BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a method of modifying powder materials used for
manufacture of machine parts by powder metallurgy, and more particularly relates to
an iron-based powder mixture for powder metallurgy and a manufacturing method thereof,
producing an improved powder mixture having stabilized apparent density, less segregation
of additives, and superior fluidity characteristics when discharged from a hopper;
which method requires less force when ejected from a die in a compacting process;
and which material contains no zinc or small amounts of zinc at most.
Description of the Related Art
[0002] Conventional powder materials used for machine parts have been mixed powders in which
the alloying powder of the components for the improvement of solid-state properties,
such as copper, nickel, graphite, and phosphorus, was mixed into an iron powder. A
lubricant such as zinc stearate was also mixed into the powder to reduce abrasion
resistance during compressed molding. However, these powder mixtures tended to experience
powder segregation, which readily occurred during transport after mixing, loading
and unloading to and from a hopper, or during molding, because the powder mixture
contained powders of different sizes, shapes, and densities.
[0003] This segregation caused fluctuations in product composition, which increased fluctuation
of dimensional changes and strength, and thus produced defective products. Furthermore,
graphite and the like, due to their properties as impalpable powders, enlarge the
specific surface area of the powder mixture, thus impairing fluidity. This impairment
lowers the injection speed to the die, which also reduces the production speed of
the green compact. Technology for preventing segregation of these powder mixtures
is disclosed in Japanese Patent Laid-Open No.56-136901 or No. 58-28231, in which a
binder is used for preventing segregation. However, the more the amount of binder
that is to improve segregation of the powder mixture, the lower the fluidity of the
powder mixture.
[0004] A powder in which graphite was adhered to the surface of the iron-based powder with
a binder of zinc stearate was disclosed in the Japanese Patent Laid-Open No. 1-219101.
Also, we have proposed a method employing a metal soap and a fatty acid as a binder
in Japanese Patent Laid-Open No. 3-162502. However, all of the above mentioned methods
included zinc and other metallic elements in the binders, which caused a major problem
since metallic elements in the binders, as oxides, contaminated the inside of the
furnace, or varied the composition of the sintered body during sintering.
[0005] To overcome these problems, some methods employ binders having no metallic elements,
as disclosed in Japanese Patent Publication No. 60-502158 and Japanese Patent Laid-Open
No. 2-217403, wherein the binders themselves do not have a lubricating function, and
thus zinc stearate was added as the lubricant in the end. Therefore, as described
before, zinc in the lubricant contaminated the inside of the furnace as an oxide or
varied the composition of the sintered body.
SUMMARY OF THE INVENTION
[0006] Accordingly, an object of the present invention is to provide an iron-based powder
mixture for powder metallurgy, and to provide a manufacturing method thereof, wherein
the powder mixture has a stabilized apparent density, less segregation, superior characteristics
of fluid flow from a hopper, and no zinc or a small amount at most, while maintaining
the powder characteristics and the green compact characteristics of the mixture.
[0007] Notwithstanding the above problems, we have created a successful iron-based powder
mixture for powder metallurgy which has overcome conventional drawbacks, and which
can be produced inexpensively in large quantities. In particular, the mixture may
be produced by adhering the alloying powder and the powder for improving machinability
to the surface of the iron-based powder as a binder which is made of a melted powder
mixture of at least one powder of an organic compound selected from a low melting
point group having a melting point of about 69-103°C and consisting of stearic acid,
oleic acid, and stearic acid amide, and a high melting point component comprising
stearic acid bisamide organic compound powder having a high melting point of about
147°C, and mixing the free powders of a lubricant into the powder mixture at a temperature
below the melting point.
[0008] Accordingly, an object of the present invention is to provide such an advantageous
mixture and a manufacturing method for its efficient production.
[0009] The present invention provides a method for producing an iron-based powder mixture
for powder metallurgy, comprising the steps of:
mixing about 0.1 % to about 1.0 % by weight of a powder of at least one organic
compound selected from a first (low-melting) group comprising stearic acid, oleic
acid amide, and stearic acid amide, about 0.1 % to about 1.0 % by weight of a powder
of an organic compound of a second (high-melting) amide comprising stearic acid bisamide,
about 0.1 % to about 3.0 % by weight of an alloying powder and/or a powder for improving
machinability, and the balance an iron-based powder;
heating the resulting powder mixture thereafter for about 30 seconds to about 30
minutes at a temperature ranging from about ten degrees C above the lowest melting
point of an organic compound of the low-melting group to a temperature not exceeding
the melting point of the higher-melting bisamide organic compound; and
subsequently cooling the mixture.
