Abstract:
A method for the production of a piston for an internal combustion engine has the following steps: producing an upper piston part having at least one joining surface, b) producing a lower piston part having at least one joining surface, c) producing a direct contact between the at least one joining surface of the upper piston part and the at least one joining surface of the lower piston part, d) heating the upper piston part and the lower piston part by induction or by a direct flow of current over the joining surfaces in the region of the joining surfaces that have been brought into direct contact, e) connecting the upper and lower piston parts with one another to produce a piston by a pressing process, and machining the piston to finish it.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Applicants claim priority under 35 U.S.C. §119 of German Application No. 10 2010 033 879.6 filed Aug. 10, 2010, the disclosure of which is incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for the production of a piston for an internal combustion engine. 
         [0004]    2. The Prior Art 
         [0005]    Such a method is described, for example, in U.S. Pat. No. 6,825,450 B2. An upper piston part and a lower piston part are connected with one another by induction welding, whereby an induction coil is positioned between the joining surfaces of the upper piston part and lower piston part. After the joining surfaces are heated, the induction coil is removed and the weld connection is produced. 
         [0006]    In this method, however, the joining surfaces cool off after the induction coil is removed, so that no optimal weld connection is achieved. Furthermore, in this method, the work has to be performed under an inert gas atmosphere, in order to prevent the heated joining surfaces from being impaired or detrimentally changed by reaction with oxygen in the air. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of the present invention to provide a method for the production of a piston for an internal combustion engine, in which an improved weld connection is achieved in the simplest possible manner. 
         [0008]    This object is achieved by a method having the following steps: 
         [0009]    a) an upper piston part having at least one joining surface is produced, 
         [0010]    b) a lower piston part having at least one joining surface is produced, 
         [0011]    c) a direct contact between the at least one joining surface of the upper piston part and the at least one joining surface of the lower piston part is produced, 
         [0012]    d) the upper piston part and the lower piston part are heated by induction or by a direct flow of current over the joining surfaces in the region of the joining surfaces that have been brought into direct contact, and 
         [0013]    e) the upper piston part and the lower piston part are connected with one another to produce a piston, by means of a pressing process. 
         [0014]    According to the invention, a direct contact between the joining surfaces of upper piston part and lower piston part is therefore produced, before heating is undertaken in the region of the joining surfaces, in order to connect the two components with one another in the region of their heated joining surfaces. Cooling of the joining surfaces before connecting upper piston part and lower piston part is thereby avoided, so that the resulting weld connection is qualitatively improved as compared with the state of the art. Furthermore, it is possible to eliminate the need for an inert gas atmosphere, since the heated joining surfaces do not come into contact with the ambient air. The joining surfaces are heated either inductively, i.e. by means of induced eddy currents, or by means of direct current flow, and subsequently connected with one another by a pressing process, i.e. by means of the action of mechanical force. The piston can be machined to finish it, if necessary. 
         [0015]    An object of the present invention is furthermore a piston that can be produced according to the method according to the invention. 
         [0016]    Finally, an object of the present invention is a piston for an internal combustion engine, having an upper piston part and a lower piston part. The upper piston part has a combustion bowl as well as an inner and an outer joining surface and the lower piston part has an inner and an outer joining surface. The upper piston part and lower piston part form a circumferential cooling channel and the inner joining surfaces have a predetermined width a. The inner joining surface of the lower piston part is formed by a circumferential support element, which has the axial length b, with b≧1.5*a. A circumferential constriction on the cooling channel side having a depth c, with c≦0.8*a is provided below the circumferential support element. 
         [0017]    According to the invention, it is therefore provided that the circumferential wall of the combustion bowl, which separates the combustion bowl from the circumferential cooling channel, has a predetermined thickness that results from a predetermined width a of the inner joining surfaces of upper piston part and lower piston part. The inner joining surface of the lower piston part is formed by a circumferential support element of the lower piston part, the axial length b of which amounts to at least one and a half times the width a of the inner support surfaces of lower piston part and upper piston part. Below the circumferential support element of the lower piston part, a circumferential constriction on the cooling channel side is furthermore provided, the depth c of which amounts to at most 0.8 times the width a of the support surfaces of lower piston part and upper piston part. 
