Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Consolidation of powder prior to sintering
Reexamination Certificate
2001-02-15
2003-03-11
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Consolidation of powder prior to sintering
C425S078000
Reexamination Certificate
active
06531090
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a powder compact and a method for manufacturing a magnet, and also relates to a powder press used for compaction of powder and a method for driving the powder press. The present invention particularly relates to a compaction technique especially suitable for production of a compact having a shape in which the size measured in the pressing direction (direction in which uniaxial pressure is applied) is greater than the size in the direction perpendicular to the pressing direction (for example, a rod shape and a cylinder shape).
In the field of powder metallurgy, various methods have been employed for imparting a shape to powder. Among them, in particular, in the field of manufacture of sintered magnets, widely used is a method for compacting magnetic alloy powder (magnet powder) with a powder press.
A conventional method for producing a green compact of magnetic alloy powder will be described with reference to relevant drawings.
FIGS. 1A through 1C
are cross-sectional views schematically illustrating the operation of a powder press of a withdrawal type. The press includes: a die
2
having a through hole for formation of a cavity
1
: and an upper punch
3
and a lower punch
4
for compacting powder in the through hole. The press further includes an upper ram
5
and a lower ram
6
coupled to driving devices not shown. In the illustrated conventional example, the upper ram
5
is driven upward and downward together with the upper punch
3
, while the lower ram
6
is driven upward and downward together with the die
2
. The lower punch
4
is kept at a fixed position with respect to a main body
10
of the press.
Using the press having the above construction, a compact is conventionally produced in the following manner.
As shown in
FIG. 1A
, the top end portion of the lower punch
4
is located inside the through hole of the die
2
to define the cavity
1
. The cavity
1
is filled with material powder. Next, as shown in
FIG. 1B
, the upper punch
3
is lowered to allow an end portion thereof to be inserted in the through hole of the die
2
, so that the powder is compacted between the upper punch
3
and the lower punch
4
(uniaxial compaction). Thus, a green compact
7
of the filled powder is produced. Thereafter, as shown in
FIG. 1C
, a step of ejecting the compact
7
from the die
2
(“draw-out step” or “push-out step”) is performed. In the illustrated conventional example, the die
2
is lowered while the lower punch
4
and the compact
7
are kept unmoved, and the upper punch
3
is lifted.
The above operation will be described in detail with reference to
FIGS. 2A and 2B
.
In
FIG. 2A
, lines A and B represent positions of the upper punch
3
and the die
2
, respectively, and line C represents pressure P applied to the top end face of the compact
7
. The compact
7
receives pressure, not only from the upper punch
3
, but also from the lower punch
4
and the die
2
. Herein, however, for convenience, only the pressure applied from the upper punch
3
to the compact
7
is specifically called “compact pressure”, and the magnitude of the compact pressure is denoted by “P”. The pressure P in
FIG. 2A
refers to this “compact pressure”.
“S
1
”, “S
2
”, “S
3
”, and “S
4
” in
FIGS. 2A and 2B
respectively represent the step of compacting powder, the step of lifting the upper punch
3
at a minimal speed, the step of ejecting the compact
7
, and the step of lifting the upper punch
3
at a high speed. These steps will be described in order as follows.
In the step S
1
, the filled powder is compacted by applying a large pressure P
C
to the powder to form the compact
7
. The compaction in a narrow definition is completed in this step. The compact
7
is in the state of being pressed inside the die
2
. At time t
1
, the step S
2
is started where the upper punch
3
is lifted gradually at a minimal speed. With this gradual lift of the upper punch
3
, the compact
7
in the pressed state expands as an elastic body in the direction opposite to the pressing direction. Once the compact pressure P reaches P
H
(>0), the minimal-speed lift of the upper punch
3
is halted.
At time t
2
, the step S
3
of ejecting the compact
7
is started. During this step, the compact
7
is held between the upper punch
3
and the lower punch
4
, and the pressure P
H
of substantially a constant value is kept applied to the compact
7
from the upper and lower punches
3
and
4
.
At time t
3
, at which the compact
7
has been completely ejected from the die
2
, the step S
4
is started where the upper punch
3
is lifted at a high speed. With this highspeed lift of the upper punch
3
, the compact pressure P abruptly drops, and becomes zero when the upper punch
3
is detached from the top end face of the compact
7
.
The above compacting method is called a hold-down method (see “Powder Compaction and Processing—From Powder to Nearnet Shape” ed. by Japan Society for Technology of Plasticity and Japanese Laid-Open Patent Publication No. 6-81006), which has a feature that the compact
7
is ejected from the die
2
while a constant holding pressure (P
H
) is applied to the compact
7
from the upper punch
3
. This method can prevent “detaching fracture” of the compact
7
, a phenomenon that may be generated in the course of ejecting the compact
7
from the die
2
.
Hereinafter, the mechanism of generation of detaching fracture will be described with reference to
FIGS. 3A and 3B
.
FIG. 3A
schematically illustrates the state where the die
2
that has just started moving downward applies friction to the periphery of the compact
7
.
FIG. 3B
schematically illustrates the state where the top end portion of the compact
7
is exposed outside as the lowering of the die
2
proceeds. Since the pressed compact
7
is an elastic body, it tends to expand in the direction shown by arrow Q
1
as a pressure P
1
applied to the compact
7
from the upper punch
3
decreases (springback phenomenon). Once the pressure P
1
is removed, leaving the top end face of the compact
7
free, the compact
7
is forced to expand outward from the die
2
. At the same time, the periphery of the compact
7
receives strong friction from the die
2
. As a result, local strain occurs inside the compact
7
, forming a crack
8
. This crack
8
causes generation of detaching fracture.
To prevent generation of detaching fracture, in the hold-down method, application of a predetermined holding pressure P
H
to the compact
7
is continued until completion of the step S
3
of ejecting the compact
7
. This conventional hold-down method has been employed for compaction of high-hardness powder such as ceramic powder and intermetallic compound powder that are high in hardness, hard to develop plastic deformation, and poor in ductility, and has delivered sufficient effects.
However, the above conventional method has the following problem. When the pressed density of the compact is comparatively low as in the case of manufacturing an anisotropic rare earth magnet, buckling (collapse) of the compact tends to be generated. In the case of manufacturing an anisotropic rare earth magnet, powder is aligned in a magnetic field during compaction. In this case, a lubricant is added to the magnet powder, and also the compacting density is reduced by compacting the powder at a low pressure, to thereby improve the alignment of powder particles. In this case, since the compact strength is weakened, buckling may be generated in the resultant compact even with application of a comparatively small pressure.
In recent years, with expanding use of magnets, there arises the need for producing compacts having a shape elongated in the pressing direction (direction of movement of the punch). Herein, for convenience, the size of a compact measured in the pressing direction is called the “compact height”, and a typical size of the compact measured in the direction perpendicular to the pressing direction is called the “compact width” or “compact diamet
Costellia Jeffrey L.
Jenkins Daniel J.
Nixon & Peabody LLP
Sumitomo Special Metals Co. Ltd.
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