Patent Publication Number: US-2015063902-A1

Title: Mounting unit and method for its production

Description:
The invention relates to a mounting unit having a mounting part and a welding element as well as a method for producing the mounting unit. 
     The fixing of a mounting part, for example a sheet metal-like body part to a basic structure of a motor vehicle, frequently takes place by point welding or resistance welding. In particular in automobile construction, mounting parts made of light-weight materials such as aluminum are used, for example, for weight saving reasons. If in this case a mix of materials is produced which is not able to be welded, i.e. if the material of the mounting part may not be welded to the material of the basic structure, the mounting part is provided with one or more welding elements which consist of a material which is compatible in terms of welding technology with the material of the basic structure, for example steel. The welding element is fixed in a through-bore of the mounting part, wherein the mounting part has a mounting face and an outer face facing away therefrom. If the mounting unit and/or the mounting part is fixed to a surface of the basic structure denoted hereinafter as the mating surface, the mounting face faces this mating surface. 
     In conventional connections of the type under discussion, disclosed for example in WO 2012/041515 A1, a disk-shaped welding element is provided, said welding element being inserted by a positive and/or non-positive connection in the through-bore of the mounting part. The welding element, configured as a disk substantially completely filling up the through-bore, has a relatively large mass which, in particular, with thick mounting parts has a negative effect on the total weight of the mounting unit. The weight saving achieved by the use of a light-weight mounting part is at least partially compensated as a result. Moreover, welding elements and/or welding element blanks of variable thicknesses corresponding to the thickness of the mounting part have to be used. 
     Proceeding therefrom, it is the object of the invention to propose a mounting unit in which the use of welding elements of reduced weight is possible without compromising the strength of the connection between the mounting unit and the welding element. Moreover, the welding elements in their initial form, i.e. the welding element blanks, are intended to be able to be produced easily and able to be used for a wide range of thicknesses of the mounting parts. 
     The object of the invention is also to specify a method for producing the mounting unit which may be carried out easily. 
     The first object is achieved by a mounting unit as claimed in claim  1  and the second object by a method as claimed in claim  14 . 
     A mounting unit according to the invention comprises at least one welding element configured as a sleeve, wherein the outer end of the sleeve arranged on the outer face of the mounting part bears a radially protruding flange. Said flange is supported with at least one part of its flange lower face on the outer face of the mounting part. By means of this design a positive connection is ensured between the welding element and the mounting part in a direction extending toward the outer face. 
     The part of the sleeve extending away from the flange, i.e. the shaft thereof protruding from the flange lower face, has a length which is at least as great as the thickness of the mounting part in the edge region of the hole of the through-bore. The inner end of the shaft arranged on the mounting face, therefore, terminates flush with the mounting face of the mounting part or protrudes beyond said mounting face with a set overhang. A front face present on the inner end of the shaft which serves as a welding surface for welding to the mating surface of the basic structure may, therefore, be brought into contact with the mating surface of the basic structure for the purpose of producing a welded connection. 
     The shaft of the sleeve has a contour shape corresponding to the cross-sectional shape of the through-bore, whereby the production of a positive connection and/or frictional connection between the wall of the through-bore and the outer wall of the shaft, for example by a radial widening of the shaft, is facilitated. 
     The shaft is retained in the through-bore by a further positive connection and namely by a positive connection in a direction facing toward the mounting face. The welding element in this case is able to be fixed in the through-bore of the mounting part with the strength required for the subsequent fixing of the mounting unit to the basic structure. The mounting unit, therefore, may be transported without the risk of the loss of a welding element. Moreover, the subsequent mounting on a basic structure is facilitated in that the mounting unit does not first have to be provided with welding elements which might be associated with a corresponding cost in terms of logistics and technical process. 
     By the sleeve-like design of the welding element, the quantity of material thereof and accordingly the weight thereof are reduced relative to a disk-shaped welding element practically completely filling the through-bore of the mounting part. The strength of the welded connection in this case is, however, comparable with a disk-shaped welding element which is connected by a central welding zone to the basic structure, or is even greater. Firstly, a sufficiently large welding surface is available with the annular front face. Secondly, relative to a central punctiform welding surface and/or welding zone a greater resistance of the welded connection results with regard to tilting of the mounting part relative to the central longitudinal axis of the welding element and/or the through-bore. In other words, in the proposed connection, a greater pull-through force may be transmitted as the welding element at the connecting point with the mounting part is subjected only to shear stress and not additionally to bending stress. 
     In the method according to the invention for producing the mounting unit, a sleeve is provided as a welding element blank which bears at its one end a radially protruding flange, wherein the shaft of the sleeve protruding from the flange lower face has a contour shape corresponding to the cross-sectional shape of the through-bore and a length which is at least as great as the thickness of the mounting part in the edge region of the hole. The welding element blank is thus introduced into the through-bore such that the flange bears against the outer face of the mounting part and the inner end of the shaft on the mounting side is aligned with the mounting face or protrudes therefrom with an overhang. A further advantage of the sleeve-like design of the welding element is that it may be connected positively to the mounting part in a simple manner by means of a punch supplied from the mounting face. To this end, at least one partial region of the sleeve shaft extending away from the welding surface is radially widened and at the same time is plastically deformed, creating a positive connection between the shaft and the mounting part in a direction extending away from the mounting face and toward the flange. 
    
