Note: Descriptions are shown in the official language in which they were submitted.
11198~7
INFILTRATED MOLDED ARTICLES OF SPHERICAL
NON-REFRACTORY METAL POWDERS
-
This invention relates to a process for forming infil-
trated, molded metal articles made from metal powders and the
articles so made. In another aspect, it relates to a process for
making infiltrated, molded metal articles by using metal powders
and a binder comprising a thermoplastic material. In a further
aspect, it relates to a powder metallurgy process for forming molds
or dies and to articles so made. In yet a further aspect, it re-
lates to a process for making dental prostheses and to the articles
so made.
In the art of powder metallurgy a significant advancement
is that disclosed in United States Pat. No. 3,823,002 (Kirby et al).
Kirby et al disclose the production of molded refractory articles
from a plastic mixture of refractory multimodal granules and a fugi-
tive thermoplastic organic binder, which mixture is shaped to form
a green article that is heated to remove the binder and form a por-
ous skeleton having no perceptible necking between the largest
contiguous granules present in the skeleton, the latter then
being infiltrated with a molten infiltrant of a second metal
with a melting point less than one-half of the refractory powder.
Briefly, this invention comprises, in one aspect,
molding a plastic mass comprising a mixture of spherical,
non-refractory metal powder and a heat-fugitive organic binder
comprising a thermoplastic material to form a green article
replica of a master, heating the molded green article to drive
off or remove the binder and lightly sinter the non-refractory
particles and form a porous, non-refractory, monolithic skele-
tal article having necking between contiguous particles
thereof, and infiltrating the skeletal article with a melt of
111~38~7
metal having a melting point which is at least 25 Kelvin
lower than the lowest melting point of the lowest melting
said non-refractory spherical metal powder.
The resulting shaped, monolithic, metal article of
5 this invention is a homogeneous infiltrated article comprising
as a major portion a first continuous phase of spherical,
non-refractory metal particles which are metallurgically
integral at their contiguous points of contact in the form
of a skeleton of interconnected globules with perceptible
10 necking, when viewed by a light microscope, between the
largest contiguous particles thereof, and a second continuous
phase of a metal which has a melting point of at least 25
- Kelvin lower than the lowest melting point of lowest melting
said spherical non-refractory particles and occupies the
15 volume of said article not occupied by said skeleton of spheri-
cal particles, said article thereby comprising two intermeshed
matrices and being substantially void free.
; Unless the context indicates otherwise, "homogeneous"
as: used herein means that when a representative cross-section
20 Of either the interior or the peripheral portion of the molded,
infiltrated article is examined with a light microscope at a
magnification at which the two phases are discernible, e.g.
150X, no significant deviation appears in the number of
spherical, non-refractory particles in a given area, and that
25 the infiltrant is uniformly dispersed around and between the
non-refractory spherical particles, and that there is no unique
axis or densification of the spherical particle in any portion
of the article (especially in the peripheral portion, i.e.,
the portion adjacent the surface of the article), such as that
30 indicative of the use of pressure to introduce coherence to
the non-refractory spherical metal particles. These homogeneous
-2-
8~
articles are essentially free of interior and surface defects
and therefore exhibit uniform physical, chemical, electrical
and mechanical properties. In addition, the two intermeshed
homogeneous matrices impart additional desirable properties,
5 e.g. resistance to wear and to impact.
Some shrinkage occurs in the process of this inven-
tion. Exactly how much shrinkage occurs depends upon the
process parameters chosen, especially the material used to pro-
duce a mold from the master and the temperature at which light
10 sintering is accomplished. Once the magnitude of process
shrinkage has been determined for given process parameters,
it can be compensated for, e.g. by machining a master to
oversize. With compensation for process shrinkage, a pre-
cision tolerance, i.e. the percent deviation of the final in-
15;filtrated article from blue print specification, of betterthan about + 0.2% can be obtained, e.g. + 0.1.
The homogeneity and precision tolerance of the non-
refractory metal articles of this invention means that these
articles are particularly well-suited for applications where
20 close dimensional tolerances are desirable such as articles
with intric~te or complex shapes and surfaces with fine details,
e.g. dental prostheses and injection molding dies.
In the accompanying drawing,
FIG. 1 is a flow diagram showing the manufacture of
25 a molded article of this invention; and
FIG. 2 is a pen-and-ink sketch of a photomicrograph
of ~n infiltrated non-refractory skeleton of a molded article
of this invention.
In the practice of this invention, a metal powder
30 composed of spherical particles of non-refractory metal is
used to make a monolithic skeleton or matrix thereof. "Non-
-3-
refractory" as the term is used herein means metals with
melting temperatures in the range of about lOOO~C to 1800C
(1273K to 2073K). "Spherical" as used herein means es-
sent~,ally spherical and is inclusive of spheroidal, oblate,
5 or prolate. Minor deviation from precise sphericity does not
adversely affect the use of powders in this invention. Re-
presentative non-refractory metals useful in this invention
include iron, cobalt, and nickel and their alloys. Typical
alloying elements for such alloys include chromium, molyb-
10 denum, tungsten, carbon, silicon and boron, and combinationsthereof. Unless otherwise in~icated, it is to be understood
that "metal" as used herein includes elemental metal and
alloys. The production of spherical metal particles, useful
in the practice of this invention, is described in the art
15 e.g. U.S. Patents 3,988l524l 3l258l817 and 3,041,672.
