Note: Descriptions are shown in the official language in which they were submitted.
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D-8235C-3630
HOT PRESSED PERMANENT MAGNET
HAVING HIGH AND LOW COERCIVITY REGIONS
-
This invention relates to hot worked unitary
permanent magnets having at least two separate regions
of different magnetic alignment. More particularly,
this invention relates to iron, neodymium and/or
praseodymium, and boron containing permanent magnet
bodies that have been hot worked so as to contain
distinct regions of different alignment -- e.g., one of
relatively high apparent coercivity and one of rela-
tively high remanence.
Background
High energy product, high coercivity
permanent magnet compositions comprising, for example,
iron, neodymium and/or praseodymium, and boron and
methods of making them are disclosed in Canada Serial
No. 426,770 filed April 26, 1983, Canada Serial No.
436,006 filed September 2, 1983 and Canada Serial No.
453,220 filed May 1, 1984, all by John J. Croat and
assigned to the assignee of this application. An
illustrative composition, expressed in atomic
proportions~ is Ndo 13(Feo.gsBo.o5)o-87
composition containing a specific stable intermetallic
phase and that possesses high coercivity when ~ormed as
fine crystallites about 20 to 400 nanometers in largest
dimension.
Melts of suitable iron-light rare earth
metal-boron compositions can be very rapidly quenched,
such as by melt spinning, to produce a solid material,
e.g., a thin ribbon. When the rate of cooling has been
controlled to produce a suitable fine crystalline
microstructure (20 r.m to 400 nm), the material has
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excellent permanent magnet properties. On the other
hand, faster cooling (overquenching) produces a
material with smaller crystallites and lower
coercivity. However, as disclosed, such overquenched
material can be annealed to form the suitable crystal
size with the associated high coercivity and high
energy product.
An interesting and useful property of this
neodymium-iron boron composition (for example) is that
it is substantially magnetically isotropic. A fine
grain, melt spun ribbon can be broken up into flat
particles. The particles can be pressed in a die at
room temperature to form a unitary body of about 85% of
the material's density. Bonding agents can be employed
before or after the compaction. The making of such
bonded magnets is disclosed in Canada Serial No.
443,441 filed December 15, 1983 by Robert W. Lee and
John J. Croat and assigned to the assignee hereof. It
was surprising to find that such bonded magnets
displayed no preferred magnetic direction. Values of
intrinsic coercivity or maximum energy product were not
dependent upon the direction of the applied magnetic
field. There was no advantage in grinding the ribbon
to very fine particles and magnetically aligning the
particles before compaction.
Such magnetically isotropic materials are
very useful because they can be easily pressed (without
magnetic alignment) into bonded shapes. The shapes can
be magnetized in the most convenient direction.
The iron, neodymium, boron type compositions
have also been processed by hot pressing and hot
working so that at least a portion of the grains or
crystallites was physically aligned producing at least
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partial magnetic alignment. As disclosed in Canada
Serial No. 451,851 filed April 12, 1984 by Robert W.
Lee, such hot worked materials had a preferred
direction of magnetization. In one form of the
practice disclosed in that application, a molten
material containing, in terms of atomic proportions,
0-13( e0-95so.05)0.87 is cooled extremely rapidly, as
by melt spinning, to form a thin ribbon of solid
material that did not have permanent magnet properties.
The material was amorphous in microstructure. The
ribbon was broken into particles of convenient size for
a hot working operation. The particles were heated
under argon to about 700C or higher in a die and
pressed with punches in the die under pressure of at
least 10,000 psi. Such hot working, hereafter termed
hot pressing, consolidated the particles into a fully
dense body.
If the hot working is stopped at the point at
which the material is consolidated to full density, the
result is a slightly magnetically aligned magnet with
the easy magnetization direction parallel to the press
direction. The demagnetization curve (room tempera-
ture, second quadrant, 4~M versus H plot) of such a
densified magnet is like that of curve A in Figure 1.
When the fully dense compact is repressed
under like conditions of elevated temperature and
pressure in a larger die cavity, the compact undergoes
considerable plastic strain in the plane perpendicular
to the press direction. This second stage of hot
working is termed die upsetting, and it produces
substantial magnetic alignment with the easy direction
of magnetization transverse to the plastic strain
direction. The demagnetization curve for such a die
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upset body is like that of curve B in Figure 1.
