxially pressed magnets made from
unground Nd
0.2(Fe
0.95B
0.05)
0.8 alloy ribbon particles. The ribbons initially had a cross-section of approximately
2mm (width) by 30 microns (thickness). The alloy ribbon as melt-spun was easily fractured
into small pieces preparatory to compaction. The relationship of compact density to
uniaxially applied pressure for fractured Nd-Fe-B ribbon particles pressed in the
direction of the axis of a right circular cylindrical compact is shown in Figure 3.
The compaction curve becomes flatter above about 1,103,162 kPa (160,000 pounds per
square inch) at a density of approximately 83 percent (6.24-grams per cm
3) of the ribbon density (7.53 grams per cm
3).
[0038] Figures 5 and 6 are scanning electron micrographs of isostatically compacted, epoxy
bonded magnets made in accordance with this example. In the micrographs, the lighter
regions are Nd-Fe-B melt-spun ribbon while the dark regions are epoxy resin or voids.
The white line in the lower right-hand corner of each micrograph represents a length
of 100 micrometers. Both are plan views of a section of isostatically pressed melt-spun
ribbon that was not ground prior to compaction. The ribbon segments each contain many
crystallites.
[0039] It is clear from Figures 5 and 6 that the melt-spun ribbon fractures and compacts
in a manner such that individual ribbon segments line up with their long edges substantially
parallel to one another. The flat planes of the particles lie facing one another with
very little space therebetween. This probably accounts for the high compaction densities.
It has been found that, by disposing a sample in an elastic tube, stopping the ends,
and isostatically exerting a pressure of 1,103,162 kPa (160,000 pounds per square
inch), a compact density of 87% (6.55 grams per cm
3)is achieved. The arrangement of the relatively large ribbon segments also seems to
provide the high density compacts with good green strength. Thus with reasonable care
they can be handled prior to bonding without breaking or chipping.
[0040] Spherical powder particles of a like alloy do not compact well under like conditions.
The green compacts are so weak that they cannot be handled prior to bonding.
[0041] Figure 5 especially points out that there are several different regions of ribbon
segments oriented parallel to one another in each compact. For example, the particles
in the region labeled 50 are oriented at an acute angle with respect to the particles
in the region labelled 52.
[0042] Figure 6 shows an enlarged section of a compact where the close packing arrangement
of the ribbon segments to one another is clearly visible.
[0043] Thus, it has been unexpectedly found that melt-spun ribbons of rare earth-iron alloys
are relatively easy to compact to densities over 80 percent employing ordinary uniaxial
or isostatic pressing means. The compacts have very high green strengths. It has also
been found that there is.no apparent advantage in pre-milling the alloy compositions.
In fact,over-milling ribbon samples was found to adversely affect the magnetic properties
of the material, i.e., reduce the remanent magnetization and energy product of magnets
made from the over-milled materials.
[0044] It has also been found that the use of conventional die and powder metal lubricants
such as powdered boron nitride does not either adversely or positively affect the
compact. However, in practice such lubricants may be desirable to minimize die wear.
[0045] Figure 4 qualitatively compares the second quadrant hysteresis of the bonded Nd-Fe-B
magnets of the preceding examples with bonded and magnetically prealigned Sm
2Co
17 and (Sm, mischmetal) Co
5 magnets. Oriented Sm
2Co
17 magnets made from near domain size powder particles, magnetically aligned during
compaction, sintered, heat-treated and then finally magnetized exhibit the highest
remanent magnetization, B
r of approximately 11 kiloGauss. Sintered oriented Sm-Co
5 magnets (substantially 100% density) have a B
r of approximately 8.5 kiloGauss.
[0046] The unoriented Nd-Fe-B magnets of this invention fall about midway between the prealigned
and bonded Sm
2Co
17 type and the SmCo
5 type magnets. Our magnets are far superior to unaligned bonded Sm-Co magnets.
[0047] Oriented ferrite magnets have much lower remanent magnetization than the bonded magnets
of the present invention and Alnico magnets have much lower coercivities. Given the
tremendous cost and processing advantages of the magnets of the present invention,
the fact that they approach the magnetic strength of the best oriented rare earth-cobalt
magnets makes them highly commercially adaptable.
[0048] The strength of the magnets of the present invention is obviously a function of the
quality, i.e., the intrinsic magnetic properties of the constituent melt-spun rare
earth-iron alloy. Melt-spun alloys with higher coercivities and remanent magnetization
values would produce even stronger hard magnets than those disclosed herein.
[0049] In conclusion, novel bonded magnets have been produced from fractured and compacted
melt-spun rare earth-iron alloy ribbons. The.magnets are magnetically isotropic. They
do not have to be magnetically prealigned yet they have properties rivalling those
of much more expensive bonded samarium cobalt magnets.
[0050] The method of the present invention may be used to make cylindrical magnets; arcuate-shaped
magnets, irregularly shaped magnets, square magnets,and magnets of almost any shape
which can be formed by powder metal compaction methods. Never before has it been possible
to efficiently and inexpensively produce such high quality permanent magnets of such
varying shape from relatively inexpensive starting materials.
[0051] While the invention has been described in terms of specific embodiments thereof,
other forms may be readily adapted by one skilled in the art. Accordingly, the invention
is to be limited only by the following claims.