U.S. patent application number 11/163030 was filed with the patent office on 2007-04-05 for powder metallurgy methods and compositions.
This patent application is currently assigned to APEX ADVANCED TECHNOLOGIES, LLC. Invention is credited to Dennis L. Hammond, Richard R. Phillips.
Application Number | 20070077164 11/163030 |
Document ID | / |
Family ID | 37902132 |
Filed Date | 2007-04-05 |
United States Patent
Application |
20070077164 |
Kind Code |
A1 |
Hammond; Dennis L. ; et
al. |
April 5, 2007 |
POWDER METALLURGY METHODS AND COMPOSITIONS
Abstract
The present invention provides metal powder compositions for
pressed powder metallurgy and methods of forming metal parts using
the metal powder compositions. In one embodiment, the metal powder
composition according to the invention includes a blend of primary
metal particles, one or more liquid phase forming materials or
precursors thereof, a lubricant and an organic acid that is capable
of reacting with an oxide of a metal in the primary metal particles
to form an organic metal salt that decomposes when the metal powder
composition is sintered under reducing or non-oxidizing conditions.
During a "delubing" step, the organic acid reacts with an oxide of
a metal in the primary metal particles to form an organic metal
salt that decomposes into a base metal or a metal-carbide during
sintering.
Inventors: |
Hammond; Dennis L.;
(Richfield, OH) ; Phillips; Richard R.; (St.
Marys, PA) |
Correspondence
Address: |
RANKIN, HILL, PORTER & CLARK, LLP
925 EUCLID AVENUE, SUITE 700
CLEVELAND
OH
44115-1405
US
|
Assignee: |
APEX ADVANCED TECHNOLOGIES,
LLC
4857-A W. 130th Street
Cleveland
OH
|
Family ID: |
37902132 |
Appl. No.: |
11/163030 |
Filed: |
October 3, 2005 |
Current U.S.
Class: |
419/29 ;
419/36 |
Current CPC
Class: |
B22F 3/1039 20130101;
B22F 2998/10 20130101; B22F 2998/00 20130101; B22F 2998/00
20130101; B22F 3/1021 20130101; B22F 3/1039 20130101; B22F 2201/01
20130101; B22F 2201/02 20130101; B22F 2201/20 20130101; B22F
2998/10 20130101; B22F 1/007 20130101; B22F 3/02 20130101; B22F
3/1021 20130101; B22F 3/1039 20130101; B22F 2998/10 20130101; B22F
1/02 20130101; B22F 1/0062 20130101; B22F 3/02 20130101; B22F
3/1021 20130101; B22F 3/1039 20130101 |
Class at
Publication: |
419/029 ;
419/036 |
International
Class: |
B22F 3/12 20060101
B22F003/12 |
Claims
1. A method of forming a metal part comprising the steps of: (i)
providing a metal powder composition comprising a blend of primary
metal particles, one or more liquid phase forming materials or
precursors thereof and an organic acid; (ii) placing the metal
powder composition within a die cavity; (iii) applying pressure to
the metal powder composition contained within the die cavity to
form a green compact; (iv) heating the green compact in a
non-oxidizing atmosphere to delube the metal powder composition and
cause the organic acid to react with an oxide of a metal on the
primary metal particles and form an organic metal salt; and (v)
heating the delubed green compact to a peak sintering temperature
at a heat up rate of 60.degree. F./min or higher in a non-oxidizing
atmosphere to decompose the organic metal salt into a base metal
and/or a metal carbide and form the metal part.
2. The method according to claim 1 wherein the metal powder
composition further comprises a lubricant.
3. The method according to claim 1 wherein the heating steps (iv)
and (v) are conducted in a reducing atmosphere.
4. The method according to claim 1 wherein heating step (v) is
conducted in a vacuum furnace, a continuous furnace or a microwave
furnace.
5. The method according to claim 1 wherein the organic acid is free
of sulfur, halogens, nitrogen and phosphorous.
6. The method according to claim 1 wherein the organic acid is a
fatty acid.
7. The method according to claim 1 wherein the organic acid is
present in the metal powder composition at a loading of from about
0.1% to about 4.0% by weight.
8. The method according to claim 1 wherein the primary metal
particles comprise one or more metals selected from the group
consisting of iron, copper, chromium, aluminum, nickel, bismuth,
cobalt, manganese, niobium, titanium, molybdenum, tin and
tungsten.
9. The method according to claim 1 wherein the liquid phase forming
materials are selected from the group consisting of Fe--C--Mn,
Fe--C, Fe--C--Si, Fe--Mn, Fe--P, Fe--S, Co--C, Mo--C, Mn--C, Ni--C,
Fe--B and Fe--Cr.
10. The method according to claim 1 wherein the metal powder
composition comprises precursors of liquid phase forming materials
selected from the group consisting of graphite, ferro phosphorous,
copper phosphorous, boron, silica, manganese sulphide, manganese,
silicon, phosphorous, sulfur, boron, chromium, cobalt and/or
molybdenum.
11. The method according to claim 1 wherein the primary metal
particles comprise iron and the organic acid comprises citric
acid.
12. The method according to claim 3 wherein the reducing atmosphere
comprises a mixture of hydrogen and nitrogen or vacuum with or
without partial pressure.