[0010] The invention further relates to a method for producing an iron-based powder mixture
for powder metallurgy, comprising the steps of:
mixing about 0.1 % to about 0.5 % by weight of at least one lubricant-free powder
selected from the group consisting of stearic acid, oleic acid amide, stearic acid
amide, stearic acid bisamide, and a heated mixture of stearic acid amide and stearic
acid bisamide, and mixing this lubricant-free powder with the iron-based powder mixture,
and
mixing the resulting powder mixture for about 30 seconds to about 30 minutes at
a temperature below the melting temperature of any component.
[0011] The present invention is further directed to an iron-based powder mixture for powder
metallurgy, comprising:
a melted mixture, as a binder, which comprises about 0.1 % to about 1.0 % by weight
of a powder of at least one organic compound selected from a first (low-melting) group
consisting of stearic acid, oleic acid amide, and stearic acid amide, and about 0.1
% to about 1.0 % by weight of a powder of a (high-melting) organic compound comprising
stearic acid bisamide; and
the balance of which is an iron-based powder, to the surface of which is adhered
about 0.1 % to about 3.0 % by weight of an alloying powder and/or a powder for improving
machinability.
[0012] The invention further relates to an iron-based powder mixture for powder metallurgy
comprising a lubricant containing about 0.1 % to about 0.5 % by weight of at least
one lubricant-free powder selected from the group consisting of stearic acid, oleic
acid amide, stearic acid amide, stearic acid bisamide, and a heated mixture of stearic
acid amide and stearic acid bisamide, and/or wherein about 0.01 % to about 0.25 %
by weight of a free powder of zinc stearate are mixed without causing adhesion by
melting to the surface of the iron-based powder.
[0013] The expression "free powder" as used herein indicates a powder which is not adhered
by melting to the iron-based powder surface, but is simply physically blended in the
mixture.
[0014] The expression "heated mixture" as used herein indicates a powder which can be obtained
by heating, melting, mixing, cooling and then crushing a powder of not less than two
organic compounds.
[0015] According to the present invention, particle segregation can be prevented by the
adhesion, by means of the binder, of the alloying powder and/or the powder for improving
machinability to the surface of the iron-based powder.
[0016] In consideration of the characteristics required of the product, the following materials
are used in the required amounts:
A pure iron powder and/or alloyed iron powder, processed by methods such as pulverization
or atomization, may be used as the iron-based powder; a graphite powder or an alloying
powder may be used as the powder for an alloy; and talc or metallic sulfide may be
used as the powder for improving machinability of the sintered body.
[0017] Not only the alloying powder and/or the powder for improving machinability and the
stearic acid bisamide can be adhered to the surface of the iron-based powder, but
also the fluidity of the iron-based powder can be improved by using, as a binder,
at least one melted compound of the first group in which the stearic acid (melting
point 69°C), oleic acid amide (melting point 76°C), and stearic acid amide (melting
point 103°C) having a low melting point are included. Furthermore, by partially melting
the powder of stearic acid bisamide (melting point 147°C) of a high melting point
and combining it with the low-melting powder of the organic compound of the first
group as the binder, and heating to melt the one but not the other, the fluidity of
the iron-based powder mixture can be improved and the force required for ejection
of the product from the die can be significantly reduced.
[0018] Further, by combining the fatty acid such as stearic acid and the fatty acid amide
such as stearic acid bisamide, the fluidity of the mixture can be improved and the
alloying powder and/or the powder for improving machinability can be adhered to the
surface of the iron-based powder, with the beneficial result that the force required
for ejection of the iron-based powder from the die can be significantly reduced.