    
     
       The invention together with further advantageous embodiments is now described in more detail with reference to the accompanying drawings, in which (primarily in sectional view): 
         FIG. 1A  shows a first exemplary embodiment of a mounting unit which comprises a mounting part penetrated by a through-bore and a sleeve-like welding element inserted into the through-bore, 
         FIG. 1B  shows a plan view in the direction of the arrow IB of  FIG. 1A , 
         FIGS. 2A-D  show the production of the mounting unit of  FIG. 1A  based on a welding element blank and a mounting part, 
         FIG. 3  show the detail III of  FIG. 1A  which shows a first variant of the positive connection between the welding element and the mounting unit, 
         FIG. 4  shows an illustration corresponding to  FIG. 3  which shows an alternative positive connection between the welding element and mounting part, 
         FIGS. 5A , B show an opposing positioning of two mounting units with mounting parts of different thicknesses, 
         FIGS. 6A-6D  show an alternative production method for the mounting unit according to  FIG. 1A , 
         FIG. 7  show a variant of the exemplary embodiment of  FIG. 1 , 
         FIG. 8  shows a further variant of the exemplary embodiment of  FIG. 1 , 
         FIG. 9A  shows a variant of the mounting unit of  FIG. 1 , which comprises a welding element with a central bore, 
         FIG. 9B  shows the welding element blank for the mounting unit of  FIG. 9A , 
         FIG. 10A  shows the welding element blank and the mounting unit for a further variant of the mounting unit of  FIG. 1 , 
         FIG. 10B  shows the finished mounting unit, i.e. the parts of  FIG. 10A  in the mounted state, 
         FIG. 11  shows the welding of a mounting unit to a basic structure by means of welding electrodes, 
         FIGS. 12A-12D  show the production of a second exemplary embodiment of a mounting unit, 
         FIG. 13  shows a mounting unit corresponding to  FIG. 12D  but with a mounting unit of greater thickness, 
         FIG. 14  shows the welding of a mounting unit according to  FIG. 12D  or  FIG. 13B  to a basic structure, 
         FIG. 15  shows a mounting unit welded to a basic structure. 
     
