NO174007B - PROCEDURE FOR THE PREPARATION OF SOLID COMPOSITION MATERIAL, SPECIAL METAL MATRIX WITH CERAMIC DISPERGATES - Google Patents

Abstract

Composites of materials in which the matrix material does not spontaneously or readily wet the disperse phase and in which the volume fraction of the disperse phase is less than that formed in a packed bed of dispersate particles can be made effectively by an indirect method of infiltrating a packed bed of dispersate particles, using pressure or other mechanical force as needed to overcome poor wetting and form an intermediate concentrated composite. The concentrated composite is then mixed with additional matrix-forming material to produce the finally desired composite. The technique is particularly valuable for composites with ceramic dispersates and metal or alloy matrixes. When the matrix is predominantly aluminum and the dispersates are predominantly silicon carbide, a barrier coating of a material that retards reaction between silicon carbide and alumina is useful.

Description

The present invention relates to a method for the production of solid composite material, preferably metal matrix composites (MMC), which comprises an uninterrupted metal matrix material which has a majority of separate particles, i.e. dispersants, dispersed through the matrix, and which has a particularly even dispersion of the dispersants and which at the same time mainly have no porosity. The invention is particularly directed to the production of metal matrix composites which have ceramic dispersants distributed substantially uniformly throughout the matrix phase. The present invention provides a solution to the problems of exceptionally high porosities in the metal matrix composites and exceptionally large aggregation of particles, which problems have prevented raw composites from having properties comparable to powder, metallurgical forging or extruded composites.

It is well known that the composite materials often have better mechanical properties than either the matrix or the dispersants alone. The extent of improvement in properties has generally been found to be a function of the degree of distribution of the dispersants. The production of composites with reproducible and particularly uniform dispersion of the dispersants has proven to be very difficult.

A method for producing composites, in which a matrix material is obtained from a fluid form, is to disperse disperses in a pre-liquid and then form composites by solidifying the liquid part, i.e. the dispersion medium, of the dispersion. In practice, however, it has been quite difficult to carry this out due to the density differences that are normally present between the dispersants and the dispersion medium. These differences generally lead to an uneven distribution of the dispersants, not only in the liquid dispersion, but also in the finished composite.

US-PS 4,735,656 recommends overcoming the density-segregation problem by (i) mixing the metal particles with ceramic particles and then (ii) heating the mixture to a temperature high enough to produce partial melting of the metal (so that on cooling melted into a compact matrix), but not so high that it causes the ceramic particles to float in it. The care required to use this process makes it unsuitable for large-scale commercial operations. The process also does not exclude the presence of voids in the finished product.

Another problem with using the simple mixing concept is that many of the most desired dispersants are difficult to wet using known pre-fluids of desired matrix phases. International patent application WO 87/06624 teaches the use of specific dispersing and/or sweeping vanes to promote high shear mixing, while minimizing the introduction of gases into the mixture as well as the retention of gases at the interface rafts between the dispersants and the matrix material. US-PS 4,662,429 teaches the addition of lithium to an aluminum matrix alloy melt to enable the wetting and dispersion of the dispersants in the matrix alloy. EP-PS 87 201 512.8 describes composites of a zinc aluminum alloy reinforced with silicon carbide powder which "surprisingly" have good mechanical properties without the difficulties often experienced with other similar composites.

The difficulties in producing high quality composites, and some of the methods attempted to address the problems, are generally reviewed in "Solidification, Structures, and Properties of Cast Metal-Ceramic Particle Composites", P.K. Rohatgi et al., 31 International Metals Reviews 115-39 (1986). A method described in more detail by B.C. Pai et al., 13 Journal of Materials Science 329-35 (1978), involves compressing dispersers with powdered matrix material to form a pellet, introducing the pellet under agitation under the surface of the fluid matrix material for the time necessary to both to melt the pellet and to disperse the dispersants in the total amount of the fluid-for-material, and then solidification of the dispersion. J. Cisse et al., in 6B Metallurgical Transactions 195-97 (1975) similarly describes the use of a "prealloy" of sintered aluminum powder rods containing 10 wt% alumina.

A. Mortensen et al., Journal of Metals. February 1988, pages 12-19, also reviews the field and refers to Eohatgi et al., listing a number of techniques for introducing the particles, including pre-infiltration of a packed substrate of particles to form a pellet or "pre- alloy" and to redisperse and dilute it into a melt of a matrix material or its precursor.

All prior art methods first produce composites containing significant porosity, unless the dispersants are easily wetted by the fluid matrix material. If the dispersants are easily wetted, the resulting properties of the composite will often degrade as a result of chemical reactions between the matrix material and the dispersants.

An aim of the present invention is to provide a method for producing composites which contain as little porosity as possible. It is a further aim to minimize the time required to prepare the composites so that chemical degradation of the dispersers of the finished matrix material or a precursor thereof in the composite is prevented, or at least significantly reduced.