Examination of the demagnetization curves of Figure 1
shows that the hot pressed magnet (curve A) and -the die
upset magnet (curve B) have substantially different
degrees of magnetic alignment. The hot pressed magnet
has relatively higher coercivity and lower remanence in
the press direction than the die upset magnet. The die
upset magnet has a higher maximum energy product, but
it is easier to demagnetize than the hot pressed body
of the same composition. There are applications for
magnets in which it is desirable to incorporate both
characteristics in a unitary magnet body.
It is an object of the present invention to
provide a method of hot working a permanent magnet body
to produce at least two regions spaced along a surface
dimension of the body and having different desired
magnetic alignments. Generally speaking, one of the
regions will have higher apparent coercivity but lower
remanence than the other region. We particularly
contemplate application of the method to rapidly cooled
compositions comprising iron, neodymium and/or
praseodymium and boron.
It is another object of our invention to
provide a unitary magnetic structure that has been
selectively hot worked such that it contains separate
regions of differing magnetic alignment. An example of
such a magnetic structure is an arcuate magnet for a
permanent magnet motor. It is contemplated that one or
both of the circumferential edges of the arc would be
worked so as to have relatively high coercivity and the
central portion of the arc would have a relatively
higher remanence. Again, we especially contemplate
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that the magnet would be of t,he above described
iron-light rare earth metal boron compositions.
Brief Summary
In accordance with a preferred embodiment of
our invention, these and other objects and advantages
are accomplished as follows.
The starting material is a rapidly quenched
composition comprising iron, neodymium and/or
praseodymium, and boron. An example of a suitable
composition is one consisting, in terms of atomic
0.13(FeO.gsBo.os)o 87- The starting
material is amorphous in microstructure or character-
ized by an extremely fine crystalline structure. It is
preferred to start with such an amorphous or fine
grained microstructure so that the hot working can be
carried out without loss of suitable coercivity in the
final product.
Particles of melt spun material are placed in
the cavity of an open-ended die between two opposing
punches. The particles are heated in the die to a
temperature, suitably about 700C or higher, and com-
pacted at a suitable pressure to form a fully densified
body. Split-ram punches may be employed, as will be
described, such that the consolidated body has a step-
wise variation in thickness over its cross sectionwithin the die. While still in the hot die, the
respective punch edges (of the split ram) may then be
brought into alignment and moved in concert to subject
a portion of the consolidated part to further hot
working. Such hot working strains the thick portion of
the irregular consolidated part differently than the
thin portion. The different strain over the cross
section produces two regions of generally different
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magnetic alignment. The region that is most highly
strained in the direction perpendicular to the press
direction is more highly aligned and has a demagneti-
zation curve like that of curve B in Figure 1. The
region that was not deformed (or less deformed) by the
second press operation has less magnetic alignment and
displays magnetic properties like that of curve A in
Figure 1.
This generally illustrates one practice of
hot working different regions of an iron, neodymium
and/or praseodymium, and boron composition such that a
unitary body is formed having regions intentionally
produced of different magnetic alignment. A typical
application for our practice is an arcuate motor magnet
in which the leading edge of the arcuate magnet (in a
motor that rotates in one direction only) is subjected
to a higher demagnetization force than the rest of the
arc. In such application the leading edge of the arc
would be processed so as to have high apparent coerciv-
ity (measured radially with respect to the arc) and therest of the arc would be hot worked so as to have
relatively high remanence and maximum energy product.
A better understanding of our invention will
be gained from a detailed description thereof as
follows. Reference will be had to the drawings in
which:
Figure 1 is a second quadrant, room
temperature, 4~M versus H plot of a hot pressed magnet
(curve A) and a die upset magnet (curve B);
Figures 2(a)-(c) are schematic drawings,
partly in section, of two different die sets showing a
sequence of die operations for forming a hot worked
arcuate magnet;
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Figures 3(a) and (b) are schematic repre-
sentations, partly in section, showing a split-ram die
in two different modes of operation;
Figures 4(a) and (b) are schematic cross
sections of a hot pressed compact and a die upset
permanent magnet processed in accordance with the
subject invention;
Figure 5 illustrates in cross section a hot
worked arcuate magnet containing adjacent regions of
different magnetic alignment in accordance with the
subject invention;
Figures 6(a) and (b) are schematic repre-
sentations illustrating the making of an arcuate magnet
like that depicted in Figure 5 in accordance with an
embodiment of the subject invention; and
Figure 7 illustrates yet another die forming
practice of forming a permanent magnet having at least
two regions of different magnetic alignment.
