Abstract:
A process to form laminated cookware vessels starts with individual metal sheets, which are first formed into individual fluid containing vessels. The fluid containing vessels are nested together and bonded in a multi-step process that deploys laser welding to form the vessels rim. The process provides great flexibility in combining different materials and varying the laminated construction of the bottom as compared to the surrounding sidewall of the cookware vessel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     None  
       BACKGROUND OF INVENTION  
       [0002]     The present invention relates to an improved process for fabricating laminated cookware articles.  
         [0003]     Laminated cookware articles are well known. They typically deploy copper and/or aluminum as one or more core layers, with surrounding layers to form the exposed interior and/or exterior surface of the cookware.  
         [0004]     Among other benefits, the copper and/or aluminum core layers enhances the thermal performance of the cookware; enabling both a faster heating of the foodstuffs and a more uniform temperature distribution. Outer layers of the laminate, that surrounds the copper and/or aluminum core, can provide an exterior surface that is easier to clean or maintain a particular desired appearance in the kitchen.  
         [0005]     Such laminated articles of cookware are fabricated starting with pre-laminated sheet stock. Methods of making sheet stock suitable for eventually forming cookware are disclosed in U.S. Pat. No. 6,427,904 to Groll, titled “Bonding of Dissimilar Metals”, as well as U.S. Pat. No. 6,109,504, also to Groll, and titled “Copper Core Cooking Griddle and Method of Making Same”. The &#39;504 teaches the desirability of forming a sheet stock laminate of stainless steel/copper/stainless steel useful for fabricating cookware via the sequential reduction of thickness by repeated hot roll bonding steps. The preferred composition is a diffusion bonded composite of 304L grade stainless steel outer layers with an inner core of high purity C-102 grade copper. However, explosion bonding is initially used to laminate the three layers.  
         [0006]     The cookware is then fabricated from the laminated sheet stock by first cutting or trimming the sheet stock into round shape. The round trimmed pieces are then deformed or drawn in a die to form a fluid containing cookware vessel. However, as the laminating process itself is cumbersome, the laminated stock material is expensive, adding to the cost of the final product. Moreover, a large portion of this expensive material is lost as waste trim. The trimmed material being laminated it also difficult to recycle.  
         [0007]     Accordingly, it would be desirable and is a first object of the invention to provide an alternative process to forming laminated cookware that does not require the use of pre-laminated sheet stock.  
       SUMMARY OF INVENTION  
       [0008]     In the present invention, the first object of providing a cost effective method of forming clad cookware is achieved by providing a first substantially planar sheet comprising at least one layer of a first metal, then providing a second substantially planar sheet comprising at least one layer of a second metal, drawing the first planar sheet to form a first preform that is a fluid containing vessel, drawing the second planar sheet to form a second preform that is a fluid containing vessel that nests within the first preform (such that each fluid containing vessel has a bottom surface and surrounding sidewalls extending upward therefrom), nesting the second perform within the first perform to form a subassembly, bonding the interface between the first and second perform to form a first bonded preform. The first bonded preform is laser welded to fully bond the materials of the first and second preforms along an annulus that circumscribes the surrounding sidewalls at the portion thereof intended to form the rim of the article of cookware. Generally, the article of cookware is trimmed at this location after laser welding.  
         [0009]     The above process can be extended to include the bonding and laser welding of a third preform formed from a third sheet of metal, thus encapsulating laminate of one metal layer between two other metal layers in the final clad structure.  
         [0010]     The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]      FIG. 1  is a cross-sectional elevation of an article of cookware according to a first embodiment of the invention.  
         [0012]      FIG. 2  is a schematic diagram illustrating the steps in the process used to fabricate the article of cookware of  FIG. 1 .  
         [0013]      FIG. 3  is a cross-sectional elevation of an article of cookware according to a second embodiment of the invention.  
         [0014]      FIG. 4  is a schematic diagram illustrating the steps in the process used to fabricate the article of cookware of  FIG. 3 .  
         [0015]      FIG. 5  is a cross-sectional elevation of an article of cookware according to another embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]     Referring to  FIGS. 1 through 5 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved clad article of cookware, generally denominated  100  herein.  
