Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Powder metallurgical composition comprising carbon:
black as flow enhancing agent.
FIELD OF THE INVENTION
The invention relates to iron-based powder metallurgical
compositions. More particularly, the present invention
relates to compositions containing flow agents to improve
flowability, but also to improve apparent density.
BACKGROUND OF THE INVENTION
Powder metallurgical compositions are well known for the
production of powder metallurgical parts. Production of
powder metallurgical parts involves filling of the powder
in a compaction tool, compaction of the powder and
subsequent sintering of the compacted body. A
prerequisite for filling of the powder is that the powder
is free-flowing and has a sufficient flow. A high flow
rate of the powder is essential to obtain a high
production rate giving lower production costs and a
better economy for each part produced.
Another factor which is essential for the production
efficiency and economy is the apparent density. Apparent
density is essential for the tool design. Powder with low
apparent density needs higher filling height which
results in unnecessarily high pressing tools, and this in
turn will result in longer compaction strokes and lower
pressing performances.
Agents which improve the flow properties are previously
known. Thus the US patent 3 357 818 discloses that
silicic acid may be used to this end. The US 5 782 954
discloses that metal, metal oxides or silicon oxide can
be used as flow agents.
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It is an object of the present invention to
provide a powder metallurgical composition with improved
powder properties such as flowability and apparent density.
SUMMARY OF THE INVENTION
It has unexpectedly been found that by adding a
small amount of carbon black, to an iron-based powder
composition, the properties of the powder composition can be
improved. Additionally the addition of controlled amounts
of carbon black will not deteriorate the properties of green
and sintered parts prepared from the new iron-based
composition but these properties may even be improved.
According to another aspect of the present
invention, there is provided a powder metallurgical
composition comprising an iron or iron-based metal powder, a
lubricant and/or a binder, and carbon black, wherein the
amount of the carbon black is between 0.001 and 0.2% by
weight, the particle size of the carbon black is below 100
nm, and the carbon black has a specific surface area above
100 m2/g.
According to still another aspect of the present
invention, there is provided a method of increasing apparent
density of a powder metallurgical composition comprising an
iron or iron-based metal powder, a binder, carbon black, and
optionally a lubricant, the method comprising adding an
amount of the carbon black to said powder metallurgical
composition, wherein the amount of the carbon black is
between 0.001 and 0.2% by weight, the particle size of the
carbon black is below 100 nm, and the carbon black has a
specific surface area above 100 m2/g.
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2a
According to yet another aspect of the present
invention, there is provided a powder metallurgical
composition comprising an iron or iron-based metal powder, a
binder, carbon black, and optionally a lubricant, wherein
the amount of the carbon black is between 0.001 and 0.2% by
weight, the particle size of the carbon black is below 100
nm, and the carbon black has a specific surface area above
100 m2/g.
DETAILED DESCRIPTION OF THE INVENTION
Generally powder metallurgical compositions
contain an iron or iron-based powder and a lubricant. The
compositions may also include a binding agent, graphite and
other alloying elements. Hard phase material, liquid phase
forming material and machinability enhancing agents may also
be included.
The iron-based powder may be of any type of iron-
based powder such as water-atomised iron powder, reduced
iron powder, pre-alloyed iron-based powder or diffusion
alloyed iron-based powder. Such powders are e.g. the iron
powder ASC100.29, the diffusion alloyed iron-based powder
Distaloy"" AB containing Cu, Ni and Mo, the iron-based powder
AstaloyT'A CrM and Astaloy"" CrL pre-alloyed with Cr and Mo,
all available from Hoganas AB, Sweden.
The amount of carbon black in the iron-based
powder composition according to the invention is between
0.001
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and 0.2% by weight, preferably between 0.01 and 0.1%. The
primary particle size of the carbon black is preferably
below 200 nm, more preferably below 100 nm and most
preferably below 50 nm. The specific surface area is in a
preferred embodiment between 150 and 1000 m2/g measured
by the BET-method. However, other types of carbon black
having other surface areas and primary particle sizes are
possible to use.
Carbon black is normally used as filler in rubber
material and as colour pigments. It is also used for its
electrical conductivity, in products for reducing static
electricity. Carbon black in combination with iron or
iron-based powders is disclosed in US patent 6 602 315.
