US2620555A - Contact alloys - Google Patents

Description

Patented Dec. 9, 1952 CONTACT ALLOYS No Drawing. Application May 5, 1945, Serial No. 592,084

.1 Claims. (Cl. 229-1823) This invention relates to alloys and to electrical contacts produced from such alloys.

The principal purpose of this invention is to provide an improved composition or alloy which is particularly well suited for the production of electrical contacts.

A further object of this invention is to provide a method of making the improved composition or alloy whereby the electrical contact characteristics of the composition will be substantially the same as the electrical contact characteristics of contacts prepared from the metal constituting the predominate proportion of the composition.

Further object and advantages will become apparent from the following description and claims.

Tungsten has long been considered a desirable material for the production of electrical contacts because of certain of its inherent, physical, chemical and electrical characteristics. Tungsten metal has been worked and treated by various methods and means intended to cause certain of the desired properties to become more or less pronounced or to change the nature of the metal to secure optimum performances under specific conditions. Tungsten contacts are commonly made by sintering pressed tungsten powder at relatively high temperatures of the order of 3000 0., working the ingot as by swaging or rolling and sawing contact discs from tungsten rods or punching discs from rolled tungsten sheet or ribbon. Producing these high sintering temperatures involves considerable expense, and the sawing and punching to form the contact discs results in an appreciable scrap loss thereby increasing the production and material costs.

Considerable effort has been expended to eliminate the steps of forming a relatively large body of tungsten, working the ingot and forming the contact disc by punching or sawing. What would appear to be the most direct method of producing contact discs of substantially pure tungsten is to form the discs by pressing tungsten powder into the desired form and sintering the pressed pellet. Such method, however, is entirely"unsatisfactory because of the high sintering temperatures required to obtain the grain growth or crystallization which is necessary for proper electrical contact performance. One of the production difiiculties which prevents the use of such method is the lack of suitable refractory materials in the type of sintering furnaces necessary.

Alloying materials may be used to reduce sintering temperatures and it has been proposed to employ certain relatively low boiling metals such as nickel, vanadium or gold. It is well known in the art that the performance of electrical contacts formed of such compositions are for the most part unsatisfactory. It has therefore been suggested that these low boiling metals be employed to assist the grain growth of the tungsten and then be removed by heating to a sufiiciently high temperature to vaporize most of the low boiling metal. The disadvantage of such method lies in the fact that the vaporization temperatures of the metals approach the sintering temperature of pure tungsten and the vaporization of these metals results in the formation of porous finished products. For electrical contact purposes it is desired that the material be as dense as possible.

In the method of forming bodies of tungsten or molybdenum at relatively low temperatures as proposed in Patent No. 2,030,229, a low melting point metal or alloy is employed as a cementing or bonding agent. In this method the low melting point metal or alloy acts in a manner similar to cement in a concrete mix. The refractory metal corresponds to the usual aggregate, such as crushed stone and sand. In this prior method it is recommended that the refractory metal be first sintered or melted and then hammered or rolled before comminuting. This treatment is for the purpose of renderin the refractory metal less likely to be recrystallized or altered in its form during the preparation of the finished product. The powdered or granulated refractory metal is mixed with a soft or low melting metal or alloy and then pressed to the desired form. The mixture is then heated or sintered only sufiiciently to melt the soft or low melting metal and to preserve the original structure of the refractory metal. During the sintering operation, the refractory metal maintains its original state of aggregation.

I have discovered that dense tungsten or molybdenum compositions having the relatively large grain or crystal structure suitable for electrical contacts may be formed at greatly lower sintering temperatures by the use of not more than 4%, preferably about 0.1% to about 1% of certain alloys which are capable of dissolving tungsten and molybdenum but which are relatively insoluble in tungsten and molybdenum. These alloys have a relatively low melting. Point. that is, a melting point of the order of themelting point of nickel and far below that of tun sten. It is essential that the amount of added alloy be small so that its presence in the finished compositions will not affect adversely the contact characteristics for various applications as compared to pure tungsten contacts.

One of the elements of my alloy is selected from the following group of metals: nickel, cobalt, iron, platinum and palladium. In some cases I may use two or more of these metals in my alloy. These metals have the property of dissolving tungsten and molybdenum, but they do not dissolve appreciably in tungsten or molybdenum.

