Abstract:
The present invention provides a method of joining a sheet metal part to a metal tube to form an assembly. In an overlapping configuration, the parts are held together under a clamping force while a self-piercing rivet is driven through the assembly by a welding electrode. The rivet pierces and passes through the sheet metal part and then through at least substantial portion of the metal tube to thereby form a mechanical interlock. An electrical current is then applied to this mechanical retention to melt a portion of the rivet as well as a portion of the metal surrounding the joining region. The flow of electrical current is then stopped after several welding cycles where the now melted material is allowed to solidify upon cooling, thereby forming a weld.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a method of joining a sheet metal part to a surface of a metal tube. More specifically, the invention relates to a joining method in which the sheet metal is initially staked to the tube using a penetrating metal fastener and then a welding current is momentarily passed through the fastener and tube to fuse the embedded fastener to the sheet and tube.  
         BACKGROUND OF THE INVENTION  
         [0002]    Shaped metal tubes are desirable for use in automotive body assemblies due to their strength and relatively low weight. In constructing automotive body assemblies that use bent, hydroformed tubes or the like, it is sometimes necessary to join a sheet metal body member to such a tube. However, it is often difficult to use traditional spot welding, riveting, or bolting practices to make sheet-to-tube assemblies.  
           [0003]    Thus, it is an object of the present invention to provide a method of joining a tubular part to a sheet metal part with a flat attachment location for an automotive body assembly or the like. It is a further object of the present invention to provide a method of joining a tubular part to a flat part where the joint is robust and will contribute to the structural integrity of the assembly.  
         SUMMARY OF THE INVENTION  
         [0004]    This invention is applicable to joining a sheet metal part having a relatively flat attachment surface to a surface of a metal tube. A suitable fixture may be provided to hold the attachment surface of the sheet metal part against the tube, especially when the joining operation is to be repeated like, for example, in a vehicle body manufacturing line.  
           [0005]    The joining operation comprises initially staking the sheet metal part to the tube with a self-piercing rivet or the like. The shank of the rivet or other staking connector is driven through the sheet metal part into the wall of the tube. Preferably, the shank of the rivet is a hollow cylinder with a tapered, sharpened end for penetrating the sheet metal thickness and the wall of the tube. The head of the rivet is driven against the outer surface of the sheet metal part and the shank of the rivet penetrates well into the wall of the tube, or even through it. But since the tube is normally an elongated body, there is no access to the buried end of the rivet shank to form it over against the inside of the tube wall to secure a riveted joint. In accordance with the invention, the rivet is permanently secured in the sheet metal and tube by passing an electrical current through the rivet and tube to fuse the rivet to the surrounding sheet and tube metal.  
           [0006]    The fusing step is carried out by applying one electrode against the head of the rivet and an opposing electrode against the opposite wall of the tube. A suitable high amperage electrical current, which is like a welding current, is passed between the electrodes. This current is suitably a 60 Hz alternating current or a rectified 60 Hz alternating current and the electrodes are best made of copper or a copper alloy. The welding current passes between the electrodes around the local circumference of the tube and through the rivet and adjoining material of the sheet metal part. The presence of small gaps between the rivet and the surrounding sheet metal and metal tube provide relatively high electrical resistance to the current. The metal at the respective gaps is rapidly heated and fused. After several cycles of the 60 Hz current, the electrical current is stopped. A nugget of fused material between the rivet, sheet metal part, and tube secures the initially staked, mechanical joint. Of course, more than one such joint may be required to securely join a given sheet metal part to a tube.  
           [0007]    In a preferred embodiment of the invention, the electrode for contact with the rivet head is used like a hammer to drive the rivet through the sheet metal part into the tube wall, and the counter-electrode on the opposite side of the tube is used like an anvil to hold the tube against the impact of the self-piercing rivet. The counter-electrode is shaped, if necessary, to conform to a contour in the tube wall so as to better serve both the anvil function and electrical contact function of that electrode.  