Commercially available non-refractory spherical particles
or powders which can be used in this invention include alloys
number 1, 21, and 157 sold by Cabot Corp. under the "Stellite"
trademark, Special Metals Corporations' Co-6 alloy sold under
20 the "Vertx" trademark, and stainless steel type 410 (American
Iron and Steel Institute specification). These commercially
available powders generally exhibit a mono-modal size distri-
bution curve and comprise a mixture of fractions of small
particle sizes and fractions of larger particle sizes. Because
2~ of their commercial availability, these mono-modal powders
are preferred in the practice of this invention and the pro-
perties of the molded articles of this invention can be achieved
without requiring the use of multi-modal powders. Mixtures of
these commercially available powders can be used in the practice
30 of this invention. The size of the spherical metal powders
useful in this invention is a broad distribution of about 1 to
-4-
200 ~m (micrometers) diameter particles with those less than
44 ~nl (-325 mesh U.S. Sieve Size) in diameter being preferred
for optimum surface finish. Commercially available spherical
meta] particles may contain a small proportion of particles
5 with a diameter of less than 1 ~m; such small particles do
not adversely affect this invention as long as the proportion
of such particles is not sufficient to prevent contact between
the larger particles present and therefore interfere with effi-
cient packing. The calculated surface area of spherical par-
10 ticles falling within the size range preferred in the practiceof this invention is about 1.8xlO 2 m2/g to 14.2xlO 2 m2/g.
The desired surface geometrics of the infiltrated
molded article will be a principle factor in determining the
particle size and size distribution of spherical particles to
15 be used in making such articles. If intricate detail or high
surface finish is desired, the particle size distribution
chosen will have a larger proportion of small diameter par-
ticles; conversely, if little detail or a rough surface finish
is re~uired, a distribution with a larger proportion of large
20 diameter spherical particles may be employed.
The use of spherical metal particles produces a
number of significant advantages over irregularly shaped
metal particles. Irregularly shaped granules, because of
the possibility of multiple, interparticle contacts between
25 any two given particles, tend to form interparticle mechanical
bridges which adversely affects their flow characteristics.
In contr~st, any two spherical particles are capable of but
a single interparticle contact and therefore do not mechani-
cally bridge. Hence, the irregularly shaped particles neither
30 flow as readily nor do they fill intricate mold details as
completely as spherical particles, even when vibrated. Higher
--5--
8~7
loading of the organic binder is possible with spherical par-
ticles. "Loading" refers to the mass of particles that may
be carried in a given amount of softened organic binder.
Spherical particles pack more efficiently than irregularly
5 shaped particles and therefore less binder is required for
a given mass of spherical particles. Better packing also
produces metal skeletons with a more uniform porosity prior to
infiltration. By "porosity" we mean the interstitial passage-
ways between the lightly sintered spherical metal particles
10 of which the skeleton (or first continuous phase) is composed.
The volume of the infiltrated article to be occupied
by the skeleton of spherical metal particles will also deter-
mine the particle size and size distribution of particles
chosen. The infiltrated article will contain as the major
15 portion thereof lightly sintered spherical metal particles,
with at least 60 volume percent preferably, (and more preferably,
at least 65 volume percent) and not in excess of about 80
volume percent spherical metal particles. The volume percent
of the article occupied by spherical metal particles is con-
20 trolled by the degree of loading of the organic binder. Varia-
tion of particle size and size distribution to adjust loading
is known in the art, e.g. see R. K. McGeary, J. Am Ceram. Soc.
44, 513-22 (1961).
Organic binders suitable for use in this invention
25 are those which melt or soften at low temperatures, e.g. less
than 180C, preferably less than 120C, thereby providing the
metal powder-organic binder mixture with good flow properties
when warmed and yet allow the powder-binder mixture to be
solid at room temperature so that a green article molded there-
30 from can be normally easily handled without collapse or defor-
mation. The binders used in this invention are those which
-6-
are heat fugitive, that is, which burn off or volatilize when
the green article is heated without causing internal pressures
on the resulting non-refractory skeletal article due to its
vaporization and without leaving substantial binder residue
5 on the skeletal article resulting from such heating step.
Organic thermoplastics, or mixtures or organic thermo-
plastics with organic thermosets, are mixed with non-refractory
spherical metal powders to form a moldable pastelike or plastic
mass when the resulting binder-powder mixture is heated.
10 Examples of thermoplastic binders include paraffin, e.g.
"Gulf Wax" (household grade refined paraffin), a combination
of paraffin with a low molecular weight polyethylene, mixtures
containing oleic or stearic acids or lower alkyl esters thereof,
e.g. "Emerest" 2642 (polyethylene glycol distearate, average
15 molecular weight of 400) as well as other waxy and paraffinic
substances having the softening and flow characteristics of
paraffin.
Representative thermosetting materials which can be
used in combination with thermoplastics as binders include
20 epoxide resins, e.g. diglycidyl ethers of bisphenol A such as
2,2-bis~p-(2,3-epoxypropoxy)phenyl]-propane, which can be used
with appropriate curing catalysts. Care must be exercised so
as not to thermally induce cross-linking during the mixing and
molding steps when thermoplastic-thermoset mixtures are used
25 as binders. Once the softened thermoplastic-thermoset binder
mixture and the spherical metal particles have been placed
in the warmed mold and vibrated, curing may be initiated by
further warming the mold. Thermoplastic-thermoset binder
mixtures tsnd to produce green articles that have stronger
30 green strength and thus are more handleable than green articles
-7-
3~7
made with just a thermoplastic as the binder.
