CA1125629A - Multiple member clad metal products and methods of making the same - Google Patents

Abstract

ABSTRACT

A new clad metal product free from orange peel effect is provided consisting of a core having at least one layer each of copper and of aluminum coated aluminum alloy and at least one outer cladding layer of stainless steel on the layer of aluminum coated aluminum alloy.

Description

This inventlon relates to multiple member clad metal products and methods of making the same and particularly to a copper core aluminum and stainless steel clad product.
The use of laminated or clad metal products of three laminates or less is well recognized in the industry.
For example, stainless steel utensils with a copper bottom layer are well known as are also aluminum cored stainless s~eels.
These products have all been available for some time past and have been extensively used for production of cookware. One of the perennial problems with these prior art materials was and is an inability to get quick and even heat transfer over the entire pan area. These prior art materials are generally far superior to single layer metals but still far ~rom perfect.
This new product overcomes the basic problems o~ the prior art materials discussed above. It will more rapidly and evenly distribute heat. It eliminates the problem of unsightly oxidation of copper in the prior art copper clad stainlass steel utensils if used as a core. It provides for superior heat distribution as compared with prior art stainless steel clad aluminum. It combines all the unique thermal properties of copper and aluminum with the corrosion and abrasion resistance properties of stainless steel.
A unique and totally unexpected quality of the metal products of the invention of our earlier applications and of cooking utensils made from the product of those inventions are their property of reducing food adherence to the surface. All metal cooking utensils have been subject to the problem of sticking food stuffs to their surfaces when subject to heat.
Teflon Trademark of Du Pont Company for its polytetrafluor-ethylene resin and other synthetic coatings have been applied ~,~9~ r6~

to prevent this in the past. Uowever, the material of this invention is substantially the equal of Teflon coated utensils in preventing sticking.
Tests to determine the various thermal characteristics of many metals used by the utensil trade were conducted to determine any differences that may exist in their capability of transferring heat evenly. The following test procedures were used to evaluate materials for use in utensils: -A singular gas fired heat source similar to a Bunsen burner was placed at an equal distance under each utensil. The amount of gas available to the burner was constant. The flame ~-~; impinged directly at the center of the outside bottom of the utensil causing a 3" circular contact. Thermocouples were attached on the inside center of the pan extending to the outside rim at l/2 inch increments, including up the side wall.
All utensils were of a similar shape; i.e., 10 inch fry pan.
Note should be made of the excellent heat transfer characteristics of the solid aluminum; copper/aluminum clad stainless; stainless clad aluminum/copper core products, and stainless clad aluminum ~-ply pxoduct. Since copper is consid-` ered the ultimate in thermal conductivity and is generally considered to be 100%, its ability to transfer heat is well known. Aluminum in its pure state is generally considered to rank second in heat transfer and possesses about 57% that of copper. Cast iron also used to cookware has approximately 16.6%
the thermal conductivity of copper whereas carbon steel such as ~ used in the core of stainless clad carbon steel as manufactured ;; by our U.S. Patents Nos. 2,718,690 and 2,758,368 contains 15.1%
of the conductivity of pura copper. Further, stainless steel Type 304 (18% chrome, 8% nickel~ has approximately 3.83% the :
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thermal conductivity of pure copper. Ceramic materials similar to glass are total resistors of thermal transfer and have conductivities of approximately 1% that of pure copper.
A direct computation of the percentages of each metal used in the cross section of a clad metal potentially usable for cookware combined with the percentage of thermal conductivity in relation to copper would give one the heat transfer values necessary to determine the thermal conductivity of the as bonded clad metal. The thickness or mass of the metals used in the body of a utensil is also important when considering cooking performance, therefore, we recommend a minimum thic~ness of .050" (1.2 mm.) be used for said application.
Tests were conducted to determine food release char-acteristics of various materials. Similar sized utensils were placed on top of a gas or electrically heated range containing a center thermocouple capable of touching the exact center of the outside bottom of the utensil. The inside of the utensil was prepared by coating lightly with cooking oil. Flour was then sprinkled on the utensil surface covexed with the cooking oil. Excess amounts of flour were removed by gravity throw-off.
The resultant thin coating of flour on the surface of the utensil was then observed for color. The utensil was placed on the burner possessing the thermostat and heat was applied by adjust-ing the burner setting to 350F. The utensils were subjected to heat for a time element of 2-1/2 minutes, then were removed from the burner. An attempt to remove the heated flour from the ~
surface of the utensil was made by placing said utensil under a ~-faucet of water at a pressure of 35 lbs. Water temperature of approximately 110F. was used for rinsing. Time of removal of the flour by the described rinsing process was recorded. The 3. `~