         [0018]    The width a depends on the size and the dimensions of the piston in an individual case. It is essential that the length b of the support element and the depth c of the constriction are dimensioned as a function of the width a of the support surfaces. This structure represents an optimal compromise between the demands concerning the stability of the piston and the need to dimension the support surfaces to be welded so as to be rather small, in order to optimize the inflow and outflow of heat and pressure during the welding process. With the configuration according to the invention, the result is achieved that the circumferential wall of the combustion bowl does not give way during the welding process, so that when the contact pressure is taken away, no cracks occur in the weld seam. Furthermore, the configuration according to the invention brings about the result that in engine operation, the ignition pressure and the ignition heat are passed into the lower piston regions particularly well. 
         [0019]    In a preferred embodiment of the method according to the invention, a narrowing is provided in the region of the at least one joining surface of the upper piston part and/or of the at least one joining surface of the lower piston part. This narrowing is filled up, in step e), with the material of the upper piston part or lower piston part, respectively, preferably to the nominal cross-section of the joining surfaces. In this way, the formation of a bead along the weld seam is avoided in an advantageous manner. The narrowing can be configured as a groove, notch, or constriction, for example. 
         [0020]    Steps d) and/or e) can be carried out, in a particularly advantageous manner, in an ambient atmosphere, i.e. it is possible to do without inert gas or the introduction of the components into a vacuum before heating the joining surfaces. In this way, the method according to the invention is further simplified. 
         [0021]    In step e), the pressing process can preferably be combined with a rotation process, i.e. the upper piston part and lower piston part are rotated relative to one another, for example by a few degrees of angle, in order to further strengthen the weld connection. 
         [0022]    Another preferred further development of the method according to the invention provides that the upper piston part has a combustion bowl as well as an inner and an outer joining surface, the lower piston part has an inner and an outer joining surface, and a circumferential cooling channel is formed when upper piston part and lower piston part are connected. Thus, a cooling channel piston can be produced in a particularly simple manner. Since the joining surfaces of the upper piston part are accessible, either from the outer surface of the upper piston part or from the combustion bowl, the method according to the invention is well suited for the production of such cooling channel pistons. 
         [0023]    In this connection, the method according to the invention allows, in an advantageous manner, the modifications that the inner and outer joining surfaces of upper piston part and lower piston part are disposed in the same plane or that the joining surfaces of upper piston part and lower piston part are disposed in at least two different planes. An offset of the joining surfaces therefore does not represent a problem. 
         [0024]    The upper piston part and/or the lower piston part can be cast parts or forged parts, for example, and can be produced from a steel material or a cast iron material, for example. 
         [0025]    It is advantageous if the constriction extends all the way to the bottom of the cooling channel, in order to further optimize carrying away heat and pressure, taking the material volume into consideration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention. 
           [0027]    In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
           [0028]      FIG. 1  shows a section through a first embodiment of a piston according to the invention, before welding; 
           [0029]      FIG. 2  shows the piston according to  FIG. 1  in section, rotated by 90° as compared with  FIG. 1 ; 
           [0030]      FIG. 3  shows the piston according to  FIG. 1  after welding; 
           [0031]      FIG. 4  shows the piston according to  FIG. 3  in section, rotated by 90° as compared with  FIG. 3 ; 
           [0032]      FIG. 5  shows a section through another exemplary embodiment of a piston according to the invention, before welding; 
           [0033]      FIG. 6  shows the piston according to  FIG. 5  in section, whereby the representation is rotated by 90° as compared with  FIG. 5 ; 
           [0034]      FIG. 7  shows the piston according to  FIG. 5  after welding; 
           [0035]      FIG. 8  shows the piston according to  FIG. 7  in section, rotated by 90° as compared with  FIG. 7 ; 
           [0036]      FIG. 9  shows a section through another embodiment of a piston according to the invention; and 
           [0037]      FIG. 10  shows an enlarged partial view of the piston according to  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0038]    Referring now in detail to the drawings,  FIGS. 1 to 4  show a first exemplary embodiment of a method according to the invention, using a piston  10 . The piston  10  is a two-part box piston, having a circumferential cooling channel. However, the present invention is also suitable for other piston types, of course. 