         [0017]     In accordance with the present invention,  FIG. 1  illustrates a first embodiment of the invention in which an article of cookware  100  has a bottom cooking surface  110  surrounded by an upward extending sidewall  120  to form a fluid containing vessel. The article of cookware  100  generally also includes one or more sideward extending handles, which are not shown in the drawing. The sidewall  120  and bottom  110  have a laminated construction in which the entire inner cooking surface  101  is lined with a layer of stainless steel  105  that is in turn surrounded by an exterior layer  109  of copper cladding  106 . A portion of the copper cladding  106   a  is about 1.5 mm thick at the bottom cooking surface  110 , whereas another portion of the copper cladding  106   b  in the sidewall has a thickness that is preferably no more than about 90% of thickness at the bottom cooking surface, that is less than about 1.3 to 1.4 mm. The inner lining of stainless steel is preferably grade  304 . The interior stainless steel lining  105  protects the interior surface of the copper from tarnishing with use, providing a surface that is easier to clean after cooking. The stainless steel layer  105  also strengthens the article of cookware  100  such that the copper layer  106  does not need to be thicker than about 1.5 mm, helping to reduce the cookware weight without a significant degradation in thermal responsiveness. The cookware weight is further reduced by making the sidewall portion  106   b  of copper layer  106  in the sidewall  112  thinner than the copper layer  106   a  in the bottom-cooking surface  110 , which is required for thermal performance. Preferably, the copper layer  106   a  in the bottom surface  110  has a thickness of about 1.5 mm, whereas the portion of the copper layer  106   b  in the sidewall  120  has a thickness of about 1.2 mm. The stainless steel layer  105  that forms the interior surface  101  of the cooking vessel preferably has a constant thickness of about 0.6 mm, resulting in a total thickness of about 2.10 mm for the bottom cooking surface  110 . In contrast, the sidewall  120  has a total thickness of about 1.8 mm. More generally, it is preferable that the copper in the sidewall is no more than about 80% of the thickness of the copper in the bottom of the pan  
         [0018]      FIG. 2  illustrates another embodiment of the invention in which a novel sequence of steps is used to fabricate the article of cookware  100 , shown in  FIG. 1 . The process described with respect to  FIG. 2  has two advantages. First, it provides a cost savings compared to forming a cooking vessel by deforming a sheet of clad metal comprising a uniform layer of stainless steel bonded to a uniform layer of copper. Further, the process allows the copper exterior to be made thinner in the sidewall than in the bottom of the pan, where the extra thickness of the copper results in improved temperature uniformity across the bottom surface  110  during cooking. This construction reduces the weight of the pan, as compared to deploying a copper layer with a constant thickness of 1.5 mm The resulting cookware article is lighter and thus easier for the user or consumer to handle.  
         [0019]     In step  201 , shown in  FIG. 2A , a substantially planar sheet of copper  205 , or an alloy thereof, is drawn to form a fluid containing vessel or outer shell  210 . In this embodiment, outer shell  210  will become the exterior of the completed cooking vessel  100 .  
         [0020]     In step  202 , also shown in  FIG. 2A , a substantially planar sheet of stainless steel, preferably grade  304  alloy,  215  is drawn to form a fluid containing liner or inner shell  220 . However, to the extent that it is desirable to utilize the completed article of cookware with induction cooking, stainless steel grade  430  is preferred.  
         [0021]     In step  203 , shown in  FIG. 2B , the inner shell  220  is nested within the outer shell  210 , forming subassembly  225 . A brazing compound is applied to at least one of the exterior of the inner shell  220  or the interior of the outer shell  210  prior to the nesting.  
         [0022]     Also in step  203 , to complete the brazing process, the temperature of the subassembly is raised to melt the brazing compounding, which upon cooling forms a metallurgical bond at interface  214 , uniting the inner shell  220  and the outer shell  210 . Pressure is applied to compress the inner and outer shells against each other at the common interface  214 , facilitating the consolidation and flow of the liquid brazing compound. It should be appreciated that each of the shells  220  and  210  are drawn in steps  201  and  202  with sufficient dimension tolerances to facilitate complete insertion of the inner shell  220  in the inner shell  210 . A slight gap is also provided to accommodate the solid brazing compound (as well as for the eventual wicking of the molten brazing compound or liquid flux) at the common interface,  214 , of subassembly  230 .  
