Abstract:
A method of manufacturing welded metal sheets is presented that leaves exposed (i.e., visible) surfaces of the sheets substantially free of any weld marks following welding, without any additional steps performed at the area of the weld following the weld. Thus, manufacturing efficiency may be increased and costs lowered. An apparatus with three stacked metal sheets which may be welded according to the method of manufacturing is also disclosed herein.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a method of welding three metal sheets and an apparatus formed using the same. 
       BACKGROUND OF THE INVENTION 
       [0002]    Welding operations are often utilized as a means for connecting metal components. There are many types of welding processes, such as spot welding, laser welding, and friction stir welding. Typically, weld marks are apparent at the area of a weld due to the high temperatures and physical changes in the material following a weld. Therefore, it is often necessary to perform “clean-up” processes following welding in order to minimize the appearance of the weld marks, especially in applications where the aesthetic appearance of the welded component is important. Such additional processes increase manufacturing time and cost. 
       SUMMARY OF THE INVENTION 
       [0003]    A method of manufacturing welded metal sheets is presented that leaves exposed (i.e., visible) surfaces of the sheets substantially free of any weld marks, without any additional steps performed at the area of the weld following the weld. Thus, manufacturing efficiency may be increased and costs reduced. An apparatus with three stacked metal sheets which may be welded according to the method of manufacturing is also disclosed. 
         [0004]    The method of manufacturing includes forming a first projection portion extending from one side of a first metal sheet, and a second projection portion extending from an opposing side of the same first metal sheet. The projection portions may be formed to a desired shape using a punch and die set. Prior to forming the projections, the metal sheets may be coated, such as with a zinc coating, for corrosion protection. Under the method, the first metal sheet with the projections formed thereon is placed between a second and a third metal sheet (i.e., the sheets are stacked) such that the first projection portion extends toward an inner surface of the second metal sheet and the second projection portion extends toward an inner surface of the third metal sheet. Next, welding electrodes are placed adjacent the metal sheets, in alignment with the projection portions. The first metal sheet is then welded to the second and third metal sheets at the projection portions. The exposed outer surfaces of the second and third metal sheets are substantially free of weld marks, because the projection portions weld to inner surfaces of the second and third sheets. The substantial absence of weld marks is also due to the weld parameters enabled under the method, such as utilizing welding electrodes with substantially flat weld contact areas that span the entire width of the area of the inner sheet having the projection portions, which distributes heat and force more evenly, energizing the electrodes for not more than about 4 milliseconds, and using a weld force of not more than about 200 pounds also contributes to the absence of weld marks. At most, the method may result in a surface depression on the outer surfaces of the second and third metal sheets of not more than 0.1 millimeters, much less than the 0.3 to 1.0 millimeter depressions typically resulting from welding processes. Furthermore, because of the relatively short weld time, no cooling period or cooling processes are required before the welding electrodes may be reused to weld another area of the stacked sheets or another set of stacked metal sheets, such as on a production line. The method may be especially useful for automotive body panels, home appliances, and other products with high surface appearance requirements. 
         [0005]    Pursuant to the method, an apparatus may be produced that includes three stacked metal sheets including two outer metal sheets juxtaposed on either side of an inner metal sheet. The inner metal sheet has a first projection portion extending toward an inner surface of one of the outer metal sheets and a second projection portion extending toward another inner surface of the other outer metal sheet. The inner surfaces of the outer metal sheets are welded to the inner metal sheet at the respective projection portions such that the outer surfaces of the outer sheets are characterized by a substantial absence of weld marks. The projection portions may have different shapes that are configured to enhance the goal of achieving a secure weld without substantially affecting the visible appearance of the outer sheets. For example, triangular or rounded extensions may be used. Also, two projection portions may extend toward one of the outer sheets on either side of another projection portion extending toward the other outer metal sheet. This balanced design may help to alleviate any twisting of the inner metal sheet that may occur during formation of the projection portions. 
