Patent Abstract:
A method for the production of a piston has the following method steps: (a) providing a blank of a piston base body, having an outer joining surface, an inner joining surface and a circumferential lower cooling channel part that runs between the two joining surfaces, (b) providing a blank of a piston ring element, having an outer joining surface, an inner joining surface and a circumferential upper cooling channel part that runs between the two joining surfaces, (c) forming a circumferential widened region on at least one joining surface, the widened region extending toward the related cooling channel part, (d) connecting the blank of the piston base body with the blank of the piston ring element by way of their joining surfaces, by friction welding, to produce a piston blank, and (e) machining the piston blank further and/or finish-machining it to produce a piston.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. patent application Ser. No. 13/065,511 filed on Mar. 23, 2011, which claims priority under 35 U.S.C. §119 of German Application No. 10 2010 033 878.8 filed Aug. 10, 2010 and German Application No. 10 2011 100 521.1 filed May 5, 2011, the disclosures of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for the production of a piston for an internal combustion engine, having a piston base body and a piston ring element. The piston base body has at least a piston skirt, and the piston ring element has at least a piston crown, a circumferential top land, and a circumferential ring belt provided with ring grooves. The piston base body and the piston ring element form a circumferential, closed cooling channel. The present invention furthermore relates to such a piston for an internal combustion engine. 
     2. The Prior Art 
     Friction-welded pistons having cooling channels in the piston head that are open toward the bottom and can be closed off by means of a sheet-metal cover are described in German Patent Application No. DE 10 2004 019 012 A1 and International Application No. WO 2007/128265 A1. In this connection, piston base body and piston ring belt each have only one joining surface. In WO 2007/128265 A1, it is proposed that the joining surfaces are not in contact over their complete area before friction welding, in order to reduce the size of the friction-welding bead that is present below the cooling channel after friction welding, in a controlled manner, so that it is easier to remove subsequent to friction welding. German Patent Application No. DE 10 2004 019 012 A1 discloses a piston base body and a piston ring element whose joining surfaces form a cavity, in order to accommodate excess material during friction welding. 
     However, neither of these two methods is suitable for producing pistons having a closed, circumferential cooling channel, since the typical pair of rolled-in friction-welding beads formed during friction welding projects radially into the cooling channel. These circumferential friction-welding beads take up a lot of space in the cooling channel. Thus, the volume of the cooling channel is excessively reduced, and the flow of the cooling oil in the cooling channel is hindered. In the case of pistons having a comparatively large combustion chamber bowl, the cooling channel can be configured to be so narrow, in the radial direction, that it would not even be able to accommodate the friction-welding beads. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a friction-welding method for the production of a piston having a closed cooling channel, in such a manner that the cooling channel of the finished piston does not experience any excessive volume reduction. 
     The solution consists in a method having the following steps: (a) making available a blank of a piston base body, in which an inner joining surface and an outer joining surface and a circumferential lower cooling channel part that runs between the two joining surfaces are pre-machined; (b) making available a blank of a piston ring element, in which an inner joining surface and an outer joining surface as well as a circumferential upper cooling channel part that runs between the two joining surfaces are pre-machined; (c) forming a circumferential widened region on at least one joining surface, whereby the widened region extends toward the related cooling channel part; (d) connecting the blank of the piston base body with the blank of the piston ring element by way of their joining surfaces, by means of friction welding, to produce a piston blank; (e) machining the piston blank further and/or finish-machining it to produce a piston. The piston according to the invention has the features that the piston base body and piston ring element are connected with one another by friction welding, and the cooling channel is free of friction-welding beads, to a great extent. 
     In material strength studies, it has been shown that when beads are rolled in, great excessive increases in notch stress occur; these are attributable to the sharp notches at the exit of the rolled-in beads. In the case of the newly developed method listed above, these sharp edges are avoided. As a result, clear increase in strength, which ranges between 85 and 100% of the base material strength, is achieved, and thus greater freedom in designing individual designs is made possible. 