Composites that have the most desirable properties for multiple purposes are made when the dispersants in them are so well-dispersed that most of them do not touch another dispersant particle. This class of composites is characterized by having "separate" dispersants or as a "separate dispersion". It has now been found that many of the difficulties in the prior art in the production of separate composites can be overcome by using a method for the production of a final composite product having a plurality of separate ceramic dispersants in a solid metal matrix, characterized in that it comprises: a ) the formation of a concentrated dispersion of the ceramic dispersants in a first fluid material which is part of said solid metal matrix material, where the formation takes place by pressure infiltration of a packed porous layer of the separated solid ceramic particles with the first fluid metallic material; b) mixing at least a part of the concentrated dispersion into a quantity of a second fluid metal material which is another part of said solid metal matrix; c) treating the product of step (b) such that the ceramic particles in the concentrated dispersion are separated and dispersed in a mixture of both the first and second fluid metal materials; and d) solidifying the product from step (c) to form the final composite product.

In the present invention, a concentrated dispersion is thus used to form the more diluted dispersion of the desired composition by mixing it with further matrix fluid precursor, and dispersing it in this. If the mixing is done while the dispersion medium of the concentrated dispersion is still fluid, this design is referred to as a "continuous" process. It will nevertheless sometimes be appropriate to cause the dispersion medium of the concentrated dispersion to solidify, to produce a "concentrated composite" before the start of the mixing step to produce the final, desired composite. Composites made in this manner are designated as being made from "concentrated composite" designs of the invention.

The attached drawings show:

Fig. 1 shows a cross-section of a type of device which is applicable for carrying out the present invention, Figs. 2-3 and 6-7 show microstructures of composites produced according to the invention, or of comparable composites and Figs. 4 and 5 show cross-sections of a other type of device which is applicable for the execution of the present invention.

It has been found that satisfactorily concentrated dispersants according to the present invention can be produced by packing dispersants in a porous bed, where most dispersants touch at least one other dispersant, and then infiltrating the packed bed with a fluid precursor to the desired finished matrix, so that (i) the acceptable uniform distribution of dispersants characterized by the packed bed is maintained during the infiltration and (ii) most, if not all, of the gas present in the volume between the particles in the bed is removed during the infiltration stage. In this way, the infiltrated packed dispersant bed becomes a concentrated dispersion of dispersants which is applicable in the present invention. Preferably, the concentrated dispersion will not have more than about 5 vol-# voids and/or gases. Even more preferred is that the porosity is substantially completely eliminated from the concentrated composite. Dispersions that have these characteristics are prepared to have higher concentrations of dispersants than is normally desirable in the finished product.

If the packed porous bed of dispersants is evacuated before infiltration, fluid infiltration into the bed can be carried out if desired from several directions. Often it is more convenient to omit the evacuation stage and infiltrate the dispersant bed only from a single direction, while allowing the removed air to flow through an open portion of the bed. When the fluid precursor does not wet the dispersants immediately, a successful infiltration can still be achieved by applying a pressure to the fluid. Although the infiltration process generally dissolves some of the contacts between the particles in the packed dispersions, the resulting dispersion is more concentrated than the finished dispersion produced from it.

The infiltration of the packed porous bed of dispersants is usually carried out under the minimum suitable conditions of both temperature and pressure. By using a low temperature, possible reaction between the dispersant powder and the matrix precursor will be minimized. Therefore, the temperature of the matrix precursor fluid used should be about 25 to about 200°C, preferably about 50 to about 175°C, and most suitably about 75 to about 125°C above its melting point. The disperse gates are preferably preheated to the temperature of the matrix precursor fluid to prevent metal cooling or solidification that may otherwise occur, and reduce or prevent complete infiltration. With regard to pressure, the use of pressure has been found to be appropriate to reduce/eliminate porosity in the concentrated dispersion, but at the same time increases the cost of performing the infiltration. The pressure used is therefore chosen by balancing the level of porosity in the concentrated dispersion and cost. In general, an overpressure of at least 172 kPa will be preferred, while still higher overpressure is more preferred, as the particle size of the dispersants is reduced. Also to help reduce the porosity due to the use of too low a pressure, it has been found desirable to increase the overpressure to about 1380 to 2070 kPa near the end of the infiltration process.

In order to promote good displacement of gases between the particles at relatively low infiltration pressure during the formation of composites, it has been found expedient to incorporate known wetting agents into the matrix precursor materials used to form the concentrated dispersion. The specified wetting agents and the amounts used will normally depend on the particular matrix material and the dispersant, and can be determined by means of routine experimentation. For example, when making a composite of silicon carbide and an aluminum alloy, tin and potassium hexafluorozirconate are preferred wetting agents since they are known to produce wetting of silicon carbide and can be readily added to the aluminum alloy precursor material.

As discussed above, the mixing of the concentrated dispersion with additional precursor fluid is performed in a manner that avoids the difficulties of dispersing small particles directly into an open container of fluid. With the favorable concentrated dispersions according to the present invention, the mixing becomes very simple. Parts of the concentrated dispersion can simply be placed on top of a second matrix fluid, if the dispersants are denser than the matrix, or covered with the second matrix fluid, if the dispersants are less dense than the matrix. A combination of gravity and agitation then mixes the dispersants into a total amount of precursor fluid matrix. It has been found that concentrated dispersants prepared by the methods described here often have very favorable properties, such that at some temperatures they behave as if the dispersants were so well bound to the matrix that each dispersant tends to carry a significant amount of the matrix material when it is moved.

Preferably, a non-vortex generating mixer will be used to minimize any air entrapment. The mixer must of course be made of a material that does not react with or degrade the matrix-precursor fluid. Suitable materials will depend on the particular composition of the composites. With an aluminum alloy matrix, e.g. mixers made of graphite or steel coated with a non-wetting coating or spray such as carbon, sodium chloride or mica coating are preferred. For some alloys, Cotronics 902 machinable ceramic from Cotronics Co., Brooklyn, NY can be used.

