Abstract:
A method for forming an electrical resistance weld between a metal tube and a metal sheet is disclosed. In the method, a plurality of generally circular protrusions having generally triangular cross sections is formed upon the sheet. Thereafter, electrodes are utilized to flex the tube, while an electrical current is generated for electric resistance welding the sheet to the tube.

Description:
TECHNICAL FIELD 
     The present invention relates to electrical resistance welding of hydroformed metal tubing with metal sheet for assembling automotive structures. 
     BACKGROUND OF THE INVENTION 
     Recent developments in metal forming technology have permitted the manufacture of automotive vehicle frames and other support members using hydroformed metal tubes. Hydroformed tubes are attractive for automotive vehicles because they afford lightweight and high integrity structures. In many applications, it is desirable to weld such tubes to sheet metal. However, in some instances, the closed nature of such hydroformed tubes imposes practical constraints on localized welding. Conventional spot welding may be employed. However, to improve process efficiency, particularly where selective control over weld nugget formation is desired, and to extend the useful lives of electrodes, there is a need for techniques alternative to conventional spot welding for achieving high integrity localized welding of hydroformed metal tubes to sheet metal. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved method for electrical resistance welding a hydroformed metal tube with a metal sheet to make an automotive vehicle structure. A hydroformed metal tube is contacted with a protrusion pattern that has been formed on the metal sheet. The protrusion pattern is defined by a plurality of generally circular and concentric protrusions having triangular cross sections. Using resistance welding electrodes, a portion of the tube is flexed for contacting the outermost protrusion of the pattern. While the tube is in the flexed state, an electrical current is applied to initiate melting from the outermost concentric protrusions radially inward. Upon solidification, a weld nugget results having an outside diameter approximating the outside diameter of the protrusion pattern. The present invention thus meets the need in the art for an improved welding technique affording selective control over nugget formation, reduced energy consumption and prolonged electrode life. 
     These and other aspects and advantages of the present invention will become apparent upon reading the following detailed description in combination with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a sectional view of a hydroformed tube being welded to a metal sheet employing a protrusion pattern of the invention, prior to flexing of the tube. 
     FIG. 1B illustrates another sectional view of the tube and sheet of FIG. 1A during flexing of the tube. 
     FIG. 2A illustrates a side sectional view of the protrusion pattern in a metal sheet. 
     FIG. 2B illustrates a top view of the protrusion pattern of FIG.  2 A. 
     FIG. 2C illustrates a sectional view of only the protrusions of the pattern of FIGS. 2A and 2B. 
     FIG. 3 illustrates a side sectional view showing formation of the protrusion pattern. 
     FIG. 4 illustrates welding of an automotive vehicle roof panel to a hydroformed frame with a plurality of welds. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1A and 1B, a section of a hydroformed metal tube  10  is welded to a metal sheet  12 . The tube  10  includes a generally continuous wall  14  having an inner surface  16  and an outer surface  18  separated by a first thickness t 1 , which is about 0.6 mm to about 3.0 mm and preferably about 1.2 mm. The inner surface  16  of the tube  10  generally defines a passageway  20  that extends along a length of the tube  10 . The outer surface  18  includes at least one generally planar wall  22  for welding to the sheet  12 . The wall  22  is flexed during welding as shown in FIG.  1 B. 
     The metal sheet  12  includes a first generally planar surface  24  and a second generally planar surface  26  separated by a second thickness t 2 , which is about 0.6 mm to about 3.0 mm. Preferably, the metal sheet  12  is formed of 0.8 mm gage coated (e.g., galvanized, galvannealed or the like) steel. 
     Referring to FIG. 2B, the metal sheet  12  includes a protrusion pattern  28  that includes a plurality of generally circular protrusions  28   a,    28   b,    28   c,    28   d,  which are generally concentric relative to each other and respectively have progressively smaller diameters D 1 , D 2 , D 3 , D 4 . 