     The idea according to the invention consists in configuring the joining surfaces in such a manner that the region of the joining surfaces on the cooling chamber side can accommodate excess material during friction welding. As a result, the radial expanse of the cooling channel is maintained practically unchanged during friction welding, in the region of the friction-welding seam. Using the method according to the invention, it is possible to produce multi-part pistons having a closed, circumferential cooling channel that is capable of functioning, by friction-welding methods. 
     The present invention is suitable for all the piston construction variants according to the claims. The piston ring element or its blank can particularly have a combustion chamber bowl. The piston ring element or its blank, instead, can also have at least one wall region of a combustion chamber bowl. Then, the piston base body or its blank has at least one crown region of a combustion chamber bowl, so that the two components jointly form the complete combustion chamber bowl. 
     A preferred embodiment consists in that in step (d), the blank of the piston base body or the blank of the piston ring element is put into rotation, the blank of the piston base body and the blank of the piston ring element are pressed together, at a speed of rotation of 1500 rpm to 2500 rpm, at a contact pressure, with reference to the joining surfaces, of 10 N/mm 2  to 30 N/mm 2 , the rotation is stopped after 1 second to 3 seconds, while maintaining the contact pressure, and subsequently, the blank of the piston base body and the blank of the piston ring element are pressed together at a contact pressure, with reference to the joining surfaces, of 100 N/mm 2  to 140 N/mm 2 . These method parameters promote the avoidance of typical friction-welding beads, so that the formation of the circumferential widened region requires particularly little work effort, because of the smaller dimensions under these circumstances. 
     The widened regions provided according to the invention can be produced in different ways. In particular, in step (c), widened regions can be made on the inner and outer joining surface of the blank of the piston base body and/or on the inner and outer joining surface of the blank of the piston ring element. 
     Furthermore, the one circumferential widened region can be formed in any desired manner, for example in the form of a slanted surface, a chamfer, or a bowl. The widened regions can be formed with an axial expanse of 1.0 mm to 1.5 mm and/or with a radial expanse of at least 0.5 mm, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
       In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
         FIG. 1  shows a blank of a piston base body and of a blank of a piston ring element, for the production of a piston according to one embodiment of the invention, in section; 
         FIG. 2  shows an enlarged detail representation of the joining surface region according to  FIG. 1 ; 
         FIG. 3   a  shows the piston blank produced from the components according to  FIG. 1 , for a piston according to the invention, in section; 
         FIG. 3   b  shows an enlarged detail representation of the joining region according to  FIG. 3   a;    
         FIG. 4   a  shows the piston according to the invention, produced from the piston blank according to  FIG. 3   a;    
         FIG. 4   b  shows an enlarged detail representation of the joining region of the piston according to  FIG. 4   a;    
         FIG. 5  shows a blank of a piston base body and of a blank of a piston ring element for the production of a piston according to another embodiment of the invention, in section; 
         FIG. 6  shows another exemplary embodiment of a blank of a piston base body and of a blank of a piston ring element for the production of a piston according to the invention, in section; 
         FIG. 7   a  shows the piston blank produced from the components according to  FIG. 5  and  FIG. 6 , respectively, for a piston according to the invention, in section; and 
         FIG. 7   b  shows an enlarged detail representation of the joining region of the piston according to  FIG. 7   a.    
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawings,  FIGS. 4   a  and  4   b  show a finished piston  10  according to the invention. Piston  10  consists of a piston base body  11  and a piston ring element  12 . The two components can consist of any metallic material, for example according to DIN EN 10083 or DIN EN 10267, which can be subjected to hardening and tempering and is suitable for friction welding. 
     In the exemplary embodiment, the piston base body consists of a steel material, for example AFP steel. The piston base body  11  has a piston skirt  15  that is provided, in known manner, with pin bosses  16  and pin bores  17  for accommodating a piston pin (not shown), as well as skirt regions  18  having working surfaces (not shown). In the exemplary embodiment, the piston ring element  12  is also produced from a steel material, for example 42CrMo4. The piston ring element  12  has a piston crown  19  as well as a circumferential top land  21 . The piston base body  11  and the piston ring element  12  form a circumferential ring belt  22  for accommodating piston rings (not shown), a circumferential, closed cooling channel  23 , as well as a combustion chamber bowl  24 . 
