Patent Publication Number: US-2011073572-A1

Title: Method to reduce thermal degradation of adhesive in weldbonding

Description:
This application claims priority from German Patent Application 102009042973 filed Sep. 25, 2009. 
     FIELD OF THE INVENTION 
     The present invention relates to a method to reduce the thermal degradation of the adhesive used in the weldbonding of metal sheets. 
     BACKGROUND OF THE INVENTION 
     It is known to place a polymer adhesive or sealant between two sheets and then attach the sheet together by applying electric current via resistance weld electrodes to create an electric resistance spot weld and simultaneously heat the sheets. In some cases the heat can cure the adhesive. Or the adhesive may have been cured previously at ambient temperature. The spot weld and the adhesive bond each contribute to a high strength attachment between the metal plates. However, the temperature required to create the metal-to-metal weld is much higher than the temperature the adhesive can sustain and therefore can result in a large area of degradation of the adhesive bond caused by overheating of the adhesive. In order to compensate for the degradation, the welds must be placed farther apart or larger amounts of adhesive must be used, thus increasing the cost and complexity of weldbonding applications. 
     SUMMARY OF THE INVENTION 
     A method for weldbonding together metal sheets includes applying adhesive on the surface of a first sheet and placing a second sheet atop the first sheet. The sheets are heated at a selected location to a high temperature forming a metallic weld nugget between the first sheet and the second sheet. The heating of the sheets also heats the adhesive. One or both of the sheets is then cooled in the area surrounding the selected location of heating so that the high temperature needed to create the metallic weld nugget is prevented from transferring so far beyond the selected location as to overheat the adhesive layer and thereby degrade the quality of the adhesive bond. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is an elevation view taken in section showing two metal sheets with a layer of adhesive between the metal sheets. 
         FIG. 2  is a side elevation view of weld electrodes applied to the metal sheets. 
         FIG. 3  is a section view taken in the direction of arrows  3 - 3  of  FIG. 2 . 
         FIG. 4  is a perspective view having parts broken away and in section and showing a second embodiment of the invention. 
         FIG. 5  is a perspective view having parts broken away and in section and showing a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     The following description of certain exemplary embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , a first metal sheet  10  is coated with a heat adhesive  12  and then a second metal sheet  14  is placed atop the adhesive  12 . 
     In  FIG. 2 , a weld-making tool Includes a first or lower electric resistance spot welding electrode  16  and a second or upper electric resistance spot welding electrode  18 . The first spot welding electrode  16  is applied to the first sheet  10  and the second spot welding electrode  18  is applied to the second sheet  14 . 
     Squeeze pressure is applied to the electrodes  16  and  18  and weld current is applied between the first electrode  16  and the second electrode  18  to create a resistance spot weld nugget  22  between the metal of the first sheet  10  and the second sheet  14 . The heat created in making the resistance spot weld nugget  22  heats the metal sheets  10  and  14 . The adhesive may be a heat curable adhesive that is cured by the heat. Or the adhesive may be curable at ambient temperature. 
     We have observed that exposing the adhesive  12  to a temperate exceeding about 250 degrees Centigrade will degrade the performance of the adhesive. Because the formation of the resistance spot weld nugget  22  requires the spot heating of the metal sheets  10  and  14  to a temperature of about 1550 degrees Centigrade, in the case of steel sheets, the adhesive will be degraded in a region surrounding the location of the spot weld nugget  22 . 
     Referring to  FIGS. 2 and 3 , a cooling manifold  26  is mounted on the second electrode  18 . The cooling manifold  26  is annular in shape and friction fits onto the second electrode  18 . The cooling manifold  26  has a plurality of axial flow passages or nozzles  32  that are connected to a coolant source  36 . The coolant source  36  provides a flow of coolant, such as air or a fluid mist or water, or other gas or liquid that flows onto the second sheet  14  in the region surrounding the resistance spot weld nugget  22 . The coolant will provide cooling of the metal sheet  14  and therefore reduce the size of the region of degradation of adhesive  12  that would otherwise occur around the electrodes. It will be understood that the flow rate and temperature and duration, as well as the heat dissipating characteristics of the particular coolant, will determine the extent to which the cooling and reduction in degradation is achieved. Thus with proper selection of coolant flow and by properly locating the flow onto the surface of the metal, the temperature can be controlled in a manner which is conducive to forming of the resistance weld and also achieving a high quality adhesive bond. Although there will be a region of adhesive degradation directly surrounding the resistance weld this region of degradation will be minimized and the region of undegraded adhesive bonding can be maximized. For example, in a typical application, the weld current can be conducted in order to heat the sheets, and then after a slight delay, the coolant flow can be initiated to begin a cooling of the sheets so that the propagation of heat into the sheets is controlled in a manner to properly make the metal-to-metal electric resistance weld, and yet the sheets are not unnecessarily overheated in a manner that would degrade the adhesive over a larger than necessary region. The electrodes  16  and  18  can remain in contact with the metal sheets  10  and  14  during the coolant flow, or the electrodes can be lifted out of contact but remain poised over the weld during an additional period of coolant flow as needed to achieve the optimum cooling of the sheets  10  and  14  to achieve optimum weldbonding. 