[0019] Referring to the fatty acids of the first or low-melting group, the amount of powder
of the organic compound, the heated and melted mixture as a binder ranges between
about 0.1 and 1.0 % by weight. When the amount of the powder is less than about 0.1
% by weight, a ratio of the amount of graphite contained in the total mixture, which
was heated and mixed, to the amount of graphite contained in the powder from about
100 to 200 mesh in the mixture (hereinafter defined as the degree of graphite adhesion)
is reduced below about 50 %; also the force required for ejecting the product from
a die after compacting decreases significantly. When the amount of powder is more
than about 1.0 % by weight, the fluidity of the mixture in flowing from the supply
hopper deteriorates.
[0020] One reason for substantially excluding zinc from the binder is to prevent contamination
on the surface of the sintered body during sintering.
[0021] In addition, from about 0.1 to 3.0 % by weight of an alloying powder and/or a powder
for improving machinability may be added. In this case, when the amount of the powder
added is less than about 0.1 % by weight, no significant advantage is realized because
of the small amount applied. On the other hand, when the amount of the added powder
exceeds about 3.0 % by weight, the degree of adhesion of the alloying powder and the
powder for improving machinability is reduced to about 50 % or less, which reduces
the efficiency of the mixture.
[0022] The iron-based powder mixture of the present invention can be obtained by mixing
and then heating the iron-based powder, the alloying powder and/or the powder for
improving machinability together with the aforementioned specific organic compounds
of the first (low-melting) and second (high-melting) groups. The preferable heating
temperature ranges from about 10°C above the melting point of the selected lower-melting
component or the one having the lower melting point when there is more than one component
of the first group which has a low melting point (the group comprises stearic acid,
oleic acid amide or stearic acid amide which melt at about 69°, 76° and 103°C, respectively)
to the melting point of the stearic acid bisamide which has a relatively high melting
point of about 147°C. In other words, as an example, when stearic acid (69°C) is selected
to be heated with the stearic acid bisamide, the minimum heating temperature should
be about 69 + 10 = 79°C up to the 147°C melting point of the stearic acid bisamide.
When the heating temperature is less than the above, the adhesion of the alloying
powder and/or the powder for improving machinability to the surface of the iron-based
powder is insufficient. On the other hand, when the heating temperature is higher
than the melting point of the stearic acid bisamide, the fluidity of the iron-based
powder deteriorates and the compounds having the lower melting point degenerate, which
increases the cost of the processing facilities and their operation. Because the heating
temperature is higher than the melting points of the lower-melting compounds of the
first group, the powders of the compounds of the first (low-melting) group are substantially
completely melted. Thus these melted compounds cause adhesion, as a binder, of the
alloying powder and/or the powder for improving machinability to the surface of the
iron-based powder. On the other hand, since the heating temperature is lower than
the melting point of the higher-melting stearic acid bisamide it melts only partially
if at all and adheres well to the surface of the iron-based powder.
[0023] By maintaining these heating and processing conditions, the fluidity of the iron-based
powder is enhanced and the sintered body may easily be ejected from the die after
compacting.
[0024] The required heating and mixing time ranges from about 30 seconds to about 30 minutes.
A heating and mixing time of less than about 30 seconds causes non-uniform adhesion
of the alloying powder and/or the powder for improving machinability to the surface
of the iron-based powder. On the other hand, a heating and mixing time of more than
about 30 minutes causes peeling of the adhered powders. Further, the preferable heating
and mixing time ranges from about 5 to 20 minutes.
[0025] The organic compounds of both groups are, of course, non-metallic; therefore, a compacted
body made of the iron-based powder mixture of the present invention does not contaminate
the inside of the furnace by generation of dust containing metallic element and/or
contaminate the surface of the sintered body by the metallic elements. The kind and
amounts of the organic compounds to be used are based upon the kind, shape, and particle-size
construction of the iron-based powder and the kind, shape, and added amount of the
alloying powder and/or the powder that is added for improving machinability.
[0026] The iron-based powder mixture according to the present invention can achieve better
ejecting force from the die and/or fluidity by adding a lubricant. The added lubricant
may comprise a room temperature free powder selected from the group consisting of
stearic acid, oleic acid, stearic acid amide, stearic bisamide, and a heated mixture
of stearic acid amide and stearic acid bisamide; or a small amount of the free powder
of zinc stearate; or a free powder of any of these organic compounds and a small amount
of zinc stearate.