     The piston base body  11  and the piston ring element  12  are connected with one another by friction welding. It is particularly evident from  FIG. 4   b  that the circumferential, closed cooling channel  23  nevertheless does not have any typical friction-welding beads. The entire volume of the cooling channel  23 , as originally provided, is therefore available for cooling the piston  10  according to the invention during engine operation. Furthermore, the flow of the cooling oil in the cooling channel is not impaired. 
     The piston  10  according to the invention is produced in the manner described below. 
     According to  FIGS. 1 and 2 , first a pre-machined blank  11 ′ of a piston base body  11  as well as a pre-machined blank  12 ′ of a piston ring element  12  are made available. The blanks  11 ′,  12 ′ essentially correspond to the finished piston base body  11  and the finished piston ring element  12 , respectively, so that the same structures are provided with the same reference symbols, and in this regard, reference is made to the above description of  FIG. 4   a . The essential difference consists in that no ring belt is machined out, but rather a smooth mantle surface  25  on the blank  11 ′ of the piston base body  11  as well as a smooth mantle surface  26  on the blank  12 ′ of the piston ring element  12  are provided. 
     The blanks  11 ′,  12 ′ can be cast, forged, or sintered by means of powder metallurgy, depending on the selection of the material. In the exemplary embodiment, the crown region  27  a part  28  of the wall region of the combustion chamber bowl  24  is pre-machined, for example lathed, into the blank  11 ′ of the piston base body  11 . Furthermore, a circumferential cooling channel part  23   a  of the cooling channel  23  is pre-machined. This results in an outer joining surface  29  and an inner joining surface  31 . In the exemplary embodiment, the remaining part  28 ′ of the wall region of the combustion chamber bowl is machined, for example lathed, into the blank  12 ′ of the piston ring element  12 . Furthermore, a circumferential upper cooling channel part  23   b  of the cooling channel  23  is machined in. This results in an outer joining surface  32  and an inner joining surface  33 . The outer joining surface  29  of the blank  11 ′ corresponds to the outer joining surface  32  of the blank  12 ′. In corresponding manner, the inner joining surface  31  of the blank  11 ′ corresponds to the inner joining surface  33  of the blank  12 ′. This means that the two blanks  11 ′,  12 ′ can be connected with one another along their joining surfaces  29 ,  31  and  32 ,  33 , respectively, to form a piston blank  10 ′. 
     In the exemplary embodiment, a circumferential widened region  34   a ,  34   b , in the form of a chamfer, is formed out at the two joining surfaces  29 ,  31  of the blank  11 ′ as well as at the two joining surfaces  32 ,  33  of the blank  12 ′, in each instance. The widened regions  34   a ,  34   b  extend in the direction of the cooling channel part  23   a  of the blank  11 ′. In corresponding manner, the widened regions  34   b  extend in the direction of the cooling channel part  23   b  of the blank  12 ′. The maximal axial expanse of the widened regions  34   a ,  34   b  each amounts to about 1.0 mm in the exemplary embodiment, while the radial expanse of each of the widened regions  34   a ,  34   b  amounts to about 0.5 mm. When the joining surfaces  29 ,  31  and  32 ,  33 , respectively, of the blanks  11 ′,  12 ′ come into contact with one another at the beginning of the friction-welding process (see below), the widened regions  34   a ,  34   b  form two joins, in the exemplary embodiment, that lie opposite one another, having a maximal axial expanse of about 2 mm, which can accommodate excess material. Of course, widened regions having a different geometry can also be combined with one another. 
