U.S. patent application number 15/750703 was filed with the patent office on 2018-08-23 for method for manufacturing sintered body and sintered body.
The applicant listed for this patent is Sumitomo Electric Industries, Ltd., SUMITOMO ELECTRIC SINTERED ALLOY, LTD.. Invention is credited to Tetsuya Hayashi, Tomoyuki Ishimine, Toshihiko Kaji, Terukazu Tokuoka.
Application Number | 20180236548 15/750703 |
Document ID | / |
Family ID | 60001295 |
Filed Date | 2018-08-23 |
United States Patent
Application |
20180236548 |
Kind Code |
A1 |
Ishimine; Tomoyuki ; et
al. |
August 23, 2018 |
METHOD FOR MANUFACTURING SINTERED BODY AND SINTERED BODY
Abstract
A sintered body manufacturing method includes: a preparation
step of preparing a raw material powder containing an iron-based
metal powder; a molding step of subjecting the raw material powder
to uniaxial pressing using a die to produce a green compact having
an overall average relative density of 93% or more; a machining
step of machining the green compact to produce a machined compact;
and a sintering step of sintering the machined compact to obtain a
sintered body.
Inventors: |
Ishimine; Tomoyuki;
(Itami-shi, JP) ; Hayashi; Tetsuya;
(Takahashi-shi, JP) ; Tokuoka; Terukazu;
(Takahashi-shi, JP) ; Kaji; Toshihiko; (Itami-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sumitomo Electric Industries, Ltd.
SUMITOMO ELECTRIC SINTERED ALLOY, LTD. |
Osaka-shi
Takahashi-shi |
|
JP
JP |
|
|
Family ID: |
60001295 |
Appl. No.: |
15/750703 |
Filed: |
April 4, 2017 |
PCT Filed: |
April 4, 2017 |
PCT NO: |
PCT/JP2017/014145 |
371 Date: |
February 6, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 1/0059 20130101;
B22F 2998/10 20130101; B22F 2999/00 20130101; B22F 2003/247
20130101; B22F 2009/0828 20130101; B22F 3/10 20130101; B22F 5/08
20130101; B22F 5/10 20130101; B22F 5/008 20130101; C22C 38/00
20130101; B22F 5/06 20130101; B22F 3/03 20130101; C22C 33/02
20130101; B22F 1/0007 20130101; B22F 5/085 20130101; B22F 2003/026
20130101; C22C 33/0264 20130101; B22F 2998/10 20130101; B22F
2009/0828 20130101; B22F 1/0059 20130101; B22F 3/02 20130101; B22F
2003/247 20130101; B22F 3/10 20130101; B22F 2999/00 20130101; B22F
2009/0824 20130101; B22F 2201/30 20130101; B22F 2999/00 20130101;
B22F 2009/0824 20130101; B22F 2201/05 20130101; B22F 2999/00
20130101; B22F 2009/0824 20130101; B22F 2201/01 20130101 |
International
Class: |
B22F 3/10 20060101
B22F003/10; B22F 3/03 20060101 B22F003/03; B22F 5/08 20060101
B22F005/08; C22C 33/02 20060101 C22C033/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2016 |
JP |
2016-077069 |
Claims
1. A method for manufacturing a sintered body, the method
comprising: a preparation step of preparing a raw material powder
containing an iron-based metal powder; a molding step of subjecting
the raw material powder to uniaxial pressing using a die to produce
a green compact having an overall average relative density of 93%
or more; a machining step of machining the green compact to produce
a machined compact; and a sintering step of sintering the machined
compact to obtain a sintered body.
2. The method for manufacturing a sintered body according to claim
1, wherein, in the machining step, the green compact is machined
into a helical gear shape.
3. The method for manufacturing a sintered body according to claim
1, wherein the uniaxial pressing is performed at a pressure of 600
MPa or higher.
4. The method for manufacturing a sintered body according to claim
1, wherein the machining step is performed using a cutting
method.
5. The method for manufacturing a sintered body according to claim
1, wherein the machining step is performed while compressive stress
is applied to the green compact in such a direction that tensile
stress acting on the green compact from a working tool is
counteracted.
6. A sintered body composed of an iron-based material comprising:
an average relative density of the whole sintered body being 93% or
more.
7. The sintered body according to claim 6, wherein the sintered
body is a helical gear.