         [0023]     Shown schematically in  FIG. 2C  is step  204 , an “ironing process” to reduce the thickness of the sidewall  120 . “Ironing” is done by the continued deep drawing of subassembly  225  in a set of dies with the clearance between male and female die members that is smaller than the actual combined thicknesses of the sidewall  120 . As the copper outer layer  106   b , is much softer than stainless steel  105 , only the thickness of the copper layer  106   b  is reduced. As it can be difficult in the brazing process of step  203  to fully reflow the liquid flux over the entire areas to be bonded in interface  214 , air and moisture can be trapped within this gap. The “ironing process” has another advantage in that it gradually expels air and moisture trapped at the common interface  214 . As the stainless steel layer  105  is not drawn the “ironing”  204 , it will remain the same height as when formed in  202 , defining rim  241 . However, as the wall thickness of the copper layer  106   b  is reduced, the height of this wall will increase from that resulted from forming step  210 .  
         [0024]     While it is possible to initially form both the inner shell  220  and outer shell  210  with a predetermined difference in initial wall heights with the intention that they become uniform during the “ironing” process of step  204 , it is preferable to trim the sidewall  120  to define the final rim height after the “ironing” process. This trimming step may utilize conventional mechanical cutting tools, water jet cutting, laser cutting and the like.  
         [0025]     When the trimming step is performed after “ironing” it is more preferable to utilize laser welding to fully bond and thus tightly seal the inner shell  220  to the outer shells  210  at the intended rim position, shown schematically as step  205  in  FIG. 2D . In step  205 , the laser beam  250  is focused to heat the intended trim area Laser welding is well known in the art of metal fabrication. One of ordinary skill in this art can readily determine the optimum laser welding conditions appropriate to the thickness, absorption and heat capacity of the copper and stainless steel layers at the weld location by routine experimentation.  
         [0026]     After trimming the article of cookware, it is preferably polished to achieve the desired aesthetic appearance. After the trimming and polishing steps in the fabrication process shown in  FIG. 2A-2D , one or more handles are generally attached to sidewall  120 .  
         [0027]     Ironing is a preferred but not limiting embodiment, depending on the ease and integrity of the bond formed in the initial brazing process.  
         [0028]     It should be appreciated that a multilayer laminated sheet of metal may be used to form one or more of the preforms that are nested inside each other and then bonded together. This may be desired when a particular pair of metal is more difficult to join by the inventive process, but a third metal is readily bonded by the inventive process after the corresponding preforms are nested together.  
         [0029]      FIG. 3  illustrates another embodiment of the invention in which an article of cookware  100  has a bottom cooking surface  110  surrounded by an upward extending sidewall  120  to form a fluid containing vessel. The sidewall and bottom have a laminated construction in which the entire inner cooking surface  101  is lined with a layer of stainless steel  105 . Stainless steel layer  105  is surrounded on the exterior surface by a layer of copper cladding  306 . The copper cladding  306  is 1.5 mm thick in the bottom portion  306   a , whereas the thickness in the sidewall portion  306   b  is about 1.2 mm thick. An outer stainless steel protective layer  340  surrounds the inner copper cladding  306 . The inner and outer linings of stainless steel are preferably grade  304 , and more preferably have a constant thickness of about 0.6 mm. As in the cookware article  100  of  FIG. 1 , this cooking vessel advantageously deploys thinner copper in the sidewalls  120  than is required in the bottom-cooking surface  110  to achieve a substantially uniform temperature, thus reducing the total weight of the cookware article.  
         [0030]     Further, the fabrication processes used to form cookware article  100  of  FIG. 3 , as illustrated in  FIG. 4 , has a lower manufacturing cost savings than constructing a comparable article of cookware starting from a triple ply clad sheet that comprise a stainless steel/copper/stainless steel construction.  
         [0031]     In step  401 , shown in  FIG. 4A , a substantially planar sheet of stainless steel, preferably grade  304  alloy,  405  is drawn to form an fluid containing inner liner  410 .  
         [0032]     In step  402 , shown in  FIG. 4A , a substantially planar sheet of copper  415 , or an alloy thereof is drawn to form a fluid containing vessel or middle shell  420 .  
         [0033]     In step  403 , also shown in  FIG. 4A , a substantially planar sheet of stainless steel  425 , preferably grade  430  alloy, is drawn to form a fluid containing vessel or outer shell  430 . Grade  430  stainless steel grade is preferred so that the completed article of cookware can be used for induction cooking.  