           [0008]    An advantage of the subject joining process is that much of the joining effort is applied from the outside of the sheet metal part and only the anvil/counter electrode need be brought into contact with the diametrically opposite side of the tube. Many of the sheet metal parts and tubes to be joined in automotive body construction are of complex shape and full two-sided access to the surfaces of the parts at the joining location may be difficult.  
           [0009]    These and other objects and advantages of this invention will become apparent from a detailed description of the preferred embodiment that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a tilted, side view, partly in cross-section of a self-piercing rivet having a hollow tapered piercing shank adapted to penetrate a sheet metal part and the wall of a tubular part.  
         [0011]    [0011]FIG. 2 is a side view, partially in cross-section, showing the clamping stage of the present invention where the sheet and tube are secured for joining.  
         [0012]    [0012]FIG. 3 is a side view, partially in cross-section, showing the self piercing rivet being driven through the sheet metal part into the tube in accordance with the present invention.  
         [0013]    [0013]FIG. 4 is a side view, partially in cross section, showing the completed riveting process and the application of electrical current to weld and secure the riveted sheet-to-tube joint. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    The present invention provides a method of joining a metal tube to a sheet metal part such as for use in automotive body assemblies. The method of this invention uses features of self-piercing riveting practices and resistance spot welding methods to provide an improved method for joining a tube with only single-sided access to the joining location. The practice of the invention will be illustrated using a flat portion of a sheet metal part and a square tube but the subject process is applicable to round tubes and the like. Sheet metal parts of complex shape with a flattened portion for joining to the tube are applicable as well.  
         [0015]    The present invention is sequentially shown in FIGS.  2 - 4 . The tooling shown in these Figures, however, is not necessarily drawn to scale. The Figures represent idealized views of the joining process for purposes of illustration.  
         [0016]    Starting with FIG. 2, a joint is to be made between a flat portion of a generic sheet metal part  30  and upper side wall  31  of a tubular part  32  that is square in cross-section. The flat portion of the sheet metal part  30  is placed against a wall surface of the tubular part  32  at a predetermined joining location. These parts are then held together, under a clamping force, to maintain positioning of the parts during the joining operation. In this process a riveting step and a fusion step are used. Accordingly, tools used in the riveting and welding may be used in the clamping. Thus, in FIG. 2, for example, a round, hollow cylinder  44  is used to apply the clamping force. The flat end  46  of cylinder  44  presses against the upper flat surface  34  of sheet metal part  30 . As will be seen, cylinder  44  will be used in the riveting step of the process. A clamping force is also applied to the opposite side wall  33  of tube  32 . In this case, the clamping force is applied by a copper slab, or back-up, which serves as a counter-electrode  40  in a subsequent welding step.  
         [0017]    While subjected to the clamping force, the tubular part  32  is initially mechanically joined to the sheet metal part  30  by means of self-piercing riveting. Any suitable self-piercing fastener will be suitable in practicing the present invention. For purposes of illustration, a self-piercing rivet in describing this invention is shown in FIG. 1. An example of a suitable self-piercing rivet  10  is shown in FIG. 1. Self-piercing rivet  10  comprises a round hollow shank  12  and a flat rivet head  14 . Thus, axially extending through shank  12  is a central hole  16 , which defines the inner surface of shank  12 . Shank  12  terminates in a tapered sharpened edge  22  for piercing the sheet metal part  30  and wall  31  of tube  32 . Piercing edge  22  is suitably hardened to penetrate metal.  
         [0018]    At the opposite end of hollow shank  12  is flat rivet head  14  that provides an impact surface for driving rivet  10  into the sheet metal part  30 . Specifically, rivet head  14  is a flat-headed, annular flange where its top surface  18  comprises a diameter that is significantly larger than shank  12 . Below top surface  18 , head  14  circumferentially tapers into shank  12 . Tapered surface  20  permits rivet  10  to be driven into the sheet metal part  30  until top surface  18  lies smoothly against part surface  34  (as shown in FIG. 4).  