Detailed Description
Our invention is applicable to permanent
magnet compositions that can be magnetically aligned by
plastic deformation of the material at elevated
temperatures. An example of a family of preferred
compositions to which our method is applicable is the
transition metal-rare earth metal-boron materials
described in the above-identified patent applications.
Our practice is particularly applicable to compositions
in which the transition metal component is iron or iron
and (one or more of) cobalt, nickel, chromium or
manganese. Cobalt is interchangeable with iron up to
about 40 atomic percent. Chromium, manganese and
nickel are interchangeable in lower amounts, preferably
less than about 10 atomic percent. Zirconium and/or
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titanium in small amounts (up to about 2 atomic percent
of ~he iron) can be substituted for iron. Very small
amounts of carbon and silicon can be tolerated where
low carbon steel is the source of iron for the
composition. The composition preferably comprises
about 50 atomic percent to about 90 atomic percent
transition metal component -- largely iron.
The composition also comprises from about 10
atomic percent to about 50 atomic percent rare earth
component. Neodymium and/or praseodymium are the
essential rare earth constituents. As indicated, they
may be used interchangeably. Relatively small amounts
of other rare earth elements, such as samarium,
lanthanum, cerium, terbium and dysprosium, may be mixed
with neodymium and praseodymium without subs-tantial
loss of the desirable magnetic properties. Preferably,
they make up no more than about 40 atomic percent of
the rare earth component. It is expected that there
will be small amounts of impurity elements with the
rare earth component.
The overquenched composition contains about 1
to 10 atomic percent boron.
The overall composition may be expressed by
l-x l-y y x
component makes up 10 to 50 atomic percent of the
composition (x = 0.5 to 0.9), with at least 60 atomic
percent of the rare earth component being neodymium
and/or praseodymium. The transition metal (TM) as used
herein makes up about 50 to 90 atomic percent of the
overall composition, with iron representing at least
about 60 atomic percent of the transition metal
content. The other constituents, such as cobalt,
nickel, chromium and manganese, are called "transition
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metals" insofar as the above empirical formula is
concerned.
Boron is present in an amount of about 1 to
10 atomic percent (y = about 0.01 to 0.11) of the total
composition.
For convenience, the compositions have been
expressed in terms of atomic proportions. Obviously
these specifications can be readily converted to weight
proportions for preparing the composition mixtures.
For purposes of illustration, our invention
will be described using compositions of approximately
the following atomic proportions:
Ndo 13(Feo.gsBo.o5)u.87
However, it is to be understood that our method is
lS applicable to other compositions as described above.
Depending on the rate of cooling, molten transition
metal-rare earth-boron compositions can be solidified
to have microstructures ranging from:
(a) amorphous (glassy) and extremely fine
grained microstructures (e.g., less than
20 nanometers in largest dimension)
through
(b) very fine (micro) grained micro-
structures (e.g., 20 nm to about 400 nm)
to
(c) larger grained microstructures.
Thus far, large grained microstructure materials with
useful permanent magnet properties have not been
produced by rapid solidification from a melt. Fine
grain microstructures, where the grains have a maximum
dimension of about 20 to 400 nanometers, have useful
permanent magnet properties. Amorphous materials do
not. However, some of the glassy microstructure
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materials can be annealed to convert them to fine grain
permanent magnets having isotropic magnetic properties.
Our invention is particularly applicable to such over-
quenched, glassy materials. It is also applicable to
"as-quenched" high coercivity, fine grain materials
provided the materials are exposed only for short
times, e.g., less than five minutes, at high
temperatures, over 700C, during the hot working.
Suitable overquenched compositions can be
made by melt spinning. Melt spinning is described in
the above applications and will not be repeated here.
It is also practiced commercially to produce nonmag-
netic or soft magnetic alloys. We prefer to use melt
spun materials that have been cooled at a rate such
that an amorphous or extremely fine crystal structure
is produced. In the case of the iron-neodymium-boron
compositions, we prefer to start with a rapidly
solidified structure having a grain size smaller than
about 20 nanometers. We then heat and work the
material in a die at a temperature of about 700-750C
to consolidate particles of the material into a fully
densified mass and then to selectively deform the
consolidated material plastically to achieve regions of
different magnetic alignment. Such processing is
carried out fairly rapidly so that excessive grain
growth does not occur and the permanent magnet
characteristics lost.