         [0006]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic illustration in side view of a projection welding system including three stacked metal sheets with an inner sheet having a first embodiment of opposing projection portions with a substantially triangular shape; 
           [0008]      FIG. 2  is a schematic illustration in cross-sectional side view of the three stacked metal sheets of  FIG. 1 ; 
           [0009]      FIG. 3  is a schematic illustration in cross-sectional side view of a die set used to form the projection portions in the inner sheet of  FIGS. 1 and 2 ; 
           [0010]      FIG. 4  is a schematic illustration in cross-sectional side view of a second embodiment of three stacked metal sheets with an inner sheet having opposing projection portions of a substantially rounded shape; 
           [0011]      FIG. 5  is a schematic illustration in cross-sectional view of a third embodiment of three stacked metal sheets with an inner sheet having opposing projection portions of substantially triangular shape with one extending toward an upper sheet and two extending toward a lower sheet; 
           [0012]      FIG. 6  is a schematic illustration in plan view of the inner metal sheet of  FIGS. 1 and 2  showing the opposing projections; 
           [0013]      FIG. 7  is a schematic illustration in cross-sectional view of the three stacked metal sheets of  FIGS. 1 ,  2  and  6  after welding; 
           [0014]      FIG. 8  is a schematic illustration in plan view of the welded metal sheets of  FIG. 7  illustrating the absence of weld marks on an exposed outer surface of the upper sheet; and 
           [0015]      FIG. 9  is a flow chart illustrating a method of welding metal sheets 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  shows a projection welding system  10  capable of welding three stacked metal sheets together without leaving weld marks on the exposed outer surfaces of the outer sheets, as described below. The stacked metal sheets include an inner sheet  12  nested between two outer sheets  14 ,  16 . The stacked metal sheets  12 ,  14 ,  16  are held securely between a stationary member  18  and an adjustable clamp  20 . Welding electrodes  22 ,  24  are placed in contact with the outer sheets  14 ,  16 . Due to the special construction of the inner sheet  12  and the electrodes  22 ,  24 , the welding process secures the sheets  12 ,  14 ,  16  to one another in an efficient and dependable manner, with evidence of the weld being nearly invisible on the exposed surfaces of the connected metal sheets  12 ,  14 ,  16 . 
         [0017]    Referring to  FIG. 2 , a cross-sectional view of the stacked sheets  12 ,  14 ,  16 , prior to welding, reveals first and second projection portions  26 ,  28  formed in the inner metal sheet  12 . The first projection portion  26  is substantially triangular in shape and extends outward from surface  29  of inner sheet  12  toward an inner surface  30  of metal sheet  14 . The second projection portion  28  is also substantially triangular in shape and extends outward from surface  31  of inner sheet  12  toward an inner surface  32  of metal sheet  16 . Referring to  FIG. 3 , a die set  34  used to form the projection portions  26 ,  28  on the inner sheet  12  includes a punch  36  movable by action of an upper die  38  toward a lower die  40  such that substantially triangular cavities  42  and formations  44  create the projection portions  26 ,  28  in the inner sheet  12  when the previously flat inner sheet  12  of  FIGS. 1 and 2  is placed between the upper and lower dies  38 ,  40 . Referring to  FIG. 6 , the inner sheet  12  is shown from above and rotated 90 degrees with respect to  FIG. 2 . The first projection portion  26  appears as an elevation while the second projection portion  28  appears as a depression. Each of the sheets  12 ,  14 ,  16  is preferably but not necessarily coated with a zinc coating  52  on either side thereof, as illustrated in  FIG. 2 , to improve corrosion resistance as well as to promote the ability to draw the projection portions  26 ,  28 . 
         [0018]    Referring again to  FIG. 1 , the electrodes  22 ,  24  are specifically designed with a substantially flat contact portion  54 ,  56 , respectively, that spans the width W (see  FIG. 2 ) of the inner sheet  12  from the beginning to the end of the projection portions  26 ,  28 . The flat contact portions  54 ,  56  allow current flowing through the electrodes  22 ,  24  (when energized) to be distributed across the entire width W of the projection portions  26 ,  28 , better distributing the heat and force load of the electrodes  22 ,  24 , to achieve a secure weld, as illustrated in  FIG. 7 , with the first projection portion  26  melting into the inner surface  30  of metal sheet  14  and the second projection portion  28  melting into the inner surface  32  of metal sheet  16 . A force load of 200 pounds with current applied for 4 milliseconds was found to achieve welds of sufficient integrity for uses such as in automotive body panels. As shown in  FIG. 7 , a surface depression D of 0 to 0.1 mm is formed at the outer surface  48  of outer sheet  14 . This surface affect is not apparent in the schematic plan view of sheet  14  in  FIG. 8 . The deformation of outer surface  50  of outer sheet  16  is similarly no more than a 0.1 mm depression in the area of the weld. The minimal surface depression achieved with the projection welding methods described herein is a function of the force applied to the stack of sheets  12 ,  14 ,  16  with the electrodes  22 ,  24 , the relatively short time span for which current is applied, and the electrode conditions. Neither the length of time of applied current nor the temperature of the metal sheets  12 ,  14 ,  16  at the area of the projection portions  26 ,  28  where the weld occurs are factors affecting surface depression D. The weld time and temperature are minimal in comparison to other welding techniques. These factors affecting surface depression with the present method and system are in contrast to the greater number of factors affecting surface depression with typical resistance welding, which typically runs between 0.3 to greater than 1 millimeter. For such typical welding processes, in addition to weld force, applied current and electrode conditions, such factors also include welded metal properties, the length of time the current is applied, the angle of the weld (i.e., angle of the electrodes relative to the metal sheets), the electrode size, and the quality of the electrode dressing, as is understood by those skilled in the art. 