         [0039]    The piston  10  is composed of an upper piston part  11  and a lower piston part  12 , which can be produced, for example, from a steel material or a cast iron material, for example by casting or forging. The piston  10  has a piston crown  13  having a combustion bowl  14 . Piston crown  13  and combustion bowl  14  are formed partly by the upper piston part  11  and partly by the lower piston part  12 . The top land and ring grooves along the outer wall region  18  are not shown, for the sake of clarity. The lower piston part  12  has a piston skirt  15  and pin bosses  16  with pin bores  17  for accommodating a piston pin (not shown). 
         [0040]    The upper piston part  11  has an inner joining surface  21  and an outer joining surface  22 . The inner joining surface  21  is configured in a ring-shaped, circumferential manner in the region of the combustion bowl  14 . The outer joining surface  22  is configured below the wall region  18 . 
         [0041]    The lower piston part  12  also has an inner joining surface  23  and an outer joining surface  24 . The inner joining surface  23  is configured in a ring-shaped, circumferential manner, corresponding to the inner joining surface  22  of the upper piston part  11 , in the region of the combustion bowl  14 . In the exemplary embodiment, the outer joining surface  14  is configured as an extension of the piston skirt  15 . The inner joining surfaces  21 ,  23  of the upper piston part  11  and of the lower piston part  12 , respectively, are disposed offset from the outer joining surfaces  22 ,  24  of the upper piston part  11  and of the lower piston part  12 , respectively. The upper piston part  11  and the lower piston part  12  form a circumferential cooling channel  25 . 
         [0042]    The piston  10  is produced from the upper piston part  11  and the lower piston part  12  as follows. First, as is evident from  FIGS. 1 and 2 , a direct contact is produced between the corresponding inner joining surfaces  21 ,  23  of the upper piston part  11  and of the lower piston part  12 , respectively, as well as between the outer joining surfaces  22 ,  24  of the upper piston part  11  and of the lower piston part  12 , respectively. The inner joining surfaces  21 ,  22  and the outer joining surfaces  23 ,  24 , respectively, therefore lie directly on top of one another. An induction coil  31  is positioned in the region of the combustion bowl  14  and assigned to the inner joining surfaces  21 ,  23  of upper piston part  11  and lower piston part  12 . A further induction coil  32  is positioned in the region of the outer wall  18  and assigned to the outer joining surfaces  22 ,  24  of upper piston part  11  and lower piston part  12 , respectively. The upper piston part  11  and the lower piston part  12  are heated by induction in the region of their joining surfaces  21 ,  23  and  22 ,  24 , respectively, until the material in this region becomes plastically deformable. Then, upper piston part  11  and lower piston part  12  are connected with one another by a pressing process, whereby upper piston part  11  and lower piston part  12  can be rotated by a few degrees of angle relative to one another. 
         [0043]      FIGS. 3 and 4  show the piston  10  obtained after upper piston part  11  and lower piston part  12  are connected. Small circumferential beads  26  are formed along the weld seams, which beads have been formed from material exiting at the sides during the pressing process when upper piston part  11  and lower piston part  12  were connected. The piston  10  can be machined further, particularly by introducing ring grooves and removing the beads  26 , which are accessible from the outside. 
         [0044]      FIGS. 5 to 8  show another exemplary embodiment of a method according to the invention, using a piston  110  composed of an upper piston part  111  and a lower piston part  112 . The piston  110  is almost identical with the piston  10  according to  FIGS. 1 to 4 , so that the same reference symbols were used for identical structures, and are used to the description relating to  FIGS. 1 to 4 . 