High shear mixing is preferred to disperse any small clusters or agglomerates of dispersants remaining from the concentrated dispersion, and also to provide a more uniform distribution of the dispersants in the finished composite. During mixing, the temperature is normally kept within the range at which the mixtures containing the dispersants exhibit thixotropy, so that efficient mixing is achieved, while at the same time the possibilities of dispersant resegregation, due to the density differences when the mixed material is moved away from the mixing zone, are reduced.

Certain procedures for performing the mixing step have been found either convenient or preferred. In carrying out the continuous embodiment of the invention, as shown in examples 9 and 10 below, it is often appropriate to provide a reservoir which can be pressurized by the concentrated dispersion from which it is injected into at least part of the other for - the running fluid. To enable intermixing, it is generally preferred to inject the concentrated dispersion into a flowing stream of the second precursor fluid. For the concentrated composite design, for. of which Example 1 below describes a suitable device, a portion of the concentrated composite may be held mechanically below the surface by a unit of second precursor fluid, maintained at a temperature high enough to re-liquefy at least a portion of the matrix of the concentrated composite , and parts of the two components of the concentrated composite may be mixed into the second precursor fluid when liquefaction occurs. Alternatively, the concentrated composite may be heated to and maintained at a temperature necessary for partial liquefaction of its matrix, and further fluid precursor added by admixture.

In the method according to the present invention, the first separated dispersions are provided which are (i) substantially free of pores, and (ii) have a substantially uniform dispersion of the dispersants. New dispersions are provided here which contain no more than about 40 volume-# of dispersants, and also no more than about 5 volume-# of voids, pores and gases.

In connection with this invention, it should be understood that a "precursor of a matrix material" refers to materials that can be converted into the desired matrix material by chemical or physical treatment without dislocation of dispersants in the material. A liquid alloy or a thermoplastic resin is therefore a precursor to the solid alloy or resin to which it hardens on cooling, or fluid mixtures of polyfunctional isocyanates, and polyfunctional alcohols are precursors to polyurethanes which they form by chemical reaction after mixing, or fluid acrylated materials, are precursors of polymers that they form after they are exposed to an electron beam. "Matrix" is used here to refer to a continuous phase of dispersion or composite, be it in a fluid or in a solid form.

The solid composite materials produced by the method according to the present invention are comprised of two primary materials; the matrix and the dispersants. Matrix materials which are applicable according to the present invention include metals, metal alloys and thermoplastic resins. Suitable metals and metal alloys include aluminium, aluminum alloys, magnesium, magnesium alloys, bronze, copper, copper alloys, zinc and zinc alloys. Suitable thermoplastic resins include polyester polyurethanes, polyether polyurethanes, and acrylic polymers and copolymers. Preferably, the matrix material is selected from aluminum, aluminum alloys, magnesium, magnesium alloys, and magnesium-aluminum alloys. Preferably, the matrix material is an aluminum alloy containing less than 12 wt-#, preferably less than 8 wt-#, and most preferably less than 1.5 wt-# of silicon.

Suitable dispersants for use here are ceramic materials such as silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, titanium, silicon oxide, boron carbide, borides, carbides, silicon compounds, diamond and the like. These materials are characterized by having a modulus of elasticity, strength and wear resistance that is significantly higher than that of the matrix materials. Preferably, the dispersants are either silicon carbide, silicon nitrite, aluminum nitrite or aluminum oxide. Preferably, the dispersants are silicon carbide. The dispersants, regardless of particular chemical compositions, are used in the form of fine particles, which generally have an average particle size of 0.1 to 45 microns, preferably 3 to 20 microns, and preferably 7 to 15 microns.

The concentrated dispersion will generally be prepared so that it contains as much dispersant as possible. Therefore, 25 to 85 wt.#, preferably 45 to 60 wt.#, and most preferably 53 to 56 wt.# dispersants will be used.

Some of the most suitable applications of this invention are in the manufacture of composites of silicon carbide, dispersed in aluminum or magnesium alloys. Such materials are suitable construction materials for use in such as aircraft fuselages and other components where a combination of low density, high toughness and high bending resistance at temperatures just below the melting point of the alloy is needed. Previously, it has been very difficult to produce composite products having about 20 wt-# silicon carbide, the most suitable mechanical range, with the substantially uniform dispersion of silicon carbide particles needed. This was particularly the case when the silicon carbide particles had a size of less than 10 microns, and/or had a wide spread of sizes. Such composites can easily be produced according to the present invention.

It is known that alloys of aluminum containing from 1 to 4% silicon give stronger composites when reinforced with ceramic materials such as silicon carbide, than aluminum alloys containing less silicon. This is so even though unreinforced silicon-aluminum alloys containing less than 1% silicon are stronger than those containing more than 1% silicon. The difficulties in making composites with low-silicon aluminum alloy and silicon carbide are believed to be caused by a reaction between the low-silicon aluminum alloy and the silicon carbide dispersers to form aluminum carbides. The aluminum carbide formation weakens the matrix/particle interface and makes SiC less effective as a reinforcement material.

