Abstract:
An apparatus and method for fastening dissimilar metals like steel and aluminum utilizes a spot welding machine. The metals are stacked with an aluminum body captured between steels. Heat from the welder&#39;s electric current softens the lower melting point aluminum allowing an indentation of the steel layer to penetrate the aluminum and weld to an opposing steel layer. The process may be used to join stacks with several layers of different materials and for joining different structural shapes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/839,478, entitled Apparatus and Methods fort Joining Dissimilar Materials, filed Jun. 26, 2013, which is incorporated by reference in its entirety herein. 
     
    
     FIELD 
       [0002]    The present invention relates to welding apparatus and methods and more particularly, to methods for joining dissimilar materials, such as dissimilar metals. 
       BACKGROUND 
       [0003]    Various fasteners, apparatus and methods for joining and assembling parts or subunits are known, such as welding, riveting, threaded fasteners, etc. In some instances, there is a need to cost effectively join dissimilar metals, such as aluminum parts, subunits, layers, etc., to other parts, subunits, layers, etc. made from other materials, such as steel (bare, coated, low carbon, high strength, ultra high strength, stainless), titanium alloys, copper alloys, magnesium, plastics, etc. Solutions for these fastening problems include mechanical fastener/rivets in combination with an adhesive and/or a barrier layer to maintain adequate joint strength while minimizing corrosion, e.g., due to the galvanic effect present at a junction of dissimilar metals. Direct welding between aluminum and other materials is not commonly employed due to intermetallics generated by the aluminum and the other materials, which negatively affect mechanical strength and corrosion resistance. In cases where direct welding is employed, it is typically some type of solid-state welding (friction, upset, ultrasonic, etc.) or brazing/soldering technology in order to minimize the intermetallics, but the mechanical performance of such joints is sometimes poor or only applicable to unique joint geometries. 
         [0004]    In the automotive industry, the incumbent technology for joining steel to steel is resistance spot welding (RSW), due to cost and cycle time considerations (less than 3 seconds per individual joint and which may be performed robotically). Known methods for joining aluminum to steel, include: use of conventional through-hole riveting/fasteners, self-pierce riveting (SPR), use of flow drill screws (FDS or by trade name of EJOTS), friction stir spot welding/joining (FSJ), friction bit joining (FBJ), and use of adhesives Each of these processes is more challenging than steel-to-steel resistance spot welding (RSW). For example, when high strength aluminum (above 240 MPa) is coupled to steel using SPR, the aluminum can crack during the riveting process. Further, high strength steels (&gt;590 MPa) are difficult to pierce, requiring the application of high magnitude forces by large, heavy riveting guns. FSJ is not widely employed in the automotive industry since joint properties (primarily peel and cross tension) are low compared to SPR. In addition, FSJ requires very precise alignment and fitup. As the thickness of the joint increases, the cycle times for the process can increase dramatically where a 5 mm to 6 mm joint stack-up may require 7 to 9 seconds of total processing time, which is well above the 2 to 3 second cycle time of RSW when fabricating steel structures. FBJ employs a bit which is rotated through the aluminum and is then welded to the steel. This process requires very precise alignment and fit-up similar to FSJ and high forging forces are required for welding to steel. FDS involves rotating a screw into the work pieces, plasticizing one of the sheets, which then becomes interlocked with the screw&#39;s thread. FDS is typically applied from a single side and requires alignment with a pilot hole in the steel sheet, complicating assembly and adding cost. Alternative fasteners, apparatus and methods for joining and assembling parts or subunits therefore remain desirable. 
       SUMMARY 
       [0005]    The disclosed subject matter relates to methods for fastening metal members. In a first embodiment a first electrically conductive body made of a first material is fastened to a second electrically conductive body made from a second material dissimilar to the material of the first body using electrical resistance welding including the steps of: placing the first and second bodies together in physical and electrical contact, the first material having a lower melting point than the second material; placing an electrically conductive third body that is made of a third material that is weldable to the second material and which has a higher melting point than the first material in physical and electrical contact with the first material to form an electrically conductive stack inclusive of at least a portion of the first body, the second body and the third body; applying an electrical potential across the stack, inducing a current to flow through the stack and causing resistive heating, the resistive heating causing a softening of a least a portion of the first body; urging a softened portion of the third body through the softened portion of the first body toward the second body; and after the portion of the third body contacts the second body, welding the third body to the second body. 
         [0006]    In another aspect of the present disclosure, the first material includes at least one of aluminum, magnesium and alloys thereof. 
         [0007]    In another aspect of the present disclosure, the second material includes at least one of steel, titanium and alloys thereof. 
         [0008]    In another aspect of the present disclosure, the third material includes at least one of steel, titanium and alloys thereof. 
         [0009]    In another aspect of the present disclosure, a portion of the third body covers an upwelled portion of the first body that is displaced when the portion of the third body is urged through the first body. 
         [0010]    In another aspect of the present disclosure, the first body, the second body and the third body are in the form of layers proximate where the third body is welded to the second body. 
         [0011]    In another aspect of the present disclosure, the layers are sheet metal. 
         [0012]    In another aspect of the present disclosure, at least one of the first body, the second body and the third body is in the form of a structural member. 
         [0013]    In another aspect of the present disclosure, the electrical potential is applied in the course of direct resistance welding. 
         [0014]    In another aspect of the present disclosure, the electrical potential is applied in the course of indirect resistance welding. 
         [0015]    In another aspect of the present disclosure, the electrical potential is applied in the course of series resistance welding. 