     As shown in FIGS. 2A-2C, each of the protrusions  28   a - 28   d  have substantially identical generally triangular cross sections with a base  30  and an apex  32 . Each base  30  has a length  1  of about 0.4 to 0.6 millimeters, and each of the protrusions  28 ( a )- 28 ( d ) has a height h from the center point of its base  30  to the apex  32  of about 0.15 to about 0.25 millimeters. 
     Referring to FIG. 3, the protrusion pattern  28  is formed in the sheet  12  with a tool  34  (e.g., a punch or die) having four concentric channels  36 ,  38 ,  40 ,  42  for complementarily defining the pattern  28 . 
     Referring again to FIGS. 1A and 1B, for welding, at least one of the protrusions  28   a,    28   b,    28   c,    28   d  is contacted with the wall  22  of the tube  10 . Current from welding electrodes  44 ,  46  is then applied to the sheet  12  and the tube  10  for resistance welding. For instance, a first copper ball or b-nose welding electrode  44  is placed into contact with the second surface  26  of the metal sheet  12  opposite the protrusion pattern  28  extending from the first surface  24 . A copper back-up welding electrode  46  is placed into contact with a second surface  48  of the tube  10  near the desired weld location. The second welding electrode  46  includes a surface  50  that generally mates with the second surface  48  of the tube  10  near the desired weld location for supporting the tube  10 . 
     As shown in FIG. 1B, at least during the initial welding, the electrodes  44 ,  46  apply a force upon the tube  10  and upon the sheet  12  sufficient to flex the wall  22  of the tube  10  toward the second surface  48  of the tube  10  or away from the inner part of the protrusion pattern  28 , thus maintaining the outermost protrusion  28   a  in contact with the flexed wall  22  of the tube  10 , but leaving a space between the surface  22  of the tube  10  and at least one but, preferably, all of the innermost protrusions  28   b - 28   d.    
     While the wall  22  of the tube  10  is flexed, current is applied to the protrusion pattern  28  for heating and melting the outer concentric protrusion  28   a.  As the outer protrusion  28   a  melts, the next inner protrusion  28   b  contacts the flexed wall  22  and is heated and melted followed by the other two inner protrusions  28   c,    28   d  respectively. 
     Subsequent cooling results in a weld nugget, which attaches the sheet  12  to the tube  10 . The nugget has an outer diameter of about the same diameter D 1  of the outer protrusion  28   a,  and the nugget is formed without substantial loss or migration of metal from the weld location. 
     Advantageously, consecutively melting the protrusions  28   a - 28   d  of the protrusion pattern  28  and then solidifying the melted metal in the manner prescribed above helps assure that even the most inner protrusion  28   d  is melted and then solidified such that an internally continuous weld nugget is formed. 
     Also advantageous, the electrodes  44 ,  46  can melt the protrusions  28   a - 28   d  quickly while applying minimal force to the sheet  12  and tube  10 , thereby increasing the useful lives of the electrodes  44 ,  46  and decreasing the likelihood of permanent deformation of the sheet  12 . Such technique is particularly attractive for welding more brittle metals such as coated high strength steel. 
     By way of example, in FIG. 4 there is a vehicle  52  having a hydroformed tube roof rail  54  with a 1.2 millimeter wall thickness and a 0.8 millimeter thick coated steel sheet roof panel  56 . The roof panel  56  (or an associated roof rail bracket) is formed with the protrusion pattern  28  of FIGS. 2A-2C spaced at weld locations  58  adjacent an edge of the panel  56 . Using an electrode force of about 222 N for locally flexing the roof rail  54  at the weld site, approximately 8 kA of current is applied for about seven cycles (1 cycle={fraction (1/60)} second). Upon completion, weld nuggets at the weld locations  58  weld the roof panel  56  or an associated roof rail bracket to the roof rail  54 . 
     It should be understood that the invention is not limited to the exact embodiment or construction which has been illustrated and described but that various changes may be made without departing from the spirit and the scope of the invention.