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, iiG~ -stick resistance was concluded as a direct relation with time of removal of same by said water rinse. The color of the flour is affected by heat concentration. The greater the heat concentra-tion the darker burnt appearance of the flour. Utensil metals with high conductivity produced little or no heat tint on the flour surface. The results, corrected to uniform base of 0.125 thickness, are listed below:

Color Appearance Removal Product of Flour Time 1. Cu/Al/SS (.125 thick) Cream 22 secs.

2. Teflon Coated Al ~.125 thick) Cream 23 secs.

3. Al (.125 thick) Cream 25 secs.

4. SS Clad ~1 (.125 thick) Cream 29 secs.

5. SS/Al/Cu/Al/SS (.110 thick) Cream 33 secs.

6. SS/Al/SS (.110 thick) Tan 37 secs.

7. SS Al Bottom (.035 SS/.090 Al) Tan-Brown 48 secs.

8. Porcelain coated Steel (.085 thick) Tan-Brown 39.4 secs.

9. SS Carbon Steel (Iron Core) (.056 thick) Dark Brown 63.0 secs.

10. High temperature glass Dark Brown 105 secs.
The food release characteristics of stainless/aluminum/
copper utensils were ~-astly superior to all other metal and glass surfaces. Release characteristics of synthetic surfaces improve with thickness and were equal when .125" aluminum thickness was used. Utensils of equal thickness made from copper/aluminum/
stainless possessed release characteristics superior to Teflon coated aluminum of equal thickness and utensils of stainless, 4. , :~Z5~

aluminum copper, aluminum, stainless were in the same range as Teflon coated aluminum. The outstanding thermal properties of the clad metals resul-ting from the practices taught by this invention are herein described. This characteristic is most important to the establishment of the superiority of the metal combinations herein produced by this invention.
We have discovered that, when pure aluminum alone is used as the cladding for copper, there are instances where an orange peel effect occurs. This orange peeling cannot be con- --trolled by any mechanical or heat treatment known to us. We have discovered, however, that the orange peel effect can be eliminated by using an aluminum alloy base with a thin coating of pure aluminum on the sides to be bonded to the copper and/or stainless steel. This eliminates the problem or orange peeling while giving the very excellent bonding characteristic of pure aluminum. We have found that the base may be an aluminum alloy, e.g. 3003, 300~, 5005 or any other such aluminum product con-sidered by ~he trade to be an alloy rather than pure aluminum.
As the thin pure aluminum coating on the aluminum alloy we may use, for example, EC, 1100, 1130l 1230, 1145, 1175, or 7072 grades of aluminum. This pure aluminum should be clad onto the surfaces of the aluminum alloy which are to be bonded to the copper and stainless steel.
In this specification the word "sheet" or "sheets"
is synonymous with coil or sheet coil as used by the metals industry.
In this invention, we accordingly provide an aluminum clad copper in which the aluminum may be clad on one or both sides of the copper. Said aluminum clad copper or copper cored aluminum may then be clad with stainless steel on one or both .

