US2740191A - Manufacture of plated powdered metal articles - Google Patents

Description

A ril 3, 1956 .1. F. KUZMICK MANUFACTURE OF PLATED POWDERED METAL ARTICLES Filed Oct. 8. 1951 MANUFACTURE OF PLATED POWDERED METAL ARTICLES Jerome F. Kuzmick, Upper Montclair, N. 5., assignor to Ekstrand & Tholand, Inc., New York, N. Y., a corporation of New York Application October 8, 1951 Serial No. 250,337 7 Claims. (Cl. 29- 420) The present invention relates to the production of plated articles or bodies, hereinafter referred to as compacts, of powder metal, and has particular reference to porous Still to plated compacts of the above character to which the plating metal is applied by deposition from a solution, by electroplating or chemical deposition, or a combination of both.

'In one of its phases the invention has for a primary object the production of improved plated compacts in which the plating is not subject to the usual deterioration characteristic of plated In another of its aspects the invention has as an object the production of plated compacts in which an improved bond between the plating and the main body of material is obtained, which will permit certain desirable working operations to be performed on the compact withoutdamaging the plating or adversely affecting the bond between the plating metal and the metal of the main body. In still another of its aspects the invention has for an object the production of plated compacts theplating of which comprises an alloy of difierent metals, the properties of which are such as to preclude their deposit in alloyed form by deposition from a solution.

Other and more detailed objects of the inventionrand the manner of carrying the invention into effect, together with the advatages to be derived from its use, will best be understood by reference to the ensuing portion of this specification and the accompanying drawings in which there is disclosed by way of illustration but without limitation some characteristic examples of carrying the invention into eiTect. p 4 a To produce a compact by ordinary powder metallurgy methods a mass of metal powder, which may be of one or a mixtureof different metals, is first pressed in a suitable die, at normal or only slightly elevated temperature, with suificient pressure to produce a so-called green piece capable of beinghandled. The green piece is then sintered in a furnace at appropriate temperature, below the melting point of the metal or of the predominant metal in the case of a mixture. Usually this is done in the presence of a reducing or protective atmosphere, such' as hydrogen gas, in the furnace. The resultant sinteredcompaet may then lie-directly used or it ma subseqii'iit'ly be further de'nsified and sized by an additionalwas varied from that show 2 cold pressing or coining operation. Even when final coining operations are employed it is very diificult to produce parts which are very dense and the usual commercial parts produced by the above methods will show a porosity which may run to as high as 40% Many attempts have been made to obtain a satisfactory plated surfa e on such sintered and porous parts in order to provide a corrosion resistant surface, a surface of improved appearance oidifierent color than that of the body of the compact, or for other reasons; Such attempts have, however, been unsuccessful in the past to the extent that the body of the compact and the plating have deteriorated by corrosion, flaking and the like, in relatively short time. 7 p

I have found that the cause of the rapid deterioration of such parts when plated by the usual methods by deposition from a solution is due to the fact that it is not possible to completely wash the solution (the electrolyte in the case of electroplating), out of the pores of the porous material and that the residual salts entrapped in the body in a relatively short time cause internal corrosion of the part, with resultant deterioration of the surface.

In accordance with the present invention this highly undesirable condition is' eliminated and a stable and lasting plated surface obtained by removing the entrapped salts through the use of annealing heat treatment subsequent to the plating operation and operative to remove the salts by volatiliz'atioii, which may or may not be accompanied by decomposition due to heat, depending upon the specific nature of the entrapped residues.

Since the siriter'ing operation necessary to produce a finished compact having the required ultimate mechanical strength is itself a heat treatment capable of driving off the entrapped salts, the invention may advantageously be carried out without additional steps and thus as economically as with the previously used unsuccessful methods, by reversing the order of steps of treatment of the initially formed green compacth In accordance with this procedure, the compact is plated in its green state and is thereafter subjected to the usual sintering operation required to 'produc'e a cohesive finished compact of required mechanical stre'n'g'th.