The spherical metal powder and organic binder are
preferably mixed in a warmed blending device, e.g., a sigma
blade mixer, the temperature being sufficiently high to soften
5 the organic binder thereby allowing the powder and binder to
be homogeneously mixed. The particular amount of binder used
depends upon the particle size and size distribution of spher-
ical metal particles employed. Sufficient binder should be
used, e.g. 2 to 10 parts by weight if 100 parts metal powder
10 is employed, such as will permit the spherical particles to
flow into and optimally occupy the mold, thereby eliminating
bulk and surface density variations in the molded article.
The powder-binder mixture can be warmed to form a plastic
mass and directly transferred into a flexible mold. Alternat-
15 tively, the warm metal powder-organic binder mixture can be
cooled and the resulting solid milled into a granular, free-
flowing state, (such a granular material being referred to as
"pill dust"), and later warmed and poured into the mold.
In order to provide a mold for molding the pill dust
20 or warm plastic mass into a desired shape, a pattern or replica
is made from a master. A molding material is poured around
the master in a suitable container, the molding material cured,
and the master withdrawn to form a mold which is capable of
reproducing substantially identical copies of the master, in-
25 cluding ~ine details and cross sections, in accordance withthis invention.
The metal articles produced in the practice of this
invention can have a working surface (that is, -the working
portion) that comes into contact with and effectuates a de-
30 formation in a material to be worked, and a support portionthat maintains the working surface in the proper position to
; -8-
produce the desired deformation. For example, a core pin,
produced according to this invention, can be used to form a
hole in an injection molded plastic part. The working surface
of such a core pin is that portion that actually comes into
5 contact with the plastic material to be molded and the support
portion holds the core pin in position so that the desired
hole is produced.
The preferred master has the working surface and
support portion mounted on the extending out of or away from
10 a base. The base may be the remainder of the material from
which the working surface-support portion was produced, or
the working surface-support portion may be mounted on a
separate base after production. A mold of the master is
produced by placing the master in a suitable container,
lS pouring the molding material around the master and curing the
molding material. If the preferred master is used, in the
later light sintering step, a one-piece porous metal skeleton
with a working surface-support portion mounted on a base will
be produced. This is desirable because the metal skeleton
20 so produced may be infiltrated by passing the infiltrant metal
through the base prior to entering the body of the porous
metal skeleton beneath the support portion-working surface.
Infiltrating the metal skeleton through the base permits the
infiltrant to solubilize, i.e. to become enriched with the
25 metal of which the working surface-support portion is composed,
prior to infilt~ating the body of the skeleton beneath the
working surface-support portion. Such enrichment of the infil-
trant metal reduces dimensional changes that would occur if
the body of the skeleton were to be infiltrated with unenriched
30 infiltrant metal and the skeleton metal were to be signifi-
,, _ g _
cantly solubilized in this unenriched infiltrant. After in-
filtration in this manner, the base may be completely removed
or machined to a desired configuration to be used as the sup-
port portion for the working surface. In this latter instance
5 the base functions as both the support portion and base and
therefore the working surface may be mounted directly on the
base.
The molding materials which can be used in the prac-
tice of this invention are those which cure to an elastic or
10 flexible rubbery form and generally have a Shore A durometer
value of about 25-60, and reproduce the fine details of the
master part without significant dimensional change, e.g. with-
out more than 1 percent linear change from the master. The
molding materials should not be degraded when heated to molding
15 temperatures, e.g. 180C, and should have a low cure tempera-
ture, e.g. room temperature. A low temperature curing molding
material will form a mold which maintains close dimensional
control from master to mold. A high temperature curing mold-
ing material will generally produce a mold having dimensions
20 substantially different from those of the master. To maintain
dimensional control, it is preferable that the mold material
have a low sensitivity to moisture. Examples of suitable
molding materials are curable silicone rubbers, such as those
described in Bulletin "RTV" 08-347 of January, 1969, of the
25 Dow Corning Co., and low exotherm urethane resins. Such mold-
ing materials cure to an elastic or rubbery form having a low
post cure shrinkage.
The amount of molding material used to form a mold
of the master can vary depending on the particular molding
30 material used and the shape of the master. It has been found
that about 10-14 cubic centimeters of molding material for
--1 0
11~L9~47
each cubic centimeter of the master will form a mold which
retains the desired flexible properties and also has suffi-
cient strength to support the small hydrostatic head produced
by the plastic powder-binder mass in the mold before solidi-
5 fication of the binder.
The molding conditions, hereinafter discussed, for
molding ~he articles of this invention permit the use of an
inexpensive soft, elastic or rubbery mold because the only
pressure applied is the hydrostatic head of the plastic pow-
lO der-binder mixture in the mold, which pressure is very small
and causes negligible distortion. The mild molding conditions
thus help ensure a precisely molded green article even though
a highly deformable mold is used. In addition, the molding
technique results in a molded green article with a uniform
15 density because of the advantageous flow characteristics of
the spherical powder.
