Process For Making Carbon-aluminum Composites

Abstract

A process for impregnating a mass of carbon particles with metal, preferably an aluminum alloy, to form a solid billet suitable for machining therefrom articles having advantageous durability and lubricity characteristics, particularly as apex seals for Wankel engines. A mass of carbon particles is heated to a temperature at or above the melting point of the metal in a closed crucible of permeable refractory material such as porous carbon, and the molten metal is poured over the porous crucible into a second crucible wherein the porous crucible becomes enveloped by the molten metal but the molten metal does not penetrate the porous crucible. The atmosphere surrounding the molten metal and porous crucible is then pressurized to force the molten metal through the porous crucible and thus cause controlled impregnation of the mass of particles.

Description

BACKGROUND OF THE INVENTION

This invention relates to the manufacture of a seal material for use in sealing moving parts of expansion chambers such as the combustion chambers of rotary piston internal combustion engines. More particularly, the invention relates to a method for making a billet or blank of a composite carbon-aluminum alloy material having advantageous properties, from which seals such as apex seals may be machined.

The apex seals for rotary piston engines (e.g., Wankel engines) are fitted at each apex of the rotary piston or trochoid and are adapted to make a planetary motion with the piston while being urged against the inner surface of the stator by the combined action of the elastic force of springs disposed behind the apex seal, gas pressure in the operating chamber and centrifugal force produced by the rotation and orbiting of the piston. The seal moves in sliding contact with the inner surface of the stator and maintains an airtight seal between adjacent expansion chambers on opposite sides of the apex. Therefore, the apex seal used for this purpose must have excellent mechanical strength at all temperatures encountered and sufficient lubricity that it does not produce an excessive wear on the inner surface of the stator. In view of these considerations and others, the quality of the apex seal has a significant bearing on the performance and durability of the engine.

Many and various materials have been proposed for the apex seal such as those shown in U.S. Pat. No. 3,619,430 and in British Pat. No. 1,234,634. The most successful material presently available comprises carbon particles impregnated with an aluminum silicon alloy.

Carbon is an excellent bearing material due to its heat resistance, wear resistance and corrosion resistance as well as other advantages such as high thermal conductivity and low thermal expansion. Accordingly, carbon in various forms and combinations is used for many types of mechanical elements, particularly sliding members in view of its wear resistance and lubricity. The use of carbon products as sliding members and the like, however, is limited because of their inherently low durability, particularly where they are subjected to considerable vibration and impact.

Methods have been devised for manufacturing apex seals of a mass of carbon particles impregnated with a continuous phase of aluminum or an aluminum alloy and while aluminum is somewhat inferior in wear resistance to many other metals and has a relatively low melting point, the resulting composite material has increased strength and better resistance against vibration and impact. One reason that the aluminum-impregnated carbon type apex seal has superior characteristics is that a high ratio of impregnation can be achieved with aluminum and the aluminum in the resulting product is intimately and tightly bonded to the carbon due to the formation of aluminum carbide at the carbon-aluminum interface.

One impregnating process is accomplished in an autoclave by immersing a porous press-molded blank made from carbon particles and a binder in a molten aluminum or aluminum alloy bath. The autoclave is evacuated and the pre-pressed blank is impregnated with the molten metal in a nitrogen or argon atmosphere in order to prevent the carbon from oxidizing. Following immersion of the carbon blank a high pressure is applied to force the molten metal into the voids in the carbon blank. For suitable properties, it is necessary that the impregnation of the carbon blank with the alloy be as complete as possible. It has been found that a sufficient impregnation of metal is difficult to achieve without using high temperatures and pressures over relatively long times.

An autoclave and related equipment for achieving the impregnation is disclosed in U.S. Pat. No. 3,599,601. Using the apparatus shown in that patent, a sintered or otherwise preformed molded part is loaded in a dip cage formed of refractory material having a plurality of slits formed therein to permit entry of molten metal. The product is heated in the dip cage to the desired temperature and then with the autoclave partially evacuated, lowered into a bath of molten metal. A pressure is applied and the molten meal is thus forced through the porous article to be impregnated.