         [0016]    In another aspect of the present disclosure, the stack includes a plurality of bodies having a melting point less than a melting point of the second and third bodies. 
         [0017]    In another aspect of the present disclosure, the second body and the third body are monolithic, the second body distinguishable from the third body by a fold and further including the steps of folding to make the fold and inserting the first body into the fold to make the stack prior to the step of applying an electrical potential across the stack. 
         [0018]    In another aspect of the present disclosure, the folding results in a J shape. 
         [0019]    In another aspect of the present disclosure, the folding results in a U shape. 
         [0020]    In another aspect of the present disclosure, the step of folding is conducted a plurality of times to make a plurality of folds. 
         [0021]    In another aspect of the present disclosure, the folding results in an S shape. 
         [0022]    In another aspect of the present disclosure, the folding results in a W shape. 
         [0023]    In another aspect of the present disclosure, a plurality of bodies are inserted into the plurality of folds. 
         [0024]    In another aspect of the present disclosure, the step of welding simultaneously generates a plurality of welds. 
         [0025]    In another aspect of the present disclosure, the folding results in a T shape with a bifurcated bottom portion and a top portion, and the step of inserting includes inserting the first body into the bifurcated bottom and the step of welding is conducted across the stack of the first body and the bifurcated bottom portion. 
         [0026]    In another aspect of the present disclosure, further conducting the step of fastening another body to the top portion of the T shape. 
         [0027]    In another aspect of the present disclosure, a force applied during the steps of urging and welding is adjustable is adjustable and further comprising the step of adjusting the force. 
         [0028]    In another aspect of the present disclosure, the steps of adjusting the current and the force can be made to accommodate different thickness of the first body, second body and third body. 
         [0029]    In another aspect of the present disclosure, the third layer and the second layer are not pierced during the steps of applying, urging and welding. 
         [0030]    In another aspect of the present disclosure, a structure has a first electrically conductive body, a second electrically conductive body and a third electrically conductive body positioned proximate one another in physical and electrical contact, the first body having a lower melting point than the second and third bodies and being positioned between the second and third bodies, the second body being welded to the third body by electrical resistance welding extending through the first body, the first body being captured between the second body and the third body. 
         [0031]    In another aspect of the present disclosure, the first body is in the form of an elongated channel and the second body is in the form of a web that extends across the elongated channel and folds back over itself at a fold defining the third body, a portion of the first body positioned in the fold and retained in the fold by the welding of the second body to the third body. 
         [0032]    In another aspect of the present disclosure, the first body is in the form of a plate, the second and third bodies are in the form of beams having an L shaped cross-section, the first body being sandwiched between the second and third bodies. 
         [0033]    In another aspect of the present disclosure, the structure further includes a plurality of plates and beams of L shaped cross-section. 
         [0034]    In another aspect of the present disclosure, the first body is in the form of an I beam, the second body is in the form of an elongated channel insertable into a hollow defined by the I shape of the first body and the third body is in the form a plate positioned on a top portion of the I shape. 
         [0035]    In another aspect of the present disclosure, the first, second and third bodies are each tubular, the second body capable of being inserted coaxially into at least a portion of the third body, the first body having dimensions permitting the insertion thereof between the second and third bodies. 
         [0036]    In another aspect of the present disclosure, the first and second bodies are each tubular, the second body having dimensions permitting the insertion thereof within the first body, the third body being a plate positioned against the exterior of the first body adjacent the second body. 
         [0037]    In another aspect of the present disclosure, the first and second bodies have at least one of a rectangular and circular cross-sectional shape. 
         [0038]    In another aspect of the present disclosure, the first body is in the form of a tube, the second body is in the form of plate positioned against the interior of the first body, the first body having an opening with dimensions permitting the insertion there through of the second body, the third body being in the form of a plate positioned against the exterior of the first body proximate the second body, sandwiching the first body there between. 
         [0039]    In another aspect of the present disclosure, the first body is in the form of an elongated channel and the second body is in the form of a channel that inserts into a hollow of the first body, the third body being in the form of a plate, the plate positioned proximate the second body sandwiching the first body there between. 
         [0040]    In another aspect of the present disclosure, the first body is in the form of an elongated channel and the second body is in the form of a tube that inserts into a hollow of the first body, the third body being in the form of a plate, the plate positioned proximate the second body, sandwiching the first body there between. 
         [0041]    In another aspect of the present disclosure, the first body is in the form of an elongated tube and the second body is in the form of a C shaped bracket that inserts into a hollow of the first body, the third body being in the form of a plate, the plate positioned proximate the second body, sandwiching the first body there between. 
         [0042]    In another aspect of the present disclosure, the first body has an aperture allowing the insertion of welding electrodes. 
         [0043]    In another aspect of the present disclosure, the first body is tubular and the second body is tubular, the first body having a side aperture allowing the insertion of the second body at an angle relative to the first body, the third body being in the form of a plate, the plate positioned proximate the second body, sandwiching the first body there between. 
         [0044]    In another aspect of the present disclosure, the first body has a tab extending therefrom proximate the side aperture. 
         [0045]    In another aspect of the present disclosure, the structure further includes a fourth body similar to the second body, the second and fourth bodies being mitered and joining at the aperture. 
         [0046]    In another aspect of the present disclosure, the structure is replicated a plurality of times to form a truss structure. 
