U.S. patent application number 10/200094 was filed with the patent office on 2004-01-22 for iron-based powdered metal compositions.
Invention is credited to Morris, Robert Craig.
Application Number | 20040013557 10/200094 |
Document ID | / |
Family ID | 30443485 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040013557 |
Kind Code |
A1 |
Morris, Robert Craig |
January 22, 2004 |
IRON-BASED POWDERED METAL COMPOSITIONS
Abstract
Molding compositions and forming processes for normally
rust-prone iron-based powders, and articles produced therefrom.
Metal alloy systems that can be successfully formed using the
processes of the invention, include elemental iron and iron alloys
including low and medium alloy steels, tool steels and a number of
specialty iron-base alloys.
Inventors: |
Morris, Robert Craig;
(Flanders, NJ) |
Correspondence
Address: |
SHELDON KAVESH, Ph. D.
16 N. POND RD.
WHIPPANY
NJ
07981
US
|
Family ID: |
30443485 |
Appl. No.: |
10/200094 |
Filed: |
July 19, 2002 |
Current U.S.
Class: |
419/36 |
Current CPC
Class: |
B22F 3/227 20130101;
B22F 1/10 20220101; B22F 2998/00 20130101; B22F 3/225 20130101;
B22F 2998/00 20130101; C22C 33/02 20130101; B22F 2998/00 20130101;
B22F 1/052 20220101; B22F 2998/00 20130101; B22F 3/227 20130101;
B22F 2998/00 20130101; B22F 1/052 20220101 |
Class at
Publication: |
419/36 |
International
Class: |
B22F 003/10 |
Claims
What is claimed is:
1. A molding composition comprising: a) at least one metal powder
selected from the group consisting of elemental iron, an iron-base
alloy containing less than 10 wt. % chromium, and an iron-based
intermetallic compound; and b) a binder comprising a gel forming
material, a solvent for said gel forming material; and at least one
corrosion inhibiting compound selected from the group consisting of
inorganic nitrates and inorganic nitrites.
2. The molding composition of claim 1, wherein said gel forming
material is at least one member selected from the group consisting
of agaroids, proteins, starches, methyl cellulose, polyvinyl
alcohol, polyacrylamide, and polyvinyl pyrrolidone.
3. The molding composition of claim 1, wherein said gel forming
material is an agaroid.
4. The molding composition of claim 1, wherein said gel forming
material is an agar, agarose or a mixture thereof.
5. The molding composition of claim 1, wherein said gel forming
material has a gel strength of at least about 200 g/cm.sup.2
measured at a temperature of 23.degree. C. on a gel comprising 1.5
wt % of the gel-forming material in 98.5 wt % solvent for said gel
forming material.
6. The molding composition of claim 1, wherein said solvent for
said gel forming material is at least 50 wt. % water.
7. The molding composition of claim 1, wherein said solvent for
said gel forming material is at least 50 wt. % water additionally
containing at least one member selected from the group consisting
of methanol, ethanol, n-propanol, iso-propanol, ethylene glycol,
and glycerine.
8. The molding composition of claim 1, wherein said solvent for
said gel forming material is at least 75 wt. % water.
9. The molding composition of claim 1, wherein said solvent for
said gel forming material is at least 90 wt. % water.
10. The molding composition of claim 1, wherein said corrosion
inhibiting compound is at least one member selected from group
consisting of the nitrate of lithium, sodium, potassium, calcium,
magnesium, zinc, cobalt, iron, chromium, and copper, and the
nitrite of lithium, sodium, potassium, calcium, magnesium, zinc,
cobalt, iron, chromium, and copper.
11. The molding composition of claim 1, wherein said corrosion
inhibiting compound is at least one member selected from group
consisting of the nitrite of lithium, sodium, potassium, calcium,
magnesium, zinc, cobalt, iron, chromium, and copper.
12. The molding composition of claim 1, wherein the pH of said
binder is from about 7 to about 11.
13. The molding composition of claim 1, wherein the pH of said
binder is from about from 8.5 to about 9.5.
14. The molding composition of claim 1, additionally comprising at
least one pH buffering compound selected from the group consisting
of a borate salt of an alkali metal element, an alkali earth
element, or ammonia.
15. The molding composition of claim 1, additionally comprising at
least one pH buffering compound selected from the group consisting
of the metaborate (BO.sub.2.sup.-) or tetraborate
(B.sub.4O.sub.7.sup.-2) salt of lithium, sodium, potassium,
magnesium, calcium, strontium, barium, zinc, and ammonia.
16. The molding composition of claim 1, wherein said metal powder
is from about 50 wt. % to about 96 wt. % of the molding
composition.
17. The molding composition of claim 1, wherein said gel forming
material is from about 0.2 wt % to about 5 wt % of the molding
composition.
18. The molding composition of claim 1, wherein said corrosion
inhibiting compound is from about 0.05 wt. % and about 5 wt. % of
said binder.
19. The molding composition of claim 1, wherein said borate
compound is from 0.05 wt. % to about 3.0 wt. % of said binder.
20. The molding composition of claim 1 further comprising at least
one member selected from the group consisting of coupling agents,
biocides, lubricants, fluidizing agents, and dispersants.
21. The molding composition of claim 1 further comprising a ceramic
powder.
22. A process comprising the steps of: a) injecting a molding
composition selected from the group consisting of the molding
composition recited in claim 1 and the molding composition recited
in claim 14 into a mold at a temperature above the gel point of
said gel-forming material; b) cooling said molding composition in
said mold to a temperature below the gel point of said gel-forming
material to produce a self supporting molded article; c) removing
said article from said mold; d) substantially removing said solvent
from said molded article; and e) sintering said molded article in a
protective atmosphere and under such conditions of time and
temperature as are required to produce a final density greater than
about 90% of the theoretical density.
23. A process comprising the steps of: a) feeding a molding
composition selected from the group consisting of the molding
composition recited in claim 1 and the molding composition recited
in claim 14 into an extruder; b) extruding said molding composition
through a shape forming die; c) cooling at least the surface of
said extrudate to a temperature below the gel point of said
gel-forming material to produce a shaped article with at least a
self supporting skin; d) substantially removing said solvent from
said shaped article; and e) sintering said shaped article in a
protective atmosphere and under such conditions of time and
temperature as are required to produce a final density greater than
about 90% of the theoretical density.