The method of the present invention affords an improved process which can be accomplished more quickly, more efficiently, is accurately controlled, eliminates the need for preforming the part or parts to be impregnated, permits a higher alloy to carbon ratio, and affords other features and advantages heretofore not obtainable.

SUMMARY OF THE INVENTION

It is among the objects of the invention to provide an improved method for impregnating a carbonaceous product with a molten metal.

Another object is to provide an improved method for making combustion chamber seals for rotary piston-type internal combustion engines.

These and other objects are achieved through a method for impregnating carbon particles with a molten metal, preferably an aluminum or aluminum alloy, which includes the following steps:

1. placing a quantity of carbon particles in a porous crucible formed of refractory material such as porous carbon and densifying the particles by vibration or jarring,

2. heating the carbon powder in the porous crucible to a temperature within or above the melting point range of the metal in an inert atmosphere,

3. placing the heated porous crucible and carbon particles in an impermeable crucible formed, for example, of ceramic material,

4. heating the metal within or above its melting point range,

5. pouring the metal in molten form over the porous crucible and into the impermeable crucible,

6. holding the porous crucible in the molten metal for at least half an hour in order to establish thermal equilibrium at a temperature within the melting point range of the metal,

7. placing the two crucibles in a pressure chamber such as an autoclave, and pressurizing the atmosphere to a pressure between 2,000 and 4,500 psi for a very short time, 1 to 10 minutes, to force the molten metal to penetrate the porous carbon crucible and impregnate the mass of carbon particles, and

8. removing the porous crucible, quenching and cooling to room temperature and then breaking away the porous crucible to obtain a billet of impregnated material.

When the metal is an aluminum-silicon alloy, the billet is rough machined into blank parts and further heat treated. It should be heated to approximately 935.degree.F. for about 8 hours, then heated at about 450.degree.F. for about 4 hours, and then final machined into apex seals. This treatment imparts dimensional stability and the desired grain structure. When other alloys and metals are used, further annealing and heat treatment should be employed as is appropriate to the particular alloy or metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an initial portion of the method of the invention using apparatus that is illustrated in diagrammatic form merely for the purpose of illustrating the method;

FIG. 2 is a sectional view showing the apparatus in a manner similar to FIG. 1 and illustrating a subsequent step in the practice of the invention;

FIG. 3 is still another sectional view illustrating apparatus in a manner similar to FIGS. 1 and 2 and showing a still further step in the practice of the method of the invention; and

FIG. 4 is a sectional view in diagrammatic form illustrating the breaking away of the material forming the impregnated porous crucible from the carbon-alluminum composite product made in accordance with the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated above, the invention resides in a method for making carbon-aluminum impregnates suitable for use in fabricating seals for various types of mechanical equipment such as seals used in the trochoid of rotary piston-type internal combustion engines. The method first requires the selection of appropriate materials. It has been found that a particularly advantageous form of particulate carbon material is calcined coconut char which is a particularly hard and abrasion-resistant type of char. This material preferably has a particle size of no more than 100 microns, and preferably no more than about 40 microns. Other hard, abrasion-resistant chars such as those made from walnut shells or other nut shells may also be used.

The molten aluminum alloy used as the impregnate is, in accordance with the preferred aspect of this invention, an aluminum-silicon alloy having a silicon content ranging from 5 to 35 percent by weight. The silicon in the alloy forms small grains which give the alloy strength at high temperatures. Preferably this is a hyper-eutectic alloy having 14 to 19 percent silicon. Silicon is soluble in aluminum to about 11 percent by weight, after which the silicon crystallizes out as tiny grains or crystals. A particularly advantageous alloy is an aluminum-silicon material sold the Aluminum Association trade designation "A-390." This is a hyper-eutectic alloy in the sense that it has more silicon than the eutectic point, in the case of A-390, about 17 percent silicon. Simple binary alloys of aluminum and silicon are not believed to be desirable. I prefer 2 to 5 percent copper or nickel and other elements found in A-390 and similar alloys.

