Abstract:
A system for and method of projection weld-bonding a plurality of workpieces, includes the steps of securing at least one adhesive layer having a plurality of projections embedded therein intermediate the workpieces, and engaging the workpieces with a resistance welding apparatus such that only the projections fuse to form the weld pool, and the layer cures to form an adhesive seal around the welds, together the adhesive layer and projections cooperatively forming a reinforced joint.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This U.S. Non-Provisional patent application claims the benefit of and is a continuation-in-part from pending U.S. Non-Provisional application Ser. No. 11/937,518, entitled SYSTEM FOR AND METHOD OF PRODUCING INVISIBLE PROJECTION WELDS filed on Nov. 9, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to resistance welding systems and bonding methods, and more particularly concerns a resistance welding system for and method of weld-bonding a plurality of workpieces utilizing projections embedded within an adhesive layer. 
         [0004]    2. Discussion of Prior Art 
         [0005]    Resistance mash welding (e.g., conventional spot or seam welding) remains the most common method of joining metallic workpieces in various industries, including automotive manufacture and construction. In this method, the workpieces  1 , 2  are typically secured in a fixed condition, and then engaged by two electrodes  3 , 4 , as shown in  FIG. 1 . The electrodes  3 , 4  function to co-extensively transmit a sustained force and an electric current through the workpieces until the combined resistance at their interface generates sufficient heat energy to produce a molten weld pool therebetween. Undesirably, however, exterior anomalies and aesthetic concerns are also often experienced. For example, depressions  5  caused by the force exuded upon the workpieces ( FIG. 1   a ), whiskers (i.e., short pieces of material sticking through the root side of the weld joint), and spatters (i.e., satellites formed by loose droplets of molten material during welding) are just a few of the common by-products of resistance welding processes. 
         [0006]    These aesthetic concerns are typically addressed during a finishing process, wherein depressions are filled and surfactants are milled prior to painting. Invariably, however, these finishing processes result in increased costs, including but not limited to additional material and labor. The need to address aesthetic concerns also results in a longer period of manufacture, thereby impacting productivity. Even where a finishing process is provided, traces of the exterior anomalies remain and are often easily detectable through the paint. 
         [0007]    Finally, another concern relating to fusion welding involves the production of relatively brittle inter-metallic areas that form within the joint when workpieces of dissimilar material (such as aluminum and steel) are melted together. These areas typically present lower load bearing strength in comparison to the homogenous areas of the joint. 
         [0008]    More recently, other methods of metallurgically joining workpieces have been developed that utilize other less aesthetically impacting technology, such as thermal laser brazing, some forms of solid state (e.g., friction, ultrasonic, or explosive) welding, and diffusion bonding. It is appreciated, however, that these methods present more complex and therefore costly technologies in comparison to conventional resistance welding. As such, these technologies have achieved limited market penetration and are relegated to relatively small subsets of applications. 
         [0009]    Yet another conventional method of joining workpieces is adhesive bonding. This method utilizes an epoxy or adhesive layer to join the workpieces  1 , 2 . It is appreciated that adhesive bonding does not require the energy input of welding to coalesce the base material and thereby form the joint. It is further appreciated that, adhesive bonding forms a better seal that separates the interior of the assembly from outside contaminants, and results in less surface deformation than do prior art welding applications of comparable extent. However, it is also appreciated that this method of joining typically provides lower overall strength in comparison to welded joints. 
         [0010]    Thus, there remains a need in the art for a facilely implemented method of joining a plurality of workpieces that combines the benefits of welding and adhesive bonding applications, and more particularly, reduces exterior surface anomalies and aesthetic concerns, while maintaining the superior strength of welding and the protective seals of adhesive bonding. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    Responsive to this need, an improved method of weld-bonding a plurality of similar or dissimilar workpieces that eliminates exterior surface anomalies is presented. The method involves the use of an adhesive layer having a plurality of projections embedded therein. The inventive system and method disclosed herein is useful among other things for providing a facilely implemented solution that requires no new or additional resistance welding equipment. 