The powder-binder mixture or pill dust, warmed 10C
to 20C or more above the softening point of the binder com-
ponent, can be fed into the vibrating elastic mold that has
2~ been preheated to approximately the same temperature as the
powder-binder mixture, and the mold and its contents can then
be evacuated. By choosing the proper size distribution of
spherical non-refractory particles and a suitable organic
binder, the consistency of the powder-binder mixture is such
25 that when heated above the melting point of said binder in a
vacuum, the mixture can be molded with only slight vibration
to ~nsure removal of air pockets or gas bubbles~
After filling the warmed, evacuated mold, vibration
of the mold is discontinued and the mold is isothermed, e.g.
30 maintained at a constant temperature 10C to 30C above the
softening point of the binder, for a sufficient period, e.g.
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.
1119~3L/l7
about 1 to 24 hours, to ensure wliform complete filling of
the mold. The mold and its contents are vibrated for a short
period prior to cooling.
Cooling the mold and its contents to room tempera-
5 ture solidifies the organic binder and forms the green moldedarticle. If the binder melts at a faixly low temperature,
e.g. 35C - 40C, then it is necessary to cool, e.g. to 0C
to 5C, the mold and its contents to the point where the binder
becomes fairly rigid, preferably in a desiccator to reduce
10 moisture condensation. The solid green article can be easily
demolded by application of a vacuum to the exterior of the
flexible mold. Vacuum demolding allows easy demolding of
shapes that have undercuts. The resulting, demolded, green
article is a faithful replica of the master. This molded
15 article has good green strength due to the hardened matrix
of organic binder supporting the non-refractory spherical
metal particles. The non-refractory powder is homogeneously
dispersed in the organic binder matrix, conducive to forming
a green article with uniform density (because of the uniform
20 distribution of powder within the binder) and to forming a
skeleton therefrom with corresponding uniform porosity when
the binder is removed.
The uniform density of the green molded article is
important in the subsequent firing and infiltration steps.
25 A uniform green density will minimize or prevent shape dis-
tortions when the green molded article is heated and infil-
trated. Also, a uniform density will minimize or prevent
the formation of localized pockets of infiltrant material which
otherwise would make the ultimate finished non-refractory
30 article exhibit unstable and non-uniform electrical or phy-
sical properties.
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, -.,
1119~7
To form the skeletal matrix, the green molded
article is preferably packed in a gently vibrating bed of
non-reactive refractory powder, e.g. alumina or silica, to
preve!nt sagging and loss of dimension upon heating in a pro-
5 gram~lable furnace to a temperature of about 900C to 1400C.
Heating the molded green article removes the organic binder
and lightly sinters or tacks the non-refractory particles to
form a metallurgically integral, handleable, porous, non-
refractory, monolithic article, or skeleton. The term "metal-
10 lurgically integral" as used herein means that there is sol1dstate interatomic diffusion, i.e. there is a solid state bond
formed, between contiguous spherical metal particles. This
heating step, in addition to removing the binder, causes first
stage of sintering of the spherical particles, i.e. formation
15 of interparticle necks, thereby producing a monolithic
article. Programmed heating is preferably employed so as to
cause only minimal spherical particle sintering or tacking
at their contiguous points of contact. Programmed heating
avoids the significant shrinkage that would occur if heating
20 and sintering were continued beyond the first stage, thereby
producing undesirable skeleton shrinkage and increase in
density as interstitial pore volume decreased and the parti-
cles became joined by larger necks. Programmed heating also
avoids the introduction of internal and external cracks other-
25 wise produced by rapid evolution of gaseous binder if thegreen molded article were to be rapidly heated to the light
sintering temperature. Small green molded articles are gen-
erally capable of being heated at a more rapid rate than
larger articles. A heating schedule found suitable for arti-
30 cles as large as 5 cm cubes when, for example, polyethyleneglycol distearate is used for the organic binder, is as fol-
lows:
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Step 1 from room temperature to 200C (about 43C
per hour)
Step 2 from 250C to 400C (about 7.5C per hour)
Step 3 from 400C to the light sintering temperature
(about 100C per hour).
This programmed heating is carried out under a protective
atmosphere, e.g. hydrogen-argon, nitrogen, hydrogen-nitrogen,
hydrogen, dissociated ammonia, and other neutral or reducing
atmospheres known in the powder metallurgy art to prevent oxi-
10 dation of the metal particles.
Heating the green molded article to a temperature in
excess of about 1020C when alumina is used as the refractory
non-reactive support material may cause some alumina to adhere
to the green molded article. For this reason, when a final
15 light sinter temperature in excess of 1020C is intended, the
light sintering process may be stopped at 1020C, and the re-
sulting coherent, handleable molded article may be cooled and
removed from the alumina bed. Alumina adhering to the surface
of the article is gently removed and the article heated to
20 the desired final light sintering temperature without the
n~cessity of support in non-reactive refractory powder. Where
light sintering temperatures of less than 1020C are employed,
surface adhering support material can be removed by gentle
brushing with a camel's hair brush.
To ensure complete filling of the interstitial pore
volume, if a mass of infiltrant metal in excess of the cal-
culated interstitial pore volume is used, excessive wetting
of the skeleton and accumulation or buildup of the infiltrant
on the exterior surface of the article, or "blooming" often
30 will result. If excessive skeleton wetting is minimized by
using slightly less infiltrant than necessary to completely
-14-
ill9~47
fill the voids of the metal skeleton, this will leave unin-
filtrated voids in the final composite and thereby reduce its
mechanical strength and uniformity of electrical and physical
properties.