         [0047]    In another aspect of the present disclosure, the structure further includes a fourth body similar to the second body and the first body has a second aperture, the second and fourth bodies inserting into the aperture and second aperture, respectively, along skew lines. 
         [0048]    In another aspect of the present disclosure, further comprising a coating on at least one of the first material, the second material and the third material. 
         [0049]    In another aspect of the present disclosure, the coating is at least one of aluminum alloy, galvanized, galvaneal and anti-corrosion paint. 
         [0050]    In another aspect of the present disclosure, the coating is an adhesive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    For a more complete understanding of the present disclosure, reference is made to the following detailed description of exemplary embodiments considered in conjunction with the accompanying drawings. 
           [0052]      FIG. 1  is a diagrammatic, cross-sectional view sequentially showing the joining of three layers of material by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0053]      FIG. 2  is a diagrammatic, cross-sectional view sequentially showing the joining of three layers of material by electrical resistance welding, the middle layer having a coating on each side, in accordance with an embodiment of the present disclosure. 
           [0054]      FIG. 3  is a diagrammatic, cross-sectional view showing the joining of three structures by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0055]      FIG. 4  is a diagrammatic, cross-sectional view showing the joining of four structures by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0056]      FIG. 5  is a diagrammatic, cross-sectional view showing the joining of five structures by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0057]      FIG. 6  is a diagrammatic, cross-sectional view showing the joining of two structures, one of which has a “J” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0058]      FIG. 7  is a diagrammatic, cross-sectional view showing the joining of three structures, one of which has a “J” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0059]      FIG. 8  is a diagrammatic, cross-sectional view showing the joining of four structures, one of which has a “J” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0060]      FIG. 9  is a diagrammatic, cross-sectional view showing the joining of two structures, one of which has an “S” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0061]      FIG. 10  is a diagrammatic, cross-sectional view showing the joining of three structures, one of which has an “S” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0062]      FIG. 11  is a diagrammatic, cross-sectional view showing the joining of two structures, one of which has a “U” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0063]      FIG. 12  is a diagrammatic, cross-sectional view showing the joining of three structures, one of which has a “U” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0064]      FIG. 13  is a diagrammatic, cross-sectional view showing the joining of three structures, one of which has a “W” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0065]      FIG. 14  is a diagrammatic, cross-sectional view showing the joining of two structures, one of which has a “T” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0066]      FIG. 15  is a diagrammatic, cross-sectional view showing the assembly of four intersecting structures into a “+” shaped configuration by four “L” shaped brackets, by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0067]      FIG. 16  is a diagrammatic, perspective view of a composite beam formed from mating structures and joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0068]      FIGS. 17   a  and  17   b  are exploded and perspective views, respectively, of an assembly joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0069]      FIGS. 18   a  and  18   b  are diagrammatic, cross-sectional views showing the sequential assembly of a first structure to a plate using “T” shaped brackets joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0070]      FIGS. 19 and 20  are an exploded view of an assembly structures to be joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0071]      FIG. 21  is a perspective view of an assembly of the structures of  FIGS. 19 and 20 . 
           [0072]      FIG. 22  is a cross-sectional view of the assembly of  FIG. 21  taken along section line  22 - 22  and looking in the direction of the arrows. 
           [0073]      FIG. 23  is an exploded view of an assembly of structures to be joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0074]      FIG. 24  is a cross-sectional view of a stack-up of the structures shown in  FIG. 23 . 
           [0075]      FIG. 25  is a diagrammatic, cross-sectional view of a stack-up of alternative structures for those shown in  FIG. 24  and ready to be welded in accordance with an embodiment of the present disclosure. 
           [0076]      FIG. 26  is a diagrammatic, cross-sectional view of a stack-up of alternative structures for those shown in  FIG. 24  and ready to be welded in accordance with an embodiment of the present disclosure. 
           [0077]      FIG. 27  is a diagrammatic, cross-sectional view of a stack-up of alternative structures for those shown in  FIG. 24  and ready to be welded in accordance with an embodiment of the present disclosure. 
           [0078]      FIG. 28  is an exploded view of an assembly of structures to be joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0079]      FIG. 29  is a diagrammatic, cross-sectional view of a stack-up of the structures shown in  FIG. 28  ready to be welded in accordance with an embodiment of the present disclosure. 
           [0080]      FIG. 30  is a diagrammatic, cross-sectional view of a stack-up of structures ready to be welded in accordance with an embodiment of the present disclosure. 
           [0081]      FIG. 31  is a diagrammatic, cross-sectional view of a stack-up of structures ready to be welded in accordance with an embodiment of the present disclosure. 
           [0082]      FIG. 32  is a perspective view of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0083]      FIG. 33  is a diagrammatic, cross-sectional view of a stack-up of structures for forming the assembly of  FIG. 32  ready to be welded in accordance with an embodiment of the present disclosure. 
           [0084]      FIG. 34  is a diagrammatic, cross-sectional view of a stack-up of structures ready to be welded in accordance with an embodiment of the present disclosure. 
           [0085]      FIG. 35  is a diagrammatic, cross-sectional view of a stack-up of structures ready to be welded in accordance with an embodiment of the present disclosure. 
           [0086]      FIG. 36  is a perspective view of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0087]      FIG. 37  is a perspective view of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0088]      FIG. 38  is a diagrammatic, cross-sectional view of a stack-up of the structures of the assembly of  FIG. 37  ready to be welded in accordance with an embodiment of the present disclosure. 
