CA1072265A - Manufacture of improved electrical contact materials - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Improved electrical contact materials and a method for manufacturing same are disclosed. The improved materials com-prise mechanically elongated billets composed of at least a first metal and oxide of a second metal which is more readily oxidizable than said first metal. The billets include a plur-ality of first and second zones, the zones extending the entire length of said billet and being substantially symmetrical and coaxial about the longitudinal axis of the billet. The first zones are substantially devoid of metal oxide and the second zones are substantially composed of metal-metal oxide. The first zones are more readily deformable than the second zones so that said billet will deform symmetrically about its long-itudinal axis when headed in the direction normal to said axis. The zones devoid of the metal oxide serve as slip zones and render the contact material readily formable into any desired electrical contact shape.

Description

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The present invention relates to electrical contact materials, methods of making electrical contact materials, and electrical contacts made in accordance with such methods and/or using such materials. The present invention is particularly related to electrical contact materials comprising metal-metal oxides.

The use of metal-metal oxides in the electrical contact industry is well known. Such materials, particu-larly those of the silver-cadmium oxide family, have the advantage of substantially reducing the tendency for sticking in make-and-break contact applications. Further, electrical contacts fabricated from such materials exhibit good ar~ interruption characteristics, low contact resis-tance, and high resistance to electrical erosion. However, certain of such materials such as, for example, silver-cadmium oxide, which have superior functional quali~ies for electrical contact applications, do not lend themselves to low cost production techniques such as heading. Low formability of such materials tends to result in cracking ~during the heading operation and an accompanying loss of functional quallties. It is recognized that a high metal oxide content is desirable to cause dispersion hardening and to lend erosion resistance to contacts abrica~ed from such materials but the adverse effect of the metal oxide on formability has heretofore resulted in a limitation on the !: percentage of metal oxide possible in the final product.
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~07Z~65 Various techniques have been disclosed in the prior art for fabricating electrical contacts from such : metal-metal oxide materials. For example, in British Patent No. 1,397,319, there is disclosed a method for fabricating contact shapes by compacting silver cadmium oxide powder unde.r pressure to form a compacted body, sintering t~e compacted body and thereafter forming the compacted body into the desired contact shape. This technique has been found to result in three distinct ~manufacturing problems. First, due to the fact that many integral particles are being consolidated, there is an extremely large surface area which is quite difficult to keep free of contamination. Seconclly, during consolidation of the particles, interfaces are formed having random directionality extending to the sur.face of the consolidated body where they act as stress raisers.~ Thirdly, the metal oxide particle size and distribution throughout t~e final . j . , product is very non-uniform and therefore difficult .~o control by virtue of varying oxidation paths inherent when variously sized integral particles are present.
Certain of the problems inherent in the technique noted above have been alleviated to some extent by other prior art techniques wherein a strip of silver-metallic alloy is manufactured by melting and casting bars of the alloy material and then rolling the bars to form strips of approximately the desired shape and thickness of the ', ' , ' , j, .
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~O~Z~5 final product followed by internal oxidation of the strip and, where necessary, a secon~ rolling operation to the final size. Still another technique involves extrusion of a pre~oxidized silver-metallic oxide material in the form of shot-grain or pellets. This technique, which is disclosed in U.S. Reissue Patent No. 27,075, generally comprises the steps of forming a silver-cadmium metallic shot, internally oxidizing the shot, compacting the oxidized shot, extruding the shot, and cold working the extruded shape to the final desired size.

