Process and composition for metallizing aluminum with another metal

July 9, 1963 s. τ. ross etal 3,096,567

PROCESS AND COMPOSITION FOR METALLIZING

ALUMINUM WITH ANOTHER METAL

Filed Nov. 20, 1959

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3,096,567

Patented July 9, 1963

United States Patent Office

3,096,567

PROCESS AND COMPOSITION FOR METALLIZING ALUMINUM WITH ANOTHER METAL

Stuart T. Ross, Costa Mesa, Calif., and Henry M. Luehmann, Mount Clemens, Mich., assignors to Chrysler Corporation, Highland Park, Mich., a corporation of Delaware

Filed Nov. 20, 1959, Ser. No. 854,394 23 Claims. (Cl. 29—155.5)

This invention relates to the metalizing of a base metal with a second metal electronegative with respect to the base metal by thermal replacement procedures and particularly concerns the copperizing or silverizing of aluminum by such method.

Aluminum is used widely in many forms for commercial products but there are applications where marked advantages would be secured were the aluminum faced with another metal over part or all of its surface area. For example, in the field of heat exchangers such as automotive radiators, aluminum strips and sheets could be used for making the tubes, fins, tanks and headers of such a structure in place of copper or brass, but aluminum per se is difficult to solder. The provision of a copper layer would enable soft soldering of such structures.

Aluminum is also extensively used for electrical parts but difficulities are encountered where soft soldered electrical connections must be made. This is especially the case where electrical connections must be made to terminals on thin aluminum sheet stock as in the case of coils made from thin aluminum sheets or condensers made therefrom. The same is true where heavier stock is to be used for electrical current transmission apparatus such as bus bars and bus ducts. In examples of this kind it would be a great advantage to provide the aluminum with a facing of another metal such as copper or silver to which electrical connections could be made, as by soft soldering, without difficulty.

Insofar as we are aware, silver has not heretofore been successfully applied to aluminum by thermal replacement or other methods and known processes for applying copper to aluminum have not been entirely satisfactory for the above purposes. Thus electroplating of copper on aluminum requires special and expensive procedures and the direct fusion of loose dry copper salts on aluminum has not been too practical. Moreover, Patent No. 2,737,463 which is a joint patent of one Lawton and one of the present applicants, describes a method by which aluminum is treated with cuprous chloride carried to the aluminum suspended in a paste of rosin plasticized with a terphenyl-type compound after which the coating is heated at 300° F. to drive off excess plasticizer and then at 750° F. to form a copper coating. In working with this patented process, it is found to have many disadvantages. Firstly, if as stated in the patent, the composition is used at temperatures above 90° F. the suspension will break or tend to break and cause an uneven distribution of the copper salt. Moreover, the vehicle does not air dry readily but must be heated to around 300° F. to avoid tackiness and to drive off excess plasticizer and hence breaking of the suspension will occur in this heating. Furthermore, such heating leaves a hard brittle coating that lacks flexibility. The composition is therefore difficult to utilize with material that must be coiled or uncoiled in processing or that must be fed over rollers. Moreover, unless heat treated as stated, the coating may be wiped off or smeared in handling by an operator or on the apparatus by which the material is processed.

The process was also believed to require rosin as a flux for the copper. The rosn-terphenyl composition readily absorbs moisture under humid atmospheric conditions. Such facilitates the bringing of moisture to the copper salt and to the copper salt aluminum interface, with a resultant chemical reaction at room temperature reducing the cuprous chloride to finely divided non-adherent copper particles precluding the formation of a satisfactory 5 copper layer by thermal replacement now contemplated. According to the present invention, the metalizing of metallic aluminum surfaces with a second metal by the use of metallic salts, particularly cuprous chloride and/or silver chloride is effectively accomplished by thermal re10 placement reaction of the salt with the natural or synthetic oxides of the aluminum and with the metallic aluminum per se, without the need for supplementary fluxes as in the above patent and with the use of a minimum quantity of metal chloride salt. Moreover, the invention avoids 15 the disadvantages noted above with respect to the salt carrier by employing liquid carriers for the salts in the form of quick drying resin-solvent compositions providing relatively stable suspensions that dry to a flexible nontacky state with or without extraneous heat.

By our invention the metal salt particles which will be cuprous chloride when a copper coating is desired and a mixture of silver chloride and cuprous chloride when a silver coating is desired, are suspended in the resinsolvent vehicle which coats them with a film of the com25 position which when dry resists the penetration of moisture. The metal salts will be in sufficient amount to form a slurry. The resin-salt-solvent composition is then suitably applied as by dipping, printing, brushing or spraying to the base aluminum where it quickly dries to a non30 tacky, flexible state with or without supplemental heating.

The resin securely bonds the metal salt particles to the aluminum to facilitate further processing.

The coated base is now heated to a temperature sufficient to melt the metal salt and volatilize the vehicle. 35 The melted salt then acts upon the base aluminum to form thereon a layer of its metal.

Although we cannot depict with certainty the action belived to occur when this layer is formed and we do not wish to be bound to any specific theory of operation, 40 we believe based upon our observations and thermodynamic considerations that a complex action involving a plurality of substantially simultaneously occurring reactions takes place to which more specific reference will now be made.

Thus when the metal salt is cuprous chloride, it is believed to act as a flux, reacting when molten, with the thin surface film of aluminum oxide (A1₂O₃) always present on the base, in an endothermic reaction producing volatile aluminum chloride (Al₂Cl_e) and non-adher₅₀ ent copper oxide (Cu₂O), the reaction being represented as follows:

(1) heat

AI₂O₃ + 3Cu₂C1i ----» AlsClrf + 3Cu₂O

From a fluxing theory it is also believed feasible that 55 some dissolution of the aluminum oxide may occur in the presence of the molten cuprous chloride and such may even occur in lieu of the stated reaction or as indicated in combination therewith.