Surface blooming can be reduced or prevented in
this invention by coating the exterior surface of the lightly
sintered metal skeleton with a thin layer of zirconia powder,
e.g. by lightly spraying the exterior of the metal skeleton
with a suspension of zirconia powder in a readily evaporated
10 or volatilized carrier, e.g. acetone. The zirconia powder
coating reduces surface buildup of the infiltrant and permits
the use of a mass of infiltrant metal in excess of that neces-
sary to just fill the interstices of the metal skeleton with-
out the occurrence of blooming (or uninfiltrated voids). Con-
15 tact between those exterior areas of the skeleton where infil-
tration is to occur, e.g. the base, and the zirconia powder is
to be carefully avoided, e.g. by covering such areas with
masking tape. The zirconia coating step may be used selec-
tively or eliminated if some amount of surface blooming is
20 desired, e.g. to produce a molded article that appears as
though it was formulated completely from the infiltrant metal,
e.g. a decorative art object with a cobalt alloy metal skeleton
infiltrated with silver or a silver alloy.
The porous metal skeleton (preferably zirconia-
25 t~ea~eddas described above) is infiltrated or infused with a
metal or alloy that melts at a temperature below the melting
point of the spherical metal particles of which the metal
skeleton is composed and preferably has the properties dis-
cussed below. Surprisingly, infiltration can be accomplished
30 without substantial dimensional change by using as the infil-
trant a metal which melts at a temperature that is as little
-15-
l~g~7
as 25K less than the melting point of the lowest melting
skeleton particles. When the infiltrant melting point, MPi,
and the melting point of the metal of which the spherical
particles is composed, MPSp, are both expressed in degrees
5 Kelvin, workable MPi/MPSp ratios of as high as .98, with .95
or less being preferred, can be used. As this ratio decreases,
dimensional changes also decrease, which means the lower limit
of the infiltrant metal melting point-skeleton metal melting
point ratio is determined by the desired properties of the
10 final infiltrated articles.
Infiltrants with the preferred properties discussed
below generally have melting points greater than about 700
Xelvin and therefore the lower limit of the melting point
ratio is about .5 with .6 being preferred.
Infiltration of the metal skeleton occurs uniformly
by capillary action without pressure applied to the infiltrant
and without the formation of localized pools of infiltrant
material in the non-refractory skeleton. The non-refractory
metal skeleton can be supported on a bed of refractory, non-
20 reactive powder. The bed arranged so that the solid infil-
trant material, which may be in the form of powder, shot or
bars, is not in direct contact with the metallic skeleton.
As the infiltrant melts, it flows under the influence of
gravity toward that area of the metal skeleton through which
25 infiltration is to occur, e.g. the base, contacts the skeleton
while liquid, and enters the skeleton by capillary action.
Direct contact between the solid infiltrant material and the
metal~lic skeleton can cause bonding of the two during heating.
In addition, differences in the thermal coefficients of ex-
30 pansion or sintering rate between the infiltrant and theskeleton will cause stress and possible cracking of the base
-16-
of the skeleton. No contact between the solid infiltrant and
the metal skeleton is therefore preferred. Because the infil-
trant is uniformly distributed throughout the non-refractory
skeleton body, uniform strength and acceptable electrical
5 characteristics are obtained, with minimal shape distortion
of the final infiltrated object due to the thermal-expansion
coefficient differences discussed above.
The metal infiltrant used will be chosen to suit
the end use for the finished part. When an electrical dis-
10 charge machining electrode is desired, infiltrants havinggodd electrical conductivity, e.g. copper, silver and alloys
of these metals, can be used. Where a harder or stronger
finished article is desired, e.g. as for structural parts,
molds or dies, the infiltrant material as well as the spheri-
15 cal metal particles can be composed of hardenable alloys which
can be further treated to increase the hardness and strength
of the article. Still other metals and alloys having a
melting point below that of the non-refractory skeleton can
be used as infiltrants.
The choice of infiltrant metal is preferably those
metals in which the skeleton metal is substantially insoluble.
Gross dimensional changes and distortion would occur if the
l,nfiltrant substantially dissolved the skeleton metal. Major
solubilization of the skeleton metal in the infiltrant can be
25 minimized by using an infiltrant metal that has been saturated
with the metal out of which the skeleton particles were manu-
factured. As discussed above, solubilization can also be
minimized by infiltrating the metal skeleton through a base.
Additionally, the molten infiltrant metal should wet the non-
30 refractory skeleton metal in order to achieve capillary in-
filtration. Excess infiltrant metal, e.g. up to a volume
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~119847
about 25% greater than the calculated total interstitial pore
volume, can be used if the exterior of the metal skeleton has
been coated with zirconia powder prior to infiltration.
The length of time at infiltration temperature and
5 the infiltration temperature used will be a function of the
size, the wetting characteristics, and the interstitial pore
size of the non-refractory metal skeleton. At a temperature
slightly above the melting point of the infiltrant, thirty
minutes is sufficient to infiltrate a cube shaped skeleton
10 with a volume as large as 130 cc.