This patent discloses a composition wherein an alloying
powder is bound to an iron-based powder by binder, to
which carbon black may be added. US 6 602 315 does not
disclose any content, particle size or effect of carbon
black and is only relevant to the binding material. Also
in patent application JP 7-157838 a powder composition
containing carbon black is disclosed. Here the purpose of
carbon black is to deoxidize a base-material.
The compositions according to the present invention may
also include alloying elements chosen from the group
consisting of graphite, Cu, Ni, Cr, Mn, Si, V, Mo, P, W,
S and Nb
In order to enhance the compressibility of the powder and
to facilitate ejection of the green component a lubricant
or a combination of different lubricants may be added to
the powder metallurgical composition. The lubricant may
be present as a particulate powder or bonded to the
surface of the iron-based powder. By adding a bonding
agent dissolved in a solvent followed by evaporation of
the solvent the lubricant may be bonded to the surface of
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the iron-based powder. The binder may also be added in
its natural liquid state with a capacity of forming a
film around the iron-based powder. Another alternative is
to use the lubricants as binding agents by heating the
composition above the melting point of the lubricant or
above the melting point of at least one of the lubricant
components followed by cooling the composition to a
temperature below the melting point.
The lubricants may be selected from the group consisting
of fatty acids, amide waxes such as ethylene
bisstearamide (EBS), or other derivates of fatty acids
such as metal stearates, polyalkylenes such as
polyethylene, polyglycols, amide polymers, or amide
oligomers. Preferably the lubricants are selected from
the group consisting of polyalkylenes, amide waxes, amide
polymers or amide oligomers.
The binders are selected from the group consisting of
cellulose ester resins, high molecular weight
thermoplastic phenolic resins, hydroxyalkylcellulose
resins, and mixtures thereof. Preferably binders are
selected from the group of cellulose ester resins and
hydroxyalkylcellulose resins.
Other possible additives are machinability improving
agents, hard phase material and liquid phase forming
agent.
According to a preferred embodiment carbon black is used
as flow agent in bonded mixtures, i.e. mixtures, wherein
finer powder of e.g. alloying element particles are
bonded by means of a binding agent to the surface of the
iron or iron-based powder particles, as these mixtures
often have poor flow properties. When used in bonded
mixtures carbon black is preferably added after the
binding operation has been effectuated. The binding
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operation may be accomplished by heating the mixture
during mixing to a temperature above the melting point of
the binding agent and cooling the mixture until the
binder has solidified. The binder may also be added
5 dissolved in a solvent. The binding operation is in this
case accomplished by evaporating the solvent by means of
heating or by vacuum. The composition is compacted and
sintered to obtain the final powder metal part.
The invention is further illustrated by the following
non-limiting. examples:
Example 1
Three types of carbon black were selected with various
specific areas and particle sizes according to table 1.
The specific surface area was determined by the BET-
method. The particle size was measured by electron
microscopy and refers to the primary particle size of the
carbon black.
Table 1
Type Specific Primary particle
surface area size (nm)
(m2/g)
CB1* 1000 30
CB2* 250 18
CB3* 150 23
* available from Degussa AG, Germany
Iron-base powder ASC100.29, available from Hoganas AB,
Sweden, was mixed with 0.77% by weight of graphite, 0.8%
of a binder/lubricant system (consisting.of 0.2% of
TM,
polyethylene(Polywax 650) and 0.6% of ethylene bis-
stearamide (EBS)). The mixture was heated during mixing
TM
to a temperature above the melting point of Polywax and
subsequently cooled. At a temperature below the melting
TM
point of Polywax, 0.03% of carbon black was added. Three
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different types of carbon black, according to table 1,
were tested. Two mixtures were prepared as reference
mixtures. Reference mixture C was prepared according to
the test mixtures with the exception that 0.8% of
graphite and no flow agent was added. In reference
mixture R 0.8% of graphite and 0.06% of Aerosil A-200,
available from Degussa AG, was added.
Powder properties were measured. Flow property was
measured using the standard method, Hall-flow cup
according to ISO 4490 and the apparent density, AD, was
measured using standard method ISO 3923.
The results of the powder properties are presented in
table 2.