My alloy also includes one or more metals of the following group of metals: copper and silver. The metals of this group do not dissolve tungsten or molybdenum and they do not dissolve appreciably in tungsten or molybdenum. They constitute with the first named group of metals alloys which have lower melting points than have those metals themselves. Ordinarily, I prefer to employ the metal or metals of the second group in lesser quantity than the metal of the first named group.

, These alloys are highly satisfactory for the purposes of my invention and dissolve tungsten and molybdenum but are relatively insoluble in tungsten and molybdenum. The metals of the alloys shall each be present in a substantially significant proportion and in excess of such quantitles as may be present as impurities. I have found that for the various applications of the contact compositions of this invention the presence of the low melting point alloys in the relatively small quantities has no objectionable effects upon the contact characteristics of the compositions as compared to pure tungsten contacts.

The compositions of my invention may be produced by providing an intimate mixture of powdered tungsten or molybdenum with powder of an alloy of the type above described or with powders of the constituent metals. The mixture of powders is then pressed to form rigid compacts of the desired shape, preferably being somewhat larger than the desired size of the finished product, the larger size being provided to compensate for the shrinkage which occurs during sintering. The compacts are then sintered in a suitable atmosphere or in a vacuum at such a temperature and for such a period of time which will result in the reuuired or desired grain growth and which will produce a dense body having large crystals or grains of refractory metal. I have found that sintering temperatures above 1200 C. are satisfactory but I prefer to use temperatures between about 1450 C. and '1600 C. The particular sintering temperature will vary somewhat with the different alloys but must always be sufficiently high to melt the alloy to permit a solution of the refractory metal in the alloy and a crystallization of the refractory metal from the solution. At these temperatures the sintering period may vary from about 30 minutes to two or three hours.

It is advisable in the preparation of the compositions of this invention to employ the alloy or alloying constituents in very finely divided form so that not withstanding the small amount of alloy in the mixture it will be thoroughly and intimately distributed throughout the mixture. A convenient method of attaining such wide distribution between the tungsten or molybdenum and the alloying metals includes introducing the alloying metals in the form of solutions of suit- .fer was noted.

4 able salts such as chlorides or nitrates. Thus I may add solutions of nitrates of two or more of the alloying metals in suitable quantity to a quantity of tungstic acid, H2WO4. The solutions should be sufilciently dilute to form with the tungstic acid a relatively thin slurry. This slurry is dried with occasional stirring to ellect uniform distribution. The powder is then reduced by heating in a reducing atmosphere at a temperature around 1000 C. for an hour or so. The reduced powder is pressed in dies under a pressure of about 20 or 30 tons or more per square inch to form self-supporting pellets or discs somewhat larger than their desired finished size. The pellets or discs are then sintered at a tempera-turebetween l450 and 1600 C.for asufficient period of time to effect the desired sintering and crystal growth. The temperature should be sufficiently high to melt the alloy formed by the alloying metals. Due to the solubility of tungsten and molybdenum in the alloy, there is a tendency for small tungsten or molybdenum crystals to dissolve in the alloy and for large tungsten or molybdenum crystals to grow with the aid of such dissolved metal. The net result is that tungsten or molybdenum particles recrystallize completely and a dense, integral fabric of sintered tungsten or molybdenum is obtained.

As a specific example of my improved procedure, I give the following: 3 grams of copper were dissolved in hydrochloric acid to which a small amount of nitric acid was added. The solution was diluted with water and 17.5 grams of nickel chloride (NiClaSI-IzO) were added. The solution was mixed with 1000 grams of tungstic acid (H2WO4). The solution was made sufiiciently dilute to provide a thin slurry. The solution was dried with occasional stirring to insure a thorough distribution of the ingredients. The dry mixture was reduced in a flowing stream of hydrogen at a temperature not exceeding 1000 C.

The reduced powder is then pressed into pellets or discs of desired shape at a pressure of around 20 or 30 tons or more per square inch and heated in a neutral or reducing atmosphere, or in a vacuum at a temperature around 1300 C. to 1350" C. The sintering may be carried out for a period of time as low as ten minutes in some cases and as long as two or three hours in other cases. Of course, sintering may be efifected more rapidly at higher temperatures such as. around 1600 C., since higher temperatures cause more rapid dissolution of small particles of tungsten (or molybdenum) in the molten alloy.