         [0012]    The method is useful for producing invisible fusion welds, which makes it ideal for exterior product welds (i.e., welds wherein the exterior surface of one or both of the engaged workpieces present an exterior product surface). It is appreciated that decreasing the amount of and more preferably eliminating exterior surface anomalies reduces the need for and extent of a finishing process, and thereby results in a reduction of the afore-mentioned costs. 
         [0013]    The method is further useful for providing a sealed joint that forms a barrier to outside contaminants, such as oil, grease, water, and particulate matter. The inventive method produces a combined welded and adhesively bound joint that presents greater structural strength in comparison to welding or adhesive bonding individually. Where used in an automotive setting, such as roof deck construction, it is also appreciated that the invention produces better weld quality in that a larger bonding area is realized, and enables the roof ditch width to be reduced. Finally, it is appreciated that the inventive process of embedding a plurality of projections in a layer of adhesive material eliminates the time consuming need to fabricate the projections, and thereby eliminates the need for a fabrication station and/or hardware. 
         [0014]    A first aspect of the invention concerns a method of weld-bonding a plurality of workpieces defining apposite exterior most surfaces utilizing at least one continuous adhesive layer comprising adhesive material and a plurality of projections embedded therein. The method comprises the steps of securing the layer in a welding position relative to one of the workpieces, and then securing the remainder of the workpieces relative to the layer and workpieces, so as to present a fixed relative condition. In the condition, each projection and the layer(s) are intermediately positioned between adjacent workpieces, such that each projection and the adjacent workpieces cooperatively define at least one initial axis of engagement. The method generally concludes by appositely engaging the surfaces along the axis with a resistance welding apparatus to deform and fuse the projections, and heat the adhesive material past a minimum temperature, so as to cooperatively form the joint. 
         [0015]    Thus, a second aspect of the invention concerns an article of manufacture adapted for use with the inventive weld-bonding process. The article of manufacture comprises a layer of adhesive material and a plurality of spaced metal projections embedded therein. 
         [0016]    Other aspects and advantages of the present invention, including preferred projection configurations, as well as methods performing the associative weld-bonding will be apparent from the following detailed description of the preferred embodiment(s) and the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0017]    Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein: 
           [0018]      FIG. 1  is an elevation view of a prior art resistance spot welding apparatus and a plurality of workpieces, in a before welding condition; 
           [0019]      FIG. 1   a  is an elevation view of the prior art apparatus and the workpieces shown in  FIG. 1 , in an after welding condition, particularly illustrating exterior surface depressions; 
           [0020]      FIG. 1   b  is an elevation view of a prior art welding apparatus having a “C”-shaped structural frame; 
           [0021]      FIG. 2  is an elevation view of a resistance welding system in accordance with a preferred embodiment of the present invention, wherein a free-body projection presenting a circular cross-sectional configuration is intermediately positioned between first and second workpieces; 
           [0022]      FIG. 2   a  is an elevation view of the system shown in  FIG. 2 , after the welding force and before the current load have been applied to the workpieces and projection; 
           [0023]      FIG. 2   b  is an elevation view of the system shown in  FIG. 2 , after the welding force and current load have been applied to the workpieces and projection; 
           [0024]      FIG. 3  is an elevation view of a free-body projection having spaced top and bottom curvilinear surfaces in accordance with a preferred embodiment of the present invention, intermediately positioned between first and second workpieces; 
           [0025]      FIG. 4  is a perspective view of a free-body projection having a diamond cross-section with chamfered edges in accordance with a preferred embodiment of the present invention engaged by a dual-electrode welding apparatus (in partial view), particularly illustrating projection-workpiece and electrode-workpiece interfaces; 
           [0026]      FIG. 4   a  is an elevation view of the projection and workpieces shown in  FIG. 4 , particularly illustrating the projection intermediately positioned between first and second workpieces; 
           [0027]      FIG. 5  is a perspective view of an annular projection having a square horizontal cross-section, in accordance with a preferred embodiment of the present invention; 