Similarly, when the silicon oxide dispersant is used with a low silicon aluminum alloy, they can react with the aluminum to form silicon and alumina. When titanium dispersions are used to reinforce low-silicon alloys, they can react to form Ti-Al and aluminum oxide. Correspondingly, as is known, other harmful reactions can occur with different ceramic/metallic combinations.

The difficulties caused by such harmful interactions between desired dispersants and either the precursor of the matrix material or the matrix material itself, such as occur when silicon carbide dispersants are used to reinforce a low-silicon aluminum, can be overcome with a design of the present invention. The deleterious reaction is sufficiently inhibited, or ideally prevented, by covering the dispersant particles with a material which (i) does not react with either the precursor or the matrix material, but (ii) will form an adherent coating on the dispersants and (iii) will assist in the wetting of the dispersants of the matrix precursor.

The suitability of the particulate dispersant coating materials can be "determined by routine experimentation by dividing particular dispersants to attempt to infiltrate a packed, porous bed thereof with the desired matrix precursor fluid, and if the infiltration succeeds, an attempt is made to disperse the concentrated dispersion into further matrix precursor material. Examples of suitable coatings for use with low-silicon aluminum alloys include metals, metal oxides, metal nitrides, metal carbides, and metal borides. When a metal coating such as copper, molybdenum, nickel, zinc, tin, or titanium is used, it is preferably extremely thin , eg up to about 2 microns, to minimize any harmful inter-metallic interactions. Metal oxides that can be used here include silicon oxides, aluminum oxides, chromium oxides, nickel oxides, copper oxides, mullite, spinels, titanium compounds, magnesium silicate, lithium silicate etc. Suitable metal nitrides include silicon nitride, titanium-Ni tritite, boron nitrite and aluminum nitrite. Suitable metal silicon components include molybdenum, copper and titanium silicon compounds.

Silicon dioxide, which can be formed on silicon carbides by heating in air, is currently preferred, partly because it is particularly convenient to produce from only heating silicon carbide powder at about 1300°C for about 30 minutes. Alumina which can conveniently be coated on silicon carbide particles from a crystallized sol to form the coating is also preferred.

Such coatings have generally not been found necessary for finished composites made with magnesium-based alloys, because the formation of harmful products has not been found to be extensive, even when magnesium is alloyed with aluminum.

The use of the invention can further be understood from the following examples, where all parts and percentages are by weight unless otherwise specified.

Example 1

A quartz tube 16 cm long and having an internal diameter of 2.2 cm was coated internally for a length of about 15 cm with a suspension of colloidal graphite available from Acheson Colloids, Ltd., Brantford, Ontario, Canada, under the brand name AQUADAG. A support rod smaller than the inside diameter of the quartz tube, but having at one end a gas-permeable plug of porous refractory clay chip that fits snugly inside the coated tube, was then inserted from the uncoated end and positioned so that the porous plug was about 10 cm from the opening of the coated end. The container thus formed by the coated pipe end and the porous plug was filled with grit F600 green silicon carbide to a packing density of about 50 vol-#, by means of a vibrating table in contact with the container. (The size distribution of grit F600 silicon carbide is fully described in the publication of the Federation of European Producers of Abrasives, hereinafter referred to as "FEPA"). The particular amount of grit F600 used in this experiment was measured with a Coulter Counter and had 50% of its volume of particles with a size greater than 9.1 microns; 3$ of the volume was made up of grains larger than 15.3 microns, 94$ of the volume was made up of grains larger than 4.8 microns, and the intermediate 75$ of the volume was made up of grains with sizes between 6.2 and 12, 2 microns. The intermediate 75$ of the volume is defined as the part of the sample roof that does not include the largest and smallest particles which each accounted for 12.5$ of the total volume.

The upper surface of the packed bed of SiC was covered with a layer of porous alumina paper (product APA1 from Zircar Products, Florida, New York) and this end of the container was then wrapped with aluminum foil. The porous alumina paper is attached tightly enough to prevent the packed bed from falling out when the container is turned upside down and to serve as a filter to exclude oxides or other unwanted products when molten metal is later infiltrated into the packed bed. The aluminum foil allows the protected end of the container to be immersed in the molten aluminum alloy without contaminating the contents with a layer of oxide that forms spontaneously on molten aluminum alloys. Shortly after immersion, the aluminum foil melts. The amount of aluminum foil is so small that the composition of the molten aluminum alloy does not change appreciably.

The shrouded container with its packed bed was then placed in a gas-tight drying cabinet which was evacuated to a pressure of no more than 0.01 bar and then backfilled with argon. The thus produced container was placed inside a device illustrated in fig. 1. The quartz tube 1 now has the alumina paper 5 at the bottom of the packed bed 4, with the porous refractory plug 3 and support rod 2 on top. The pipe 1 is connected via a gas-tight attachment 6 to a channel 7 which allows the inflow or outflow of gas from the space above the porous plug independently of the space 8 in the upper part of the device.