sides of the aluminum~
Thus, this invention provides for a clad metal product consisting essentially of a core having a member from the group consisting of copper and copper alloys; and at least one layer of aluminum alloy oontaining less than 99% alumi~um coated with substantially pure aluminu~ containing at least 99% alumlnum, said core forming a major thickness portion of said composite and at least one outer cladding layer of stainless steel konded to said aluminum opposite the copper so that the aluminum layers lie between the stainless steel and the copper with substantially pure aluminum ak,utting the copFer and the stainless steel.
In a further emkodiment, this invention provides for a method of making a comFosite clad metal product comprising the steps of assembl-ing a copper sheet, at least one aluminum alloy sheet ooated with substantially pure aluminum on the surfaces to be joined and at least one sheet of stainless steel with at least one aluminum alloy sheet between the copper and stainless steel sheets and subjecting the assembled sheets bo sufficien-t pressure to produce one of a reduction of 2% followed by a reduction of akout 5% to 25% or a single reduction of akout 20% to 70%.
The aluminum must ke substantially pure aluminum, (such as Type 1100, 1130, 1230, 1145, 1175 or 7072) coated on an aluminum alloy such as Type 3003, 3004, 5005. We preferably use an aluminum such as Type 1145 coated as a layer clad on one or koth sides of aluminum alloy such as Types 3003, 3004, 5005O All members of the composite are preferably cleaned and conditioned on their surface by abrasive grinding to remove : all oxides, however, this can be eliminated for the stainless steel but must be used on the pure aluminum cladding surface of the aluminum alloy. Preferably the product may be made by forming a pre-composite of aluminum and copper, whether the pre-kond be oopper clad one side of pure aluminum clad aluminum alloy or aluminum on koth sides of a copper core, and thereafter konding a stainless steel cladding to one or both sides of the aluminum clad aluminum alloy. This pre-formed cladding or core is :

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preferably cleaned and condi-tioned on the pure aluminum surface or surfaoes by abrading or wire brushing or any suitable mechanical method to re~ove all oxides, however, this can be eliminated for the stainless steel. In this practice the pure alu~inum clad aluminum alloy clad copper, or pure aluminum clad aluminum alloy clad copper core has been cleaned as describ-ed above, cold bonded under pressure, e.g. by heavy reduction in the level of 40-80% in one or multiple passes on a rolling mill, then post heat treated if desired to increase the strength of the union between same.
The pre-formed cladding or core consisting of pure aluminum clad aluminum alloy and copper is heated along with the stainless steel to a temperature of about 300-800F., reduced 20-70% in one pass or alternately reduced up to 5~ in one roll stand followed by a reduction of 10~25% in a second roll stand, and then reheated - 6a -.

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to about 600-800F., preferably at 700F. to permit diffusion to occur between the adjacent layers of metals. This diffusion operation causes an increase in the bond strength between the three dissimilar metals, and also causes a certain amount of stress relieving. The resulting product is readily deep drawn into cooking vessels.
As a second alternative, the product may be made by forming a pre-composite cladding or core of pure alumimlm clad aluminum alloy and copper and thereafter applying the stainless steel cladding to o~e or both sides of the pure aluminum surface.
In this practice, the pure aluminum clad aluminum alloy and copper sheets are abrasively cleaned and conditioned, brought together into contact, heated to 300-700F. and subjected to 30-70% reduction. The pure aluminum clad aluminum alloy and copper composite pre-form may be post heated to increase the strength of the union there between. The resulting copper/
aluminum compact is then abrasively cleaned on its pure aluminum surface or surfaces and is used as a cladding or core for a stainless cladding operation. In this step the assembly of cladding or core and stainless steel is again heated to about 300-800F., subjected preferably to a first reduction of about 2-5%, and then subjected to a second reduction in the range of 5-25~, alternatively the assembly may be reduced in a single reduction pass of about 20-70~. The product is then heat treated to about 700F. to cause annealing and diffusion as discussed above.
In a third alternative, one or two sheets of pure aluminum clad aluminum alloy are mechanically cleaned, heated to ; 300-800F. and brought into contact with one or both surfaces of sheet of copper which is at room temperature, and reduced by 7.
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either the one or two step reductions discussed above. The pre-form composite of pure aluminum clad aluminum alloy and copper may be heat treated if desired to increase the strength of the union between same~ This cladding or core is then placed between sheets of stainless steel, reduced and annealed following the practices set forth abo~e.
A fourth alternatiue would include the manufacture of the pre-form pure aluminum clad aluminum alloy clad on one or both sides of the copper by cold or hot-bonding, as discussed in the first alternative, and then the cold bonding of said pre-form of pure aluminum clad aluminum alloy and copper on one or both sides with stainless steel by use of a heavy reduction step at the level of 30-70% in one or two passes. The resulting composite of stainless, copper and aluminum is heat treated as discussed in alternative #1.
In a fifth alternative, all member, i.e., copper, pure aluminum clad aluminum alloy and stainless steel, of the composite are preferably cleaned and conditioned on their exposed surfaces by abrasive grinding to remove all oxides~
however, this can be eliminated ~or the stainless steel. The pieces are heated separately or are brought together prior to heating, heated to a temperature of about 300-800F., reduced 20~70~ in one pass or alternately reduced up to about 5% in one roll stand followed by a reduction of 10-25%
in a second roll stand and then reheated to about 600-800F.
(preferably at 700F.) to permit diffusion to occur between the adjacent layers of metal. The diffusion operation causes an increase in bond strength between the three dissimilar metals.
This invention can, perhaps, be best understood by 8.