24. A self supporting shaped article having a composition selected
from the group consisting of the composition recited in claim 1 and
the composition recited in claim 14.
25. A shaped article produced by the process of claim 22.
26. A shaped article produced by the process of claim 23.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to molding compositions and forming
processes for normally rust-prone iron-based powders, and articles
produced therefrom. Metal alloy systems that can be successfully
formed using the processes of the invention, include elemental iron
and iron alloys including low and medium alloy steels, tool steels
and a number of specialty iron-base alloys.
[0003] 2. Description of the Related Art
[0004] The prototypical process for forming metal powders is Metal
Injection Molding (MIM). The steps of fabrication of metal or
ceramic-metallic (GERMET) parts are the following:
[0005] i. Metal and/or ceramic powders are blended with a
thermoplastic binder material and shredded or pelletized to create
an injection molding feedstock with thermoplastic properties.
[0006] ii. The thermoplastic feedstock is injection molded in a
fluid state using methods and tools typical of conventional plastic
injection molding, and removed from the mold in a solid state.
[0007] iii. The "green" state as-molded parts are subjected to
thermal and/or chemical processes to remove the binder phase.
[0008] iv. The remaining "brown" state metal or CERMET parts are
sintered at higher temperatures to effect consolidation and
densification of the molded object.
[0009] Several methods, processes, and binder systems have
previously been described for fabrication of rust prone iron-based
metal alloys and CERMET materials containing them. Each of these
processes has one or more disadvantages that prevents important
applications.
[0010] For example, commonly utilized polymer or wax binder MIM
processes, such as the methods described by Achikita et al. in U.S.
Pat. No. 5,250,254, are limited to small parts, weighing no more
than a few hundred grams, and with maximum section thickness of
less than 10 millimeters. This limitation is imposed by the
difficulties associated with binder removal prior to sintering. The
manufacture of larger parts is prevented or rendered uneconomical
by dimensional instability, cracking, or simply the long times
needed for binder removal from larger and thicker sections. In
addition, great care must by taken when using wax or resin binders
to avoid an undesirable out-of-specification increase in the carbon
content of the alloy as a result of incomplete removal of the
hydrocarbon binder phase.
[0011] Fanelli et al., in U.S. Pat. No. 4,734,237, disclose
agaroid-based aqueous binders for molding of metal and ceramic
powders. The development of aqueous-binder molding compositions,
including those disclosed by Fanelli et al., has largely removed
the part size restrictions imposed by wax and polymer binders since
the binder phase in these largely consists of water which is easily
removed by evaporation under ambient conditions. In the special
case of agar-based binders, the carbon content problem associated
with wax and polymer binders is also effectively addressed since
the agar component of the binder is largely gasified at relatively
low temperatures during the early stages of the sintering cycle.
Further reduction in carbon content is easily achieved by employing
an oxidizing atmosphere in the early stages of the sintering heat
treatment as taught by Zedalis in U.S. Pat. No. 5,985,208. Carbon
content can also be reduced by heat treatment in hydrogen.
[0012] Borate and polyamine additives to enhance the gel strength
of agar-based aqueous binders have been disclosed by Sekido et al.
in U.S. Pat. No. 5,258,155, Rohrbach et al. in U.S. Pat. No.
5,286,767 and Fanelli et al. in U.S. Pat. No. 5,746,957.
[0013] Zedalis et al., in U.S. Pat. No. 6,268,412, incorporated
herein by reference to the extent not incompatible herewith,
disclose molding compositions and processing steps for injection
molding of non-rust-prone stainless steel articles using water-base
agaroid binder systems. Stainless steels, a family of iron-based
alloys containing between 10.5 and 28 atomic % chromium, are
compatible with water-based binder systems, since the high chromium
content confers great resistance to oxidation in the presence of
water.
[0014] When rust-prone iron-base alloy powders are substituted for
the stainless steel powders in the process taught by Zedalis, the
resulting molding feedstock is chemically unstable and must be
molded and dried within hours or days, or the water will react with
the iron-base alloy powder to form rust, thereby substantially
altering and degrading the rheological properties, sintering, and
shrinkage behavior of the feedstock.
[0015] It is commonly observed that ferrous alloys progressively
oxidize or rust in the presence of air and moisture. The essential
chemistry of rust formation, as described in The Metals Handbook,
Volume 1, 8.sup.th Edition, published by the American Society for
Metals, (1961), p257, follows. In the first step of the reaction,
iron reacts with water to form ferrous and hydroxyl ions and
hydrogen:
Fe+2H.sub.2O=Fe.sup.+++2OH.sup.-+H.sub.2 (1)
[0016] In a second step, oxygen, if present, reacts with the
ferrous ions to produce ferric ions which precipitate out of
solution as insoluble ferric hydroxide FeO(OH), otherwise known as
rust. Since the rust deposit does not form a protective layer,
reaction 1 is free to proceed until the metallic iron is consumed
or equilibrium is reached.
[0017] The equilibrium constant for reaction 1 is:
K=[Fe.sup.++][OH.sup.-].sup.2P.sub.H2 (2)
[0018] where the square brackets indicate the concentration of the
species and P.sub.H2 is the partial pressure of hydrogen.
[0019] Equation 2 suggests that the equilibrium concentration of
Fe.sup.++ can be suppressed by increasing the hydroxyl ion
concentration, equivalent to increasing the pH, and or increasing
the hydrogen partial pressure. Numerous hydroxyl ion sources,
including alkali metal hydroxides and carbonates, ammonia, and
various organic amines have been used to inhibit rusting of ferrous
alloys in applications involving intermittent or continuous
exposure to water.