With aluminum oxide (A1₂O₃) being removed expos60 ing nascent aluminum to the action of the molten cuprous chloride blanket, two additional reactions exothermic in character, and self-sustaining are then believed to occur. According to the first, the cuprous chloride will react with the oxide free aluminum to form nascent copper 65 and additional volatile aluminum chloride (AI₃C1₆) which volatilizes from the reaction zone. This reaction is represented by the following:

(2) 2A1+3Cu₂C1₂-&gt;6Cu+AI₂C1₆7 ~θ This nascent copper readily alloys with the oxide free aluminum and thereby decreases the amount of oxide free aluminum available for further reaction.

3,096,567

According to the second reaction, the oxide free aluminum is also believed to readily reduce the copper oxide (Cu₂O) formed in the initial reaction to additional nascent copper and aluminum oxide (A1₂O₃). This reaction is represented by the following: 5 (3) 2A1+3Cu₂O-&gt;A1₂O₃+6Cu

This additional free copper also readily alloys with oxide free aluminum thus further reducing the amount of oxide free aluminum available for reaction. The aluminum io oxide (A1₂O₃) formed in the latter reaction is again reduced by cuprous chloride (Cu₂Cl₂) to start the above cycle all over again.

The above three reactions are believed to occur when a copper coating is desired and when the cuprous chlo- 15 ride is present in sufficient amount to enter into the initial reaction and with an excess to enter into the further exothermic replacement reaction with the oxide free aluminum whereby some of the aluminum combines with the chloride of the cuprous chloride salt and is released 20 as aluminum chloride gas and the aluminum thus used up is replaced by nascent copper from the salt which forms a smooth continuous layer of copper over the aluminum surface in the reaction zone.

The three reactions can occur simultaneously with the 25 initial removal of aluminum oxide (A1₂O₃) by reaction with the cuprous chloride (Cu₂Cl₂). The reactions will terminate when there is a reduction in temperature to below the melting point of the cuprous chloride.

If any of the aluminum chloride (A1₂C1₆) produced 30 in the above Reactions 1 and 2 does not escape from the treated surface a reaction which is the reverse of (1) may occur to reduce copper oxide (Cu₂O) still present therein to cuprous chloride (Cu₂Cl₂) which may be subsequently washed off. This reaction is represented as 35 follows:

(4) Al₂Cl₆+3Cu₂O-»Al₂O₃+3Cu₂Cl₂

The reactions will also cease when all of the oxide free surface aluminum has been alloyed with copper 40 since then there is no further oxide free aluminum for reaction with the cuprous chloride. With no oxide free aluminum available to reduce the copper oxide Reaction No. 3 also stops. The reactions will also cease when all of the cuprous chloride is used up. This will only occur ⁴⁵when there is an insufficiency of this salt present.

It has also been noted that during prolonged heating there is a tendency of the copper and aluminum to diffuse in a direction to more nearly satisfy equilibrium conditions. This is hereinafter explained in connection with ⁵⁰the character of interface produced by the invention.

When a silver coating is desired, the silver chloride salt acts in the same way as the cuprous chloride salt as evident from the following reactions:

(1) Al₂O₃+6AgCl-&gt; Al₂Cl_ej+3Ag₂O (2) 2Al+6AgCl-&gt;Al₂Cl_ef+6Ag

For reasons which will hereinafter appear, cuprous chloride is also preferably present with the silver chloride when a continuous silver coating is desired. The cuprous chloride clears away some of the aluminum oxide so as to make oxide free aluminum available for exothermic reaction with the silver chloride salt. The reactions in this instance may be represented by the following:

(1) (Surface oxides are dissolved)—

Al₂O34~3Cu₂Cl₂-&gt; Α1₂Ο₆) -|-3Cu₂O (2) (Surface aluminum metal is replaced by silver)— 70

2Al--6AgCl-s&gt; 6Ag+Al₂Cl_e-r (3) In addition a reaction similar to No. 3 above may occur producing some free copper, but to a negligible extent because of the limited use of cuprous chloride. 75

This free copper will immediately react with the excess of silver chloride present to reform cuprous chloride and deposit additional free silver to aid the coating process. This reaction explains the noticeable absence of any copper in the final silver coating.

The cuprous chloride in each instance acts as a flux for the aluminum in that it enables removal of the aluminum oxide. Once the aluminum oxide is removed, the surface of the aluminum base is essentially metallic aluminum and it reacts with more of the cuprous chloride or the silver chloride as the case may be, the molten layer of these salts serving during this period to exclude air. The resin vehicle during the period that heating is taking place is volatilized without leaving undesirable inert residues in the metal layer that is formed on the base. In some cases a deposit of soot or expended flux in the form of chlorides and oxides will form on the coating metal but such will readily wash off after the metal overlay is formed and the base is cooled. Some of it may be more readily removed by quenching the coated article in water. Moreover, it is believed that some of the carbon given off during volatilization of the vehicle will act as a catalyst in promoting the initial reaction between the metal chloride and aluminum oxide and lower the temperature essential to make the reaction goThe formed copper or silver layer as the case may be, will be metallurgically bonded at its interface with the aluminium base by an intermetallic zone comprising a compound or compounds of copper and aluminum, or silver and aluminum. Stated otherwise, the formed layer will comprise an intermetallic interface covered with a layer of copper or silver. In some cases depending upon the extent of heating of the treated base and the metal salt employed, there may be more than two layers, stratas, or zones, only the top layer being of the coating metal and the remainder intermetallic compounds of the coating metal and aluminum. The closer the zone is to the base aluminum, the higher will be its concentration of aluminum. With sufficient heating, all layers or zones including that at the top may be made of intermetallic character.