    
    
     The mounting units  1  shown in the illustrations comprise a mounting part  2  which, for example, is a sheet metal-like shaped part which is intended to be fixed to a basic structure  5  ( FIG. 15 ) namely a frame structure of a motor vehicle or the like. The mounting part  2  has a mounting face  3  which in the final mounted state, i.e. when the mounting unit  2  according to  FIG. 15  is welded to a mating surface  4  of the basic structure  5 , faces the basic structure and/or the cited mating surface. The mounting part  2  is penetrated by a least one through-bore  6 . A welding element S configured in the form of a sleeve  7  is positively fixed in the through-bore  6 . The sleeve  7  has on its outer end  8  facing away from the mounting face  3 , a radially protruding flange  9 . Said flange extends transversely to the central longitudinal axis  26  of the through-bore  6  and/or the welding element S and with its flange lower face  10  bears fully, or as is the case in the mounting unit  1  of  FIG. 11 , partially against the outer face  13  of the mounting part  2 . The outer face  13  extends at least in the edge region of the hole of the through-bore  6  parallel to the mounting face  3  and transversely to the aforementioned central longitudinal axis  26  and is remote from the mounting face  3 . The part of the sleeve  7  extending away from the flange lower face, the shaft  14  thereof, extends into the throughbore  6  and has a contour shape which corresponds to the cross-sectional shape of the through-opening  6 . In the exemplary embodiments shown in the illustrations, the through-bore  6  has a circular cross-sectional shape. The outer wall  15  of the shaft  14  bears substantially flat against the bore wall  16 . 
     The length  17  of the shaft  14  is at least as great as the thickness  18  of the mounting part  2  in the edge region of the hole of the through-bore  6  ( FIG. 3 ). If the shaft length  17  is greater than the thickness  18 , the inner end  19  of the sleeve  7  and/or of the shaft  14  arranged in the region of the mounting face  3  protrudes with a set overhang  20  from the mounting face  3 . The size of the aforementioned overhang is not arbitrary but is predetermined so that in the final mounted state according to  FIG. 15  a defined intermediate space  23  is present between the mounting part  2  and the basic structure  5  in which, for example, an insulating layer  24  is arranged. Said insulating layer serves, for example, to prevent contact corrosion by preventing direct contact between the mounting part  3  and the basic structure  5  and the penetration of moisture in the intermediate space  23 . An intermediate space  23  between the mounting part  3  and the basic structure  5 , however, is not required in every case, for example when contact corrosion is not a risk. The inner end  19  of the sleeve  7  is then aligned virtually with the mounting face  3  so that for producing a welded connection the front face  25  present at the aforementioned end may be brought into contact with the mating surface  4  of the basic structure and may be welded thereto. 
     The positive fixing of the welding element S in the through-bore  6  takes place by means of two positive connections acting in opposing directions. Firstly, may be cited the flange  9  which bears against the outer face  13  of the mounting part  2  and/or radially overlaps the edge region of the hole of the through-bore  6  on the outer face. In this manner, a positive connection in a direction R 1  extending parallel to the central longitudinal axis  26  of the through-bore  6  and extending toward the outer face  13  is ensured. In the final mounted state according to  FIG. 15 , the direction R 1  faces at right angles to the mating surface  4  of the basic structure  5 . 
     A second positive connection which is effective in a direction R 2  opposing the direction R 1  is achieved in that a portion of the bore wall  16  extending away from the mounting face  3  and the outer wall  15  of at least the inner shaft end  19  widens conically in a complementary manner toward the mounting surface  3  of the mounting part  2 . This design of the positive connection is thus expedient if the material of the mounting part  3  has a lower hardness and/or is more easily deformable than the material of the welding element S. The positive connection may then be produced in a simple manner by at least the inner end  19  of the welding element S and/or of the originally cylindrical shaft  14 ′ of a welding element blank forming the subsequent welding element S, being substantially conically widened with a widening tool, wherein in this case the originally cylindrical bore wall  16  is plastically deformed by widening and at the end has a conical shape complementary to the conical deformation of the shaft  14 . 