[0020] Rusting can be further inhibited by passivation of the
exposed ferrous alloy surface. Typically, passivation involves a
thin but impervious layer of gamma iron oxide formed, in-situ, by
reaction of the iron with oxidizing ions. Pourbaix, in Atlas of
Electrochemical Equilibria in Aqueous Solutions, Pergamon Press,
New York P. 312 (1966), states that passivation of iron is
difficult at a pH below 8, relatively easy at a pH above 8 and very
easy between pH 10 and 12. Above pH 13, however, iron will corrode
by hyperferrate ion formation. Passivation of ferrous alloy
surfaces can be accelerated by the deliberate addition of oxidizers
to aqueous environments. Nitrite and nitrate salts have been used
in this manner as rust-inhibiting additives in cooling water and
other process water applications. pH buffers, salts formed by
reaction of weak acids with strong bases, are frequently employed
with nitrite and nitrates to maintain pH in the proper range. The
Metals Handbook, Vol 1, 8.sup.th Ed., American Society for Metals,
P. 279, 1961 states that sodium nitrate-borate combinations have
been used to inhibit corrosion in diesel engine cooling systems and
in low pressure, hot water recirculating systems. In this case,
sodium borate (formed by reaction of the weak acid H.sub.3BO.sub.3
with the strong base NaOH), functions as a pH buffer. In a similar
fashion, calcium nitrite is frequently added to concrete
formulations to inhibit rusting of embedded steel reinforcing bars.
In this case, the required alkaline environment is synergistically
provided by the calcium oxide component of the Portland cement
concrete.
[0021] Behi et al. in U.S. Pat. No. 6,261,336, specifically
addressed the problem of rust formation in aqueous injection
molding feedstocks containing rust-prone ferrous alloy powders, and
taught that these materials can be stabilized against rust
formation by the addition of alkaline sodium silicate to the
aqueous binder. It was shown by Behi that carbonyl iron powder
feedstocks containing appropriate amounts of sodium silicate are
somewhat stable against rust formation and attendant hydrogen
evolution, and that the stability is further enhanced by the
addition of potassium borate. While this was an improvement over
unstabilized iron-based aqueous binder feedstocks, experience with
the sodium silicate stabilized feedstocks has revealed that the
sodium silicate addition unfortunately renders the feedstock
pellets tacky and difficult to feed into the barrel of an injection
molding machine. Moreover, residual SiO.sub.2 and/or iron silicate
inclusions, resulting from decomposition of higher loadings of the
sodium silicate during sintering, may be undesirable for
applications requiring maximum ductility and fatigue resistance in
the final sintered steel part.
[0022] A need exists for methods enabling molding of rust prone
iron-based alloys that avoid the size limitations of the prior art
wax and polymer based binders and the processing and ductility
limitations of sodium silicate stabilized aqueous binders.
SUMMARY OF THE INVENTION
[0023] In one embodiment, the invention is a molding composition
comprising at least one metal powder selected from the group
consisting of elemental iron, an iron-base alloy containing less
than 10 wt. % chromium, and an iron-based intermetallic compound;
and a binder comprising a gel forming material, a solvent for said
gel forming material; and at least one compound selected from the
group consisting of inorganic nitrates and inorganic nitrites.
[0024] In another embodiment, the invention is a process comprising
the steps of: injecting an aforedescribed molding composition into
a mold at a temperature above the gel point of said gel-forming
material; cooling said molding composition in the mold to a
temperature below the gel point of said gel-forming material to
produce a self supporting molded article; removing said article
from the mold; substantially removing said solvent from said molded
article; and sintering said molded article in a protective
atmosphere and under such conditions of time and temperature as are
required to produce a final density greater than about 90% of the
theoretical density.
[0025] In another embodiment, the invention is a process comprising
the steps of: feeding an aforedescribed molding composition into an
extruder; extruding said molding composition through a shape
forming die; cooling at least the surface of said extrudate to a
temperature below the gel point of said gel-forming material to
produce a shaped article with at least a self supporting skin;
substantially removing said solvent from said shaped article; and
sintering said shaped article in a protective atmosphere and under
such conditions of time and temperature as are required to produce
a final density greater than about 90% of the theoretical
density.
[0026] In yet another embodiment, the invention comprises shaped
articles comprising a metal powder selected from the group
consisting of elemental iron, an iron-base alloy containing less
than 10 wt. % chromium, and an iron-based intermetallic compound
produced by one of the aforedescribed processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows the effect of NaNO.sub.2 concentration on the
gel strength of 2 wt. % agar.
[0028] FIG. 2 shows the effect of pH on the gel strength of 1.5 wt.
% of a typical food-grade agar.
[0029] FIG. 3 shows the effect of potassium tetraborate on the gel
strength of a 2 wt. % agar gel containing 0.5 wt. % NaNO.sub.2.
DETAILED DESCRIPTION OF THE INVENTION
[0030] Iron-base articles are formed according to this invention
from normally rust-prone powdered materials. In one embodiment, the
invention is a molding composition comprising at least one metal
powder selected from the group consisting of elemental iron, an
iron-base alloy containing less than 10 wt. % chromium, and an
iron-based intermetallic compound; and a binder comprising a gel
forming material, a solvent for said gel forming material, and at
least one compound selected from the group consisting of inorganic
nitrates and inorganic nitrites.
[0031] It will be understood that an iron-base alloy or an
iron-based intermetallic compound is one in which a plurality of
the metal atoms are iron.
[0032] Preferably, the metal powder molding composition
additionally contains a soluble pH-buffering compound and may
contain a ceramic powder.
[0033] The powder particles comprising the metal powder molding
composition are preferably of a spheroidal shape having a weight
average particle size of about 1 to about 50 microns (micrometers).
More preferably, the weight average particle size is about 2 to
about 20 microns and most preferably about 5 to about 15 microns.
In some cases, where maximum solids loading is required, it is
desirable to employ powders with a bimodal particle size
distribution, for example with one population of particles in the
5-15 micron size range and another population of particles with
sizes in the 50-100 micron range. The metal powder particles are
also preferably substantially dense and free of trapped gas pockets
and voids. The iron-base alloy powder is preferably made by the
well-known processes of gas or water atomization, or carbonyl
deposition, but other methods of powder manufacture may be used if
the preferred ranges of particle size, shape and density are
achieved at an acceptable cost.
[0034] Some representative standard iron-base alloys, which are
normally susceptible to rusting but which, in powder form, can be
conveniently compounded and molded using the methods of the
invention, are listed in Tables I to III. These Tables are
abstracted from The Metals Handbook, Volume 1, 8.sup.th Edition,
published by the American Society for Metals, (1961). It will be
understood that many other standard and specialized iron-base
alloys, in addition to those listed in Tables I to III, can also
benefit from application of the invention.