         [0019]    Projection portions of various shapes may be used equally as well as the triangular projection portions  26 ,  28 . For example,  FIG. 4  shows another embodiment of stacked metal sheets  112 ,  114 ,  116 , shown prior to welding. The inner metal sheet  112  is formed with projection portions  126 ,  128  which are substantially rounded. Such projection portions  126 ,  128  may be formed using a die pair similar to that of  FIG. 3  with differently shaped cavities and formations, as is well understood by those skilled in the art.  FIG. 5  shows yet another embodiment of stacked sheets  212 ,  214 ,  216  within the scope of the invention. In this embodiment, a first projection portion  226  extends toward an inner surface of the metal sheet  214 , a second projection portion  228  extends toward the inner surface of the metal sheet  216 , and a third projection portion  230  extends toward the inner surface of the metal sheet  216 , with the first projection portion  226  being between the projection portions  228  and  230 . The addition of projection portion  230  may alleviate twisting of the inner metal sheet  212  in the area of the projection portions  226 ,  228 ,  230  about the plane formed by the inner metal sheet  212  in comparison to embodiments with only two projection portions. Either of the embodiments of  FIGS. 4 and 5  may be used in the projection welding system  10  of  FIG. 1  in lieu of stacked sheets  12 ,  14 ,  16  to accomplish the welding with virtually no weld marks apparent on the exposed outer surfaces of the outer sheets  114 ,  116  and  214 ,  216 , respectively. 
         [0020]    Referring to  FIG. 9 , a method of welding metal sheets  300  is described for purposes of discussion with respect to the projection welding system  10  of  FIG. 1  and the stacked metal sheets  12 ,  14 , and  16 . However, it should be understood that the method  300  is not limited to use with these particular devices and components. The method  300  includes step  302 , providing a first metal sheet (inner metal sheet  12 ) having projection portions  26 ,  28 , extending outward from opposing surfaces  29 ,  31 , respectively. Step  302  may optionally include step  304 , coating the metal sheets  12 ,  14 ,  16  with coating, such as a zinc coating. Step  302  may also includes as step  306 , forming the projection portions with a punch and die, following step  304 . Steps  304  and  306  may alternatively be performed by one or more different entities than the entity undertaking step  302 . 
         [0021]    Following step  302 , the method  300  includes step  308 , placing second and third outer metal sheets  14 ,  16  adjacent the respective opposing surfaces  29 ,  31  of the first (inner) metal sheet  12  to form a set of stacked sheets. Next, the method  300  includes step  309 , placing welding electrodes  22 ,  24  adjacent the metal sheets  12 ,  14 ,  16  in alignment with the projection portions  26 ,  28 . Steps  308  and  309  are in preparation for step  310 , welding the projection portions  26 ,  28  to respective inner surfaces  30 ,  32  of the outer metal sheets  14 ,  16 . Step  310  is accomplished such that the outer surfaces  48 ,  50  of the outer sheets  14 ,  16  are left with a substantial absence of weld marks following the weld (i.e., with no more than a surface depression D (of  FIG. 7 ) in the range of 0-0.1 mm). Notably, step  310  may be carried out with welding electrodes  22 ,  24  having substantially flat contact portions  54 ,  56  spanning the projection portions  26 ,  28 , with a weld force of approximately 200 pounds and the electrodes  22 ,  24  energized for approximately 4 milliseconds. Because step  310  may be carried out with such a relatively low weld force and duration, the electrodes may be reused in step  312  for another welding operation, such as welding a subsequent set of stacked metal sheets, or a subsequent set of projections on the same stacked metal sheets, without any specific cooling processes or cooling period necessary prior to the reuse. 
         [0022]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.