     A further possibility for producing a positive connection between the mounting part  2  and the welding element S in the direction R 2  is that a radial outer region  27  of the inner shaft end  19  radially overlaps a rear engagement surface  28  of the mounting part  3 , wherein the rear engagement surface  28  is remote from the outer face  13  of the mounting part  2  and extends transversely to the central longitudinal axis  26  of the through-bore  6  ( FIG. 4 ). This type of positive connection is expedient if the material of the mounting part  2  has a greater hardness and/or is less easily plastically deformable than the material of the welding element S. When producing the mounting unit  1  the inner end  19  of the shaft inserted in the through-bore  6  is radially widened with a widening tool. This results substantially only in a plastic deformation of the shaft  14  such that the radial outer region  27  of the shaft  14  radially overlaps the rear engagement surface  28 . In the exemplary embodiment shown in  FIG. 4 , the rear engagement surface is a region of the mounting face  3  extending away from the bore wall  16 . Such a positive connection is also possible if no set overhang  20  of the shaft  14  is provided. The rear engagement surface  28  is then, for example, a radial shoulder inside the throughbore  6  (not shown). 
     The set overhang  20  is adjusted by an axial compression of the welding element. In the mounted state, therefore, an axially compressed welding element S is present, the shaft  14  thereof protruding with the set overhang from the mounting face  3  of the mounting part  2 . The use of a more or less compressed welding element S has the advantage that, depending on the thickness  18  of a mounting part  2  to be connected to a basic structure  5 , a single welding element blank  40 , i.e. one with the same overall size and/or shaft length  17 ′, may be used. An initial overhang  46  of the welding element blank  40  ( FIG. 2C ) may be axially compressed by adapting to the respective thickness  18  of the mounting part  2  to such an extent that the inner end  19  of the shaft  14  in the mounted state protrudes with the set overhang  20  from the mounting face  3 . 
     In the mounting unit of  FIG. 1A , the inner end  19  of the shaft  14  is closed by a transverse wall  29 . An edge region of the transverse wall  29  forms with the sleeve wall  30  a V-shaped fold  33  opening toward the outer sleeve end  8 . The front face  25  which serves as a welding surface is arranged on the connecting region between the sides of the fold  33  arranged in a V-shape. The central region  34  of the transverse wall  29  located radially inside the fold  33  extends in the exemplary embodiment of  FIG. 1A  in a plane parallel to the mounting face  3  of the mounting part  2 . The welding element S is in an axially compressed state. On its side remote from the outer face  13  of the mounting part  2 , a depression  49  is present, said depression being surrounded by the central region  34  and the fold  33 . 
     In the variant of  FIGS. 7 and 8  the central region  34  of the transverse wall  29  does not extend in one plane but is in the shape of a dish  35  opening toward the outer face  13  of the mounting part. The lower face  36  of the bottom of the dish remote from the outer face  13  of the mounting part  2  is aligned with the front face  25  of the sleeve  7  and forms an additional welding surface by which the welding element S may be welded to the mating surface  4  of the basic structure  5 . In the variant of  FIG. 8 , the central region  34  of the transverse wall  29  is also configured in the form of a dish  35 . The lower face  36  of the dish, however, is also set back in the direction of the outer face  13  of the mounting part  2 , so that an axial spacing  37  is present between the plane spanned by the front face  25  of the sleeve  7  and the lower face  36 . In the final mounted state according to  FIG. 15 , by means of the spacing  37 , an intermediate space (not shown) is kept free, said intermediate space serving, for example, for receiving a layer of adhesive  38  ( FIG. 8 ). In the variant of  FIG. 9A , the transverse wall  29  is penetrated by a central bore  39 . Only one radial external region of the transverse wall  29  is present, said radial external region forming the radial inner side of the fold  33 . A central region  34  of the above-mentioned type is thus not present. 
     During the production of a mounting part  1  initially a sleeve  7 ′ is provided as a welding element blank  40 . The sleeve  7 ′ corresponds substantially to the subsequent sleeve  7  of the finished mounting part  1 . The sleeve wall  30 ′ thereof is not yet radially widened, and thus still has the original cylindrical shape. The flange  9  already described above is integrally formed on the outer end  8  thereof. The subsequent inner end  19  of the sleeve  7 ′ is closed by a transverse wall  29 ′ which extends in a plane extending transversely to the central longitudinal axis  43  of the welding element blank. A mounting part  2  which is penetrated by at least one through-bore  6  is provided. The through-bore  6  has a circular cylindrical cross-sectional shape, wherein the diameter  45  thereof is slightly greater than the outer diameter  44  of the shaft  14 ′. The shaft  14 ′ may thus be easily inserted into the through-opening  6 . At the end of the insertion process, the flange  9  bears with its flange lower face  10  against the outer face  13  of the mounting part  2  ( FIG. 2C ). The length  17 ′ of the shaft  14 ′ is dimensioned such that the shaft  14 ′ protrudes with an overhang  46  from the mounting face  3  of the mounting part  2 . 