           [0089]      FIG. 39  is a perspective view of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0090]      FIGS. 40 and 41  are exploded and diagrammatic, cross-sectional views of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0091]      FIGS. 42 and 43  are side and perspective views, respectively, of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
           [0092]      FIGS. 44 and 45  are perspective and diagrammatic, cross-sectional views of an assembly of structures joined by electrical resistance welding in accordance with an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0093]      FIG. 1  shows the joining of three layers of material  10 ,  12 ,  14  in accordance with an embodiment of the present disclosure. The layers  10 ,  12 ,  14  may be dissimilar, e.g., dissimilar metals, like steel and aluminum. For example, the outer layers  10  and  14  may be a steel alloy and the intermediate layer  12  an aluminum alloy. As shown, the two outer layers  10 ,  14 , which are compatible for the purpose of welding, may be welded to one another through the intermediate layer  12 , to form a laminate structure L 1 . This is shown in sequential stages labeled A-E. As shown at stage A, this process may be conducted at a conventional spot welding station having opposing electrodes, the tips  16  and  18  of which are shown at stage A embracing the stack-up of layer  10 ,  12 ,  14  before welding. At stage B, opposing forces F 1 , F 2  exerted by the conventional welding machine (not shown) move the tips  16 ,  18  towards one another, and an electric potential is applied between the electrodes  16 ,  18  giving rise to a current I passing through the electrodes and layers  10 ,  12 ,  14 . The forces F 1 , F 2  and current I are applied throughout the stages B-D and the magnitude and duration of each may be varied depending upon the requirements at each stage. For example, the current I required to heat/plasticize the aluminum layer  12  during the transition from stage A to stage C, may be less than that required to weld steel layer  10  to steel layer  14  as occurs during stages C and D. Similarly, the forces F 1  and F 2  may be varied to accommodate changing processing requirements. 
         [0094]    The current I heats each of the layers  10 ,  12 ,  14  to a temperature at which the aluminum layer  12  plasticizes and can be displaced/pierced by the upper and lower layers  10 ,  14  as they are urged toward one another by the electrodes  16 ,  18 . The aluminum layer  12  is heated resistively by current I and also through conduction from the layers  10 ,  14 . The layers  10 ,  14  have lower heat and electrical conductivity than the aluminum layer  12 , such that a low current typically achieved with a resistance spot welder suitable for making resistance spot welds in steel can be used to generate the heat required to plasticize the aluminum layer  12 , as well as to weld layer  10  to layer  14 , as described below. Since the aluminum alloy layer  12  has a lower melting point than the steel alloy layers  10 ,  14 , the aluminum layer  12  reaches a plastic state permitting displacement by the converging layers  10 ,  14 , which form converging depressions  10 D,  14 D (U-shaped in cross-section) proximate the electrodes  16 ,  18  responsive to the forces F 1 , F 2  and current I, allowing the converging layers  10 ,  14  to penetrate the aluminum layer  12 . The convergence of the layers  10 ,  14 , as shown at stage B, results in a displacement of the aluminum alloy of layer  12  at the area of convergence of the layers  10 ,  14  such that a ring-shaped thickening  12 T (shown diagrammatically in dotted lines in stage B only) is formed, causing upwellings  10 U and  14 U in the softened layers  10 ,  14  proximate the depressions  10 D,  14 D. As shown at stages C and D, the layers  10 ,  14  converge completely, forcing the aluminum alloy of layer  12  out at the surface areas of convergence  10 C,  14 C, whereupon the layers  10 ,  14  begin to melt at the area of contact  10 C,  14 C and a zone M of molten metal begins to form at the interface of the layers  10  and  14 . The zone M is the weld material or “nugget” where the metal of the layers  10 ,  14  liquify and commingle. In accordance with one embodiment, the current I is applied until weld zone M&gt;3*sqrt (minimum gauge of outer layers  10 ,  14 ). As shown at stage E, after having accomplished welding at stage D, the forces F 1 , F 2  and current I can be removed and the electrode tips  16  and  18 , withdrawn, whereupon the molten zone M hardens to weld W. 
         [0095]    As shown in  FIG. 2 , the foregoing process can be conducted with barrier layers  20 ,  22 , e.g., an adhesive layer of surface pre-treatment or paint/primer (not shown) applied to the upper and lower surfaces of layer  12 , or to the surfaces of layer  10 ,  14  which would otherwise contact layer  12 , so long as the barrier layer(s)  20 ,  22  do not prevent the current I from flowing, impeding electrical resistance heating. In this manner, the contact between joined, dissimilar metals of layers  10 ,  12 ,  14  can be reduced, along with unwanted galvanic interaction and corrosion. Since the process of joining in accordance with the present disclosure is attributable to gradual displacement of the layers  10 ,  12 ,  14  during the penetration and welding phases B-D, the process accommodates a range of thicknesses of layers  10 ,  12 ,  14 . 