     To connect the two blanks  11 ′,  12 ′, these are braced so as to align, in known manner. Then, one of the two blanks  11 ′,  12 ′ is put into rotation, until a speed of rotation of 1,500 rpm to 2,500 rpm is achieved. Now, the blanks  11 ′,  12 ′ are brought into contact with one another by way of their joining surfaces  29 ,  31  and  32 ,  33 , respectively, and pressed together at a constant initial contact pressure, with reference to the joining surfaces  29 ,  31  and  32 ,  33 , respectively, of 10 N/mm 2  to 30 N/mm 2 . The rotational movement and the constant contact pressure produce a friction that heats up the joining surfaces  29 ,  31  and  32 ,  33 , respectively. The speed of rotation and the contact pressure are selected in such a manner, as a function of the materials used, so that the joining surfaces  29 ,  31  and  32 ,  33 , respectively, heat up to a temperature close to the melting point of the material or the materials. When this has been reached (after 1 to 3 seconds, depending on the material or materials), the rotation is ended, while maintaining the initial contact pressure, i.e. the spindle used for rotation is braked and stopped as quickly as possible (within less than 1 second, if at all possible). During this process, the contact pressure is maintained. After movement has been stopped, the contact pressure is increased to a joining pressure, with reference to the joining surfaces  29 ,  31  and  32 ,  33 , respectively, that is a multiple of the initial contact pressure, of 100 N/mm 2  to 140 N/mm 2 , and the blanks  11 ′,  12 ′ are pressed together under this joining pressure for about 5 seconds. In this connection, the excess material is taken up into the joins described above. 
       FIGS. 3   a  and  3   b  show the piston blank  10 ′ that has been produced in this manner. The piston blank  10 ′ essentially corresponds to the finished piston  10 , so that the same structures are provided with the same reference symbols, and reference is made to the above description of  FIG. 4   a  in this regard. As the result of the friction-welding process described above, the piston blank  10 ′ does not have any typical friction-welding bead  35  along the friction-welding seams as well as on the mantle surfaces  25 ,  26  of the blanks  11 ′ and  12 ′, respectively, as well as on the wall region  28 ,  28 ′, in each instance. It can particularly be seen in  FIG. 3   b  that the cooling channel  23  formed from the cooling channel parts  23   a ,  23   b  of the blanks  11 ′ and  12 ′, respectively, does not have any typical rolled-in friction-welding beads along the friction-welding seams. The melted, excess material released during the friction-welding process described above, which would form rolled-in friction-welding beads in the state of the art, was accommodated by the joins by the widened regions  34   a ,  34   b , during the friction-welding process. 
     The piston blank  10 ′ is machined further or machine-finished in known manner, depending on the configuration of the blanks  11 ′,  12 ′. For example, the outer shape, surfaces, combustion chamber bowl, pin bores, etc. can be machine-finished. In particular, the ring belt  22  is machined in, and the friction-welding beads  35  are removed. In the end result, the finished piston according to  FIGS. 4   a  and  4   b , as described above, is obtained. 
       FIG. 5  shows an alternative embodiment of a blank  111 ′ of a piston base body  11 , as well as of a blank  112 ′ of a piston ring element  12  for a piston  10  according to the invention. The blanks  111 ′,  112 ′ essentially correspond to blanks  11 ′,  12 ′ according to  FIG. 1 , so that the same structures are provided with the same reference symbols, and reference is made to the above description of  FIG. 1  in this regard. It is pointed out that the representation according to  FIG. 5  is rotated by 90° as compared with the representation according to  FIG. 1 . 
     Analogous to the blanks  11 ′,  12 ′ according to  FIG. 1 , the blank  111 ′ has an outer joining surface  129 , and the blank  112 ′ has a corresponding outer joining surface  132 . Also analogous to the blanks  11 ′,  12 ′ according to  FIG. 1 , the blank  111 ′ has an inner joining surface  131 , and the blank  112 ′ has a corresponding inner joining surface  133 . This means that the two blanks  111 ′,  112 ′ can be connected with one another along their joining surfaces  129 ,  131  and  132 ,  133 , respectively, to produce a piston blank  110 ′. 
     In the exemplary embodiment, a circumferential widened region  134   b  in the form of a slant is formed on both joining surfaces  132 ,  133  of the blank  112 ′ of the piston ring element  12 , in each instance. The widened regions  134   b  extend in the direction of the cooling channel part  23   b  of the blank  112 ′. The maximal axial expanse of each of the widened regions  134   b  amounts to about 1 mm, in the exemplary embodiment. When the joining surfaces  129 ,  131  and  132 ,  133 , respectively, come into contact with one another, at the beginning of the friction-welding process described above, the widened regions  134   b  form a clear space, in the exemplary embodiment, in the shape of a right triangle, with a maximal axial expanse of about 1 mm, in which the melted material is distributed. Of course, widened regions having a different geometry can also be combined with one another. 
