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 April 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:
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.
[0006] 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
[0007]
[Fig. 1] Figure 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.
[Fig. 2] Figure 2 is a schematic perspective view of an assembly described in a production
example and including a planetary carrier and planetary gears.
[Fig. 3] Figure 3 is a schematic side view of a planetary gear described in the production
example.
[Fig. 4] Figure 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]
[0008] 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.
[0009] 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.
[0010] 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]
[0011] 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
[0012]
- <1> A sintered body manufacturing method according to an embodiment comprises:
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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017]
<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.
[0018] 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.
[0019]
<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.
[0020] When the green compact is produced in the above pressure range, the green compact
obtained can have a high density and excellent machinability.
<4> In another mode of the sintered body manufacturing method according to the embodiment,
the machining step is performed using a cutting method.
[0021] 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.
<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.
[0022] 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.
<6> A sintered body according to another embodiment,
the sintered body composed of an iron-based material comprising:
an average relative density of the whole sintered body being 93% or more.
[0023] 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.
<7> In one mode of the sintered body according to this embodiment, the sintered body
is a helical gear.
[0024] 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.
<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° or more with respect to an axial
line of the helical gear.
[0025] 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° 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° or more.
Details of Embodiments of the Present Invention
[0026] 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>
<<Summary of sintered body manufacturing method>>
[0027] The sintered body manufacturing method according to the embodiment comprises the
following steps.
S1. Preparation step: A raw material powder containing an iron-based metal powder
is prepared.
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.
S3. Machining step: The green compact is machined to produce a machined compact.
S4. Sintering step: The machined compact is sintered to obtain a sintered body.
S5. Finishing step: Finish machining is performed so that the actual dimensions of
the sintered body are closer to its design dimensions.
[0028] These steps will be described in detail.
<<S1. Preparation step>>
[Metal powder]
[0029] 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 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.
[0030] The average particle diameter of the metal powder is, for example, from 20 µm to
200 µm inclusive and from 50 µm to 150 µm 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 µm or more, the flowability of the raw
material powder can be easily ensured. When the average particle diameter of the metal
powder is 200 µm 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]
[0031] 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.
[0032] 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>>
[0033] 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.
[0034] 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]
[0035] 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.
[0036] 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 µm × 600 µm = 300,000 µm
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>>
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
<<S4. Sintering step>>
[0041] 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°C to
1,400°C and from 1,200°C to 1,300°C inclusive. The sintering time is, for example,
from 15 minutes to 150 minutes inclusive and from 20 to 60 minutes inclusive.
[0042] 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>>
[0043] 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>>
[0044] 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.
[0045] 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 µm × 600 µm = 300,000 µm
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>
[0046] 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>>
[0047] 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
3.
[0048] 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.
- A cylindrical green compact for a planetary gear 2 diameter: 50 mm, height: 20 mm
- A disk-shaped green compact for the first member 31 diameter: 130 mm, height: 35 mm
[0049] A cylindrical green compact for the second member 32 diameter: 130 mm, height: 35
mm
[0050] 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 µm × 600 µm = 300,000 µm
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
3.
[0051] 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° 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. 1. 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.
[0052] 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.
[0053] 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 µm × 600
µm = 300,000 µm
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
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%.
[0054] 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.
<<Sample B: conventional sintered body manufacturing method>>
[0055] 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° or more. With the rotary
press, the available molding pressure was much lower than 600 MPa.
[0056] 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
3). In portions other than the teeth 20, the average relative density was about 89.7%
(average bulk density: 7.0 g/cm
3). The overall average relative density of sample B was about 89%.
[0057] 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>
[0058] 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
[0059]
- 1
- assembly
- 2
- planetary gear, 20 tooth
- 3
- planetary carrier
- 31
- first member, 31b boss portion
- 32
- second member, 32s disk portion, 32b bridge portion
- 100
- cutting tool
- 200
- green compact, 201 machining chips, 202 metal particle
- 300
- solidified metal body, 301 machining chips