After infiltration, the article is cooled and the
exterior zirconia coating is removed, e.g., by peening with
a glass bead peening apparatus (Empire Abrasive Equipment
Corp. Model No. ~-20) at a pressure of 1.4 to 2.8 kg~cm
15 using an 8 mm diameter orifice. If an age hardenable infil-
trant is employed, e.g. copper alloyed with nickel (15%) and
tin (7%), or if the metal skeleton is hardenable the infil-
trated article may be subjected to a low temperature aging
cycle to increase hardness and/or wear resistance. Lastly,
20 excess infiltrant or the superfluous base is machined or cut
away from the shaped composite or working surface producing
the finished infiltrated molded metal article.
A tabulation of representative systems of spherical
metal particles and infiltrants is shown in Table I. The
25 melting points in degrees Kelvin and the ratio of the melting
points is tabulated.
Table II contains the elemental compositions of the
metals in Table I.
-18-
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An optlcal examinatlon Or the working surface Or
the rlnlshed artlcle at a magnlrlcation Or 150X reveals a
contlnuous matrlx Or essentially spherlcal partlcles or lnter-
connected globules ln contact wlth and surrounded by a con-
tlnuous phase of infiltrant. ~o evldence of surface coldwork, e.g. dlsturbed sur~ace metal as produced ln conven-
tional machlnlng operatlons, is seen.
FIG. 2 ls a vlew Or a metallurglcally pollshed ln-
terior cross sectlon Or an infiltrated article of thl~ ln-
vention at a magnification Or 600X. A continuous matrlx 3of essentially spherical metal particles 4 with a distribu-
tion Or sizes ls clearly evident. qhe continuous phase Or
metal inflltrant 6 ln contact wlth and lntermeshed wlth the
skeleton Or spherical metal particles or globules ls seen
with neck formation 7 between particles. At this magnlfica-
tlon, de~lation ~ from spherlclty become apparent. These
deviations are the result of partlal dlssolutlon Or the skele-
ton metal in the molten inriltrant and are characteristic of
the infiltrated metal articles of thls inventlon. Such dis-
solutlon causes the slight loss Or sphericlty and impartsthe somewhat globular, erroded appearanc~ to the spherical,
interconnected non-refractory spherical metal particles.
l'he process of thls lnvelltlon descrlbed above is
lllustrated ln FIG. 1. A master 11 machlned to compensate
for lnherent shrlnkage is molded 12 using a flexlble moldlng
materlal such as "~TV" sillcone rubber. The moldlng materlal
ls cured by the appropriate process depending upon the flex-
ible moldlng compound used, and the machined master is demolded
13 rrom the cured, solid rubbery mold 14. Non-refractory
spherlcal metal powder 16, e.g. "Vertx" Co-6 cobalt-based
powder, Or the appropriate size distrlbutlon ls mixed with
a thermoplastic binder 17, e.g. paraffin or a mixture of a
thermoplastic and a thermosetting binder, and heated 18. The
resulting mass optionally may be allowed to cool 19 to a solid
21, and milled 22 into pill dust, 23, which requires heating
5 24 before feeding the powder-binder mass 26 to the heated
molld 27 or the heated powder-binder or mass 26 can be passed 25
from step 18 directly to the mold 27. The mold 14 is appro-
priately heated 28 prior to filling with the heated mass 26.
The mold and its contents are evacuated while being vibrated
10 29 to remove air bubbles and completely pack the mold 30.
The completely filled mold is isothermed 31 and vibrated 32
briefly prior to cooling 33. Vacuum demolding 35 produces a
rigid handleable green molded article 35.
The resulting green molded article 35 is packed in
15 a non-reactive refractory powder and programably fired 37 to
drive off the thermoplastic binder and cause the metal par-
~icles to lightly sinter to form a porous metal article 38.
The porous article is surface treated 39 and placed in a con-
tainer suitable for infiltration 41 with, for example, copper
20 42. After cooling 43, the resulting infiltrated article 44
may be dressed at the site of infiltration to remove irregular-
ities 45. After vacuum demolding the green molded article 35,
the flexible mold may be recycled 14 to produce another arti-
cle.
The infiltrated non-refractory metal articles of
this invention are uniformly dense, tough, impact resistant
and essentially free of internal and surface defects. They
exhibit uniform physical mechanical and electrical properties
; and a precision tolerance of better than + 0.2% can be achieved.
30~These articles are particularly useful for applications
where tough non refractory articles with close dimensional
-22-
tolerances are required, such as articles having intricate
or complex shapes and surfaces with fine detail, e.g. dental
prostheses, dies for metal die casting, and dies for plactic
injection molding.
Objects and advantages of this invention are illus-
trated in the following examples which should not be construed
to limit the scope of this invention. All parts are by weight
unless otherwise specified.
-23-
1119~7
EXAMPLE I
One hundred parts of a less than 149 ~m (-100 mesh
U.S. Sieve) spherical metal powder cobalt-based alloy
("Vertx" Co-6 sold by Special Metals Corp.) was mixed with 3.5
5 parts of polyethylene glycol distearate ("Emerest" 2642, m~~p.