     As the next method step, generally a widening and plastic deformation of a partial region and/or axial portion of the shaft  14 ′ extending away from the inner end  19  is undertaken by means of a widening tool, namely the punch  47  supplied from the mounting face  13 . The result of this measure is a positive connection between the radially widened shaft  14  and the mounting part  2  in a direction R 2  facing toward the mounting face  13 . In the case shown in  FIGS. 2A-D , the subsequent inner end  19  of the shaft  14 ′ is closed by a transverse wall  29 ′. According to  FIG. 1A  a punch  47  is used to form a mounting unit  1 , said punch having a central circular projection  42  with a conically extending side wall  50  and being surrounded by an annular recess  53 . The projection  42  and the recess  53  surrounding said projection together form a punch surface which is complementary to the side of the welding element  7 , S remote from the outer face  13  of the mounting part. 
     By the cooperation of a counter holder  48  applied to the flange  9 , a central region of the transverse wall  29 ′ is forced by the projection  42  into the interior of the sleeve  14 ′, wherein at the same time due to the conical side wall  50  of the projection  42  the sleeve  7 ′ is radially widened and the initial overhang  46  is shortened to the set overhang  20 . The extent of the respective shortening of the initial overhang  46  is dependent on the thickness  18  of the mounting part  2 . The thinner the mounting part  2 , the further the transverse wall  29 ′ of the welding element blank  40  is pressed by the punch  47  toward the plane E spanned by the flange  9  and the deeper the side of the transverse wall  29  remote from the outer face  13  of the mounting part  2 , through the depression  49  produced by the plastic deformation under discussion. In the mounting unit  1  of  FIG. 5B  with the thicker mounting part  2  an axial spacing  54  is present between the plane E and the transverse wall  29  and/or the central region  34  thereof. Conversely, in the mounting unit  1  of  FIG. 5A  with the thinner mounting part  2  the central region  34  of the transverse wall  29  is aligned with the plane E. The difference in thickness between the mounting parts  2  is thus compensated by means of the fold  33  formed by the axial compression of the welding element blank  40 : with the reducing thickness  18  of the mounting part  2 , the length of the side  55  of the fold located radially on the inside increases until it has the same length as the side of the fold  33  located radially on the outside, formed by the sleeve wall  30  corresponding to the state according to  FIG. 5A . 
     In the example shown in  FIGS. 2A-D  the material of the mounting part  2  is more flexible and thus more deformable than the material of the welding element blank  40 . Accordingly, the bore wall  16  is deformed in a complementary manner to the conical widening of the sleeve wall  30 , which may be clearly derived from  FIG. 3 . If the mounting part consists of a material which is harder and/or harder to deform plastically than the welding element S, an alternative production method is provided as may be derived from  FIGS. 6A-D . Due to the aforementioned difference in hardness, the region of a mounting part  2  having the through-bore  6  may be used as a drawing die  55  for producing the welding element blank  40  from a sheet metal blank  56 . To this end, the sheet metal blank  56  is pressed onto the through-opening  6  and namely concentrically to the central longitudinal axis  26  thereof and by means of a deep drawing punch  57  a radial internal region of the sheet metal blank  56  is pressed through the through-opening  6 , forming the sleeve wall  30  and the transverse wall  29 ′ of the welding element blank  40 . The flange  9  of the welding element S is formed from a radial external region of the sheet metal blank  56 , wherein said flange is pressed by a flange-like radially protruding part  58  of the deep drawing punch  57  flat against the outer face  13  of the mounting part  2 . In this variant of the method, therefore, the shaping of the welding element blank  40  and the insertion thereof into the through-bore  6  take place in a single method step. 
     The plastic deformation of the welding element blank  40  for the purpose of the positive connection with the mounting part  2  takes place according to  FIG. 6C , i.e. as in the above described manner (see description of  FIGS. 2C and 2D ). However, due to the harder material of the mounting part  2  the bore wall  16  of the through-bore  6  is not conically widened. Instead, the radial outer region  27  of the inner shaft end  19  of the welding element S is forced radially outwards by the punch  47 , wherein it radially overlaps the mounting surface  13  acting as a rear engagement surface  28 , forming the positive connection acting in the direction R 2  (see also  FIG. 4 ). 
