US3573071A - Cemented metal powder and fibers and method of making a composite article therefrom - Google Patents

Abstract

THE DISCLOSURE PERTAINS TO A NEW COMPOSITION OF MATERIALS IN THE FORM A POROUS CEMENTED COMPOSITE AND A METHOD OF MAKING THE SAME. METAL POWDER SUCH AS COPPER IS MIXED WITH A CEMENT BINDER SUCH AS PORTLAND CEMENT. WHEN PREPARED AS A SLIP, POURED AND CURED, THE CEMENTED METAL POWDER HAS SUFFICIENT GREEN STRENGTH TO HOLD TIS SHAPE MAKING IT SUITABLE FOR CASTING PURPOSES. BY TREATING THE GREEN COMPOSITE WITH A HEADENING AGENT, SUCH AS AMMONIUM HYDROXIDE, IT IS HARDENED AND STRENGTHENED SO AS TO WITHSTAND GRIDING, POLISHING, MACHINING OR OTHER OPERATIONS NORMALLY ASSOCIATED ONLY WITH WORKING ON SOLID METALS. A METAL FIBER MAY BE MIXED WITH OR SUBSTITUTED FOR THE METAL POWDER WITH GOOD RESULTS.

Description

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COGH/LL ATTOENEYS March 30, 1971 HQRTQN ET AL 3,573,071

CEMENTED METAL POWDER AND FIBERS AND METHOD OF MAKING A COMPOSITE ARTICLE THEREFROM Filed Oct. 18, 1968 2 Sheets-Sheet 1 [Ni f ROBERT A. O TIMOTHY 1..

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United States Patent CEMENTED METAL POWDER AND FIBERS AND METHOD OF MAKING A COMPOSITE ARTICLE THEREFROM Robert A. Horton, Chesterland, and Timothy L. Coghill, Mentor, Ohio, assignors to Precision Metalsmiths, Inc.

Filed Oct. 18, 1968, Ser. No. 768,868 Int. Cl. B28b 7/34 US. Cl. 10638.3 19 Claims ABSTRACT OF THE DISCLOSURE The disclosure pertains to a new composition of materials in the form a porous cemented composite and a method of making the same. Metal powder such as copper is mixed with a cement binder such as Portland cement. When prepared as a slip, poured and cured, the cemented metal powder has sufficient green strength to hold its shape making it suitable for casting purposes. By treating the green composite with a hardening agent, such as ammonium hydroxide, it is hardened and strengthened so as to withstand grinding, polishing, machining or other operations normally associated only with working on solid metals. A metal fiber may be mixed with or substituted for the metal powder with good results.

PRIOR ART Composites of non-metals and metals are common in pyrometallurgical applications such as sintering where a metal and non-metal are compacted and heated with a binder to form a bond.

Wet processes for making composites are generally limited to mixtures of two or more non-metals which cure or air dry to a maximum strength and hardness. Non-metallic composites usually do not have a metallic luster or any of the other properties associated with sintered metallic composites, including strength and hardness.

SUMMARY OF THE INVENTION This invention relates generally to metallic, cemented composites, and more specifically to the formation of metal powder composites comprised of a cementitious matrix having a metal powder cemented therein.

In accordance with the preferred embodiments, the present invention provides a wet process and a new composite made thereby of a metal powder in a porous cementitious matrix which is hardened by treating it with an ammonium compound. After drying the cemented composite has a metallic appearance and is hardened to the point that it can be worked like a solid metal.

The matrix may be any one or a combination of well known inorganic cements, binders, sols or gels such as Portland cement, or a gypsum product such as plaster of Paris, or a gelatinous silica such as formed by some of the common alkali metal silicates.

The metal powder which has proved most satisfactory is copper. Other metal powders may be used such as nickel, cobalt, or zinc. Generally, those transitional or heavy metals forming complex ions with ammonia are believed to be suitable. Beside pure metals, alloy powders may also be used such as bronze powder.