36C) and the resulting metal powder-binder mixture was
warmed to 66C. The resulting plastic mass was transferred
to a cubical cavity (5.08 cm) of a flexible mold made of
cured "RTV" silicone ruhber which was heated to 66C. The
10 mold was evacuated to 3 Torr and maintained at 66C for 10
minutes, while being vibrated by a Model J 50A Jogger
vibrating at a rheostat setting of 80 to 90. The mold and
its contents were then repressurized and transferred to an
ove~ to be isothermed at 38C for 24 hours. After this iso-
15 thermal treatment, the mold was again vibrated (with arheostat setting of 40) for 5 minutes and allowed to cool to
room temperature over a 2 hour period. The cooled mold and
its contents were placed in a desiccator containing anhydrous
calcium sulfate and cooled to about 4C for 1 hour. The cooled
20 mold and its contents were removed from the desiccator and
the green article was immediately demolded using vacuum
demolding. The resulting green article was placed in a
graphite boat containing alumina powder ("Alcoa" grade A-100)
and vibrated slightly to lightly pack the non-reactive
~ 25 refractory powder around the green article. The boat and
: its contents were placed in a retort in an electric, cam-
controlled Lindberg furnace, and the retort was slowly
evacuated to prevent the alumina powder from scattering
within the furnace. A vacuum of about 0.5 Torr was sufficient
30 to remove most of the rea~ctive gases and the furnace was
rapidly backfilled with an atmosphere of argon containing
~l-24-
11~9~'~7
5~ hydrogen. A dynamic gas atmosphere was maintained during
the heating cycle at a flow rate of 85 liters/hour. The
furnace was heated from room temperature to 250C at a rate
of 43C per hour, from 250C to 350C at a rate of 7.5C
5 per hour, from 350C to 1000C at a rate of 100C per hour,
and maintained at 1000C for l/2 hour to degrade and remove
the binder and lightly sinter the spherical metal particles.
Heating was discontinued and the boat and its contents were
allowed to cool to room temperature under the dynamic gas
10 atmosphere in the furnace. The lightly sintered skeletal
article was removed from the alumina bed and gently brushed
with a camel hair brush to remove any surface adhering alumina.
The surface of the article was then sprayed with an aerosol
suspension made up of 10 g. of zirconia powder (about 1 to 5
15 ~m diameter) in 100 ml acetone. The skeletal article was a
cube, and about 0.5 cm of the portion of the 4 faces, adjacent
one face or base, was covered with masking tape while the
exposed remainder of the five faces was sprayed with the
aerosol suspension. The face or base was not covered with
20 masking tape because it was directed away from the zirconia
spray and therefore protection of this face was unnecessary.
After removal of the masking tape, the green skeletal arti-
cle was placed at the base of a sloped alumina bed located
in a graphite boat. An amount of copper powder ("Gould"
25 type R-64, -100 mesh) was placed on the alumina bed so that
upon melting, the liquid copper would flow by gravity down-
ward toward that portion of the skeletal article not covered
with zirconia powder, contact the metal skeleton, and infil-
trate through the unsprayed exterior surface. The boat and
30 its contents were placed in a,molybdenum~ound ~ t~iæ'~fur-
-25-
..,
1~198~7
nace, and the furnace was evacuated to 0.05 Torr and back-
filled with hydrogen. A dynamic hydrogen atmosphere was
maintained at a flow rate of 141 liters/hour while the tem-
perature was raised from room temperature to 1100C over a
5 2 hour period and maintained at that temperature for 1/2 hour.
After infiltration, the resulting infiltrated article was
cooled and the exterior zirconia coating was removed by
peening it with less than 44 ~m glass beads (-325 mesh)
through a 8 mm orifice at 1.4 to 2.8 kg/cm2 pressure. The
10 peened article was sectioned, metallographically polished,
and, when examined at 50X and 750X, the article appeared
homogeneous with necking between contiguous sintered
spherical particles and no internal cracks, gross porosity
or other discontinuities were observed. Figure 2 is a
15 representation of the appearance of the articles.
EXAMPLES 2 - 17
A number of runs ¦Examples 2-17) were carried out
by the procedure as described in Example I to make other in-
filtrated articles of this invention, these further runs
20 being summarized in Table III. In each of these further
runs, 100 parts of sphexical metal powder were utilized in
making infiltrated articles in the form of impact test bars
5.08 cc in size. Where light sinter temperatures in execss
of 1020C were used, the green molded article was program-
25 ably heated to about 1020C, cooled, removed from the non-
reactive refractory support and then reheated to the light
sinter temperature indicated. Sectioned plate infiltrants
were commercially obtained metals while sectioned slab infil-
trants were laboratory prepared metals. The results of
30 metallurgical Rockwell "C'l and Rockwell "B" hardness tests
-26-
1~984'7
are given in Table III, as we l as Charpy impact tests on
notched and unnotched specimens. Rockwell "B" and "C"
hardness tests were conducted according to ASTM Specifica-
tion E18-74. Impact tests were conducted according to ASTM
5 Specification E23-72. Simple beam, Type A, Charpy impact
specimens were used but were modified so that cross-section
dimensions of 0.399 + 0.003 in. (1.01 cm + .008 cm)were used.
Specimens showing unnotched impact strength were not notched.
In Table III, no internal flaws were exh~bited on
10 fracture faces nor on metallographically polished sections
of the 131 cc cubes in Examples 9 and 10. This is due in
part to the uniform density of the finished infiltrated
articles.
-27-
1119134~7
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29
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H ~ ~ i~ 'I ~ '~ ~3
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g o o
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3~
EXAMPLE 18
Using the procedure of Example 1, a core pin, suit-
able for plastic extrusion r was prepared. The core pin was
approximately 1/8 inches (Q.32 cm) in diameter and was used
5 to produce an end-to-end cylindrical hole running along the
axis of a cylindrical plastic part about 1/2 inch (.27 cm)
long with an exterior diameter of .32 inches (.813 cm).