Reference has already been made to Figure 1
which graphically depicts the demagnetization
properties of a hot pressed iron-neodymium-boron magnet
(curve A) and a die upset magnet (curve B) of the same
composition. The hot pressed magnet is only moderately
magnetically alignec and possesses a relatively high
~Z~432Z:
degree of coercivity in the press direction. The die
upset magnet has been signifcantly plastically
deformed. The material has attained a relatively high
degree of alignment. Its coercivity in the press
direction has thereby been decreased but its remanence
increased.
In our work we have observed that our hot
worked magnets display magnetic alignment in a
direction perpendicular to the direction of plastic
flow. When such plastic flow happens to be perpen-
dicular to the press direction (the direction of
movement of the punches), the magnetic alignment is
parallel to the press direction. When we measure
magnetic properties, e.g., coercivity and remanence, of
such anisotropic magnets, we detect and report the
values measured in the preferred (aligned) direction
unless otherwise stated.
Reference is made to Figure 2 to illustrate
the method of forming a die upset magnetically aligned
arcuate permanent magnet. Particles of melt spun
material are loaded into the cavity formed by open
ended die 10 and vertically aligned opposing punches 12
and 14 such as is shown in Figure 2(a). The die and
its contacts are heated by an induction heater (not
shown) to a temperature at or near 700C. The punches
12 and 14 compact the particulate material under a
pressure of, for example, 15,000 psi to form the
substantially fully densified body 16 depicted in the
die cavity in Figure 2(a). The arrows within the
outline of body 16 indicate that the densified compact
is substantially unaligned. However, there is a slight
alignment preference in the direction of compaction.
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The magnetic properties of this compact are like that
depicted in curve A of Figure 1.
The compact 16 is then transferred to a
larger die 18 with opposing punches 20 and 22 as shown
in Figure 2(b). The cavity is likewise heated to
maintain the compact 16 at a temperature at or near
700C. The compact 16 is then plastically deformed by
the punches 20 and 22 to form die upset arcuate body
24. The material flows laterally, but the direction of
alignment, and easy magnetization, is transverse to the
plastic flow, i.e., generally in the direction of
pressing as shown by the arrows in Figure 2(c). The
resulting arcuate magnet 24 is wider and thinner than
compact 16. Magnet 24 has a high degree of magnetic
alignment which is substantially uniformly radial with
respect to the center of curvature. Magnet 24 is shown
in section in Figure 2(c). In perspective view it
would appear like the magnet 36 shown in Figure 4(b).
Such arcuate magnet 24 could be produced by the
practice described in the above cited Lee application.
In accordance with our invention, we provide
an arcuate magnet or other permanent magnet structure
in which there are at least two regions in the unitary
body having different magnetic alignment. In one
embodiment of our invention, this is accomplished by
first making a densified hot pressed compact having
sections of different thickness. A convenient practice
is to form a compact having abrupt or stepped
differences in thickness. This can be accomplished by
using a split ram die as depicted in Figure 3. As
shown, such a die uses a conventional die body 25, but
both the upper 26 and lower 28 punches are split and
the two portions (26', 26" and 28', 28") of each punch
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13
can either move in concert as shown in Figure 3(a) or
move separately as shown in Figure 3(b). Such a split
punch or split ram die arrangement can be employed to
hot press and consolidate particulate melt spun
material to form a compact like that depicted in Figure
4(a).
The arcuate hot pressed compact 30 of Figure
4(a) is of substantially uniform density and random
magnetic alignment (or disalignment), but it has a
stepwise change in thickness. The perspective view
(Figure 4(a)) of arcuate compact 30 shows a relatively
thick portion 32 and adjacent thin section 34. The
compact 30 has a chord length L. It can be produced in
a split ram die in which the split punches are operated
as shown in Figure 3(b). The formation of a hot
pressed, densified compact 30 of such configuration
permits the making of a die upset arcuate of uniform
thickness but having regions of different magnetic
alignment. The compact of Figure 4(a) is pressed in a
wider section die cavity, heated to a temperature of
about 700C and hot worked into a longer (chord length
L' > L as seen in Figure 4) but thinner arcuate magnet
36 such as depicted in Figure 4(b). Since section 32
of the Figure 4(a) compact 30 was thicker than section
34, it undergoes more plastic deformation and flow.
Therefore, this portion 32 of compact 30 undergoes
considerable lateral strain to form arcuate region 38
of die upset magnet 36 in Figure 4(b). Thus, region 38
of the final arcuate magnet 36 is highly magnetically
aligned as illustrated in Figure 4(b). Conversely,
region 34 of compact 30 undergoes relatively little
deformation and thus region 40 of magnet 36 is
substantially unaligned. Region 40 has magnetization
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14
characteristics like that of curve A of Figure 1, and
region 38 has magnetic characteristics like that of
curve B. Thus, the right circumEerential edge portion
40, as seen in Figure 4(b), of the arcuate magnet 36
has a higher coercivity than the rest of the one-piece
magnet. This characteristic is particularly useful in
arcuate pole pieces of a DC motor where the
demagnetization forces act most severely on the leading
edge of the magnet.