Further the invention contemplates a cemented metal composite in which metallic fibers or filaments are mixed with the metal powder constituent. The metallic fibers react with the ammonium solution just as the powder to harden and strengthen the composite. In some instances, it may be desirable to use metal fibers alone instead of powder as the metallic constituent of the composite.

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The invention contemplates a method of making a cemented metal composite which comprises the steps of mixing metal particles with a quantity of inorganic binder and a liquid to form a slip or grog, drying at least partially to set the binder forming a green composite and treating the green composite with a solution of an ammonium compound such as ammonium hydroxide or any one of several ammonium salts such as ammonium chloride, nitrate, sulfate, carbonate or others. The hardening treatment may be by painting, dipping, vacuum impregnation or other techniques of penetrating the ammonium solution into the porous binder so as to come into contact with the metal particles.

Among the advantages of the invention are: (1) provision of a composite material which will readily duplicate all the surface characteristics of a mold or pattern; (2) by adjusting the cement binder ingredients and curing times the amount of shrinking may be controlled; (3) a useful intermediate material results which has a strength in the partially or fully cured states enabling it to be handled or worked, e.g., as a sculpture medium; (4) molds made from the composite material are lighter than solid metal molds and have better heat conductivity than plaster, plastic or metal-filled plastic molds; and (5) after hardening, the composite can be drilled, machined, ground, polished, or otherwise worked upon as a solid metal would be and has a metallic lustre and appearance.

The cemented composites made in accordance with the invention have a variety of uses, but will be described herein as a material for making dies used for forming wax patterns in an investment casting process. The dies may be prepared by pouring the composite over a positive model or pattern to produce the desired cavity. The model may be made of Wax, plastic, rubber, or almost any other material capacle of being easily shaped inasmuch as the method of making the dies is a wet process requiring no external heating. After curing, the die is separated from the model which may be reused in making any number of dies.

Those familiar with investment casting will appreciate that molten wax is injected in these dies to form the wax patterns which are removed and invested with a suitable investment composition which sets up around the patterns to form a refractory mold. The wax patterns are subsequently burned out leaving a cavity in the refractory mold for receiving the molten metal out of which the part to be made is cast. Investment casting is ideally suited for smaller sized metal parts of intricate or complicated shapes and in which the requirements for surface smoothness and dimensional tolerances are high. For example, investment casting is widely used in the manufacture of turbine blades, as will be described herein as an illustrative application of the invention.

The uses of the new material are varied, however, and include such things as the repair of castings and other metal parts, the use as a back-up material for metal shells such as spray metal molds or electroformed molds, electrically conductive mortars, and heat transfer cements or heat sinks. The cemented composite is also highly suitable as a sculpture medium capable of being shaped with sculpturing tools while in a green or cured state and thereafter hardened in its permanent form.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial view of one stage in the process of making a die for use in the investment casting of a turbine blade in which the die is formed from the cemented metal composite in accordance with the invention;

FIG. 2 depicts a vacuum chamber in which the die is placed after curing and in which an ammonium solution 3 is impregnated in the die hardening it for use; FIG. 2a shows the die after hardening;

FIG. 3 is a fiow sheet showing the preferred method used in preparing and hardening the novel composite material;

FIG. 4 is a greatly exaggerated view of the composite material showing the grains of metal powder held in a cured, porous cementitious matrix ready for the hardening steps;

FIG. 5 depicts the composite material in FIG. 4 after it has been impregnated with the ammonium hardening agent in accordance with the invention;

FIG. 6 depicts, in exaggerated proportion, a composite material which includes metal powder as well as metal rfibers in a cementitious matrix hardened by an emmonium compound; and,