Most recently, in U.S. Patent Nos. 3,932,935 a~d 3,932,936, a method for manufacturinq silver-metallic oxide materials from which electrical contacts can be prepared has been disclosed which comprises extrusion pressing an 1~ assembled plurality of plates or wires of silver-metallic oxide materials-which had been previously prepared by ....
either internal oxidation or powder metallurgical tech-niques. The extruded product thus produced is said to exhibit good formability when subsequently shaped into an electrical contact. While it is disclosed that the fibrous structure of the metallic oxide stratum ~hat results from this technique will have a favorable effect on the subse-quent handling of the extruded product when it is formeù
into an electrical contact/ such stratum exists ln only a single plane, thus resulting in less -than a complete uniformity of metallic oxide distribution throughout the final product. This lack of complete uniformity can lead to the presence of stress raisers and consequently to an - adverse effect on formabllity.
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' : '' . ., : .' " ' -' At present, several hundred different contact materials are presently manufactured in order to fill market requirements. Generally, a number of different functional qualit:ies are needed in the contact material in order to provide the best performance results in a given application. For example, in the household circuit breaker market, silver-molybdenum-tungsten, silver-molybdenum, and silver-cadmium oxide materials are most widely used for the electrical contacts. On the other hand, in the appliance contact market, fine silver, silver-copper alloys, and silver-cadmium oxide materials are used extensively. In either case, each type of refractory material contributes a particular desirable quality to the contact material incorporating same. However, manufacturing limitations often cause compromises to be made and the optimum contact material composition cannot always be used.
~ leretofore, the manufacture of composite contact materials has been accomplished by incorporating a desired additive material into the basic metal-metal oxide material ,~0 in the form of a powder such as disclosed in British Patent - No. 1,397,319 discussed above, U.S. Patent No. 3,15&,469 and U.5. Patent No. 3,827,883. Alternatively, the material has been incorpora-ted by melting and casting it along with the basic materials into the form of an ingot prior to rolling the ingot to form a slab from whîch electrical contacts are then manufactured. This latter technique is disclosed in U.S. Patent No. 3,694,197. It has also been suggested, in UOS. Patent No. 3,821,848, that composite contact materials can be fabricated by metallurgically bonding a layer of a desired additive material directly to a metal-metal oxide electrical contact.

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Each of the prior art techniques for manufacturing com-posite electrical contact materials suffers from the same draw-backs as those set forth above with respect to the manufacture of electrical contact materials in general. Further, the man-ufacture of composite electrical contact materials accordingto the prior art techniques does not result in as uniform a distribution of the additive ma-terial throughout the finished product as would be desireable. Again, this lack of complete uniformity can lead to the presence of stress raisers and to an adverse effect on formability, particularly in the case where the additional material is added prior to internal oxid-; ation and becomes partially oxidized as well.
; According to one aspect of the present invention, an im-proved electrical contact material is provided comprising a mechanically elongated billet composed of at least a first metal and the oxide of a second metal which is more readily oxidi.zable than the first metal, the billet including a plur-ality of alternating first and second zones, the zones extending the entire length of the billet and being substantially symmet-rical and coaxial about-the longitudinal axis of the billet, the first zones being substantially devoid of metal oxide and the second zones being substantially composed of metal-metal oxide, and the first zones are more readily deformable than the second zones so that the billet will deform symmetrically about its . 25 longitudinal axis when headed in the direction normal to said ..
axis.
By one variant thereof the billet included additional zones : -alternating with said first and second zones, said additional , zones also extending the entire lenght of said billet and being substantially symmetrical and coaxial about the longitudinal axis of said billet, said additional zones being composed sub-. .................. . . . . . . .
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stantially of m~terials selected from the group comprisingme-tals, metal oxides, metal alloys, and mi~tures of same and having compositions different from said first and second ~ones.
By one var~ation thereor, -the cross-sec-tion of said billet is circular or rectangular.
By another variation, the first metal is silver.
By another variation, the second metal is selected from the group consisting of cadmium, ~inc, and tin.
According to another aspect of the present invention, a method is provided for making an electrical contact material comprising a mechanically elongated billet composed of at least a first metal having good electrical conductive properties and the oxide of a second metal which is more readily oxidizable than ~aid first metal, the method comprising the step of: forming the billet with a plurality of alterna-ting first and second zones -which extend the entire length of the bille-t and are substantially symmetrical and coaxial about the longitudinal axis of the billet, the first zones being substantially clevoid of metal oxide and the second zones being substantially composed of metal-metal oxide, and the first zones are more readily deformable than the second zones so that said billet will deform symmetrically about its longitudinal axis when headed in the direction normal to said axis.
By a variant thereof, the plurality of alternating first and --second zones are formed by providing a metallic alloy strip com-posed of at least said first metal and said second metal, internally oxidizing said strip to obtain a s-trip composed of said first metal and the oxide of said second metal, the center of said strip being at least partially depleted of said second metal during internal oxidation and being substantially devoid of metal oxide, ..
winding said strip to form a wrapped billet, and mechanically elongating said wrapped billet. ~Z2~5 ~i s ~1 ~
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- By another variant, the me-thod includes the further step of providing at least one additional strip of a material selec-ted from the grQUp com.prising metals, me-tal oxides, metal alloys, and mixtures of same, in overlying relationship to said s-trip of said first metal and the oxide of said second metal prior to winding of said strips to form said wrapped billet, said add-itional strip having a composition different than the compos-ition of said first metal.
By one variation thereaf, the wrapped billet is formed about a core of metal-metal oxide wire.
By another variation, the first metal is silver.
By another variation, the second metal is selected from the group consisting of cadmium, zinc, and tin, especially where it is cadmium.
By a specific variation, the first metal is silver and the second metal is cadmium.
In order that the invention may be fully understood, it will be described with reference to the accompanying drawings, in which:
Figure 1 is a flow diagram setting forth the process steps 20 of a preferred embodiment of the present invention; ~-Figure 2 is a phoLomicrograph at lOOX of a partial cross-section of an extruded billet of silver-cadmium oxide produced : -in accordance with an aspect of the present invention;
Figure 3 is a schematic îllustration of the macrostructure of a cross-sectional area of an extruded product obtained accord-ing to prior art techniques;
Figure 4 is a schematic illustration of the macrostructure of a cross-sectional area of an extruded product obtained accord-in to the process of an aspect of the present invention;
Figure S is a schematic illustration of the macrostructure of a vertical cross-section of an electrical contact manufactured according to prior art techniques; and ,~- 7~ ZZ65 , . ~ . , ,, ~ . .