In order to illustrate the performance of contacts made in accordance with the present invention as compared to the performance of standard grade tungsten contacts, distributor tests were run. The procedure in the tests was as follows: The contacts were mounted in the distributors, the dwell having been correctly adjusted and the contact resistancesymbolized by Rc-measured at 5 amperes by the voltage-ammeter method. The distributors were of the 6-lobe type, running at from 525 to 585 R. P. M. The input voltage was set at 8 /2 volts, and circuit constants adjusted so that the peak break current, as measured on an oscillograph, was 4 to 5 amperes. Then at every 25-hour interval thereafter until the test run of hours had been completed, Re was measured and the contact surface condition and the amount of'traris- At the conclusion of the test the dwell, spring tension, peak current, Rc, input voltage, surface Contact test data Av. Dura- Av Fina Av. Av. Final Material tion of test i hms) Pitting Point (hours) Grade Condition Regular Tungsten. 100 0.0015 1.0 gray. New Alloy 100 0. 005 1. 2 gray.

! Pitting Grade Classification:

1. Flat or nearly so.

2. Smooth, shallow pit.

3. Deep, sharp edged pit.

Among the various alloys which may be used, containing metals of the two classes set forth above, are the following: In addition to the nickel-copper described in the preceding example; cobalt-copper, nickel-cobalt-copper, platinum-copper, platinum-silver, platinum-silvercopper, palladium-silver, platinum-palladiumsilver, palladium-copper. It will, of course, be understood that this list is not intended to be limitative of the invention, since other alloys consisting of one or more of the metals of each of the two classes of alloying metals defined above may be used with highly satisfactory results.

This application is a continuation-in-part of my copending application entitled Contact Alloys, Serial No. 460,443, filed October 1, 1942, which application is a continuation-in-part of my application entitled Contact Alloys, Serial No.

99.9% by weight of tungsten, the remainder being made up of an alloy of nickel and copper, in a proportion by weight of about 3 parts of nickel to about 2 parts of copper, said body consisting essentially of relatively large grains of recrystallized tungsten with the major portion oi the alloy segregated at the grain boundaries.

2. An electrical make and break contact comprising a pressed and sintered-to-shape body consisting essentially of from 99.0% to about 99.9% by weight of tungsten, the remainder being made up of an alloy of platinum and. copper, in a proportion by weight of about 3 parts of platinum to about 2 parts of the copper, said body consisting essentially of relatively large grains of recrystallized tungsten with the major portion of the alloy segregated at the grain boundaries.

3. An electrical make and break contact comprising a pressed and sintered-to-shape body consisting essentially of from 99.0% to 99.9% by weight of tungsten, the remainder being made up of an alloy of palladium and silver in a proportion by weight of about 3 parts of palladium to about 2 parts of silver and being present in an amount sufficient to produce a dense product, said body consisting essentially of relatively large grains of recrystallized tungsten with the major portion of the alloy segregated at the grain boundaries.

4. An electrical make-and-break contact comprising a pressed and sintered-to-shape body consisting essentially of from 99.0% to about 99.9% by Weight of a metal from a first group consisting of tungsten and molybdenum, the remainder of the body consisting of an alloy of at least two metals, at least one of the metals being taken from a second group consisting of nickel, cobalt, iron, platinum, and palladium, and at least another of the metals being taken from a third group consisting of copper and silver, the metals of the second and third groups being in a proportion by weight of about 3 parts to 2 parts, respectively, said body consisting essentially of relatively large grains of recrystallized metal of the first group with the major portion of the said alloy segregated at the grain boundaries.

WILLIAM H. LENZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,229,960 Humphries June 12, 1917 1,346,192 Gebauer July 13, 1920 1,809,780 Gebauer June 9, 1931 2,030,229 Schwarzkopf Feb. 11, 1936 FOREIGN PATENTS Number Country Date 447,567 Great Britain May 21, 1936 497,747 Great Britain Dec. 28, 1938