       FIG. 6  shows a further exemplary embodiment  211 ′ of a piston base body  11 , as well as of a blank  212 ′ of a piston ring element  12  for a piston  10  according to the invention. The blanks  211 ′,  212 ′ essentially correspond to the blanks  11 ′,  12 ′ according to  FIG. 1 , so that the same structures are provided with the same reference symbols, and reference is made to the above description of  FIG. 1  in this regard. It is pointed out that the representation according to  FIG. 6  is rotated by 90° as compared with the representation according to  FIG. 1 . 
     Analogous to the blanks  11 ′,  12 ′ according to  FIG. 1 , the blank  211 ′ has an outer joining surface  229 , and the blank  212 ′ has a corresponding outer joining surface  232 . Also analogous to the blanks  11 ′,  12 ′ according to  FIG. 1 , the blank  211 ′ has an inner joining surface  231 , and the blank  212 ′ has a corresponding inner joining surface  233 . This means that the two blanks  211 ′,  212 ′ can be connected with one another along their joining surfaces  229 ,  231  and  232 ,  233 , respectively, to produce a piston blank  110 ′. 
     In the exemplary embodiment, a circumferential widened region  234   a ,  234   b  in the form of a slant is formed on both joining surfaces  229 ,  231  of the blank  211 ′ as well as on both joining surfaces  232 ,  233  of the blank  212 ′. The widened regions  234   b  extend in the direction of the cooling channel part  23   a  of the blank  211 ′. In corresponding manner, the widened regions  234   b  extend in the direction of the cooling channel part  23   b  of the blank  212 ′. The maximal axial expanse of each of the widened regions  234   a ,  234   b  amounts to about 1 mm, in the exemplary embodiment. When the joining surfaces  229 ,  231  and  232 ,  233 , respectively, of the blanks  211 ′,  212 ′ come into contact with one another, at the beginning of the friction-welding process described above, the widened regions  234   a ,  234   b  form two clear spaces that lie opposite one another, in the exemplary embodiment, in the shape of an equilateral triangle, with a maximal axial expanse of about 2 mm, in which the melted material is distributed. Of course, widened regions having a different geometry can also be combined with one another. 
     With the friction-welding method described above, essentially the same piston blank  110 ′ as the one shown in  FIGS. 7   a  and  7   b  is obtained from the blanks  111 ′,  112 ′ according to  FIG. 5  and from the blanks  211 ′,  212 ′ according to  FIG. 6 . The piston blank  110 ′ essentially corresponds to the piston blank  10 ′ according to  FIGS. 3   a  and  3   b , so that the same structures are provided with the same reference symbols, and reference is made to the above description of  FIG. 3   a  in this regard. As the result of the friction-welding process described above, the piston blank  110 ′ has the friction-welding beads or thickened regions shown in  FIG. 7   b . Also in the cooling channel  23  formed from the cooling channel parts  23   a ,  23   b  of the blanks  111 ′ and  112 ′, respectively, as well as the blanks  211 ′ and  212 ′, respectively, contains friction-welding beads or thickened regions, as indicated above. The melted, excess material released during the friction-welding process described above, which would form friction-welding beads in the state of the art, was taken up by the clear spaces formed by the widened regions  134   b  or  234   a ,  234   b , respectively, so that a distribution of the melted material occurs, which ensures that the friction-welding beads or thickened regions, which are formed in the direction toward the cooling chamber, are smaller than the friction-welding beads or thickened regions that are situated on the sides facing away from the cooling chamber. 
     The piston ring  110 ′ is machined further or machine-finished in known manner, depending on the configuration of the blanks  111 ′,  112 ′ or the blanks  211 ′  212 ′, respectively. For example, the outer shape, surfaces, combustion chamber bowl, pin bores, etc. can be machine-finished. In particular, the ring belt  22  is machined in. In the end result, the finished piston  10 , as described above in connection with  FIGS. 4   a  and  4   b , is obtained. 
     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.

Technology Classification (CPC): 5