METHOD TO PRODUCE COMPOSITE MATERIAL WITH A HARD INNER LAYER WITH DEEP DRAW CAPABILITY

Abstract

A method for producing a composite metallic material having a soft outer layer of a first metal or metal alloy and a hard inner layer of a second metal or metal alloy includes first selecting the soft layer and the hard layer according to the desired properties of the combined layers. The soft layer is then bonded with the hard layer. Finally, the bonded layers are annealed at a temperature within the range of 700-1200 degrees Fahrenheit to secure the bond and enhance formability.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Provisional Patent Application No. 62/009,773, filed on Jun. 9, 2014, the disclosure of which is relied upon and incorporated herein by reference.

FIELD OF THE INVENTION

A method to produce a highly formable metallic composite material with hard inner layers (e.g., stainless metal) and a soft outer layer (e.g., copper) is developed incorporating a unique method for bonding and annealing the layers.

BACKGROUND OF THE INVENTION

Traditionally, hard metallic layers (e.g., stainless steel) bonded/adhered to soft layers (e.g., copper) could not be fully annealed for certain applications because fully annealing the stainless steel will result in unacceptable surface finish or properties. For example, in the cookware industry, stainless steel is used as the inner layer of the vessel to ensure a nonreactive surface during cooking and to facilitate cleaning. However, the stainless steel lacks good heat transfer characteristics. The desired heat transfer characteristics can be provided by an aluminum or copper outer layer (e.g., C11000) because of the excellent heat conductivity of these materials. In this case, fully annealing the stainless steel inner layer will cause large grains in the soft aluminum or copper outer layer. The large copper grains in return, cause an unacceptable surface finish (e.g., orange peeling) after forming operations.

A common method to circumvent the grain growth issue is by using alloyed copper (such as C19400) which pin grain boundaries and reduce grain growth. But alloyed copper is not only considerably more expensive and not widely used, but also results in a much lower thermal/electrical conductivity and performance in most applications. FIG. 1 provides a comparison of different copper properties and the associated price.

SUMMARY OF THE INVENTION

A method for producing a composite metallic material having a soft outer layer and a hard inner layer is described herein. The method includes the steps of first identifying the soft layer of a metal or metal alloy and the hard layer of a metal or metal alloy according to the desired properties of the combined layers. The soft layer is then bonded with the hard layer. Finally, the bonded layers are annealed at a temperature within the range of 700-1200 degrees Fahrenheit to secure the bond and enhance formability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table providing comparison of different copper properties and the associated prices.

FIG. 2 is a flow chart illustrating the process for joining materials as described herein.

FIG. 3 is a diagram illustrating the bonding mill for cladding a soft metallic layer with a hard metallic layer.

FIG. 4 is a table identifying a non-exclusive list of hard metallic layers that can be combined with soft metallic layers according to the process described herein.

DETAILED DESCRIPTION OF THE INVENTION

A cold-roll-bonding and annealing process is described herein to produce a clad metallic material composite 10 that includes a soft outer layer 12 of a metal or metallic alloy and a hard inner layer 14 of a metal or metallic alloy that will allow the use of any type of soft outer layer (e.g., C1100 copper) and which is formable in deep draw or comparable operations. As a bit of background, deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered “deep” drawing when the depth of the drawn part exceeds its diameter.

The clad composite 10 includes two roll bonded metal layers with the soft outer layer 12 (such as aluminum, copper or related alloy materials) and the hard inner layer 14 (such as stainless steel, steel, titanium or related alloy materials). A multitude of clad combinations are available to combine the unique surface properties of the various metals and metal alloys to suit the particular application or design needs, such as providing the desired light weight, heat transfer, and/or strength characteristics. In this disclosure, a hard layer is considered to be any metal with a Rockwell hardness on the B scale of greater than HRB 50 and a soft layer is any metal with a Rockwell hardness on the B scale of less than HRB 50.

The cold roll bonding process is used to produce a bi-layer of hard/soft composite material 10. The bond between the metallic layers 12, 14 can then be secured with a heat treating/sinter process. The sinter process does not fully anneal or recrystallize the hard inner layer but secures the bond between the layers 12, 14. As long as the hard layer 14 is used as the inner layer in any forming process, a sinter anneal is adequate as the stainless steel inner layer only experiences compressive forces. Referring to the same example above, cookware products have stainless steel on the inside (cooking surface) and copper on the outside. In such a case, the stainless steel will only see compressive forces and full anneal heat treatment is not required. Pure copper is sufficient in this case resulting in a significant increase in the thermal conductivity. This will also result in lower production costs and reduced manufacturing cycle time.

The steps involved in this process 100 are shown in flowchart of FIG. 2. Specifically, the user will select two layers 12, 14; namely, the user will select a clean soft layer (step 102) and a clean hard layer (step 104) according to the desired properties for the resultant material composite 10. Once the two layers 12, 14 are chosen, the next step 106 is for the layers 12, 14 to be bonded using a conventional cold-roll bonding process. The bonding process 106 that is employed is similar to known cold-roll bonding process, in which the two layers 12, 14 are bond/clad on a four-high mill 20, as shown in FIG. 3. Such a process is described in prior patents, such as U.S. Pat. Nos. 6,475,675 and 8,420,225, which are incorporated herein by reference.

The bonded layers 10 are then sintered/annealed in step 108, but not according to conventional annealing practices. That is, the bonded layers 10 are sintered/partially-annealed at about 700° F.-1200° F. to improve the bond/peel strength between the individual layers 12, 14. In the known processes, the bonded layers 10 are traditionally manufactured by annealing the copper and the stainless steel at high temperatures (1800° F.), which causes the grain growth in the copper and hence the need for an alloyed copper.