13. A metal part formed according to the method of claim 1.
14. The metal part according to claim 13 wherein the metal part
comprises a carbon or low-alloy steel having a sintered density of
greater than 95% of theoretical density.
15. A metal powder composition for use in pressed powder metallurgy
comprising a blend of primary metal particles, one or more liquid
phase forming materials or precursors thereof and an organic acid
that is capable of reacting with an oxide of a metal in the primary
metal particles to form an organic metal salt that decomposes when
the metal powder composition is sintered under reducing or
non-oxidizing conditions.
16. The metal powder composition according to claim 15 further
comprising a lubricant.
17. The metal powder composition according to claim 15 wherein the
organic acid is free of sulfur, halogens, nitrogen and
phosphorous.
18. The metal powder composition according to claim 15 wherein the
organic acid is a fatty acid.
19. The metal powder composition according to claim 15 wherein the
organic acid is present in the metal powder composition at a
loading of from about 0.1% to about 4.0% by weight.
20. The metal powder composition according to claim 15 wherein the
primary metal particles comprise one or more metals selected from
the group consisting of iron, copper, chromium, aluminum, nickel,
bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and
tungsten.
21. The metal powder composition according to claim 15 wherein the
liquid phase forming materials are selected from the group
consisting of Fe--C--Mn, Fe--C, Fe--C--Si, Fe--Mn, Fe--P, Fe--S,
Co--C, Mo--C, Mn--C, Ni--C, Fe--B and Fe--Cr.
22. The metal powder composition according to claim 15 wherein the
metal powder composition comprises precursors of liquid phase
forming materials selected from the group consisting of graphite,
ferro phosphorous, copper phosphorous, boron, silica, manganese
sulphide, manganese, silicon, phosphorous, sulfur, boron, chromium,
cobalt and/or molybdenum.
23. The metal powder composition according to claim 15 wherein the
primary metal particles comprise iron and the organic acid
comprises citric acid.
24. The metal powder composition according to claim 15 wherein the
reducing atmosphere comprises a mixture of hydrogen and nitrogen or
vacuum with or without partial pressure.
25. A metal part formed of the metal powder composition according
to claim 15.
26. The metal part according to claim 25 wherein the metal part
comprises a carbon or low-alloy steel having a sintered density of
greater than 95% of theoretical density.
27. A method of forming a metal part according to the invention
comprises: (i) providing a metal powder composition comprising
primary metal particles comprising a major amount of one or more
metallic elements having relatively low viscosity when molten and
an organic acid; (ii) placing the metal powder composition within a
die cavity; (iii) applying pressure to the metal powder composition
contained within the die cavity to form a green compact; (iv)
heating the green compact in a non-oxidizing atmosphere to delube
the metal powder composition and cause the organic acid to react
with an oxide of a metal on the primary metal particles and form an
organic metal salt; and (v) heating the delubed green compact to a
peak sintering temperature in a non-oxidizing atmosphere to
decompose the organic metal salt into a base metal and/or a metal
carbide and form the metal part.
28. The method of forming a metal part according to claim 27
wherein the metallic elements having relatively low viscosity when
molten in the primary metal particles are selected from the group
consisting of copper and aluminum.
29. The method according to claim 27 wherein the metal powder
composition further comprises a lubricant.
30. A metal powder composition comprising primary metal particles
comprising a major amount of one or more metallic elements having
relatively low viscosity when molten and an organic acid that is
capable of reacting with an oxide of a metal in the primary metal
particles to form an organic metal salt that decomposes when the
metal powder composition is sintered under reducing or
non-oxidizing conditions.
31. The metal powder composition according to claim 30 wherein the
metallic elements having relatively low viscosity when molten in
the primary metal particles are selected from the group consisting
of copper and aluminum.
32. The metal powder composition according to claim 30 further
comprising a lubricant.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The present invention relates to methods and compositions
for use in pressed powder metallurgy.
[0003] 2. Description of Related Art
[0004] In pressed powder metallurgy, a substantially dry metal
powder composition is charged into a die cavity of a die press and
compressed to form a green compact. Pressing causes the metal
powder particles in the metal powder composition to mechanically
interlock and form cold-weld bonds that are strong enough to allow
the green compact to be further processed. After pressing, the
green compact is removed from the die cavity and sintered at a
temperature that is below the melting point of the major metallic
constituent of the metal powder composition, but sufficiently high
enough to strengthen the bond between the metal powder particles,
principally through solid-state diffusion. Some metal powder
compositions include minor amounts of other metals and/or alloying
elements that melt during sintering to facilitate liquid phase
sintering of the non-melting major constituent of the metal powder
composition. This increases the bonding strength between the metal
powder particles and typically increases the final density of the
sintered part.
[0005] In most pressed powder metallurgy applications, it is
necessary to add a lubricant to the dry metal powder composition
before it is pressed to form the green compact. The most commonly
used lubricants in pressed powder metallurgy are ethylene
bis-stearamide wax and zinc stearate, but other lubricants are also
sometimes used. The lubricant helps the individual metal powders
flow into all portions of the die cavity, allows for some particle
to particle realignment during pressing and also serves as a
release agent that facilitates removal of the green compact from
the die cavity after pressing. The least amount of lubricant
necessary to obtain good flow and release is used.