The total thickness of the nascent metallic layer and intermetallic zones will vary with the time and temperature of heating, the mode of heating, the character of the aluminum base, and the sufficiency of available metallic salt for continuing the thermal replacement reaction. Once melting of the salt takes place and a reaction with the aluminum oxide occurs which will be recognized by the aluminum chloride vapor given off, formation of the metal facing layer will immediately follow the time required for producing a complete silver layer being somewhat greater than that for copper.

For example, when obtaining a nascent copper layer, on .040 aluminum sheet heating of the salt coated base at a temperature between 800° to 900° F. for about three minutes will usually produce a total thickness between 0.00022 to 0.00042. Of this the nascent copper layer will have a thickness between about 0.00010” and 0.00020. On the other hand, when obtaining a copper facing on .005 aluminum foil, only between 5 to 10 seconds of heating between 750° and 800° F. is required to produce a complete coating but of somewhat lesser thickness. When employing the apparatus shown in the drawing and hereinafter described, the coated foil remains in the heating zone for about one minute.

In the case of a nascent silver layer on a .040 aluminum base heating the salt coated base at a temperature of between 780° F. to 820° F. for fifteen minutes will usually produce a silver layer of about 0.0004” in thickness and an intermetallic layer of less than 0.0001 thickness. However, complete silver coatings of lesser thickness have been possible in five minutes.

An object of our invention is therefore to provide a novel and satisfactory method for metallurgically coating

8,096,567 aluminum articles with a metal electronegative thereto by the use of a salt of such metal which when brought to a molten condition will react with the metallic aluminum of the article to form an overlay of the metal of the salt on the article. 5

Another object is to provide a method as in the preceding object wherein a metal salt, which may in certain cases be the metal overlay forming salt itself is present to flux the aluminum to remove oxides of aluminum and leave metallic aluminum for reaction with the metal overlay io forming salt.

An additional object is to provide articles of aluminum with an overlay of copper and/or silver metallurgically bonded thereto by an intermetallic compound of aluminum and such metal. 15

A further object is to provide a process as in the preceding objects wherein the overlay metal forming salt is cuprous chloride or silver chloride and the fluxing metal salt is cuprous chloride.

Still another object is to provide a process as in the preceding objects wherein the metal salts are applied to the article in finely divided particle form suspended in a quick drying resin-solvent vehicle requiring a minimum of the salt to produce a slurry thereof, and which vehicle will bond the particles to the surface of the aluminum article and air dry to a resin film, coating the metal salt particles and inhibiting the penetration of moisture, by absorption, therethrough to the salt particles.

A particular object is the provision of a method for copperizing and/or silverizing aluminum articles or material which comprises treating the aluminum article with a liquid slurry comprising finely divided cuprous and/or silver chloride suspended in a synthetic resin-solvent composition or vehicle containing the metal salt in a concentration sufficient to react with all of the aluminum oxides of the surface of the aluminum article in the portion to be coated and with enough of the metallic aluminum to produce a continuous metal facing comprising copper and/or silver upon the article metallurgically bonded thereto. 40

Another object is to provide a process in accordance with the preceding object whereby the stated result is obtained upon heating the article and coating to a temperature at least the melting temperature of the metal salt thereby effecting the reaction between metal salt and aluminum oxide and between metal salt and surface metallic aluminum and replacement of the latter with the metal of the salt.

Another object is to provide a process as in the preceding object wherein the resin is of a slow burning character containing little or no non-volatile inert substances such that the resin-solvent composition may be volatilized during heating without effecting undesirable contamination of the resultant metal facing with an ash residue causing discontinuity in the metal facing.

Still another object is to provide a process as in the preceding object where the resin consituent of the vehicle is a polybutyl methacrylate resin.

A particular object is to provide a process as in the preceding objects wherein the composition is applied to an aluminum base previously coated with a resin film compatible with and soluble in the resin-solvent sytsem of the suspending vehicle.

A specific object is to provide a method and composition for metalizing an aluminum base material with copper and/or silver wherein the aluminum is faced or coated with a liquid suspension of copper and/or silver chloride salt in a resin-solvent vehicle, which coating may be dried to a tough adherent flexible non-tacky layer facilitating handling and flexing of the coated base material without danger of smudging, smearing, cracking or other injury to the coated surface and which coating upon subsequent heating to a predetermined elevated temperature, at least the melting point of the metal salt, will cause the metal of the salt to be metallurgically bonded to the aluminum surface as a facing substantially free of permanent contamination by residue ash of the vehicle which is simultaneously volatilized during such heat treatment.

Another specific object is to provide a novel method and composition for treating a thin flexible aluminum strip or web of aluminum as by coating with a composition comprising a suspension of copper and/or silver chloride salt in a quick drying vehicle consisting essentially of a synthetic organic resin dissolved in a volatile solvent, which coating may be rapidly and substantially completely dried to a tough, non-tacky, flexible layer, facilitating coiling of the strip in closed or open coils and feeding of the coated strip with its dry coated side in contact with roller feed mechanisms or the feeding of the coated material in the form of individual pieces over a conveyor system, all without smearing or wiping off of the coating and/or cracking or peeling thereof during such process.

A further object is to provide an article, such as an 20 aluminum web and/or strip, treated as in the preceding object, and to heat treat the same at an elevated temperature to produce by thermal replacement reaction between the metal salt, aluminum oxide and aluminum metal of the base aluminum, a continuous layer of pure metal of 25 the salt and intermetallic compounds of the metal of the salt and aluminum, both metallurgically bonded to the aluminum in the area coated by the resin salt composition and to effect substantially complete volatilization of the constituents of the resin vehicle without undesirable 30 contamination of the metal layer by non-volatile ash residue of the vehicle.