FIG. 7 represents the structure showing a modification in which a metal powder is compacted without using a cementitious matrix, but in which the physical connection between the individual grains of metal powder has been strengthened chemically by the penetration of an ammonium hardening agent between the grains.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the casting of one-half of a wax injection die 10 for a turbine blade is shown. A positive master pattern 12, of the turbine blade is previously made up from some easily shaped material such as the composition which is the subject matter of US. Pat. No. 3,296,006, issued Jan. 3, 1967, entitled Pattern Material Composition. Of course other materials such as rubber, plastic, Wax, wood or any easily shaped material capable of holding a given form may be used. The master pattern is like the finished blade in surface and shape, except that the dimensions will be such as to take into account the shrinkage and expansion encountered in molding the wax patterns and casting with the particular metal from which the blade is to be made. A mold 14 is used to form the die 10. The mold 14 has a parting line 18 at the dihedral edges of the blade pattern 12 which is supported on a modeling clay 20 leaving the upper half of the pattern exposed. The cemented powder metal slip used for making the die 10 is poured into the mold 14 covering the exposed half of the pattern 12. It should be understood that only one-half of the die 10 is shown being made in FIG. 1 and that the other mating half will be made in the similar fashion.

After air drying for approximately 12 hours, the die 10 is removed from the mold 14. During this time the composite slip has cured, at least partially, to permit handling. When quite dry in appearance, the composite die 10 is porous enough to be impregnated with the hardening agent. When dry, it is placed in a vacuum chamber 26 containing the hardening agent 27 which, in accordance with the invention, is an ammonium solution of one of the ammonium bases or salts mentioned hereinafter. The chamber 26 is evacuated causing the ammonium solution 27 to penetrate throughout the porous cement matrix of the die 10. The immersion time will depend to some extent on the ability of the matrix to resist being dissolved While immersed in the hardening agent. Usually from one to five minutes immersion time is satisfactory. If the matrix is particularly soluble or subject to chemical attack by ammonia, the hardening agent can be poured over, painted, or sprayed on the die 10, but the increase in hardness will not be as uniform throughout as when vacuum conditions are used to obtain deep penetration of the hardening agent.

After the die 10 has been impregnated it is removed from the vacuum chamber 26 and air dried for several hours to harden. FIG. 2a shows the die 10 after drying and hardening being dressed with a grinding wheel 28 in the manner of a solid metal casting. Prior to impregnation, the cemented metal composite can be scratched with a fingernail. FIG. 2a is illustrative of the marked increase in hardness which occurs when the invention is practiced.

DESCRIPTION OF THE PREFERRED METHOD The preferred method of the invention involves the preparation of a slurry or slip composed of a binder or cement, a metal powder and a carrier liquid or solvent (see flow sheet in FIG. 3). According to Example I below, the slip suitable for making the die 10 for use in making wax patterns in investment casting may compris a mixture of 230 grams of copper powder with 42 ml. of a cement consisting essentially of sodium silicate and aqueous colloidal silica. This mixture is vacuumed to remove air bubbles and then poured under vibration over the master pattern 12.

The following formulation is exemplary.

Example 1 One volume of sodium silicate solution is diluted with 3 volumes of water. One volume of this solution is added slowly to 2 volumes of Ludox AM with rapid stirring. Ludox AM is an alumina-modified aqueous colloidal silica dispersion, manufactured by E. I. du Pont Company. Forty-two ml. of this solution is then mixed with 230 grams of copper powder (purified grade, manufactured by Matheson, Coleman & Bell Division of the Matheson Company). The slip is vacuumed to remove air bubbles and then poured under vibration into the mold 1 4. Any excess liquid rising to the top is drained off. After curing for approximately 24 hours, the die 10 is impregnated under vacuum in the chamber 26 with a solution of 20% by Wgt. dibasic ammonium phosphate in water and allowed to dry.

Example II To test the decorative appearance of articles, the same slip as prepared in Example I was poured into a rubber mold to make reproductions of a small sculptured plaque. After setting and subsequent drying, the plaques were impregnated with a hardening agent according to the following formulation: a solution of 500 ml. of water, 500 ml. of ammonium hydroxide solution and 120 grams ammonium carbonate. After drying the impregnated plaques were polished to bring out a distinctive metalliccopper appearance.