           [0028]      FIG. 6  is a perspective view of an annular projection having a circular horizontal cross-section, in accordance with a preferred embodiment of the present invention; 
           [0029]      FIG. 7  is a perspective view of a free-body projection having an “H”-shaped vertical cross-section, in accordance with a preferred embodiment of the present invention; 
           [0030]      FIG. 8  is an elevation view of the projection shown in  FIG. 7 ; 
           [0031]      FIG. 9  is a perspective view of a lower workpiece, a projection recently placed in the welding position, and a roll dispenser comprising a dispensing reel, a wound tape having a plurality of embedded projections therein, a projection ejector, and a receiving reel, in accordance with a preferred embodiment of the invention; 
           [0032]      FIG. 10  is a side elevation view of a portion of the tape shown in  FIG. 9 ; 
           [0033]      FIG. 10   a  is a cross-section of the portion of tape shown in  FIG. 10 , taken along the line A-A therein; 
           [0034]      FIG. 11  is a perspective view of a lower workpiece, a projection and an encircling portion of tape recently placed in the welding position, and a roll dispenser comprising a dispensing reel, a wound tape having a plurality of embedded projections therein, a modified projection ejector and tape cutter, and a receiving reel, in accordance with a preferred embodiment of the invention; 
           [0035]      FIG. 12  is a schematic elevational view of a plurality of workpieces, an adhesive layer having embedded spherical projections therein, and a plurality of electrodes in a before weld-bonding condition, in accordance with a preferred embodiment of the present invention; 
           [0036]      FIG. 12   a  a schematic elevational view of the workpieces, layer, projection and electrodes shown in  FIG. 12 , in a post weld-bonding condition; 
           [0037]      FIG. 13  is a planar view of an elongated layer, particularly illustrating a plurality of spherical projections, adhesive material, and constant projection spacing, in accordance with a preferred embodiment of the invention; 
           [0038]      FIG. 13   a  is a planar view of a cross-shaped layer having reduced projection spacing adjacent the outermost lateral edges, in accordance with a preferred embodiment of the invention; and 
           [0039]      FIG. 13   b  is a planar view of a layer presenting a planar sheet configuration, particularly illustrating a plurality of elongated projections or wires in a mesh configuration, and discontinuous adhesive material in a radial band pattern, in accordance with a preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    The present invention concerns a system  10  ( FIGS. 2-13   b ) for and method of producing an invisible spot or seam weld  12  ( FIG. 3 ) between a plurality of workpieces  14 , 16 , such as a two-sheet “stack-up” of automotive sheet metal. The inventive system  10  is configured to produce the invisible weld  12  respective to the exterior of the constructed workpiece assembly (compare  FIGS. 1   a  and  2   b ). That is to say, exterior surface deformations or anomalies, such as surface depressions, are not formed during the inventive resistance welding method described herein. It is appreciated that the invention, therefore, increases the aesthetic appeal and reduces manufacturing costs associated with the assembled product. The invention is adapted for use with conventional resistance mash welding devices, such as the apparatus  18  generally depicted in  FIG. 1   b , and does not require additional welding equipment and/or modifications. 
         [0041]    In the illustrated embodiments, a plurality of two workpieces  14 , 16  of equal thickness is shown; however, the inventive system  10  may be utilized to invisibly weld a greater plurality, or structural components having variable thickness or otherwise configuration by modifying and applying the teachings of the system  10  as required. The workpieces  14 , 16  preferably present planar configurations ( FIGS. 2 and 4 ) defining generally flat surfaces and peripheral edges, and may be formed of a wide range of metals, including steel and aluminum alloy. In the welding position, the workpieces  14 , 16  present oppositely engageable exterior surfaces  14   a , 16   a , and interior surfaces  14   b , 16   b  apposite and parallel to the respective exterior surface ( FIG. 2 ). 
         [0042]    As illustrated and further described herein, the inventive weld  12  is produced by engaging at least one free-body projection  20  positioned intermediate the workpieces  14 , 16  with a resistance welding apparatus  18 . The apparatus  18  may present a single-sided welding apparatus, so as to streamline the assembly process. In this configuration, a conductive backing block (not shown) may be provided to support the lower workpiece  16  either adjacent the weld  12  or at a convenient location spaced from the joint. If the workpieces  14 , 16  and projection  20  present sufficient stiffness, then a support is not necessary. 