[0006] The lubricant is conventionally removed from the green
compact by gradually heating the green compact at a relatively low
heating rate (e.g., .about.15.degree.F./min) until the lubricant
melts, boils and/or decomposes. This "delubing" is typically
accomplished during an initial heating or preheating stage at the
beginning of the sintering process. This can be accomplished in a
batch furnace or in a continuous furnace. In a continuous furnace,
the green compact is placed on a conveyor that moves the part
slowly into and through a sintering oven. The slow movement of the
conveyor allows the temperature of the green compact to increase at
a slow rate, allowing the lubricant to melt and then boil and then
gas off. Most of the remaining lubricant residue is decomposed and
burned out as the temperature of the green compact increases. Some
small quantity of the lubricant may diffuse into the base metal and
contribute carbon to the final part. The lubricant is completely
removed from the green compact at a temperature that is
substantially lower than the final sintering temperature. In a
batch furnace, the temperature is gradually increased to remove the
lubricant prior to sintering that may be programmed to run at
different conditions.
[0007] To maximize the opportunity for the individual metal
particles to bond to each other, it has long been the practice to
sinter the green compact at a peak sintering temperature for a
significant amount of time, typically on the order of 30 minutes or
more. Allowing the part to soak or dwell at the peak sintering
temperature for this period of time is believed to increase the
likelihood that individual metal particles will bond via
solid-state diffusion. The slow movement of the conveyor or the
temperature profile in a batch furnace insures that the green
compact receives a lengthy soak or dwell time in the hot zone of
the sintering oven.
[0008] Ideally, the sintered density of a final part would be 100%
of the theoretical density of the metallic constituents of the
metal powder composition used to form the part. However, the
sintered density of parts formed from most metal powder
compositions does not approach 100% of theoretical density. Using
conventional carbon or low alloy steel metal powder compositions
and pressed powder metallurgy methods, a sintered density of about
93% to 94% of theoretical density can be achieved.
BRIEF SUMMARY OF THE INVENTION
[0009] The present invention provides metal powder compositions for
pressed powder metallurgy and methods of forming metal parts using
the metal powder compositions. In one embodiment of the invention,
the metal powder composition comprises a blend of primary metal
particles, one or more liquid phase forming materials or precursors
thereof, a lubricant and an organic acid that is capable of
reacting with an oxide of a metal in the primary metal particles to
form an organic metal salt that decomposes when the metal powder
composition is sintered under reducing or non-oxidizing conditions.
Preferably, the primary metal particles comprise iron, which may be
alloyed with other metals/elements such as, for example, carbon,
copper, manganese, phosphorus, silicon, sulfur, nickel, chromium,
bismuth, cobalt, niobium, molybdenum, tungsten, tin, aluminum and
titanium. The liquid phase forming materials are preferably
selected from the group consisting of Fe--C--Mn, Fe--C, Fe--C--Si,
Fe--Mn, Fe--P, Fe--S, Co--C, Mo--C, Mn--C, Ni--C, Fe--B and Fe--Cr,
or precursors thereof selected from the group consisting of
graphite, ferro phosphorous, copper phosphorous, boron, silica,
manganese sulphide, manganese, silicon, phosphorous, sulfur, boron,
chromium, cobalt and/or molybdenum.
[0010] A first method of forming a metal part according to the
invention comprises: (i) providing a metal powder composition
comprising a blend of primary metal particles, one or more liquid
phase forming materials or precursors thereof, a lubricant and an
organic acid; (ii) placing the metal powder composition within a
die cavity; (iii) applying pressure to the metal powder composition
contained within the die cavity to form a green compact; (iv)
heating the green compact in a non-oxidizing atmosphere to delube
the metal powder composition and cause the organic acid to react
with an oxide of a metal on the primary metal particles and form an
organic metal salt; and (v) heating the delubed green compact to a
peak sintering temperature at a heat up rate of 60.degree. F./min
or higher in a non-oxidizing atmosphere to decompose the organic
metal salt into a base metal and/or a metal carbide and form the
metal part. The conversion of the metal oxide on the surface of the
primary metal particles to an organic metal salt during the
delubing step creates a "clean" surface on the primary metal
particles that is receptive to both liquid phase bonding and
subsequent diffusion bonding. The rapid heating rate during the
sintering step ensures that the liquid phase formers have adequate
time to create liquid phase bonds between the primary metal
particles before the constituents of the liquid phase diffuse into
the particles.
[0011] In a second embodiment of the invention, the metal powder
composition comprises primary metal particles comprising a major
amount of one or more metallic elements having relatively low
viscosity when molten, a lubricant and an organic acid that is
capable of reacting with an oxide of a metal in the primary metal
particles to form an organic metal salt that decomposes when the
metal powder composition is sintered under reducing or
non-oxidizing conditions. Preferably, the primary metal particles
comprise copper or aluminum, which may be alloyed with conventional
alloying elements.