     The production of the variant  FIG. 9A  may take place in the same manner as described above. Only one welding element blank  40  is used, the transverse wall  29 ′ thereof, as shown in  FIG. 9B , being penetrated by a central bore  39 ′. The above-described production method may also be used for the variant according to  FIGS. 7 and 8 . However, for the shaping of the dish  35  opening toward the outer face  13  of the mounting part  2 , a [ ] with a complementary shaped projection (not shown) and a punch  47  with a corresponding recess (not shown) are required. 
     In  FIG. 10B  a mounting unit  1  is shown in which an insulating layer  59   a  is present between the flange  9  and the outer face  13  of the mounting part  2  and/or on the flange lower face  10  in order to prevent, for example, contact corrosion. Such an insulating layer  59   b  may also be present in a gap between the shaft  14  of the sleeve  7  and the bore wall  16  and/or on the outer wall  15  of the shaft  14 . An insulation of this type is expedient, in particular, in a mounting part which consists of carbon fiber reinforced plastics. This material—carbon fiber reinforced plastics—behaves electrochemically as a precious metal. 
     For producing a mounting unit  1  with an insulating layer  59   a  and/or  59   b,  for example the corresponding surfaces of the welding element blank  40 , i.e. for example the flange lower face  10  and the shaft  14 , may be provided with a corresponding coating. If, however, such a welding element blank  40  is inserted into the through-bore of the mounting part  2 , there is the risk that insulating material present on the outside of the shaft  14 ′ is at least partially abraded again. In order to avoid this, a welding element blank  40  is used, the flange lower face  10  thereof forming with the outer wall  15  of the shaft  14 ′ an acute angle a. An insulating material  59 ′ is applied to the flange lower face  10  in a quantity which is larger than is required for forming the subsequent insulating layer  59   a  between the flange  9  and the mounting part  2 . When pressing the blank  40  using a method according to  FIGS. 2A-D , initially the outer edge  70  of the flange  9  comes to bear against the outer face  13  of the mounting part  2 . If the flange  9  in the continuation of the production method is pressed with its flange lower face  10  against the outer face  13  and at the same time is deformed such that it extends in a [ ] transversely to the central longitudinal axis  26  of the through-bore, the insulating material  59 ″ is pressed on the flange lower face  10  into a gap  60  present between the shaft  14  and the bore wall  16 , forming the insulating layer  59   b.  So that a gap  60  is present with a corresponding receiving capacity for the insulating material  59 ′, the external diameter  44  of the shaft  14 ′ and the diameter  45  of the through-bore  6  are adapted to one another accordingly. 
     The welding of a mounting unit  1 , and/or a welding element S, 7  connected to a mounting part  2 , to the basic structure  5  takes place by means of a first welding electrode SE 1  applied to the flange side of the welding element, and a second welding electrode SE 2  ( FIG. 11  and  FIG. 14 ) in contact with the basic structure. The contact surface  63  present on the flange side, with which the first welding electrode SE 1  is in electrical contact with the welding element S, 7 , has a larger surface than the front face  25  of the inner end  19  of the welding element which serves as a welding surface, i.e. is welded to the basic structure. By means of the cited surface ratio it is ensured that a high flow density prevails in the region of the front face  25  so that the metals may be melted and welding may take place in the region of the front face  25 . 
     In the example shown in fig. lithe contact surface  63  is present on an annular bead  71  of the flange  9 . The annular bead  71  is a region of the flange  9  which has been bulged outwardly in the direction R 2 . In contrast to the variant of  FIG. 1A , only one partial region  64  of the flange  9  extending radially outwardly away from the annular bead  71  bears against the mounting face  13  and/or the edge region of the hole of the through-bore  6 , but not the lower face of the annular bead  71 . The effect of this design is that none of the welding current or at least only a small proportion of the welding current flows via the mounting part  2  and thus the edge region of the hole, located in plan view according to the arrow  65  below the flange  9 , does not heat up as much. 