         [0043]    More preferably, the system  10  includes a dual-electrode welding apparatus  18  (as generally shown in the illustrated embodiments), such as the type having a “C”-shaped structural frame  22  ( FIG. 1   b ). In this configuration, the apparatus  18  includes a first electrode  24 , a transport mechanism (not shown), and an identical back-up electrode  26 . As known in the art, the electrodes  24 , 26  oppositely engage the workpieces  14 , 16 , to cooperatively impart a welding force thereupon and complete an electric potential. Thus, the electrodes  24 , 26  are preferably configured to contact the workpiece surfaces  14   a , 16   a  adjacent the projection  20 , so as to maximize the applied force to and minimize the travel path of the current through the projection  20 . As further described herein, the preferred apparatus  18  is operable to transmit the force and current load non-concurrently, wherein the force drive mechanism (also not shown) is actuated first. 
         [0044]    Where seam welding is desired, the apparatus  18  includes wheel electrodes that rollingly engage the workpieces  14 , 16 , as known in the art. The projection width is preferably less than the electrode wheel width, but a maximum lateral dimension is not defined. In this configuration, it is appreciated that elongated and even complex sinuous welds can be produced. It is also appreciated that the invention provides the added benefits of determining the precision of weld formation by the placement and configuration of the projection rather than by the accuracy of the electrode wheel path. 
         [0045]    The interior surfaces  14   b , 16   b  of the workpieces are spaced by and abut the free-body projection  20 . As a result, the projection  20  and workpieces  14 , 16  cooperatively define top and bottom points of contact, p, and at least one axis of engagement, α, passing through the points ( FIG. 2 ). As previously mentioned, once the projection  20  has been properly positioned, and the workpieces  14 , 16  and projection  20  are secured in a relatively fixed condition (e.g., by clamping), the exterior surfaces  14   a , 16   a  are engaged by the welding apparatus  18 , so as to transmit the force and current co-axially with the axis or axes of engagement. It is appreciated that an adhesive  20   a  affixed to the projection  20  or the workpieces  14 , 16 , or magnetism may be utilized to help retain the projection in the welding position (prior to clamping). 
         [0046]    The preferred projection  20  and workpieces  14 , 16  are cooperatively configured such that the projection  20  deforms and completely fuses prior to any deformation of the workpieces  14 , 16  at or near their exterior surfaces  14   a , 16   a . To that end, the projection  20  consists of material having a mean melting temperature less than that of the workpiece material(s); and more preferably less than ninety percent of the melting temperature of the workpiece material. Once molten, the projection  20  predominately forms the weld pool. It is appreciated, however, that a small quantity of workpiece material also fuses along the projection-workpiece interfaces, as part of a “wetting” process. The wetting process enables the formation of metallurgical bonds between the projection  20  and workpieces  14 , 16 . 
         [0047]    Suitable projection materials include mild steel, aluminum alloys, silicon-bronze wire, or a combination thereof. The applied material is selected based upon the physical and chemical properties, including the relative “wettability,” hardness and melting temperatures, of the workpiece material(s). For example, where the workpiece material is electrogalvanized steel, a silicon-bronze projection  20  is preferably utilized, as it is appreciated that such combination of materials produce sufficient wetting along the projection-workpiece interfaces. In another example, where the workpieces  14 , 16  are formed of hard steel, the projection  20  preferably consists of mild steel having a 5 to 10 micron (i.e.,  10   −6  m) thick electrogalvanized zinc coating, as it is appreciated that the zinc coating facilely wets brazed workpiece material. 