Fig. 1 shows a inicropho tograph of a cross section of a sintered compact, v

I Fig. 2 reveals a microphotog raph of a similar cross section of a sintered compact wherein the sintering time n in Fig. 1.

As an example of this procedure, but Without limitation, I have produced articles according to one phase of the invention as follows:

1) A metal powder compact was formed by compressing in a suitable die under a pressure of 30 tons per square inch a mixture of 99% iron powder and 1% graphite, powder, giving a green compact having a porosity m ('2) The green-compact was prepared for plating in accordance withconventional methods by pickling in a warm 10% solution of hydrochloric acid and washing in warm water. 7

(3) The prepared compact was then nickel plated by conventional electroplating methods and in accordance with usual standard practice using a cathode current density of approximately 18 amperes per square foot with the following electrolyte:

, Grams/liter NiSO4.7HzO NH4Cl H3303 15 The plating operation was continued until a deposit approximately .001 inch" thick was obtained'on the campact.

present invention may mass and in such cases, a short presintering is (4) The plated piece was then sintered in a convenonal sintering furnace in a hydrogen atmosphere. The intering cycle was 30 minutes to 2000 F., 1 hour at 000 F. and 30 minutes cooling to room temperature.

(5) The sintered compact had a very bright smooth urface and no blistering or separation of the plating ocvurred. Also when a cross section of the compact was :xamined microscopically with a 400 diameter magnifization, the structure found was as shown in the micro- )hotograph of Fig. l. The temperature and duration of :he sintering cycle operated to volatilize and drive off the residual salts from the plating operation and as also will be observed from Fig. l, the sintering operation also caused the plating metal to penetrate ordifiuse into the porous body of the compact to a obtainable merely through plating deposition. This will be evident by reference to Fig. 2 which shows to the same magnification the cross section of a compact prepared in the same manner as above described except that the sintering operation was limited to a -minute period at sintering temperature. Comparison of the two specimens shows clearly the additional diffusion resulting from the full sintering operation.

A fully sintered compact made in accordance with the above described method was subsequently subjected to a coining operation under a pressure of 50 tons per square inch. rt was found that the coolirng operation did not disrupt the bond between the plated metal and the iron body, even though as a result of the operation the compact Was compressed to smaller dimensions and its density was increased. The apparent reason for this is that the nickel plating, having been entirely annealed during the sintering operation and thoroughly bonded by the diffusion which occurred, was capable of taking the displacement caused by the coining without cracking or flaking from the body of the compact.

in order to test the efiicacy of the plating against corrosion three specimens were prepared, one of which was produced in accordance with the previously described method, a second of which, having a body of the same ferrous material, was nickel plated in the conventional manner after sintering and the third of which, having a body of the same ferrous material as the other two, was left unplated. These three specimens were placed in a sealed container above a saturated salt solution. The unplated specimen began to show a uniform coating of rust within 48 hours and was badly corroded throughout within a week. The conventionally plated specimen began to show pockets of corrosion within 24 hours after the test com- I menced. The specimen plated in accordance with the present invention showed no signs of corrosion after one months exposure to the salt atmosphere.

As previously mentioned the specific way in which the be most economically practiced .is by plating a green compact without intermediate operations and thereafter performing the entire required sintering operation. However, in some instances it may be desirable to vary this procedure. For certain kinds of parts made by powder metallurgical methods the green parts may be too fragile for production handling in a plating operation, due to thin sections, high porosity desired in the final product, the nature of the metal forming the body of the compact or other reasons. In such cases it may be desirable for mechanical reasons, to enhance the self-supporting strength of the green compact by subjecting the green compact to a preliminary sintering operation to increase its mechanical strength to an extent permitting the piece to be handled in the subsequent plating operations and final sintering without danger of breakage.