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reference to the following examples of products and processes according to this invention and by the accompanyin~ Figure showing in section a composite product according to this inven-tion.
Example I
A sheet of copper having a thickness of 0.010 inch was abrasively cleaned with a wire brush and placed between two sheets of Type 3003 Aluminum each 0.075 inch thick, coated with a thin layer of Type 11~5 Aluminum on both sides which were similarly abrasively cleaned. The copper and the pure aluminum clad aluminum alloy are reduced approximately 50-65~ to bond same to~ether. The .050 inch thick pre-form of aluminum clad copper was abrasively cleaned on the exposed pure aluminum sur-face, then brought into contact with two sheets of Type 304 stainless steel 0.010 inch thick, and placed with the stainless steel against the cleaned pure aluminum surfaces. The assembly of metals was heated to 700F. and passed through a rolling mill to reduce the thickness to 0. 065 inch, then passed through a second rolling mill and reduced to a thickness of 0.050 inches.
20 The resulting product was then reheated to 700F. for a sufficient time to produce diffusion between the adjacent metal layers in their entire interfaces. The final annealed product was deep drawn into cooking vessels such as frypans.
` Where it is desired to have one surface of stainless steel and the other of copper, the pre-form consists of a copper clad aluminum material in khe same procedures as outlined above are followed but only one side of the copper ls clad.
E~ample II
A sheet of 0.010 inch copper was abrasively cleaned and placed between two abrasively cleaned sheets of 0.125 inch .. .
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Type Alclad (Trademark of the Aluminum Company of America for an aluminum alloy coated with aluminum) 300~ Aluminum coated with Type 1145 Aluminum. The metals were heated to 450F. and given a first rolling pass at about 2~ reduction, and then rolled to about 0.110 inch thickness in a final pass. The rolled product was then reheated to 700F. to permit diffusion between the metal layers to occur. The final product was cut into blanks and deep drawn into cooking vessels.
Where it is desired to have one surface of stainless steel and the other-of aluminum or copper, the same procedures outlined above are followed, omitting the coating which is not to be used.

Example III -A sheet of copper having a thickness of 0.110 inch was abrasively cleaned with a wire brush and placed between two sheets of Type 3003 Aluminum alloy each 0~075 inch thick, coated with 1145 Aluminum and similarly abrasively cleaned. Two sheets of Type 304 stainless steel, 0.010 inch thick was abrasively cleaned, was placed on each side of the pure aluminum clad aluminum alloy sheets. The assembly was heated to 700F.
` and passed through a rolling mill to reduce the thickness to 0.1~0 inch. The resulting product was then reheated to 700F. i-~
for a sufficient time to produce diffusion between adjacent metal layers at their interfaces. The final annealed product was deep drawn into cooking vessels.
We have found that the product of this invention can be utilized in the cooking vessel field in a variety of ways and combinations. For example, a nine-ply material shown in the accompanying Figure, consisting of a copper core 10 clad ., ~ , .. 10.