Table 2
ID Powder composition Flow AD
(s/50g) (g/cm3)
C ASC100.29+0.8%C+0.8%lubricant 30.0 3.06
R ASC100.29+0.8%C+0.8%lubricant+ 25.4 3.11
0.06% A-200
CB1 ASC100.29+0.77%C+0.8%lubricant+ 23.0 3.29
0.03 CB1
CB2 ASC100.29+0.77%C+0.8%lubricant+ 26.4 3.15
0.03 CB2
CB3 ASC100.29+0.77%C+0.8%lubricant+ 25.8 3.14
0.03 CB3
The tests show that the addition of carbon black to a
powder metallurgical mixture improves the flow rate and
AD compared to the mixture without any flow agent.
Addition of CB1 improves flow and AD compared to addition
of known flow agent whereas addition of CB2 and CB3 gives
about the same flow improvement but a higher AD compared
to addition of flow agent A-200.
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Example 2
Carbon black type CB 1 was selected in order to determine
the optimal added amount to the iron-based powder
mixture. The mixtures were prepared according to the
description of example 1. Added amounts of alloying
elements, binder/lubricant, flow agent and graphite are
shown in table 3.
Reference mixtures, R1 without flow agents and R2 with a
commercial available flow agent, which is Aerosil A-200
available from Degussa AG, were prepared.
Table 3
ID Powder composition Flow AD
(s/50g) (g/cm3)
B1 ASC100.29+2%Cu+0.8%C+0.8%lubricant+ 20.9 3.48
0.025%CB1
B2 ASC100.29+2%Cu+0.8%C+0.8%lubricant+ 20.8 3.49
0.03%CB1
B3 ASC100.29+2%Cu+0.8%C+0.8%lubricant+ 21.1 3.46
0.04%CB1
B4 ASC100.29+2%Cu+0.8%C+0.8%lubricant+ 21.6 3.43
0.06%CB1
R1 ASC100.29+2%Cu+0.8%C+0.8%lubricant 29.6 3.19
R2 ASC100.29+2%Cu+0.8%C+0.8%lubricant 24.5 3.28
+0.06% A-200
Test pieces according to ISO 2740 were compacted at a
pressure of 600 MPa at ambient temperature and sintered
at 1120 C in an 90/10 N2/H2 atmosphere. In table 4 the
mechanical properties are presented for the powder
compositions according to table 3.
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Table 4
ID TS (MPa) YS (Mpa) A ( o)
B1 610 444 2.12
B2 603 442 1.98
B3 596 438 1.93
B4 536 411 1.49
R1 603 437 2.22
R2 545 397 1.93
As can be seen from table 4 an added amount of 0.06 % of
carbon black will influence the tensile strength, TS,
yield strength, YS, and elongation, A. The influence on
the mechanical properties is negligible when amounts of
0.04 % by weight, and lower, of carbon black were added,.
Example 3
Example 3 shows that the new flow agent can be used in
compositions for warm compaction. One test mixture, B5,
and one reference mixture, R3, of 3 000 grams,
respectively, were prepared as follows.
As a reference mixture 60 grams of a copper powder, 24
grams of graphite, 13.5 grams of a high temperature
lubricant Promold" available from Morton International of
Cincinnati, Ohio, USA and remaining iron powder, ASC
100.29, was thoroughly mixed during heating to 45 C.
Furthermore, 4.5 grams of a cellulose ester resin
dissolved in acetone was added and the mixture was mixed
for 5 minutes. During a second mixing period of 10-30
minutes, while maintaining a temperature of 45 C of the
material, the solvent was evaporated. Finally, as a flow
agent 1.8 grams of Aerosil A-200 was added and
thoroughly mixed.
As a test mixture 60 grams of a copper powder, 23.1 grams
of graphite 13.5 grams of a high temperature lubricant
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Promold available from Morton International of
Cincinnati, Ohio, USA and remaining iron powder, ASC
100.29, was thoroughly mixed during heating to 45 C.
Furthermore, 4.5 grams of a cellulose ester resin
dissolved in acetone was added and the mixture was mixed
for 5 minutes. During a second mixing period of 10-30
minutes, while maintaining a temperature of 45 C of the
material, the solvent was evaporated. Finally, as a flow
agent 0.9 grams of carbon black CB1 was added and
thoroughly mixed.
Flow and AD of both the mixtures were measured according
to ASTM B 213 at a temperature of 120 C. In table 5 it
can be seen that a substantial increase in AD was
achieved for the powder mixture according to the
invention, substantially the same flow rate was achieved
for the composition containing the new flow agent
compared to the composition containing a known flow
agent.