To facilitate compacting and also to reduce die wear,

greater extent than that it is common practice to include a small percentage of v lubricant, usually a metallic soap of a fatty acid such as zinc stearate or equivalent, as a constituent of the powder desirable not only to enhance mechanical strength as discussed above, but also as a means for removing the lubricant prior to plating and final sintering. If a green compact containing appreciable lubricant is plated and thereafter heat treated, the lubricant has two deleterious effects. One is that the presence of the lubricant at the surface of the compact gives to the latter a relatively waxy characteristic which renders/more difficult the initial deposition of a coating of plating metal for final bonding to the body of the compact by the subsequent heat treatment. The other is that if lubricant is present in a plated green compact which is thereafter sintered, the lubricant in effect boils off during the sintering operation in a manner different from the way in which residual salts from the plating operation volatilize and are driven offand in a way which may result in substantial puncturesin or blistering of the plating. ()ne of the factors contributing to this is that the lubricant is distributed throughout the mass of the compact and is quantitatively greater than the amount of residual salts from the plating operation which are substantially confined to the boundary layer of the compact. v Since in any event, in order to achieve the objects of the invention, the piece must be subjected to heat treatment after the plating operation, it is ordinarily de'sirable, if presintering is required prior to plating, to con-- fine such operation to a short preliminary treatment which may advantageously be at lower than usual sintering temperature, in the interest of economy.

By way of example, but without limitation, presintering which will accomplish the desired objects may comprise heating in a neutral or reducing atmosphere for five to ten minutes at a'temperature of 1500 F. or even lower, as contrasted with usual sintering procedures which, for ferrous metal compacts for example, heat at 2000 F. or thereabouts for periods of from twenty minutes to an hour depending upon the specific material and diodesired ultimate physical properties.

However, it is to be noted that insofar as obtaining the desired ultimate results sought by the invention there is nothing to preclude a full sintering operation before plating, followed by annealing heat treatment after plating of whatever temperature and duration may be required to drive off the residual entrapped salts.

In another of its aspects the invention permits the production of a uniformly diffused alloy metal plating on a metal powder compact. As is well known certain metals which are capable of forming desirable alloys have entirely different chemical and electro chemical properties which make it impossible to codeposit them in the form of an alloy coating by deposition from solution. I have found, however, that the desirable result of producing a uniform alloy coating can be obtained by the present invention by plating a green or presintered compact in a series of separate plating operations with the different metals which it is desired to have form an alloy and thereafter sintering the piece. The sintering operation not only drives otfthe residual salts but in addition, due to the temperature and duration of the heat treatment, causes the separately deposited plating metals to alloy with each other as well as to format firm and permanent bond with the main body of the piece..

As an example of this a green iron compact pressed from powder without addition of lubricant formed as previously described, was electroplated with nickel to form a coating .001 inch in thickness. This was followed by a second plating operation in which the nickel coated piece was electroplated with a layer of .0005 inch of copper. After this operation the piece, of course, had a copper colored appearance. The specimen was then sintered in a'hydrogen atmosphere, the samecycle as previously described being employed. The surface color of the resultant sintered piece was no longer copper-y but instead had a silvery appearance. A microscopic study of the cross section of-the piece and chemical-analysis of its coating revealed that the copper and nickel had diffused to form a 'cupro-ni'ckel alloy of approximately the composition of well known Monel metal. It will be evident that the same procedure may be employed to produce other alloy coatings, such for example as nickelchromium, copper-tin and so forth, by separately plating a piece with the different constitutent plating metals and thereafter bonding and alloying them by subsequent heat treatment. It will further be evident that insofar as the nature of the final coating is concerned, that is, whether it is to be pure metal or an alloy, it is immaterial whether the plating operation of operations are carried out on a completely green compact or one that has previously been subjected to a sintering operation. The criterion in all cases is that of heat treatment subsequent to the platmg.