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on each side with an aluminum alloy sheet 11 coated on each side with pure aluminum 12 which is in turn coated on both sides with stainless steel 13 can be used to form cooking vessels which have the desirable characteristics of stainless steel on the exposed surfaces but have a uniformity of heat trans~er unattainable in any prior art materials.
A seven or eight-ply material having a copper core coated on each side with pure aluminum clad aluminum alloy and having one outer layer of stainless is very satisfactory for making cooking vessels which are to be porcelainized or `
coated after forming on the exposed pure aluminum surface or used as discs attached to the bottom of utensils made con-ventionally from solid or clad metal. Here again the high heat transfer effects are achieved. In such case where discs are used the coating of pure aluminum on the surface to be exposed is omitted.
Finally, a five-ply material can be made using the procedures outlined above having a copper layer coated on one side with a pura aluminum clad aluminum alloy core and having a copper layer of stainless steel bonded to the pure aluminum surface providing a product which may be formed into cooking utensils having a copper exterior sur~ace for decorative and heat transfer purposes, and the other exterior surface of stainless steel with a core of aluminum. Such a product is illustrated in the accompanying Figure wherein a copper core 10 is clad with aluminum alloy sheets 11 coated on both sides with pure aluminum 12, and with outer sheets of stainless steel 13.
We ha~e discussed the composition of this invention in this application primarily in terms of cookware hecause . .,' ~

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this is the area where the largest amounts of such metals are used, however, the composition of this application can be used in any of a great variety of industrial uses where heat transfer is of importance and where resistance to discoloration, oxidation, etc., are desired.
In the foregoing specification we have set out certain preferred embodiments and practices of our invention, however, it will be understood that this invention may be otherwise embodied within the scope of the following claims. ~
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Claims (54)

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

1. A clad metal product consisting essentially of a core having a member from the group consisting of copper and copper alloys; and at least one layer of aluminum alloy containing less than 99% aluminum coated with substantially pure aluminum containing at least 99% aluminum, said core forming a major thickness portion of said composite and at least one outer cladding layer of stainless steel bonded to said aluminum opposite the copper so that the aluminum layers lie between the stainless steel and the copper with substantially pure aluminum abutting the copper and the stainless steel.

2. A clad product as claimed in claim 1 having a layer of aluminum alloy coated with substantially pure aluminum on each side of the copper so that the substantially pure aluminum abuts the copper and one outer cladding layer of stainless steel on one of said aluminum layers with the aluminum between the stainless steel and the copper.

3. A clad metal product as claimed in claim 2 in which the core is made up of a layer of copper, with layers of aluminum alloy coated with substantially pure aluminum on each side, said substantially pure aluminum abutting the copper, and a layer of stainless steel on each side with the layers of aluminum between the stainless steel and the copper on each side.

4. A clad product as claimed in claim 1 consisting essentially of a layer of aluminum alloy coated with pure aluminum on both sides and an outer cladding of copper on one side and an outer cladding of stainless steel on the other side so that the aluminum layer coated with substantially pure aluminum lies between the stainless steel and the copper, with each of the copper and stainless steel in contact with said substantially pure aluminum coating.

5. Method of making a composite clad metal product comprising the steps of assembling a copper sheet, at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined and at least one sheet of stainless steel with at least one aluminum alloy sheet between the copper and the stainless steel sheets and subjecting the assembled sheets to sufficient pressure to produce one of a reduction of 2% followed by a reduction of about 5% to 25% or a single reduction of about 20% to 70%.

