Note: Descriptions are shown in the official language in which they were submitted.
O 91J17276 Z~)8155~ pcr/us91/o3l6l
,
DESCRI PTION
FILLER MATERIALS FOR METAL MATRIX COMPOSI~ES
echnical Field
The present invention relates to a novel process for forming a
filler material which can be used in various metal matrix composite
formation processes for forming metal matrix composite bodies.
Particularly, an infiltration enhancer and/or an infiltration enhancer
precursor and/or an infiltrating atmosphere are in communication with a
filler material or a preform, at least at some point during the process,
which permits molten matrix metal to spontaneously infiltrate the filler
material or preform. Such spontaneous infiltration occurs without the
requ;rement for the application of any pressure or vacuum. The amount
of matrix metal provided is sufficient only to coat, to a desired
thickness, substantially all of the filler material. The coated filler
material is thereafter comminuted for use in any desired metal matrix
composite formation process.
Backqround Art
Composite products comprising a metal matrix and a strengthening
or reinforcing phase such as ceramic particulates, whiskers, fibers or
the like, show great promise for a variety of applications because they
combine some of the stiffness and wear resistance of the reinforcing
-;~ phase with the ductility and toughness of the metal matrix. Generally,
a metal matrix composite will show an improvement in such properties as
strength, stiffness, contact wear resistance, coefficient of thermal
-. 30 expansion (C.T.E.), density, thermal conductivity and elevated
temperature strength retention relative to the matrix metal in
; monolithic form, but the degree to which any given property may be
improved depends largely on the specific constituents, their volume or
weight fraction, and how they are processed in forming the composite.
In some instances, the composite also may be lighter in weight than the
;~ matrix metal per se. Aluminum matrix composites reinforced with
ceramics such as silicon carbide in particulate, platelet, or whisker
form, for example, are of interest because of their higher specific
stiffness (e.g., elastic modulus over density), wear resistance, thermal
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WO 91/~7276 2~8 ~_~'f~ - 2 - PCI`/US91/0316
conductivity, low coefficient of thermal expansion ~C.T.E.) and high
temperature strength and/or specific strength (e.g., strength over
density) relative to aluminum.
Various metallurgical processes have been described for the
fabrication of aluminum matrix composites, including methods based on
powder metallurgy techniques and liquid-metal infiltration techniques
which make use of pressure casting, vacuum casting, stirring, and
wetting agents. With powder metallurgy techniques, the metal in the
form of a powder and the reinforcing material in the form of a powder,
whiskers, chopped fibers, etc., are admixed and then either cold-pressed
and sintered, or hot-pressed. The maximum ceramic volume fraction in
silicon carbide reinforced aluminum matrix composites produced by this
method has been reported to be about 25 volume percent in the case of
whiskers. and about 40 vnlllmP nercpnt- in the cacp nf narticlllates
The production of metal matrix composites by powder metallurgy
techn~ques utilizing conventional processes imposes certain limitations
with respect to the characteristics of the products attainable. The
volume fraction of the ceramic phase in the composite is limited
typically, in the case of particulates, to about 40 percent. Also, the
pressing operation poses a limit on the practical size attainable. Only
relatively slmple product shapes are poss;ble without subsequent
processing (e.g., forming or machining) or without resorting to complex
presses. Also, nonuniform shrinkage during sintering can occur, as well
as nonun;formity of microstructure due to segregation in the compacts
and grain growth.
U.S. Patent No. 3,970,136, granted July 20, 1976, to J. C. Cannell
et al., describes a process for forming a metal matrix composite
; incorporating a fibrous reinforcement, e.g. silicon carbide or alumina
whiskers, having a predetermined pattern of fiber orientation. The
composite is made by placing parallel mats or felts of coplanar fibers
in a mold with a reservoir of molten matrix metal, e.g., aluminum,
between at least some of the mats, and applying pressure to force molten
metal to penetrate the mats and surround the oriented fibers. Molten
metal may be poured onto the stack of mats while being forced under
pressure to flow between the mats. Loadings of up to about 50% by
volume of reinforcing fibers in the composite have been reported.
The above-described infiltration process, in view of its
dependence on outside pressure to force the molten matrix metal through
the stack of fibrous mats, is subject to the vagaries of pressure-
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91/17276 ~ PCT/US91/0316t
induced flow processes, i.e., possible non-uniformity of matrix
formation, porosity, etc. Non-uniformity of properties is possible even
though molten metal may be introduced at a multiplicity of sites within
the fibrous array. Consequently, complicated mat/reservoir arrays and
flow pathways need to be provided to achieve adequate and uniform
penetration of the stack of fiber mats. Also, the aforesaid pressure-
infiltration method allows for only a relatively low reinforcement to
matrix volume fraction to be achieved because of the difficulty inherent
in inf;ltrating a large mat volume. Still further, molds are required
to contain the molten metal under pressure, which adds to the expense of
the process. Finally, the aforesa;d process, limited to infiltrating
aligned particles or fibers, is not directed to formation of aluminum
metal matrix composites reinforced with materials in the form of
randomly ori~nt~ p~rtirlPc; whic~PrS ~r fihPr5
In the fabrication of aluminum matrix-alumina filled composites,
aluminum does not readily wet alum;na, thereby making it difficult to
form a coherent product. Various solutions to this problem have been
suggested. One such approach is to coat the alumina with a metal (e.g.,
nickel or tungsten), which is then hot-pressed along with the aluminum.
In another technique, the aluminum is alloyed with lithium, and the
alumina may be coated with silica. However, these composites exhibit
variations ;n propert;es, or the coatings can degrade the filler, or the
matrix contains lithium which can affect the matrix propert;es.
U.S. Patent No. 4,232,091 to R. W. Gr;mshaw et al., overcomes
certain difficulties in the art which are encountered in the production
of aluminum matrix-alumina composites. This patent descr;bes applying
pressures of 75-375 kg/cm2 to force molten aluminum (or molten aluminum
alloy) into a f;brous or whisker mat of alumina which has been preheated
to 700 to 1050C. The maximum volume ratio of alumina to metal in the
resulting solid casting was 0.25/1. Because of its dependency-on
outside force to accomplish infiltration, this process is subject to
many of the same deficiencies as that of Cannell et al.
European Patent Application Publication No. 115,742 describes
making aluminum-alumina composites, especially useful as electrolytic
~ 35 cell components, by filling the voids of a preformed alumina matrix with
;~ molten aluminum. The application emphasizes the non-wettability of
alumina by aluminum, and therefore various techniques are employed to
wet the alumina throughout the preform. For example, the alumina is
coated with a wetting agent of a diboride of titanium, zirconium,
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hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium,
titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inert
atmospheres, such as argon, are employed to facilitate wetting. ~his
reference also shows applying pressure to cause molten aluminum to
penetrate an uncoated matrix. In this aspect, infiltration is
accomplished by evacuating the pores and then applying pressure to the
molten aluminum in an inert atmosphere, e.g., argon. Alternatively, the
preform can be infiltrated by vapor-phase aluminum deposit;on to wet the
surface prior to filling the voids by infiltration with molten aluminum.
To assure retention of the aluminum in the pores of the preform, heat
treatment, e.g., at 1400 to 1800DC, in either a vacuum or in argon is
required. Otherwise, either exposure of the pressure infiltrated
material to gas or removal of the infiltration pressure will cause loss
of aluminum frnm t.h~ hn~v .
The use of wetting agents to effect infiltration of an alumina
component in an electrolytic cell with molten metal is also shown in
European Patent Application Publication No. 94353. This publication
describes production of aluminum by electrowinning with a cell having a
cathodic current feeder as a cell liner or substrate. ]n order to
protect this substrate from molten cryolite, a thin coating of a mixture
of a wetting agent and solub;l;ty suppressor is applied to the alum;na
substrate prior to start-up of the cell or while immersed in the molten
aluminum produced by the electrolytic process. Wetting agents disclosed
are titanium, zirconium, hafnium, s;licon, magnesium, vanadium,
chromium, niobium, or calcium, and titanium is stated as the preferred
agent. Compounds of boron, carbon and nitrogen are described as being
useful in suppressing the solubility of the wetting agents in molten
aluminum. The reference, however, does not suggest the production of
metal matrix composites, nor does it suggest the formation of such a
composite in, for example, a nitrogen atmosphere.
In addition to application of pressure and wetting agents, it has
been disclosed that an applied vacuum will aid the penetration of molten
aluminum into a porous ceramic compact. For example, U.S. Patent No.
3,718,441, granted February 27, 1973, to R. L. Landingham, reports
infiltration of a ceramic compact (e.g., boron carbide, alumina and
beryllia) with either molten aluminum, beryllium, magnesium, titanium,
vanadium, nickel or chromium under a vacuum of less than 10-6 torr. A
vacuum of 10-2 to 10-6 torr resulted in poor wetting of the ceramic by
the molten metal to the extent that the metal did not flow free~y into
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the ceramic void spaces. However, wetting was said to have improved
when the vacuum was reduced to less than 10-6 torr.
U.S. Patent No. 3,864,154, granted February 4, 1975, to G. E.
Gazza et al., also shows the use of vacuum to achieve infiltration.
This patent describes loading a cold-pressed compact of AlB12 powder
onto a bed of cold-pressed aluminum powder. Additional aluminum was
then positioned on top of the AlB12 powder compact. The crucible,
loaded with the AlB12 compact "sandwichedN between the layers of
aluminum powder, was placed in a vacuum furnace. ~he furnace was
evacuated to approximately 10-5 torr to permit outgassing. The
temperature was subsequently raised to llOO-C and maintained for a
period of 3 hours. At these conditions, the molten aluminum penetrated
the porous AlB12 compact.