Table 5
ID Flow (s/50g) AD (g/cm )
R3 21.3 3.25
B5 22.0 3.35
Example 4
Example 4 shows that the new flow agent can be used in
combination with different iron-based powders. The
mixtures were prepared according to the method of example
1 and the same binder/lubricant system as in example 1
was used. The iron-based powder used and amount of
additives are shown in table 6. The identifications RA,
RB, RC, RE and RF indicate that the mixtures are
reference mixtures containing 0.06% flow agent Aerosil A-
200, available from Degussa AG. The identifications C, E,
and F indicate that the mixtures are reference mixtures
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without any flow agents. Carbon black CB1 was used in all
mixtures. The iron or iron-based powder used were:
ASC 100.29, an atomised plain iron powder from Hoganas
AB.
5 Distaloy AB, a diffusion alloyed iron-based powder
containing Cu, Ni and Mo from Hoganas AB.
Astaloy CrM, a pre-alloyed iron-based powder containing
Cr and Mo from Hoganas AB.
Astaloy CrL, a pre-alloyed iron-based powder containing
10 Cr and Mo from Hoganas AB.
Table 6
ID Powder mixture composition
RA ASC 100.29+ 2% Cu powder+ 0.8% graphite+ 0.8%
lubricant+ 0.06% A-200
Al ASC 100.29+ 2% Cu powder+ 0.77% graphite+ 0.8%
lubricant+ 0.03% CB 1
RB Dist AE+ 0.8% graphite+ 0.8% lubricant+ 0.06%
A-200
B1 Dist AE+ 0.77% graphite+ 0.8% lubricant+ 0.03%
CB 1
C ASC100.29+0.8%C+0.8% lubricant
RC ASC100.29+0.8%C+0.8% lubricant+0.06% A-200
C1 ASC100.29+0.77%C+0.8%lubricant+0.03% CB1
E Ast.CrM +0.4%C+0.8%lubricant
RE Ast.CrM +0.37%C+0.8%lubricant+0.06% A-200
El Ast.CrM +0.37%C+0.8%lubricant+0.03% CB1
F Ast.CrL +0.6%C+0.8%lubricant
RF Ast.CrL +0.57%C+0.8%lubricant+0.06% A-200
F1 Ast.CrL +0.57%C+0.8%lubricant+0.03 CB1
The powder properties of the powder mixtures were
measured. Test pieces according to ISO 2740 were
compacted at a pressure of 600 MPa at ambient temperature
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and sintered at 1120 C 90/10 N2/H2 atmosphere. Mechanical
properties such as green strength, GS, dimensional
changes, DC, as well as sintered density, SD, were
determined and the results are presented in table 7.
Table 7
ID Flow (s/50g) AD (g/cm) GS (MPa) DC% SD [g/Cm3]
RA 24.8 3.13 11.3 0.18 7.01
Al 22.6 3.35 12.8 0.18 7.04
RB 24.8 3.17 12.3 -0.15 7.12
B1 23.1 3.43 13.3 -0.15 7.13
C 30 3.06
RC 25.4 3.11 11.6 -0.03 7.06
C1 23.0 3.29 12.6 -0.00 7.07
E 31.9 2.82
RE 27.5 2.93 13.8 -0.25 6.94
El 23.9 3.08 16 -0.24 6.94
F 33.1 2.78
RF 28.4 2.88 12.2 -0.13 6.99
F1 26.5 2.96 14.6 -0.11 6.99
Table 7 shows that carbon black gives improved flow, AD
and green strength in mixtures having different base
powders compared to mixtures containing a known flow
agent.
Example 5
Example 5 shows that the new flow agent also improves
flow of a plain mixture without any binding agents (not
bonded mixture). Three mixtures containing the iron
powder ASC100.29, 2 % of a copper powder, 0.5 % of
graphite, 0.8% of ethylene bisstearamide as lubricant and
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different amounts of carbon black, CB1, according to
table 8 were prepared. A mixture without any carbon black
was used as reference mixture. The flow rate was measured
on the different mixtures.
Table 8
Flow rate
ID CB1 (%) (s)
Reference 0 34.2
1 0.06 31.0
2 0.08 30.3
As can be seen from table 8 additions of carbon black to
not bonded mixtures improve the flow rate.