6. The method of making a composite clad metal product comprising the steps of first joining together a copper sheet and at least, one aluminum alloy sheet coated with sub-stantially pure aluminum on the surfaces to be joined under pressure sufficient to provide a reduction to 80%, post-heat treating the same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded aluminum-copper composite on at least one sheet of stainless steel with the aluminum adjacent to the stainless steel, heating the stacked metals to a temperature between 300°F. to 800°F. and subjecting the stacked metals to sufficient pressure to a first reduction of about 2% followed by a reduction of about 5 to 25%.

7. A method of making a composite metal product com-prising assembling a copper sheet, at least one aluminum sheet coated with substantially pure aluminum on the surfaces to be joined and at least one stainless steel sheet with the aluminum sheet between the copper and stainless steel sheets, heating said metal sheets to a temperature between 300°F to °F and then joining the assembled heated sheets under pressure sufficient to produce a reduction of about 20 to 70%.

8. The method as claimed in claim 6 or 7 wherein the sheets being joined consist of a central copper sheet, an aluminum alloy sheet coated with substantially pure aluminum on each surface on opposite surfaces of the copper sheet and a stainless steel sheet on at least one surface of the aluminum opposite the copper.

9. A method as claimed in claim 6 wherein the pre-bonded aluminum-copper composite and the stainless steel sheet are heated in separate furnaces and then stacked one on the other.

10. A method as claimed in claim 6 or 7 wherein the sheets are joined by passing through a rolling mill.

11. A method as claimed in claim 6 or 7 wherein the sheets are post heat-treated to about 700°F.

12. The method of making a composite clad metal product comprising first joining together a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined under pressure sufficient to provide a reduction to 80%, post-heat treating same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded composite on at least one sheet of stainless steel, subjecting the stacked metals to a first reduction of about 2% followed by a reduction of about 5 to 25%.

13. The method of making a composite product comprising cleaning a copper sheet, at least one aluminum sheet coated with substantially pure aluminum on the surfaces to be joined and at least one stainless steel sheet to be joined including mechanical cleaning of said aluminum surfaces to be joined, assembling said sheets with the aluminum sheet between the copper and stainless steel sheets and joining the assembled sheets under pressure sufficient to produce a reduction of about 20% to 70%.

14. The method as claimed in claim 12 or 13 wherein the sheets being joined consist of a central copper sheet, an aluminum alloy sheet coated with substantially pure aluminum on each surface on opposite surfaces of the copper sheet and a stainless steel sheet on at least one surface of the aluminum opposite the copper.

15. A method as claimed in claim 12 or 13 wherein the sheets are joined by passing through a rolling mill.

16. A method as claimed in claim 12 or 13 wherein the sheets are post-heat treated at about 700°F. after being first joined.

17. The method of making a composite clad metal product comprising the steps of first heating a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined to 300°F to 800°F, then joining said heated sheets together under pressure sufficient to provide a reduction of 20 to 70%, post heat-treating same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded composite on at least one sheet of stainless steel with the stainless steel in contact with the cleaned aluminum surface, heating the stacked metals to a temperature between 300°F. to 800°F., and subjecting the stacked metals to sufficient pressure to a first reduction of about 2% followed by a reduction of about 5 to 25%.

18. The method of making a composite clad metal product wherein a copper sheet, at least one aluminum sheet coated with substantially pure aluminum on the surfaces to be joined and at least one stainless steel sheet are first heated to 300°F to 800°F and then assembled with the aluminum sheet between the copper sheet and the stainless steel sheet and the assembled sheets are joined under pressure sufficient to produce a reduction of about 20 to 70%.

19. The method as claimed in claim 17 or 18 wherein the sheets being joined consists of a central copper sheet and an aluminum alloy sheet coated with substantially pure aluminum on each surface on opposite surfaces of the copper sheet and a stainless steel sheet on at least one surface of the aluminum opposite the copper.

20. A method as claimed in claim 17 or 18 wherein the pre-bonded aluminum-copper composite and the sheet of stainless steel are heated in separate furnaces and then stacked one on the other.