1TABLE I Representative Carbon Alloy Steels Chemical composition
limits for major alloying elements SAE (atomic %) No. C Mn Si Ni Cr
Mo 1010 0.08-0.13 0.30-0.60 0.15-0.35 -- -- -- 1330 0.28-0.33
1.60-1.90 0.15-0.35 -- -- -- 4023 0.20-0.25 0.70-0.90 0.15-0.35 --
-- -- 4024 0.20-0.25 0.70-0.90 0.15-0.35 -- -- 0.20-0.30 4130
0.28-0.33 0.40-0.60 0.15-0.35 -- 0.80-1.10 0.15-0.25 4320 0.17-0.22
0.45-0.65 0.15-0.35 1.65-2.00 0.40-0.60 0.20-0.30 4817 0.15-0.20
0.40-0.60 0.15-0.35 3.25-3.75 -- 0.20-0.30 4820 0.18-0.23 0.50-0.70
0.15-0.35 3.25-3.75 -- 0.20-0.30 9310 0.08-0.13 0.45-0.65 0.15-0.35
3.00-3.50 1.00-1.40 0.08-0.15
[0035]
2TABLE II Representative Tool Steels. Chemical composition limits
for major alloying elements AISI (atomic %) No. C Mn Si Cr Ni Mo W
V Co Molybdenum high-speed steels M1 0.78- 0.15- 0.20- 3.50- 0.3
8.20- 1.40- 1.00- -- M30 0.75- 0.15- 0.20- 3.50- 0.3 7.75- 1.30-
1.00- 4.50- Tungsten high speed steels T1 0.65- 0.10- 0.20- 3.75-
0.3 -- 17.25 0.90- -- T15 1.50- 0.15- 0.15- 3.75- 0.3 1.00 11.75
4.50- 4.75- Intermediate high-speed steels M50 0.78- 0.15- 0.20-
3.75- 0.3 3.90- -- 0.80- -- Chromium hot-work steels H10 0.35-
0.25- 0.80- 3.00- 0.3 2.00- -- 0.25- -- H19 0.32- 0.20- 0.20- 4.00-
0.3 0.30- 3.75- 1.75- 4.00- Tungsten hot-work steels H21 0.26-
0.15- 0.15- 3.00- 0.30 -- 8.50- 0.30- -- H26 0.45- 0.15- 0.15-
3.75- 0.3 -- 17.25 0.75- -- Molybdenum hot-work steels H42 0.55-
0.15- -- 3.75- 0.3 -- 8.50- 0.30- -- Air-hardening, medium-alloy
cold-work steels A2 0.95- 1.00 0.50 4.75- 0.3 0.90- -- 0.15- -- A10
1.25- 1.60- 1.00- -- 1.55- 1.25- -- -- -- High-carbon,
high-chromium, cold-work steels D2 1.40- 0.60 0.60 11.00 0.3 0.70-
-- 1.10 -- D5 1.40- 0.60 0.60 11.00 0.3 0.3 -- 1.00 2.50- Low
carbon mold steels P4 0.10- 0.20- 0.40 2.00- 0.35 -- -- -- -- P6
0.05- 0.35- 0.10- 1.25 3.25- -- -- -- --
[0036] Other standard alloys within the scope of the invention
include Powder Metallurgy (P/M) and Metal Injection Molding (MIM)
alloys listed in MPIF Standard 35: Materials Standards For P/M
Structural Parts, published by Metal Powder Industries Federation,
(1997) and in Materials Standards For Metal Injection Molded Parts,
published by Metal Powder Industries Federation, (1993). Such alloy
compositions are given in Table III below.
3TABLE III Representative P/M and MIM Alloys Chemical composition
limits for major alloying elements Material (atomic %) designation
Fe Ni C Mo Cu F-0000 97.7-100 -- 0.0-0.3 -- -- FC-0200 93.8-98.5 --
0.0-0.3 0.3-0.6 FN-0200 92.2-99.0 1.0-3.0 0-0.3 -- 0-2.5 FL-4605
94.50-97.50 1.70-2.00 0.40-0.70 0.40-0.80 -- FLN-4205 93.95-97.75
1.35-2.50 0.4-0.7 0.50-0.85 -- FLN2-4405 93.30-97.90 1.00-3.00
0.4-0.7 0.7-1.0 -- FLN4-4405 91.30-95.90 3.00-5.00 0.4-0.7 0.7-1.0
-- MIM-4600 94.9-98.5 1.5-2.5 0.0-0.1 0.0-0.5 -- MIM-4650 94.4-98.1
1.5-2.5 0.4-0.6 0.0-0.5 -- MIM-2700 88.9-93.5 6.5-8.5 0.0-0.1
0.0-0.5 --
[0037] The iron-base metal powders are initially mixed with
gel-forming material and a solvent at a temperature sufficient to
insure dissolution of the gel-forming material in the solvent. This
molding composition is proportioned to be fluid enough to enable it
to be readily supplied to a die or mold by any of a variety of
techniques, and especially by injection molding or extrusion.
Generally, the amount of metal powder in the mixture is about 50
percent to about 96 percent by weight of the mixture. Preferably,
the metal powder constitutes about 80 percent to about 95 percent
by weight of the mixture, and most preferably constitutes about 90
percent to about 94 percent by weight of the mixture.
[0038] The gel-forming material employed in the binder is a
material that exhibits a gel strength of at least about 200
g/cm.sup.2, measured at a temperature of 23.degree. C. on a gel
comprising 1.5 wt % of the gel-forming material in 98.5 wt %
solvent. This is the minimum value of gel strength necessary to
produce a self-supporting article having sufficient green strength
to be handled at ambient temperature without the need for special
handling equipment. Preferably gel strength is at least about 400
g/cm.sup.2. Higher values of gel strength can be particularly
useful in producing parts with complex shapes, thinner
cross-sections, and/or higher weights. Furthermore, higher gel
strengths may enable the use of smaller amounts of the gel-forming
material in the molding composition.
[0039] The gel strength of the gel-forming material is measured by
using an apparatus commonly employed in the manufacturing of
industrial gums. The standard apparatus consists of a rod having a
1 cm.sup.2 circular cross section, one end thereof which is
suspended above one pan of a twin pan balance. A large container is
placed on the other pan of the balance. A smaller container on the
pan, above which is suspended the rod, is filled with about 200 ml
(volume) of a gel having about 1.5 wt % of the gel-forming material
dissolved in a solvent. The empty container is then balanced
against the gel-containing container. The rod is then lowered into
contact with the top surface of the gel. Water is then metered into
the empty container and the position of the balance pointer is
continuously monitored. When the top surface of the gel is
punctured by the rod, the balance pointer rapidly deflects across
the scale and the water feed is immediately discontinued. The mass
of water in the container is then measured and the gel strength,
weight (force) per unit area, is calculated.