Example III In another test a negative silicone rubber master mold of a part having an irregular shaped base, approximately inch thick and 1 /2 inches in length, with a serrated boss flanked by two triangular protuberances approximately Ma inch thick and having a peak height of approximately fit inch was prepared from the following formulation.

A gelling agent was prepared by diluting one volume of sodium silicate solution with six volumes of water. One volume of this solution was added slowly with rapid stirring to 700' volumes of an acid hydrolyzed ethyl silicate 40 solution of the following formulation:

Vol.

(1) Eethyl silicate 40 21 (2) Synasol solvent 17 (3) 1% H01 by wgt. in H O 2 The Synasol solvent is proprietary denatured ethanol, obtained from Union Carbide Chemicals Company.

Fifteen ml. of this was then mixed with grams of copper powder. The slip was vacuumed to remove air bubbles and then poured with vibration, avoiding air entrapment, into a negative silicone mold. Some liquid came to the surface during and after vibration which was poured off. After 1% hours the part was lifted and allowed to air dry. After drying for a considerable length of time the part was impregnated with a solution comprising 21 ml. of water, 21 ml. of ammonium hydroxide solution, and 7 grams ammonium carbonate. Following additional drying the part was polished to bring out the metallic copper appearance.

Example W In another test of a different formulation, a slip was poured into a negative silicon rubber master model to produce a rectangular shaped part approximately 2% inches long inch wide and /2 inch deep. The following formulation was used:

A mix Was prepared by adding 5 grams of RV. Stone to 45 grams of copper powder. The RV. Stone is a proprietary name given to dental plaster based on alpha gypsum and manufactured by Pre-Vest, Inc. of Cleveland, Ohio. To this dry mixture was added 12 ml. of water and the resulting slip was vacuumed to remove entrapped air. With vibration the slip was poured into the negative master model and after drying approximately 20 minutes the part was lifted and allowed to air dry. After air drying the part was impregnated under vacuum with a solution comprising 7 grams of ammonium chloride dissolved in 50 ml. of water. The ammonium chloride is reagent A.C.A., granular, obtained from Matheson, Coleman & Bell. After additional drying the part was polished with a fine abrasive paper to a metallic copper appearance.

' Example V In the preparation of a wax injection die similar to Example I, the hardening agent Was mixed directly with the metal powder and cement binder. The following formulation was used:

Fifteen grams of RV. Stone were mixed with 200 grams of copper powder and this dry mix was then used in the preparation of a slip by adding a solution containing 22% ammonium chloride by weight in water. The slip was vacuumed to remove air bubbles and poured with vibration onto a positive silicone rubber master model to produce the wax injection mold. The mold was approximately 2% inches in length, had a width of approximately 1 /2 inches and a depth of inch. The excess liquid which came to the surface during and after vibration was decanted. After eighteen hours the mold was lifted from the master model and polished subsequent to additional drying to bring out the metallic copper appearance;

Example VI A series of strength tests were run on the formulation of Example I to illustrate the increase in strength that occurs as a result of impregnation. Test bars having a 3 cross-section were broken on a 2.32" span using center loading. The bars were made according to the following steps.

A slip according to the formulation in Example I is poured into a silicone rubber mold to produce the test bar sizes indicated and allowed to set over night before lifting. After lifting the test specimens were air dried 24 hours, after which some of them were impregnated according to Example I. After impregnation those samples were additionally dried for 24 hours before testing.

The unimpregnated bars had an average modulus of rupture of approximately 28 p.s.i. The bars impregnated with a 20% by weight dibasic ammonium phosphate solution according to Example I had an average modulus of rupture of 1,065 p.s.i., an increase of 730%.

Example VII In another test, specimens impregnated with the ammonium hydroxide and ammonium carbonate solution of Example 11 for two minutes under vacuum followed by five minutes without vacuum had an average modulus of rupture of 1,850 p.s.i. or an increase of 1,340%.

