Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-10-19
2002-08-13
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C148S108000, C148S302000, C419S066000, C425S078000
Reexamination Certificate
active
06432158
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a compact (i.e., green compact) of rare earth alloy powder, a rare earth magnet, and a powder compacting machine. More particularly, the present invention relates to a powder pressing method for a rare earth magnet that has a form requiring multi-stage filling and compacting of rare earth alloy powder.
When magnetic powder is filled in a cavity of a powder compacting machine (a press machine) and simply compressed, the magnetic moments of powder particles are only randomly oriented. If a magnetic field is formed in the cavity and magnetic powder filled in the cavity is compressed in the magnetic field, a compact with powder particles aligned in a desired direction can be produced. If the compact is made of rare earth alloy powder excellent in magnetic properties, a high-performance anisotropic magnet can be manufactured from the compact.
FIG. 1
illustrates a typical compacting machine used for the case of orienting magnetic powder particle in a radial direction. The machine in
FIG. 1
includes a die
10
having a through hole, a magnetic core
12
having an outer circumference facing the inner wall of the through hole of the die
10
, a cylindrical lower punch
14
inserted into the through hole of the die
10
from below, and a cylindrical upper punch
16
inserted into the through hole of the die
10
from above. The magnetic core
12
is composed of an upper core
12
a
and a lower core
12
b
that fit in core holes of the upper punch
16
and the lower punch
14
, respectively. The upper core
12
a
and the lower core
12
b
are made of a ferromagnetic material, while the upper punch
16
and the lower punch
14
are made of a nonmagnetic material (e.g., core
12
).
The die
10
shown in
FIG. 1
has a layered structure composed of an upper portion made of a ferromagnetic material (magnetic portion
10
a
) and a lower portion made of a nonmagnetic material (nonmagnetic portion
10
b
). A cylindrical space is defined between the outer circumference of the core
12
and the inner wall of the magnetic portion
10
a
of the die
10
. The cylindrical space can be blocked with the upper punch
16
and the lower punch
14
on the top and bottom sides thereof, respectively. The outer circumference of the core
12
, the inner wall of the die
10
, and top end face of the lower punch
14
form a “cavity” into which powder is filled. Magnetic powder
24
filled in the cavity is sandwiched by the upper punch
16
and the lower punch
14
and thus compacted by compression. In this case, the cavity is defined by the top end face of the lower punch
14
, the outer circumference of the core
12
, and the inner wall of the magnetic portion
10
a
of the die
10
. A cylindrical sleeve
11
made of a nonmagnetic material may optionally be provided on the inner wall of the through hole of the die
10
to ensure that no step will be formed between the ferromagnetic portion and the nonmagnetic portion and that a compact will not be injured by such a step during removal from the die. In this case, the cavity is defined by the top end face of the lower punch
14
, the outer circumference of the core
12
, and the inner wall of the sleeve
11
.
An upper coil
20
and a lower coil
22
are provided for forming a radial magnetic field inside the cavity. A magnetic field generated by the upper coil
20
and a magnetic field generated by the lower coil
22
repel each other in and around the center portion of the magnetic core
12
, thereby forming a radial magnetic field that expands from the center portion of the core
12
radially toward the die
10
. The arrows in
FIG. 1
represent magnetic fluxes in the magnetic materials.
In order to improve the degree of alignment of magnetic powder in a compact to be produced, an intense radial magnetic field must be formed in the cavity. In order to increase the intensity of the radial magnetic field, it is desirable to increase electric power supplied to the coils
20
and
22
, as well as optimizing the size and material of the core
12
. However, increase in the electric power supplied to the coils will raise production cost and also cause a trouble of generating heat. Optimization of the size and material of the core is difficult because the core size is defined by the inner diameter of a magnet to be produced and improvement of the core material is limited.
In view of the above, when an axially elongated cylindrical magnet is to be manufactured, a multi-stage compacting process is employed where a powder filling step and a pressing step are repeated a plurality of times to ensure that an aligning magnetic field with a sufficient intensity is applied. In the multi-stage compacting process, when a long cylindrical compact is to be produced, a cycle of powder filling/compression in the magnetic field is repeated to sequentially produce axially divided portions of the compact. Accordingly, the cavity length per cycle is small and thus the intensity of the radial magnetic field formed in the cavity can be increased.
A conventional multi-stage compacting process will be described with reference to
FIGS. 1
,
2
A and
2
B.
First, as shown in
FIG. 1
, the magnetic powder
24
filled in the cavity is pressed in the presence of a magnetic field to produce a first-stage compact
26
(first-stage compression step). Thereafter, as show in
FIG. 2A
, magnetic powder
24
is filled in a cavity formed on the upper surface of the first-stage compact (denoted by
26
) and pressed in the presence of a magnetic field (second-stage compression step). In the second-stage compression step, the cavity is defined by the top surface of the first-stage compact
26
, the outer circumference of the core
12
, and the inner wall of the magnetic portion
10
a
of the die
10
. As show in
FIG. 2B
, by the second-stage compression step, a second-stage compact
28
is formed on the first-stage compact
26
. The two compacts are integrated to form a compact
30
.
By repeating the powder filling step and the compression step a plurality of times in the manner described above, an anisotropic ring magnet having a desired axial length can be manufactured beyond the limitation of the axial length L (see
FIG. 1
) of the magnetic portion
10
a
of the die
10
. This method for manufacturing an anisotropic ring magnet by multi-stage compacting is disclosed in Japanese Laid-Open Publication No. 9-233776, for example.
The anisotropic magnet manufactured by the above conventional method has the following problem. Disorder in alignment arises at the boundary of the first-stage compact
26
and the second-stage compact
28
, resulting in degradation in magnetization at the boundary.
FIG. 3
is a graph showing the surface magnetic flux density (Bg) at the outer circumference of a ring magnet (a cylindrical magnet) manufactured by the conventional multi-stage compacting method. The ring magnet manufactured and evaluated had an outer diameter of 16.4 mm, an inner diameter of 10.5 mm, and an axial length of 20 mm as measured after surface finishing. In the graph, the surface magnetic flux density (Bg) at the outer circumference of the magnet is shown by the solid line. The measurement was made using a gauss meter by scanning the surface of the magnet with a measuring probe. In the graph in
FIG. 3
, values in a region B correspond to values measured on the second-stage compact
28
, while a values in a region C correspond to values measured on the first-stage compact
26
.
FIG. 4
is a perspective view of the cylindrical magnet of
FIG. 3
, denoted by
32
. The left-hand side of the magnet
32
(corresponding to the compact
30
) in
FIG. 4
corresponds to the upper portion of the compacting machine (upstream portion with respect to the pressing direction).
As is apparent from the graph in
FIG. 3
, a large drop in surface magnetic flux density (Bg) is observed at the boundary of the first-stage and second-stage compacts
26
and
28
. Actually, the surface magnetic flux density (Bg) at the boundary is about 60% or
Harada Tsutomu
Morimoto Hitoshi
Tanaka Atsuo
Costellia Jeffrey L.
Mai Ngoclan
Sumitomo Special Metals Co. Ltd.
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