[0027] In the present invention the organic compounds which separately comprise the heated
and melted mixture previously described, and the room temperature powder mixture,
are then mixed. The degree of adhesion of the alloying powder and/or the powder for
improving machinability is improved by the heated and melted mixture; the ejecting
force from the die is reduced by lubricating action of the room temperature powder
mixture.
[0028] The amount of the lubricant powder added to the mixture should not be less than about
0.1 % by weight and not more than about 0.5 % by weight. When the added amount is
less than about 0.1 % by weight, the die ejecting force does not improve markedly
after compacting. On the other hand, when the added amount of lubricant is more than
about 0.5 % by weight, the fluidity from the hopper of the mixture decreases.
[0029] The added amount of zinc stearate lubricant should preferably not be less than about
0.01 % by weight nor more than about 0.25 % by weight. When the added amount is less
than about 0.01 % by weight, fluidity of the mixture when fed from the hopper cannot
be improved. On the other hand, when the added amount is more than about 0.25 % by
weight contamination occurs on the surface of the sintered body.
[0030] The required time for adding these free powders to the iron-based powder and mixing
ranges between about 30 seconds and about 30 minutes at room temperature. Less than
about 30 seconds results in incomplete mixing, and more than about 30 minutes causes
deformation of the particles of the free powders which diminishes the effect of reducing
the ejecting force exerted on the compacted body from the die. Accordingly, the preferable
adding and mixing time ranges from about 5 to 20 minutes.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0031] A detailed description of the present invention will now be given in conjunction
with the accompanying tables.
Practical Example 1
[0032] Stearic acid or oleic acid amide or stearic acid amide of the first group, and stearic
acid bisamide of the second group, as a binder, were added in amounts shown in Table
1, into an atomizing iron powder for powder metallurgy. The powder had an average
particle diameter of 78 µm.
[0033] Then 0.8 % by weight of a graphite powder having an average particle diameter of
16 µm, was also added as an alloying powder, into the atomized iron powder. The powder
was mixed with heating and (partial) melting for 20 minutes at 120°C and then cooled.
[0034] Then 1.5 % by weight of a copper powder was added as the alloying powder into sample
No. 8, and talc having main components of MgO and SiO₂ with an average particle diameter
of not more than 44 µm were added as a powder for improving machinability into sample
No. 9.
[0035] Reduced iron powder, instead of atomized iron powder, with an average particle diameter
of 78µm was used in Sample No. 10.
[0036] In the comparative example the atomized iron powder was the same powder used in the
practical example 1 of the present invention. Each organic chemical powder of the
first and second groups was the only powder added as a binder.
[0037] Furthermore, zinc stearate used for a conventional lubricant was employed by mixing
at a room temperature without heating as sample No. 5 of a comparative example.
[0038] The results of practical example 1 are shown in Table 1. The ejecting force shown
in Table 1 indicates the ejecting force needed for ejecting a 25 mm-diameter and 20
mm-height tablet from a die, wherein the tablet was made of the powder provided in
both the practical example and the comparative example, and compacted at 5 t/cm² of
the compacting pressure.
[0039] The degree of graphite (C) adhesion in the powder is represented by the ratio of
the amount of C in the powder of particle size ranging from 100 to 200 mesh of this
mixture to the amount of C of the total mixture which was heated, melted, and mixed.
[0040] The degree of carbon adhesion is indicated as the ratio of (C analysis value in 100-200
mesh)/(C analysis value in the total mixture) x 100(%).
[0041] Under the above conditions, the degree of talc adhesion was measured by the same
method as the one used for the carbon.
[0042] The fluidity characteristics of the powder are represented by the fluidity time of
a 100 g mixture from a 5.1 mm diameter orifice provided at the bottom center of a
container which is 40 mm in diameter and 100 mm high, to which a 100 g mixture of
powder mixture at room temperature was added.
[0043] In the present invention in which the above described specific organic compound was
melted, the powder mixture for the powder metallurgy, in comparison with the conventional
comparative example, had a high degree of graphite adhesion, and achieved less segregation
and less ejecting force and superior fluidity.
[0044] In addition, samples No. 1 to No. 3 of the comparison example, to which only the
lower melting point organic compound was added, had deteriorated fluidity.