Still another object is to provide a metallurgically bonded copper and/or silver facing layer on a pliable aluminum base strip and to thereafter subject the assembly 35 to rolling to effect a smooth finish on the facing metal and tempering of the strip to a predetermined degree of hardness.

A further specific object is to provide a novel method for providing aluminum strips with a metal facing adapted for soft soldering by conventional procedures.

Other objects and advantages of our invention will appear from the following description and from the drawing showing our invention for purposes of illustration and not limitation, applied to a continuous flexible strip of 45 aluminum to be provided with a metalizing facing on the opposite sides thereof.

In the drawing is a schematic showing of a form of apparatus for carrying out the process of our invention and whereby both faces of an aluminum foil strip may be 50 provided wtih a metallurgically bonded metallic facing such as of copper and/or silver by a continuous process.

In accordance with the present invention, the surface of an article or base member composed of aluminum and/ or an aluminum alloy predominantly of aluminum, such 55 as a web 10 of aluminum foil about 0.005 to 0.009 thick, and of any suitable width, fed from a supply roll 12 is coated with a composition such as a slurry of finely divided cuprous and/or silver chloride salt particles in suspension in a resin-solvent composition by passing the ^eo web 10 through a trough 14 or the like containing such a coating composition 16 and while holding the web submerged in the composition 16 by a roller 18. The slurry is preferably agitated during movement of the web by a conventional agitating device such as an electric stirrer ⁶⁵ or mixer 20 to maintain the metal salt in uniform suspension.

The coated web 10, in this instance coated on both sides by a layer 21 of the composition 16, is then and while still in motion subjected to air drying at room tem70 perature or is given accelerated drying by passage through a suitable drying device such as an air blower or oven 22, operating at a relatively low temperature, for instance a temperature below the boiling point of the solvent preferably 220° F. all without breaking the suspension or 75 causing bubbling of the vehicle. The drying will in either

3,096,567 case be sufficient to drive off the solvent and dry the coating to a non-tacky film 23 such that the coated web may be passed over a roller 24 with a coated side in contact therewith without smudging or smearing of the coating and metal salt. The unreacted coating 23 will at this time be firmly bonded to the aluminum web with the particles of metal salt held in contact with the aluminum by a film of resin and will have sufficient flexibility and strength to operate over the roller 24 without cracking, peeling, smudging or smearing.

When the coating composition is air dried, sufficient distance will be provided between the rollers 18 and 24 to make this possible. It will be understood that air drying may be aided by directing currents of dry air across the coated web 10, 21 and that such air currents may if desired, be above room temperature. Dry air is preferred in order to minimize absorption of moisture by the coating.

After passing around the roller 24, the web 10 with its dried resin bonded metal salt coating 23 is preferably passed downwardly through an oven or furnace 28, for instance an electric furnace controlled to provde a suitable elevated temperature, for instance above 750° F., sufficient to melt the metal salt of the coatings 23 and effect a reaction thereof with the surface oxides of the base aluminum web and with the metallic aluminum thereof whereby the oxide is removed and some of the metallic aluminum replaced by a layer of the metal of the salt. During this heating operation, the resin constituents of the coating composition are burned off and volatilized so as to leave a metallic coating 30 of copper and/or silver metallurgically bonded to the aluminum and substantially uncontaminated by the presence of inert ash residue but which may have some removable slag and unreacted salt at its surface which may be washed off. After leaving the furnace 28, the web 10 with its facing 30 of copper and/or silver is passed between suitable rubber drive rollers 31, the latter connected to a suitable power source such as a motor and reduction gear or pulleys (not shown) after which the metalized web is preferably passed over a pulley or roller 32 suspended by suitable spring means 34 from a support 36 and then into a wash tray 38 containing a shallow layer of water or other wetting and cleaning fluid 39, for instance a sodium chloride brine adapted to dissolve excess metal chloride salt and loosen and carry off soot or other residue. Here the metalized web 10—30 is fed under a roller 40 which holds it in the liquid and then passes between a pair of counter rotating stiff tampico cleaning brushes 42, 44 positioned above the roller 40 and above the level of any liquid in the tank 38. Obviously, the metalized web

10—30 will be wetted by the water or cleaning brine and brushed by the brushes 42, 44 to remove any stubborn surface dirt, salt, soot or slag deposited or formed in processing.

The brushes 42 and 44 are shielded on their upper segmental portions with suitable splash shields 46, 48 and secured on the shields 46, 48 above the brushes 42, 44 and in adjacency to the web 10—30 are suitable spray headers 50, 52 which direct a stream of water downwardly under pressure against the opposite sides of the metalized sheet to remove any final vestige of loose material. The water from the sprays runs into the tank 38 and drains from the tank with the cleaning liquid through the base outlet 54 into a sump 56 from which it may be pumped by a sump pump 58 through a filter (not shown) and if desired recirculated. The wetted sheet after leaving the final water spray may be air dried or passed through an oven 60 which may be supplied with suitable blower devices (not shown) and operated at a suitable temperature for instance 212° F. to 220° F. whereby any liquid on the web is rapidly evaporated.

From the oven 60 the dried web is passed over a further roller 62 and then fed to the bight of a rolling mill generally identified by the numeral 66 comprising a plu8 rality of sets of suitably driven rollers whereby the web, now substantially dead soft, is treated to minimize or eliminate any roughening in the metal coating layer and produce a substantially smooth shiny surface, and restore 5 a suitable amount of temper and springiness lost during the heating operations. From the rolling mill 66 the web may be fed to a variable speed coiler device generally identified by the numeral 68 for winding into rolls of suitable size for subsequent use.