[0040] The gel strength measurements reported herein in FIGS. 1 and
3 were obtained by this general method with the exceptions that the
agar concentration was 2% and the indenting rod had a smaller
cross-sectional area of 0.0792 cm.sup.2 (corresponding to a
diameter of 0.125 inches). The measurements reported in FIG. 2 were
obtained using a standard 1 cm.sup.2 indentor.
[0041] Gel forming materials include agaroids, proteins, starches,
methyl cellulose and synthetic polymers such as polyvinyl alcohol,
polyacrylamide, and polyvinyl pyrrolidone. Preferred gel forming
materials are agariods, other natural gums and synthetic water
soluble polymers. An agaroid is defined as a gum resembling agar
but not meeting all of the characteristics thereof. (See H. H.
Selby et al., "Agar," Industrial Gums, Academic Press, New York,
N.Y., 2nd ea., 1973, Chapter 3, p. 29). As used herein, however,
agaroid not only refers to any gum, resembling agar, but also to
agar and derivatives thereof such as agarose. An agaroid is
particularly useful because it exhibits rapid gelation within a
narrow temperature range, a factor that increases the rate of
production of molded articles. The gel point of the gel-forming
material is preferably about 10.degree. C. to about 60.degree. C.
and most preferably is about 30.degree. C. to about 45.degree.
C.
[0042] The preferred gel-forming materials are those which are
readily water soluble and comprise an agaroid, or more preferably,
agar, and the most preferred gel-forming materials consist of agar
or agarose.
[0043] The gel-forming material is provided in an amount from about
0.2 wt % to about 5 wt % based upon the total solids in the molding
composition. More than about 5 wt. % of the gel-forming material
may be employed in the molding composition without adverse effects
but this increases costs. Most preferably, the gel-forming material
comprises between about 1 percent and about 3 percent by weight of
solids in the molding composition.
[0044] The solvent for the gel-forming material may be one of a
number of polar liquids, including water, low molecular weight
alcohols, polyhydric solvents such as ethylene glycol and
glycerine, and mixtures thereof, depending upon the composition of
the gel-forming material. It is most preferable to employ a solvent
that can also provide fluidity to the molding composition at
elevated temperature, thus enabling the molding composition to be
easily supplied to a mold. Aqueous solvent systems are particularly
suited for serving as both solvents for the gel-former and
providing the desired rheological properties to the molding
composition. Water is also easily removed from the molded body
prior to and/or during firing.
[0045] Preferred are aqueous solvents containing at least about 50
wt. % water. More preferred are solvents containing 75 wt. % water.
Most preferred are aqueous solvents containing at least about 90
wt. % water. Water is the preferred solvent for agaroid and many
other gel-forming materials.
[0046] Generally, the solvent is about 3 percent to about 50
percent by weight of the molding composition depending upon the
viscosity desired. Where the gel-former is an agaroid and the
solvent is water, the water is between preferably about 4 percent
to about 20 percent by weight of the mixture, with about 5 percent
to about 10 percent by weight being more preferred.
[0047] However, many iron and iron-base alloy powders react with
water in the preferred aqueous binders to form rust. It has been
found that the iron and iron-containing powders can be rendered
stable against rusting, without adverse effects on gel strength or
processibility by incorporation of small amounts of soluble
inorganic nitrites or nitrates in the aqueous binder. Preferred
inorganic nitrites and nitrates are those of lithium, sodium,
potassium, calcium, magnesium, zinc, cobalt, iron, chromium, and
copper. The nitrite or nitrate concentration, expressed as a weight
percentage of the binder is preferably in the range of from 0.05%
to 5.0%, more preferably in the range of from 0.1% to 2.0%, and
most preferably in the range of from 0.15% to 1%. Surprisingly, low
concentrations of nitrite salts do not degrade the gel strength of
agar gels as illustrated in FIG. 1.
[0048] The pH of the binder is preferably held in the range of from
7 to 11 and more preferably in the range of from 8.5 to 9.5.
Maintaining the pH in this range improves stability against rusting
while preserving gel strength. Higher pH would be more beneficial
in facilitating iron surface passivation and inhibiting rusting,
but could be damaging to gel strength of the preferred agaroid gel
forming compounds, FIG. 2 shows the detrimental effect of
increasing pH on the gel strength of a typical food grade agar.
[0049] Preferably, the metal powder molding composition includes a
soluble pH buffering compound. Preferably, the pH buffering
compound is at least one borate salt of an alkali metal element, an
alkali earth element, or ammonia. Most preferably, the pH buffering
compound is a borate salt comprising at least one member selected
from the group consisting of the metaborate (BO.sub.2.sup.-) or
tetraborate (B.sub.4O.sub.7.sup.-2) salt of lithium, sodium,
potassium, magnesium, calcium, strontium, barium, zinc, and
ammonium. The borate concentration, expressed as a weight
percentage of the binder phase, is preferably of from about 0.05%
to about 3.0%, more preferably in the range of from 0.15% to 2.0%,
and most preferably in the range of from 0.2% to 1.0%.
[0050] Binder pH can be adjusted, for example, by changing the
molar ratio of boric acid to base in the borate salt. For example,
a 0.5 wt. % solution of potassium tetraborate
(K.sub.2B.sub.4O.sub.7) in distilled water has a pH of about 9.2
while a 0.5 wt. % solution of potassium metaborate (KBO.sub.2) has
a pH of about 10.8. Borate salts provide the synergistic benefit of
increasing the gel strength of agar-based binders, as illustrated
in FIG. 3, which shows the effect of potassium tetraborate on agar
gel strength.
[0051] Pelletized aqueous agar-binder injection molding feedstocks,
containing nitrite or nitrate and borate binder additives do not
exhibit any tacky or sticky properties, as have been noted with the
sodium silicate stabilized feedstocks disclosed by Behi, and are
easily fed by gravity through the hopper of an injection molding
machine or extruder. Thus, the nitrite (or nitrate)-borate additive
combination is particularly synergistic and beneficial for the
production of useful aqueous-binder molding compositions comprising
normally rust-prone iron-base alloy powders.