The tube with its packed bed was then submerged, as shown in fig. 1, in a bath of molten A357 aluminum alloy (containing about 7% silicon) 9, with a melting point of 610°, was held in a graphite crucible 10 at a temperature of 700°C by means of a traditional heating element 11. The crucible and heater is inside a gas-tight space formed by the container 12, which is protected from the heat and by the heated element 11 of insulation 13. Space 8 was first filled with argon gas at atmospheric pressure. After preheating for 5 minutes the tube containing the packed bed, the thermocouple 15 showed that the temperature of the molten metal 9 had returned to the desired value of 700°C after being cooled by the introduction of the packed bed and its container, the pressure within the furnace above the layer of molten alloy was increased at a rate of 1.36 bar/minute by supplying additional argon gas through inlet channel 14. This pressure caused the fluid alloy to flow through the packed bed from the bottom, thereby replacing gas from the top of the bed through the porous plug into the separate channel 7.

When the pressure reached 13.6 bar, the increase in pressure was stopped and the pressure was maintained at this level for 5 minutes. The pressure throughout the furnace was then lowered via outlet 15, and the tube containing the packed bed of SiC, now infiltrated with molten alloy, was removed and cooled in air to form a concentrated composite, which was separated from the quartz tube by pushing from one end and was then cut into pieces using a diamond saw. A microcut of a polished section of the prepared concentrated composite is shown in fig. 2.

In a separate graphite crucible, 91 g of A357 alloy was melted and heated to 720°C, and a piece weighing 62 g of the concentrated composite, prepared as described above, was placed on top of the molten alloy. After 10 minutes, the material was stirred with a graphite rod to aid in a preliminary breaking up of the concentrated composite, and a rotating graphite stirrer, preheated to 600°C, was then immersed in and used to stir the melt for 5 minutes at 300 rpm . minute. The crucible was then removed from the furnace and its contents emptied into another crucible and cooled. The resulting composite of this invention with 20 volume-# SiC was examined under a microscope after a polished section was prepared. It showed good distribution of the SiC dispersate throughout the composite, with clear intimate contacts at most SiC metal interfaces and little porosity. This is shown in a microcut of the composite in fig. 3.

Example 2.

In Example 2, particles of grit F600 silicon carbide were placed in the bottom of a steel crucible to provide a packed bed of about 50 vol-# SiC. A quantity of molten alloy containing 90$ Mg and 10$ Aluminium, sufficient to infiltrate the entire packed bed, was poured over the bed and the crucible with its contents placed inside a pressurizable furnace, at 700"C. Compressed argon was then fed to the furnace until the pressure reached 34 bar This was necessary to ensure that the molten alloy impregnates the entire packed bed except for a small pocket at the bottom into which air originally present in the packed bed was been relocated.

A portion of the fully impregnated concentrated composite prepared as described immediately above was softened at 700°C and mixed with a further amount of the melt, 90% Mg and 10% Al alloy, selected to result in a finished composite of 20 volumes -# SiC. Mixing was first carried out with a handheld stirrer until the concentrated composite showed the necessary low viscosity to allow effective mechanical mixing. Mixing was then continued with a double helical stirrer operating at 400 rpm. This prevented gas from entering through the vortex formation. After about 5 minutes of stirring, the result was a semi-solid, pourable slurry. This material was then cast and allowed to solidify. A highly dispersed, finished composite was formed.

Example 3

In example 3, this was carried out in the same way as in example 2, except that grit F500 was used instead of grit F600 silicon carbide.

Example 4

In example 4, this was carried out in the same way as in example 2, except that grit F320 was used instead of grit F600 s i1 i s iumcarb i d.

Example 5-7

The preparation of the concentrated composite for these examples was carried out in approximately the same manner as in examples 2-4, respectively, except that (i) a somewhat different device allowing evacuation as well as pressurization of the space within the container for dispersants was used; (ii) first, solid alloy material was added to the container above the packed bed; (iii) the space within the container for the dispersants was evacuated after the solid metal and dispersants had been placed inside the furnace maintained at 900°C, prior to pressurization to cause infiltration; and (iv) a commercial grade pure magnesium alloy was used for the matrix.

The concentrated composite was mixed with additional molten matrix alloy in a special container in an argon atmosphere, using a powerful stirrer similar to a turbine moving at 2000-3000 revolutions per minute. my. A system of baffles in the container prevented significant gas entrainment during mixing. The dispersion was mixed for about 5 minutes at a temperature of about 700°C. The stirrer was then immediately removed and the dispersion immediately cast into an approximately 9 mm deep copper mold. A favorably dispersed composite with about 15 wt-# of dispersants was the result.

Example 8

This was carried out in the same manner as in Examples 5-7, except that a finer particle size of silicon carbide, about 3 microns in size, was used.

Example 9

Example 9 illustrates the use of the invention in continuous casting and is carried out by the device shown in section in fig. 4. Molten alloy 100 and concentrated composite are fed continuously into a chamber 102 maintained at a temperature that will keep the mixture at least partially fluid. From chamber 102, the mixture is pumped and mixed by a rotor 103 to a mixing area 104, where it is subjected to vigorous agitation. High shear forces in the area 104 are achieved in a narrow gap 108 between the chamber wall 105 and a rotor 106. Both the chamber wall and the rotor have surfaces that comprise a conical truncated cone with the same tapering angles, so that the gap width, and the corresponding shear speed, can be adjusted by vertical displacement between the rotor and the chamber wall. The well-dispersed dispersion flows out into region 107 and can be fed into a crucible for the solidification process, continuous casting of an extrusion blank, or the like.

The concentrated composite itself can alternatively be rotated vigorously in a bath of molten alloy, so that parts of the concentrated composite are peeled off at the interface while the matrix of the concentrated composite is softened under the influence of the higher temperature of the bath of molten alloy.