It will be understood that the foregoing apparatus may be modified to provide for coating of only a single face of the aluminum strip or only predetermined portions of one or both sides of the strip by suitable transfer roll or printing roll or other known applicator devices.

Moreover, the metallic salt coating composition may be applied over a facing of insulation material for example a film of polyurethane plastic or other resin compatible with the vehicle of the coating composition such that the vehicle and the metallic salt of the suspension may be 20 commingled with the resin of the film by the softening or dissolving action of the solvent of the vehicle or upon heating. The latter processing is desirable for example when making aluminum strips for electrical coils and providing the strips with copper and/or silver coated 25 ends or intermediate areas to which lead wires may be soft soldered. In such cases the coated portions or ends will preferably be placed in a swedging die or passed between rollers as aforesaid to provide sufficient strength to support a copper wire to be soldered to these portions.

Moreover, the above apparatus may include suitable conveyor mechanisms for carrying individual aluminum strips or pieces coated or to be coated with the copper and/or silver composition. These coatings may also be applied by dipping, brushing, spraying, trowelling or ex35 truding. It will also be evident that a previously metalized strip may be recoated to repair a previous coating. The vehicle for the metal chloride salts of the composition 16 is essentially a liquid resin-solvent system or medium capable of holding the chloride salt in suspension 40 during coating and drying and of dispersing the chloride salts uniformly along the surface of the aluminum base metal and of coating the particles of salt with resin so as to securely bond them to the surface to be treated until such a temperature of the coated base is reached that the 15 metal salt becomes molten and may react with the oxides and the metallic aluminum surface of the aluminum base.

The resin solvent system must be capable of quick drying in air at room temperature or with drying aids such as a 50 warm blast of dry air or mild heating by a furnace at temperatures below that which will cause bubbling of the coating and movement or precipitation of the salt away from the base metal. It has been found for example, that the coating composition is readily air dried in a dis55 tance of 12 feet when the web is moved at a speed of about one foot per minute.

As described above, the composition should dry to an adherent thin, tough, flexible film, non-tacky or sticky in character so as to avoid smudging or smearing of the 6b coating composition during handling or contact with conveying apparatus or when wound into coils.

During heating of the coated base material to the melting point of the metal chloride salt the resin binder will enable holding of the salt particles to the base to facilitate 65 initial reaction of the salt and aluminum oxide of the base material. It will slowly burn off during the removal of the aluminum oxide and the accompanying thermal replacement reaction between the metal salt and the metallic aluminum surface whereby a layer of the metal of the 70 salt is formed in metallurgically bonded relation to the aluminum base. By the time the complete metal layer is formed, the volatile constituents including solids of the vehicle will be driven off and depending upon the vehicle will leave a clean metal surface or one covered more 15 or less by deposits of soot, i.e. carbon, excess unreacted

3,096,567 salt and residual oxides (expended flux in the form of oxides) readily washable from the metal face. The vehicles we use will not contaminate the metallic layer with inert material such as silicates or other non-volatile constituents that will create discontinuity in the coating. 5

The resin binder of the vehicle will also protect the metal chloride salt from moisture and will preferably itself not serve as a carrier for substantial quantities of moisture such that premature chemical reaction with the aluminum base, at room temperature will not occur reduc- ] q ing the salt to metal particles without metallurgical bonding taking place.

The salt carrier or vehicle will preferably have a resin binder content of not less than about 2% to 3% by weight of the coating composition in order to provide 15 adequate binding power and coating of the individual metal chloride particles. Greater quantities may be used with increasing viscosity of the carrier and the deposit of larger quantities of surface residues upon heating. The latter is not objectionable since it protects the metal facing 20 from oxidation during cooling and will readily wash off. In general, an upper limit of about 20% is preferred by reason of cost.

The resin constituent will be organic in character and may be thermoplastic or thermosetting preferably the 25 former in view of the greater flexibility generally obtained from such resins. Moreover, the resin should preferably burn slowly at the temperatures at which the metal chloride salt melts, as a rapidly combustible resin will be apt to generate gases that will blow the metal chloride salt 30 off the aluminum surface or the resin may burn off before the salt has melted.

In processing, the base with its coating composition will preferably be heated at a temperature between 750° F. and 900° F., better still 850° F., which has been found 35 to be sufficient to melt the chloride salt and effect the replacement reaction and sufficient to pyrolize the dried resin film of the vehicle.

The preferred vehicle will be thermoplastic polybutyl methacrylate resin in an aromatic solvent such as toluene. 40 Such a vehicle readily forms a suspension upon stirring with the metal chloride salt particles and will rapidly dry to a film of the desired characteristics. Moreover, it will pyrolize to less than 0.02% non-volatile ash.

Other suitable synthetic resin-solvent vehicles are cellu- 45 lose esters and ethers such as cellulose acetate in ethyl or butyl alcohol or an ester such as ethyl acetate; cellulose acetate butyrate in ethyl acetate; ethyl cellulose in ethyl alcohol or ethyl acetate; polystyrene in toluene; a polyester such as the reaction product of phthallic anhy- βθ dride and ethylene glycol in methyl ethyl ketone or toluene; polyethylene in trichloroethylene; polymethylmethacrylate (Plexiglas) in methylethyl ketones; polyvinyl acetate in ethyl acetate; phenyl formaldehyde in butanol; urea formaldehyde in butanol; and melamine formalde- 5.5 hyde in butanol. It will be understood that other known solvents may be used with these resins.