An aluminum alloy is preferred because it readily "wets" and penetrates the carbon char particle powder. Other metals and alloys may be adapted to this process such as copper or other metals listed in column 2 of U.S. Pat. No. 3,599,601. Aluminum-magnesium alloys may be used. However, for the immediate purposes of this invention, I prefer a metal which is principally aluminum, by which I mean at least 50 percent aluminum by weight.

The porous baked carbon crucible preferably has a porosity of at least about 20 percent of the total volume thereof. Crucibles of lower porosities, as low as 5 percent by volume, may be employed but they are not as suitable because they require a longer impregnation time and because they are more expensive to manufacture. The porous crucible is preferably made of a nongraphitized carbon.

The porosity of the mass of carbon particles is of a higher order, as at least 20 percent and preferably around 50 percent by volume. After being placed in the porous crucible, the carbon particles are densified by vibration or jarring and the above porosities are to be taken after such densification. The preferred porosity range is 30 to 60 percent by volume.

Referring more particularly to the drawings which show apparatus for practicing the invention, in diagrammatic form only, a preferred embodiment of the method will be illustrated and described using materials found to be particularly advantageous. Initially the mass of carbon particles, or more specifically, the calcined coconut char 10, is placed in a porous ceramic crucible 11 preferably formed of fairly thin-walled baked carbon material and a lid 12 formed of the same material is placed thereon (FIG. 1). The powder is densified by vibration or jarring. The porous crucible 11 preferably has a porosity of about 20 percent by volume and the carbon powder preferably has a porosity of about 50 percent by volume.

The crucible 11 is heated in an induction furnace to a temperature in excess of the melting point of the metal or within the melting point range, in the case of the aluminum-silicon alloy, around 1,100.degree.F. The heating is done in a nitrogen or argon or other inert gas atmosphere ot reduce or eliminate the oxygen. The crucible 11 may be heated by itself or it may be placed inside the ceramic crucible and the assembly heated in the induction furnace in an inert atmosphere.

The metal 13 is also heated to a molten condition, again in this instance around 1,100.degree.F. in an induction furnace. The heating may advantageously be accomplished in the same furnace and the temperature should be very carefully monitored. Preheating of the porous crucible and carbon powder to that high a temperature, however, is only preferable. Preheating to lower temperatures, about 350.degree.F, may be employed and is beneficial in that it drives out some gas and moisture from the carbon powder.

When the metal is heated, it may also be purified as is well known in the art. In the case of an aluminum-silicon alloy, chlorine gas may be bubbled through the molten alloy to eliminate entrapped oxygen and hydrogen. Alternatively, degassing tablets may be added to the melt. Also, to control grain size, a nucleating agent such as aluminum phosphide may be added.

The volume of metal to be melted should be weighed and selected in accordance with a predetermined amount of material needed depending upon the volume of the mass of carbon particles 10. After the necessary heating of the carbon particles 10 and the metal 13 has been accomplished, the porous crucible 11 is carefully fitted into an impermeable ceramic crucible 15 preferably formed of graphite-clay material and having spacers 16 located on the bottom thereof as indicated in FIG. 1. A lid 21 provided with holes 22 is placed on the ceramic crucible 15 and over the porous crucible to keep it in place.

Referring next to FIG. 2, it will be seen that the molten alloy 13 is poured from the ceramic crucible 14 over the lid 21 so that it overflows down and around the walls of the porous crucible 11 to fill the spaces within the impermeable ceramic crucible 15. The spacers 16 serve to support the porous crucible 11 above the floor of the impermeable ceramic crucible 15 so that the molten metal essentially surrounds all surfaces of the porous crucible 11.