         [0096]    In one example, stages B and C may have an associated force F H  of a magnitude of, e.g., from 600 to 2000 pounds and a current level I H  of a magnitude of, e.g., from 4,000 to 24,000 amperes, that is appropriate for plasticizing the layer  12  of aluminum having a thickness of 2 mm and welding layer  10  of low-carbon steel with an average thickness of 2.0 mm to layer  14  of 780 MPa galvanized coated steel with a thickness of 1.0 mm. These magnitudes of force and current are just exemplary and are dependent upon the dimensions and compositions of the layers  10 ,  12 ,  14 . The duration of time to transition from stage B to C may be in the order of 0.2 to 2.0 secs. Pursuing this example further and using the same dimensions and properties of the layers  10 ,  12 ,  14 , stage D may utilize an associated force F W  of a magnitude of, e.g., from 500 to 800 pounds and a current level I W  of a magnitude of, e.g., from 6,000 to 18,000 amperes, that is appropriate for initiating the melting of the layers  10 ,  14  to form a molten weld zone M. The magnitude of force F W  may be changed to a force F T  (not shown) of a magnitude of, e.g., from 600 to 1,000 pounds and a current level I T  (not shown) of a magnitude of, e.g., from 3,000 to 12,000 amperes at stage D to form an expanded weld zone to temper the weld and to render it with an average cross-sectional diameter of 4 mm to 6 mm. The completion of stage D may take, e.g., 0.1 to 0.5 secs. 
         [0097]    While the foregoing examples refer to outer layers  10 ,  14  made from steel, these layers may be from other materials, such as titanium. Similarly, the intermediate layer  12  may be an aluminum alloy or another material, such as a magnesium alloy. In order to penetrate an intervening layer like layer  12 , the outer layer  10  and/or  14  should be made of a material with a higher melting point than the intervening layer(s)  12  penetrated during the heating/penetrating phase, e.g., stages B and C ( FIG. 1 ). In order to conduct the welding phase, e.g., stage D, the layers  10 ,  14  must be compatible to be resistance welded. For example, if the layer  10  is made from high strength (&gt;590 MPa) galvanized steel, then the layer  14  may be made, e.g., from standard, low-carbon steels, high strength steels (&gt;590 MPa) or stainless steel grades. 
         [0098]    In one example of a welding operation conducted in accordance with the present disclosure, a commercially available electric spot welding machine, such as a 250 kVA AC resistance spot welding pedestal welding station available from Centerline Welding, Ltd. was employed to conjoin three layers  10 ,  12 ,  14 , layers  10  and  14  being 0.7 mm 270 MPa galvanized steel and layer  12  being a 1.5 mm 7075-T6 aluminum alloy as shown and described above relative to  FIG. 1 . The upper electrode tip  16  and the lower electrode tip  18  were standard, commercially available electrodes. 
         [0099]    Aspects of the present disclosure include low part distortion, since the layers to be fastened, e.g.,  10 ,  12 ,  14 , are held in compression during the weld and the heat affected zone is primarily restricted to the footprint of the electrodes  16 ,  18 . The conjoined layers  10 ,  12 ,  14  trap intermetallics or materials displaced by penetration of the intermediate layer  12 . 
         [0100]    The weld formed between layers  10  and  14  does not pierce the surface of those layers proximate the weld, preserving appearance, corrosion resistance and water impenetrability. During penetration of layer  12 , e.g., at stages B and C of  FIG. 1  and the welding phase, stage D, intermetallics are displaced from the weld zone M. The methodology and apparatus of the present disclosure is compatible with conventional RSW equipment developed for steel sheet resistance welding. The layers  10 ,  14  may optionally be coated (galvanized, galvaneal, hot-dipped, aluminized) to improve corrosion resistance. 
         [0101]    The welding process of the present disclosure does not require a pilot hole, but can also be used with a pilot hole in the intermediate layer  12 . Pilot holes may also be used to allow electrical flow through dielectric layers such as adhesive layers or anti-corrosive coatings/layers  20 ,  22 . The weld quality resulting from use of the process can be tested in accordance with quality assurance measurements applied to the cavity left by the weld, i.e., by measuring the dimensions of the cavity. Ultrasonic NDE techniques may also be utilized on the side(s), e.g., of layers  10   14  to monitor the weld quality. 
         [0102]    Compared to FDS (EJOTS), SPR, and SFJ, the apparatus of the present disclosure used to fasten layers of dissimilar materials has a smaller footprint, allowing access to tighter spaces. The apparatus and method of the present disclosure uses lower compressive forces as compared to SPR insertion forces since the layers  10 ,  12 ,  14  are heated/softened during stages B-D of  FIG. 1 . The methods and apparatus of the present disclosure provide the ability to join high strength aluminums (which are sensitive to cracking during SPR operations) and to join high and ultra high strength steels, since there is no need to pierce the steel metal with the fastener but rather, spot welding is employed. 
         [0103]    The apparatus and method of the present disclosure does not require rotating parts and is conducive to resolving part fit-up issues since the overall process is similar to conventional resistance spot welding (RSW) with respect to how the component layers/parts are fixtured. In addition, the process can be conducted quickly, providing fast processing speeds similar to conventional RSW. The apparatus and methods of the present disclosure can be applied to use on both wrought and cast aluminum products and may be used to produce a compatible metal joint rather than a bimetallic weld, as when welding aluminum to steel, which may have low joint strength. As noted below, the apparatus and methods of the present disclosure may be used to conjoin multiple layers of different materials. 
         [0104]      FIG. 3  shows that the process of the present disclosure may be used to join three structures  30 ,  32 ,  34  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, structure  32  may be a box-shaped hollow beam, e.g., made from aluminum alloy with a leg  32 L that is captured between the L-shaped structures  30 ,  34 . The structure  32  may be fabricated, cast, forged or extruded. Multiple welds W may be made along the length of the structures  30 ,  32 ,  34 , as required for the application. The structures  30 ,  32 ,  34  are shown in cross section and in three dimensions in  FIG. 3 . Figures described below, may show the cross-sectional view only for simplicity of illustration. 