     In addition,  FIGS. 2 and 3  show that each of the electrodes  16  and  18  have an internal cooling chamber  38  that receives coolant through a pipe  40  to control the temperature of the electrodes. This cooling of the electrodes  16  and  18  contributes to the reduction in degradation provided by the flow from the cooling manifold  26 . The coolant provided by the pipe  40  is preferably a liquid, but can also come from the coolant source  36 . 
     It will be understood that a cooling manifold similar to the cooling manifold  26  can also be provided on the first electrode  16  to cool the first sheet  10 . In addition, the cooling manifold can be mounted on the electrode as shown in the drawings, or alternatively, the cooling manifold can be mounted on the electrode holder or other structure of the welding apparatus. In addition it will be understood that the electrodes will be moved along the sheets to make a series of such metal weld nuggets. 
       FIG. 4  shows another embodiment of the invention. In  FIG. 4 , a first metal sheet  50  is coated with adhesive  52  and then a second metal sheet  54  is placed atop the adhesive  52 . In  FIG. 4  the weld-making tool is a friction stir welding tool  58  that is mounted within a tool holder  60  and is rotating at high speed. The tool  58  has been plunged into the first metal sheet  50  and the second metal sheet  54  and is traversing a path along the sheets in the direction of arrow  62 . The high speed rotation of the tool  58  softens and stirs the metal of the sheets  50  and  54  to create a continuous weld nugget  64  that permanently attaches together the metal sheets  50  and  54 . In addition, the heat created by the friction stir welding will heat the metal sheets  50  and  54  and the adhesive  52 . 
     A cooling manifold  65  is mounted on the tool holder  60  and includes a nozzles  66 ,  68  and nozzle  70  that are connected to a coolant source  72 . The coolant source  72  provides a flow of coolant, such as air or a fluid mist or water that flows onto the second metal sheet  54  in the region alongside and following the friction stir weld nugget  64 . The coolant will provide cooling of the metal sheet  54  and therefore reduce the size of the region of degradation of adhesive  52  that would otherwise occur around the weld nugget  64 . It will be understood that the flow rate and temperature and duration, as well as the heat dissipating characteristics of the particular coolant, will determine the extent to which the cooling and reduction in degradation is achieved.  FIG. 4  also shows the nozzle  66  positioned behind the tool  58  so that the nozzle  66  follows along behind the tool  58  as the tool  58  moves in the direction of arrow  62  and the weld nugget  64  is cooled after its creation. The nozzles  68  and  70  are positioned along the left and right side of the path of the movement of the tool  58 . Any number of such nozzles can direct the coolant flow onto the metal sheets at a selected distance from the weld tool  58 , or only in a path following behind the weld tool  58 , with the number and location of the nozzles chosen so that cooling provided by the coolant does not adversely affect the heating of the sheets that is required to achieve the friction stirring of the metal sheets and yet will optimize the cooling of the sheets to minimize the degradation of the adhesive bond. 
       FIG. 5  shows another embodiment of the invention. In  FIG. 5 , a first metal sheet  90  is coated with adhesive  92  and then a second metal sheet  94  is placed atop the adhesive  92 . A first electrode roller  98  is carried by a tool holder  100  and is pressed against the second metal sheet  94 . A second electrode roller  104  is carried by a tool holder  108  and is pressed against the first metal sheet  90 . The electrode rollers  98  and  104  roll along the metal sheets  90  and  94 , which can be accomplished either by moving the tools holders  100  and  108  along the sheets, or by moving the sheets  90  and  94  between the electrode rollers  98  and  104 . Weld current is conducted between the first electrode roller  98  and the second electrode roller  104  to create a weld nugget  110  that extends in a continuous path within the first sheet  90  and the second sheet  94 . The heat created in making the resistance weld nugget  110  simultaneously heats the metal sheets  90  and  94  and the adhesive  92 . 
     A cooling manifold  114  is mounted on the tool holder  100  and is connected to of a coolant source  118 . A plurality of nozzles, shown for example at  122  and  124  are mounted on the tool holder  100  and are connected with the cooling manifold  114  to provide a flow of coolant, such as air or a fluid mist or water that flows onto the metal sheet  94  in the region alongside and following the electrode roller  98 . Likewise, a cooling manifold  128  is mounted on the tool holder  108  and connects with a coolant source  130  and nozzles  134  and  136  to spray coolant onto the metal sheet  90 . The coolant will provide cooling of the metal sheets  90  and  94  and therefore reduce the size of the region of degradation of adhesive  92  that would otherwise occur around continuous weld nugget  110 . It will be understood that the flow rate and temperature and duration, as well as the heat dissipating characteristics of the particular coolant, will determine the extent to which the cooling and reduction in degradation is achieved. 
     Thus it is seen that the invention will optimize weldbonding of metal sheets by allowing the creation of high temperature to make the metal-to-metal weld nugget, and yet control the metal temperatures to achieve optimal heating without excessive adhesive degradation. 
     Although the drawings show the example of two metal sheets being weldbonded together, the aforedescribed weldbonding method can be used when two or three or more sheets are being attached together. In addition, rather than simple metal sheets, one or both of the metal sheets can be a laminated metal of metal-polymer-metal construction, and the coolant flow will minimize the possibility that the heating will degrade the polymer layer of the laminated metal.