         [0048]    To further prevent exterior surface deformation, the projection  20  is configured so as to present minimal top and bottom projection-workpiece interfaces, as determinable by the lateral cross-section and depth of the projection  20 . Each projection-workpiece interface, pwi, presents an area substantially smaller than (e.g., less than seventy-five, and more preferably less than twenty-five percent of) each of the electrode-workpiece interfaces, ewi ( FIG. 4 ). The projection  20  presents a width profile, as measured along its height, h, that maintains this ratio as the projection fuses. It is appreciated that the smaller areas of the projection-workpiece interfaces compared to the areas of the electrode-workpiece interfaces, result in greater pressure being exerted upon the projection  20 . More preferably, to further increase this ratio, modified top and bottom electrodes  24   a , 26   a  ( FIG. 4   a ) defining flat workpiece engaging surfaces substantially (e.g., 1.5 to 3 times) greater in diameter than those of standard size electrodes are utilized. 
         [0049]    In one suitable configuration, the projection  20  presents curvilinear engagement surfaces that provide singular points of contact, p. For example, the projection  20  may define a purely circular cross-section, as shown in  FIG. 2 . Alternatively, the curvilinear surfaces may be vertically spaced or elongated as shown in  FIG. 3 , so as to increase projection volume, maintain a single lateral point of contact, and reduce the maximum lateral projection width. It is appreciated that an initial single point of contact, as in a spherical or ellipsoidal projection  20  maximizes the pressure at and therefore minimizes the welding force required to initially deform the projection  20 . 
         [0050]    Other projection configurations include polygonal cross-sectional shapes, such as the diamond configuration shown in  FIGS. 4 and 4   a . In this configuration, the edges of the diamond are preferably chamfered to present flat workpiece engaging surfaces  20   a  not more than 1 mm in width; and the projection  20  is oriented so as to engage the workpieces  14 , 16  along the flat engaging surfaces  20   a.    
         [0051]    The projection  20  further defines an overall longitudinal length, l ( FIG. 7 ) that changes during fusion based on the longitudinal configuration of the projection versus the height of engagement. In this regard, it is appreciated that a segment of wire, for example, presents a generally constant l, while a spherical projection  20  will present a constantly changing l as it fuses. The height ( FIG. 8 ) and length ( FIG. 7 ) of the projection  20  are sized to produce the desired weld joint size/area, and are more specifically determined based on the workpiece material and application. For example, where the workpieces  14 , 16  consists essentially of steel, the workpiece thickness is between 0.6 and 2 mm, and the application makes the provision of an effective joint highly critical, the projection length is preferably within the range 5 to 20 mm. More preferably, the projection length is approximately 9 mm for workpiece thickness within a range of 0.6 to 1.2 mm, and approximately 12 mm within a range of 1.2 to 2 mm. The projection diameter is within the range 0.6 to 2 mm, and is more preferably 0.9 mm for workpiece thickness within the range 0.6 to 1.2 mm, and 1.4 mm for thickness within the range 1.2 to 2 mm. 
         [0052]    In another embodiment, the projection  20  may present an annular longitudinal configuration having a wall thickness within the range of 1 to 3 mm. Shown in  FIGS. 5 and 6  are square and circular embodiments of this configuration. Where spot welding is to be performed, the annular projection  20  presents a maximum outside diameter not greater than the minimum lateral dimension of the electrode-workpiece interfaces. It is appreciated that in this configuration the weld footprint (i.e., effective area of the weld) is maintained, even though the amount of projection material to be fused, and therefore welding force and current load required are reduced. 
         [0053]    Finally, in yet another embodiment shown in  FIGS. 7-9 , the projection  20  may present an “H”-shaped vertical cross-section formed by a cross member  28  that bisects and interconnects two preferably parallel outer members  30 , 32 . In this configuration, the projection  20  is oriented so as to engage the workpieces  14 , 16  along the tops and bottoms of the parallel outer members  30 , 32 . Thus, initial projection-workpiece interfaces, in this configuration, are limited to the wall thickness, T, and depth, d. As shown in  FIG. 8 , the cross member  28  presents a width, l, and a height or thickness, t; while the outer members  30 , 32  further present a height, h. More preferably, the cross member length and outer member height are cooperatively configured, such that l is equal to h times a multiple within the range of 3 to 8. For example, h may be within the range of 0.7 to 2 mm, T within the range of 0.5 to 1.5 mm, l within the range of 3 to 8 mm, t within the range of 0.2 to 0.5 mm, and d within the range of 0.6 to 1.2 mm. 