In a modification of the invention, a composite is prepared using vmetal powder and metal fibers which react with the ammonium compounds to produce the increase in hardness and strength desired. According to Example VIII below, copper powder and copper fibers 6 are mixed with a cement binder although it is understood that any metal powder and fiber of the class referred to as heavy metals, such as cobalt, nickel, or zinc, may be used.

Example VIII Strength tests were run on a formulation comprising a mixture of copper powder and copper fibers, the binder of Example I, and a hardening solution of ammonium hydroxide and ammonium carbonate.

The copper powder was blended with 16% copper fibers (based on the weight of the powder). Individual copper fibers measured 0.0015 in. x 0.002 in. x 0 .062 in. as obtained fro-m Fiberfil Division, Rexall Chemical Co.

After molding, setting, lifting, and drying, the test bars were impregnated with a hardening agent according to the following formulation: a solution of 420 ml. of water, 420 ml. of ammonium hydroxide solution and 140 grams ammonium carbonate.

. After drying the bars had an average modulus of rupture of 2,860 p.s.i.

The following is an example of a cemented metal composite prepared from a metal powder other than copper and in which the hardening agent is added directly to the mixture of binder and metal powder thereby skipping the intermediate setting and impregnation steps according to the preferred method represented by the flow sheet in FIG. 3.

Example IX Forty-five grams of cobalt powder obtained from Matheson, Coleman & Bell was mixed with 5 grams of RV. Stone. Fourteen ml. of a 22% by weight aqueous ammonium chloride solution was added to the mixture. After vacuuming to remove air entrapped therein, the mix was poured with vibration into a circular container to produce a sample patty. After drying, the patty was removed from the container and after subsequent additional drying, was polished to reveal a shiny, metallic cobalt appearance.

Example X In a test using zinc metal dust according to the process steps in which impregnation is used as in the preferred method, the following formulation applies:

Forty-five grams of zinc metal dust obtained from Mallinckrodt Chemical Works was mixed with 5 grams of RV. Stone. This dry mixture was used in the preparation of a slip by the addition of twelve ml. of water. The slip was vacuumed to remove air and bubbles. Vibration was employed in pouring the mix into a cylindrical container to produce a sample patty. After drying over night, the specimen was lifted from the container and allowed to further air dry. The patty was impregnated under vacuum with a solution of 14 grams ammonium chloride in 50 ml. of water. After subsequent air drying the patty was polished to a metallic Zinc appearance.

While metal powder composites have been prepared from copper, zinc and cobalt powders, nickel can also be used. In addition to the relatively pure metals, alloy powders may also be used, for example, irregular bronze /10 and CLT 88/ 8/4 copper-lead-tin alloy, manufactured by American Metal Climax, Inc., are two wellknown pre-alloyed metal powders which may be used. Other metals or alloys of metals may be found to work with varying degrees of success depending upon the type of reaction bond created in the presence of the hardening agent. While it is not known what produces the increase in hardness in the presence of ammonia, one theory is that it is a result of the formation of complex coordinate covalent bonds of the metal with ammonia. The metals which are frequently referred to as the transitional metals in the Periodic Table, such as cobalt, nickel, copper and zinc are examples. The lighter metals such as aluminum and those ambivalent metals exhibiting non-metallic properties are not generally successful.

Suitable hardening agents in addition to those given in the formulations in the preceding examples include various solutions of ammonium salts such as the fluoride, bifiuoride, silicofluoride, sulfate, bisulfate, sulfite, persulfate, thiosulfate, sulfamate, carbonate, bicarbonate, stearate, bromide, chloride, oxalate, tartrate, formate, acetate, chromate, dichromate, mono-ammonium phosphate, di-ammonium phosphate, dibasic ammonium citrate and possibly others.

Somewhat less effective but still useful as a hardening agent are such double salts as sodium ammonium phosphate, ferric ammonium sulfate and aluminum ammonium sulfate, and such other ammonia derivatives as urea, tetramethyl ammonium hydroxide, tetrakis (Z-hydroxyethyl)-ammonium hydroxide, organic ammonium silicates and numerous other ammonium compounds or compound derivatives. Moreover the various impregnating or hard ening agents can be used either alone or in combination with one another. The agent will normally be dissolved in water, but other solvents such as Synasol proprietary alcohol can also be used. This may be desirable in the event that the cementitious matrix has poor resistance to water or the impregnating agent is insoluble in water.