[0045] Likewise, sample No. 4 of the comparison example, to which only the high melting
point organic compound was added, had reduced ejecting force but deteriorated degree
of graphite adhesion. Sample No. 5 of comparative example, to which zinc stearate
powder was added by conventional room temperature mixing has the deteriorated ejecting
force and degree of graphite adhesion.

Practical Example 2
[0046] The identical iron powder, binder, and alloy powders of practical example 1 were
used. The added amounts are shown in Table 2. In practical example 2, the identical
copper powder of practical example 1 was used as the alloying powder in sample No.
8, and the identical talc of practical example 1 was used as the powder for improving
machinability. The same heating temperatures and times as those in practical example
1 were applied.
[0047] The lubricants were mixed into the above obtained iron-based powder mixture for 10
minutes at room temperature.
[0048] Added free powders as the above mentioned lubricants were stearic acid, oleic acid,
stearic acid amide, stearic acid bisamide, and a heated mixture of stearic acid amide
and stearic acid bisamide.
[0049] In the related comparison example, the same atomized iron powder as the one used
in the practical example was used, and powders of organic compounds in the first and
second groups were the only powders added as a binder. The degree of C adhesion, fluidity,
and ejecting force of the obtained mixture were measured in the same manner as in
practical example 1. The result of the measurement is shown in Table 2. All the practical
examples showed 85 % or more of the degrees of C and Talc adhesions, preferable fluidity,
and low ejecting force. On the contrary, in the comparison example, fluidity deteriorated.

Practical Example 3
[0050] The identical iron powder, binder, and alloying powder as in practical example 1
were used, and the added amount of each of these is shown in the Table 3. In the sample
No. 3, the identical copper powder of practical example 1 was used as the alloying
powder. In sample 5, the identical talc of practical example 1 was used as the alloying
powder. The iron powder, binder, and alloying powder were mixed with heating and melting
for 10 minutes at 115°C, then cooled and mixed with zinc stearate as a lubricant for
10 minutes at room temperature. In the comparative example, the identical atomized
iron powder of the practical example were used, and zinc stearate in an amount exceeding
the appropriate range was added as a lubricant. Then, the degree of C adhesion, fluidity,
and ejecting force of the obtained mixture were measured in the same manner as that
of practical example 1. The result of the measurements is shown in Table 3.
[0051] In the practical example 3 of the present invention, advantageous characteristics
of the degree of adhesion, fluidity, ejecting force and the surface condition of the
sintered body were obtained. On the other hand, the surface condition of the sintered
body of the comparison example was inferior to practical example 3 of the present
invention.

Practical Example 4
[0052] The identical iron powders, binders, and alloying powders of practical example 1
were used and the added amounts are shown in table 4. In test sample No. 8, the identical
copper powder of practical example 1 was used as an alloying powder. In practical
example 4, the heating temperature and time were the same as practical example 3.
The free powders of stearic acid, oleic acid amide, stearic acid amide, stearic acid
bisamide, the heated mixture of stearic acid amide and stearic acid bisamide, and
zinc stearate were added as lubricants. These lubricants were added into the above
mentioned iron-based powder mixture and mixed for 10 minutes at room temperature.
In the comparison example, the identical atomized iron powder of the practical example
was used, and the lubricants were added as shown in Table 4. The degree of C adhesion,
fluidity, and ejecting force of the obtained mixture were measured in the same manner
as practical example 1. The result of the measurement is shown in Table 4. The degree
of C adhesion, fluidity, ejecting force, and the surface condition of the sintered
body of the practical example 4 of the present invention showed superior characteristics
against the comparison examples in which the fluidity and the surface condition of
the sintered body, in particular, were inferior due to an excessive amount of the
room temperature mixture excepting zinc stearate and the zinc stearate.

[0053] According to the present invention, an iron-based powder mixture for powder metallurgy
has advantageous characteristics. In comparison with conventional mixtures, the iron-based
powder mixture has a stable level of powder metallurgy product and improved machinability
due to reduced segregation of the alloying powder and the powder for improving machinability.
It has a stabilized filling condition in the die due to superior fluidity of the powder
mixture in flowing from the hopper. There is less damage to the molded body, thanks
to the reduced force of ejection from the die. There is less and less contamination
in the sintering furnace and surface of sintered body because of the use of reduced
amounts of metallic elements such as binders and lubricants.