The binder is dissolved in the solvent and the finely divided metal chloride salt particles are uniformly distributed throughout the vehicle by agitation which is con- co tinued until a uniform suspension is obtained, but the particles are not dissolved.

The metal chloride salts are preferably used in finely divided form preferably at least of a size to pass a —325 mesh screen. This assures better coverage of the base cs surface. It is found advantageous to mill the salt particles in the vehicle such increasing the fineness of the powdered chloride salt and assuring excellent wetting of each particle with the binder of the vehicle and still better coverage.

The concentration of cuprous chloride and/or silver chloride salt suspended in the vehicle for obtaining a copper or silver coating respectively should be sufficient to react with all of the surface aluminum oxide of the base aluminum to be copper or silver coated (stoichiom10 etric amount based on aluminum oxide) with an excess to effect thermal replacement thereafter with the surface metal aluminum of the base and form a complete facing of the metal.

A visual test that has proven practical is to employ a composition in which the concentration of metal salt suspended in the vehicle will be sufficient to produce by a single or multiple application, preferably one, a dried layer of salt that completely masks the aluminum surface to be treated. When the latter is obtained there is sufficient salt present for the reaction.

A salt concentration of at least about 30% by weight of the coating composition with an upper limit of 85% producing a layer of salt at least 0.0005 thick will usually accomplish the result with a single coating. The percentage will vary with the character of vehicle and particle size of the metal salt employed. The heating time with these concentrations will depend upon the character of base structure, the vehicle, and mode of heating. It will vary from about five seconds to one hour.

When preparing a coating composition for facing the aluminum with silver and silver chloride salt may in certain cases be used alone as when a continuous coating is not required. However, it is found best to mix a quantity of cuprous chloride along with the silver chloride content aforesaid to act as a flux on the aluminum oxide and to assure the obtaining of a continuous layer of silver. The amount of cuprous chloride required is sufficient to remove the aluminum oxide at the temperature of operation. Although as little as 5% of cuprous chloride can be employed for this purpose it is preferred to use an amount of cuprous chloride between about 20% to 50% by weight of the combined salts as the silver chloride particles are soft and lumpy and with the presence of a substantial amount of cuprous chloride it becomes possible to ball mill the mixture to reduce the silver chloride to a particle size, about —325 mesh, whereby it will have greater covering power. Ball milling assures greater uniformity in distribution of the two salts. Instead of mixing the cuprous chloride with the silver chloride as aforesaid the two salts may be first melted down in a ratio of about 4 to 5 parts by weight of cuprous chloride with 5 parts by weight of silver chloride to form a lower melting point eutectic which is then ground to a fine particle size in the order of —325 mesh and added to and preferably ball milled with additional silver chloride to produce a cuprous chloride concentration within the above limits preferably using at least about 40% by weight of the eutectic. Moreover, instead of using cuprous chloride in the above mixture of salts as the flux, alkali halides such as lithium chloride and/or lithium fluoride may be added up to about 10% by weight of the final mixture. The use of cuprous chloride is preferred since it does not require heating of the aluminum above a temperature of 700° F. to 900° F.

Silver chloride will not ordinarily operate alone as does cuprous chloride because upon heating to the melting point the silver chloride does not wet the aluminum surface sufficiently to form a continuous layer but draws up into typical cornucopia shaped beaded areas possibily because of high surface tension effects or because of repellant forces between dissimilar molecules. By using a mixture as above, the surface oxides of aluminum are dissolved by reaction with the cuprous chloride and/or other additive fluxes aforesaid and the surface aluminum is then wetted by the silver chloride. Unless the cuprous chloride or other additive flux is used, one finds on cooling under the cornucopia shaped beads, areas which have been silver coated but which are not continuous as desired.

Although we have described the application of the coating composition to a web by passing through a tank containing this composition, it will be understood that the composition may be applied by other known methods such as brushing, dipping or spraying after which the

3,096,667 ¹¹ coating is dried and then heated to obtain the reaction and coating. Moreover, the heating temperature and time will vary with the precise composition of the coating; silver coatings usually requiring higher temperatures up to about 900° F. Usually at least about 750° F. will be required in all cases. Some eutectics may permit of a lower reaction temperature.

The character of the metal layer produced by the metal salt, especially in the case of copper coating, will be greatly dependent upon the temperature at which heating occurs and the period of heating. At high temperatures and with extensive heating there is a tendency once the outer metal layer is formed, for a multiple layer structure to be created wherein the outermost layer is constituted only of the metal of the salt and in which the underlying layer or layers are constituted of intermetallic compounds of the metal of the salt and the aluminum with the greatest concentration of aluminum being in the compound layer forming the interface between the coating and the aluminum base. By extensive heating all of the metal of the salt may be alloyed with aluminum with the concentration of aluminum, increasing from the exterior of the coated surface inwardly.

For example, when obtaining copper coated aluminum at temperatures between 750-900° F. the coating layer will comprise three strata (1) copper on the surface, (2) an intermediate alloy strata of Al₄Cu₉. and (3) a final alloy strata at the interface of Tl₂Cu. By sufficient heating diffusion of the elements can occur whereby all strata are composed of copper-aluminum alloy usually Al₂Cu.

It should be noted that the intermetallic compounds are poor conductors of heat and electric current so that where a better conductor than aluminum is desired as for high frequency electrical work it is desirable to maintain the pure copper or silver surface layer. Moreover, the thicker the Al₂Cu layer the more brittle will the structure be at the interface.