The assembly of the impermeable ceramic crucible 15, porous carbon crucible 11, carbon particles 10 and molten aluminum 13 is then maintained at a temperature approximately above or within the melting point range of the metal for a time sufficient to equalize the temperature between the crucibles and the molten metal. There is no pressure on the system and the molten metal does not penetrate the porous crucible 11. Finally, the assembly is placed in a quick-opening autoclave 17 (see FIG. 2) and a lid 18 is tightly attached to provide a hermetic seal.

After this thermal equilibration, the space within the autoclave 17 is then pressurized using a pump 19 to provide a pressure between about 2,000 to 4,500 psi for from 1 to 10 minutes and preferably 2 to 5 minutes. This is a pneumatic pressure, air or an inert gas such as nitrogen or argon. A pressure of 4,200 psi is believed to be particularly advantageous. The high pressure in the autoclave atmosphere forces the molten metal 13 to penetrate the porous carbon crucible 11 and lid 12 and then to impregnate the mass of carbon particles 10. Any scum that may form on the surface of the molten metal will be filtered out by the porous crucible 11 and thus not penetrate into the mass of carbon particles. The extent of impregnation of the voids or spaces within the mass of carbon particles 10 is essentially complete and any gas originally contained within the mass of particles is either dissolved into the molten metal or expelled through the porous crucible 11.

The application of pressure is continued for a short period of time, preferably 2 to 5 minutes, just barely sufficient to cause complete impregnation of the mass of particles 10. As soon as the impregnation is believed to be complete, the pressure is reduced to normal, the lid 18 removed and the assembly within the autoclave removed and quenched to room temperature.

The duration of the application of pressure is minimized so as to carefully control the reaction of aluminum and carbon which forms aluminum carbide. While the presence of aluminum carbide in the product of amounts of around 3 to 6 percent is desirable, quantities in excess of such percentages are to be avoided for the reason that excess aluminum carbide reacts with water vapor thus causing erosion of the seal material. The reaction between the molten aluminum and carbon to form an aluminum carbide interface facilitates impregnation by the molten metal.

After quenching, the billet of impregnated material is broken away from the porous crucible (FIG. 4). In the case of the aluminum-silicon alloy, the billet is then cut up into blanks and further heat treated and annealed. The billet may be annealed for from 6 to 10 hours at 900.degree. to 1,000.degree.F. and then from 3 to 5 hours at 400.degree. to 500.degree.F., preferably at the times and temperatures previously noted, and then quenched in air to room temperature. After heat treatment, the blanks are finally machined into apex seals.

As should be apparent, the process of the present invention has particular utility in the manufacture of aluminum silicon alloy impregnated seals. The A-390 alloy solidifies within a range of 1,050.degree. to 1,200.degree.F.

In dealing with such alloys, the temperature of impregnation, by which I means the temperature at which the air pressure is introduced into the autoclave, should be within the melting point range, about 1,100.degree.F. When the temperature is much over 1,100.degree.F. and towards the top end of the melting point range, there is too much of a reaction between the aluminum and carbon resulting in the formation of too much aluminum carbide and a part of poor physical properties.

While pressures in the order of 3,500 to 4,500 psi are preferred, I am able to impregnate a powder with pressures down as low as 100 psi. When there is no problem of chemical reactions between the metal and carbon powder, I believe that such lower pressures may be satisfactory for production operations. I prefer the higher pressures in the case of aluminum-silicon alloys because I can obtain impregnation in shorter times, more surely in production.

The following example illustrates the invention:

Calcined coconut shell charcoal was placed in a porous carbon crucible. The material had the following particle size range:

6% above 32 micron 50% above 20 micron 95% above 8 micron

It was vibrated to a density of 0.8 grams per cubic centimeter.