Il C P~t"nt No. 3,35~,976, gr2nt~d J~nu~r~ 23, lg68 to John. N.
Reding et al., discloses the concept of creating a self-generated vacuum
in a body to enhance penetration of a molten metal into the body.
; Specifically, it is disclosed that a body, e.g., a graphite mold, a
steel mold, or a porous refractory material, is entirely submerged in a
molten metal In the case of a mold, the mold cavity, which is filled
with a gas reactive with the metal, communicates with the externally
located molten metal through at least one orifice in the mold. When the
mold is immersed into the melt, filling of the cavity occurs as the
self-generated vacuum is produced from the reaction between the gas in
the cavity and the molten metal. Particularly, the vacuum is a result
of the formation of a solid oxidized form of the metal. Thus, Reding et
al. disclose that it is essential to induce a reaction between gas in
the cavity and the molten metal. However, utilizing a mold to create a
vacuum may be undesirable because of the inherent limitations associated
with use of a mold. Molds must first be machined into a particular
shape; then finished, machined to produce an acceptable casting surface
on the mold; then assembled prior to their use; then disassembled after
their use to remove the cast piece therefrom; and thereafter reclaim the
mold, which most likely would include refinishing surfaces of the mold
or discarding the mold if it is no longer acceptable for use. Machining
of a mold into a complex shape can be very costly and time-consuming.
Moreover, removal of a formed piece from a complex-shaped mold can also
be difficult (i.e., cast pieces having a complex shape could be broken
when removed from the mold). Still further, while there is a suggestion
that a porous refractory material can be immersed directly in a~molten
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metal without the need for a mold, the refractory material would have to
be an integral piece because there is no provision for infiltrating a
loose or separated porous material absent the use of a container mold
(i.e., it is generally believed that the particulate material would
typically disassociate or float apart when placed in a molten metal).
Still further, if it was desired to infiltrate a particulate material or
loosely formed preform, precautions should be taken so that the
infiltrating metal does not displace at least portions of the
particulate or preform resulting in a non-homogeneous microstructure.
Each of the above-discussed processes could be enhanced by
providing a filler material which exhibits enhanced wetting properties
relative to the filler materials actually utilized in the processes.
DescriDtinn nf (`.nmmnnlv Qwnerl l.l.S. P~tentC ~n~l P~t~n~ ~nnlir~t;nnc
The subject matter of this application is related to that of
several other copending and co-owned patent applications and issued
Patents. Particularly, these other copending patent applications and
issued Patents describe novel methods for making metal matrix composite
matèrials (hereinafter sometimes referred to as "Commonly Owned Metal
Matrix Patents and Patent Applications").
A novel method of making a meta1 matrix composite material is
disclosed in Commonly Owned U.S. Patent Application Serial No.
07/049,171, filed May 13, 1987, in the names of White et al., and
entitled aMetal Matrix Composites", now United States Patent No.
4,828,008, which issued on May 9, 1989, and which published in the EPO
on November 17, 1988, as Publication No. 0291441. According to the
method of this White et al. invention, a metal matrix composite is
produced by infiltrating a permeable mass of filler material (e.g., a
ceramic or a ceramic-coated material) with molten aluminum containing at
least about 1 percent by weight magnesium, and preferably at least about
3 percent by weight magnesium. Infiltration occurs spontaneously
without the application of external pressure or vacuum. A supply of the
molten metal alloy is contacted with the mass of filler material at a
temperature of at least about 675C in the presence of a gas comprising
from about 10 to 100 percent, and preferably at least about 50 percent,
nitrogen by volume, and a remainder of the gas, if any, being a
nonoxidizing gas, e.g., argon. Under these conditions, the molten
aluminum alloy infiltrates the ceramic mass under normal atmospheric
- pressures to form an aluminum (or aluminum alloy) matrix compostte.
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91/17276 PCT/US91/03161
When the desired amount of filler material has been infiltrated with the
molten aluminum alloy, the temperature is lowered to solidify the alloy~
thereby forming a solid metal matrix structure that embeds the
reinforcing filler material. Usually, and preferably, the supply of
molten alloy delivered will be sufficient to permit the infiltration to
proceed essentially to the boundaries of the mass of filler material.
The amount of filler material in the aluminum matrix composites produced
according to the White et al. invention may be exceedingly high. In
this respect, filler to alloy volumetric ratios of greater than 1:1 may
be achieved.
Under the process conditions in the aforesaid White et al.
invention, aluminum nitride can form as a discontinuous phase dispersed
throughout the aluminum matrix. The amount of nitride in the aluminum
matrix may varv depen~ing ~n cllrh f~rtnrc ~c tPm.pPr~t~ y
composition, gas composition and filler material. Thus, by controlling
one or more such factors in the system, it is possible to tailor certain
properties of the composite. For some end use applications, however, it
may be desirable that the composite contain little or substantially.no
aluminum nitride.
It has been observed that higher temperatures favor infiltration
but render the process more conducive to nitride formation. The White
et al, invention allows the choice o~ a balance between infiltration
k;netics and nitride formation.
An example of suitable barrier means for use with metal matrix
composite formation is described in Commonly Owned U.S. Patent
; Application Serial No. 07/141,642, filed January 7, 1988, in the names
of Michael K. Aghajanian et al., and entitled aMethod of Making Metal
Matrix ~omposite with the Use of a Barrier," now U.S. Patent No.
4,935,055, which issued on June 19, l99O, and which published in the EPO
on July 12, 1989, as Publication No. 0323945. According to the method
of this Aghajanian et al. invention, a barrier means ~e.g., particulate
titanium diboride or a graphite material such as a flexible graphite
foil product sold by Union Carbide under the trade name Grafoil~) is
disposed on a defined surface boundary of a filler material and matrix
alloy infiltrates up to the boundary defined by the barrier means. The
~- barrier means is used to inhibit, prevent, or terminate infiltration of
the molten alloy, thereby providing net, or near net, shapes in the
resultant metal matrix composite. Accordingly, the formed metal matrix
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composite bodies have an outer shape which substantially corresponds to
the inner shape of the barrier means.
The method of U.S. Patent No. 4,828,008 was improved upon by
Commonly Owned and Copending U.S. Patent Application Serial No.
07/517,54~, filed April 24, 1990, which is a continuation of U.S. Patent
Application Serial No. 07/168,284, filed March 15, 1988 (now abandoned),
in the names of Michael K. Aghajanian and Marc S. Newkirk and entitled
"Metal Matr;x Composites and Techniques for Making the Same", and which
published in the EPO on September 20, 1989, as Publication No. 0333629.
In accordance with the methods disclosed in this U.S. Patent
Application, a matrix metal alloy is present as a first source of metal
and as a reservoir of matrix metal alloy which communicates with the
first source of molten metal due to, for example, gravity flow.
Particularly lln~Pr the rnnditinns desrribed in this patent applir~tinn,
the first source of molten matrix alloy begins to infiltrate the mass of
filler material under normal atmospheric pressures and thus begins the
formation of a metal matrix composite. The first source of molten
matrix metal alloy is consumed during its infiltration into the mass of
filler material and, if desired, can be replenished, preferably by a
continuous means, from the reservoir of molten matrix metal as the
spontaneous infi1tration continues. When a desired amount of permeable
filler has been spontaneously infiltrated by the molten matrix alloy,
the temperature is lowered to solidify the alloy, thereby forming a
solid metal matrix structure that embeds the re;nforcing filler
material. It should be understood that the use of a reservoir of metal
is simply one embodiment of the invention described in this patent
application and it is not necessary to combine the reservoir embodiment
with each of the alternate embodiments of the invention disclosed
therein, some of which could also be beneficial to use in combination
with the present invention.
The reservoir of metal can be present in an amount such that it
provides for a sufficient amount of metal to infiltrate the permeable
mass of filler material to a predetermined extent. Alternatively, an
optional barrier means can contact the permeable mass of filler on at
least one side thereof to define a surface boundary.
Moreover, while the supply of molten matrix alloy delivered should
be at least sufficient to permit spontaneous infiltration to proceed
essentially to the boundaries (e.g., barriers) of the permeable mass of
filler material, the amount of alloy present in the reservoir could
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exceed such sufficient amount so that not only will there be a
sufficient amount of alloy for complete infiltration, but excess molten
metal alloy could remain and be attached to the metal matrix composite
body. Thus, when excess molten alloy is present, the resulting body will
be a complex composite body (e.g., a macrocomposite), wherein an
infiltrated ceramic body having a metal matrix therein will be directly
bonded to excess metal remaining in the reservoir.
Further improvements in metal matrix technology can be found in
commonly owned and copending U.S. Patent Application Serial No.
07/521,043, filed May 9, 1990, which is a continuation-in-part
application of U.S. Patent Application Serial No. 07/484,753, filed
February 23, 1990, which is a continuation-in-part application of U.S.
Patent Application Serial No. 07/432,661, filed November 7, 1989 (now
abandoned!, wh;~`.h ic a rnntinl-!atinn-in-nart appliratl~on nf U.~. P~t~nt
Application Serial No. 07/416,327, filed October 6, 1989 (now
abandoned), which is a continuation-in-part application of U.S. Patent
Application Serial No. 07/349,590, filed May 9, 1989 (now abandoned),
which in turn is a continuation-in-part application of U.S. Patent
Application Serial No. 07/269,311, filed November 10, 1988 (now
20 abandoned), all of which were filed in the names of Michael K.