The following examples illustrate compositions which may be used in accordance with the present invention. These are not to be taken as limiting the invention. In all cases, the proportions given in the examples are parts by weight and unless other use stated the coating composition was made into a slurry:

Example 1

A vehicle was prepared containing 3 parts by weight of polybutyl methacrylate resin dissolved in 97 parts of toluene. Such a resin solvent vehicle can be made from a commercially available resin solvent composition such as Pierce &amp; Stevens Binder #9658 which is a resin-solvent composition containing about 21% by weight of solids consisting of polybutyl methacrylate and thinning this down with toluene to about a 3% solids composition. To 1975 milliliters of this vehicle is added 900 grams of cuprous chloride salt having a size to pass through a minus 325 mesh screen which is ball milled in a gallon size ball mill for about 72 hours in the vehicle to increase the fineness of the powdered salt and assure wetting of each particle with the resin binder of the vehicle. After milling, the composition is agitated to assure a suspension of the cuprous chloride particles in the vehicle. The composition will have a specific gravity of about 1.25' (30° Be.). The composition was then applied to an aluminum strip base by dipping and the composition air dried in place for 12 minutes after which the base with its coating was heated to a temperature between 750° F. to 800° F. for about one minute to effect a reaction between the cuprous chloride and aluminum oxide and metallic aluminum of the base to form a layer of copper metallurgically bonded by a copper-aluminum interface to the base aluminum. The reaction and metal layer formation will be recognized by the fusing of the salt and evoluation of white fumes of aluminum chloride. Usually a few seconds of heating at this temperature will suffice to produce the copper layer. After cooling, the copperized aluminum base was washed with a solution of sodium chloride brine and then with clean water and dried. A soft tsrip with a smooth clean copper coating was obtained free of inert residue.

The coated strip was then passed through a set of steel rolls and given a 20% reduction in thickness. This treatment produced a substantial amount of hardness in the strip while still retaining sufficient flexibility to be coiled. The copper facing was shiny.

Example 2

An aluminum foil strip was treated as in Example 1 using a composition prepared by mixing two pounds of cuprous chloride salt in one quart of a resin-solvent composition containing about 21% solids consisting of polybutyl methacrylate resin and obtainable commercially under the name of Pierce and Stevens B-9658 and diluting the mixture with toluene to a specific gravity of 1.25 (30° Be.) while ball milling to obtain a uniform mixture of the salt and vehicle in the form of a suspension. Similar good results were obtained.

Example 3

An aluminum foil strip was coated on one side with a suspension of one part by weight of cuprous chloride salt in two parts by weight of a vehicle prepared by mixing 10 parts by weight of butanol and one part by weight of resin-solvent composition containing 50% solids by weight consisting of melamine formaldehyde resin obtainable commercially under the name of Monsanto Chemical Company “Resimene No. 876.” The coated strip was dried by gentle heating and heated to a temperature between 750° F. to 800° F. to effect the reaction and metal layer formation. A satisfactory coating of copper without inert particle contamination was obtained.

Example 4

An aluminum foil strip was coated and heated as in Example 3 with the following composition to form a copper coating: one part by weight of cuprous chloride suspended in two parts by weight of an epoxy resinsolvent vehicle prepared by dissolving one part by weight of the solid condensation product of bisphenol A and spichlorohydrin (Shell Chemical Company “Epon 1009”) in 20 parts by weight of methyl ethyl ketone solvent. A satisfactory coating of copper without inert particle contamination was obtained.

Example 5

An aluminum foil strip was coated and heated as in Example 3 with the following composition to form a copper coating: one part by weight of cuprous chloride suspended in two parts by weight of a vinyl resin solution prepared by dissolving one part by weight of polyvinyl acetate resin (Bakelite “Ayaf”) in 20 parts by weight of methyl ethyl ketone. A satisfactory coating of copper was obtained without inert particle surface contamination.

Example 6

An aluminum foil strip was coated and heated as in Example 3 with the following composition to form a copper coating: one part by weight of cuprous chloride suspended in two parts by weight of a vinyl resin solution prepared by dissolving one part by weight of polyvinyl butyral resin in twenty (20) parts by weight of butyl alcohol.

Example 7

An aluminum foil strip was coated and heated as in Example 3 with the following composition to form a copper coating: one part by weight of cuprous chloride suspended in two parts of a polyamide resin (condensation product of a dibasic acid and diamine) solution prepared by dissolving one part by weight of polyamide resin (General Mills “Vcrsamide 900”) in a mixture of 20 parts by weight of ethyl alcohol and 20 parts by

3,006,567 ¹³ weight of isopropanol and reducing the liquid composition to 21 parts by weight of evaporation. The coating composition used here was in the form of a paste suspension.

Example 8

An aluminum foil strip was coated and heated as in Example 3 with the following composition: one part by weight of cuprous chloride suspended in two parts by weight of a polyamide resin (condensation product of a dibasic acid and a diamine) solution prepared by dissolving one part by weight of a polyamide resin (General Mills “Versamide 930”) in 20 parts by weight of isopropanol.

Example 9

An aluminum sheet was provided with a metallurgically bonded silver coating as follows: A mixture of 4 parts by weight of silver chloride and one part by weight of cuprous chloride was ball milled to a size passing a minus 325 mesh screen. This mixture was then suspended in one part by weight of a commercial resin-solvent vehicle (Pierce and Stevens No. 9571) comprising 3½ parts by weight of polybutyl methacrylate resin dissolved in 96½ parts of a suitable solvent such as a mixture of about 60% xylene, and 40% saturated hydrocarbons such as toluene. The base was then coated with the composition, air dried and then heated at a temperature of between 775° F. to 800° F. for a period of 5 to 15 minutes. The silver coating obtained was smooth, continuous and free of contaminating inert particles.