Various binder materials may be used and from the present knowledge it appears that any of the common inorganic bonding agents used in various cements and plasters that harden through reaction with water are satisfactory. Also colloidal dispersions that can be gelled by one means or another, and other binders that are formed in situ by reaction between suitable ingredients contained in the mix are most satisfactory. Exemplary binders which have been found suitable are Portland cement, aluminous cements, plaster of Paris, colloidal silica, sodium silicate, magnesium phosphate and other metallic phosphates and hydrolyzed organic silicates that deposit gelatinous silica. Even binders that are attacked and softened by the hardening agent may be used if care is taken to limit the immersion period by removing the body from the solution before it becomes too weak to handle and before any loss of detail occurs.

Referring to FIG. 4, an exaggerated schematic of the cemented metal powder structure is shown depicting the cement matrix 30 after it has set up around the metal powder grains 32, but prior to hardening. In this condition the composite is relatively soft and can be scratched with the fingernail. FIG. shows the same structure after impregnation with an ammonium compound producing a chemical reaction at the interface of the metal powder grains 32. The increase in strength and hardness is believed to be due to the formation of a stable metallic ammonia complex 34 which develops around each metal grain and at the interface forms a chemical bond. In this condition, the composite is hard enough to be Worked as though it were solid metal. Polishing bring out a metallic-like lustre characteristic of the particular metal powder.

In the modification of the invention discussed in connection with Example VIII, a metal powder and fiber composite is formed as depicted in exaggerated fashion in FIG. 6. Metal powder grains 32a are mixed with metal fibers 36 and bonded in a cement matrix 30a. After setting and lifting, the composite is impregnated with an ammonium solution and dried. An ammonia complex 34a is formed which is coextensive with the metal powder 32a and fibers 36. Any suitable fiber could be used to reinforce the matrix, but the results are particularly good when the fibers enter into the ammonia hardening reaction. Thus fibers comprised of one of the metals mentioned that is reactive with ammonium compounds are preferred. The ammonia complex 34a is a more extensive formation throughout the composite as represented by FIG. 6 than occurs when the reaction is between discrete grains of metal powder as depicted in FIG. 5.

FIG. 7 is another modification of the invention in which no binder or cement matrix is used, at least in the sense used in the preparation of the composite of FIGS.

8 4, 5 and 6. In this modification, the powder grains 32b are compacted or otherwise held in situ while an ammonium solution is impregnated therein producing the ammonia complex 34b at the interface of the grains which produces the hardening or strengthening.

Modifications and changes may be made to the invention as will be apparent to those skilled in the art to which it pertains which modifications and changes are to be regarded as reasonable equivalents thereof and are intended to be covered by the appended claims except insofar as limited by the prior art.

What is claimed is:

1. The method of making a cemented metal composite article comprising the steps of (a) preparing a slip by mixing a metal particle constituent and a binder constituent in a mixture in which at least a major portion of the metal particle constituent is selected from the group consisting of copper, cobalt, nickel and zinc and the metal particle constituent predominates over said binder constituent, the latter forming in situ to deposit a porous relatively soft matrix and at least a sutficient amount being used to cause the individual metal particles to ahere (b) curing the binder in air for a time sufficient to develop a green strength to permit handling of the composite article (c) hardening the composite article by treating it with an ammoniacal solution which reaches the metal particles through the pores of the matrix rigidifying and strengthening the composite article.

2. The method according to claim 1 wherein the ammoniacal solution is capable of penetrating the porous binder matrix and the composite article is immersed in the liquid under vacuum conditions to insure uniform penetration thereof throughout.

3. The method according to claim 1 wherein the ammoniacal solution is applied to the composite article by painting or brushing it on the surface.