8. The sintered body according to claim 7, wherein the helical gear
has teeth inclined 30.degree. or more with respect to an axial line
of the helical gear.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for manufacturing
a sintered body and to a sintered body.
[0002] The present application claims priority from Japanese Patent
Application No. 2016-077069 filed on Apr. 7, 2016, the entire
contents of which are incorporated herein by reference.
BACKGROUND ART
[0003] PTL 1 discloses a metallic member manufacturing method (a
sintered body manufacturing method) comprising: calcining a compact
prepared by pressure molding of a metal powder; machining the
calcined compact; and then subjecting the machined compact to main
firing. In the manufacturing method in PTL 1, the calcined compact
prepared by calcining the compact has higher mechanical strength
than the uncalcined compact, is less likely to chip during
machining, and is therefore easily machined. The calcined compact
has a lower hardness than the sintered body subjected to the main
firing and is therefore easily machined. Specifically, in the
manufacturing method in PTL 1, the green compact is calcined to
increase its mechanical strength, and then the calcined compact is
machined, so that chipping and cracking are less likely to occur
during the machining.
CITATION LIST
Patent Literature
[0004] PTL 1: Japanese Unexamined Patent Application Publication
No. 2007-77468
SUMMARY OF INVENTION
[0005] The sintered body manufacturing method of the present
disclosure comprises: [0006] a preparation step of preparing a raw
material powder containing an iron-based metal powder; [0007] a
molding step of subjecting the raw material powder to uniaxial
pressing using a die to produce a green compact having an overall
average relative density of 93% or more; [0008] a machining step of
machining the green compact to produce a machined compact; and
[0009] a sintering step of sintering the machined compact to obtain
a sintered body.
[0010] The sintered body of the present disclosure is an iron-based
sintered body having an overall average relative density of 93% or
more.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 shows schematic illustrations of machining with a
cutting tool, an upper illustration showing how a green compact is
machined with the cutting tool, a lower illustration showing how a
solidified metal body is machined with the cutting tool.
[0012] FIG. 2 is a schematic perspective view of an assembly
described in a production example and including a planetary carrier
and planetary gears.
[0013] FIG. 3 is a schematic side view of a planetary gear
described in the production example.
[0014] FIG. 4 shows the planetary carrier described in the
production example, an upper illustration being a schematic front
view, a lower illustration being an A-Across section of the upper
illustration.
DESCRIPTION OF EMBODIMENTS
Problems to be Solved by the Disclosure
[0015] In the metal member manufacturing method in PTL 1, since the
green compact is calcined, particles of the metal powder are
sintered to some extent. Although the hardness of the calcined
compact is lower than the hardness of the sintered body subjected
to the main firing, the calcined compact has a certain hardness.
Therefore, the technique in PTL 1 is susceptible to improvement in
machinability. Moreover, since the particles of the metal powder
are sintered during the calcination, machining chips must be melted
in order to reuse the machining chips.
[0016] In the metal member manufacturing method in PTL 1, pressure
molding, calcination, machining, and main firing are performed
sequentially, and the number of steps for obtaining the metal
member is large. Therefore, the technique in PTL 1 is susceptible
to improvement in metal member productivity.
[0017] One object of the present disclosure is to provide a
high-productivity sintered body manufacturing method in which an
unsintered green compact can be easily machined.
Advantageous Effects of the Disclosure
[0018] In the sintered body manufacturing method of the present
disclosure, the unsintered green compact can be easily machined,
and therefore the sintered body of the present disclosure can be
manufactured with high productivity.
Description of Embodiments of the Present Invention
[0019] <1> A sintered body manufacturing method according to
an embodiment comprises: [0020] a preparation step of preparing a
raw material powder containing an iron-based metal powder; [0021] a
molding step of subjecting the raw material powder to uniaxial
pressing using a die to produce a green compact having an overall
average relative density of 93% or more; [0022] a machining step of
machining the green compact to produce a machined compact; and
[0023] a sintering step of sintering the machined compact to obtain
a sintered body.
[0024] In the above sintered body manufacturing method, the green
compact is produced by uniaxial pressing using the die. In the
uniaxial pressing, the raw material powder can be molded under
application of very high contact pressure. Therefore, a green
compact having a high and uniform relative density with no brittle
portions present locally can be easily obtained. The green compact
obtained by uniaxial pressing is excellent in mechanical strength,
and chipping and cracking are less likely to occur during
machining. Specifically, since the green compact obtained by
uniaxial pressing can be subjected to the machining step without
calcination, the sintered body manufacturing method can produce the
sintered body with high productivity.