         [0105]      FIG. 4  shows that the process of the present disclosure may be used to join four structures  40 ,  42 ,  44 ,  46 , by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, two L-shaped intermediate structures  42 ,  44 , e.g., made from aluminum alloy are captured between two L-shaped structures  40 ,  46 , e.g., made from steel and conjoined at weld W. When mentioned herein, “steels” shall include various types of steel, including stainless steels and titanium alloys. “Aluminum alloys” shall include magnesium alloys. 
         [0106]      FIG. 5  shows that the process of the present disclosure may be used to join five structures  50 ,  52 ,  54 ,  56 ,  58  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, two L-shaped intermediate structures  52 ,  56 , e.g., made from aluminum alloy are captured between three L-shaped structures  50 ,  54 ,  58  e.g., made from steels, etc. Weld W 1  joins structure  50  to structure  54  and weld W 2  joins structure  54  to structure  58  capturing structures  52  and  56  there between, respectively. 
         [0107]      FIG. 6  shows that the process of the present disclosure may be used to join two structures  60 ,  62  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, an L-shaped intermediate structure  62 , e.g., made from aluminum alloy is captured in a “J” portion  60 J of structure  60 , e.g., made from steel, and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. In this instance, the weld W is established between the opposing portions of the “J” portion  60 J. 
         [0108]      FIG. 7  shows that the process of the present disclosure may be used to join three structures  70 ,  72 ,  74  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, two intermediate structures  72 ,  74 , e.g., made from aluminum alloy, are captured in a “J” portion  70 J of structure  70 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. The weld W is established between the opposing portions of the “J” portion  70 J. 
         [0109]      FIG. 8  shows that the process of the present disclosure may be used to join four structures  80 ,  82 ,  84 ,  86  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, two intermediate structures  82 ,  86 , e.g., made from aluminum alloy are captured along with structure  84  (steel) in a “J” portion  80 J of structure  80 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. In this instance, weld W 1  is established between intermediate steel structure  84  and structure  80  and weld W 2  is established between another side of intermediate structure  84  and J-shaped portion  80 J of structure  80 . 
         [0110]      FIG. 9  shows that the process of the present disclosure may be used to join two structures  90 ,  92  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, an intermediate structure  92 , e.g., made from aluminum alloy is captured in the bottom curve  90 C 2  of an S-shaped portion  90 S of structure  90 , e.g., made from steel, and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. In this instance, weld W 1  is established between the opposing portions of curve  90 C  1  of the structure  90  and weld W 2  is established between the opposing portions of curve  90 C 2  of the structure  90 , capturing structure  92  therein. 
         [0111]      FIG. 10  is a diagrammatic, cross-sectional view showing the joining of three structures,  100 ,  102 ,  104 , structure  100  having an “S” configuration, by electrical resistance welding in accordance with an embodiment of the present disclosure. An intermediate structure  102 , e.g., made from aluminum alloy is captured in the top curve  100 C 1  of an S-shaped portion  100 S of structure  100 , e.g., made from steel. Intermediate structure  104 , e.g., made from aluminum alloy, is captured in the bottom curve  100 C 2  of an S-shaped portion  100 S of structure  100 . Both structure  102  and  104  are retained in S-shaped portion  100 S by electrical resistance welding in accordance with an embodiment of the present disclosure. Weld W 1  is established between the opposing portions of curve  100 C 1  and weld W 2  is established between the opposing portions of curve  100 C 2  of the structure  100 . 
         [0112]      FIG. 11  shows that the process of the present disclosure may be used to join two structures  110 ,  112  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, an intermediate structure  112 , e.g., made from aluminum alloy is captured in a U-shaped structure  110 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. In this instance, the weld W is established between the opposing portions of the U-shaped structure  110 . 
         [0113]      FIG. 12  shows that the process of the present disclosure may be used to join three structures  120 ,  122 ,  124  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The intermediate structures  122 ,  124 , e.g., made from aluminum alloy are captured in a U-shaped structure  120 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. The weld W is established between the opposing portions of the U-shaped structure  120 . 
         [0114]      FIG. 13  shows that the process of the present disclosure may be used to join three structures  130 ,  132 ,  134  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The intermediate structures  132 ,  134 , e.g., made from aluminum alloy, are captured in the U-shaped structures  130 U 1  and  130 U 2  which make up the W-shaped structure  130 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. The welds W 1 , W 2  and W 3  are established between the opposing portions of the U-shaped structures  130 U 1  and  130 U 2  which make up the W-shaped structure  130 . 
         [0115]      FIG. 14  shows that the process of the present disclosure may be used to join two structures  140 ,  142  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . In this instance, an intermediate structure  142 , e.g., made from aluminum alloy is captured in a split T-shaped structure  140 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. In this instance, the weld W is established between the opposing bottom portions  140 B 1  and  140 B 2  of the T-shaped structure  140 . 
         [0116]      FIG. 15  shows that the process of the present disclosure may be used to join eight structures  150 ,  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . Intermediate structures  152 ,  156 ,  160  and  164 , e.g., made from aluminum alloy are captured between four L-shaped structures  150 ,  154 ,  158  and  162 , e.g., made from steel and retained there by electrical resistance welding in accordance with an embodiment of the present disclosure. The welds W 1 , W 2 , W 3  and W 4  are established between the opposing L-shaped structures  150 ,  154 ,  158  and  162 . 