[0012] A second method of forming a metal part according to the
invention comprises: (i) providing a metal powder composition
comprising primary metal particles comprising a major amount of one
or more metallic elements having relatively low viscosity when
molten, a lubricant and an organic acid; (ii) placing the metal
powder composition within a die cavity; (iii) applying pressure to
the metal powder composition contained within the die cavity to
form a green compact; (iv) heating the green compact in a
non-oxidizing atmosphere to delube the metal powder composition and
cause the organic acid to react with an oxide of a metal on the
primary metal particles and form an organic metal salt; and (v)
heating the delubed green compact to a peak sintering temperature
in a non-oxidizing atmosphere to decompose the organic metal salt
into a base metal and/or a metal carbide and form the metal part.
The conversion of the metal oxide on the surface of the primary
metal particles to an organic metal salt during the delubing step
creates a "clean" surface on the primary metal particles that is
receptive to both liquid phase bonding and subsequent diffusion
bonding. Because no liquid phase forming materials or precursors
thereof are present in the composition, the heating rate during
sintering is not critical.
[0013] Metal parts formed using the metal powder compositions and
methods according to the invention exhibit a substantially higher
sintered density than metal parts formed from metal powder
compositions that do not comprise an organic acid, and such higher
densities can be reached in less time and at lower energy costs.
For example, it is possible to form carbon steel or low alloy steel
metal parts that have a sintered density that approaches 100% of
theoretical density. Subsequent heat treatment of metal parts
formed from the metal powder compositions and methods of the
invention substantially improve the mechanical properties of the
parts, which in some cases are better than can be achieved using
non-powder metallurgical processes such as forging and casting.
[0014] The foregoing and other features of the invention are
hereinafter more fully described and particularly pointed out in
the claims, the following description setting forth in detail
certain illustrative embodiments of the invention, these being
indicative, however, of but a few of the various ways in which the
principles of the present invention may be employed.
DETAILED DESCRIPTION OF THE INVENTION
[0015] In a first embodiment of the invention, metal powder
compositions according to the present invention comprise a blend of
primary metal particles, one or more liquid phase forming materials
or precursors thereof, a lubricant and an organic acid that is
capable of reacting with an oxide of a metal in the primary metal
particles to form an organic metal salt that decomposes when the
metal powder composition is sintered under reducing or
non-oxidizing conditions. Throughout the instant specification and
in the accompanying claims, the term "primary metal particles"
refers to the principal metal powder component of the metal powder
composition by weight. The primary metal particles can comprise a
single metallic element, or can be alloys, agglomerations or blends
of two or more metallic elements. Suitable metals include, for
example, iron, copper, chromium, aluminum, nickel, bismuth, cobalt,
manganese, niobium, titanium, molybdenum, tin and tungsten. Iron is
a particularly preferred metal and is the major constituent of
steel.
[0016] The primary metal particles tend to have surfaces that are
oxidized, typically as a result of contact with oxygen in the
atmosphere or with water vapor. Primary metal particles comprising
iron, which are frequently used in pressed powder metallurgy to
form steel parts, have surfaces that are oxidized to form iron
oxide. Applicants believe that metal oxides on the surface of
primary metal particles may interfere with solid-state diffusion
bonding between such particles during sintering. The metal oxides
on the surface of the primary metal particles may also inhibit the
formation of liquid phase alloys, which can be used to solder, weld
or otherwise bind the individual metal particles together.
[0017] A variety of organic acids are known to react with metal
oxides to produce organic metal salts. For example, acetic acid
will react with iron oxide to form ferrous acetate. Similarly,
citric acid will react with iron oxide to form ferrous citrate.
Lactic acid will react with iron oxide to ferrous lactate. And,
malic acid, tartaric acid, oxalic acid, oleic acid, and stearic
acid will react with iron oxide to form ferric malate, ferrous
tartrate, ferrous oxalate, ferric oleate and ferrous stearate,
respectively.
[0018] Organic acids suitable for use in the invention are those
which are strong enough to react with metal oxides on the surface
of the primary metal particles to produce metal salts, and which
are compatible with the mixing, filling and compaction and
sintering steps of the pressed powder metallurgy process.
Preferably, the organic acid or acids used in the invention do not
leave undesirable residues or by-products when decomposed during
delubing and sintering. Accordingly, organic acids that are free
of, or contain very little, sulfur, nitrogen, phosphorous and
halogens are preferred.
[0019] Fatty acids are particularly suitable organic acids for use
in the invention. A non-exhaustive list of fatty acids is set forth
in Section 7-28 ("Properties of Selected Fatty Acids") of the CRC
Handbook of Chemistry and Physics, 76th Edition (1995), which is
hereby incorporated by reference. It will be appreciated that other
organic acids can also be used. Many organic acids are listed in
Section 8-45 to 8-55 ("Dissociation Constants of Organic Acids and
Bases") of the CRC Handbook of Chemistry and Physics, 76th Edition
(1995), which is also hereby incorporated by reference. The organic
acids identified in that list that are compatible with pressed
powder metallurgy and which are free of, or contain very little,
sulfur, nitrogen, phosphorous and halogens can be used.
[0020] Citric acid is the presently most preferred organic acid for
use with metal powder compositions for low alloy steel and carbon
steels as well as stainless steel, copper and aluminum. Other
particularly useful organic acids include acids that have a pKA
value low enough to react with metal oxides and which are solids at
press conditions (typically .about.140.degree. F. and higher).