         [0034]     In step  404 , shown in  FIG. 4B , the fluid containing inner liner  410  is nested within the middle shell  420 . A brazing compound is applied to at least one of the exterior of the inner liner  410  or the interior of middle shell  420 . Further, the middle shell  420 , including inner line  410 , is nested within outer shell  430 , forming subassembly  445 . Likewise, a brazing compound is applied to at least one of the exterior of the middle shell  420  and the interior of the outer shell  430 .  
         [0035]     It should be appreciated that each of the liner  410  and shells  420  and  430  are drawn in steps  401 ,  402  and  403  with sufficient dimension tolerances to facilitate complete insertion in the nested arrangement of subassembly  445 , with a slight gap at each interface to accommodate the brazing compound and the eventual wicking of the molten brazing compound.  
         [0036]     Also in step  404 , to complete the brazing process, the temperature of the subassembly is raised to melt the brazing compound, which upon cooling forms a metallurgical bond at interfaces  454  and  455 , substantially bonding each liner or shell to the next larger shell in subassembly  445 . Liner  410  and shells  420  and  430  are also pressed together enabling the consolidation and flow of the liquid brazing compound at their respective common interfaces  454  and  455 .  
         [0037]     It should be appreciated that the liner  410  and shells  420  and  430  can be nested in an alternative sequence and be braised in multiple, rather than a single step, if desired. Preferably, the subsequent “ironing” process of step  406  of  FIG. 4C  is done after the bonding of the three liner/shells formed in steps  401 ,  402  and  403  by brazing in step  404 . As described with respect to  FIG. 2C , the “ironing process” not only reduces the copper thickness in sidewall  120 , but also expels trapped air and moisture from interface  454  and  455 .  
         [0038]     As in forming cooking vessel  100  in  FIG. 2 , laser welding in step  407  is carried out after “ironing” in step  406 , following by trimming to form rim  460 , as indicated by the dotted line in  FIG. 4D .  
         [0039]     After trimming, the article of cookware is polished to the aesthetically desired final finish One or more side handle are generally attached after the trimming and polishing steps in the fabrication process.  
         [0040]     It should be appreciated that alternatives to the embodiments described with respect to  FIGS. 2 and 4  include substituting aluminum for copper. Further embodiments included a construction wherein a titanium, including alloys thereof, and aluminum or aluminum alloy preforms are bonded to each other. In such instances it would be preferable if the titanium or titanium alloy preform was used as the inner shell, with the aluminum or aluminum alloy preform as the outer shell. Such a bonded preform can be anodized by conventional processes after the bonding steps, thereby rendering the outer aluminum shell into the harder anodized aluminum, while providing a more chemically resistant titanium metal as the inner cooling surface.  
         [0041]     In accordance with another aspect of the present invention,  FIG. 5  illustrates another embodiment of the invention in which an article of cookware  100  has a bottom cooking surface  110  surrounded by an upward extending sidewall  120  to form a fluid containing vessel. The sidewall and bottom have a laminated construction in which the entire inner cooking surface is lined with a layer of stainless steel  105  and the outside of the article of cookware is a copper cladding  106 . At the bottom of the article of cookware  100  is disposed a layer of aluminum  504 , or an alloy thereof, having a thickness of between about 2 mm to about 7 mm, disposed between the interior stainless steel lining  105  and the exterior copper cladding  106 . As the aluminum layer  504  only extends across the bottom-cooking surface  110 , the upward extending sidewall  112  comprises a laminate of copper  106   b  and stainless steel  105 .  
         [0042]     The above construction is highly advantageous as the aluminum layer  504 , depending on the relative thickness with respect to the copper layer, helps to spread heat laterally. However, as the aluminum  504  is not disposed within the sidewall  112  of the cooking vessel, the lateral spread of heat is predominantly in the bottom of the cookware. Further, this construction avoids having to construct an article of cookware from an expensive triple laminated sheet of copper/aluminum/stainless steel.  
         [0043]     The article of cookware  100  in  FIG. 5  can be fabricated by impact bonding an aluminum slab or sheet that is pre-cut into a circle to one or both of the stainless steel or copper layers shown in  FIG. 2 . The step of impact bonding either can be carried out before or after the stainless steel or copper sheets are formed into vessels by the drawing process described in steps  201  and  202  of  FIG. 2 .  
         [0044]     While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.