Aluminum Association A-390 aluminum-silicon alloy was melted and chlorine gas bubbled through it. Aluminum Association A-390 alloy has the following composition:

16 to 18% silicon 4-5% copper 0.5 max iron 0.45-0.65 magnesium .2% max titanium .1% max manganese .1% max zinc trace phosphorus balance aluminum

The porous carbon crucible was placed in a ceramic crucible and heated to 1,100.degree.F. in a nitrogen atmosphere. The molten A-390 was poured over it and the assembly soaked at 1,100.degree.F. for 30 minutes to equalize the temperature. Then it was placed in a quick-opening autoclave. Then air at 4,200 psi was admitted to the autoclave for 2 minutes. Thereafter, the pressure was released and the autoclave opened up, and the porous crucible removed from the molten metal and quenched to room temperature.

Thereafter, the billet was broken out and cut into rough blocks which were heated at 935.degree.F. for 8 hours and then at 450.degree.F. for 4 hours. The heat-treated blocks were final machined into apex seals.

The material had the following physical characteristics:

apparent density 2.06 gm/cc hardness RG 90 transverse strength 27,000 psi resistance .000033 ohm-inches

Seals made from this material were tested in a Wankel engine. The engine was a 32 cubic inch engine with an 81/2 to 1 compression ratio. The seals lasted for 159 hours with a wear of 0.0042 inches. The seals broke due to a malfunction of another component.

The above-described seal material contained approximately 38 percent carbon by weight. A suitable carbon weight percentage range is 35 to 45 percent. The aluminum-silicon alloy and carbon reaction products comprise the balance. I could not measure the amount of carbide reaction products. The aluminum-silicon alloy contained about 17 percent silicon, the desired range being between 14 and 20 percent silicon, from 0 to 5 percent copper or nickel and less than 1 percent other elements selected from magnesium, titanium and iron.

The carbon powder density was 0.8 grams/cc and the alloy density was 2.7 grams/cc. The density of the seal material was 2.1 grams/cc. A suitable density range is 1.9 to 2.3 grams/cc.

The electrical resistance ranges from 20 to 60 micro-ohm inches. The transverse strength should be at least 25,000 psi. The Rockwell G hardness should be in the range of 80-100.

The foregoing seal material functions as described as a Wankel engine apex seal, a most difficult application.

While the invention has been shown and described in connection with a specific embodiment thereof, this is intended for the purpose of illustration rather than limitation and other variations and modifications of the specific method herein shown and described will be apparent to those skilled in the art all within the intended spirit and scope of the invention. Accordingly, the patent is not to be limited to the specific embodiment of the method of the invention herein shown and described nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced by the invention.

Claims

I claim:

1. A process for impregnating a mass of carbon particles with an alloy of aluminum, silicon and at least one other metal comprising the steps of:

melting a quantity of said alloy,

placing a mass of carbon particles in a porous crucible formed from a refractory material, said particles being smaller than 40 microns and having a porosity range of 30 to 60 percent by volume, the porosity of the porous crucible being at least 20 percent by volume,

placing the porous crucible in an impermeable crucible,

pouring the molten alloy over said porous crucible and into said impermeable crucible,

equalizing the temperature between said molten alloy and said crucibles,

subjecting said crucibles, carbon particles and molten alloy to a gas pressure of between 2,000 psi and 4,500 psi for from one to five minutes to force said molten alloy to penetrate the porous crucible and impregnate the carbon particles,

rapidly cooling the porous crucible to room temperature, and then

removing the thus formed billet of alloy-impregnated carbon particles.

2. A process as defined in claim 1 wherein said metal comprises a hyper-eutectic aluminum-silicon alloy, said particles comprise calcined coconut shell char, and said alloy is at a temperature of about 1,100.degree.F. when the pressure is applied.

3. A process as defined in claim 1 wherein the billet is annealed for from 6 to 10 hours at 900.degree. to 1,000.degree.F. and then further held for from 3 to 5 hours at 400.degree. to 500.degree.F. and then rapidly cooled to room temperature.

4. A process as defined in claim 1 wherein the porous crucible is made of a non-graphitized carbon and the billet is removed from the crucible by breaking away the crucible.