         [0045]    The significant difference between the piston  10  according to  FIGS. 1 to 4  and the piston  110  according to  FIGS. 5 to 8  as well as between the methods according to the invention used for their production consists in that the upper piston part  111  and the lower piston part  112  have narrowings  127 ,  128  in the region of the joining surfaces  121 ,  123  and  122 ,  124 , respectively, in the assembled but not yet welded state. In the exemplary embodiment, the narrowings  127 ,  128  are configured as constrictions, and are produced, in the exemplary embodiment, by introducing bevels  129  on the corresponding joining surfaces  121 ,  123  and  122 ,  124 , respectively. During the pressing process when connecting upper piston part  111  and lower piston part  112 , the narrowings are filled with material that exits out of the weld seams at the side. In  FIGS. 7 and 8 , it can be seen that after upper piston part  111  and lower piston part  112  are connected, no beads are present in the region of the weld seams, but rather, an extensively smooth surface has been formed. 
         [0046]      FIGS. 9 and 10  show another exemplary embodiment of a piston  210  according to the invention. The piston  210  essentially corresponds to the piston  10  according to  FIGS. 1 to 4 , so that reference is made to the above description in this regard. The same reference symbols were used for structures that agree with one another. 
         [0047]    The piston  210  is composed of an upper piston part  211  and a lower piston part  212  that can be produced, for example, from a steel material or a cast iron material, for example by means of casting or forging. The piston  210  has a piston crown  13  having a combustion bowl  14 , whereby piston crown  13  and combustion bowl  14  are formed partly by the upper piston part  11  and partly by the lower piston part  12 . The top land and ring grooves along the outer wall region  18  were not shown, for the sake of clarity. The lower piston part  12  has a piston skirt  15  and piston pins  16  with pin bores  17  for accommodating a piston pin (not shown). 
         [0048]    The upper piston part  211  has an inner joining surface  21  and an outer joining surface  22 . The inner joining surface  21  is configured in a ring-shaped, circumferential manner in the region of the combustion bowl  14 . The outer joining surface  22  is configured below the wall region  18  in the exemplary embodiment. 
         [0049]    The lower piston part  212  also has an inner joining surface  23  and an outer joining surface  24 . The inner joining surface  23  is configured in a ring-shaped, circumferential manner in the region of the combustion bowl  14 , corresponding to the inner joining surface  22  of the upper piston part  211 . In the exemplary embodiment, the outer joining surface  24  is configured as an extension of the piston skirt  15 . The inner joining surfaces  21 ,  23  of the upper piston part  211  and of the lower piston part  212 , respectively, are disposed offset from the outer joining surfaces  22 ,  24  of the upper piston part  11  and of the lower piston part  12 , respectively, in the exemplary embodiment. The upper piston part  211  and the lower piston part  212  form a circumferential cooling channel  15 . 
         [0050]    The inner joining surfaces  21 ,  23  of upper piston part  211  and lower piston part  212 , respectively, have a predetermined width a. The inner joining surface  23  of the lower piston part  212  is formed by a circumferential support element  233 . The support element  233  has an axial length b that amounts to at least one and a half times the width a of the inner joining surfaces  21 ,  23 : b≧1.5*a. The support element  233  delimits the cooling channel  25 , on the one hand, and the combustion bowl  14 , on the other hand. A constriction  234  on the cooling channel side is provided below the support element  233 . The depth c of the constriction  234  amounts to at most 0.8 times the width a of the inner support surfaces  21 ,  23 : c≦0.8*a. 
         [0051]    This structure guarantees the stability of the piston  210  according to the invention, and, at the same time, the slimmest possible configuration of the support surfaces  21 ,  23  as well as of the support element  234 , in order to obtain an optimal pressure welding connection. 
         [0052]    Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.