21. A method as claimed in claim 17 or 18 wherein the sheets are joined by passing through a rolling mill.

22. A method as claimed in claim 17 or 18 wherein the sheets are post-heat treated to about 700°F. after being first joined.

23. The method of making a composite clad metal product comprising the steps of first heating a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined to 300°F to 800°F
then joining said heated sheets together under pressure sufficient to provide a reduction of 20 to 70%, post-heat treating same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded composite on at least one sheet of stainless steel with the stainless steel in contact with the cleaned aluminum surface, subjecting the stacked metals to a first reduction of about 2% followed by a reduction of about 5 to 25%.

24. The method of making a composite clad metal product wherein a copper sheet, at least one aluminum sheet coated with substantially pure aluminum on the surfaces to be joined and at least one stainless steel sheet are first heated to 300°F to 800°F, then assembled with the aluminum sheet between the copper and the stainless steel sheets, the assembled sheets are joined under pressure sufficient to pro-duce a reduction of about 20 to 70% and the product is post-heat treated.

25. The method as claimed in claim 23 or 24 wherein the sheets being joined consist of a central copper sheet and an aluminum alloy sheet coated with substantially pure aluminum on each side of the copper sheet and a stainless steel sheet on at least one surface of the aluminum opposite the copper.

26. A method as claimed in claim 23 or 24 wherein the stacked metals are heated in separate furnaces.

27. A method as claimed in claim 23 or 24 wherein the sheets are joined by passing through a rolling mill.

28. A method as claimed in claim 23 wherein the sheets are post-heat treated to about 700°F.

29. The method of making a clad metal product characterized by a high rate of thermal transmission and by resistance to corrosion comprising the steps of:
a. mechanically cleaning the surfaces of each of a sheet from the group consisting of copper and copper alloys, and at least one aluminum alloy sheet coated with substantially pure aluminum;
b. placing said mechanically cleaned sheets one on top of the other and applying at least one sheet of stainless steel on the aluminum;
c. heating said sheets to a temperature of about 300°F. to 800°F;
d. joining said sheets under pressure sufficient to reduce the total thickness of said sheets an approximately 2% in the first reduction step and between 5 and 25% reduction in the second reduction step;
e. stress relieving the joined sheets at a temperature of about 700°F.

30. The method of making a clad metal product characterized by a high rate of thermal transmission and by resistance to corrosion comprising the steps of:
(a). mechanically cleaning the surfaces of each of a sheet from the group consisting of copper and copper alloys, and at least one aluminum alloy sheet coated with substantially pure aluminum;
(b). placing said mechanically cleaned sheets one on top of the other and applying sufficient pressure to join the sheets to form a pre-bonded composite;

(c). applying at least one sheet of stainless steel on the aluminum;
(d). heating said sheets to a temperature of about 300°F. to 800°F.;
(e). joining said sheets under pressure sufficient to reduce the total thickness of said sheets approximately 2% in a first reduction step and between 5% and 25% reduction in a second reduction step; and (f). stress relieving the joined sheets at a temperature of about 700°F.

31. The method as claimed in claim 29 or 30 wherein the stacked metals are subjected to a reduction of approximately 20 to 70%.

32. The method as claimed in claim 29 or 30 wherein the sheets are joined by passing through a rolling mill.

33. A method as claimed in claim 30 wherein the pre-bonded aluminum-copper composite and the stainless steel sheet are heated in separate furnaces and then stacked one on the other.

34. The method of making a composite clad metal product comprising the steps of first joining together a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined under pressure sufficient to provide a reduction to 80%, post-heat treating the same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded aluminum-copper composite on at least one sheet of stainless steel with the aluminum adjacent to the stainless steel, heating the stacked metals to a temperature between 300°F. to 800°F. and subjecting the stacked heated metals to sufficient pressure to produce a reduction of about 20%
to 70%.

35. The method of making a composite clad metal product comprising the steps of first joining together a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined under pressure sufficient to provide a reduction to 80%, post-heat treating same, cooling, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded composite on at least one sheet of stainless steel, subjecting the stacked metals to a pressure sufficient to produce a reduction of about 20% to 70%.