Example 10

Example 10 illustrates a design of the invention with another continuous method and can be understood with the help of fig. 5, which shows a section of the device suitable for the invention. A fixed chamber 201 capable of withstanding the pressures involved is provided with a conventional device for maintaining different temperatures in different regions of its interior, and contains two inlets 202 and 203, and an outlet 204. At inlet 202, molten metal is supplied 205 under pressure. At inlet 203, the dispersants 206 are supplied at a suitable speed, and also under pressure by means of a press, supply screw or other suitable devices.

During operation of the continuous process of this invention, chamber 201 is maintained at a temperature which will maintain molten metal in regions 205 and 206, and a temperature too low to melt the metal "used on top of region 206, which constitutes a packed bed of dispersants The flow of dispersants from zone 203 is maintained in a downward direction by mechanical pressure applied against the packed bed of dispersants, but this does not prevent metal from filling the spaces between the particles of the packed bed of dispersants in the lower parts of the inlet region for dispersers, where the temperature is necessary high to keep the metal molten. A zone of concentrated composite of this invention is therefore formed in the region 207, but upward penetration of the metal is limited by its solidification in the upper parts of the inlet 203, which forms a more or less clear boundary between region 207 containing concentrated composite and region 206 with only dispersate and gas.

In the area where the molten metal 205 contacts the concentrated composite 207, the flow rate of the metal accelerates by means of a constriction formed by a bulge 208 in the chamber wall. In the region between 208 and 207, the concentrated composite is continuously entrained downstream by the rapidly flowing molten metal and is sheared and dispersed into the flowing metal. At a sufficient distance downstream from the constriction, an area 209 of substantially homogeneous and non-porous dispersion is obtained. This dispersion may be cast continuously from outlet 204 to produce a solid, continuous extrusion blank 210 of the finished desired composite. The volume fractions of the metal matrix and dispersants are controlled by regulating the relative feed rate of the dispersants and molten metal at their inlets, 203 and 202 respectively.

Instead of a constriction, separate mechanical or electromagnetic stirrers can be used to disperse the concentrated composite into additional matrix precursors.

Example 11

Example 11 was carried out in the same way as example 1, except that the SiC particles used were a mixture of equal volumes of FEPA grit F400, grit F500 and grit F600. Grit F600 had the same size distribution as in Example 1. Grit F500 material had 3 volume-# particles greater than 22.5 microns, 50 volume-# particles greater than 13.7 microns, 94 volume-56 particles greater than 8.7 microns , and the bulk 75 volume-# particles with sizes between 10.6 and 17.7 microns. These measurements were performed by a Coulter Counter. Using the same measurement technique, the grit F400 material had 3 vol% particles larger than 25 microns, 50 vol-5é particles larger than 17 microns, 94 vol-56 particles larger than 12 microns and the bulk 75 vol-# particles with sizes between 13 and 20.5 microns. 75% by volume of the mixture had particles between 7.8 and 19 microns, 3% by volume of the mixture had particles less than 5.2 microns, and 94% by volume of the mixture had particles greater than 21 microns. The finished composite had an essentially even distribution of all particle sizes of SiC within the matrix when this was studied in section.

Example 12

Example 12 was the same as Example 1, except that the dispersants used were boron carbides instead of silicon carbide. Good redistribution of the concentrated composite was achieved in the finished composite.

Example 13

Example 13 was conducted in the same manner as Example 1, except that (i) 100 g of concentrated composite and 215 g of additional A354 alloy were used to provide a 15 volume # composite;

(ii) melting temperature during mixing of the concentrated composite into additional molten alloy was only 607°C instead of 700°C; and the final stirring was performed for only 2.5 minutes instead of 5. The differences in temperature significantly increased the apparent viscosity during mixing of the concentrated composite with additional matrix material and some large air pores were introduced during stirring and preserved in the finished composite. Therefore, even if the SiC dispersants were again well dispersed inside the finished composite, the result was less preferred than in example 1.

Example 14

Example 14 was carried out in the same manner as Example 1, except that (i) alloy used was type 6061 alloy or an A357 and (ii) 100 g of concentrated composite and 149 g of additional molten alloy were used in the final mixing step.

Type 6061 alloy contains 0.6$ Si, 1% Mg, 0.3% Mn, and 0.2$ Cr, balance Al. Presumably due to the very low silicon content, the dispersions of the concentrated composite within the finished composite were not quite as good as in example 1. A microsand of the finished composite in this example is shown in fig. 6.

Example 15

Example 15 was carried out in the same way as example 14, except that the alloy used was 10% Si - 90% Al. The result is in strong contrast to those in example 14, in that the distribution of the SiC dispersants inside the finished composite was very even.

Example 16

Example 16 was carried out in the same way as Example 14, except that the alloy used contained 99.9% Al. The dispersions of silicon carbide in the finished composite were less uniform than that obtained in Example 14.

Example 17

Example 17 was carried out in the same manner as Example 16, except that (i) the temperature both for the preparation and for the mixing of the concentrated composite was increased to 800°C; and (ii) 71 g of concentrated composite and 152 g of additional alloy were used in the mixing stage, which should have produced a finished composite with only 15 wt-# SiC. The mixing of concentrated composite with additional metal nevertheless proved to be impossible in practice. X-ray diffraction of a sample of the concentrated composite showed the presence of aluminum carbide at the interfaces between the dispersants and the matrix. This is believed to be the reason why the concentrated composite was so difficult to break up.