         [0054]    In operation, the weld  12  is preferably formed by a welding apparatus  18  operable to transmit the welding force for a minimum period (e.g., 300 ms) prior to transmitting the current load ( FIGS. 2-2   b ). As shown in intermediate  FIG. 2   a , it is appreciated that under a pure force load the projection  20  may undergo noticeable deformation, as occasioned by a harder workpiece material. More preferably, however, the projection  20  does not show deformation under the applied force load. It is appreciated that the generated stresses also facilitate fusion once the current load is applied, which thereby results in energy conservation. The force and current loads are then concurrently applied for a sustained period sufficient to fuse the projection  20  (e.g., 5 to 50 ms). Immediately upon the complete fusion of the projection  20 , the force and current loads are terminated, so that deformation does not begin to form at the exterior surfaces  14   a , 16   a  ( FIG. 2   b ). Both periods are preferably optimized through trial and error for a given application (i.e., set of variables) and recorded in a storage medium (not shown). 
         [0055]    In a second mode of operation, the preferred system  10  is configured to autonomously position the projection  20  in an assembly-line setting; and to that end, includes a roll dispenser  34 , such as the type used to place rivets during conventional rivet bonding applications. As shown in  FIGS. 9-11 , the roll dispenser  34  includes a dispensing reel  36  storing a wound tape  38  having a plurality of equally spaced embedded projections  20  therein, and a receiving reel  40 . An ejector (or “gun”)  42  is utilized to remove the projections from the tape  38  ( FIG. 9 ). The dispenser  34  is configured to translate into a placement position once the lower workpiece  16  has been properly secured, and out of the placement position once a projection  20  has been properly ejected and positioned. After the upper workpiece  14  is secured atop the projection  20 , the weld  12  is produced, the joined workpieces  14 , 16  are removed, and a new lower workpiece  16  has been properly secured, the tape  38  is advanced one projection spacing, and the dispenser  34  is re-turned to the placement position. In an exemplary configuration ( FIGS. 9-11 ), the tape  38  is advanced by drabbing a plurality of periphery holes  44  defined by the tape  38  with prongs  46  presented by the receiving reel  40 . Alternatively, it is appreciated that the dispenser  34  may present a fixed station, wherein the workpiece and newly positioned projection  20  perform the translation. The tape may be 10 to 15 mm wide and 0.5 mm thick. 
         [0056]    The dispenser  34  and apparatus  18  are preferably programmably controlled, and present a closed-loop feedback control system  10 . In this configuration, for example, the system  10  may further include at least one sensor  48  ( FIG. 11 ) operable and oriented to detect whether the workpieces  14 , 16  and/or projection  20  has been properly positioned. The sensor  48  is communicatively coupled (e.g., connected by hard-wire or short-range wireless technology) to the dispenser  34  and apparatus  18  through a controller (not shown). It is appreciated that this facilitates a mass assembly process, wherein invisible projection welding is performed to join a large plurality of sets of workpieces over a welding period. Moreover, the system  10  may be programmably configured to access the storage medium, so as to recall previously determined optimized periods for a given application. 
         [0057]    In a third mode of operation, the tape  38  is formed of material that forms an adhesive sealant when heated to a minimum temperature. In this configuration, the mode further includes positioning the projection  20  and an encircling portion  50  of the tape in the weld position. The portion  50  is produced, for example, by cutting the portion  50  from the remainder of the tape  38  with a modified ejector  42   a  ( FIG. 11 ). The portion  50  is secured in the fixed condition in addition to the still embedded projection  20 . When the workpieces  14 , 16  are engaged by the welding apparatus  18  to fuse the projection  20 , the portion  50  is heated to the minimum temperature. As a result, an adhesive barrier is formed that completely encases the weld  12 , and once cured during a finishing/painting process, further bonds the workpieces  14 , 16 . Thus, it is appreciated that this configuration significantly increases the capacity of the joint and seals it from harmful impurities, such as moisture, oil, and dirt, and conditions, such as galvanic corrosion. 