4. The method according to claim 1 wherein the ammoniacal solution is an aqueous solution of an ammonium salt or base.

5. The method according to claim 1 wherein the binder is selected from a group consisting of colloidal silica, hydrolyzed ethyl silicate, aluminous cements and alkali or alkaline earth metal phosphates, sulfates or silicates.

6. The method according to claim 1 wherein the metal particles are in powder form and contain a predominate amount of a metal selected from the group consisting of copper, cobalt, nickel and zinc.

7. The method according to claim 1 where the metal particles are in fibrous form and contain a predominate amount of a metal selected from the group consisting of copper, nickel, cobalt and zinc.

8. The method according to claim 1 wherein the metal particles are a mixture of powder and fibers and contain a predominate amount of a metal selected from the group consisting of copper, nickel, cobalt and zinc.

9. The method of making a cemented metal article comprising the steps of mixing a metal particle mass with a minimum quantity of binder to form a mixture in which the metal particle mass will become agglomerated with the addition of water, said particle mass being substantially entirely composed of a metal selected from the group consisting of copper, cobalt, nickel and zinc and hardening the metal particle mass by reacting it with an ammoniacal solution containing in excess of about 14 percent of an ammonium compound to form a stable complex reaction bond between at least a majority of said metal particles.

10. The method according to claim 9 wherein the binder is selected from a group consisting of colloidal silica, hydrolyzed ethyl silicate, aluminous cement, and alkali or alkaline earth metal phosphates, sulfates, or silicates prepared as an aqueous Slip and drying in situ to deposit a porous, relatively soft matrix in which the metal particles are relatively uniformly distributed.

11. The method according to claim 9 wherein the binder is a silica or silicate gelation and the metal particle mass is formed substantially entirely of copper powder.

12. The method according to claim 9 comprising the steps of curing the binder for approximately 24 hours and impregnating the metal particle mass under vacuum conditions with an ammoniacal solution containing an ammonium compound selected from the group consisting of dibasic ammonium phosphate, ammonium hydroxide, ammonium carbonate and ammonium chloride.

13. An article prepared according to the method of claim 12 which after impregnation with the ammoniacal liquid and drying for approximately 24 hours has an average modulus of rupture as high as 1,000 psi 14. The method of bonding a metal particle mass comprising the steps of retaining the particle mass in a confined space so that individual particles are in relatively close packed relationship,

said particles being composed predominately of a metal selected from the group consisting of copper, cobalt, nickel and zinc, and

penetrating the particle mass with a solution containing an ammonium compound in concentrations sufficient to form a stable complex reaction bond between at least a majority of said metal particles.

15. The method according to claim 14 wherein the metal is selected from the group consisting of copper, cobalt, nickel and zinc and the ammonium compound is in concentrations of at least about 14% by weight or volume.

16. An article prepared according to the method of claim 10 in which the minimum proportion of metal to binder is in the order of about 9 grams of metal for each 1 gram of binder.

17. An article according to claim 16 in which the complex reaction bond is formed by reaction with ammonium chloride in minimum concentrations of in the order of 7 grams of ammonium chloride dissolved in milliliters of water.

18. An article prepared according to the method of claim 1 in which the minimum proportion of metal to binder in the slip is in the order of 5 grams of metal per unit volume of binder.

19. An article according to claim 18 which hardens in the presence of an aqueous solution of an ammonium compound selected from the group consisting of dibasic ammonium phosphate, ammonium carbonate and ammonium hydroxide present in a minimum concentration of in the order of 20% by weight.

References Cited UNITED STATES PATENTS 2,251,610 8/1941 Grande 106-38.3

2,380,945 8/1945 Collins 10638.3UX

2,887,392 5/1959 Lolley 106-383 3,309,212 3/1967 Lubalin 10638.3X

3,314,806 4/1967 Emblem 106-38.35X

3,340,024 9/1967 Mahar 22192 LORENZO B. HAYES, Primary Examiner US. Cl. X.R.

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