     In the above-described welding element S, 7 , the inner end  19  of the shaft  14  is at least partially closed by a transverse wall  29 . In a further variant, a transverse wall  129  is present on the outer end  8  of the shaft  14  ( FIG. 12D ). In this case the transverse wall  129  protrudes radially beyond the shaft  14 , forming the flange  9 . As in the variant described further above, with the internal transverse wall  29 , the welding element is fixed to the mounting part  2  and/or in the through-bore due to the flange radially overlapping the edge of the through-bore and by a first positive connection in the direction R 1 . 
     Also the second positive connection acting in the direction R 2  is effected in the same manner as in the first variant. To this end, at least the inner end  19  of the sleeve  7  and/or the welding element S is radially widened, wherein a positive connection is present corresponding to  FIG. 3 , if the welding element S, 7  relative to the mounting part  2  consists of a harder, less easily deformable material. At least one longitudinal portion of the shaft  14  adjacent to the front face  25  is radially widened wherein the outer wall  15  and/or the aforementioned longitudinal portion thereof is conically formed. The bore wall  16  cooperating with the outer wall and/or a longitudinal portion thereof is deformed in a complementary manner. When the mix of materials is reversed, however, a positive connection is present according to  FIG. 4 . 
     The production of a mounting unit  2  with a welding element S, 7  with an outer transverse wall  129  takes place as  FIGS. 12B and 12C  show, principally in the same manner as described further above. The radial widening of the inner end  19 ′ of the welding element blank  40  takes place by means of a punch  47  of the type shown in  FIGS. 2C and 6C . Said punch also comprises a projection  42  with a conical side wall  50  and an annular recess  53  surrounding the projection. The counter holder  48  required for axial compression of the welding element blank  40 , bears at least flat against the flange  9 . If the punch  47  and the counter holder  48  are moved in a relative manner toward one another, the shaft  14 ′ of the aforementioned blank  40  is axially compressed. In this case, due to the conical side surface  50  of the projection  42  in addition to the axial compression a radial widening of the inner end  19 ′ of the shaft  14 ′ takes place, forming the positive connection which is effective in the direction R 2 . The sleeve wall  30 ′ of the welding element blank  40  is in this case thickened as a result of the compression. The longitudinal portion of the sleeve wall  30  adjacent to the front face  25  of the welding element S, 7  of the mounting unit  1  forms a thickened region  66 , the inner wall thereof  67  being shaped in a complementary manner to the conical side wall  50  of the punch projection  42 . 
     During the production of the mounting unit  1  under discussion, a welding element blank  40  which is suitable for mounting parts  2  of variable thicknesses  18  is also used. The axial extent  68  of the thickened region  66  is smaller, the greater the thickness  18  of the mounting part  2 . Thus in the mounting unit  1  of  FIG. 12D  with the thinner mounting part  2  the aforementioned axial extent  68  is greater than in the mounting unit  1  of  FIG. 13  with a thicker mounting part  2 . 
     The welding of a mounting unit according to  FIG. 12D  and  FIG. 13  takes place once again using two welding electrodes SE 1 , SE 2  ( FIG. 14 ). The contact surface  163  with which the welding element S, 7  is brought into contact with the first welding electrode, placed on the flange side thereof, is once again larger than the front face  25  of the shaft  14 . Moreover, the contact surface  163  protrudes in a domed manner from the side of the transverse wall  129  remote from the mounting part  2 , wherein it is formed by the surface of a disk-shaped projection  69  of the transverse wall  129 . The projection does not overlap the edge of the through-bore  6  so that the welding current is not conducted via the edge region of the hole to the welding face and/or front face  25 . The edge region of the hole of the mounting part  2  arranged below the flange  9 , viewed in the direction of the arrow  65 , is not subjected to as much heating as a result. 
       FIG. 15  shows a mounting unit in the final mounted state in which it is welded to the mating surface  4  of a basic structure  5  by means of the welding element S, 7 . The indirect connection of the mounting part  2  to the basic structure  5  results, on the one hand, from the radial overlap of the flange  9  of the welding element S, 7  in the edge region of the hole of the through-bore  6  and, on the other hand, from the welded connection  72  of the front face  25  of the welding element S, 7  with the mating surface  4  of the basic structure  5 . A material layer is present in the intermediate space  23  between the mounting part  2  and the basic structure  5 , said material layer for example acting as an insulating layer  24 , for example for avoiding contact corrosion, and/or as an adhesive layer for increasing the load-bearing capacity. 
     LIST OF REFERENCE NUMERALS 
       1  Mounting unit
 