Examples of suitable alternative acids to citric acid include, for
example, oxalic acid, tartaric acid, malic acid and low-melting
acids that are partially solublized in higher melting acids or
other organic materials that decompose into constituents that are
similar to citric acid or the other acids identified above.
[0021] The amount of organic acid present in the metal powder
composition will depend on the amount of metal oxide to be removed,
and the ability of the organic acid to remove the metal oxide
during the delubing/sintering cycle(s). Loadings from about 0.1% by
weight to about 4% by weight are typically sufficient. Ideally, a
stoichiometric amount of acid would be added relative to the oxides
on the surface of the metal particles, plus an excess of about 10
mole percent, if press conditions would allow it. To insure
adequate distribution of the organic acid in the metal powder
composition, it is preferable that the organic acid be micronized
to an average particle size of about 30 .mu.m or less (e.g., via
milling). When used neat (i.e., not blended with other materials),
it is preferable for the organic acid to be mirconized close in
time prior to use so that the micronized particles do not have an
opportunity to degrade upon exposure to atmospheric moisture.
[0022] In the most preferred embodiment of the invention, the
organic acid is used in combination with a lubricant (e.g., by
creating a masterbatch comprising a blend of the lubricant, the
organic acid and, optionally, other components of the powder metal
composition such as graphite). Conventional lubricants such as
ethylene bis-stearamide wax and zinc stearate can be used, but the
lubricant described in U.S. Pat. No. 6,679,935, which is hereby
incorporated by reference, is most preferred. Such a lubricant
transforms from a solid to a liquid due to shear in the press,
spreads and makes a uniform coating of lubricant, liquid phase
forming materials and/or precursors and organic acid on the surface
of the primary metal particles. The lubricant, due to its liquid
nature, becomes less viscous as the temperature rises, and the
molten lubricant can serve as an effective vehicle or solvent for
the organic acid and the liquid phase forming materials and/or
precursors thereof. It will be appreciated that some organic acids,
particularly longer chain fatty acids, can serve as both a
lubricant and a compound that assists in the removal of metal
oxides from the surface of the metal powder particles.
[0023] Throughout the instant specification and in the accompanying
claims, the term "liquid phase forming materials" refers to
metallic alloys that, when present between adjacent primary metal
particles in a liquid (molten) state during sintering, assist in
forming a liquid phase bond (e.g. solder/weld-type bonds) between
the primary metal particles. Liquid phase forming materials are
separate and distinct from the primary metal particles, and are
blended therewith to form a substantially homogeneous composition.
Iron is the predominant metallic constituent of low alloy dry
powder steel metal compositions, and the presence of carbon,
manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt
and/or molybdenum on the surface of the oxide-free metal particles
can lead to the liquid phase forming materials such as, for
example, Fe--C--Mn, Fe--C, Fe--C--Si, Fe--Mn, Fe--P, Fe--S, Co--C,
Mo--C, Mn--C, Ni--C, Fe--B and Fe--Cr. Precursors to liquid phase
forming materials thus include graphite, ferro phosphorous, copper
phosphorous, boron, silica, manganese sulphide, manganese, silicon,
phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum.
Liquid phase forming materials and precursors thereof are
conventionally used in powder metallurgy. The presence of an
organic acid in compositions according to the first embodiment of
the invention and the rapid heating of the delubed green compact
during sintering allows for the use of reduced amounts of the
liquid phase forming materials to achieve parts having higher
sintered density, which can approach theoretical density.
[0024] The metal powder compositions according to the invention can
be processed using conventional dry powder metal techniques. The
powder metal composition is typically placed into a cavity and
pressed to form a green part. The green part is then heated to
remove the lubricant during a "delubing" step. The present
compositions can be "delubed" using conventional delubing
techniques. Delubing should be conducted in a non-oxidizing
atmosphere.
[0025] As the green compact is heated during delubing, the organic
acid present in the metal powder compositions according to the
invention reacts with the metal oxides on the surface of the
primary metal particles to form organic metal salts. Without being
bound to a particularly theory, applicants believe that any one or
more of three distinct reaction mechanisms may occur during the
heating of the green compact, which facilitate the removal of the
metal oxide layer from the surface of the primary metal particles:
melt fusion; ionic; and/or vapor. In the melt fusion reaction
mechanism, the organic acid would melt and boil on the surface of
the primary metal particles, reaching temperatures that allow for a
direct neutralization reaction. In the ionic reaction mechanism,
the organic acid would partially dissolve in residual water that is
bonded or adhered to the surface of the primary metal particles
forming a hot ionic acid that dissolves the metal oxide as the
temperature rises. In the vapor reaction mechanism, the organic
acid would become volatile and scavenges the metal oxide layer as
it escapes from the green compact.
[0026] Although the exact mechanism of the reaction between the
organic acid and the metal oxide on the surface of the primary
metal particles is not definitively known at present, applicants
believe that the organic acid effectively removes all or some part
of the metal oxides from the surface of the primary metal
particles. The "cleaned" surfaces of adjacent primary metal
particles are in contact with each other, which allows for better
necking in the solid phase, because there is less hindrance or
interference to diffusion bonding caused by the presence of a metal
oxide at the interface between the particles. Applicants believe
that some localized liquid phase sintering also probably occurs
(even in the absence of liquid phase forming materials or
precursors thereof), because the non-oxidized surfaces of adjacent
metal particles are more reactive.