Figure 5 is an end view of a two-region
magnet 42 illustrating a general principle of our
invention. One (or both) arcuate edge (region 46) of
magnet 42 has a magnetic orientation as schematically
illustrated by the direction of the arrows of the
Figure that are oriented at an angle theta (~ ~ 0) with
respect to the radial direction of the arc. The
remaining portion 44 of the magnet has been worked such
that it is magnetically oriented radially with respect
to the center of curvature, as shown by the arrows in
region 44. Thus, both regions 44 and 46 are highly
aligned and have relatively high remanence in the
alignment direction. However, edge region 46 is more
difficult to demagnetize by the reverse field generated
by a motor armature. Such two-part magnet is another
embodiment of our invention. A two-part magnet like
that depicted in Figure 5 can be produced by a practice
illustrated in Figures 6(a) and (b). A die upset
permanent magnet 48 of the warped configuration
depicted in Figure 6(a) is produced. A hot pressed
compact is first made and then die upset into the
Figure 6(a) configuration with strain occurring in the
direction indicated. Although the die upset magnet 48
is warped, the magnetic orientation is parallel across
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the entire end section. While still warm, the warped
body 48 is bent in a die 50 between opposed punches 52
and 54 into an arcuate permanent magnet like 42 in
Figure 5. The reversed bending of the warped starting
magnet produces an arcuate magnet (like 42) having
regions (like 44 and 46) of different magnetic
alignment as illustrated in Figure 5.
Figure 7 illustrates yet another die forming
practice of producing a two-part permanent magnet in
accordance with our invention. A hot-pressed uncurved
compact 56 of stepwise difference in thickness is first
produced. The compact 56 has a relatively thick
portion 58 and a thin portion 60. This is accomplished
in die 62 using split punches 64 and 66 operated in
stepped relationship. The punches are then slightly
withdrawn and employed in concert to produce thinning
of the thick portion 58 of the compact and thickening
of the thin portion 60 of the compact 56. The result
in this instance is a flat body 68 (dashed lines)
having regions of different magnetic alignment
resulting from regions of different plastic flow. In
Figure 7 the arrows depict strain direction rather than
magnetization direction. The latter would be perpen-
dicular to the strain as previously stated.
Thus in accordance with our invention, we
produce a unitary body of magnetic material that has
two or more regions of different magnetic alignment.
It is preferred that the regions be separated along a
surface direction rather than one region being
contained within another. This separation of regions
along a surface dimension is illustrated by regions 38
and 40 in arcuate magnet 36 and regions 44 and 46 in
arcuate magnet 42. In both cases the regions are
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separated in the circumferential direction of the arc.
The regions of different magnetic alignment are
produced by selectively hot working different parts of
the body of rnagnetic material in different ways.
Different portions of the body are caused to undergo
different degrees of strain at elevated temperature or
the strain is induced in different directions. This
can be accomplished, for example, by starting with a
densified compact of varying thickness and die
upsetting it to produce a product of uniform thickness.
In another embodiment, a body of magnetic material of
uniformly parallel alignment may be bent at elevated
temperature ~as illustrated in Figure 6) to produce our
two-part magnet structure.
lS Our invention can also be practiced by using
a previously hot pressed compact in combination with
particles of melt spun ribbon. By hot working the
compact and particles in different regions of the same
die, the compact may be die upset (for example3 and the
particles hot pressed to full density (or nearly full
density) with it to form a unitary body having regions
of different alignment. In this embodiment one may
employ different compositions for the initial compact
and for the added particles.
Our practice is particularly useful in making
transition metal-rare earth-boron magnets of the type
described above. However, it may also be utilized with
other magnetic compositions that can be magnetically
aligned by plastic deformation at a suitable elevated
temperature.
The terms "permanent magnet" or "hard magnet"
as used herein mean a material having a significant
16
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intrinsic coercivity at room temperature, e.g., greater
than 1000 oersted.
While our invention has been described in
terms of a few specific embodiments thereof, it will be
appreciated that other forms could readily be adapted
by one skilled in the art. Accordingly, the scope of
our invention is to be limited only by the following
claims.