 2  Mounting part
 
 3  Mounting face
 
 4  Mating surface
 
 5  Basic structure
 
       6  Through-bore 
       7  Sleeve 
       8  Outer end (of  7 ) 
       9  Flange 
       10  Flange lower face
 
 13  Outer face
 
       14  Shaft 
       15  Outer wall (of  14 )
 
 16  Bore wall
 
       17  Length (of  14 ) 
       18  Thickness (of  2 ) 
       19  Inner end (of  7 ) 
       20  Set overhang
 
 23  Intermediate space
 
 24  Insulating layer
 
 25  Front face
 
 26  Central longitudinal axis
 
 27  Radial outer region (of  7 )
 
 28  Rear engagement surface
 
 29  Transverse wall
 
 31  Inner face (of  29 )
 
 30  Sleeve wall
 
       33  Fold 
       34  Central region (of  29 ) 
       35  Dish 
       36  Lower face 
       37  Spacing 
       38  Adhesive layer
 
 39  Central bore
 
 40  Welding element blank
 
       42  Projection (on  47 ) 
       43  Central longitudinal axis (of  40 )
 
 44  Outer diameter (of  7 ′)
 
       45  Diameter (of  6 ) 
       46  Overhang 
       47  Stamp 
       48  Counter holder 
       49  Depression 
       50  Shaft 
       53  Recess 
       54  Axial spacing 
       55  Drawing die 
       56  Sheet metal blank
 
 57  Deep drawing punch
 
       58  Part (of  53 ) 
       59  Insulating layer 
       60  Gap 
       63  Contact surface
 
 64  Partial region
 
       65  Arrow 
       66  Thickened region (of  14 )
 
 67  Inner wall
 
 68  Axial extent
 
       69  Projection 
       70  Outer edge
 
 71  Annular bead
 
 72  Welded connection
 
 129  Transverse wall
 
 163  Contact surface
 
S Welding element
 
     R 1  Direction 
     R 2  Direction