While in substantially all cases I have found the process above described satisfactory from a commercial standas sodium and potassium cyanide, the residual salt is relatively difiicult to remove at a commercially satisfactory rate of speed by heating alone. In order to obviate the disadvantage of prolonged heating beyond the time required for a sintering operation, this ditficulty may be avoided by using other appropriate salts for the electrolytic bath, but if such substitution is not desirable for economic or other reasons, the difiiculty is readily avoided by subjecting the plated article to the'action of an appropriate acid solution operative to convert the cyanide salt to a salt more readily volatilized by heat alone. By way of example, a plated article containing residual sodium cyanide as a result of the plating process mayadvanand in certain instances'other procedures may be preferred. As is'well known, a superior quality of nickel plating is often obtainable by first plating with copper, to provide a thin acid, the latter being materially in quantity required. During the immersion period the solution was agitated and kept at a temperature of approximately 90" C. After removal from the bath the compact was Washed in water and thereafter plated with nickel by electroplating procedure of the same nature as that noted in connection with the example previously given herein. Following the electroplating the compact was washed and then subjected to annealing treatment copper and nickel resulted, but sentially a nickel plating because of the relatively very coating of copper deposited from the bath.

insofar as the present inthin It will be understood that the nature producing essencarbide bits and the like,

While in accordance with usual and good practice, compacts are desirably thoroughly washed after plating and before final heat treatment, such washing is not essential to the practice of the present invention. However,

ing' thoroughly dry out. the article is allowed after promptly place them in the heat treating furnace for final treatment.

Also while for purposes of explaining the nature of the invention only examples of compacts of ferrous metal nature have been given, such examples are not to be considered as limiting with respect to the composition of the body to be plated since the invention is applicable to porous bodies of any metal or alloy capable of being fabricated by usual pressing and sintering methods and which can be plated.

It is important to note that the applicability of the invention Is to the plating of porous bodies,

the principles of the present invention, even if such becluded salts were present. In contrast, the nature of the plating deposited from solution on a porous Body is relatively porous, and while as a result of the final heat treatment the porosity of the plating may be materially reduced, this does not occur at a sutiiciently early stage of the invention.

While in some instances, very hard refractory metallic articles such as tool bits of cemented carbides, produced from powder by operations resulting in a product which lem of occluded plating salts and the nature of the initial plating is such as to prevent the subsequent action which is basically characteristic of this invention.

It is further to be pointed out that the term powder as herein employed is intended to cover all particle sizes ithin the range capable of acts by the pressing and tie of powder metallurgy.

From the foregoing it will be apparent that the invenon may be carried into effect by numerous different spe ific procedures applied to a wide variety of materials ombined in many different ways and that through the ise of the invention new and improved forms of compact rot heretofore obtainable may be produced. The invenion is accordingly to be understood as embracing all meth- M18 and products falling within the scope of the appended :laims.

This application is a continuation-impart with respect to and replaces my copcnding application Serial No. 727,771 filed February ll, 1947, now abandoned, and relates back thereto as to all common subject matter.

What is claimed is:

1. The method of making a plated powder metal compact which comprises forming a self-supporting porous powder body having less than the ultimately required mechanical strength, plating the surface of the body with metallic material of different chemical composition than that of the powder by deposition from a solution in which the body is immersed in the absence of material pres sure and from which in the pores of the body, whereby to confine the deposited material substantially to the surface layer of the body and minimize the depth of penetration of the plating solution, and thereafter subjecting the plated body to heat treatment at a sintering temperature to effect the sintering necessary to produce the required ultimate mechanical strength and to volatilize and remove from said body said residual salts while limiting the time and tem perature of the heat treatment to retain porosity in the body of the compact and the plated surface thereof.

2. The method as set forth in claim 1 in which said plating solution comprises a salt resistant to volatilization at normal sintering temperature and in which the plated compact is treated with an acid solution to convert said salt to another and more readily volatilized salt prior to said heat treatment.

3. The method of making a plated powder metal compact which comprises pressing and sintering a body of malleable metallic powder to form a porous compact, plating the surface of the'cornpact with malleable metallic material of different chemical composition than that of the powder by deposition from a bath in which the compact is immersed in the absence of material pressure and from which bath residual salts are deposited in the being formed into metal comsintering operations characterpores of the compact, whereby to form a porous plating of the deposited material confined substantially to the surface layer of the compact and to minimize the depth of penetration of the plating solution, and thereafter subjecting the plated compact to heat treatment for a period of time and at a temperature sufiiciently high to volatilize and remove from the compact said residual salts.

solution residual salts are deposited 4. The method as set forth in claim 3 in which the plated compact is maintained in a wet state until subjected to the subsequent heat treatment for the removal of the residual salts.