         [0117]      FIG. 16  shows a composite beam  170  formed from mating structures  172 , e.g., made from aluminum, and structure  174  made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . A series of welds, W 1 , W 2 , W 3 , W 4 , etc., along the U-shaped portions  174 U 1  and  174 U 2 , retain the structure  170  together. 
         [0118]      FIGS. 17   a  and  17   b  show composite beam  180  formed from mating structures  182 , e.g., made from aluminum and T-shaped structures  184 ,  184 ′ made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  (not shown) that function as described above in reference to  FIG. 1 . As in described in relation to  FIG. 14 , spot welds of portions  184 B 1  and  184 B 2  extending through the structure  182  may be used to secure structures  184  to the I-beam structure  182 . The same approach is applicable to structure  184 ′. Slots S accommodate the center web C of the I beam structure  182 . The upper portions, e.g., 184T, may be used as mounting flanges to spot weld a plate  186 , e.g., made from steel, as shown by welds W in  FIG. 17   b.    
         [0119]      FIGS. 18   a  and  18   b  show a composite structure  190  with a similar makeup as structure  180  shown in  FIGS. 17   a ,  17   b , with structure  190  formed from mating structures  192 , e.g., made from aluminum and T-shaped structures  194 ,  194 ′ made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . Spot welds WT of portions  194 B 1  and  194 B 2  extend through the extension  192 A (with a similar arrangement applying to  194 ′) and  192 B to secure structures  194 ,  194 ′ to the structure  192 . The upper portions  194 T  194 ′T may be used as mounting flanges to spot weld a plate  196 , e.g., made from steel, as shown by welds WS in  FIG. 18   b.    
         [0120]      FIGS. 19-22  show a composite structure  200  formed from a hollow beam structure  202 , e.g., made from aluminum, a tapered tubular structure  204  made, e.g., from fabricated or cast steel and a collar structure  206 , e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The structure  204  has a base portion  204 B, a tapered portion  204 T and a nipple portion  204 N that slideably receives the hollow beam structure  202  there over. The collar structure  206  is slideably received over the structure  202 . Spot welds W extend through the hollow beam structure  202  to join the collar structure  206  to the nipple portion  204 N to secure the assembly  200  together by electrical resistance welding. The welds W could be described as rivets, which rivet the collar structure  206  and the beam structure  202  to the nipple portion  204 N. As shown in  FIG. 20 , this welding/riveting operation can be conducted by a single weld gun with electrodes  16 ,  18  positioned on opposite sides of the structure  200  to simultaneously conduct welding in the areas A 1  and A 2 , resulting in welds W 1 , W 2 , as shown in  FIG. 22 . The welds W 3 , W 4  could likewise be simultaneously conducted, the simultaneous generation of multiple welds reducing the total number of repositioning operations of the workpiece/welding apparatus required to complete the welding/riveting operation. 
         [0121]      FIGS. 23 and 24  show a composite structure  210  formed from a hollow beam structure  212 , e.g., made from aluminum, a tubular structure  214  made from steel and plates  216 A,  216 B, e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The structure  214  may have any given length relative to structure  212 , but in the embodiment depicted should have overlap with the plates  216 A,  216 B in order to permit spot welding the plates to the structure  214 , which may be slideably received within structure  212 . The resulting composite  210  has properties attributable to each of the structures  212 ,  214  and  216 A,  216 B. In one alternative, the tubular structure  214  may be subdivided into a plurality of separate tubular structures, e.g., a first disposed in the hollow beam  212  proximate one end and the other disposed at the other end or in an intermediate position, allowing additional plate(s)  216  to be attached at the other end or in an intermediate position(s). 
         [0122]      FIGS. 25-27  show variations  210 A,  210 B,  210 C on the composite structure  210  shown in  FIGS. 23 and 24 . More particularly, the internal structures  220  ( FIG. 25 ),  222  ( FIG. 26 ),  224  ( FIG. 27 ), show three different cross-sectional shapes.  FIGS. 25 and 26  show a welding stack-up arrangement for direct welding, wherein the current passes between  16 A and  18 A and  16 B and  18 B, respectively. The welding may be of the push-pull type, permitting four welds to be conducted simultaneously. Note that for simplicity of illustration, the areas where welding would be conducted are not shown in  FIG. 25  and the figures following  FIG. 25 , but such areas are like the areas A 1 , A 2  of  FIG. 20 , which are proximate the electrodes  16 ,  18  and in  FIG. 25-27  would be proximate the electrodes  16 A,  16 B,  18 A,  18 B.  FIG. 27  shows an alternative electrode arrangement wherein electrodes  16 A and  16 B define a current path including a single electrode  18 A on the other side of the stack-up  210 C. Alternatively, the hollow beam (tube) structure  212  may be formed from a sheet wrapped around the internal structures  220 ,  222 ,  224 . 
         [0123]      FIGS. 28 and 29  show composite structure  220  formed from a hollow beam structure  222 , e.g., made from aluminum, a plate  224  and a plurality of disks  226 , e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The hollow beam structure  222  has a plurality of openings  222 H through which the disks  226  may be inserted and accessed by an electrode  18  in order to permit spot welding the disks  226  to the plate  224  through the beam structure  222 . 
         [0124]      FIG. 30  shows a stack-up for a composite structure  230  formed from a hollow beam structure  232 , e.g., made from aluminum, a plate  234  and a U-shaped member (channel)  236 , e.g., made from steel, that may be joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The U-shaped member  236  may be spring loaded, i.e., the U-shape may be biased to diverge outwardly and may frictionally grip hollow beam structure  232 . The U-shaped member  236  may be inserted into hollow beam structure  232  by electro-magnetic forming, shrink-fit, mechanical contact, bonding, fastening, clinching, brazing, etc. 