Example 10

A25 aluminum sheet was provided with a metallurgically bonded silver layer as follows; 10 parts by weight of silver chloride was added to and mixed with 8.3 parts by weight of cuprous chloride and heated at 550° F. to 600° F. for 12 hours to form a eutectic. The resulting mass was cooled and then ground to a size to pass a minus 325 mesh screen. To the ground material was then added sufficient silver chloride salt to obtain a mixture (about 30 parts by weight) containing about 80% by weight concentration of silver chloride. The salt mixture was then ball milled with 10 parts by weight of the polybutyl methacrylate resin-solvent referred to in Example 9 until a suspension of the salt in the vehicle was obtained. The composition was then applied to the base, air dried and heated for fifteen minutes at a temperture between 775° F. to 800° F. After cooling, the silverized aluminum base was washed with clean water and dried. A smooth continuous silver coating free of contaminating inert particles was obtained. Upon metallographic examination the nascent silver coating was found to be bonded to the aluminum base by an intermetallic compound comprising aluminum and silver. The nascent silver layer produced a thickness of about .004.

Example 11

An aluminum foil strip 0.002 thick for use in making aluminum foil coil alternator rotors was coated on both faces with an electrical insulating layer or film comprising a 30% solids polyurethane-phenolic resin composition in a solvent commercially available as “Carwin No. 8029” made by the Carwin Polymer Products, Inc. of North Haven, Conn. The solvent was evaporated and the resin coating dried by heat. The strip with its dried coating was then brush coated at predetermined spaced intervals adjacent edge portions of the strip and over small areas thereof with a cuprous chloride slurry prepared by suspending 43 parts by weight of cuprous chloride in 57 parts by weight of a commercially available resin-solvent vehicle, as used in Example 9 (3½ parts by weight of resin to 96½ parts of solvent) from which sufficient solvent was evaporated to produce a resin-solvent composition of 5% solids.

The brush coating was quickly air dried at room tem14 perature or by mild heating and then heated at 750“ F.845° F. long enough (about 7 seconds) to cause the salt to melt and coat the metallic aluminum underlying the polyurethane layer with copper by thermal replacement as described above. In this action the xylene-toluene solvent of the salt composition will dissolve the polyurethane-phenolic film and carry the cuprous chloride to the underlying metal where upon subsequent heating a thermal replacement action can occur to produce the copper layer. The copper facings were then wiped clean and sprayed with a mixture of 4 parts by weight of 30 tin-70 lead solder and 1 part by weight of a solder flux made up by mixing 7.2 parts by weight of ammonium chloride, 6.7 parts by weight of urea [CO(NH₂)₃], 44.5 parts by weight of Carbowax 4000 (a water soluble wax), and sufficient hexylene glycol (about 100 parts by weight) to form a sprayable mixture. Two pre-tinned copper wires were then soldered spaced apart in contact with the prepared copper faced aluminum surface by applying a heating iron to the assembly for about 7 seconds. The terminals thus made were brushed clean and a coating of polyurethane-phenolic resin sprayed on the uninsulated portion of the strip where the strip had been copper coated and soldered. The strip was then severed intermediate the two copper wires so as to form the beginning of one coil and the end of another.

Example 12

An aluminum transmission control valve was coated on its bearing surface by dripping in a suspension prepared from 908 parts by weight of cuprous chloride and 820 parts by weight of a commercial resin solvent vehicle such as used in Example 9, The coating was air dried and the valve then heated for 13 minutes at 750° F. to effect the thermal replacement reaction and formation of the copper layer. The valve was then washed in dilute ammonium chloride and then in clean water to remove all residue. The valve was tested in an experimental transmission known as the Chrysler A-323 transmission for 80 hours to 200° F., 122 hours to 300° F. and 45 hours to a temperature between 325° F. to 350° F. The valve operation was satisfactory. The coated surface was noted to be somewhat blackened after testing but this was attributed to action of oil addition agents.

Example 13

An alumium sheet of 2S aluminum was coated by dipping in a liquid slurry suspension of 70 parts by weight of cuprous chloride in 30 parts by weight of a resinsolvent vehicle such as used in Example 9. The coating was air dried with the aid of a heatlamp and then heated for 3 minutes at a temperature of 900° F. to produce a nascent copper coating on the base aluminum metallurgicaly bonded thereto by an intermetallic compound of copper and aluminum. The nascent copper layer had a thickness of between .00010 to .00020. The intermetailic layer was composed of two compounds of aluminum and copper, to wit, Al₄Cu_e and Al₂Cu. The thickness of this layer was between .00012 and .00022. The coated member was tested on a reciprocating wear test machine against an SAE 51100 bar member having a 17-21 microinch surface finish. The average scoring load in four runs was 570 p.s.i. whereas the average scoring load for uncoated 2S aluminum was 400 p.s.i. The surface finsh of the copper coated specimen remained good at the conclusion of testing.

Example 14

An aluminum foil strip was prepared as in Example 11 using in place of the polyurethane-phenolic resinsolvent composition, a resin-solvent composition composed of polyvinyl chloride in a suitable solvent such as toluene or methyl ethyl ketone and including a plasticizing agent such as polyvinyl acetate.

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Example 15

An aluminum foil strip was prepared as in either of Examples 11 and 14 but applying the base resin coating to only one side of the strip and applying the metal salt composition to such coated side.

From the foregoing description of our invention it will be seen that we have provided a novel process apparatus and composition for obtaining copper and/or silver coatings upon aluminum base materials and for the adaptation of aluminum foil for the production of electrical devices such as alternators wherein electrical terminals must be provided in electrical connection with the aluminum. It will be understood that various changes and modifications may be made without departing from the spirit and intents of our invention and all such modifications and changes as may come within the scope of the appended claims are contemplated.