5. The method of making a plated metal compact which comprises pressing and sintering a body of malleable metallic powder to forma porous compact, plating the surface of the compact with malleable metallic material of different chemical composition than that of the powder by deposition from a bath comprising a salt resistant to volatilization at hightemperature and in which the compact is immersed in the absence of material pressure and from which both residual salts are deposited in the pores of the compact, whereby to confine the deposited material substantially to the surface layer of the compact and minimize the depth of the penetration of the plating solution, treating the plated compact with an acid solution to convert the residual .salt to other and more readily volatilized salt, and thereafter subjecting the plated compact to heat treatment for a period of time at a temperature sufficiently high to volatilize and remove from the compact said other salt.

6. The method of making a plated powder metal compact which comprises pressing and sintering a body of malleable metallic powder to form a porous compact, plating the surface of the compact with a first metal of difierent chemical composition than that of the powder by chemical deposition from a first bath, thereafter further plating the compact with a second malleable metallic material of different composition than that of the powder by electrolytic deposition from a second bath, said compact being immersed in said baths in the absence of material pressure to confine the deposited material substantially to the surface layer of the compact and to minimize the depth of penetration of the plating solutions, residual salts being deposited in the pores of the compact from said solutions, and thereafter subjecting the plated compact to heat treatment for a period of time and at a temperature sufficiently high to volatilize and remove from the compact said residual salts.

7. The method as defined in claim 6 in which the compact is formed of ferrous metal and is first plated by chemical deposition with copper and thereafter electroplated with nickel.

References Cited in the file of this patent UNITED STATES PATENTS 1,268,987 McMullen June 11, 1918 2,289,897 Balke July 14, 1942 2,370,242 Hensel Feb. 27,1945 2,409,295 Marvin Oct. 15, 1946 2,456,779 Goetzel Dec. 21, 1948 2,463,039 Kellogg Mar. 1, 1949 2,491,839 Tinker Dec. 20, 1949 2,520,373 Price Aug. 29, 1950 6. The process as defined by claim 5 in which said solvent contains in the range of about 25 to 50% by volume, based on said methyLethyl ketone, of said aromatic hydrocarbon.

References Cited in the file of this patent UNITED STATES PATENTS 2,045,742 Winning et a1 June 30, 1936

Claims (1)

1. THE METHOD OF MAKING A PLATED POWDER METAL COMPACT WHICH COMPRISES FORMING A SELF-SUPPORTING POROUS POWDER BODY HAVING LESS THAN THE ULTIMATELY REQUIRED MECHANICAL STRENGTH, PLATING THE SURFACE OF THE BODY WITH METALLIC MATERIAL OF DIFFERENT CHEMICAL COMPOSITION THAN THAT OF THE POWDER BY DEPOSITION FROM A SOLUTION IN WHICH THE BODY IS IMMERSED IN THE ABSENCE OF MATERIAL PRESSURE AND FROM WHICH SOLUTION RESIDUAL SALTS ARE DEPOSITED IN THE PORES OF THE BODY, WHEREBY TO CONFINE THE DEPOSITED MATERIAL SUBSTANTIALLY TO THE SURFACE LAYER OF THE BODY AND MINIMIZE THE DEPTH OF PENETRATION OF THE PLATING SOLUTION, AND THEREAFTER SUBJECTING THE PLATED BODY TO HEAT TREATMENT AT A SINTERING TEMPERATURE TO EFFECT THE SINTERING NECESSARY TO PRODUCE THE REQUIRE ULTIMATE MECHANICAL STRENGTH AND TO VOLATILIZE AND REMOVE FROM SAID BODY SAID RESIDUAL SALTS WHILE LIMITING THE TIME AND TEMPERATURE OF THE HEAT TREATMENT TO RETAIN POROSITY IN THE BODY OF THE COMPACT AND THE PLATED SURFACE THEREOF.

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