[0052] Nitrite and/or nitrate salts and agar can be utilized as
nutrients by various bacteria. Bacterial attack of the molding
compound during storage could result in loss of corrosion
protection, and degradation of the binder strength. Thus, it is
important that the molding compound be kept free of bacterial
contamination, especially if there is to be a significant storage
period between compounding and molding. This can be accomplished
either by storage under sterile conditions, since compounding at
temperatures approaching 100 degrees C. is expected to produce a
sterile as-compounded product, or by the addition of suitable broad
spectrum biocides to the formulation, the latter approach being
preferred. Preferred biocide additives include benzoate salts such
as n propyl p hydroxybenzoate (CAS No. (94-13-3), and methyl
p-hydroxy benzoate, also known as Methyl Parabem (CAS No. 99-76-3).
These particular biocides are highly effective at concentrations of
0.01-0.5 wt %, based on the water content of the molding compound,
and are preferably used in combination to increase the range of
bacterial species addressed. Other biocides can also be used
provided that they are effective and do not materially degrade the
properties or performance of the molding compound.
[0053] The molding composition may also contain a variety of other
additives, which can serve a number of useful purposes. For
example, coupling agents and/or dispersants may be employed to
ensure a more homogeneous mixture. Lubricants such as glycerin and
other monohydric and polyhydric alcohols may be added to assist in
feeding the mixture along the bore of an extruder barrel and/or
reduce the vapor pressure of the liquid carrier and enhance the
production of the near net shape objects. Small molecule sugars,
such as glucose, sucrose, fructose etc., can be used to increase
the fluidity of agar-based molding compositions, as described by
Behi in U.S. Pat. No. 6,262,150. These fluidizing agents can be
used to increase the volume fraction of metal powder in
agaroid-based molding compositions leading to reduced shrinkage in
the sintering step.
[0054] The amount of additives will vary depending on the additive
and its function within the system. However, the additives must be
controlled to ensure that the gel strength of the gel-forming
material is not substantially destroyed. For example, Table IV
below shows the deleterious effect on the gel strength of agar in
aqueous solution by LICA-38J (Kenrich Petrochemicals, Inc.), an
additive that may be used to enhance the processing of the metal
powder in the molding formulation.
4TABLE IV Additive Concentration on Agar Gel Strength at 23.degree.
C Additive Agar Weight %: Gel Strength None 3.85 1480 .+-. 7
g/cm.sup.2 0.95 wt % LICA-38J 3.80 1360 .+-. 7 g/cm.sup.2
[0055] In another embodiment, the invention is a process comprising
the steps of: injecting an aforedescribed molding composition into
a mold at a temperature above the gel point of said gel-forming
material; cooling said molding composition in the mold to a
temperature below the gel point of said gel-forming material to
produce a self supporting molded article; removing said article
from the mold; substantially removing said solvent from said molded
article; and sintering said molded article in a protective
atmosphere and under such conditions of time and temperature as are
required to produce a final density greater than about 90% of the
theoretical density.
[0056] In another embodiment, the invention is a process comprising
the steps of: feeding an aforedescribed molding composition into an
extruder; extruding said molding composition through a shape
forming die; cooling at least the surface of said extrudate to a
temperature below the gel point of said gel-forming material to
produce a shaped article with at least a self supporting skin;
substantially removing said solvent from said shaped article; and
sintering said shaped article in a protective atmosphere and under
such conditions of time and temperature as are required to produce
a final density greater than about 90% of the theoretical
density.
[0057] In yet another embodiment, the invention comprises shaped
articles comprising a metal powder selected from the group
consisting of elemental iron, an iron-base alloy containing less
than 10 wt. % chromium, and an iron-based intermetallic compound
produced by one of the aforedescribed processes.
[0058] A wide range of molding or extrusion pressures may be
employed. Generally, in an injection molding process, the molding
composition is delivered to the mold at pressures from about 20 psi
(137 kPa) to about 3,500 psi (24 MPa), although higher or lower
pressures may be employed depending upon the molding technique
used. Most preferably, the molding pressure is in the range of
about 100 psi (690 kPa) to about 1500 psi (10 MPa). In an extrusion
process, the molding composition is delivered to the shape forming
die at pressures of about 20 psi (137 kPa) to about 1000 psi (6.9
MPa).
[0059] The temperature of the molded or extruded shape, upon
removal from the mold or exit from the extrusion die, is preferably
at or below the gel point of the gel-forming material at least at
its surface. The gel point of the gel-forming material in the
present invention is preferably from about 10.degree. C. to about
60.degree. C., and most preferably is from about 30.degree. C. to
about 45.degree. C. The mold or die temperature is maintained at
less than about 40.degree. C., and is preferably less than about
25.degree. C. The appropriate temperature of the molding
composition can be achieved during or after the mixture is a
supplied to the mold or die, and especially by cooling in the mold
or die.
[0060] After the part has been formed and cooled, the green body
thus formed, is a self-supporting body, requiring no special
handling. It may be dried to substantially remove the solvent
before being placed into a sintering furnace or it may be dried in
the furnace.
[0061] In the furnace, the body is sintered in a reducing
atmosphere to produce the final dense product. Before being brought
to sintering temperature, the green body may first be heated in air
or vacuum to moderate temperatures of about 250.degree. C. to about
600.degree. C. to assist in removal of the small amount of organic
matter in the body. The sintering times and temperatures (sintering
schedules) are regulated according to the powdered material
employed to form the part. Sintering schedules are well known in
the art for a multitude of iron-base materials. For example, Zhang
and German discuss the sintering of MIM Fe--Ni alloys in The
International Journal of Powder Metallurgy, 38, pp. 51-61 (2002).
The fired products produced by the present invention can be very
dense, net or near net shape products.
[0062] As noted previously, an advantage of the processes of the
present invention over prior art processes for molding of
rust-prone iron based alloys is the use of gel forming binders
rather than wax and polymer binders such as described in U.S. Pat.