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Figure 6 is a schematic illustration of the macrostru~ture of a vertical cross-sec-tion of an electrical contact manufactured according to the present inven-tion.
Before specifically turning to the d-rawings, it is noted that, according to an aspect of the present invention electrical contac-t materials are manufactured by providng a metallic alloy strip composed of at least a first metal which has good electrical conductive properties and a second metal which is more readily oxidizable than the first metal and internally oxidizing the strip to obtain a strip which is composed of the first metal and ; the oxide of the second metal. During internal oxidation of the strip, the center of the s-trip becomes at least partially depleted of the second metal and is substantially devoid of the metal ox- ~
ide. Subsequent to internal oxidation, the strip is wound to rom a wrapped billet and the wrapped billet is mechanically el-ongated whereby a ductile metal-metal oxide product is obtained.
Mechanical eleongation can be effected according to known tech-niques such as, for example, by .: :

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z~s extrusion, swaging, or rod rolling. The product thus obtained can then be shaped according to conventional methods into any desired final contact shape.
Metals having good electrically conductive properties that can be used as the first metal in accor-dance with the present invention include silver, gold, palladium, platinum, and aluminum. The second material, which will be more readily oxidizable than the first material, can be selected from the group comprising cadmium, tin, zinc, lead, thallium, copper, thorium~
indium, titani~m, berylium, magnesium, calcium, strontium, barium, uranium, and zirconium. The preferred composition, however, for the final product is silver-cadmium oxide since it is generally recognized as exhibiting the best arc .
I ~ interruption characteristics, low contact resistance, and high resistance to electrical erosion of any other mate~
rials presently used in the manufacture of electrical contacts.
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The strip of metal-metal oxide material which is to be wrapped and mechanicalIy elongated in accordance with ' . . .
~ the present invention can be prepared by various techniques ' .
~; including melting and casting a metal-metal alloy into an -ingot, rolling the ingot into a strip, and then subse-quently subjecting the rolled strip to internal oxidation. ~`
However, if desired, the strip can be prepared by blending , ; powders of the first and second metals, sintering the blended powders to form an ingot, and then rolling the -ingot to form a strip prior to internal oxidation.
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The metallic alloy strip can be subjected to a suitable cleaning operation, either prior to or subsequent to internal oxidation, or both. The shape of the strip, having a relatively small surface area when compared to separate integral particles of material compacted to form elec~rical contacts according to certain of the prior art techniques, lends itself quite readily to such cleaning and the consequent removal of any surface contaminants.
The lnternally oxidized metal-metal oxide strip 1~ can be wound into a wrapped billet either with or without a center core of a similar metal-metal oxide wire. The cross-section of the billet can be any desired shape so long as it is wrapped symmetrically about the longitudinal axis of the billet. In any event, it has been found that the resultant product after mechanical elongation exhibits a series of annular laminar flow lines of the oxide mate-rial throughout the silver matrix giving a coaxial and symmetrical distribution of the oxide throughout the mechanically elongated billet. It has been found ~hat such a distribution results in superior formability of the mechanically elongated product when compared with metal-metal oxide materials made according to the prior art techniques. Specifically, although not wishing to be bound by any specific theory, it is believed that the superior formability results from the fact that during internal oxidation of the metallic alloy strip, zones depleted of metallic oxide which causes dispersion hardening of the final product are formed, thus leaving slip zones of the first more malleable metal. When the internally oxidized strip is wound to form the wrapped billet, each separate g l~ZZ65 . ~, . . . - ~ . .

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wrap has its own depletion zone thus, in effect, causing a series of coaxial and symmetrical slip zones devoid of the metallic oxide throughout the coiled billet. Basically, since the zones which are devoid of the metal oxide are far more malleable than the dispersion hardened metal oxide, the mechanically elongated product will deform symmetri- .
cally about the longitudinal axis of the product when it is headed in the direction normal thereto. The slip zones thus enable much more efficient heading than obtainable with contact ~aterials including randomly distributed dispersion hardened areas of the metallic oxide as obtained according to prior art techniques. ..
; . As noted hereinabove, while silver-metallic oxide materials manufactured in accordance with the technique .d.isclosed in U.S. Patent Nos. 3,932,935 and 3,932,936 may exhibit what is said to be a fibrous structure of the metallic oxide stratum and that such structure enhances the formability.of products so produced, it can readily be seen , that such structure cannot possibly give the coaxial and ~ .symmetrical slip zones which extend the entire longitudinal ~ length of the mechanicaIly elongated billet as are obtain-able in accordance with the present invention.
Still another feature of the present invention resides in the fact that composite materials can be manu-factured by placing additional strips of material in overlylng relationship to the basic internally oxidi~ed metallic alloy strip, rolling the strips together to form.a wrapped billet, and then subsequently mechanically elonga-ting the wrapped billet. The resultant product will ~ include separate coaxial and symmetrical zones of the~

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~z~s additional material alternating with the slip zones of the electrically conductive material of the first sheet and the dispersion hardened zones of metallic oxide. As should be readily apparent, the technique results in extremely uniform distribution of the additional material not other-wise obtainable according to prior art ~echniques. Further-more~ the same advantages of formability that result from the presence of the slip zones that were discused above will be obtained as well. Among the materials that contri- -bute the desired characteristics to electrical contacts that can be utilized in conjunction with the present in~ention are metal-metal oxide of different compositions than those of the basic strip, pure silver, silver graphite, tungsten, silver-tungsten, molybdenum, silver-molybdenum, silver~
iron, silver-nickel, gold, platinum9 palladium, aluminum, copper, and mixtures of same. Of course, those of ordinary skill in the art can readily determine and utilize whatever additive materials will impart particular desired charac- -~eristics to the final prod~lct.
It should also be noted that the advantages obtained by the presence of the coaxial and symmetrical slip zones of an aspect of the present invention can be achieved by alternately stacking sheets of an electrically conductive materlal e.g. silver with a dispersion hardened material such as silver~cadmium oxide, rolling ~he stacked sheets to form a wrapped billet and then mechanically elon-gating the wrapped billet. The pure silver layer will serve as a slip zo~e enabling sym~etrical heading about the long-itudinal ~-~

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~vt7z~6s axis of the mechanically elongated billet in the same fashion as that set forth aboveO Of course, any particular combination of materials desired can be wound into the form of a billet and mechanically elongated and the advantages will be obtained so long as at least one of the sheets has good electrical conductive properties and is more malleable than the other materials being utilized.
It is noteworthy that the concept of winding elongated metallic material into a coil prio~ to extruding same has been disclosed in connection with the production of titanium base metal shapes. Thus, in U.S. Patent ~o.
3,579,800 such a technique is utilized under specific conditions directly related to titanium metallurgy, in connection with the manufacture of elongated titanium base extrusions such as tubes and rods, and is said to result in a lessening of stress raisers during such manufacture which lead to cracking or splitting of the rods during subsequen~
cold-drawing operations. Specifically, the technique involves the steps of orienting a sheet of titanium base metal in its rolling direction so that the elongated grains of the metal are specifically oriented along a first axis, coiling the oriented sheet a~out a second axis perpendi-cular to the first axis to provide a plurality of convolu-tions about said second axis and to form an extrusion billet, and extruding the billet at an elevated temperature in excess of 1200F. The resulting product is used pri-marily for tubing in connection with the manufacture of aircraft. It should, however, be clear that although the broad concept of wlnding s sheet material lnto a coiL prior :