A "Stellite" No. 1 alloy spherical metal powder (less than
44 ~m) was mixed with 4.61 parts of "Emerest"2642 thermo-
10 plastic organic binder. Light sintering of the green moldedcore pin was continued at 1122C for 45 minutes. Infiltra-
tion with a copper alloy containing nickel (15%) and tin (7%)
was conducted for 45 minutes at a temperature of 1120C.
The infiltrated pin was machined to permit force
15 fitting into the movable portion of a two-part injection
mold. Mating of the core pin on the stationary portion of
the injection mold was assured by sanding the pin tip. After
installation of the core pin in the movable mold portlon, the
entire mold was installed in a VanDorn 75 ton (58,000 kg),
20 screw-type injection molding machine with an injection capa-
city of 5-1/2 ounces (156 g) of polymeric material with the
density of polystyrene. One-hundred twenty injection molded
plastic parts were produced from polystyrene. Each plastic
part was removed from the core pin and ejected from the mold
25 as the movable portion of the mold was opened. A barrel
temperature of 193C was used along with a maximum injection
force of 20,000 psi (14.1x10 kg/m2). The plastic parts
exhibited no extraneous plastic material, indicating that
complete closure of the hole had been obtained with the
30 mounted core pin flush against the stationary mold portion.
-31-
No peening, cracking or wear occurred on the core pin demon-
strating homogeneous physical characteristics of the pin.
EXAMPLE 19
A "Stellite"'21 -270 mesh (less than 53 ~m)
5 spherical metal powder was used to prepare a gage block
using the procedure of Example 1. After light sintering
at l,000C, the skeleton was sprayed with zirconia powder
dispersed in acetone. The coated block was then placed in
contact with B dental inlay casting gold of composition,
10 gold (76~), silver (14.3~), copper (7.5%), palladium (2~),
and indium (bal). Heating to 1,000C for 1/2 hour in a
hydrogen atmosphere produced infiltration of the inlay
casting gold into the spherical metal powder skeleton. No
distortion of the skeleton occurred and the shrinkage of
15 the block from the master process shrinkage averaged 0.7g%.
Rockwell "B" hardness averaged 96.
EXAMPLE 20
Two hundred g. -325 mesh (less than 44 ~m) "Vertx"
Co-6 cobalt-based spherical metal powder was mixed with 2.0
20 g of "Epon"828 thermosetting resin for 5 minutes. One-half
gram epoxy curing catalyst (Shell Oil Co., Type F-l) was
added and mixed for about two minutes. Lastly, 8.0 g. of
butyl stearate was added to produce a smooth putty consis-
tency after about five minutes additional mixing. This mix-
25 ture ~ e~ into la ,~r~,a,t~in~ mold p,r-eheated~to 66C, de-aired
under a vacuum of 1 Torr and repressurized to ambient pres-
sure. The article was then maintained at 66C for 1/2 hour
-32-
,, .. ~ , .
~1~98~7
to cure the thermosetting resin and provide rigidity to the
molded article. The article was demolded, packed in an alumina
bed and heated to 1010C in an argon atmosphere containing 5%
hydrogen as in Example 1. The li~htly sintered article in the
5 shaped of a two-inch (5.08 cm) cube was coated with an aerosol
suspension of zirconia and infiltrated with copper at 1110C
for 45 minutes.
EXAMPLE 21
~ die casting cavity in the shape of a serrated knob
10 approximately 1/2 inch (1.27 cm) in diameter and 1/2 inch
(1.27 cm) long was prepared using the procedure of Example 1.
A male model of the serrated knob was used and generation
reversal was effectuated with two-component casting material
sold under the trademark "Carbalon" 122G. The individual com-
15 ponents of the casting material were cooled to 50F (10C) anddeaired for 5 minutes under roughing pump vacuum (approximately
30 Torr). Equal portions of the two components were mixed and
poured over the male master which had been placed in a suitable
container so as to contain the casting material. The cast
20 material covering the male master was deaired for about one
minute under roughing pump vacuum and cured for about 1 hour
at 50F (10C). The male model was then demolded from the
cured female replica, and the replica was allowed to cure for
an additional 24 hours at room temperature~ The replica die
25 cavity so produced is a female pattern reversal of the original
male model.
The female replica die cavity was replicated
according to procedure of Example 1 using a "Stellite" 1 -325
mesh (less than 44 ~m diameter) cobalt-base alloy and 4.61
30 parts of "Emerest" 2642. The green molded die cavity was
-33-
11~9~7
lightly sintered at 1130C and the resulting metal skeleton
was infiltrated with a copper alloy containing nickel (15~)
and tin (7~). Infiltration was accomplished in a hydrogen
atmosphere with a 45 minutes infiltration period and an infil-
5 tration temperature of 1110C. Zinc, heated to a temperatureof 500C in an air oven, was poured into the infiltrated die
cavity. The zinc was allowed to solidify and the casting was
removed from the cavity. No reaction appeared to occur
between the zinc and the die cavity wall.
Various modifications and alterations of the inven-
tion will become apparent to those skilled in the art without
departing from the scope and spirit of the invention, and it
should be understood that this invention is not to be limited
to the illustrative embodiments and examples set forth herein.
-34-