Aghajanian et al. and all of which are entitled "A Method of Forming
Metal Matrix Composite Bodies By A Spontaneous Infiltration Process, and
Products Produced Therefrom" (an EPO application corresponding to U.S.
Patent Application Serial No. 07/416,327 was published in the EPO on
25 ~une 27, I990, as Publication No. 0375588). According to these
Aghajanian et al. applications, spontaneous infiltration of a matrix
metal into a permeable mass of filler material or preform is achieved by
` use of an infiltration enhancer and/or an infiltration enhancer
- precursor and/or an infiltrating atmosphere which are in communication
30 with the filler material or preform, at least at some point during the
process, which permits molten matrix metal to spontaneously infiltrate
the filler material or preform. Aghajanian et al. disclose a number of
matrix metal/infiltration enhancer precursor/infiltrating atmosphere
systems which exhibit spontaneous infiltration. Specifically,
35 Aghajanian et al. disclose that spontaneous infiltration behavior has
been observed in the aluminum/magnesium/nitrogen system; the
aluminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; and
the aluminum/calcium/nitrogen system. However, it is clear from the
disclosure set forth in the Aghajanian et al. applications that the
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spontaneous infiltration behavior should occur in other matrix
metal/infiltration enhancer precursor/infiltrating atmosphere systems.
Each of the above-discussed Commonly Owned Metal Matrix Patents
and Patent Applications describes methods for the production of metal
matrix composite bodies and novel metal matrix composite bodies which
are produced therefrom. The entire disclosures of all of the foregoing
Commonly Owned Metal Matrix Patents and Patent Applications are
expressly incorporated herein by reference.
Summarv of the Invention
An improved filler material for use in conventional metal matrix
composite formation processes is produced by spontaneously infiltrating
a permeable mass of filler material or a preform with a molten matrix
mPt~l, SnPrlflr~ v~ ~n infiltratlr~n enhanrer and,/or an infiltratic~n
enhancer precursor and/or an infiltrating atmosphere are in
communication with the filler material or preform, at least at some
point during the process, which permits molten matrix metal to
spontaneously infiltrate the filler material or preform. The amount of
matrix metal provided is sufficient only to coat, to a desired
thickness, substantially all of the filler material. ~he coated filler
material is thereafter comminuted for use in any desired metal matrix
composite body formation process.
In a first preferred embodiment, a precursor to an infiltration
enhancer may be supplied to at least one of, a filler material or
preform, and/or a matrix metal and/or an infiltrating atmosphere. The
supplied infiltration enhancer precursor may thereafter react with at
least one of the filler material or preform and/or the matrix metal
and/or the infiltrating atmosphere to produce infiltration enhancer in
at least a portion of, or on, the filler material or preform.
Ultimately, at least during the spontaneous infiltration, infiltration
enhancer should be in contact with at least a portion of the filler
- material or preform.
In another preferred embodiment of the invention, rather than
supplying an infiltration enhancer precursor, an ;nfiltration enhancer
may be supplied d;rectly to at least one of the preform, and/or matrix
metal, and/or infiltrating atmosphere. Ultimately, at least during the
spontaneous infiltration, the infiltration enhancer should be in contact
with at least a portion of the filler material or preform.
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This application discusses various examples of matrix metals,
which at some point during the formation of a filler material for use in
a metal matrix composite body formation process, are contacted with an
infiltration enhancer precursor, in the presence of an infiltrating
atmosphere. Thus, various references will be made to particular matrix
metal/infiltration enhancer precursor/infiltrating atmosphere systems
which exhibit spontaneous infiltration. However, it is conceivable that
many other matrix metal/infiltration enhancer precursor/infiltrating
atmosphere systems other than those discussed in this application may
behave in a manner similar to the systems discussed above herein.
Specifically, spontaneous infiltration behavior has been observed in the
aluminum/magnesium/nitrogen system; the aluminum/strontium/nitrogen
system; the aluminum/zinc/oxygen system; and the
m;nllm/~lrillm~/nit~r~g~n syst~m~ Accnrdin ,ly, ~ven, th^U ,h th~s
application discusses only those systems referred to above herein (with
particular emphasis being placed upon the aluminum/magnesium/nitrogen
system), it should be understood that other matrix metal/infiltration
enhancer precursor/infiltrating atmosphere systems may behave in a
similar manner.
In a preferred embodiment for achieving spontaneous infiltration
into a permeable mass of filler material or a preform, a desired amount
of molten matrix metal is contacted with the preform or filler material.
The preform or filler material may have admixed therewith, and/or at
some point during the process, be exposed to, an infiltration enhancer
precursor. Moreover, in a preferred embodiment, the molten matrix metal
and/or preform or filler material communicate with an infiltrating
atmosphere for at least a portion of the process. In another preferred
. embodiment, the matrix metal and/or preform or filler material
- communicate with an infiltrating atmosphere for substantially all of the
process. The preform or filler material will be spontaneously
infiltrated by molten matrix metal, and the extent or rate of
spontaneous infiltration and formation of metal matrix composite will
. vary with a given set of processing conditions including, for example,
the concentration of infiltration enhancer precursor provided to the
system (e.g., in the molten matrix alloy and/or in the filler material
or preform and/or in the infiltrating atmosphere), the size and/or
composition of the filler material, the size and/or composition of
particles in the preform~ the available porosity for infiltration into
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the preform or filler material, the time permitted for infiltration to
occ~r, and/or the temperature at which infiltration occurs.
Moreover, by varying the composition of the matrix metal and/or
the amount of matrix metal provided and/or the processing conditions,
the amount and composition of the coating formed on the filler material
or preform can be controlled. Thus, the chemical, physical and/or
mechanical properties of the coating formed on a filler material can be
controlled to meet any particular application or need. Accordingly, a
filler material which normally might not be compatible with a particular
matrix metal due to, for example, poor wettability of the filler
material by the matrix metal, can be made to be compatible by placement
of a desirable coating thereon. For example, in the case of an
aluminum/magnesium/nitrogen infiltration system, the composition of the
cnat.ing nn the fillPr m~aterial m.ay be contrnllad. Specifir~lly, the
spontaneous infiltration reaction may be carried out only to the extent
that the magnesium (infiltration enhancer precursor) is reacted with
nitrogen (infiltrating atmosphere) to form a magnesium nitride coating
on the filler. Further, spontaneous infiltration may be carried out to
form a coating of aluminum metal which reacts with the nitrogen
atmosphere to form an aluminum nitride coating. Alternatively, the
atmosphere may be switched to an inert atmosphere (e.g.~ argon) after
infiltration to provide an aluminum coating on the filler. Further, the
atmosphere may be switched to an alternate reactive atmosphere, e.g., an
oxidizing atmosphere, after infiltration to form, for example, an
alumina coating on the filler. Still further, by controlling the
processing conditions during the formation of a metal matrix composite
the nitrogen content of the formed metal matrix composite may be
tailored to meet a wide range of industrial applications.
Moreover, by controlling the composition and/or size (e.g.
particle diameter) and/or geometry of the filler material or the
material comprising the preform, the physical and/or mechanical
properties of the coated filler material can be controlled or engineered
to meet any number of industrial needs. Still further, the mechanical
; and/or physical properties (e.g., density, elastic and/or specific
modulus, strength and/or specific strength, etc.) of the coated filler
material may be tailored depending on the loading of the filler material
in the loose mass or in the preform. For example, by providing a loose
mass or preform comprising a mixture of filler particles of varying
sizes and/or shapes, wherein the density of the filler is greater than
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91/17276 2 ~ 8 .~ ~ . ~ j 6 /US91/03l61
that of the matrix metal, a higher filler loading, due to enhanced
packing of the filler material, may be achieved, thereby resulting in a
- metal matrix composite body with an increased density. By utilizing the
teachings of the present invention, the volume percent of filler
material or preform which can be infiltrated can vary over a wide range.
Once infiltration of the filler material or preform is carried out to an
extent sufficient to coat substantially all of the filler material or
preform, the formed composite body is comminuted to form a coated filler
material for use in any desired metal matrix compos;te formation
process.
Definitions
"Aluminum", as used herein, means and includes essentially pure
metal !~.9. a rP~t nurP; rr~m.mPrrially ~Y~iah un3110yPd
aluminum) or other grades of metal and metal alloys such as the
commercially available metals having impurities and/or alloying
constituents such as iron, silicon, copper, magnesium, manganese,
chromium, zinc, etc., therein. An aluminum alloy for purposes of t~is
definition is an alloy or intermetallic compound in which aluminum is
the major constituent.
"Balance Non-Oxidizinq Gas", as used herein, means that any gas
present in addition to the primary gas comprising the infiltrating
atmosphere, is either an inert gas or a reducing gas which is
substantially non-reactive with the matrix metal under the process
conditions. Any oxidizing gas which may be present as an impurity in
the gas(es) used should be insufficient to oxidize the matrix metal to
any substantial extent under the process conditions.
"Barrier" or "barrier means", as used herein, means any suitable
means which interferes, inhibits, prevents or terminates the migration,
movement, or the like, of molten matrix metal beyond a surface boundary
of a permeable mass of filler material or preform, where such surface
boundary is defined by said barrier means. Suitable barrier means may
be any such material, compound, element, composition, or the like,
` which, under the process conditions, maintains some integrity and is not
substantially volatile (i.e., the barrier material does not volatilize
; to such an extent that it is rendered non-functional as a barrier).
Further, suitable "barrier means" includes materials which are
substantially non-wettable by the migrating molten matrix metal under
the process conditions employed. A barrier of this type appears to
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56
w o 91/17276 ~ - 14 - PCT/US91/0316
exhibit substantially little or no affinity for the molten matrix metal,
and movement beyond the defined surface boundary of the mass of filler
material or preform is prevented or inhibited by the barrier means. The
barrier reduces any final machining or grinding that may be required and
defines at least a portion of the surface of the resulting metal matrix
composite product. The barrier may in certain cases be permeable or
porous, or rendered permeable by, for example, drilling holes or
puncturing the barrier, to permit gas to contact the molten matrix
metal, etc.
"Carcass" or "Carcass of Matrix Metal", as used herein, refers to
any of the original body of matrix metal remaining which has not been
consumed during formation of the metal matrix composite body, and
typically, if allowed to cool, remains in at least partial contact with
the metal m~tr;~ ~nmpncitP hndy which has bePn formPd. It shnuld bP
understood that the carcass may also include a second or foreign metal
therein.
"Filler", as used herein, is intended to include either single
constituents or mixtures of constituents which are substantially non-
reactive with and/or of limited solubility in the matrix metal and may
be single or multi-phase Fillers may be provided in a wide variety of
forms and sizes, such as powders, flakes, platelets, microspheres,
whiskers, bubblès, etc , and may be either dense or porous "Filler"
may also include ceramic fillers, such as alumina or silicon carbide as
fibers, chopped fibers, particulates, whiskers, bubbles, spheres, fiber
mats, or the 1ike, and ceramic-coated fillers such as carbon fibers
coated with alumina or silicon carbide to protect the carbon from
attack, for example, by a molten aluminum parent metal Fillers may
also include metals
"Hot-TopPinq", as used herein, refers to the placement of a
substance on one end (the "topping" end) of an at least partially formed
metal matrix composite which reacts exothermally with at least one of
- the matrix metal and/or filler material and/or with another material
supplied to the topping end This exothermic reaction should provide
sufficient heat to maintain the matrix metal at the topping end in a
molten state while the balance of the matrix metal in the composite
cools to solidification temperature
"Infiltratinq AtmosPhere", as used herein, means that atmosphere
which is present which interacts with the matrix metal and/or preform
(or filler material) and/or infiltration enhancer precursor and/or
,
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~ O 91/17276 2~S~ r~56 PCT/US91/03161
infiltration enhancer and permits or enhances spontaneous infiltration
of the matrix metal to occur.
"Infiltration Enhancer", as used herein, means a material which
promotes or assists in the spontaneous infiltration of a matrix metal
into a filler material or preform. An infiltration enhancer may be
formed from, for example, a reaction of an infiltration enhancer
precursor with an infiltrating atmosphere to form (1) a gaseous species
and/or (2) a reaction product of the infiltration enhancer precursor and
the infiltrating atmosphere and/or (3) a reaction product of the
infiltration enhancer precursor and the filler material or preform.
Moreover, the infiltration enhancer may be supplied directly to at least
one of the preform, and/or matrix metal, and/or infiltrating atmosphere
and function in a substantially similar manner to an infiltration
enhancer which has fnrmed ac; reartinn bPtwppn an jnfiltratinn enh~nrPr
precursor and another species. Ultimately, at least during the
spontaneous infiltration, the infiltration enhancer should be located in
at least a portion of the filler material or preform to achieve
spontaneous infiltration.
"Infiltration Enhancer Precursor" or "Precursor to the
Infiltration Enhancer", as used herein, means a material which when used
in combination with the matrix metal, preform and/or infiltrating
atmosphere forms an infiltration enhancer which induces or assists the
matrix me~al to ~pontaneously infiltrate the filler material or preform.
Without wishing to be bound by any particular theory or explanation, it
appears as though it may be necessary for the precursor to the
infiltration enhancer to be capable of being positioned, located or
transportable to a location which permits the infiltration enhancer
precursor to interact with the infiltrating atmosphere and/or the
preform or filler material and/or matrix metal. For example, in some
matrix metal/infiltration enhancer precursor/infiltrating atmosphere
systems, it is desirable for the infiltration enhancer precursor to
volatilize at, near, or in some cases, even somewhat above the
temperature at which the matrix metal becomes molten. Such
volatilization may lead to: (1) a reaction of the infiltration enhancer
precursor with the infiltrating atmosphere to form a gaseous species
which enhances wetting of the filler material or preform by the matrix
metal; and/or (2) a reaction of the infiltration enhancer precursor with
` the infiltrating atmosphere to form a solid, liquid or gaseous
- infiltration enhancer in at least a portion of the filler matertal or
~ ,, .
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wo 9l/17276 2~ 8.~,5~ 16 - PCT/US91/0316 ~
preform which enhances wetting; and/or (3) a reaction of the
infiltration enhancer precursor within the filler material or preform
which forms a solid, liquid or gaseous infiltration enhancer in at least
a portion of the fi~ler material or preform which enhances wetting.
"Matrix Metall' or /'Matrix Metal AlloY", as used herein, means that
metal which is utilized to form a metal matrix composite (e.g., before
infiltration) and/or that metal which is intermingled with a filler
material to form a metal matrix composite body (e.g., after
infiltration). When a specified metal is mentioned as the matrix metal,
it should be understood that such matrix metal includes that metal as an
essentially pure metal, a commercially available metal having impurities
and/or alloying constituents therein, an intermetallic compound or an
alloy in which that metal is the major or predominant constituent.
NM~/triY MPta,l,/!nfiltratinn Enhanc~r Prl~curcnr~Tnfiltratinr1
Atmosphere Svstem" or "SDontaneous sYstem//~ as used herein, refers to
that combination of materials which exhibit spontaneous infiltration
into a preform or filler material. It should be understood that
whenever a /'//' appears between an exemplary matrix metal, infiltration
enhancer precursor and infiltrating atmosphere that the /'/" is used to
designate a system or combination of materials which, when combined in a
particular manner, exhibits spontaneous inf;ltration into a preform or
filler material.
"Metal Matrix Composite" or /'MMC", as used herein, means a
material comprising a two- or three-dimensionally interconnected alloy
or matrix metal which has embedded a preform or filler material. The
matrix metal may include various alloying elements to provide
specifically desired mechanical and physical properties in the resulting
composite.
A Metal /'Different/' from the Matrix Metal means a metal which does
not contain, as a primary constituent, the same metal as the matrix
metal (e.g., if the primary constituent of the matrix metal is aluminum,
the "different/' metal could have a primary constituent of, for example,
nickel).
NNonreactive Vessel for Housinq Matrix Metal/' means any vessel
which can house or contain a filler material (or preform) and/or molten
matrix metal under the process conditions and not react with the matrix
and/or the infiltrating atmosphere and/or infiltration enhancer
precursor and/or a filler material or preform in a manner which would be
significantly detrimental to the spontaneous infiltration mechanism.
:
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f - -~ 91/172~6 - 17 - PCT/US91/03161
The nonreactive vessel may be disposable and removable after the
spontaneous infiltration of the molten matrix metal has been completed.
"Preform" or "Permeable Preform", as used herein, means a porous
mass of filler or filler material which is manufactured with at least
one surface boundary which essentially defines a boundary for
infiltrating matrix metal, such mass retaining sufficient shape
integrity and green strength to provide dimensional fidelity prior to
being infiltrated by the matrix metal. ~he mass should be sufficiently
porous to accommodate spontaneous infiltration of the matrix metal
I0 thereinto. A preform typically comprises a bonded array or arrangement
of filler, either homogeneous or heterogeneous, and may be comprised of
any suitable material (e.g., ceramic and/or metal particulates, powders,
fibers, whiskers, etc., and any combination thereof). A preform may
exi~t. PithPr 5innl!larly nr ~S an as5am.blaga.
"Reservoir", as used herein, means a separate body of matrix metal
positioned relative to a mass of filler or a preform so that, when the
metal is molten, it may flow to replenish, or in some cases to initially
provide and subsequently replenish, that portion, segment or source of
matrix metal which is in contact with the filler or preform.
aSDontaneous Infiltration", as used herein, means the infiltration
of matrix metal into the permeable mass of filler or preform occurs
without requirement for the application of pressure or vacuum (whether
externally applied or internally created).
Brief DescriDtion of the Fiqures
The following Figures are provided to assist in understanding the
invention, but are not intended to limit the scope of the invention.
~imilar reference numerals have been used wherever possible in each of
the Figures to denote like components, wherein:
Figure I shows a schematic cross-sectional view of the setup used
~` to form the metal matrix composite of Example I; and
Figure 2 is a photomicrograph at 400X magnification of the porous
metal matrix composite body which was subsequently comminuted to form
the filler material of Example I.
Detailed DescriDtion of the Invention and Preferred Embodiments
The present invention relates to forming an improved material for
use in conventional metal matrix composite formation processes by
spontaneously infiltrating a filler material or preform with molten
, ~ '
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: ' ,
:
w o 91/17276 2~ PCT/US91/031
matrix metal. Particularly, an infiltration enhancer and/or an
infiltration enhancer precursor and/or an infiltrating atmosphere are in
communication with the filler material or preform, at least at some
point during the process, which permits molten matrix metal to
spontaneously infiltrate the filler material or preform. The amount of
matrix metal provided is sufficient only to coat, to a desired
thickness, substantially all of the filler material. The coated filler
material is thereafter comminuted for use in any desired metal matrix
composite body formation process.
In a first preferred embodiment, a precursor to an ;nfiltration
enhancer may be supplied to at least one of, a filler material or
preform, and/or a matrix metal and/or an infiltrating atmosphere. The
supplied infiltration enhancer precursor may thereafter react with at
lP~t. nnP nf thP filler m.atDrial or prDfn~m. ~nd~o~ th~ m, t~jv m,Dt~l
and/or the infiltrating atmosphere to produce infiltration enhancer in
at least a portion of, or on, the filler material or preform.
Ultimately, at least during the spontaneous infiltration, infiltration
enhancer should be in contact with at least a portion of the filler
material or preform.
In another preferred embodiment of the invention, rather than
supplying an infiltration enhancer precursor, an infiltration enhancer
may be suppl;Ed directly to at least one of the preform, and/or matrix
metal, and/or infiltrating atmosphere. Ultimately, at least during the
spontaneous infiltration, the infiltration enhancer should be in contact
with at least a portion of the filler material or preform.
This application discusses various examples of matrix metals,
which at some point during the formation of a filler material for use in
a metal matrix composite body formation process, are contacted with an
infiltration enhancer precursor, in the presence of an infiltrating
atmosphere. Thus, various references will be made to particular matrix
metal/infiltration enhancer precursor/infiltrating atmosphere systems
which exhibit spontaneous infiltration. However, it is conceivable that
many other matrix metal/infiltration enhancer precursor/infiltrating
atmosphere systems other than those discussed in this application may
behave in a manner similar to the systems discussed above herein.
Specifically, spontaneous infiltration behavior has been observed in the
aluminum/magnesium/nitrogen system; the aluminum/strontium/nitrogen
system; the aluminum/zinc/oxygen system; and the
aluminum/calcium/nitrogen system. Accordingly, even though this
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! ``'O 91/1/276 PCT/US91/03161
application discusses only those systems referred to above herein (with
particular emphasis being placed upon the aluminum/magnesium/nitrogen
system), it should be understood that other matrix metal/infiltration
enhancer precursor/infiltrating atmosphere systems may behave in a
similar manner.
In a preferred embodiment for achieving spontaneous infiltration
into a permeable mass of filler material or a preform, a desired amount
of molten matrix metal is contacted with the preform or filler material.
The preform or filler material may have admixed therewith, and/or at
I0 some point during the process, be exposed to, an infiltration enhancer
precursor. Moreover, in a preferred embodiment, the molten matrix metal
and/or preform or filler material communicate with an infiltrating
atmosphere for at least a portion of the process. In another preferred
embodiment, th~ matrix m~?t.~l ~nrl~nr pr~fnrm nr fillPr m~t~ri~l
communicate with an infiltrating atmosphere for substantially all of the
process. The preform or filler material will be spontaneously
infiltrated by molten matrix metal, and the extent or rate of
spontaneous infiltration and formation of metal matrix composite w~ll
vary with a given set of processing conditions including, for example,
the concentration of infiltration enhancer precursor provided to the
system (e.g., in the molten matrix alloy and/or in the filler material
or preform and/or in the infiltrating atmosphere), the size and~or
composition of the filler material, the size and/or composition of
particles in the preform, the available porosity for infiltration into
the preform or filler material, the time permitted for infiltration to
occur, and/or the temperature at which infiltration occurs.
Moreover, by varying the composition of the matrix metal and/or
the amount of matrix metal provided and/or the processing conditions,
the amount and composition of the coating formed on the filler material
or preform can be controlled. Thus, the chemical, physical and/or
mechanical properties of the coating formed on a filler material can be
controlled to meet any particular application or need Accordingly, a
filler material which normally might not be compatible with a particular
matrix metal due to, for example, poor wettability of the filler
material by the matrix metal, can be made to be compatible by placement
of a desirable coating thereon. For example, in the case of an
aluminum/magnesium/nitrogen infiltration system, the composition of the
coating on the filler material may be controlled. Specifically, the
spontaneous infiltration reaction may be carried out only to the extent
:'
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WOgl/17276 ~'~ ~ ~5~ - 20 - PCI/US91/0316
that the magnesium (infiltration enhancer precursor) is reacted with
nitrogen (infiltrating atmosphere) to form a magnesium nitride coating
on the filler. Further, spontaneous infiltration may be carried out to
form a coating of aluminum metal which reacts with the nitrogen
atmosphere to form an aluminum nitride coating. Alternatively, the
atmosphere may be switched to an inert atmosphere (e.g., argon) after
infiltration to provide an aluminum coating on the filler. Further, the
atmosphere may be switched to an alternate reactive atmosphere, e.g., an
oxidizing atmosphere, after infiltration to form, for example, an
alumina coating on the filler. Still further, by controlling the
processing conditions during the formation of a metal matrix composite
the nitrogen content of the formed metal matrix composite may be
tailored to meet a wide range of industrial applications.
Mnrenv~r; hy ~nntrnllinn thP Cnmnncitinn 2nrl~nr Ci7P (P~9;
particle diameter) and/or geometry of the filler material or the
material comprising the preform, the physical and/or mechanical
properties of the coated filler material can be controlled or engineered
to meet any number of industrial needs. Still further, the mechanical
and/or physical properties (e g., density, elastic and/or specific
modulùs, strength and/or specific strength, etc.) of the coated filler
material may be tailored depending on the loading of the filler material
in the loose mass or in the preform. For example, by providing a loose
mass or preform comprising a mixture of filler particles of varying
sizes and/or shapes, wherein the density of the filler is greater than
that of the matrix metal, a higher filler loading, due to enhanced
packing of the filler material, may be achieved, thereby resulting in a
metal matrix composite body with an increased density. By utilizing the
teachings of the present invention, the volume percent of filler
` material or preform which can be infiltrated can vary over a wide range.
Once infiltration of the filler material or preform is carried out to an
extent sufficient to coat substantially all of the filler material or
preform, the formed composite body is comminuted to form a coated filler
material for use in any desired metal matrix composite formation
process.
~ 35 With reference to Figure 1, a simple lay-up 10 for forming a
;~ spontaneously infiltrated metal matrix composite is illustrated.
~ Specifically, a filler or preform 1, which may be of any suitable
`~ material, as discussed in detail below, is placed in a non-reactive
.
~ vessel 2 for housing matrix metal and/or filler material. A matrix
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:~ 3 91/17276 '' ~ - i `' Pcr/Us4l/o3l6l
metal 3 is placed on or adjacent to the filler or preform 1. The lay-up
is thereafter placed in a furnace to initiate spontaneous infiltration.
Without wishing to be bound by any particular theory or
explanation, when an infiltration enhancer precursor is utilized in
combination with at least one of the matrix metal, and/or filler
~aterial or preform and/or infiltrating atmosphere, the infiltration
enhancer precursor may react to form an infiltration enhancer which
induces or assists molten matrix metal to spontaneously infiltrate a
filler material or preform. Moreover, it appears as though it may be
necessary for the precursor to the infiltration enhancer to be capable
of being positioned, located or transportable to a location which
permits the infiltration enhancer precursor to interact with at least
one of the infiltrating atmosphere, and/or the preform or filler
material an~!nr mnlt.en matriX metA~l Fnr PY,AmnlP j jn s~me mAtri~
metal/infiltration enhancer precursor/infiltrating atmosphere systems,
it is desirable for the infiltration enhancer precursor to volatilize
at, near, or in some cases, even somewhat above the temperature at which
the matrix metal becomes molten. Such volatilization may lead to: (1)
a reaction of the infiltration enhancer precursor with the infiltrating
atmosphere to form a gaseous species which enhances wetting of the
f;ller material or preform by the matrix metal; and/or (2) a reaction of
the infiltration enhancer precursor with the infiltrating atmosphere to
form a solid, liquid or gaseous infiltration enhancer in at least a
portion of the f;ller material or preform which enhances wetting; and/or
(3) a reaction of the infiltration enhancer precursor within the filler
material or preform which forms a solid, liquid or gaseous infiltration
enhancer in at least a portion of the filler material or preform which
enhances wetting.
Thus, for example, if an infiltration enhancer precursor was
included or combined with, at least at some point during the process,
molten matrix metal, it is possible that the infiltration enhancer could
volatilize from the molten matrix metal and react with at least one of
- the filler material or preform and/or the infiltrating atmosphere. Such
reaction could result in the formation of a solid species, if such solid
species was stable at the infiltration temperature, said solid species
being capable of being deposited on at least a portion of the filler
material or preform as, for example, a coating. Moreover, it is
conceivable that such solid species could be present as a discernable
solid within at least a portion of the preform or filler material. If
w 0 9l/17276 2 ~8 ,~5 ~ - 22 - PCT/US91/031
such a solid species was formed, molten matrix metal may have a tendency
to react (e.g., the molten matrix metal may reduce the formed solid
species) such that infiltration enhancer precursor may become associated
with (e.g., dissolved in or alloyed with) the molten matrix metal.
Accordingly, additional infiltration enhancer precursor may then be
available to volatilize and react with another species (e.g., the filler
material or preform and/or infiltrating atmosphere) and again form a
similar solid species. It is conceiYable that a continuous process of
conversion of infiltration enhancer precursor to infiltration enhancer
followed by a reduction reaction of the infiltration enhancer with
molten matrix metal to again form additional infiltration enhancer, and
so on, could occur, until the result achieved is a spontaneously
infiltrated metal matrix composite.
In order tl Pffert Snnntanpnu~ jnfiltratinn nf the m.atri.Y mPt21
into the filler material or preform, an infiltration enhancer should be
provided to the spontaneous system. An infiltration enhancer could be
formed from an infiltration enhancer precursor which could be provided
(1) in the matrix metal; and/or (2) in the filler material or preform;
and/or (3) from the infiltrating atmosphere; and/or (4) from an external
ZO source into the spontaneous system Moreover, rather than supplying an
infiltration enhancer precursor, an infiltration enhancer may be
supplied directly to at least one of the filler material or preform,
and/or matrix metal, and/or infiltrating atmosphere. Ultimately, at
least during the spontaneous infiltration, the infiltration enhancer
should be located in at least a portion of the filler material or
preform.
In a preferred embodiment of the invention, it is possible that
the infiltration enhancer precursor can be at least partially reacted
with the in~iltrating atmosphere such that the infiltration enhancer can
be formed in at least a portion of the filler material or preform prior
to or substantially contiguous with contacting the filler material or
preform with the matrix metal (e.g., if magnesium was the infiltration
enhancer precursor and nitrogen was the infiltrating atmosphere, the
infiltration enhancer could be magnesium nitride which would be located
in at least a portion of the preform or filler material).
An example of a matrix metal/infiltration enhancer
precursor/infiltrating atmosphere system is the
; aluminum/magnesium/nitrogen system. Specifically, an aluminum matrix
metal can be contained within a suitable refractory vessel which, under
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the process conditions, does not adversely react with the aluminum
matrix metal and/or the filler material when the aluminum is made
molten. A filler material or preform can thereafter be contacted with
molten aluminum matrix metal and spontaneously infiltrated.
Moreover, rather than supplying an infiltration enhancer
precursor, an infiltration enhancer may be supplied directly to at least
one of the preform or filler material, and/or matrix metal, and/or
infiltrating atmosphere. Ultimately, at least during the spo.ntaneous
infiltration, the infiltration enhancer should be located in at least a
portion of the filler material or preform.
Under the conditions employed in the method of the present
invention, in the case of an aluminum/magnesium/nitrogen spontaneous
infiltration system, the preform or filler material should be
sufficiently n~rm~hl~ t.n pprmit the nitrnnpn-cnnt2ininn naC tn
penetrate or permeate the filler material or preform at some point
during the process and/or contact the molten matrix metal. Moreover,
the permeable filler material or preform can accommodate infiltration of
the molten matrix metal, thereby causing the nitrogen-permeated preform
` to be infiltrated spontaneously with molten matrix metal to form a metal
matrix composite body and/or cause the nitrogen to react with an
infiltration enhancer precursor to form inf;ltration enhancer in the
filler material or preform and thereby result in spontaneous
infiltration. The extent of spontaneous infiltration and formation of
the metal matrix composite will vary with a given set of process
conditions, including magnesium content of the aluminum alloy, magnesium
content of the preform or filler material, amount of magnesium nitride
in the preform or filler material, the presence of additional alloying
elements (e.g., silicon, iron, copper, manganese, chromium, zinc, and
the like), average size of the filler material (e.g., particle diameter)
comprising the preform or the filler material, surface condition and
~ type of filler material or preform, nitrogen concentration of the
-~ infiltrating atmosphere, time permitted for infiltration and temperature
at which infiltration occurs. For example, for infiltration of the
; molten aluminum matrix metal to occur spontaneously, the aluminum can be
alloyed with at least about 1 percent by weight, and preferably at least
~ about 3 percent by weight, magnesium (which functions as the
- infiltration enhancer precursor), based on alloy weight. Auxiliary
alloying elements, as discussed above, may also be included in the
matrix metal to tailor specific properties thereof. Additionally, the
2~!8.~ ~5~
WO 91/17276 - 24 - PCI`/US91/0316
auxiliary alloying elements may affect the minimum amount of magnesium
required in the matrix aluminum metal to result in spontaneous
infiltration of the filler material or preform. Loss of magnesium from
the spontaneous system due to, for example, volatilization should not
occur to such an extent that no magnesium was present to form
infiltration enhancer. Thus, it is desirable to utilize a sufficient
amount of initial alloying elements to assure that spontaneous
infiltration will not be adversely affected by volatilization. Still
further, the presence of magnesium in both of the preform (or filler
material) and matrix metal or the preform (or filler material) alone may
result in a reduction in required amount of magnesium to achieve
spontaneous infiltration (discussed in greater detail later herein).
The volume percent of nitrogen in the infiltrating atmosphere
al~n aff~.t..c fn~matinn rates nf the met21 matr-i.Y cnmnncit~ hndy.
Specifical~y, if less than about 10 volume percent of nitrogen is
present in the atmosphere, very slow or little spontaneous infiltration
will occur. It has been discovered that it is preferable for at least
about 50 volume percent of nitrogen to be present in the atmosphere,
thereby resulting in, for example, shorter infiltration times due to a
much more rapid rate of infiltration. The infiltrating atmosphere
(e.g., a nitrogen-containing gas) can be supplied directly to the filler
material or preform and/or matrix metal, or it may be produced or result
from a decomposition of a material
The minimum magnesium content required for the molten matrix metal
to infiltrate a filler material or preform depends on one or more
variables such as the processing temperature, time, the presence of
auxiliary alloying elements such as silicon or zinc, the nature of the
~-- f;ller material, the location of the magnesium in one or more components
of the spontaneous system, the nitrogen content of the atmosphere, and
the rate at which the nitrogen atmosphere flows. Lower temperatures or
shorter heating times can be used to obtain infiltration as the
magnesium content of the alloy and/or preform is increased. Also, for a
- given magnesium content, the addition of certain auxiliary alloying
elements such as zinc permits the use of lower temperatures. For
~ 35 example, a magnesium content of the matrix metal at the lower end of the
- operable range, e.g., from about 1 to 3 weight percent, may be used in
conjunction with at least one of the following: an above-minimum
processing temperature, a high nitrogen concentration~ or one or more
auxiliary alloying elements. When no magnesium is added to the preform,
:
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o 9~ 276 - 25 - 2 ~8.L ~ ~6 PCT/US91/03161
alloys containing from about 3 to 5 weight percent magnesium are
preferred on the basis of their general utility over a wide variety of
process conditions, with at least about 5 percent being preferred when
lower temperatures and shorter times are employed. Magnesium contents
in excess of about 10 percent by weight of the aluminum alloy may be
employed to moderate the temperature conditions required for
infiltration. The magnesium content may be reduced when used in
conjunction with an auxiliary alloying element, but these elements serve
an auxiliary function only and are used together with at least the
above-specified minimum amourt of magnesium. For example, there was
substantially no infiltration of nominally pure aluminum alloyed only
with 10 percent silicon at 1000C into a bedding of 500 mesh, 39
Crystolon (99 percent pure silicon carbide from Norton Co.). However,
in the presence nf m~gnPcium. cil innn h~r~ hepn fnl!n~ tn nrnmntP the
infiltration process. As a further example, the amount of magnesium
varies if it is supplied exclusively to the preform or filler material.
It has been discovered that spontaneous infiltration will occur with a
lesser weight percent of magnesium supplied to the spontaneous system
when at least some of the total amount of magnesium supplied is placed
in the preform or filler material. It may be desirable for a lesser.
amount of magnes;um to be provided in order to prevent the formation of
undesirable intermetallics in the metal matrix composite body. In the
case of a silicon carbide preform, it has been discovered that when the
preform is contacted w;th an aluminum matrix metal, the preform
containing at least about 1% by weight magnesium and being in the
presence of a substantially pure nitrogen atmosphere, the matrix metal
spontaneously infiltrates the preform. In the case of an alumina
preform, the amount of magnesium required to achieve spontaneous
infiltration is slightly higher. Specifically, it has been found that
`~ 30 when an alumina preform, when contacted with a similar aluminum matrix
metal, at about the same temperature as the aluminum that infiltrated
into the silicon carbide preform, and in the presence of the same
nitrogen atmosphere, at least about 3% by weight magnesium may be
required to achieve similar spontaneous infiltration to that achieved in
the silicon carbide preform discussed immediately above.
It is also noted that it is possible to supply to the spontaneous
system infiltration enhancer precursor and/or infiltration enhancer on a
surface of the alloy and/or on a surface of the preform or filler
j material and/or within the preform or filler material prior to
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infiltrating the matrix metal into the filler material or preform (i.e.,
it may not be necessary for the supplied infiltration enhancer or
infiltration enhancer precursor to be alloyed with the matrix metal, but
rather, simply supplied to the spontaneous system). For example, in the
aluminum/mangesium/nitrogen system, if the magnesium was applied to a
surface of the matrix metal it may be preferred that the surface should
be the surface which is closest to, or preferably in contact with, the
permeable mass of filler material or vice versa; or such magnesium could
be mixed into at least a portion of the preform or filler material.
Still further, it is possible that some combination of surface
application, alloying and placement of magnesium into at least a portion
of the preform could be used. Such combination of applying infiltration
enhancer(s) and/or infiltration enhancer precursor(s) could result in a
decrease in the total weiaht n~rcPnt nf ma9npsillm nePdPd tn n~nm.nte
infiltration of the matrix aluminum metal into the preform, as well as
achieving lower temperatures at which infiltration can occur. Moreover,
the amount of undesirable intermetallics formed due to the presence of
magnesium could also be minimized.
The use of one or more auxiliary alloying elements and the
concentration of nitrogen in the surrounding gas also affects the extent
of nitriding of the matrix metal at a g;ven temperature. For example,
auxiliary alloying elements such as zinc or iron included in the alloy,
or placed on a surface of the alloy, may be used to reduce the
infiltration temperature and thereby decrease the amount of nitride
formation, whereas increasing the concentration of nitrogen in the gas
may be used to promote nitride formation.
The concentration of magnesium in the alloy, and/or placed onto a
surface of the alloy, and/or combined in the filler or preform material,
; also tends to affect the extent of infiltration at a given temperature.
Consequently, in some cases where little or no magnesium is contacted
directly with the preform or filler material, it may be preferred that
at least about three weight percent magnesium be included in the alloy.
Alloy contents of less than this amount, such as one weight percent
magnesium, may require higher process temperatures or an auxiliary
alloying element for infiltration. The temperature required to effect
the spontaneous infiltration process of this invention may be lower: (I)
when the magnesium content of the alloy alone is increased, e.g., to at
least about 5 weight percent; and/or (2) when alloying constituents are
mixed with the permeable mass of filler material or preform; and/or (3)
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when another element such as zinc or iron is present in the aluminum
alloy. The temperature also may vary with different filler materials.
In general, in the aluminum/magnesium/nitrogen system spontaneous and
progressive infiltration will occur at a process temperature of at least
about 675-C, and preferably a process temperature of at least about
750-C-800-C. Temperatures generally in excess of 1200-C do not appear
to benefit the process, and a particularly useful temperature range has
been found to be from about 675-C to about 1000-C. However, as a
general rule, the spontaneous infiltration temperature is a temperature
which is above the melting point of the matrix metal but below the
volatilization temperature of the matrix metal. Moreover, the
spontaneous infiltration temperature should be below the melting point
of the filler material. Still further, as temperature is increased, the
ten~n~y t.n form a re2rtinn product betwDpn the matri.Y mDtal and
infiltrating atmosphere increases (e.g., in the case of aluminum matrix
metal and a nitrogen infiltrating atmosphere, aluminum nitride may be
formed). Such reaction product may be desirable or undesirable based
upon the intended application of the metal matrix composite body.
Additionallyt electric resistance heating is typically used to achieve
the infiltrating temperatures. However, any heating means which can
cause the matrix metal to become molten and does not adversely affect
spontaneous infiltration, is acceptable for use with the invention.
In the present method, for example, a permeable filler material or
preform comes into contact with molten aluminum in the presence of, at
least sometime during the process, a nitrogen-containing gas. The
nitrogen-containing gas may be supplied by maintaining a continuous flow
of gas into contact with at least one of the filler material or preform
and/or molten aluminum matrix metal. Although the flow rate of the
nitrogen-containing gas is not critical, it is preferred that the flow
rate be sufficient to compensate for any nitrogen lost from the
atmosphere due to any nitride formation, and also to prevent or inhibit
the incursion of air which can have an oxidizing effect on the molten
metal.
The method of forming a metal matrix composite is applicable to a
wide variety of filler materials, and the choice of filler materials
will depend on such factors as the matrix alloy, the process conditions,
the reactivity of the molten matrix alloy with the filler materialf and
the properties sought for the final composite product. For example,
when aluminum is the matrix metal, suitable filler materials include (a)
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oxides, e.g. alumina, magnesia, zirconia; (b) carbides, e.g. silicon
carbide; (c) borides, e.g. aluminum dodecaboride, titanium diboride, and
(d) nitrides, e.g. aluminum nitride, and (e) mixtures thereof. If there
is a tendency for the filler material to react with the molten aluminum
matrix metal, this might be accommodated by minimizing the infiltration
time and temperature or by providing a non-reactive coating on the
filler. The filler material may comprise a substrate, such as carbon or
other non-ceramic material, bearing a ceramic coating to protect the
substrate from attack or degradation. Suitable ceramic coatings include
oxides, carbides, borides and nitrides. Ceramics which are preferred
for use in the present method include alumina and silicon carbide in the
. form of particles, platelets, whiskers and fibers. The fibers can be
discontinuous (in chopped form) or in the form of continuous filament,
such as mult.ifil?,m,?nt t~wc, Fl!rthpri thP fjllPr m~tPri~l nr nreform may
be homogeneous or heterogeneous.
It also has been discovered that certain filler ~aterials exhibit
enhanced infiltration relative to filler materials having a similar
chemical composition. For example, crushed alumina bodies made by the
; method disclosed in U.S. Patent No. 4,713,360, entitled "Novel Ceramic
Materials and Methods of Making Same", which issued on December 15,
1987, in the names of Marc S. Newkirk et al., exhibit desirable
infiltration properties relative to commercially available alumina
; products. Moreover, crushed alumina bodies made by the method disclosed
in Copending and Commonly Owned Application Serial No. 819,397, entitled
"Composite Ceramic Articles and Methods of Making Same", in the names of
Marc S. Newkirk et al., also exhibit desirable infiltration properties
relative to commercially available alumina products. The subject matter
of each of the issued Patent and Copending Patent Application is herein
expressly incorporated by reference. Thus, it has been discovered that
infiltration of a permeable mass of ceramic material can occur at lower
~ infiltration temperatures and/or lower infiltration times by utilizing a
:~ crushed or comminuted body produced by the method of the aforementioned
; U.S. Patent and Patent Application.
The size, shape, chemistry and volume percent of the filler
material (or preform) can be any that may be required to achieve the
; properties desired in the composite. Thus~ the filler material may be
in the form of particles, whiskers, platelets or fibers since
~ infiltration is not restricted by the shape of the filler material.
i Other shapes such as spheres, tubules, pellets, refractory fiber cloth,
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and the like may be employed. In addition, the size of the filler
material does not limit infiltration, although a higher temperature or
longer time period may be needed for infil~ration of a mass of smaller
particles than for larger particles or vice-versa depending on the
particular reaction conditions. Average particle diameters as small as
a micron or less to about 1100 microns or more can be successfully
utilized in the present invention, with a range of about 2 microns
through about 1000 microns being preferred for a vast majority of
commercial applications. Further, the mass of filler material ~or
preform) to be infiltrated should be permeable (i.e., contain at least
some interconnected porosity to render it permeable to molten matrix
metal and/or to the infiltrating atmosphere). Moreover, by controlling
the size (e.g., particle diameter) and/or geometry and/or composition of
the fill~r mat~ri~l or th~ m~t~ri~l CnmnrjSinn thP nrPfnrm; th~ phyci,-~l
and mechanical properties of the formed metal matrix composite can be
controlled or engineered to meet any number of industrial needs. For
example, wear resistance of the metal matrix composite can be increased
by increasing the size of the filler material (e.g., increasing the
average diameter of the filler material particles) given that the filler
material has a higher wear resistance than the matrix metal. However,
strength and/or toughness may tend to increase with decreasing filler
size. Further, the thermal expansion coefficient of the metal matrix
composite may decrease with increasing filler loading, given that the
coefficient of thermal expansion of the filler is lower than the
coefficient of thermal expansion of the matrix metal. Still further,
the mechanical and/or physical properties (e.g., density, coefficient of
thermal expansion, elastic andjor specific modulus, strength and/or
specific strength, etc.) of a formed metal matrix composite body may be
tailored depending on the loading of the f~ller material in the loose
mass or in the preform. For example, by providing a loose mass or
preform comprising a mixture of filler particles of varying sizes and/or
shapes, wherein the density of the filler is greater than that of the
matrix metal, a higher filler loading, due to enhanced packing of the
filler materials, may be achieved, thereby resulting in a metal matrix
composite body with an increased density. By utilizing the teachings of
the present invention, the volume percent of filler material or preform
which can be infiltrated can vary over a wide range. Accordingly, by
practicing any of the above teachings, alone or in combination, a metal
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w o 9~ 276 2~ L~; ~ - 30 - PCI/US91/03161
matrix composite can be engineered to contain a desired combination of
properties.
It has been observed that for aluminum infiltration and matrix
formation around a ceramic filler, wetting of the ceramic filler by the
aluminum matrix metal may be an important part of the infiltration
mechanism. Further, the wetting of the filler by molten matrix metal
may permit a uniform dispersion of the filler throughout the formed
metal matrix composite and improve the bonding of the filler to the
matrix metal. Moreover, at low processing temperatures, a negligible or
minimal amount of metal nitriding occurs resulting in a minimal
discontinuous phase of aluminum nitride dispersed in the metal matrix.
However, as the upper end of the temperature range is approached,
nitridation of the metal is more likely to occur. Thus, the amount of
t.hP n;tri~le nh~SP jn tha mat~ m,2tri.Y ran ha controll~d by varving tha
processing temperature at which infiltration occurs. The specific
process temperature at which nitride formation becomes more pronounced
also varies with such factors as the matrix aluminum alloy used and its
quantity relative to the volume of filler or preform, the filler
material to be infiltrated, and the nitrogen concentration of the
infiltrating atmosphere. For example, the extent of aluminum nitride
formation at a given process temperature is believed to increase as the
ability of the alloy to wet the filler decreases and as the nitrogen
concentration of the atmosphere increases.
It is therefore possible to tailor the constituency of the metal
matrix during formation of the composite to impart certain
characteristics to the resulting product. For a given system, the
process conditions can be selected to control the nitride formation. A
composite product containing an aluminum nitride phase will exhibit
certain properties which can be favorable to, or improve the performance
of, the product. Further, the temperature range for spontaneous
infiltration with an aluminum alloy may vary with the ceramic material
used. In the case of alumina as the filler material, the temperature
for infiltration should preferably not exceed about 1000C if it is
desired that the ductility of the matrix not be reduced by the
significant formation of nitride. However, temperatures exceeding
1000C may be employed if it is desired to produce a composite with a
less ductile and stiffer matrix. To infiltrate silicon carbide, higher
temperatures of about 1200C may be employed since the aluminum alloy
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nitrides to a lesser extent, relative to the use of alumina as filler,
when silicon carbide is employed as a filler material.
Further, the constituency of the metal matrix may be modified
after formation of the metal matrix composite, and prior to or after
comminuting the metal matrix cumposite body. For example, exposure of
the formed metal matrix composite to a heat treatment may improve the
tensile strength of the metal matrix composite. (The standard test for
tensile strength is ASTM-D3552-77 (reapproved 1982).)
For example, a desirable heat treatment for a metal matrix
composite containing a 520.0 aluminum alloy as the matrix metal may
comprise heating the metal matrix composite to an elevated temperature,
for example, to about 430-C, which is maintained for an extended period
(e.g., 18-20 hours). The metal matrix may then be quenched in boiling
water at about lOO C for abol~t 20 c~.nn~c !i.~.; a T-4 he~t. tr~at.ment
which can temper or improve the ability of the composite to withstand
tensile stresses.
Moreover, it is possible to use a reservoir of matrix metal to
assure infiltration of the filler material and/or to supply a second
metal which has a different composition from the first source of matrix
metal. Specifically, in some cases it may be desirable to utilize a
matrix metal in the reservoir which differs in composition from the
first source of matrix metal. For example, if an aluminum alloy is used
as the first source of matrix metal, then virtually any other metal or
metal alloy which was molten at the processing temperature could be used
as the reservoir metal. Molten metals frequently are very miscible with
each other which would result in the reservoir metal mixing with the
first source of matrix metal so long as an adequate amount of time is
given for the mixing to occur. Thus, by using a reservoir metal which
is different in composition from the first source of matrix metal, it is
possible to tailor the properties of the metal matrix to meet various
operating requirements and thus tailor the properties of the metal
matrix composite.
A barrier means may also be utilized in combination with the
present invention. Specifically, the barrier means for use with this
invention may be any suitable means which interferes, inhibits, prevents
or terminates the migration, movement, or the like, of molten matrix
alloy (e.g., an aluminum alloy) beyond the defined surface boundary of
the filler material. Suitable barrier means may be any material,
compound, element, composition~ or the 1ike, which, under the process
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WO 91/17276 - 32 - PCIIUS91/0316
conditions of this invention, maintains some integrity, is not volatile
and preferably is permeable to the gas used with the process, as well as
being capable of locally inhibiting, stopping, interfering with,
preventing, or the like, continued infiltration or any other kind of
movement beyond the defined surface boundary of the ceramic filler.
Barrier means may be used during spontaneous infiltration or in any
molds or other fixtures utilized in connection with thermo-forming of
the spontaneously infiltrated metal matrix composite, as discussed in
greater detail below.
Suitable barrier means includes materials which are substantially
non-wettable by the migrating molten matrix alloy under the process
conditions employed. A barrier of this type appears to exhibit little
or no affinity for the molten matrix alloy, and movement beyond the
defined surface boundary of the filler material or pr~form is prevented
or inhibited by the barrier means. The barrier reduces any final
machining or grinding that may be required of the metal matrix composite
product. As stated above, the barrier preferably should be permeable or
pnrous, or rendered permeable by puncturing, to permit the gas to
contact the molten matrix alloy.
Suitable barriers particularly useful for aluminum matrix alloys
are those containing carbon, especially the crystalline allotropic form
of carbon known as graphite. Graphite is essentially non-wettable by
the molten aluminum alloy under the described process conditions. A
particular preferred graphite is a graphite foil product that is sold
under the trademark 6rafoil~, registered to Union Carbide. This
graphite foil exhibits sealing characteristics that prevent the
migration of molten aluminum alloy beyond the defined surface boundary
of the filler material. This graphite foil is also resistant to heat
and is chemically inert. Grafoil~ graphite foil is flexible,
compatible, conformable and resilient. It can be made into a variety of
shapes to fit any barrier application. However, graphite barrier means
may be employed as a slurry or paste or even as a paint film around and
on the boundary of the filler material or preform. Grafoil~ is
particularly preferred because it is in the form of a flexible graphite
sheet. In use, this paper-like graphite is simply formed around the
filler material or preform.
Other preferred barrier(s) for aluminum metal matrix alloys in
nitrogen are the transition metal borides (e.g., titanium diboride
(TiB2)) which are generally non-wettable by the molten aluminum metal
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alloy under certain of the process conditions employed using this
material. With a barrier of this type, the process temperature should
not exceed about 875 C, for otherwise the barrier material becomes less
efficacious and, in fact, with increased temperature infiltration into
the barrier will occur. Moreover, the particle size of the barrier
material may affect the ability of the material to inh;bit spontaneous
infiltration. The transition metal borides are typically in a
particulate form (1-30 microns). The barrier materials may be applied
as a slurry or paste to the boundaries of the permeable mass of ceramic
filler material which preferably is preshaped as a preform.
Other useful barriers for aluminum metal matrix alloys in nitrogen
include low-volatile organic compounds applied as a film or layer onto
the external surface of the filler material or preform. Upon firing in
nitrngen ~cnP~;~lly at tkD prnCDss conditions of thl.s inYontinn~ the
organic compound decomposes leaving a carbon soot film. The organic
compound may be applied by conventional means such as painting,
spraying, dipping, etc.
Moreover, finely ground particulate materials can function as a
barrier so long as inflltration of the particulate material would occur
at a rate which is slower than the rate of inf;ltration of the filler
material.
Thus, the barrier means may be applied by any suitable means, such
as by covering the defined surface boundary with a layer of the barrier
means. Such a layer of barrier means may be applied by painting,
dipping, silk screening, evaporating, or otherwise applying the barrier
means in liquid, slurry, or paste form, or by sputtering a vaporizable
barrier means, or by simply depositing a layer of a solid particulate
barrier means, or by applying a solid thin sheet or film of barrier
means onto the defined surface boundary. With the barrier means in
place, spontaneous infiltration substantially terminates when the
infiltrating matrix metal reaches the defined surface boundary and
contacts the barrier means.
Demonstrations of the present invention is included in the
Examples immediately following. However, this Example should be
considered as being illustrative and should not be construed as limiting
the scope of the invention as defined in the appended claim.
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WO 91/17276 3 Pcr/US9l/03161~!
2~
ExamDle 1
The following Example demonstrates a method for making a porous
body by a spontaneous infiltration technique.
Figure 1 is a cross-sectional schematic of the setup 16 used to
form a porous metal matrix composite body. Specifically, a Grade ATJ
graphite boat (Union Carbide Corporation, Carbon Product Division,
Cleveland, OH) measuring about 3 inches (76 mm) long by about 3 inches
(76 mm) wide by about 2.5 inches (64 mm) high, and having a wall
thickness of about 0.5 inch (13 mm), was lined with a graphite foil box
11. The graphite foil box 11 had been formed from a piece of PERMAFOIL~
graphite foil (TT America, Portland, OR) measuring about 6 inches (152
mm) long by about 6 inches (152 mm) wide by about 0.010 inch (0.25 mm)
thick, which was cut and folded to fit within the inner dimensions of
the grAphit~ hn~t.
A filler material mixture 12 was prepared by ball milling a
mixture comprising, by weight, about 95% 38 ALUNDUM~ 500 grit alumina
(Norton Co., Worcester, MA) and about 5% -325 mesh magnesium powder.
Specifically, the filler material mixture was placed into a plastic jar
and ball milled for about an hour, then the filler material mixture was
dried at a temperature of about 150C for about 1 hour.
About 200 grams of the filler material mixture 12 were placed
into the graphite foil box 11 within the graphite boat 10 and leveled.
About 0.75 gram of -50 mesh magnesium powder 13 were placed onto the
surface of the filler material mixture 13.
About 20 grams of a matrix metal 14 designate aluminum alloy
520, and nominally comprising by weight about < 0.25% Si, < 0.30% Fe, <
0.25% Cu, < 0.15 Mn, 7.5-10.5% Mg, < 0.15% Zn, < 0.25% Ti and the
balance Al, were placed onto the -50 mesh magnesium powder 13 covering
the filler material mixture 12. A piece of graphite foil 15 was then
placed over the top of the graphite boat 10 to complete the setup.
The setup was placed into a retort heated by a resistance
furnace, and the retort door was closed. At about room temperature, the
retort was evacuated to a vacuum of about 30 inches (762 mm) of mercury,
then the vacuum pump was shut off and a nitrogen atmosphere flowing at
about 15 liters per minute was introduced into the retort chamber. The
furnace and its contents were then heated to a temperature of about
500C at a rate of about 400C, held at about 500C for about an hour,
then heated to about 750C at a rate of about 400C per hour. After
about 3 hours at about 750C, the flowing nitrogen atmosphere and the
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power to the furnace were shut off, and the setup was cooled to roomtemperature.At about room temperature, the setup was disassembled to reveal
that a porous metal matrix composite body had been formed. A sample of
the porous metal matr;x composite body was placed in an electron
microscope and a photomicrograph was of the sample, taken in the
secondary electron mode. Figure 2 is a micrograph taken at about 400X
magnification showing the interconnected nature of the filler material
particles, as well as the porosity between the alumina filler material.
The porous metal matrix composite body was subsequently
comminuted by mechanically crushing the composite.
:
