US2229667A - Silver-lead alloy - Google Patents

Description

Jan. 28, 1941. H.) J. NESS 2,229,667

snvanmmn ALLOY Filed April so, 1938 2 Sheets-Sheet 1 3 uoeutor I f/aro/o J N255 torncl Patented Jan. 28, 1941 "UNITED. STATES PATENT- OFFICE 2,229,667 smvER-Lmn ALLOY Harold J. Ness, Bloomfield, N. J. Application April 30,- 1938, Serial No. 205,391

17 Claims. This invention relates to alloys and to the method of producing the same, and more particularly to bearing metal alloys composed primarily of silver and lead. ,5 In the development of greater power and higher speeds in internal combustion engines for automobile and aircraft and in other machines, it has been necessary to employ bearing alloys which, at the elevated temperatures obtained in such machines, have mechanical properties, such as compressive strength, tensile strength and resistance to pounding, exceeding those of the usual whitemetal bearing allows. Alloys of copper and lead have been employed to some extent to replace tin-base alloysfor such service.

In a copending application Serial No. 193,277,.

tory by itself, as a bearing metal, has bearing properties far superior to copper. Since, however, the solubility of lead in silver in the solid state is negligible, it has not been possible, heretofore, to incorporate more than about 4% of lead in silver. Any additional lead added to the silver merely segregates in relatively large masses in the silver.

One of the objects'of the present invention is to eliminate the difficulties which have heretofore accompanied the production of such silverlead alloys. J

Another object is to provide a method of, producing silver-lead alloys in which agitation of the molten mass is obtained without mechanical stirring.

Another object is to provide such a method in which the agitation of the mass continues in the mass.

Another object'is to provide a method of making silver-lead alloys in which the lead will be firmly bonded to the silver and retained in a finely divided state in a continuous network or matrix of the silver.

Still another object is to provide such a method which will result in a uniform dispersion and Another object is to provide a simple, e conomi-- cal and reliable method of producing silver-lead alloys.

A still further object is to produce a silver-lead 00 alloy of strong physical properties and high antimold and up to the time of solidification of the friction and anti-scoring properties at either low or elevated temperatures. Y

Another object is to provide a silverlead bearing metal alloy in which sweating out of the lead at high operating temperatures and/or pres-- sures will not occur. I

Another object is to produce silver-lead alloys having lead contents up to 70% or more.

Another object is to produce an oxygen free alloy of silver and lead. Another object is to produce silver-lead alloys in which the lead is contained within the grain boundaries of the silver.

Another object is to provide a process of making silver alloys in which the molten alloy will be free from dross.

Other objects and advantages will hereinafter appear.

The present invention is-based, in part, upon the discovery that when lithium is introduced into a silver-lead alloy, and precautions are observed to maintain some lithium in the alloy, a

natural circulation or agitation of the molten agitation or automatic stirring action continues,"

after it has once started, until actual solidification of the alloy upon cooling. As a consequence,

, no opportunity is afforded for the ingredients to segregate. Morever, the agitation is very rapid, apparently occurring simultaneously throughout the entire molten mass sothat a thorough stirring of the molten mass results and, upon solidification thereof, the lead of the alloy is retained in a uniform and finely subdivided form within a silver matrix and is intimately bonded thereto.

- The lithium, when maintained inv a small proportion in the silver-lead mixture by regulated conditions ofoperation, the nature of which will hereinafter appear, produces in the molten mass as the temperature thereof is raised to above a certain temperature, a violent agitation which is accompanied by considerable surface turbulence and a crackling and simmering sound. The temperature at which this agitation commences varies with the proportion of the silver and lead. in the mixture and is readily determined, for any particular mixture, from observation of the'surface of themass, since the agitation of the mass is pronounced and unmistakable. The proportion of lithium required to efiect this stirring or agitating action is very small, very definite and active-agitation and uniform and fine dispersion of the lead having been obtained in the produc-v tion of silver-lead alloys, which upon analysis showed only a trace of lithium.

The lithium may be introduced into .the' mix-' ture in a variety of ways, as for example, alloyed withthe lead, alloyedwith the silver, in a solid metallic state either alone or wrapped in other metals, such as silver, copper, lead, etc., or asa vapor. I

While the method of introducing lithium into the molten mass as an alloy of either lead or silver is highly satisfactory, this is not necessary since the pure lithium may be added directly to the fused mixture of lead and silver, either by dropping solid pieces of lithium therein or by packing it in a crucible which is inverted and pushed down into the molten mass.

Since the quantity of lithium required is extremely small, it may be added as a vapor, merely by creating a. lithium vapor or atmosphere about the crucible; in any suitable manner, as for instance, by liberation of lithium frorn the lining of a furnace, which has been previously treated with lithium, as will hereinafter appear, or by introducing lithium metal or a compound thereof into the furnace atmosphere. The pouring may be done as soon as the agitation begins, although the quality and fineness of dispersion is not affected by continued heating since no dross occurs, even at high temperatures. Due to the inherent agitation of the molten mass with continued heating thereof, it may be readily cast from holding furnaces, which are generally used when producing die or permanent mold castings.

The molten mass, as it cools in. the molds, continues to agitate until stopped by solidification thereof. There is, therefore, no opportunity for segregation of either the lead or silver to occur. The molten mass is very dense and therefore there is little or no shrinkage during solidification. 1

It has been found, however, that the mere addition of lithium to the charge does not result in the stirring action described nor in the production of a fine grain and uniformly distributed;

structure, but in addition it is necessary to prevent, at least in part, loss of lithium from the charge or to conduct the process under conditions which will supply additional lithium to the vapor at a high temperature so that the lining will take up a portion of the lithium with which it is treated. The furnace linings thus treated will remain ina suitable lithiated condition for a considerable period, when not in use, and may be used for melting lithium containing silver alloysrepeatedly without reconditioning. This conditioning of the furnace lining, it is believed, results in a saturation or near saturation of the lining by the lithium or acompound thereof whereby the lining, upon subsequent melting of a charge, is not free to readilyabsorb additional lithium from the charge, but on the contrary liberates lithium to produce a lithium containing atmosphere over and possibly through the crucible, which atmosphere retards the escape of lithium from the charge. If desired, this lithium containing atmosphere may comprise the sole means of introducing the lithium into the charge, excellent results being obtainable by merely heating a pure silver and lead mixture. in a furnace having a lithium treated lining. Sumcient silver X100;

lithium is absorbed into the charge, from the lithium containing atmosphere produced from heating of the lining, to cause pronounced agitation of the molten mass and a fine and uniform distribution of the lead in the silver upon casting. This method of introducing the lithium into the melt is, in some respects, a preferred one since in the resulting castings the lead is extremely well distributed and finely divided.

The amount of lead which may be combined with silver while retained in a uniformly dispersed and finely divided state therein may vary up to about 70% lead or even higher. ,When silver, lead and lithium alone are employedor even if small amounts of such-metals as tin, copper and zinc are added, the silver 'or silver and other alloying components, if present, form a con-.

tinuous matrix or cellular network in which the lead is dispersed as discontinuous'finely divided particles, firmly bonded to the silver, acondition which produces high tensile and compressive strength. and resistance to pounding. The grain structure of the silver-lead'alloys produced in accordance with the process. herein described assumes the grain structure of oxygen free silver with the lead finely divided and highly dispersed within the individual silver grains, each silver grain including a countless number of separate lead particles. The'alloy being produced in a lithium containing atmosphere, it is completely oxygen free and has not only grain'formation but physical properties comparable to oxygen free silver.

The number of lead particles appearing in a cross section of the silver-lead alloy of the present invention is enormous and for an alloy containing 40% lead, it may vary from, say, 50,000 to several hundred thousand per square inch and the average size of a lead particle may be of the order of magnitude of square inches or smaller although the present invention is not limited to such particle size or distribution range, the foregoing figures being given only by way of example. The presence of-lithium in the alloy in addition to its dispersing action, toughens and strengthens both the silver and lead of the alloy and increases their tensile strength, thereby further enhancing the physical properties of the alloy. Moreover, due to the encasement of the individual lead particles within the silver grains,

. the lead will not sweat out under operating contremely uniform and finely divided'whether the .casting is permitted to cool fast or slowly and therefore it may be readily cast in steel, iron or dry sand molds. substantially chemically inert to lithium. With the presentprocessthe pouring can be conducted directly through the air without oxygen absorption due to the presence of the lithium in the melt, which appears to form a protective atmosphere about the moltenstream;

In order that the invention may be more clearly understood reference will now be had to the accompanying drawings, in which:

Fig. 1 is a vertical section'of a gas fired furnace suitable for the production of a silver-lead-lithium alloy; i

Fig. 24s a photomicrograph of a silver-leadiithium alloy containing substantially 10% lead and less than 1% .of lithium, with the remainder The sand employed should be pound of lithium,

lithium alloy containing substantially 70% lead and less than 1% of lithium, with the remainder silver X100; and

Fig, 4' shows means for introducing lithium compounds into the furnace of Fig. 1.

Referring to Fig. '1, I have shown a gas'fired furnace of conventional construction having a. refractory lining in within which is contained a crucible II, which may be of graphite, preferably lined with a refractory cement such as sillimanite to prevent reaction of the lithium therewith. A refractory cover *-plate I! having a suitable gas vent I3 is provided. Gas and air are admitted through aburner ring l4 surrounding the furnace, the gas being supplied through a conduit l5 and the air through aconduit IS. A blower 11, shown diagrammatically, and suitable valves (not shown) control the amount of air and gas supplied. In order to assist in maintaining an adequate supply of lithium vapor in the furnace above and around the crucible, the incoming air is caused to flow through a chamber l8 containing solid lithium particles or a powdered comcontained between metal screens IS. A bypass sleeve permits airto be introduced independently of the lithium chamber l8, if desired. Sufficient lithium either in elemental or oxide form is carried into the furnace with the air and vaporized therein, to produce a strong lithium spectral line when the gasesescaping from the furnace are viewed with a spec troscope, thus denoting the presence of lithium in the furnace gases in an elemental form and in an ionized condition, Experimental data indicates that the presence of lithium'in the elemental form is due, at least in part, to thereducing action of carbon monoxide gas in the furnace on lithium oxide, under the-conditions prevailing in the furnace. A furnace lining which has been found suitable consists of silica. 65%, alumina and the remainder ignition loss, 1. e., moisture and other volatile material, and small percentages of iron, titanium, lime-and magnesia.

The lining is treated by heating lithium in the furnace for a substantial period at a, temperature sufiicient to cause formation of a strong lithium vapor in the furnace as previously described. The lithium for treating the' lining may be introduced into the furnace, as a charge, through the top thereof, although sufilcient lithium can be carried into the furnace with the air flowing through the, chamber l8, to effectively treat the lining. .The sufliciency of the amount of lithium or lithium compound taken up by the lining can be determined by melting a charge of silver and lead therein. If upon melting of such a charge an agitation of the mass occurs, the lining is sufiiciently treated for a successful carrying out of the present process. If no agitation occurs, it is necessary to retreat the furnace lining in the same manner. The treatment can also be effected by applying to the lining a compound of lithium, such as lithium chloride, thereon.

The compound may be applied as awater soluamount thereof to be carried in by the air flowcompound or in colloidal suspension.

. f ing through the lithium containing chamber l8,

although this is not necessary since repeated heats can be made with a single treatment of the furnace lining.

As previously stated, the charge is heated in 5 the lithium vapor containing atmosphere until circulation occurs after which it may be poured immediately or the heating may be continued for long periods until it is convenient or desirable to D0111;

The alloy may be used for coating backing strips, such as steel, in the same manner disclosed for coating such strips with a copper-lead alloy in my copending application Serial No, 193,278 filed March 1, 1938. i r

Referring next to Figs. Z'and 3 of-the drawings, it will be noted that the lead, indicated by the dark areas, is dispersed very uniformly throughout the silver and is in an extremely fine state of subdivision, whether the lead is present in a 20 low percentage (10% in Fig. 2) or a high percentage- (70% in Fig. 3). The dark or lead particles, it will 'be noted-,-are extremely fine, substantially uniformly distributed and entirely surrounded by the silver, 'thusindicating a continuous silver matrix with the lead particles-dis persed directly in-the silver grains.

As previously stated, the vcomposition of the silver and lead content of the alloy may vary within wide limits while maintaining the advan-- tages of fine structure and uniform dispersion. The relative amounts of lead and silver will be varied in accordance with the service to be performed by the alloy.

It is believed that the active portion of the furnace atmosphere consists of lithium in the metallic state and that the lithium in the silverlead alloy is also in a metallic state although these facts are difllcult of determination audit is possible that under the conditions prevailing in the-furnace some active compound of lithium may be formed which effects the stirring action and imparts other beneficial characteristics to the alloy. The term lithium" is employed in the claims, therefore, to include either metallic lithium or lithium in whatever other form it may occur in the furnace atmosphere and in the alloy.

In place of introducing metallic lithium into the furnace, "a compound thereof, such as the chloride, fluoride, hydroxide, oxide, or carbonate of lithium, or lithium containing ores such as spo umene oramblygonite, or mixtures thereof, may be used. The lithiated atmosphere may be provided in a gas fired furnace, for instance,

by introducing a small quantity of, a compound -oflithium in powdered form, into the air or gas dered lithium compounds directly into the furnace or into the air or gas conduit extending thereinto is fully disclosed in my copending application Serial No. 143,410, filed May 19, 1937, and entitled Injecting apparatus. fired furnace the lithium compounds may be added directly to the oil. either as an oil soluble A process. of treating fuel .oil with lithium compounds is dis closed in my copending application Serial No. 154,203, filed July 17,1937, and'entitled Promo;- tion of combustion." In-the' case of electric fur-' naces the lithium compounds may'be mixed with powdered carbon, such as graphite, and the mixtureblown in .a fine spray into the furnace in the 16 In an on 05 inafter appear. The desired rate or frequency of introduction of the lithium compound may be' determined by spectroscopic examination of the gases escaping from the furnace.

The use of the lithium compounds in place of metallic lithium simplifies the problem of introducing the lithium into the furnace, since the suitable compounds are more stable and more readily handled. Moreover, such compounds are normally in a pulverulent state and the amount to be introduced can be readily metered and introduced by simple, reliable and inexpensive apparatus Lithium metal, moreover, is several times as expensive as the compounds for equivalent amounts of lithium, since the metal must be produced from the compound by an expensive and involved process. Also, it has been found, when using a salt, such as lithium carbonate, introduced either continuously or at intervals throughout the process, that suitable results may be obtained in a new or unseasoned furnace, although with continued use a finer subdivision'of the lead in the silver is obtained.

In Fig. 4 I have shown apparatus for cona melting furnace to provide a satisfactory atmosphere in which to produce a silver-lead alloy. In this figure the furnace is provided with a supply of a lithium compound, such as the carbonate, by an-injecting apparatus comprising a-cylindrical container 22 of transparent material, resting in a base casting 23 and having an upper closure member 24, both carried by a vertical support 25 mounted on a base 26'. Extending into the chamber 22, through a stufiing gland 21, is a shaft 28 journaled in bearings 29 and 30. The shaft is revolved by a motor 3! through a belt 32. Mounted on the shaft 28 are several sets of blades .33a t.o 33c of decreasing length and increasing pitch. The lower blades 33a are integral with a collar 36 forming a labyrinth seal with a gland closure plate 35. A stack 36 extends downwardly from the top plate 24 and surrounds the upper blades 33c. When the container 22 is provided with a supply of a.

powdered lithium compound, the blades 33a to 33c agitate the compound, raising it in a cloud up through the stack 36, the powder then falling down around the outside of the stack.

A combustible mixture of air and gas under pressure is supplied by a blower 31 also driven by the motor 31 by a belt 38, the air and gas mixture being'conducted by a conduit 39 into a gas cleaner 40 from 'whence the stream is diverted, part passing by conduit 4| .into the base of the container22, and part being conducted by conduit 42 to. a Venturi 'nozzle 43, having communication with the container 22 by a conduit 44; The air and gas mixture directed into the container 22 becomes laden with the tinuously introducing .lithium compounds into conduits 4i and 42 control the quantity of gas mixture and compound blown into the furnace.

' By supplying a gas and air mixture to the blower,

the ratio of gas and air in the furnace is not disturbed by the fluid medium employed to convey. the powders into the furnace. In place of the mixture of air .and gas, either air alone or gas alone may be employed to convey the powdered compound into the furnace, although the use of air alone tends to create localized hot zones in the furnace and somewhat detracts from the efliciency of the action of the lithium compound in such zones. Excessive moisture in the air, such as results from high compression of the same, or in the gas employed, also detrimentally affects the action ofthe lithium compound and the maintenance of the proper lithiated atmosphere within the furnace, and should be avoided.

The lithium compound so introduced produces in the silver-lead charge a natural circulation or agitation of the molten mass in the same manner as does metallic lithium.

The furnace lining may be composed largely of silicon oxide and when such a lining is employed, the lithium compounds may be introduced into the furnace at spaced intervals since the lining serves as a-storage and liberating medium whereby the desired atmosphere is maintained within the furnace between the periods of introduction of the lithium compound. It is not necessary, however, that the 1ining be of the burned brick of low permeability and low porosity containing approximately silica and 44% alumina, having a bulk density of about 1.2- oz. per cu. inch and a fusion point of about 3200 F. A cement for bonding the bricks, such as sillimanite, cyanite, andulusite, and mullite, analyzing as follows: 'silica 38.07%, alumina 56.63%, titanium 1.14%, iron oxide 33%, ignition loss 2.78% and the remainder moisture and other impurities, has been found suitable. This cement may also be used as a coating for the bricks after their assembly in the furnace. In general, cellular insulating brick or moulded or tamped-in linings have not been as satisfactory as linings built up from hard burned refractories, apparently due to the high permeability thereof or the reaction of the lithium or-oneof its compounds with the binder employed in such moulded linings. Linings, hearths and other refractory parts of the furnace consisting of or containing silicon carbide have been found to react detrimentally with the lithium" of the atmosphere, due possibly to the nature of the binder commonly employed, causing breakdown of the refractory and to some extent, reduction in the efficiency of the lithium containing atmosphere for its intended purpose.

I prefer to introduce the lithium metal; or

- compound into the furnace mixed with the air or fuel since this insures that it will pass through the hottest part of the flame, which is at a temperature several hundred degrees above the average furnace temperature. Hence the lithium or its compound will be more readily converted into a vapor. v

- Under the conditions prevailing in the furnace the lithium compound, or at least a por-' tion thereof, breaks down liberating free metallic lithium. The reaction is apparently first the formation of lithium oxide .which reacts with carbon monoxide as follows:

The lithium is thus freed to combine with the oxygen of the furnace or any oxygen that may be included in the metal being heated and the i mally broken down to lithium oxide liberating carbon dioxide. The reaction is then repeated 1 with the lithium oxide so formed. As will be noted from the reactions set forth above each particle of lithium present in the furnace goes through the above cycle repeatedly; with it ultimately escapes with the furnace gases and it is .necessary, therefore, to supply additional lithium or lithium compounds only to replace that so escaping and that deposited on the material being heated or on parts of the furnace.

In electric furnaces or in combustion furnaces in which the chargeis in a container and not in direct contact with the furnace gases, I add to the furnace or to the container, carbon monoxide or a supply of carbon and oxygen from which carbon monoxide-may be generated, in order to obtain a reduction of the lithium oxide in accordance with the foregoing equation.

ments as nickel, copper, tin, zinc and other suitable alloying elements. as previously indicated,

may be included in minor amounts in the composition to produce desirable variations in the physical properties of the alloy. and such alloys of. silver are intended to be included under the term silver when usedin the claims.

The term alloy is used throughout the speciiicatiori and in the claims, in a broad sense, to include a solid solution, mechanical mixture or other form of combination of the various elements thereof. I

It is to be understood that these particular ex- 45 amples given of silver-lead-lithium alloys and the procedure for producing the same are to be considered in an illustrative sense and not as delining the limits-of the invention.

What I claim is: '4 Y 1. The method of forming an alloy composed at least in part of silver and lead which comprises heating the constituents of the alloy together in a molten state in the presence of lithium until agitation of the molten metal occurs.

- 2. The method of forming an alloy compose at least in part of silver and lead which comprises heating theconstituents of the alloy together in a molten state in the presence of lithium until agitation of the molten metal occurs and pouring the'molten mass into molds while agitae tion is continuing. I

3. The method of producing agitationv in .an alloy composed of silver and lead which comprises heating the metals together in a molten state with lithium. and, simultaneously providing a lithium containing atmosphere about the" molten metal.

4. The method of forming an alloy composed at least in part of silver and lead which comprises heating together silver. and lead in a molten statein the. presence of lithium vapor until agitation of the metal occurs.

15. The method of producing alloys composed at least inpart of silver and lead which comreprises heating the metals together in a molten lithium carbonate of the above reaction is ther-.

It is to be understood that silver alloys may be used in place oi pure silver and that such elelithium carbonate.

state, agitating the molten mass and continuing the agitation during cooling and up to the point of solidification of the alloy, the agitation being produced by the presence of lithium in contact with the melt. a 5

6. The method of producing silver-lead-lithium alloys which comprises melting together silver, I lead and lithium to a temperature at which agitation of the mass occurs and preventing escape of at least part of the lithium from the molten metal.

7. The method of producing silver-lead-lithi anti-friction articles comprising melting silver, lead and lithium together at a temperature sufficient to produce agitation of the molten metal, casting the metal into ingots, remelting the same as desired to a temperature sufllcient to cause reagitation and recasting the same into said articles.

8. The method of producing anti-friction al-. loys, comprising heating silver and lead in 'a molten state together with a fractional per cent of lithium and minor proportions of iron, nickel, silicon or tin, until agitation of the molten metal occurs.

9. A silver-lead-lithium alloy containing 5% to 70% lead, not over about 1% of lithium and substantially all of the remainder silver, the lithium being present in suflicient amount to render the alloy substantially homogeneous. I

10. A silver-lead-lithium alloy containing 5% to 70% lead in finely divided form, not over about 1% of lithium and substantially all the remainder silver, the lithium being present in suflicient amount to cause substantially uniform dispersion of the lead in the silver.

11. The method of producing silver-lead alloys comprising heating said silver and lead in a molten state in -a furnace, and providing lithium in the furnace atmosphere during the heating of the alloy.

12. The method of-producing agitation in an alloy composed of silver and lead which comprises heating the metals together ,in a molten state and providing an atmosphere about the molten metals which exhibits a lithium spectral line.

1'3. The'method of forming an alloy composed at least in part of silver and lead which com prises heating together in a molten state silver and lead in an atmosphere which exhibits a lithium produced by the decomposition of a lithium halide. v 1

producing a SflVIl-hflde a 16. The method of lithium alloy comprising heating silver and lead in a molten state in an atmosphere containing lithium produced by the decomposition of 17. A silver-lead-lithium alloy containing from .5% to 70% lead, detectable amounts upto 1% of lithium and substantially all of "the remainder silver, thelead being finely distributed within

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