No. 5,250,254. The benefits of this binder system include the
ability to mold larger and thicker parts and the ability to achieve
higher production rates. Usually, the processing temperatures of
the molding composition in the present invention are less than
100.degree. C., and typically about 90.degree. C. These
temperatures are substantially lower than the temperatures normally
required with the wax and polymer binders. Consequently, the
gel-forming materials of the present invention require
substantially less mold or die cooling.
EXAMPLES
Comparative Example 1
[0063] Four 100 cc gel samples containing approximately 2 wt. % of
agar were made by combining the following ingredients:
[0064] 1. 98 grams distilled water; 2 grams TIC PRETESTED.RTM. Agar
Agar 100 (TIC Gums, Inc., Belcamp, Md.)
[0065] 2. 97.5 grams distilled water; 2 grams TIC PRETESTED.RTM.)
Agar Agar 100; 0.5 grams potassium nitrite (Alpha Aesar CAS#
7758-09-0)
[0066] 3. 97 grams distilled water; 2 grams TIC PRETESTED.RTM.)
Agar Agar 100; 0.5 grams potassium nitrite (Alpha Aesar CAS#
7758-09-0); 0.5 grams potassium tetraborate (K.sub.2B.sub.4O.sub.7)
(Alpha Aesar CAS # 12045-78-2).
[0067] 4. 97 grams distilled water; 2 grams TIC PRETESTED.RTM. Agar
Agar 100; 0.5 grams potassium nitrite (Alpha Aesar CAS# 7758-09-0);
0.5 grams potassium metaborate (KBO.sub.2) (Alpha Aesar CAS #
16481-66-6).
[0068] In each case, the ingredients were stirred together in a
beaker at room temperature and then covered and placed in a covered
container of boiling water for about 30 minutes, during which time
the solutions were stirred again several times. The resulting clear
and homogeneous agar solutions were then cooled to room temperature
and allowed to rest for about 2 hours prior to gel strength
measurements. The results are shown in TABLE V below. Surprisingly,
the agar gel strength reduction, expected at higher pH values, and
illustrated in FIG. 2, is not seen with the potassium borate salt
buffer system.
5TABLE V pH and Gel Strength values of agar gels containing various
combinations of KNO2, K.sub.2B.sub.4O.sub.7 and
K.sub.2B.sub.2O.sub.4 Potassium Nitrite Conc., wt.% Buffer pH Gel
Strength, g/cm.sup.2 0 None 7 1950 0.5 None 7 2160 0.5
K.sub.2B.sub.4O.sub.7 9.5 4350 0.5 KBO.sub.2 10.8 4550
Comparative Example 2
[0069] A dry powder mixture was made by thoroughly combining 8
grams of pure iron metal powder (Atmix 10 micron pure iron powder
from Atmix Corporation, Hachinohe-shi, Aomori-ken, Japan) with 0.2
grams of powdered agar (TIC PRETESTED.RTM. Agar Agar 100). The
resulting dry powder mixture was further combined with 0.8 grams of
distilled water to yield a homogeneous damp powder mixture. This
mixture was loaded into the mixing chamber of a small homemade
screw-type mixer-extruder in which it was heated to approximately
100.degree. C. for 5 minutes, and then further mixed and compounded
by the shearing action of the rotating screw. The compounded
mixture was then extruded directly from the mixing chamber forming
a strand of about 2.5 mm diameter. The color of the freshly
compounded and extruded mixture was steel gray, similar to that of
the pure iron powder. The as-extruded steel gray strand was rubbery
and resilient.
[0070] A portion of the strand was dried at about 90.degree. C. for
20 minutes. Samples of the as-extruded (wet) and the dried strand
were separately stored in small sealed glass containers at room
temperature.
[0071] The color of the stored samples of the dried extrudate
remained steel gray and did not change perceptively over a period
of several months. In contrast, after 24 hours the color of the wet
extrudate was significantly darker, turning to a dull black color
after several days, indicative of a chemical reaction between the
water and the iron particles. After three months of storage in the
wet state, the wet strands had no resilience and had become
brittle, probably due to loss of water to the oxidation reaction
with the iron particles.
Example 1
[0072] An extruded strand was prepared from a molding composition
as described in Comparative Example 2 except that the agar solution
also contained 0.5 weight % potassium nitrite (Alpha Aesar CAS#
7758-09-0). The pH of this solution, as measured using a freshly
calibrated Checker pH meter, was 7.00. A portion of the extruded
strand was dried and both wet and dried portions were stored in
small glass containers at room temperature. Again, the color of the
freshly extruded mixture was steel gray and the texture was rubbery
and resilient. In contrast to the result in Comparative Example 2,
after three months of storage the color of the material stored in
the wet state was unchanged as compared to its initial color or the
color of the dried material. The wet stored material also retained
a rubbery and resilient texture.
Example 2
[0073] An extruded strand was prepared from a molding composition
as described in Comparative Example 2 except that the agar solution
also contained 0.5 weight % potassium nitrite (Alpha Aesar CAS#
7758-09-0) and 0.5 weight % potassium tetraborate (Alpha Aesar CAS
# 12045-78-2). The pH of this solution, as measured using a freshly
calibrated Checker pH meter, was 9.2. A portion of the extruded
strand was dried, and both wet and dried portions were stored in
small glass containers at room temperature. The color of the
freshly extruded mixture was steel gray and the texture was rubbery
and resilient. After three months of storage, the color and texture
of the material stored in the wet state were unchanged.
Example 3
[0074] An extruded strand was prepared from a molding composition
as described in Comparative Example 2 except that the agar solution
also contained 0.5 weight % potassium nitrite (Alpha Aesar CAS#
7758-09-0) and 0.5 weight % potassium metaborate (Alpha Aesar CAS #
16481-66-6). The pH of this solution measured using a freshly
calibrated Checker pH meter, was 10.8. A portion of the extruded
strand was dried, and both wet and dried portions were stored in
small glass containers at room temperature. The color of the
freshly extruded mixture was steel gray and the texture was rubbery
and resilient. After three months of storage in the wet state, the
color and texture of the material were unchanged.