~ -12-to extrusion is disclos~d, heretofore there was no recogni-, ~
tion that such a technique could be applied to the manu-facture of electrical contact materials from metal-metal oxides where a uniform distribution of the oxide throughout the final product is required in order to impart to the material a high degree of formability without attendant rupture during manufacture. Further, there was clearly no recogni~ion that the depletion zones of metal oxide which result from internal oxidation of metal-metal alloy strips where the second metal is more oxidizable than the first metal would form slip zones in a product formed by me~
chanical elonga~ion of wrapped billets of such strips and that the presence of these slip zones would result in enhanced formability of the mechanically elongated product while permitting a higher content of metallic oxide to be present therein than was possible according to prior art techniques for manufacturing electrical contacts.
In a preferred embodiment of the present inven-tion, as illustrated by the flow diagram in Figure 1, an ingot of a silver-metallic alloy, preferably unoxidized silver-cadmium, is melted and cast according to known techniques. The alloy constituents will typically range from¦ 5% cadmium tq 25% cadmium. In addition, other me~als can be present~e g. nickel or cobalt, which result in crystal refinement of the silver in the final produc~O The particular dimensions for the ingot can be of any size desired so long as such size i5 suitable for subsequent rolling to form the strip which is to be wrapped.

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The silver-metallic ingot is then rolled to a thickness of fro~ .100 inch to .001 inch and then the strip is subjected to internal oxidation under appropriate conditions which are determined by economic considerations as well as the characteristic requirements of the finish~d product desired. It has been found that the character-istics of the finished product which are related to the particle size and distribution of the cadmium oxide and the silver matrix, are affected to a significant degree by the thickness of the strip during oxidation and strips having a thickness in the aforementioned range have been found to exhibit good results in this regardO During internal oxidation of the rolled strip, the center of the strip becomes at least partially depleted of cadmium and is substantially devoid of any cadmium oxide.
After internal oxidation i5 comple~ed, the rolled and oxidiæed strip is then subjected to a cleaning opera-tîon to remove any surface contaminants that may have formed during the oxidation operationO Cleaning can be accomplished by any known chemical or mechanical means.
Further, in a preferred embodiment of the present inven-tion, the strip surfaces are cleaned by a suitable cleaning method prior to internal oxidation of the strip as well.
The relatively small sur face area of the str ip, when compared to the separate integra~ particles of alloy which are compacted to form electrical contact materials accord-ing tQ certain prio~ art techniques, permits efficient removal of surface contaminants which can adversely affect the properties of the final product.

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The oxidi~ed and cleaned strip is then wound into a tightly wrapped billet which can be mechanically elon-gated according to conventional techniques to form the final product. Typically, the billet will be ~t 5-20 inches in length and will have a diameter of from ~b~ 2-8 inches, the specific diameter being dictated only by the apparatus being utilized to effect mechanical elongation and the desired dimensions of the final product. Further, if desired, a core of silver-metallic oxide wire can be provided at the center of the billet, such core either having been previously fabricated according to the process ¦of an aspec-t oE the present invention, or obtained by any of the known prior art techniques.