[0027] The iron oxide content of most commercial low alloy steel
metal powder compositions for pressed powder metallurgy ranges from
0.05% to 0.5% by weight. Metal powders having the lowest oxygen
content provide the best compressibility and best final properties,
but are generally more expensive. Use of an organic acid according
to the present invention allows for the removal of the oxygen from
such metal particles, which is present as iron oxide. The organic
acid reacts with the iron oxide or other metals to form an organic
iron salt, which decomposes during sintering to form very finely
divided iron metal or other base starting metals, which can serve
to promote solid state sintering and localized liquid phase
sintering, or iron carbide, which can be a component of the low
alloy or carbon steel part. Thus, the present invention provides
two distinct benefits: metal particles having surfaces that have
all or some of metal oxides removed, which enhances the efficiency
of both solid state and liquid phase sintering; and a by-product
from the decomposition of the iron salt, which also enhances the
solid state or liquid phase sintering.
[0028] Applicants have discovered that it is critical that the
delubed compact be heated to the peak sintering temperature in a
reducing atmosphere or inert atmosphere at a rate of about
60.degree. F./min or more in order to obtain a metal part having a
higher sintered density than would otherwise be obtained using a
conventional metal powder composition that did not comprise an
organic acid. Applicants believe that the delubing procedure
removes all or part of the oxide layer from the surface of the
metal particles at the last possible moment before sintering, which
promotes solid-state diffusion and liquid phase sintering. Heating
at a rate lower than 60.degree. F./min does not appear to provide
any improvement in sintered density.
[0029] Applicants theorize that once the metal oxides have been
removed from the surface of the primary metal particles, the
material present at or on the surface of the metal particles become
highly receptive to solid state diffusion. If the heating rate is
slow, diffusion occurs over an extended period of time
contemporaneous with the relatively slow heating rate, allowing the
material present at or on the surface of the particles time to
diffuse into the particles, which depletes the amount of liquid
phase forming material present on the surface of the particles to
obtain liquid phase soldering, welding or bonding between the
particles. In essence, a slow heating rate assures that bonding is
accomplished predominantly or entirely by solid state diffusion,
and not by liquid phase bonding. Use of a faster heating rate, on
the other hand, reduces the time the liquid phase forming material
at or on the surface of the cleaned particles has to diffuse into
the particles, and thereby maintains sufficient amounts of liquid
phase forming material to promote liquid phase bonding between the
particles during the heating cycle. Liquid phase bonding is similar
to soldering or welding, and leads to substantial improvements in
the final density of the sintered parts. Thus, the rapid heating
rate is necessary to provide sufficient time for liquid phase
forming materials to form liquid-type bonding between the primary
metal particles. The time period during which the rapid heating
occurs may vary according to the particular heating process and
equipment being used, but is typically accomplished within about
ten minutes or less. High oven temperatures can be used (i.e., oven
temperatures of as high as about 2,650.degree. F., which is in
excess of the melting temperature of the primary metal particles)
so long as the metal part is not allowed to reach a temperature
above the melting temperature of the primary metal particles. Use
of sintering temperatures below the melting temperature of the
primary metal particles can allow for extended dwell times,
provided the heating rate is rapid. Sintering is typically
conducted in a non-oxidizing, preferably reducing, atmosphere such
as that which comprises a blend of hydrogen and nitrogen, or in
endothermic (e.g. CO--H.sub.2--N.sub.2) or inert atmospheres (e.g.,
Ar).
[0030] The first method of forming a metal part according to the
invention comprises: (i) providing a metal powder composition
comprising a blend of primary metal particles, one or more liquid
phase forming materials or precursors thereof, a lubricant and an
organic acid; (ii) placing the metal powder composition within a
die cavity; (iii) applying pressure to the metal powder composition
contained within the die cavity to form a green compact; (iv)
heating the green compact in a non-oxidizing atmosphere to delube
the metal powder composition and cause the organic acid to react
with an oxide of a metal on the primary metal particles and form an
organic metal salt; and (v) heating the delubed green compact to a
peak sintering temperature at a heat up rate of 60.degree. F./min
or higher in a non-oxidizing atmosphere to decompose the organic
metal salt into a base metal and/or a metal carbide and form the
metal part. The conversion of the metal oxide on the surface of the
primary metal particles to an organic metal salt during the
delubing step creates a "clean" surface on the primary metal
particles that is receptive to both liquid phase bonding and
subsequent diffusion bonding. The rapid heating rate during the
sintering step ensures that the liquid phase formers have adequate
time to create liquid phase bonds between the primary metal
particles before the constituents of the liquid phase diffuse into
the particles. With more efficient oxide reduction or removal,
leaner compositions reach higher densities. These leaner
compositions have a smaller time window to react, which is made
available by having an earlier removal of oxides.