         [0019]    The first step of this joining method comprises driving self-piercing rivet  10  through sheet metal part  30  and through at least a substantial portion of wall  31  of tubular part  32  to temporarily stake the parts together. This rivet driving step can be accomplished by any suitable means. However, since the rivet piercing step is to be followed by a welding step, it may be preferred to combine tooling for the two steps. For example, welding electrode  42 , shown in a broken away illustration in FIGS.  2 - 4 , may be used to hammer or drive rivet  10  through the metal layers.  
         [0020]    For the riveting step, cylinder  44  is sized to accommodate the round head  14  of rivet  10 . Welding electrode  42 , with a flat tip for hammering, is sized to the diameter of rivet head  14  can slide axially up and down in cylinder  44 . Cylinder  44  helps to locate rivet  10  on the joining portion of sheet metal part  30  and to keep it aligned as it is being driven into surface  34 . It also provides suitable clamping force on the assembled parts for the joining operation. Also, welding electrode  40  is shaped to fit closely against wall  33  to hold tubular part  32  against sheet metal part  30  and for good electrical contact with wall  33 .  
         [0021]    As shown in FIG. 3, rivet  10  is then driven into the sheet metal part  30  using electrode  42  as a rivet punch. Upon sudden and forceful impact of electrode  42  against rivet head  14 , piercing edge  22  will engage and penetrate top surface  34  of sheet  30 . Thus, rivet  10  forms its own aperture. As shown in FIG. 4, rivet  10  is pushed through sheet  30  and into tube  32  until rivet head  14  is flush with top surface  34  of sheet metal part  30 . Rivet  10  is driven into sheet  30  under a high rate and impact so that it will penetrate the sheet without significantly deforming it. The sheet metal  30  and tube  32  may deform inwardly from the impact of the rivet  10  as seen in FIG. 4. FIG. 4 illustrates the situation in which the inserted rivet  10  has pierced the sheet metal  30  without bending it and has bent the metal tube  32  inwardly. The rivet  10  has cut and retained a disk from the sheet metal part  30  and pushed in the tube wall  31 . As seen, tube  32  protrudes slightly below hollow center  16  of shank  12  of the rivet  10 . Sheet or tube material captured in hollow center  16  of rivet  10  may contribute to the weld layer as welding material. Thus, a mechanical attachment is formed between sheet  30  and tube  32 .  
         [0022]    The riveted, mechanical attachment is then secured using a resistance spot welding process. In this welding process, rivet  10  receives a pulse of high current flow from welding electrode  42  and copper slab  40 , which acts as a counter electrode. A steady application of force is also supplied by clamping element  44  and copper slab  40 . A pulse of high amperage AC (or rectified AC) current is directed through the facing electrodes  40 ,  42  to the joining location. Typically a 60 Hertz welding current is applied for several cycles of electrical current application, which amounts to less than a second of applied electrical current. Resistance to the electrical current by rivet  10  and surrounding metal of sheet  30  and tube  32  generates heat in the flow path of the current, thereby causing the metal located at the joining location to melt as well as a substantial portion of shank  12  of rivet  10 . Depending on the materials used for sheet  30  and tube  32 , the material for rivet  10  should be selected such that it has a higher resistance to electrical current than the sheet  30  and tube  32  materials. As such, that rivet  10  will contribute mostly to welding material for the joint. Rivet  10  will then weld with the surrounding metal of sheet  30  and tube  32 . Upon cooling, the molten metal solidifies to form weld nugget  48 .  
         [0023]    The method of the present invention allows a secure and strong mechanical bond and weld to be formed between a sheet metal part and a tubular part, where the tubular part may be bent and/or hydroformed. Such tubular parts are difficult to join with other metallic parts because tooling cannot reach the inside of the tube, but the method of the present invention overcomes this difficulty. In a preferred embodiment, the electrode tooling is used to clamp the pieces together, drive the rivet into the parts and weld the rivet to the parts. Such consolidation of the duties of specific pieces of equipment allows the joining process to be economically and efficiently advantageous in vehicle fabrication, and specifically in automotive body assembly processes.  
         [0024]    While the invention has been described in terms of a preferred embodiment, it is not intended to be limited to that description, but rather only to the extent of the following claims.