Example 4
[0075] An extruded strand is prepared from a molding composition as
described in Comparative Example 2 except that the agar solution
also contained 0.4 weight % potassium nitrite (Alpha Aesar CAS#
7758-09-0), 0.1 wt. % potassium nitrate (Alpha Aesar CAS#
7757-79-1) and 0.5 weight % potassium tetraborate (Alpha Aesar CAS
# 12045-78-2). The pH of this solution is about 9.5. A portion of
the extruded strand is dried, and both wet and dried portions are
stored in small glass containers at room temperature. The color of
the freshly extruded mixture is steel gray and the texture is
rubbery and resilient. After three months of storage in the wet
state the color and texture of the material are expected to be
unchanged.
Example 5
[0076] A dry powder mixture was made by thoroughly mixing 4000
grams of iron-2% nickel powder (Atmix P20F from Atmix Corporation,
Hachinohe-shi, Aomori-ken, Japan) with 88.88 grams of powdered agar
(TIC PRETESTED.RTM. Agar Agar 100) in a Paul O. Abbe, water jacket
heated, 0.5 gallon capacity sigma-blade mixer running at about 20
rpm for 5 minutes at room temperature.
[0077] The manufacturer's specifications for the iron-2% nickel
powder are shown in Tables VI and VII below.
6 TABLE VI Elemental Analysis Wt. % C 0.46 Si 0.3 Mn 0.46 P 0.015 S
0.022 Ni 2.00 Cr 0.14 O 3200 PPM
[0078]
7 TABLE VII Particle Size Wt. % +30 micron 0.46 30-20 micron 16.5
20-10 micron 28 -10 micron 55 Weight Avg. 9.17 Particle Size micron
Tap density 4.95 g/cc
[0079] A solution of 1.8 grams of potassium nitrite (Alpha Aesar
CAS# 7758-09-0) and 1.8 grams of potassium tetraborate (Alpha Aesar
CAS # 12045-78-2) in 400 cc of distilled water was then added to
the sigma mixer gradually, and mixing was continued at 20 rpm for
an additional 20 minutes, still at room temperature, producing a
loose, damp powder mixture containing about 89 wt. % Fe-2Ni metal
powder, 8.9 wt. % of the nitrite-borate solution, and 1.98 wt. %
agar. The mixer water jacket temperature was then raised to about
89.degree. C. over a period of 30 minutes, and held at this
temperature for an additional 30 minutes while continuing to mix at
20 rpm. During heating, the temperature of the mixture rose to
about 82.degree. C. and the consistency of the mixture was altered
from a loose powder to a high-viscosity fluid as the agar
dissolved. The pH of the mixture was expected to be approximately
9.5 based on the borate content. The gel point temperature was
about 40.degree. C.
[0080] The blended mixture was then allowed to cool to 38.degree.
C., at which point it was shredded twice using the coarse grater
attachment of a Hobart Kitchen Aid food processor. After shredding
and cooling, the material had a rubbery texture. The moisture
content measured after shredding, using an Arizona Instruments
Computrac Moisture Analyzer, was 7.28 wt. %, somewhat below the
targeted level of 8 wt. %. Two additional batches were compounded
and shredded in the same way, except that additional water was
added to compensate for evaporation losses. The three batches of
shredded molding compound were blended together. The moisture
content of the combined material was 8.22 wt. %. The material was
placed in a tightly sealed plastic bucket. After 5 weeks of
storage, the moisture content of material, its color and
appearance, and its texture were unaltered.
[0081] The shredded and aged material was supplied to the hopper of
a reciprocating screw Boy 22 ton injection molding machine. Several
dozen standard tensile test bars (mold cavity dimensions: overall
length 6.4", gauge section 2".times.0.5".times.0.125") were molded
using screw rotation speed of 100 rev/min., barrel and nozzle
temperature setpoints of 185.degree. F. (85.degree. C.), injection
pressure in the range of 400-800 psi, and injection speed in the
range of 1-2 inches per second. The mold temperature was maintained
at 18.degree. C.
[0082] The molded test bars, thus prepared, were air dried for 24
hours resulting in a stable residual moisture content of 0.4%.
Thirteen of the bars were then sintered in a 2 cubic foot batch
furnace (CM Furnaces, Inc) in an atmosphere of flowing hydrogen.
The sintering schedule was as shown in Table VIII below.
8 TABLE VIII Ramp at 100.degree. C./Hour to Hold for 4 Hours
600.degree. C. Ramp at 100.degree. C./Hour to Hold for 1 Hours
1149.degree. C. Ramp at 100.degree. C./Hour to Hold for 3 Hours
1360.degree. C. Ramp at 100.degree. C./Hour to Room Temperature
[0083] The width and thickness dimensions of the parts were
measured before and after sintering in order to determine the
sintering shrinkage. The sintering shrinkage in the width direction
was 15.6% with an estimated standard deviation of 0.8%. The
sintering shrinkage in the thickness direction was 15.6% with an
estimated standard deviation of 0.7%.
[0084] The theoretical density of the Fe 2Ni alloy is
approximately: 1 theor . = wt . fraction Fe .times. Fe + wt .
fraction Ni .times. Ni = 0.98 .times. 7.86 g / cm 3 + 0.02 .times.
8.90 g / cm 3 = 7.88 g / cm 3
[0085] The average density of the sintered test bars was 7.54 g/cc
with an estimated standard deviation of 0.016 g/cm.sup.3. The
measured density represented about 95.7% of the theoretical density
of the Fe 2Ni alloy.
[0086] One of the sintered test bars was sectioned, polished, and
examined metallographically. The observed structure consisted of a
homogenous continuous iron phase containing uniformly distributed
1-5 micron spherical voids. Etching with a standard 2% nital
solution revealed equiaxed 30-50 micron grains with some of the
distributed spherical voids lying on grain boundaries while others
were positioned within the grains.
[0087] Six of the test bars were subjected to tensile strength and
hardness testing. The average hardness was Rockwell B 90.8 with an
estimated standard deviation of 1.2, the average ultimate tensile
strength was 84.75 ksi (810 MPa) with an estimated standard
deviation of 0.35 ksi (2.4 MPa), and the average elongation was
17.96% with an estimated standard deviation of 1.32%. These
properties exceed the typical values for alloy MIM-4650 given by
MPIF Standard 35 published by the Metal Powder Industry
Foundation.
[0088] Having thus described the invention in rather full detail,
it will be understood that such detail need not be strictly adhered
to but that further changes and modifications may suggest
themselves to one skilled in the art, all falling within the scope
of the invention as defined by the subjoined claims.
* * * * *