The preferred technique for effecting mechanical elongation of the wrapped billet is extrusion due to the rela~ive efficiency of extrusion versus other techniques for mechanical elonqation such as swaging or rod rolling.
However, in certain instances it has been found that swaging or rod rolling are to be preferred since such technique~ permit a higher utilization of material. In this regard, the extrusion process results in the formation of a butt end on the extruded product which must be ~emoved prior to further processing. Although in the case of many extruded products the butt end can be salvaged and recycled directly, in the context of the present invention this is not possible. The reason for this is that the extrusion , ... . .
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metal oxide which cannot be simply melted down for u~sequent recycling. Rather, in order to salvage the v~luable precious metal components of the butt end, it is n~ceSSary to subject the butt end to an expensive and complex refining operation. Further, in the case of metal-metal oxide products which include cadmium as a component thereof, refining may be impracticable due to the high toxicity of cadmium and the effect on the atmosphere of cadmium fumes or vapors resulting from the refining process. Nevertheless, as can readily be appreciated, where metal-metal oxides which include precious metals e-g-~silver, gold~ palladium, and platinum are involved as contem-lplated with an aspect of the present invention, the economic significance of simply scrapping the butt end of the extrusion product can become quite significant and refining , to salvage the components becomes an economic necessity.
In view of the above and in order to avoid the necessity for refining and the possible environmental problems occasioned thereby, the less efficient techniques of swaging and rod rolling can, if desired, be employed to alleviate such problems, the latter techniques permitting ; total utilization of the material. In any event, the advantages which result from the presence of the slip zones when the elongated product is headed to form an electrical contact, are achieved regardless of whether an extruded, swaged, rolled, or otherwise mechanically elongated product . .
is used.
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, The mechanically elongated billet, typically having a diameter of from .050-2.0 inches, is shaped into a final electrical contact according to standard manufacturing techniques known in the prio~ art e.g.
heading, or bonding into a rivet or contact. The actual diameter of the billet will, or course, be determined by the particular final manufacturing operation being uti-lized. In any case, the mechanically elongated product, as well as the final electrical contact manufactured there-from, will exhibit superior formability and headability to materials obtained according to prior art techniques~ As noted above, it is believed that this superior formability results from the presence of the zones depleted of cadmium oxide which result in the formation of slip zones of essentially pure silver after the strip has been wrapped and mechanically elongatedr The slip zones permit symmet-rical heading of the mechanically elongated product about its longitudinal axis. In this regard, reference to Figures 2-6 show quite clearly, by way of comparison with the prior art structures 40 and 50, the existence of the slip zones of silver and the annular laminar flow lines of the oxide material throughout the cross-section of the product as opposed to the relatively random distribution characteristic of materials manufactured according to prior art techniques.
Specifically, in Figure 2, the cross-section of an extruded silver cadmium oxide billet is shown prior to deformation into an electrical contact shape, the edge of ' ' ~ Z~6~

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` the product being indicated by reference numeral 10 and the interface between successive wrappings of the silver-cadmium oxide strip being indicated by refe~ence nu-meral 16. As can readily be seen, the product comprises depletion zones 12 of essentially pure silver alternating throughout the cross-section of the product with dispersion hardened zones 14 of silver-cadmium oxide.
Generally, when ,a mechanically elongated product in accordance with an aspec-t of the present invention is head-ed along its longitudinal axis, the depletion zones serve as slip zones resulting in virtually uniform and symmetrical de-forma~ion of -the billet in the direction normal to the longitudinal axis of the billet. The resultant uniform deformation is believed to be apparent from a comparison of Figures 4 and 6. Thus, as represented schematically in ; Figures 4 and 6, the metal deforms quite uniformly about the axis due to the presence o the slip zones 18 which alternate with the dispersion hardlened zones so that the slip zones and dispersion hardenecl zones remain substan-tially uniformly distributed throughout the formed product resulting in more uniform mechanical and electrical proper-ties in the formed contact. This is to be distinguished from the relatively random distribution of both the elec-trically conductive material and the dispersion hardened material which is characteristic of the prior art structure portrayed schematically in Figures 3 and 5 in which head cracks 22 result during headingO

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, The followin~ Example will further serve to illustrate an embodiment of the presen-t invention:
A melt of a silver-cadmium alloy containing 86~5%
silver, 13.3% cadmium, 0.2% nickel was prepared at approxi-mately 1020C. The metal was then cast into an ingot having the dimensions 2'l x 12" x 24". The ingot was then rolled according to conventional rolling techniques to a thickness of 0.050 inch. Subsequent to rolling, the ingot was coiled with a porous separator in order to hold the wraps apart and was thereafter heated to 750C~ and held for a period of 12 hours at 20 psi in an atmosphere of oxygen. At the end of the 12 hour period the coiled strip was fully oxidized and subjected to a cleaning operation by a strip brushing machine in order to remove excess cadmium oxide and other contaminants which were formed on the surface of the strip. The cleaned strip was then tightly wound onto a 1/2" diameter rod of silver-cadmium oxide which had previously been extruded. The extrusion billet ; thus produced was 12" long and had a diamter of 3-1/2".
~o The billet was then preheated to 500C. and extruded according to normal extrusion techniques to form a .300 inch diameter wire which was then subsequently drawn and annealed to a finished size for subsequent heading or bonding into a rivet or contact.
The wire thus produced had a final composition of i 85% silver and 15~ cadmium oxide. This material exhi~ited superior formability and headability to materials containing only 10~ cadmium oxide which had been produced according to prior art techniques.