Example 18

Example 18 was carried out in the same way as Example 17, except that (i) the finished mixture was carried out at 720°C instead of 800°C, and (ii) the SiC particles were heated in air at 1300°C for 30 minutes before was infiltrated to form the concentrated composite. This treatment is known to form a layer of SiOg on the surface of the silicon carbide. The X-ray analysis of this concentrated composite confirmed that the silicon oxide surfaces strongly retard the formation of aluminum carbide, and therefore the concentrated composite can be easily dispersed in the finished mixing stage. A microcut of the finished composite produced in this way is shown in fig. 6.

Example 19

Example 19 was the same as Example 1, except that the SiC particles were coated with aluminum oxide before the formation of the concentrated composite in the following manner; a boehmite sol of 10 wt-# total solids containing 0.15 wt-# of fine α-alumina seeds, repaired in a form of incipient gelation as described in detail in U.S. Pat.

4623364, was prepared. One liter of this sol was mixed with 1 kg of FEPÅ grit 600 SiC, and the mixture then pumped through a NIRO spray dryer, which caused the SiC to become coated with an apparently uniform coating of dried alumina gel when examined in an electron- scanning

microscope. The coated particles were then heated at 1200°C for 30 minutes to convert the alumina gel to α-alumina. The transformation was confirmed by the X-ray diffraction analysis which showed that only α-SiC and α-alumina were the phases present.

The finished composite prepared in this example showed an excellent uniformity of dispersion of SiC within the matrix.

Comparative example A

In the following comparative example, SiC was coated with Cr metal by an evaporation-condensation process to 29%. Infiltration tests with both A357 and Al 99.9 were done using quartz tubes that were 20 cm long and had an internal diameter of 5 cm. The temperature of infiltrated aluminum was varied between 700°C and 800°C, and the pressure varied up to 2760 kPa. In both cases the infiltration was negligible. It is believed that the chromium metal was dissolved by the molten aluminum to form an aluminium-chromium alloy. Small additions of chromium to the aluminum significantly increased the melting temperature to solidify the metal front before infiltration can occur. Chrome metal is therefore not a suitable coating material.

Comparative example B

In the following comparative example, SiC powder, coated as in the aforementioned comparative example, was calcined in air at 1200° C. for 1/2 hour to produce the formation of chromium oxide around each particle. The calcination only converted the surface of the chromium coating into chromium oxide. This powder was packed and infiltrated with A357 as in Example 1, using a pressure of 2586 kPa and a temperature of 750°C.

89 g of this pre-composite was heated to 850°C with 209 g of A357 to give a finished composition of 14$. After 2.5 hours, the pre-composite parts had not yet broken up, thus causing little dispersion. The protective barrier of Cr£0 3 on each particle was dissolved to allow aluminum to react with chromium, as in the former comparative example, to thereby form a high melting alloy and to prevent the dispersion of the pre-composite. A surface of chromium oxide layer on a mainly chromium coating was therefore not suitable.

Example 20

In Example 20, a master alloy was prepared using A357 and SiC coated with chromium to 3.5 wt-# and calcined as in the latter comparative example. In this case, the chromium was nevertheless completely oxidized to chromium oxide. The infiltration occurred at 750° C with a pressure of 2760 kPa. 94.9 g of this concentrated dispersion sample was dispersed in 235 g of A357 to give a composite with 13.4 wt-# SiC. This took 15 minutes at 850°C. The microsands indicate a good dispersion with some remaining lumps of pre-alloy which can be broken down by improved stirring. In this case, the chromium oxide coating prevented reactions between the aluminum metal and SiC to allow dispersion of the master alloy to occur. Lack of chromium metal in the coating also prevents the melting point of the alloy from increasing, which was found to prevent dispersions of the pre-composite in the previously mentioned comparative examples.

Example 21

In Example 21, a coating of Sis^ on SiC particles was produced by heating SiC in liquid nitrogen to 1425°C. A pre-composite was prepared as in Example 1 using A357 at a pressure of 2070 kPa and a temperature of 725°C. 75 g of this pre-composite was stirred into 192 g of A357 as in Example 20 to give a finished fraction of 13.1% SiC. The dispersion occurred easily. Microsanding of the finished composite indicates good dispersion of coated SiC, especially without noticeable formation of reaction products between the metal matrix and the coated SiC dispersants.

Comparative example C

In a further comparative example, SiC was coated to 8.3% nickel metal using an electroless NiB coating solution. The infiltration was carried out using quartz tubes as in Example 19, with A357 at 725°C and applying a pressure of 1380 kPa. 9.3 g of this prealloy was dispersed in 57.9 g of 357 using a small porcelain crucible and a graphite stirring rod. The furnace temperature was set at 800° C. The pre-composite did not disperse uniformly, presumably due to the nickel-silicon alloy formation, thus indicating that a nickel metal coating is not suitable with this aluminum alloy.

Example 22

In example 22, the procedure in comparative example C was repeated by reducing the nickel content of the dispersants to 2%, and by using Al 99.9 as matrix material in both the infiltration and the dispersion. The result was a good dispersion.