The annealing process 108 is performed in a batch or continuous process and in a controlled atmosphere. Annealing can be performed in atmospheres such as hydrogen, nitrogen, or a mixture of hydrogen/nitrogen. The temperature of the sinter/anneal is determined by two factors: sensitization temperature and copper grain growth.

Concerning sensitization temperature, austenitic stainless steel (if used in this type of product) usually cannot be batch annealed as it will go through a ‘sensitization’ process where the chromium is depleted from the grain boundaries due to chromium carbide precipitation resulting in poor stress corrosion cracking performance.

With respect to copper grain growth, the temperature needs to be lower than the grain growth temperature of the copper (softer) material 12.

It should also be noted that ferritic stainless steel undergoes an embrittlement when subjected to prolonged heating between 750 degrees Fahrenheit and 1000 degrees Fahrenheit. The most severe effects are experienced around 885 degrees Fahrenheit. Such embrittlement causes decrease in the ductility/forming characteristics.

Therefore, the selected temperature is generally in the range of 700-1200° F. depending on the material selection. The range of greater than 700 degrees Fahrenheit and less than 1200 degrees Fahrenheit allows for batch annealing of Austenitic stainless steel if used in the product and use of copper alloys that have higher oxygen content in hydrogen atmospheres. Batch annealing is a traditionally less expensive process than continuous or strand annealing. Copper alloys with a high oxygen content when annealed at high temperatures (such as 1800° F.) in a hydrogen atmosphere form water vapor within the copper causing blisters in the copper surface.

After the bonded material 10 has been annealed, it is transitioned to post process in step 110. The post process 110 usually varies with the different applications, and may include further rolling, slitting, cut to length and forming operations that would follow the sintering/annealing process. For example, in the cookware industry, the annealed material 10 will be cut to sheets, formed into pots of different sizes and shapes, buffed to produce a cooking vessel. In comparison, in the electronics industry, the bonded material 10 could be stamped to different shapes and sizes and used as a heat sink or other purposes for the electronic component.

By using this process, various combinations of metal layers are possible that were not considered previously possible using prior techniques. Moreover, it is noted that the process can be used on a variety of hard and soft layers to achieve the desired properties. A few additional examples (although a variety of other examples may be clad according to the described process) are found in the table included as FIG. 4. For example, titanium and copper may be combined to provide a material composite that lightweight and corrosion resistant, which is good for the cookware and electronics industries. Steel and copper may be combined to provide a material composite that is especially good for the cookware and cable shield industries, in that the composite has improved thermal and electrical conductivity, improved strength, and can be produced at a lower cost. A third example is the combination of stainless steel with copper, which provides improved thermal conductivity and corrosion resistant, which is also beneficial in the cookware industry.

Having thus described exemplary embodiments of a method to produce metallic composite material, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations, and modifications may be made within the scope of this disclosure. Accordingly, the invention is not limited to the specific embodiments as illustrated herein, but is only limited by the following claims.

Claims

1. A method for producing a composite metallic material having a soft outer layer and a hard inner layer comprising the steps of:

a. identifying the soft layer and the hard layer according to the desired properties of the combined layers;

b. bonding the soft layer with the hard layer; and

c. annealing the bonded layers at a temperature within the range of 700-1200 degrees Fahrenheit to secure the bond and enhance formability.

2. The method as described in claim 1 wherein step a) further comprises selecting the soft outer layer from a group consisting of copper or aluminum.

3. The method as described in claim 1 wherein step a) further comprises the step of selecting the soft outer layer having a Rockwell hardness on the B scale of less than HRB 50.

4. The method as described in claim 1 wherein step a) further comprises the step of selecting the hard outer layer having a Rockwell hardness on the B scale of greater than HRB 50.

5. The method as described in claim 1 wherein step a) further comprises selecting the hard outer layer from a group consisting of steel, stainless steel, or titanium.

6. The method as described in claim 1 wherein step b) includes cold-roll bonding the soft layer with the hard layer.

7. The method as described in claim 1 wherein the annealing performed in step c) is in a continuous process and in a controlled atmosphere.

8. The method as described in claim 7 wherein the annealing is performed a controlled atmosphere selected from the group consisting of hydrogen, nitrogen, or a mixture of hydrogen and nitrogen.

9. A method for producing a composite metallic material having a soft outer layer and a hard inner layer comprising the steps of:

a. identifying the soft layer and the hard layer according to the desired properties of the combined layers;

b. feeding the soft layer and the hard layer into a bonding mill to bond the soft layer with the hard layer; and

c. annealing the bonded layers at a temperature of greater than 700 degrees Fahrenheit and less than 1200 degrees Fahrenheit to secure the bond and enhance formability.

10. The method as described in claim 9 wherein step a) further comprises selecting the soft outer layer from a group consisting of copper or aluminum.

11. The method as described in claim 9 wherein step a) further comprises the step of selecting the soft outer layer having a Rockwell hardness on the B scale of less than HRB 50.

12. The method as described in claim 1 wherein step a) further comprises the step of selecting the hard outer layer having a Rockwell hardness on the B scale of greater than HRB 50.

13. The method as described in claim 12 wherein step a) further comprises selecting the hard outer layer from a group consisting of steel, stainless steel, or titanium.

14. The method as described in claim 9 wherein step b) includes cold-roll bonding the soft layer with the hard layer.

15. The method as described in claim 9 wherein the annealing performed in step c) is in a continuous process and in a controlled atmosphere.

16. The method as described in claim 15 wherein the annealing is performed a controlled atmosphere selected from the group consisting of hydrogen, nitrogen, or a mixture of hydrogen and nitrogen.