[0031] In a second embodiment of the invention, the metal powder
composition comprises primary metal particles comprising a major
amount of one or more metallic elements having relatively low
viscosity when molten, a lubricant and an organic acid that is
capable of reacting with an oxide of a metal in the primary metal
particles to form an organic metal salt that decomposes when the
metal powder composition is sintered under reducing or
non-oxidizing conditions. Preferably, the primary metal particles
comprise copper or aluminum, which may be alloyed with conventional
alloying elements. No liquid phase forming materials or precursors
thereof are present in the composition according to the second
embodiment of the invention. However, due to the low viscosity of
the metal in the primary metal particles, the particles tend to
fuse together, likely through diffusion alone, and form high
density parts upon sintering. The absence of an oxide layer, which
is stripped and converted to a metal salt during a delube step,
yields primary metal particles having very "clean" (i.e.,
oxide-free or having very low amounts of oxide residues) surfaces,
which are capable of bonding and fusing together without the need
for liquid phase forming materials or precursors thereof.
[0032] Thus, a second method of forming a metal part according to
the invention comprises: (i) providing a metal powder composition
comprising primary metal particles comprising a major amount of one
or more metallic elements having relatively low viscosity when
molten, a lubricant and an organic acid; (ii) placing the metal
powder composition within a die cavity; (iii) applying pressure to
the metal powder composition contained within the die cavity to
form a green compact; (iv) heating the green compact in a
non-oxidizing atmosphere to delube the metal powder composition and
cause the organic acid to react with an oxide of a metal on the
primary metal particles and form an organic metal salt; and (v)
heating the delubed green compact to a peak sintering temperature
in a non-oxidizing atmosphere to decompose the organic metal salt
into a base metal and/or a metal carbide and form the metal part.
The conversion of the metal oxide on the surface of the primary
metal particles to an organic metal salt during the delubing step
creates a "clean" surface on the primary metal particles that is
receptive to both liquid phase bonding and subsequent diffusion
bonding. Because no liquid phase forming materials or precursors
thereof are present in the composition, the heating rate during
sintering is not critical.
[0033] Metal parts formed using the metal powder compositions and
methods according to the invention exhibit a substantially higher
sintered density than metal parts formed from metal powder
compositions that do not comprise an organic acid, and such higher
densities can be reached in less time and at lower energy costs.
For example, it is possible to form carbon steel or low alloy steel
metal parts that have a sintered density that approaches 100% of
theoretical density. Copper parts can also be formed in accordance
with the invention that have sintered densities approaching 100% of
theoretical density. Subsequent heat treatment of metal parts
formed from the metal powder compositions and methods of the
invention substantially improve the mechanical properties of the
parts, which in some cases are better than can be achieved using
non-powder metallurgical processes such as forging and casting.
[0034] The following examples are intended only to illustrate the
invention and should not be construed as imposing limitations upon
the claims.
EXAMPLE 1
[0035] A Stock Powder Metallurgy Composition ("Stock P/M") was
prepared by dry mixing the components set forth in Table 1 below:
TABLE-US-00001 TABLE 1 Component Weight Percent ANCORSTEEL 85 HP*
97.00% UT-3PM** 2.00% Graphite Powder 0.65% SUPERLUBE PS1000-B***
0.35% *ANCORSTEEL 85 HP is a water atomized, pre-alloyed steel
powder (approximate chemical composition in weight percent:
.about.98.93% Fe; 0.86% Mo; 0.12% Mn; 0.08% O; and <0.1%
C)available from Hoeganeaes Corporation of Cinnaminson, New Jersey.
**UT-3PM is a high-purity nickel powder for pressed powder
metallurgy applications available from Norilsk of Moscow, Russia.
***SUPERLUBE PS1000-B is a pressed powder metallurgy lubricant
capable of transforming from a solid to a liquid due to shear from
Apex Advanced Technologies of Cleveland, Ohio.
EXAMPLE 2
[0036] Test bars were formed using the Stock P/M formed in Example
1. In Sample 1, the test bar was formed solely out of the Stock P/M
formed in Example 1. In Samples 2 and 3, the test bars were formed
by blending the Stock P/M with citric acid at a 0.2% by weight
loading and a 0.4% by weight loading, respectively. Each test bar
was formed using a 50 tsi (tons per square inch) Tinius Olsen
hydraulic press. Each test bar had the following dimensions: 1/2''
wide.times.11/4'' long.times.1/4'' thick.
[0037] The green density of the pressed test bars was measured in
accordance with the procedures set forth in MPIF Standard 45 and
ASTM B331-95 (2002). The green test bars were delubed at normal
conditions and were sintered in a continuous furnace at a heat up
rate of 133.degree. F./min in the hot zone to a temperature of
2,480.degree. F. in an atmosphere consisting of 25% H.sub.2 and 75%
N.sub.2. The density of the green and sintered test bars is
reported in Table 2 below: TABLE-US-00002 TABLE 2 Sample Stock P/M
Citric Acid Green Density Sintered Density 1 100% .sup. 0% 7.24
g/cm.sup.3 7.32 g/cm.sup.3 2 99.8% 0.2% 7.15 g/cm.sup.3 7.81
g/cm.sup.3 3 99.6% 0.4% 7.11 g/cm.sup.3 7.83 g/cm.sup.3
[0038] The data reported in Table 2 shows that at a high heat up
rate (>60.degree. F./min), the presence of a small amount of
citric acid in the Stock P/M blend results in a substantial
improvement in sintered density. Specifically, the data in Table 2
shows that blending 0.4% by weight of citric acid with the Stock
P/M coupled with a heat up rate of 133.degree. F./min increases the
sintered density of the test bars from 7.32 g/cm.sup.3 to 7.83
g/cm.sup.3, which is an improvement from 93.25% to 99.75% of
theoretical density.