[0025] In the above sintered body manufacturing method, the green
compact produced has a uniform relative density of 93% or more.
Therefore, when the machined compact prepared by machining the
green compact is sintered, the change in the dimensions of the
machined compact is stabilized. Specifically, the degree of
contraction of the machined compact does not vary locally, and the
entire machined compact contracts substantially uniformly. This can
prevent the actual dimensions of the sintered body from deviating
largely from the design dimensions. Preferably, the relative
density is 95% or more.
[0026] In the above sintered body manufacturing method, since the
green compact is subjected to the machining step without sintering,
machining resistance during the machining step is low. Therefore,
the speed of machining can be about 5 to about 10 times faster than
that when a solidified metal body is machined, and the life of
tools used for the machining can be about 10 to about 100 times
longer. Since the machining resistance of the green compact is low,
the stiffness of cutting tools and shanks can be low, and long or
small-diameter cutting tools and shanks can be used for machining.
Since flexibility in selection of cutting tools and shanks is high
as described above, fewer constraints are imposed on the design of
the shape of the sintered body, i.e., its design flexibility is
high. For example, a finely machined sintered body such as a
hollowed sintered body can be produced.
[0027] In the above sintered body manufacturing method, the
machining chips generated during the machining can be reused
without melting the chips. This is because, since the green compact
is produced by cold pressure molding and is not sintered before
machining, the metal powder contained in the machining chips is not
altered.
[0028] <2> In one mode of the sintered body manufacturing
method according to the embodiment, the green compact is machined
into a helical gear shape in the machining step.
[0029] In the sintered body manufacturing method according to the
embodiment, since the green compact is machined before it is
sintered, the green compact can be easily machined into a complex
helical gear shape.
[0030] <3> In another mode of the sintered body manufacturing
method according to the embodiment, the uniaxial pressing is
performed at a pressure of 600 MPa or higher.
[0031] When the green compact is produced in the above pressure
range, the green compact obtained can have a high density and
excellent machinability.
[0032] <4> In another mode of the sintered body manufacturing
method according to the embodiment, the machining step is performed
using a cutting method.
[0033] The cutting may be performed using at least one working tool
such as a milling cutter, a hob, a broach, or a pinion cutter.
Since the green compact is excellent in machinability, the cutting
can be easily performed with high precision using any of the above
working tools.
[0034] <5> In another mode of the sintered body manufacturing
method according to the embodiment, the machining step is performed
while compressive stress is applied to the green compact in such a
direction that tensile stress acting on the green compact from a
working tool is counteracted.
[0035] When the machining is performed while the compressive stress
is applied to the green compact in such a direction that the
tensile stress acting on the green compact is counteracted, the
occurrence of chipping and cracking in the green compact can be
effectively prevented. Means for applying the compressive stress
will be exemplified in an embodiment described later.
[0036] <6> A sintered body according to another embodiment,
[0037] the sintered body composed of an iron-based material
comprising: [0038] an average relative density of the whole
sintered body being 93% or more.
[0039] The sintered body in this embodiment has an average relative
density of 93% or more and is a novel innovative sintered body.
Since the average relative density of the sintered body in the
embodiment is 93% or more, its mechanical strength compares
favorably with that of a machined product prepared from a
solidified metal body. The sintered body in this embodiment is
manufactured by the sintered body manufacturing method in the
preceding embodiment. Therefore, the sintered body can be
manufactured with higher productivity than a machined product
prepared from a solidified metal body. Preferably, the average
relative density is 95% or more.
[0040] <7> In one mode of the sintered body according to this
embodiment, the sintered body is a helical gear.
[0041] The sintered helical gear can be used as, for example, a
component of a transmission of an automobile. As described above,
the sintered body according to the embodiment has a mechanical
strength that compares favorably with that of a machined product
prepared from a solidified metal body. Therefore, the sintered body
sufficiently functions as a component of an automobile to which a
high load is applied.
[0042] <8> In one mode of the sintered body according to the
embodiment that has the helical gear shape, the helical gear has
teeth inclined 30.degree. or more with respect to an axial line of
the helical gear.
[0043] Since the above helical gear has excellent mechanical
strength, the teeth of the helical gear are less likely to be
damaged during use even when the teeth are inclined 30.degree. or
more with respect to the axial line. As the angle of the teeth with
respect to the axial line increases, the noise generated when the
helical gear is engaged with another gear is further reduced.
Preferably, the angle of the teeth with respect to the axial line
is 50.degree. or more.
Details of Embodiments of the Present Invention
[0044] A specific example of a sintered body manufacturing method
according to an embodiment of the present invention will be
described with reference to the drawings. However, the present
invention is not limited to this example. The present invention is
defined by the scope of the claims and is intended to include any
modifications within the scope and meaning equivalent to the scope
of the claims.
Embodiment 1
[0045] <<Summary of Sintered Body Manufacturing
Method>>
[0046] The sintered body manufacturing method according to the
embodiment comprises the following steps.
[0047] S1. Preparation step: A raw material powder containing an
iron-based metal powder is prepared.
[0048] S2. Molding step: The raw material powder is subjected to
uniaxial pressing using a die to produce a green compact having an
overall average relative density of 93% or more.
[0049] S3. Machining step: The green compact is machined to produce
a machined compact.
[0050] S4. Sintering step: The machined compact is sintered to
obtain a sintered body.
[0051] S5. Finishing step: Finish machining is performed so that
the actual dimensions of the sintered body are closer to its design
dimensions.
[0052] These steps will be described in detail.
<<S1. Preparation Step>>
[Metal Powder]
[0053] The metal powder is a main material forming the sintered
body, and examples of the metal powder include an iron powder and
an iron alloy powder composed mainly of iron. Typically, the metal
powder used is a pure iron powder or an iron alloy powder. The
"iron powder composed mainly of iron" means that the iron alloy
contains, as its component, elemental iron in an amount of more
than 50% by mass, preferably 80% by mass or more, and more
preferably 90% by mass or more. Examples of the iron alloy include
an alloy containing at least one alloying element selected from Cu,
Ni, Sn, Cr, Mo, Mn, and C. The above alloying elements contribute
to improvement in the mechanical properties of the iron-based
sintered body. Among the above alloying elements, Cu, Ni, Sn, Cr,
Mn, and Mo are contained in a total amount of from 0.5% by mass to
5.0% by mass inclusive and from 1.0% by mass to 3.0% by mass
inclusive. The content of C is from 0.2% by mass to 2.0% by mass
inclusive and from 0.4% by mass to 1.0% by mass inclusive. The
metal powder used may be an iron powder, and a powder of any of the
above alloying elements (an alloying powder) may be added to the
iron powder. In this case, the component of the metal powder in the
raw material powder is iron. However, the iron reacts with the
alloying element during sintering in the subsequent sintering step
and is thereby alloyed. In the raw material powder, the content of
the metal powder (including the alloying powder) is, for example,
90% by mass or more and is 95% by mass or more. The metal powder
used may be produced by, for example, a water atomization method, a
gas atomization method, a carbonyl method, or a reduction
method.
[0054] The average particle diameter of the metal powder is, for
example, from 20 .mu.m to 200 .mu.m inclusive and from 50 .mu.m to
150 .mu.m inclusive. When the average particle diameter of the
metal powder is within the above range, the metal powder is easy to
handle and is easily pressure-molded in the subsequent molding step
(S2). When the average particle diameter of the metal powder is 20
.mu.m or more, the flowability of the raw material powder can be
easily ensured. When the average particle diameter of the metal
powder is 200 .mu.m or less, a sintered body with a dense structure
can be easily obtained. The average particle diameter of the metal
powder is the average particle diameter of the particles included
in the metal powder and is a particle diameter (D50) at which a
cumulative volume in a volumetric particle size distribution
measured by a laser diffraction particle size distribution
measurement apparatus is 50%. The use of the fine-grain metal
powder allows the surface roughness of the sintered body to be
reduced and its corner edges to be sharpened.
[Others]
[0055] In press forming using a die, a raw material powder prepared
by mixing a metal powder and an internal lubricant is generally
used to prevent the metal powder from sticking to the die. However,
in this example, the raw material powder contains no internal
lubricant. When the raw material powder contains an internal
lubricant, the content of the internal lubricant is 0.2% by mass or
less based on the total mass of the raw material powder. This is
because a reduction in the ratio of the metal powder in the raw
material powder is prevented to obtain a green compact with a
relative density or 93% or more in the molding step described
later. However, the raw material powder is allowed to contain a
small amount of an internal lubricant so long as a green compact
with a relative density or 93% or more can be produced in the
subsequent molding step. The internal lubricant used can be a
metallic soap such as lithium stearate or zinc stearate.
[0056] To prevent the occurrence of chipping and cracking in the
green compact in the machining step described later, an organic
binder may be added to the raw material powder. Examples of the
organic binder include polyethylene, polypropylene, polyolefin,
polymethyl methacrylate, polystyrene, polyvinyl chloride,
polyvinylidene chloride, polyamide, polyester, polyether, polyvinyl
alcohol, vinyl acetate, paraffin, and various waxes. The organic
binder may be added as needed or may not be added. When the organic
binder is added, the amount of the organic binder added is such
that a green compact with a relative density or 93% or more can be
produced in the subsequent molding step.
<<S2. Molding Step>>
[0057] In the molding step, a die is used to uniaxial press the raw
material powder to thereby produce a green compact. The die used
for the uniaxial pressing includes a die block and a pair of
punches to be fitted into upper and lower openings of the die
block. The raw material powder filled into a cavity of the die
block is compressed by the upper and lower punches to thereby
produce a green compact. The green compact that can be formed using
this die has a simple shape. Examples of the simple shape include a
circular columnar shape, a circular tubular shape, a prismatic
columnar shape, and a prismatic tubular shape. A punch having a
projection or recess on its punching surface may be used. In this
case, a recess or projection corresponding to the projection or
recess of the punch is formed in the green compact having the
simple shape. The green compact having the simple shape is intended
to include such a green compact having a recess or projection.
[0058] The pressure (contact pressure) during the uniaxial pressing
may be 600 MPa or higher. By increasing the contact pressure, the
relative density of the green compact can be increased. The contact
pressure is preferably 1,000 MPa or higher. The contact pressure is
more preferably 1,500 MPa or higher. The upper limit of the contact
pressure is not particularly specified.
[External Lubricant]
[0059] In the uniaxial molding, it is preferable to apply an
external lubricant to inner circumferential surfaces of the die
(the inner circumferential surface of the die block and the
pressing surfaces of the punches) in order to prevent the metal
powder from sticking to the die. The external lubricant used may be
a metallic soap such as lithium stearate or zinc stearate.
Alternatively, the external lubricant used may be a fatty acid
amide such as lauric acid amide, stearic acid amide, or palmitic
acid amide or a higher fatty acid amide such as ethylene
bis-stearic acid amide.
[0060] The overall average relative density of the green compact
obtained by uniaxial pressing is 93% or more. The overall average
relative density of the green compact is preferably 95% or more,
more preferably 96% or more, and still more preferably 97% or more.
The overall average relative density of the green compact can be
determined as follows. Cross sections of the green compact that
intersect the direction of a pressing axis (preferably cross
sections perpendicular to the pressing axis direction) are taken at
a position near the center in the pressing axis direction, a
position near one end, and a position near the other end. Then the
cross sections are subjected to image analysis. More specifically,
first, images of a plurality of viewing fields are captured in each
cross section. For example, images of 10 or more viewing fields
having an area of 500 .mu.m.times.600 nm=300,000 nm.sup.2 are
captured in each cross section. Preferably, the images of the
viewing fields are captured in each cross section from positions
distributed uniformly as much as possible. Next, the captured image
of each viewing field is subjected to binarization processing to
determine the ratio of the area of the metal particles in the
viewing field, and the ratio of the area is regarded as the
relative density in the viewing field. Then the relative densities
determined in the viewing fields are averaged to compute the
overall average relative density of the green compact. The position
near one end (the other end) is, for example, a position within 3
mm from a surface of the green compact.
<<S3. Machining Step>>
[0061] In the machining step, after the green compact has been
produced by uniaxial pressing, the green compact is machined
without sintering. The machining is typically cutting, and a
cutting tool is used to machine the green compact into a prescribed
shape. Examples of the cutting include milling and lathe turning.
Examples of the milling include drilling. Examples of the cutting
tool used for drilling include a drill and a reamer, and examples
of the cutting tool used for milling include a milling cutter and
an end mill. Examples of the cutting tool used for lathe turning
include a turning tool and an indexable cutting insert. Moreover,
the cutting may be performed using a hob, a broach, a pinion
cutter, etc. A machining center that can automatically perform a
plurality of types of processing may be used for machining.
[0062] The concept of machining will be described with reference to
conceptual illustrations in FIG. 1. An upper illustration in FIG. 1
schematically shows how a green compact 200 is machined with a
cutting tool 100, and a lower illustration schematically shows how
a solidified metal body 300 is machined with the cutting tool 100.
As shown in the upper illustration in FIG. 1, in the green compact
200 formed by packing metal particles 202 under pressure, the green
compact 200 is machined such that the metal particles 202 are torn
off the surface of the green compact 200 by the cutting tool 100.
Therefore, machining chips 201 generated as a result of machining
are composed of metal powder of metal particles 202 separated from
the green compact 200. The powdery machining chips 201 can be
reused without melting. When clusters of aggregated metal particles
202 are present, the clusters may be pulverized as needed. As shown
in the lower illustration in FIG. 1, the solidified metal body 300
is machined such that the surface of the solidified metal body 300
is shaved off by the cutting tool 100. Machining chips 301
generated by machining are composed of elongated structures and
must be melted for reuse.
[0063] Before the machining, the surface of the green compact may
be coated or impregnated with a volatile or plastic solution
containing an organic binder dissolved therein in order to prevent
chipping and cracking from occurring in the surface layer of the
green compact during machining.
[0064] The green compact may be machined while compressive stress
is applied to the green compact in such a direction that the
tensile stress acting on the green compact is counteracted to
thereby prevent chipping and cracking from occurring in the green
compact. For example, when the green compact is broached to form a
machined hole, strong tensile stress acts on a portion near an
opening of the machined hole from which the broach protrudes when
it pierces the green compact. One method for applying the
compressive stress that counteracts the tensile stress to a green
compact is to stack a plurality of green compacts one on top of
another. It is preferable to dispose a dummy green compact, a plate
material, etc. below the lowermost green compact. When a plurality
of green compacts are stacked one on top of another, the lower
surface of an upper green compact is pressed against the upper
surface of a lower green compact, and compressive stress is thereby
applied to the lower surface. When broaching is performed on the
stacked green compacts from above, chipping and cracking can be
effectively prevented from occurring near the openings of the
machined hole which are formed on the lower surfaces of the green
compacts and from which the broach protrudes. When a machined
groove is formed in a green compact by milling, strong tensile
stress acts on a portion near an end of the machined groove. To
address this problem, a plurality of green compacts are arranged in
the moving direction of the milling cutter such that compressive
stress acts on portions corresponding to the ends of the
groove.
[0065] <<S4. Sintering Step>>
[0066] In the sintering step, the machined compact obtained by
machining the green compact is sintered. By sintering the green
compact, a sintered body in which the particles of the metal powder
are in contact with each other and bonded together is obtained. To
sinter the green compact, well-known conditions suitable for the
composition of the metal powder can be used. For example, when the
metal powder is an iron powder or an iron alloy powder, the
sintering temperature is, for example, from 1,100.degree. C. to
1,400.degree. C. and from 1,200.degree. C. to 1,300.degree. C.
inclusive. The sintering time is, for example, from 15 minutes to
150 minutes inclusive and from 20 to 60 minutes inclusive.
[0067] The degree of machining in the machining step may be
adjusted according to the difference between the actual dimensions
of the sintered body and its design dimensions. The machined
compact prepared by machining the high-density green compact with a
relative density or 93% or more contracts substantially uniformly
during sintering. Therefore, by adjusting the degree of machining
in the machining step according to the difference between the
actual dimensions after sintering and the design dimensions, the
actual dimensions of the sintered body can be very close to the
design dimensions. This allows time and effort in the subsequent
finish machining to be reduced. When a machining center is used for
the machining, the degree of machining can be easily adjusted.
<<S5. Finishing Step>>
[0068] In the finishing step, the surface of the sintered body is,
for example, polished. The surface roughness of the sintered body
is thereby reduced, and the dimensions of the sintered body are
adjusted to the design dimensions.
<<Outline of Sintered Body>>
[0069] With the sintered body manufacturing method described above,
a sintered body with an overall average relative density of 93% or
more can be obtained. The overall average relative density of the
sintered body is approximately the same as the overall average
relative density of the unsintered green compact. The overall
average relative density of the sintered body is preferably 95% or
more, more preferably 96% or more, and still more preferably 97% or
more. The larger the average relative density, the higher the
strength of the sintered body.
[0070] The overall average relative density of the sintered body
can be determined as follows. Cross sections of the sintered body
that intersect the pressing axis direction (preferably cross
sections perpendicular to the pressing axis direction) are taken at
a position near the center in the pressing axis direction, a
position near one end, and a position near the other end. Then the
cross sections are subjected to image analysis. More specifically,
first, images of a plurality of viewing fields are captured in each
cross section. For example, images of 10 or more viewing fields
having an area of 500 .mu.m.times.600 .mu.m=300,000 .mu.m.sup.2 are
captured in each cross section. Preferably, the images of the
viewing fields are captured in each cross section from positions
distributed uniformly as much as possible. Next, the captured image
of each viewing field is subjected to binarization processing to
determine the ratio of the area of the metal particles in the
viewing field, and the ratio of the area is regarded as the
relative density in the viewing field. Then the relative densities
determined in the viewing fields are averaged to compute the
overall average relative density of the green compact. The pressing
axis direction of the sintered body can be easily found by
observing the deformation state of the metal powder in the cross
sections of the sintered body because the sintered body has been
uniaxially pressed in its production process. The position near one
end (the other end) is, for example, a position within 3 mm from a
surface of the green compact.
Production Examples
[0071] In production examples, the sintered body manufacturing
method in the embodiment and a conventional sintered body
manufacturing method were used to produce assemblies 1 shown in
FIG. 2 and each including planetary gears 2 and a planetary carrier
3. Each planetary gear 2 is a helical gear having teeth 20
extending obliquely to an axial line as shown in FIG. 3 (see a
dash-dot line). As shown in FIGS. 2 and 4, the planetary carrier 3
includes a disk-shaped first member 31 and a second member 32
having three bridge portions 32b formed in its disk portion
32s.
<<Sample A: Sintered Body Manufacturing Method in
Embodiment>>
[0072] First, a raw material powder was prepared by mixing an Fe-2
mass % Ni-0.5 mass % Mo alloy powder with 0.3% by mass of C
(graphite) powder. The true density of the raw material powder was
about 7.8 g/cm.sup.3.
[0073] Next, the raw material powder was pressure-molded by
uniaxial pressing to produce the following three types of green
compacts. The molding pressure was 1,200 MPa for each of these
cases. [0074] A cylindrical green compact for a planetary gear 2
diameter: 50 mm, height: 20 mm [0075] A disk-shaped green compact
for the first member 31 diameter: 130 mm, height: 35 mm [0076] A
cylindrical green compact for the second member 32 diameter: 130
mm, height: 35 mm
[0077] The overall average relative densities of these three types
of green compacts were determined and found to be 93% or more. As
described in <<S2. Molding step>> above, the average
relative density of each green compact was determined as follows.
Cross sections of the green compact were taken at a position near
the center in the pressing axis direction and positions near the
opposite ends. Images of 10 or more viewing fields having an area
of 500 .mu.m.times.600 .mu.m=300,000 .mu.m.sup.2 were captured in
each cross section and subjected to image analysis. Specifically,
the average relative density of the green compact was about 96.2%.
The average relative density was converted to an average bulk
density, and the average bulk density of the green compact was 7.5
g/cm.sup.3.
[0078] Next, a commercial machining center was used to machine each
of the green compacts produced, and machined compacts having
desired shapes were thereby produced. The green compacts for the
planetary gears 2 were machined to form teeth 20 inclined
50.degree. with respect to their axial line. The green compact for
the first member 31 was machined to form a boss portion 31b by
shaving as shown in FIG. 2. Then a hole was formed at the center of
the boss portion 31b, and teeth of an internal gear were formed
inside the hole. The green compact for the second member 32 was
machined to form the bridge portions 32b by shaving. Then, as shown
in the lower illustration in FIG. 4, an inner circumferential
surface portion (a portion indicated by a black arrow) included in
a base portion of each bridge portion 32b and connected to the disk
portion 32s was formed into an R shape. When the inner
circumferential surface portion is formed into an R shape, the
strength of the bridge portions 32b can be improved. During the
machining of any of the above green compacts, no chipping and
cracking occurred in the green compacts. The machining chips
generated by machining were composed of metal powder of metal
particles separated from the green compacts.