Example 23

In Example 23, SiC was given a total of 9.556 coatings of chromium oxide-alumina solid solution. This coating was prepared by mixing an equal weight of SiC and a 30% solid sol gel of Cr(NO3)3~9H20 and AI2O3-H2O. The resulting mass was dried to a powder and heated to 1250°C to remove structural water in the coating material and to convert nitrates to an oxide. This SiC was infiltrated with A357 as in Example 1 using a temperature of 725°C and a pressure of 1100 kPa. 5.8 g of this prealloy was then stirred into 60.7 g of A357 to give a composite having 4% SiC. Dispersion was good.

Example 24

In Example 24, a b% molybdenum coating was coated on silicon carbide particles by mixing coarse molybdenum powder with F600 SiC in a suitable weight ratio and heating in a vacuum oven to vaporize the molybdenum and again deposit it on the SiC particles. The coated particles were then treated as in Example 1 with Å357 alloy to form a concentrated dispersion containing 54$ dispersants. The concentrated dispersion was then stirred into additional A357. Good straightening of the concentrated dispersion was observed and a uniform finished composite was produced.

Example 25

In Example 25, the procedure of Example 24 was repeated to prepare a b% titanium metal coating on silicon carbide dispersants, to use these dispersants to prepare a concentrated dispersion and to use the concentrated dispersion to prepare a finished uniform dispersion. The result was a good dispersion.

Example 26

In example 26, the procedure in example 25 was repeated to produce silicon carbide dispersants with a titanium metal coating and then the titanium coating was oxidized to be converted into titanium oxide. The infiltration with A357 at a pressure of 1103 kPa psi was just sufficient to produce a pore-free concentrated dispersion, but when repeated at 2070 kPa the process proceeded without problems. Good finished dispersions were produced in each case.

Example 27

In Example 27, titanium nitride coatings were coated on silicon carbide dispersants by (i) forming a silica and carbon diffusion layer by calcining SiC in air at 1100°C, (ii) coating titanium metal thereon by a packet diffusion process, and then ( iii) nitriding the dispersants at 925-1025°C to convert metallic Ti to TiN. The resulting dispersants were lightly infiltrated with pure aluminum to form a concentrated dispersion which was stirred into further pure aluminum to produce a well-dispersed, finished composite product.

Claims (16)

1. Process for the production of a final composite product which has a plurality of separate ceramic dispersants in a solid metal matrix, characterized in that it comprises: a) the formation of a concentrated dispersion of the ceramic dispersants in a first fluid material which is part of said solid metal matrix material , where the formation takes place by pressure infiltration of a packed porous layer of the separated solid ceramic particles with the first fluid metallic material; b) mixing at least a part of the concentrated dispersion into a quantity of a second fluid metal material which is another part of said solid metal matrix; c) treating the product of step (b) such that the ceramic particles in the concentrated dispersion are separated and dispersed in a mixture of both the first and second fluid metal materials; and d) solidifying the product from step (c) to form the final composite product. 2. Method according to claim 1, characterized in that the packed, porous layer is evacuated before the infiltration. 3. Method according to claim 1 or 2, characterized in that the packed, porous layer contains interparticulate gas, and said infiltration is carried out from one side of said layer at a sufficiently low speed to allow the gas, which was originally contained in the packed layer, to be replaced by the fluid material. 4. Method according to any one of claims 1-3, characterized in that the mixing of at least part of the concentrated composite with the second amount of the fluid material is carried out at a temperature that is at least 70° C higher than the melting point of the matrix in the final composite. 5. Method according to any one of the preceding claims, characterized in that the concentrated dispersion comprises 25 to 85 wt-# ceramic dispersants and 75 to 15 wt-# metal matrix precursor. 6. Method according to any one of the preceding claims, characterized in that the first fluid material and the second fluid material have the same chemical composition. 7. Method according to any one of the preceding claims, characterized in that it comprises solidifications of at least part of the first fluid material of the concentrated dispersion before step (b) is carried out. 8. Method according to any one of the preceding claims, characterized in that the first amount of fluid material contains a material which facilitates the wetting of the dispersants without promoting harmful chemical reactions. 9. Method according to any one of the preceding claims, characterized in that the dispersants mainly consist of silicon carbide and boron carbide, and the matrix material mainly consists of aluminum or magnesium. 10. Method according to any one of claims 1 to 8, characterized in that the metal matrix material contains mainly aluminum and silicon. 11. Method according to any one of claims 1 to 8, characterized in that said matrix material mainly comprises aluminium. 12. Method according to any one of claims 1 to 8, characterized in that the solid ceramic dispersants have a surface layer of a barrier material which, at the temperature of the formation of the concentrated dispersion and the temperature of the formation of the final composite, slows down a chemical reaction that would otherwise occur between the material in the dispersants and said fluid metal material. 13. Method according to claim 12, characterized in that said barrier material mainly consists of a material which is a metal, a metal oxide, a metal nitride, a metal silicon compound, a metal carbide or a metal boride. 14. Method according to claim 13, characterized in that the method is carried out so that the surface layer of the barrier material is up to 2 microns thick. 15. Method according to any one of claims 12-14, characterized in that the first fluid metal material contains mainly aluminum and less than 12% silicon. 16. Method according to any one of the preceding claims, characterized in that the final composite comprises 5 to 40 volumes-5é separated disperses and no more than 5 volumes-56 pores, voids and/or gases.