         [0125]      FIG. 31  shows a stack-up for a composite structure  240  formed from a hollow beam structure  242 , e.g., made from aluminum, a plate  244  and a hollow beam (tube)  246 , e.g., made from steel, that may be joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The hollow beam  246  may be inserted into hollow beam structure  242  by electro-magnetic forming, shrink-fit, mechanical contact, bonding, fastening, clinching, brazing, etc. 
         [0126]      FIGS. 32 and 33  show composite structure  250  formed from a hollow, cylindrical beam structure  252 , e.g., made from aluminum, a plate  254  and a hollow cylindrical support beam  256 , e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18 ′ that function as described above in reference to  FIG. 1 . The plate  254  has an arch portion  254 A that is complementarily shaped relative to the beam structure  252 . A plurality of welds W secure the plate  254  to the support beam  256 .  FIG. 33  shows the welding stackup of composite structure  250 . As can be seen, the electrode  18 ′ has a large surface area such that the electric current and heat attributable to resistive flow is distributed and does not cause melting to occur at the interface with the beam structure  252 . Electrode  16  has a normal spot welding configuration, such that it concentrates the current and heat to form a spot weld W. 
         [0127]      FIG. 34  shows a stack-up for a composite structure  260  formed from an I beam structure  262 , e.g., made from aluminum, a plate  264  and a pair of channel beams  266 A,  266 B, e.g., made from steel, that may be joined to the plate  264  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . Since both electrodes  16 ,  18  are on the same side of plate  264 , the welding set-up could be described as for single sided welding. 
         [0128]      FIG. 35  shows a stack-up for a composite structure  270  formed from a boxed I beam structure  272 , e.g., made from aluminum, a plate  274  and a pair of channel beams  276 A,  276 B, e.g., made from steel, that may be joined to the plate  274  by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . Since both electrodes  16 ,  18  are on the same side of plate  274 , the welding set-up could be described as for single sided welding. The channel beams  276 A,  276 B may be inserted in the beam structure  272  telescopically at an end, or openings  272 O may be provided in the beam structure  272  to allow insertion of the channel beams, e.g.,  276 B. 
         [0129]      FIG. 36  shows a composite structure  280  formed from a hollow beam structure  282 , e.g., made from aluminum with access windows  282 W through which brackets  284 , e.g., made from steel, may be inserted and through which electrode  18  may be inserted to perform a spot welding operation as described above for securing a plate or other steel member (not shown) placed against the outer surface of the beam structure  282  in proximity to the brackets  284 . An alternative type of bracket  286  is shown positioned at the open end of the beam  282  and may perform a similar function as brackets  284 . 
         [0130]      FIGS. 37 and 38  show composite structure  290  formed from a hollow beam structure  292 , e.g., made from aluminum, a plate  294  and a hollow beam structure  296 , e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The beam structure  292  has an opening  292 O permitting the perpendicular insertion of beam structure  296 . As shown in the welding stack-up of  FIG. 38 , the electrodes  16 ,  18  may be utilized to weld plate  294  through beam  292  to beam  296 . 
         [0131]      FIG. 39  shows a composite structure  300  formed from a hollow beam structure  302 , e.g., made from aluminum, a hollow beam structure  304  and a plate  306 , e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The beam structure  302  has side openings  302 O permitting the perpendicular insertion of beam structure  304 . The beam structure  302  has flanges  302 F extending from the beam  302  proximate the openings  302 O. The plate  306  may be welded through beam  302  and/or flanges  302 F to beam  304 . 
         [0132]      FIGS. 40 and 41  show a composite structure  310  formed from a hollow beam structure  312 , e.g., made from aluminum, a hollow beam structure  314  and plates  316 A,  316 B, e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The beam structure  312  has side openings  312 O permitting the perpendicular insertion of beam structure  314 . The beam structure  312  has flanges  312 F (four in number) extending from the beam  312  proximate the openings  312 O. The plates  316 A,  316 B may be welded through beam  312  and/or flanges  312 F to beam  314 .  FIG. 41  shows the welding stack-up of components of structure  310  prior to welding. 
         [0133]      FIGS. 42 and 43  show a composite truss structure  320  formed from hollow beam structures  322 , e.g., made from aluminum, hollow beam structures  324  and plates  326 A,  326 B, e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The beam structures  322  have side openings  322 O permitting the insertion of mitered ends of beam structures  324  where they are retained by welds W between the plates  326 A,  326 B and the structures  324 . 
         [0134]      FIGS. 44 and 45  show a composite structure  330  formed from a hollow beam structure  332 , e.g., made from aluminum, hollow beam structures  334 A,  334 B and plates  336 A,  336 B, e.g., made from steel, joined by electrical resistance welding applied by electrodes  16 ,  18  that function as described above in reference to  FIG. 1 . The beam structure  332  has side openings  322 O permitting the insertion of beam structures  334 A,  334 B there through at an angle, the beams  334 A,  334 B being at a skew orientation relative to each other. The beams  334 A,  334 B are welded in place via plates  336 A,  336 B via electrical resistance welding. As before, the spot welds extend through the aluminum structure  332  allowing the steel structures  334 A,  334 B to weld to the plates  336 A,  336 B. 
         [0135]    It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the disclosed subject matter. All such variations and modifications are intended to be included within the scope of the claims.