EXAMPLE 3
[0039] The test bars were formed using the same Stock P/M formed in
Example 1 using the same procedures as set forth in Example 2. The
green test bars were delubed at normal conditions, sintered in a
continuous furnace at a heat up rate of 50.degree. F./min in the
hot zone to a temperature of 2,480.degree. F. in an atmosphere
consisting of 25% H.sub.2 and 75% N.sub.2. The density of the green
and sintered test bars is reported in Table 3 below: TABLE-US-00003
TABLE 3 Sample Stock P/M Citric Acid Green Density Sintered Density
4 100% .sup. 0% 7.29 g/cm.sup.3 7.42 g/cm.sup.3 5 99.6% 0.4% 7.21
g/cm.sup.3 7.35 g/cm.sup.3 6 99.2% 0.8% 7.10 g/cm.sup.3 7.23
g/cm.sup.3
[0040] The data reported in Table 3 shows that the presence of
small amounts of citric acid in the Stock P/M blend does not result
in any improvement in sintered density when the heat up rate is
below 60.degree. F./min. Specifically, the sintered density of the
test bars decreased with the addition of citric acid at a heat up
rate of 50.degree. F./min due to lower green density to start.
Typically there is a direct correlation between green densities and
sintered, the lower it starts the lower it goes.
EXAMPLE 4
[0041] Test bars were formed using the same Stock P/M formed in
Example 1 using the same procedures as set forth in Example 2. The
green test bars were delubed at normal conditions, sintered in a
continuous furnace at a heat up rate of 15.degree. F./min in the
hot zone to a temperature of 2,460.degree. F. in an atmosphere
consisting of 25% H.sub.2 and 75% N.sub.2. The density of the green
and sintered test bars is reported in Table 4 below: TABLE-US-00004
TABLE 4 Sample Stock P/M Citric Acid Green Density Sintered Density
7 100% .sup. 0% 7.29 g/cm.sup.3 7.43 g/cm.sup.3 8 99.6% 0.4% 7.27
g/cm.sup.3 7.46 g/cm.sup.3 9 99.2% 0.8% 7.11 g/cm.sup.3 7.45
g/cm.sup.3
[0042] The data reported in Table 4 shows that the presence of
small amounts of citric acid in the Stock P/M blend had no
appreciable effect on the sintered density at conventional powder
metallurgy heat up rates. Specifically, the sintered density of the
test bars was relatively constant with the addition of citric acid
at a heat up rate of 15.degree. F./min.
EXAMPLE 5
[0043] The Stock P/M Composition from Example 1 was used to form
test bars as described in Example 2. One set of test bar samples
were pressed solely out of the Stock P/M Composition. A second set
of test bar samples were pressed out of the Stock P/M Composition
mixed with an additional 0.4% by weight of citric acid. All of the
test bars were delubed in a continuous furnace in an inert
atmosphere consisting of 100% nitrogen at a peak temperature below
about 410.degree. F. at a heating rate of about 16.degree. F. per
minute. The test bars were then allowed to cool to ambient
temperature (.about.72.degree. F.) and later were placed in a
microwave furnace under a reducing atmosphere and heated for 2.5
minutes. The test bars that did not include citric acid reached a
sintered density of 7.65 g/cm.sup.3 at 1356.degree.F., whereas the
test bars that did include citric acid reached a sintered density
of 7.81g/cm.sup.3 at the same temperature. Theoretical density
would be considered to be .about.7.82-7.84 g/cm.sup.3. The
temperature noted is a reference temperature only. The actual part
temperature may have been higher at the peak of heating. Rapid
heating of the test bars that included an organic acid resulted in
significantly higher sintered density than the test bars that did
not include an organic acid.
EXAMPLE 6
[0044] A powder metal grade of powdered copper (Acupowder Grade
165: .about.99.5% purity) was mixed with 0.35% by weight of Apex
Lubricant (PS1000b) and 0.1% by weight lithium stearate and pressed
into test bars as described in Example 2. Lithium stearate is
generally known and regarded in the art as an additive that helps
copper achieve higher density. A second set of test bars were
pressed out of a composition comprising the same powdered copper,
0.35% by weight of Apex Lubricant (PS1000b) and 0.4% by weight
citric acid. All of the test bars were then delubed and sintered in
one operation in a batch furnace at 15.degree. F. degrees per
minute in 100% hydrogen up to 1930.degree. F. with a 30 minute
hold-at temperature. Rapid heating after the delube step was not
required to obtain higher sintered density because there were no
alloying/liquid phase forming elements present in the composition.
The test bars that did not include citric acid reached a sintered
density of 8.05 g/cm.sup.3, whereas the test bars that did include
citric acid reached a sintered density of 8.95 g/cm.sup.3.
Theoretical density ranges from 8.92 to 8.96. By removal of the
surface oxides alone the density achieved 100% theoretical.
[0045] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
illustrative examples shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *