An investment casting technique for the formation of metal matrix composite bodies and products produced thereby

Abstract: The present invention relates to a novel method for forming metal matrix composite bodies and the novel pro­ ducts produced therefrom. A negative shape or cavity, which is complementary to the desired metal matrix composite body to be produced, is first formed. The formed cavity is thereafter filled with a permeable mass of filler material (7). Molten matrix metal (8) is then induced to sponta­ neously infiltrate the filled cavity. Particularly, an in­ filtration enhancer and/or an infiltration enhancer precur­ sor and/or an infiltrating atmosphere are also in communi­ cation with the filler material (7), at least at some point during the process, which permits the matrix metal (8) when made molten, to spontaneously infiltrate the permeable mass of filler material (7), which at some point during the processing, may become self-supporting. In a preferred em­ bodiment, cavities can be produced by a process which is similar to the so-called lost-wax process.

Description: Field of the Invention The present invention relates to a novel method for forming metal matrix composite bodies and the novel products produced therefrom. A negative shape or cavity, which is complementary to the desired metal matrix composite body to be produced, is first formed. The formed cavity is thereafter filled with a permeable mass of filler material. Molten matrix metal is then induced to spontaneously infiltrate the filled cavity. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor as well as an infiltrating atmosphere are also in communication with the filler material, at least at some point during the process, which permits the matrix metal, when made molten, to spontaneously infiltrate the permeable mass of filler material, which at some point during the processing, may become self-supporting. In a preferred embodiment, cavities can be produced by a process which is similar to the so-called lost-wax process. Background of the Invention 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, 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 sore instances, the composite also may be lighter in weight than the matrix metal per se. Aluminium matrix composites reinforced with ceramics such as silicon carbide in particulate, platelet, or whisker form, for example, are of interest because of their higher stiffness, wear resistance and high temperature strength relative to aluminium. 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 volume percent in the case of particulates. The production of metal matrix composites by powder metallurgy techniques 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 simple product shapes are possible without subsequent processing (e.g., forming or machining) or without resorting to complex presses. Also, nonuniform shrinkage during sintering can occur, as well as nonuniformity 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-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 difficulty of infiltrating 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 aforesaid 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 oriented particles, whiskers or fibers. In the fabrication of aluminum matrix-alumina filled composites, aluminum does not readily wet alumina, 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 in properties, or the coatings can degrade the filler, or the matrix contains lithium which can affect the matrix properties. U.S. Patent No. 4,232,091 to R. W. Grimshaw et al., overcomes certain difficulties in the art which are encountered in the production of aluminum matrix-alumina composites. This patent describes applying pressures of 75-375 kilograms/centimeters² to force molten aluminum (or molten aluminum alloy) into a fibrous or whisker mat of alumina which has been preheated to 700 to 1050°C. 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 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, 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. This 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 deposition 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 1800 C, 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 from the body. 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. In order to protect this substrate from molten cryolite, a thin coating of a mixture of a wetting agent and solubility suppressor is applied to the alumina 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, silicon, 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 freely into 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 AlB compact "sandwiched" between the layers of aluminum powder, was placed in a vacuum furnace. The furnace was evacuated to approximately 10⁻⁵ torr to permit outgassing. The temperature was subsequently raised to 1100 C and maintained for a period of 3 hours. At these conditions, the molten aluminum penetrated the porous AlB₁₂ compact. U.S. Patent No. 3,364,976, granted January 23, 1968, 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 dissembled 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 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. Accordingly, there has been a long felt need for a simple and reliable process to produce shaped metal matrix composites which does not rely upon the use of applied pressure or vacuum (whether externally applied or internally created), or damaging wetting agents to create a metal matrix embedding another material such as a ceramic material. Moreover, there has been a long felt need to minimize the amount of final machining operations needed to produce a metal matrix composite body. The present invention satisfies these needs by providing a spontaneous infiltration mechanism for infiltrating a material (e.g., a ceramic material), which is formed into a preform, with molten matrix metal (e.g., aluminum) in the presence of an infiltrating atmosphere (e.g., nitrogen) under normal atmospheric pressures so long as an infiltration enhancer is present at least at some point during the process. Description of Commonly Owned Patent Applications The subject matter of this application is related to that of several other copending and co-owned patent applications. Particularly, these other copending patent applications describe novel methods for making metal matrix composite materials (hereinafter sometimes referred to as "Commonly Owned Metal Matrix Patent Applications"). A novel method of making a metal matrix composite material is disclosed in Commonly Owned EP-A-291 441. According to the method of said 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 675°C 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 composite. 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 invention, aluminum nitride can form as a discontinuous phase dispersed throughout the aluminum matrix. The amount of nitride in the aluminum matrix may vary depending on such factors as temperature, alloy 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 aforementioned invention allows the choice of a balance between infiltration kinetics and nitride formation. An example of suitable barrier means for use with metal matrix composite formation is described in Commonly Owned EP-A-323 945. According to the method of this. invention, a barrier means (e.g., particulate titanium diboride or a graphite material such as a flexible graphite tape 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 composite bodies have an outer shape which substantially corresponds to the inner shape of the barrier means. The method of Patent Application EP-A-291 441 was improved upon by Commonly Owned EP-A-333 629. In accordance with the methods disclosed in this 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, under the conditions described in this patent application, 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 infiltration 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 reinforcing 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 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. Each of the above-discussed Commonly Owned Metal Matrix 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 Patent Applications are expressly incorporated herein by reference. Summary of the Invention A metal matrix composite body is produced by infiltrating a permeable mass of filler material which, at some point during the processing, may become self-supporting (i.e., may be formed into a preform). The filler material is positioned within a cavity of an investment shell. Specifically, in a preferred embodiment of the invention, a low melting or volatilizable mandrel (e.g., a wax mold) can be made such that at least a portion of the wax mold corresponds in shape to the metal matrix composite body which is desired to be formed. The wax mold can be coated by an appropriate process with, for example, a refractory material, which can be applied by, for example, painting, spraying, dip-coating, etc. Once an appropriate thickness of, for example, ceramic material, has been built up onto a surface of the wax mold, and the coated refractory material is made to be self-supporting, the wax mold can be removed from the coating by, for example, melting, volatilization, etc., and the coating can have therein a cavity which substantially corresponds in shape to the wax which has been removed therefrom. In one embodiment, the formed cavity may be coated by an appropriate technique with an appropriate barrier material which assists in defining the final shape of the metal matrix composite body to be formed. Once the barrier material has been appropriately positioned, a filler material can then be placed into at least a portion of the cavity. Moreover, an infiltration enhancer and/or an infiltration enhancer precursor as well as an infiltrating atmosphere are also in communication with the filler material, at least at some point during the process, which permits the matrix metal, when made molten, to spontaneously infiltrate the permeable mass of filler material, which at some point during the processing, may become self-supporting. In a preferred embodiment, an infiltration enhancer may be supplied directly to at least one of the filler material, and/or matrix metal and/or infiltrating atmosphere. Independent of the supplier of infiltration enhancer precursor or infiltration enhancer, ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material. It is noted that this application discusses primarily aluminum matrix metals which, at some point during the formation of the metal matrix composite body, are contacted with magnesium, which functions as the infiltration enhancer precursor, in the presence of nitrogen, which functions as the infiltrating atmosphere. Thus, the matrix metal/infiltration enhancer precursor/infiltrating atmosphere system of aluminum/-magnesium/nitrogen exhibits spontaneous infiltration. However, other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems may also behave in a manner similar to the system aluminum/magnesium/nitrogen. For example, similar spontaneous infiltration behavior has been observed in the aluminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; and the aluminum/calcium/nitrogen system. Accordingly, even though the aluminum/magnesium/nitrogen system is discussed primarily herein, it should be understood that other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems may behave in a similar manner. When the matrix metal comprises an aluminum alloy, a formed cavity can be filled with a filler material (e.g., alumina or silicon carbide particles), said filler material having admixed therewith, or at some point during the process being exposed to, magnesium, as an infiltration enhancer precursor. Moreover, the aluminum alloy and/or the filler material at some point during the processing, and in a preferred embodiment during substantially all of the processing, are exposed to a nitrogen atmosphere, as an infiltrating atmosphere. Alternatively, the requirement can be obviated if the filler material is admixed with, or at some point during the process, exposed to magnesium nitride, as an infiltration enhancer. Still further, at some point during the processing, the filler material will become at least partially self-supporting. In a preferred embodiment, the filler material becomes self-supporting before or substantially simultaneous with the matrix metal contacting the filler material (e.g., the matrix metal could contact the filler material for the first time as molten matrix metal, or, the matrix metal could contact the filler material first as a solid material, and thereafter become molten when heated). The extent or rate of spontaneous infiltration and formation of metal matrix composite will vary with a given set of process conditions including, for example, the concentration of magnesium provided to the system (e.g., in the aluminum alloy and/or in the filler material and/or in the infiltrating atmosphere), the size and/or composition of the filler material, the concentration of nitrogen in the infiltrating atmosphere, the time permitted for infiltration, and/or the temperature at which infiltration occurs. Spontaneous infiltration typically occurs to an extent sufficient to embed substantially completely the filler material or preform. In a preferred embodiment, once infiltration has been achieved, the surrounding coated ceramic material can be removed to expose a net or near net shape metal matrix composite body. Definitions " Aluminum ", as used herein, means and includes essentially pure metal (e.g., a relatively pure, commercially available unalloyed 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 this definition is an alloy or intermetallic compound in which aluminum is the major constituent. " Balance Non-Oxidizing 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 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. " 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 matrix composite body which has been formed. It should be understood that the carcass may also include a second or foreign metal therein. " Filler ", as used herein, is intended to mean 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, such as powders, flakes, platelets, microspheres, whiskers, bubbles, 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 like, 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. " Infiltrating 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 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, (1) a reaction of an infiltration enhancer precursor and the infiltrating atmosphere to form 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 filler material or preform, and/or matrix metal, and/or infiltrating atmosphere and function in a substantially similar manner to an infiltration enhancer which has formed as a reaction between an infiltration enhancer 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 (1) the matrix metal, (2) the preform or filler material and/or (3) an infiltrating atmosphere forms an infiltration enhancer which induces or assists the matrix metal to spontaneously 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 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 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. " Removable Mandrel " or " Removable Replicate ", as used herein, means a material or object which is capable of being shaped and maintaining its shape when coated with a material which is capable of forming a refractory shell and which can be removed from a formed refractory shell by, for example, melting or volatilizing or physical removal as an intact component. " Matrix Metal " or " Matrix Metal Alloy ", as used herein, means that metal which is intermingled with a filler material to form a metal matrix composite body. 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. " Matrix Metal/Infiltration Enhancer Precursor/Infiltrating Atmosphere System " or " Spontaneous System ", as used herein, refers to that combination of materials which exhibits 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, the "/" is used to designate a system or combination of materials which, when combined in a particular manner, exhibits spontaneous infiltration 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). " Nonreactive Vessel for Housing Matrix Metal " means any vessel which can house or contain molten matrix metal under the process conditions and not react with the matrix and/or the infiltrating atmosphere and/or infiltration enhancer precursor in a manner which would be significantly detrimental to the spontaneous infiltration mechanism. " 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. The mass should be sufficiently porous to accommodate spontaneous infiltration of the matrix metal 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 exist either singularly or as an assemblage. " 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. " Shell " or " Investment Shell ", as used herein, means the refractory body which is produced by coating a removable mandrel with a material which can be made to be self-supporting (e.g., by heating) such that when the mandrel is removed, the refractory body includes a cavity which substantially corresponds to the original shape of the removable mandrel. " Spontaneous 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 Description of the Figures The following Figures are provided to assist in understanding the invention, but are not intended to limit the scope of the invention. Similar reference numerals have been used wherever possible in each of the Figures to denote like components, wherein: Figure 1a illustrates a plurality of removable replicates for forming an investment shell; Figure 1b shows a removable tree for forming an investment shell; Figure 2 shows an investment shell in accordance with the present invention; Figure 3a shows the investment shell containing a suitable filler being contacted by a suitable matrix metal; Figure 3b shows the investment shell and the filler being spontaneously infiltrated; and Figure 4 is a photograph of a metal matrix composite formed in accordance with Example 1. Detailed Description of the Invention and Preferred Embodiments The present invention relates to forming a metal matrix composite body by spontaneously infiltrating a filler material with a molten matrix metal, said filler material having been formed into a particular shape. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor as well as an infiltrating atmosphere are also in communication with the filler material, at least at some point during the process, which permits the matrix metal, when made molten, to spontaneously infiltrate the permeable mass of filler material, which at some point during the processing, may become self-supporting. In accordance with the invention, a low melting or volatilizable or removable mandrel is first formed. The mandrel is then coated with a material which can rigidize to form a shell which contains therein a cavity which is complementary in shape to the removable mandrel. The mandrel can then be removed from the shell. Once the shell has been formed, the shell, optionally, can be coated on the interior cavity portion thereof with an appropriate barrier material, which serves as a barrier to the infiltration of matrix metal. Thereafter, a filler material can be placed at least partially within the formed cavity such that when molten matrix metal is induced to spontaneously infiltrate the filler material, a metal matrix composite body is produced. The produced metal matrix composite body substantially corresponds in shape to the removable mandrel. An investment shell for use in accordance with the present invention can be made by first fabricating one or more replicates (1) of the desired metal matrix composite body, as illustrated in Figure 1a. The replicates (1) may be formed of wax-coated plaster of Paris, all wax, or other suitable materials that can be removed, e.g., by melting or volatilizing, from a later formed investment shell. If the shape of the replicate permits, or if the shell is formed as a two-piece or multi-piece shell, the replicate can be physically removed and either disposed of or reused. Further, one or more of the removable replicates (1) may be attached to a trunk (2) to form a tree (3), as illustrated in Figure 1b. The trunk (2) may also be formed of wax-coated plaster, all wax or other suitably removable materials. Preferably, a cup portion (4) is also attached to the trunk (2). As will be understood from the discussion below, the cup portion (4) is formed of a suitable nonremovable material such as alumina, stainless steel, or the like. The tree (3) may then be repetitively and successively dipped in, for example, a ceramic slip or slurry and dusted with a ceramic powder to build up a refractory investment shell (5) around the tree, as illustrated in Figure 2. The thickness and composition of the investment shell (5) so built up is not critical, although the shell should be sufficiently rugged to withstand the further steps of the casting process. The shell (5) may also be formed by painting, spraying or any other convenient process, depending upon the size and configuration of the shell and the coating material employed. Once the shell (5) is formed, the tree (3) is removed, for example, by melting the wax, thereby leaving a cavity (6) within the shell (5) that faithfully corresponds to the shape or shapes of the removable mandrels. As discussed in more detail below, the investment shell (5) is preferably impermeable to the molten matrix metal. Shells which also are permeable to an infiltrating atmosphere are particularly advantageous but are not necessary to the practice of the present invention. Suitable refractory materials for forming shells have been found to be alumina, silica and silicon carbide, but other refractory materials may also be used. An investment shell should be rugged, yet easily removable when desired, without exerting excess stresses on the metal matrix composite bodies to be formed therein. For example, it has been found that glass-like materials, such as the aluminum borosilicates, although they are advantageously impermeable to matrix metal, can stress the composite bodies during their formation because of, for example, the disparity in their thermal expansion coefficients. In addition, glass-like shells can be relatively difficult to remove from the composites. The cavity (6) may then be packed with a suitable filler, which may include an infiltration enhancer precursor and/or an infiltration enhancer, and heated in the presence of an infiltrating atmosphere. It is preferable that the filler be packed only into the portions of the cavity corresponding to the replicates (1), in which case the portion of cavity (6) corresponding to trunk (2) remains unfilled. Molten matrix metal is then suitably arranged in contact with the filler (7), for example, by pouring matrix metal (8) into the shell (5) through the cup portion (4), as illustrated in Figure 3a. The investment shell (5) may be conveniently disposed in a refractory vessel (9), optionally containing a bedding material (11), which is continuously purged with infiltrating atmosphere. Under proper conditions discussed further below, the matrix metal (8) spontaneously infiltrates the filler (7) as illustrated in Figure 3b by advancing infiltration fronts (10). It will be understood that the filler may have formed rigidized preforms during the process, but such formation is unnecessary when the investment shell (5) is sufficiently strong to retain the shape desired for the finished metal matrix composite bodies, and the filler otherwise cannot lose the desired shape. Furthermore, rather than pouring molten matrix metal into the shell, solid matrix metal may be disposed in contact with the filler, and then subsequently liquefied. Furthermore, as the infiltration front advances, the matrix metal can be changed via a reservoir or the introduction of an additional matrix metal, to thereby alter the properties of different portions of the resultant metal matrix composite body. After completion of the spontaneous infiltration, the shell (5) is cooled and removed by physical removal or by chemical means which react with the shell, but not with the composite. The metal matrix composite bodies corresponding to the replicates (1) may then be separated from any remaining carcass of matrix metal. It has been found that at least for some matrix metals, rapid cooling is desirable to maintain a fine microstructure in the composite bodies. Such cooling can be achieved, for example, by removing the shell while still hot and embedding it in a bed of sand at room temperature. It will be understood that investment shell casting is an inexpensive process for producing shaped metal matrix composites. Several composite bodies may be produced simultaneously, and the investment shell itself can be quickly produced from inexpensive materials. The composite bodies produced in this way can also show good net shape capabilities ( i.e., they can require minimal finishing). For some materials employed for the investment shell, it has been found that the matrix metal can continue to infiltrate beyond the filler into the shell itself. For example, porous investment shells made from an alumina or silica slurry and a silicon carbide powder may be infiltrated by matrix metal when the filler and/or matrix metal includes magnesium. In order to prevent such excessive infiltration, a barrier means may be formed on at least a portion of the surfaces of the cavity in the shell. The barrier, which is impermeable at least to the matrix metal, prevents the spontaneous infiltration of matrix metal beyond the filler, thereby permitting the production of composites requiring minimal shape finishing. Suitable barriers are described further below. In order to effect spontaneous infiltration of the matrix metal 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 the investment shell; and/or (5) from an external 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 and/or investment shell. 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 it is possible that the infiltration enhancer precursor can be at least partially reacted with the infiltrating atmosphere such that infiltration enhancer can be formed in at least a portion of the filler material or preform prior to or substantially contiguous with contacting the preform with molten 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 filler material or preform). 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 the process conditions, does not react with the aluminum matrix metal when the aluminum is made molten. A filler material containing or being exposed to magnesium, and being exposed to, at least at some point during the processing, a nitrogen atmosphere, can then be contacted with the molten aluminum matrix metal. The matrix metal will then spontaneously infiltrate the filler material or preform. Moreover, rather than supplying an infiltration enhancer precursor, an infiltration enhancer may be supplied 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 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 system, the filler material or preform should be sufficiently permeable to permit the nitrogen-containing gas to 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 should accommodate infiltration of the molten matrix metal, thereby causing the nitrogen-permeated filler material or 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 infiltration enhancer in the filler material or preform and thereby resulting in spontaneous infiltration. The extent or rate 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, and/or filler material or preform, and/or investment shell; magnesium nitride content in the aluminum alloy, filler material or preform, or investment shell, the presence of additional alloying elements (e.g., silicon, iron, copper, manganese, chromium, zinc, and the like), average size (e.g., particle diameter) of the filler material, surface condition and type of filler material, nitrogen concentration of the infiltrating atmosphere, time permitted for infiltration and temperature at which infiltration occurs. For example, for infiltration of the molten aluminim matrix metal to occur spontaneously, the aluminum can be alloyed with at least about 1% by weight, and preferably at least about 3% 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 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 the filler material or preform and matrix metal and investment shell or any two or more of the matrix metal, filler material or preform and investment shell may result in a reduction in the required amount of magnesium to achieve spontaneous infiltration (discussed in greater detail later herein). The volume percent of nitrogen in the nitrogen atmosphere also affects formation rates of the metal matrix composite body. Specifically, 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 should be supplied to a filler containing an infiltration enhancer precursor by any suitable means such as permeation of the filler prior to its contacting the molten matrix metal, diffusion through the investment shell and any matrix metal barrier means to the filler, dissolution or bubbling through the molten matrix metal, or the like. Moreover, channels or orifices could be provided in any barrier means and the investment shell to direct infiltrating atmosphere into the system. Still further, the infiltrating atmosphere may result from a decomposition and/or recombination of one or more materials. 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 filler 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 complete 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 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 filler material or preform, 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 amount of magnesium. For example, there was substantially no infiltration of nominally pure aluminum alloyed only with 10 percent silicon at 1000°C into a bedding of 25 µm (500 mesh), 39 Crystolon (99 percent pure silicon carbide from Norton Co.). However, in the presence of magnesium, silicon has been found to promote 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 magnesium 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 with 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 acceptable spontaneous infiltration is slightly higher. Specifically, it has been found that 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 material and/or within the preform or filler material prior to 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). If the magnesium was applied to a surface of the matrix metal it may be preferred that said 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 weight percent of magnesium needed to promote 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 given 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: (1) 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) 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, 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 1200°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 tendency to form a reaction product between the matrix metal 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. Additionally, 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 is placed 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 the 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 nitride formation in the alloy matrix, 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 material, and the properties sought for the final composite product. For example, when aluminum is the matrix metal, suitable filler materials include (a) oxides, e.g., alumina; (b) carbides, e.g., silicon carbide; (c) borides, e.g., aluminum dodecaboride, and (d) nitrides, e.g., aluminum nitride. 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 from of continuous filament, such as multifilament tows. Further, the ceramic mass or preform may be homogeneous or heterogeneous. It also has been discovered that certain filler materials exhibit enhanced infiltration relative to filler materials by having a similar chemical composition. For example, crushed alumina bodies made by the method disclosed in EP-A-155 831 exhibit desirable infiltration properties relative to commercially available alumina products. Moreover, crushed alumina bodies made by the method disclosed in EP-A-193 292, also exhibit desirable infiltration properties relative to commercially available alumina products. The subject matter of each of said Patent Applications is herein expressly incorporated by reference. Thus, it has been discovered that complete 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 Patent Applications. The size and shape of the filler material can be any that may be required to achieve the properties desired in the composite. Thus, the material may be in the form of particles, whiskers, platelets or fibers since infiltration is not restricted by the shape of the filler material. Other shapes such as spheres, tubules, pellets, refractory fiber cloth, and the like may be employed. In addition, the size of the material does not limit infiltration, although a higher temperature or longer time period may be needed for complete infiltration of a mass of smaller particles than for larger particles. Further, the mass of filler material (shaped into a preform) to be infiltrated should be permeable,(i.e., permeable to molten matrix metal and to the infiltrating atmosphere.comprise a nitrogen-containing gas). The method of forming metal matrix composites according to the present invention, not being dependent on the use of pressure to force or squeeze molten matrix metal into a preform or a mass of filler material, permits the production of substantially uniform metal matrix composites having a high volume fraction of filler material and low porosity. Higher volume fractions of filler material may be achieved by using a lower porosity initial mass of filler material. Higher volume fractions also may be achieved if the mass of filler is compacted or otherwise densified provided that the mass is not converted into either a compact with close cell porosity or into a fully dense structure that would prevent infiltration by the molten alloy. 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 of the infiltration mechanism. 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 the nitride phase in the metal matrix can be controlled by varying the 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 1000°C if it is desired that the ductility of the matrix be not reduced by the significant formation of nitride. However, temperatures exceeding 1000°C 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 1200°C may be employed since the aluminum alloy nitrides to a lesser extent, relative to the use of alumina as filler, when silicon carbide is employed as a filler material. Moreover, it is possible to use a reservoir of matrix metal to assure complete 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 than 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 like, which, under the process conditions of this invention, maintains some integrity, is not volatile and preferably is permeable to the infiltrating atmosphere 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 filler material. 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 preform is prevented or inhibited by the barrier means. The barrier assists in the formation of bodies having the final shape required of the metal matrix composite product. As stated above, the barrier preferably may be permeable or porous to permit the gas of the infiltrating atmosphere to contact the molten matrix alloy. Alternatively, orifices or the like could be provided in the barrier means to facilitate flow of infiltrating atmosphere. 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 tape product that is sold under the trademark Grafoil®, registered to Union Carbide. This graphite tape exhibits sealing characteristics that prevent the migration of molten aluminum alloy beyond the defined surface boundary of the filler material This graphite tape is also resistant to heat, chemically inert, flexible, compatible, conformable and resilient. However, graphite barrier means may even be employed as a slurry or paste or even as a paint film around and on the boundary of the filler material or preform, and in this form can be readily applied to the cavity in the investment shell. Grafoil is preferred for simple composite shapes because it is in the form of a flexible graphite sheet, and thus can be readily applied to planar surfaces. 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 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. The transition metal borides are typically available in a particulate form (1-30 microns). The metal boride formation may be applied as a slurry or paste to the cavity in the investment shell, thereby defining the boundaries of the permeable mass of ceramic filler material. Further, a suitable barrier for spontaneous systems including magnesium is magnesium oxide which may be formed on the surface of the shell cavity by heating a magnesium containing mixture filling the cavity in the presence of nitrogen, then removing that mixture in the presence of, for example, air. Magnesium nitride formed at the surface of the shell cavity is thereby converted to magnesium oxide which adheres to the cavity surface. Because, at the processing temperatures employed in the present invention, magnesium is volatile, magnesium vapor can infiltrate a porous investment shell, leading to matrix metal spontaneous infiltration into the shell. The presence of magnesium oxide apparently depletes the supply of magnesium infiltration enhancer precursor and/or magnesium nitride infiltration enhancer localized at the shell cavity surface, thereby adversely affecting the spontaneous infiltration of matrix metal into the depleted region. In addition, the depletion material, such as magnesium oxide or any of the other suitable depletion materials described below, present at the surface of the shell cavity may only temporarily forestall infiltration of the shell by matrix metal for a period limited by, for example, the amount of depletion material available at the surface and the amount of infiltration enhancer and/or infiltration enhancer precursor and/or infiltrating atmosphere to be depleted before solidification of the matrix metal. It will be understood that an investment shell which does not permit infiltration of an infiltration enhancer and/or infiltration enhancer precursor and/or infiltrating atmosphere or, even if so infiltrated, is not spontaneously infiltrated by matrix metal would not require inclusion of a barrier means on the surface of the shell cavity. Indeed, only spontaneous systems containing volatile magnesium, and of such systems only those containing more magnesium than is necessary for complete spontaneous infiltration of the filler, when used with porous investment shells appear to benefit from such barriers. Impermeable, glass-like investment shells may thus be used advantageously with magnesium-containing spontaneous systems subject to the other characteristics of such shells that were elsewhere described. It will be further understood that spontaneous systems which include constituents of low volatility at process temperatures would also not require such barriers. 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 nitrogen, especially at the process conditions of this invention, 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 infiltration of the particulate material would occur at a rate which is slower than the rate of infiltration of the filler material. Thus, the barrier means may be applied by any suitable means, such as by layering the defined surface boundary with the barrier means. Such 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 upon reaching the defined surface boundary and contacting the barrier means. Various demonstrations of the present invention are included in the Examples immediately following. However, these Examples should be considered as being illustrative and should not be construed as limiting the scope of the invention as defined in the appended claims. Example 1 A removable mandrel was formed comprising a wax-coated plaster of Paris replicate of a gear, 7.6 centimeters in diameter and 6.4 centimeters thick. The plaster of Paris is available from Bondex Co., and the wax coating was CSH Max-E-Wax, commercially available from Casting Supply Company, New York, NY. The removable mandrel was dipped in a slip or slurry comprising substantially equal weight proportions of colloidal 20% alumina, supplied by Remet Co., and 5 µm (1000 grit) silicon carbide powder, supplied by Norton Co. and solid under the tradename 37 Crystolon. Other fine silicon carbide grits could also be used. The slip-coated removable mandrel was then dusted with dry, 216 µm (90 grit) silicon carbide powder (37 Crystolon) which adhered to the slurry coating. The sequential dip-dust steps were repeated three times, after which the dusting powder was changed to 408 µm (24 grit) silicon carbide (37 Crystolon). The sequential dip-dust steps were then repeated another three times. The developing investment shell was dried for 1/2 hour at about 65°C after each dip-dust step sequence. After the last dip-dust sequence the investment shell was fired in an air furnace at a temperature of about 900°C for a period of 1 hour. This firing volatilized the wax coating on the removable mandrel and weakened the plaster of Paris; after cooling to room temperature, the plaster was easily liquefied and washed out of the investment shell. The shell was then thoroughly air-dried for about 12 hours at a temperature of about 75°C. A barrier was formed on the surface of the cavity in the investment shell by first packing the cavity with a mixture of 5 µm (1000 grit) silicon carbide powder (39 Crystolon from Norton Co.) and about 10% by weight 250 µm (50 mesh) magnesium powder (Aesar, available from Johnson Mathey Co.). The so-filled investment shell was then placed in a 316 stainless steel can which was covered by a thin copper foil (available from Atlantic Engineering Co.). A stainless steel tube was introduced through the copper foil, and the interior of the can was purged by substantially pure nitrogen gas at a flow rate of about 0.25 liters/minute. The continuous purging can was then heated in a preheated electric resistance-heated furnace from about 600°C to 750°C over a time period of about 1 hour, and maintained at about 750°C for about 1 hour. The can and its contents were then removed from the furnace, and the cavity was flushed clear with water while still hot. A black coating on the surface of the cavity was thus formed. Some small portions of the coating spalled off the investment shell as the fill mixture was removed. After being thoroughly dried, the barrier-coated cavity of the investment shell was packed with a filler comprising a mixture of an alumina powder (C75-RG, available from Alcan Chemical Products, Co.) and about 5 weight percent of a 44 µm (325 mesh) magnesium powder (Aesar, available from Johnson Mathey Co.) for a total weight of about 337 grams. Hand-packing reduced the volume of the filler by approximately half, having the effect of producing higher volume fractions of filler material and a more uniformly structured composite bodies. The filler-packed investment shell was then placed in a 316 stainless steel can, and a 722g aluminum alloy ingot of standard 520 aluminum alloy was placed in the can in contact with the filler. The can was covered with a thin copper foil and the interior of the can was continuously purged with pure nitrogen gas at a flow rate of about 2 liters/minute. The can was heated in an electric-resistance heated furnace from room temperature to about 800°C over a period of about 2 hours, and maintained at about 800°C for about 0.5 hour, at the end of which time the aluminum alloy had liquefied and spontaneously infiltrated the filler. The temperature of the furnace was then reduced to about room temperature over a period of about 2 hours, thereby solidifying the metal matrix composite gear, and the investment shell was removed from the furnace. The shell was supported in a bed of sand at room temperature and was tapped off the metal matrix composite gear with hammer blows. The resulting metal matrix composite gear showed good shape fidelity, as shown in Figure 4, and required minimal surface finishing except in those areas adjacent the areas of the surface of the cavity from which the barrier coating had spalled. Some infiltration of the aluminum matrix metal into the investment shell occurred through those areas. Example 2 An investment shell was formed by the same dip-dust sequence as in Example 1 around a removable mandrel that comprised a thermoplastic foam cup. After removal of the cup mandrel from the investment shell by firing the shell at about 850°C for about 1 hour, the cavity in the shell was filled with a saturated aqueous solution of magnesium perchlorate (available from Morton Thiokol Co.). The solution was allowed to soak the shell cavity surface for about 2 minutes, after which the solution was removed from the shell cavity. The investment shell was air-dried in a furnace at a temperature of about 100°C. The temperature was then ramped up to about 750°C over a period of about 2 hours, the shell was fired at a temperature of about 750°C for about1 hour, and the temperature was ramped down over a period of about 2 hours. The investment shell cavity was then packed about half full with the filler as in Example 1 and subjected to the same subsequent process steps as in Example 1. Upon removal of the metal matrix composite cup, examination revealed good shape fidelity with minimal surface finishing needed. No extraneous infiltration of the investment shell by the aluminum matrix metal occurred. Example 3 A removable mandrel comprising a thermoplastic foam cup was used to form an investment shell. The mandrel was first dipped in a slip or slurry of equal proportions of pure calcium carbonate (available from Standard Ceramic Supply Co.) and colloidal 20 weight percent silica (available from Nyacol Co.). The slurry-coated mandrel was then dusted with silicon carbide as in Example 1, and subsequent dip-dust sequence steps were carried out as in Example 1. Further process steps leading to formation of the shell proceeded as in Example 1, with the exception that no separate barrier formation via heating and removal of a silicon carbide/magnesium mixture was performed. In general, silica is preferred for forming investment shells because such shells tend to be stronger and more rugged. Alumina is preferable for shells which undergo cavity surface barrier formation as in Example 1. The shell was then packed with a filler comprising a mixture as in Example 2, and subsequent processing proceeded as in Example 2, with equally good net shape performance shown by the metal matrix composite. Example 4 An investment shell was formed as in Example 3, with the exception that, before firing, the surface of the cavity in the shell was sprayed with a high-temperature, aluminum paint, available from Sherwin-Williams Co. and sold under the name Hi-Enamel Aluminum Color Spray Paint). The paint comprises a No. 2 aluminum paste in a silicate vehicle. The painted investment shell was then fired for a period of about 2 hours, but otherwise similar to the firing in Example 3. Subsequent processing proceeded as in Example 3. The net shape performance, i.e., the fidelity to the removable mandrel and the lack of surface finishing needed, of the resulting metal matrix composite body was even better than the bodies formed in Examples 1-3.

Claims: A method for making a metal matrix composite, comprising: (a) forming an investment shell having a cavity therein; (b) providing a substantially non-reactive filler in the cavity; (c) placing a source of matrix metal adjacent to said substantially non-reactive filler; and (d) heating said matrix metal to a temperature region above its melting point and in the presence of an infiltrating atmosphere during at least a portion of the process and at least one of an infiltration enhancer and an infiltration enhancer precursor to cause the matrix metal to spontaneously infiltrate the filler. The method of claim 1, wherein the filler comprises at least one material selected from the group consisting of powders, flakes, platelets, microspheres, whiskers, bubbles, fibers, particulates, fiber mats, chopped fibers, spheres, pellets, tubules and refractory cloths. The method of claims 1, wherein the investment shell is formed by coating a removable mandrel with a refractory material, rendering the refractory material self-supporting, and removing the removable mandrel. The method of claim 3, wherein the removable mandrel comprises a wax mold. The method of claim 3, wherein the removable mandrel is removed from the investment shell by reversibly disassembling the investment shell. The method of claim 3, wherein the refractory material comprises at least one of alumina, silica and silicon carbide. The method of claim 3, wherein the removable mandrel is coated by at least one of painting, spraying and dipping. The method of claim 3, further comprising the step of coating the cavity with a barrier for inhibiting the spontaneous infiltration of molten matrix metal. The method of claim 1, wherein the infiltrating atmosphere communicates with at least one of the filler and the matrix metal through the investment shell. The method of claim 1, further comprising the steps of providing at least a second matrix metal, and spontaneously infiltrating at least a portion of the filler with molten second matrix metal.

Inventors: BURKE JOHN THOMAS, BURKE, JOHN THOMAS
Applicants: LANXIDE TECHNOLOGY CO LTD, LANXIDE TECHNOLOGY COMPANY, LP.
Application year: 1989
Main classification: C22C1
IPC codes: B22F3, C22C1, B22D11, B22D19, C22C21, B22C9, C22C47