         [0058]    In continuation, it is appreciated that the later configuration may include a plurality of projections  120 , as shown in  FIG. 12 . More particularly,  FIGS. 12 and 12   a  show an adhesive layer  150  having a plurality of projections  120  embedded therein, in pre and post weld-bonding conditions. The layer  150  is interposed between, so as to be tangibly engaged to both workpieces  14 , 16 , along surface areas of engagement. The preferred layer  150  consists of an epoxy based adhesive material. In the welding position, the layer preferably presents an elongated shape defining a longitudinal axis equal to that of the desired joint; however, it is well within the ambit of the invention for alternative configurations to be utilized such as a planar sheet, or the cross-shaped configuration shown in  FIG. 13   a.    
         [0059]    As shown in  FIG. 12 , the projections  120  are preferably symmetrically spaced, and, where defining an average diameter, spaced a distance not less than half the diameter, so that each projection  120  freely expands during fusion and does not engage adjacent projections  120  ( FIG. 12   a ). However, it is appreciated that the spacing of the projections  120  may vary along the longitudinal length or lateral width of the layer  150 . For example, the spacing may be reduced towards the edges of the layer  150  in order to provide a stronger joint that is better configured to withstand peeling forces at these locations ( FIG. 13   a ). 
         [0060]    The projections  120  are of predetermined size (correlative to spacing), and more preferably present diameters within the range 0.5 to 1 mm. The projections  120  are formed of metal typical used during fusion welding (e.g., electro-galvanized zinc coating, aluminum alloys, steel, etc.), and more preferably consist essentially of silicon-bronze alloy. It is appreciated that the projections may be identical or present dissimilar constituencies where an aggregate joint is desired. 
         [0061]    Also shown in  FIGS. 12-13   a , each projection  120  is preferably spherical in shape, so that a single initial axis of engagement is defined between the projection  120  and workpieces  14 , 16 . However, as previously mentioned, the projections  120  may present alternative configurations, such as polygonal, cylindrical and ellipsoidal shapes, wherein a plurality of initial axes of engagement are defined. Moreover, where the layer  150  presents a planar sheet configuration, the projections  120  may present elongated wire configurations, oriented in a mesh, as shown in  FIG. 13   b . Where a mesh is utilized, it is appreciated that the planar sheet layer may present discontinuous regions of adhesive material, such as the radial bands  150   a  also shown in  FIG. 13   b.    
         [0062]    Where weld-bonding sheet metal, such as the roof of a vehicle, to a bottom sheet, such as the body side of the vehicle, it is appreciated that the present invention results in expanding the bonding area, and more particularly, in expanding to the footprint area of the electrodes (e.g., 20 mm×7 mm). The electrodes  24   a , 26   a  and layer  150  are therefore cooperatively configured accordingly. Alternatively, it is further appreciated that the electrodes  24   a , 26   a  may engage only a portion of the layer  150  at a time to sequentially form the joint. More preferably, where the electrodes  24   a , 26   a  present electrode wheels, the wheels present a lateral width greater than that of the layer  150 , and are operable to rollingly engage the workpieces  14 , 16  along the longitudinal axis of the layer  150 , so that welding is performed along the entire length of the joint in a single pass. 
         [0063]    Thus, in operation, the adhesive layer  150  is applied to a pre-positioned lower workpiece  16  such as the body side of a vehicle; the upper workpiece  14 , such as the roof of the vehicle, is then positioned over the layer  150  and secured relative thereto; and lastly the layer  150  is welded using the welding apparatus  18  in the multi-step mode previously described. Finally, because the adhesive layer  150  including the embedded projections  120  cover the entire area of the joint, the welding electrodes  24   a , 26   a  can be disengaged from the projection locations, as it is appreciated that electrode positioning need not be as precise as in the case of traditional projection welding. 
         [0064]    The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments and modes of operation, as set forth herein, could be readily made by those skilled in the art without departing from the spirit of the present invention. The inventor hereby states his intent to rely on the Doctrine of Equivalents to assess the scope of the present invention as pertains to any apparatus, system or method not materially departing from the literal scope of the invention set forth in the following claims.