1. A method for producing an iron-based powder mixture for powder metallurgy, comprising
the steps of:
mixing (A) about 0.1 % to about 1.0 % by weight of a powder of at least one organic
compound selected from the group consisting of stearic acid, oleic acid amide, and
stearic acid amide,
(B) about 0.1 % to about 1.0 % by weight of a powder of stearic acid bisamide,
(C) about 0.1 % to about 3.0 % by weight of an alloying powder and/or a powder for
improving machinability, and
(D) the balance which is an iron-based powder;
heating the resulting iron-based powder mixture thereafter for about 30 seconds
to about 30 minutes at a temperature ranging from about 10°C above the lowest melting
point of the organic compound (A) to the melting point of said stearic acid bisamide
(B), and
subsequently cooling the mixture.
2. A method for producing an iron-based powder mixture for powder metallurgy, comprising
the steps of:
mixing about 0.1 % to about 0.5 % by weight of at least one free lubricant powder
selected from the group consisting of stearic acid, oleic acid amide, stearic acid
amide, stearic acid bisamide, and a heated mixture of stearic acid amide and stearic
acid bisamide with an iron-based powder mixture (A) plus (B) plus (C) plus (D) as
defined in claim 1; and
mixing the resulting powder mixture for about 30 seconds to about 30 minutes at
a temperature below the melting points of the ingredients.
3. A method for producing an iron-based powder mixture for powder metallurgy, comprising
the steps of:
mixing about 0.01 % to about 0.25 % by weight of a free powder of zinc stearate
with said iron-based powder mixture (A) plus (B) plus (C) plus (D) defined in claim
1; and
mixing the resulting powder mixture for about 30 seconds to about 30 minutes at
a temperature below the melting points of the ingredients.
4. A method for producing an iron-based powder mixture for powder metallurgy, comprising
the steps of:
introducing into a mixer about 0.1 % to about 0.5 % by weight of at least one free
lubricant powder selected from the group consisting of stearic acid, oleic acid amide,
stearic acid amide, stearic acid bisamide, and a heated mixture of stearic acid amide
and stearic acid bisamide, about 0.01 % to about 0.25 % by weight of a free powder
of zinc stearate, and also introducing into said mixture the balance which is said
iron-based powder mixture (A) plus (B) plus (C) plus (D) as defined in claim 1; and
mixing the resulting powder mixture for about 30 seconds to about 30 minutes at
room temperature.
5. An iron-based powder mixture for powder metallurgy, comprising:
a melted binder mixture which comprises about 0.1 % to about 1.0 % by weight of
a powder of at least one organic compound selected from a lower melting group consisting
essentially of stearic acid, oleic acid amide, and stearic acid amide, and about 0.1
% to about 1.0 % by weight of a powder of an organic compound of a higher melting
group comprising stearic acid bisamide; and
the balance of said mixture comprising an iron-based powder, to the surface of
which is adhered about 0.1 % to about 3.0 % by weight of an alloying powder and/or
a powder for improving machinability.
6. An iron-based powder mixture for powder metallurgy as defined in claim 5 wherein about
0.1 % to about 0.5 % by weight of at least one free lubricant powder selected from
the group consisting of stearic acid, oleic acid amide, stearic acid amide, stearic
acid bisamide, and a heated mixture of stearic acid amide and stearic acid bisamide
is mixed without causing melting adhesion to the surface of the iron-based powder.
7. An iron-based powder mixture for powder metallurgy as defined in claim 5 wherein about
0.01 % to about 0.25 % by weight of a free powder of zinc stearate is mixed without
causing melting adhesion to the surface of the iron-based powder.
8. An iron-based powder mixture for powder metallurgy as defined in claim 5 wherein about
0.1 % to about 0.5 % by weight of at least one free powder selected from the group
consisting of stearic acid, oleic acid amide, stearic acid amide, stearic acid bisamide,
and a heated mixture of stearic acid amide and stearic acid bisamide, and wherein
about 0.01 % to about 0.25 % by weight of a free powder of zinc stearate are mixed
without causing melting adhesion to the surface of the iron-based powder.