[0079] Next, the machined compacts were sintered to produce the
planetary gears 2 and planetary carrier 3 composed of the sintered
bodies. During the sintering, no chipping and cracking occurred in
the sintered bodies. Finally, the planetary gears 2 and the
planetary carrier 3 were, for example, polished so that their
dimensions were closer to the design dimensions and their surface
roughness was reduced.
[0080] The average relative densities of the planetary gears 2 and
the planetary carrier 3 in sample A were determined and found to be
about 93% or more. As described in <<Sintered body>>
above, the average relative density of each of the planetary gears
2 and the planetary carrier 3 (sintered bodies) was determined as
flows. Cross sections were taken at a position near the center in
the pressing axis direction and positions near opposite ends.
Images of 10 or more viewing fields having an area of 500
.mu.m.times.600 .mu.m=300,000 .mu.m.sup.2 were subjected to image
analysis. Specifically, the average relative density of each of the
planetary gears 2 and the planetary carrier 3 was about 96.2%. The
average relative density was converted to an average bulk density,
and the average bulk density of each of the planetary gears 2 and
the planetary carrier 3 was 7.5 g/cm.sup.3. The viewing fields
captured in the cross sections include portions of the teeth 20 of
the planetary gears 2. The relative density of only these portions
was determined and found to be 96.2%.
[0081] The planetary gears 2 and the planetary carrier 3 in sample
A had mechanical strength comparable to that of planetary gears and
a planetary carrier formed from solidified metal bodies produced by
a melting method. It was therefore found that the planetary gears 2
and the planetary carrier 3 in sample A can be sufficiently used
for components of automobiles.
[0082] <<Sample B: Conventional Sintered Body Manufacturing
Method>>
[0083] The same raw material powder as sample A was prepared and
subjected to near net shape molding to produce green compacts
having a shape close to the shape of the planetary gears 2 and a
green compact having a shape close to the shape of the planetary
carrier 3. Since the planetary gears 2 are helical gears, a rotary
press was used for near net shape molding of the planetary gears 2.
With the rotary press, the inclination of the teeth 20 with respect
to the axial line cannot be 45.degree. or more. With the rotary
press, the available molding pressure was much lower than 600
MPa.
[0084] The near-net shaped green compacts were sintered and
subjected to finish machining to thereby produce planetary gears 2
and a planetary carrier 3 in sample B. For each of the planetary
gears 2 and the planetary carrier 3 in sample B, the relative
densities of viewing fields in cross sections were determined by
the same method as that for sample A. The relative densities were
different for different viewing fields. Specifically, in the teeth
20 of the planetary gear 2, the average relative density was about
88.5% (average bulk density: 6.9 g/cm.sup.3). In portions other
than the teeth 20, the average relative density was about 89.7%
(average bulk density: 7.0 g/cm.sup.3). The overall average
relative density of sample B was about 89%.
[0085] The mechanical strength of the planetary gears 2 and the
planetary carrier 3 in sample B was much worse than that of a
planetary gear and a planetary carrier formed from solidified metal
bodies produced by a melting method. In particular, since the
relative density of the teeth 20 of the planetary gear 2 to which
high stress is applied during use is low, the planetary gears 2 and
the planetary carrier 3 in sample B may be unsuitable for
components of automobiles.
<Applications>
[0086] The sintered body manufacturing method in the embodiment can
be preferably used to produce a sintered member having a
complicated shape that is difficult to produce only by pressure
molding using a die. The sintered body manufacturing method in the
embodiment can be used to produce, for example, sprockets, rotors,
gears, rings, flanges, pulleys, vanes, bearings, etc. used for
machines such as automobiles.
REFERENCE SIGNS LIST
[0087] 1 assembly [0088] 2 planetary gear, [0089] 20 tooth [0090] 3
planetary carrier [0091] 31 first member, [0092] 31b boss portion
[0093] 32 second member, [0094] 32s disk portion, [0095] 32b bridge
portion [0096] 100 cutting tool [0097] 200 green compact, [0098]
201 machining chips, [0099] 202 metal particle [0100] 300
solidified metal body, [0101] 301 machining chips
* * * * *