36. The method of making a composite clad metal product comprising first heating a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined to 300°F. to 800°F., then joining said heated sheets together under pressure sufficient to provide a reduction of 20 to 70%, post-heat treating same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded composite on at least one sheet of stainless steel, heating the stacked metals to a temperature between 300°F. to 800°F., and subjecting the stacked metals to sufficient pressure to produce a reduction of about 20% to 70%.

37. The method of making a composite clad metal product comprising the steps of first heating a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined to 300°F. to 800°F. then joining said heated sheets together under pressure sufficient to provide a reduction of 20 to 70%, post-heat treating same, mechanically cleaning the aluminum surface, placing said mechanically cleaned pre-bonded composite on at least one sheet of stainless steel, subjecting the stacked metals to a pressure sufficient to produce a reduction of about 20% to 70%.

38. A method as claimed in claim 34, or 35, or 36 wherein the sheets being joined consist of a central copper sheet and an aluminum alloy sheet coated with substantially pure aluminum on each surface of the copper sheet and a stainless steel sheet on at least one surface of the preformed copper/aluminum core.

39. A method as claimed in claim 33, or 34 wherein the sheets are joined by passing through a rolling mill.

40. A method as claimed in claim 30 wherein the sheets being joined consist of a central copper sheet and an aluminum alloy sheet coated with substantially pure aluminum on each surface of the copper sheet and a stainless steel sheet on at least one surface of the preformed copper/aluminum core.

41. A method as claimed in claim 35 or 36 wherein the sheets are joined by passing through a rolling mill.

42. A method as claimed in claim 33, or 34 wherein the sheets are post-heat treated to about 700°F.

43. A method as claimed in claim 35, or 36 wherein the sheets are post-heat treated to about 700°F.

44. A method as claimed in claim 28, or 33, wherein the sheets are heated in separate furnaces and then stacked one on the other.

45. A method as claimed in claim 34 or 35 or 36 wherein the sheets are heated in separate furnaces and then stacked one on the other.

46. A method of making a composite clad metal product comprising the steps of first joining together a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined under a pressure sufficient to provide a reduction of about 80%, post-heat treating the bonded aluminum copper composite, heating the pre-bonded aluminum copper composite to a tem-perature in the range 300°F. to 800°F., placing said heated aluminum-copper composite on at least one sheet of stainless steel with the aluminum abutting the stainless steel and subjecting the stacked metals to sufficient pressure to produce a first reduction of about 2% followed by a reduction of about 5% to 25%.

47. A method of making a composite clad metal product comprising the steps of first joining together a copper sheet and at least one aluminum alloy sheet coated with substantially pure aluminum on the surfaces to be joined under a pressure sufficient to provide a reduction of about 80%, post-heat treating the bonded aluminum copper composite, heating the pre-bonded aluminum-copper composite to a temperature in the range 300°F. to 800°F., placing the heated composite on at least one sheet of stainless steel with the aluminum abutting the stainless steel and subjecting the stacked metals to sufficient pressure to produce a reduction of about 20% to 70%.

48. A method as claimed in claim 41 or 42 wherein the sheets being joined consist of a central copper sheet and an aluminum alloy sheet coated with substantially pure aluminum on each surface of the copper sheet and a stainless steel sheet on at least one surface of the preformed copper/aluminum core.

49. A method as claimed in claim 47 or 48 wherein the sheets are joined by passing through a rolling mill.

50. A method as claimed in claim 47 or 48 wherein the sheets are post-heat treated to about 700°F.

51. A method as claimed in claim 47 or 48 wherein the sheets are heated in separate furnaces and then stacked one on the other.

52. A method as claimed in claim 8 wherein the sheets are joined by passing through a rolling mill.

53. A method as claimed in claim 8 wherein the sheets are post heat-treated to about 700°F.

54. A method as claimed in claim 19 wherein the sheets are post heat-treated to about 700°F.