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The zones depleted of cadmium oxide which serve as slip zones of pure silver which are distributed symmetri-cally and coaxially throughout the final product and extend along the entire length thereof, permit the extruded produc~ to be symmetrically headed about its longitudinal axis. The presence of these slip zones, therefore, permits the incoporation of a higher percentage of cadmium oxide in the final produc~ than was possible according to the prior art techniques without any sacrifice of formability~ The inclusion of a higher cadmium oxide content con~ributes to more uniform arc erosion and thus better wear characteris-tica of contacts formed from such materials. Further, because the materials made in accordance with the process of an aspect of the present invention can be uniformly and sym-metrically headed, the contac-ts so produced will exhibit better wear characteristics since the arc will distribute evenly ra-ther than focusing on a weak point such as occurs in an el-ectrical contact havin~ nOnuniform conflguration o~ the head.

Such uniformity leads to a lower temperature rise during uae and therefore to more uniform electrical properties.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved electrical contact material comprising a mechanically elongated billet composed of at least a first metal and the oxide of a second metal which is more readily oxidizable than said first metal, said billet including a plurality of alternating first and second zones, said zones extending the entire length of said billet and being substan-tially symmetrical and coaxial about the longitudinal axis of said billet, said first zones being substantially devoid of metal oxide and said second zones being substantially composed of metal-metal oxide, and said first zones are more readily deformable than said second zones so that said billet will deform symmetrically about its longitudinal axis when headed in the direction normal to said axis.

2. The electrical contact material of Claim 1, wherein said billet includes additional zones alternating with said first and second zones, said additional zones also extending the entire length of said billet and being substantially symmetrical and coaxial about the longitudinal axis of said billet, said additional zones being composed substantially of materials selected from the group comprising metals, metal oxides, metal alloys, and mixtures of same and having compositions different from said first and second zones.

3. The electrical contact material according to Claim 1 or 2, wherein the cross-section of said billet is circular or rectangular.

4. The electrical contact material according to Claim 1 or 2, wherein said first metal is silver.

5. The electrical contact material according to Claims l or 2, wherein said second metal is selected from the group consisting of cadmium, zinc, and tin.

6. The electrical contact material according to Claims 1 or 2, wherein said second metal is cadmium.

7. The electrical contact material according to Claim 1 or 2, wherein said first metal is silver and wherein said second metal is cadmium.

8 A method of making an electrical contact material comprising a mechanically elongated billet composed of at least a first metal having good electrical conductive properties and the oxide of a second metal which is more readily oxidizable than said first metal, said method comprising the step of forming said billet with a plurality of alternating first and second zones which extend the entire length of said billet and are substantially symmetrical and coaxial about the longitudinal axis of said billet, said first zones being substantially devoid of metal oxide and said second zones being substantially composed of metal-metal oxide, and said first zones are more readily deformable than said second zones so that said billet will deform symmetrically about its longitudinal axis when headed in the direction normal to said axis.

9. The method according to Claim 8, wherein said plurality of alternating first and second zones are formed by providing a metallic alloy strip composed of at least said first metal and said second metal, internally oxidizing said strip to obtain a strip composed of said first metal and the oxide of said second metal, the center of said strip being at least partially depleted of said second metal during internal oxidation and being substantially devoid of metal oxide, winding said strip to form a wrapped billet, and mechanically elongating

10. A method according to Claim 8, further comprising the step of providing at least one additional strip of a material selected from the group comprising metals, metal oxides, metal alloys, and mixtures of same, in overlying relationship to said strip of said first metal and the oxide of said second metal prior to winding of said strips to form said wra-ped billet, said additional strip having a composition different than the composition of said first metal.

11. A method according to Claims 8, 9, or 10 wherein said wrapped billet is formed about a core of metal-metal oxide wire.

12. A method according to Claims 8, 9, or 10, wherein said first metal is silver.

13. A method according -to Claims 8, 9, or 10, wherein said second metal is selected from the group consisting of cadmium, zinc, and tin.

14. A method according to Claims 8, 9, or 10, wherein said second metal is cadmium.

15. A method according to Claims 8, 9, or 10, wherein said first metal is silver, and wherein said second metal is cadmium.