Abstract:
A method for producing a piston for an internal combustion engine may include the steps of: producing a piston main body from a first blank via a deformation process; producing a piston ring part from a second blank via at least one of a deformation process and a casting process; pre-machining the first blank and the second blank, and finish machining a welding surface of the first blank and a welding surface of the second blank; connecting the pre-machined first blank and the pre-machined second blank via a welding process to form a piston body; and performing at least one of a secondary machining and a finish machining of the piston body to produce the piston.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. DE 102013004575.4, filed Mar. 18, 2013, German Patent Application No. 102013014345.4, filed Aug. 28, 2013, and International Patent Application No. PCT/DE2014/000138, filed Mar. 18, 2014, all of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method for producing a piston for an internal combustion engine, having a piston main body and a piston ring part, wherein the piston main body has a combustion depression with a dome and has piston bosses equipped with boss bores, which piston bosses are connected to one another by way of running surfaces, wherein the piston ring section has a piston crown, an encircling fire land and an encircling ring section, and wherein the piston main body and the piston ring section form an encircling cooling duct. The present invention also relates to a piston that can be produced by way of said method. 
       BACKGROUND 
       [0003]    A method for producing a piston is known from the German patent applications 10 2011 013 141 A1 and DE 10 2011 013 067 A1, in which method the blank of the piston main body is finished by forging in the entire region of the combustion depression. The contour of the combustion depression is thus not involved in the secondary machining process. 
         [0004]    It has however been found that the local heating of the piston body during the welding of the blanks results in a change in the microstructure and the dissipation of stresses in the material, which have the effect that the geometry and thus the volume of the combustion depression change. Therefore, the volume of the combustion depression in the finished piston may deviate considerably from the predefined values. Since the combustion depression is finished by forging, additional cutting machining is no longer possible. This applies in particular to combustion depressions with complex geometries. 
       SUMMARY 
       [0005]    It is the object of the present invention to further develop a generic method such that, in as simple a manner as possible, the volume of the combustion depression in the finished piston lies within the predefined tolerance range even in the case of complex geometries. 
         [0006]    The solution consists in that, in step a), during the production of the blank of the piston main body, the contour of the combustion depression outside a dome region is fully produced, excess material is formed in the dome region of the combustion depression, and in that, in step e), such an amount of the excess material in the dome region of the combustion depression is removed as to result in a predetermined volume of the combustion depression. 
         [0007]    The present invention also relates to a piston producible by way of the method according to the invention. 
         [0008]    The method according to the invention is characterized in that, during the production of the piston main body by deformation processes, excess material is formed in the region of the dome of the combustion depression, whereas the remaining region of the combustion depression is fully produced, that is to say requires no further secondary machining Through the removal of a particular amount of the excess material, the predetermined volume of the combustion depression can be accurately set after the welding of the blanks, without the need to manipulate the geometry of the combustion depression outside the region of the dome. 
         [0009]    Advantageous refinements will emerge from the subclaims. 
         [0010]    A refinement of the method according to the invention provides that, in step e), for the inspection of the volume of the combustion depression, during the removal of the excess material in the dome region, the present lowest point of the combustion depression  5  is detected, and a plane running perpendicularly to the piston central axis is applied to said lowest point, said plane being used as a starting point for the finish machining of the piston crown. 
         [0011]    It is expediently provided that, in step c), cooling duct regions are formed into the blanks and/or are finish-machined, because the cooling duct regions are no longer accessible after the welding of the blanks 
         [0012]    It is furthermore advantageous if, in step c), in the blank of the piston main body, the interior space is finish-machined and inlet and outlet openings for cooling oil are formed into the cooling duct region. 
         [0013]    The piston main body is easier to handle for this purpose than the welded piston body. 
         [0014]    Furthermore, based on the same considerations, it is recommended that, in step c), on the blank of the piston main body and/or of the piston ring part, the outer diameter be pre-machined, and/or, on the blank of the piston ring part, the piston crown be pre-machined, and/or, on the blank of the piston main body, the piston bosses be pre-machined. 
         [0015]    A preferred refinement provides that, in step e), the boss bores are formed into the piston bosses after the piston crown has been finish-machined. Since the compression high of a piston is defined by the distance between the piston crown and central axis of the boss bore, the predetermined compression height of the finished piston can be obtained in a particularly simple manner. 
         [0016]    Various deformation methods may be selected for the production of the blank of the piston main body. The blank may be forged by hot working at 1200° C. to 1300° C. and subsequently subjected to cold calibration. The blank may also be forged by hot working at 1200° C. to 1300° C. and subsequently subjected to cold working at a temperature of at most 150° C. The blank may furthermore be forged by warm working at 600° C. to 900° C. Furthermore, the blank may, after the warm working, be subjected to cold working at a temperature of at most 150° C. Finally, the blank may be forged by cold working at at most 150° C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Exemplary embodiments of the present invention will be discussed in more detail below on the basis of the appended drawings in which, in each case in a schematic illustration which is not to scale: 
           [0018]      FIG. 1  shows an exemplary embodiment of a piston produced by way of the method according to the invention; 
           [0019]      FIG. 2  shows a first exemplary embodiment of in each case one blank of a piston main body and of a piston ring part for a piston as per  FIG. 1 ; 
           [0020]      FIG. 3  shows the blanks as per  FIG. 2  after pre machining has been performed; 
           [0021]      FIG. 4  shows a welded piston body formed from the pre-machined blanks as per  FIG. 3 ; 
           [0022]      FIG. 5  shows an exemplary embodiment of a welded piston body with a further embodiment of a pre-machined blank for the piston lower part. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows an exemplary embodiment of a piston  10  according to the invention. The piston has a piston main body  11  with combustion depression  12  equipped with a central dome  13 . The piston main body  11  furthermore has, in a manner known  5  per se, piston bosses  14  with boss bores  15  for receiving a piston pin (not illustrated), and an interior space  24 . The piston bosses  14  are connected to one another by way of running surfaces  16 . 
         [0024]    The piston  10  furthermore has a piston ring part  17  with a piston crown  18 , with an encircling fire land  19  and with an encircling ring section  21  with annular grooves for receiving piston rings (not illustrated). An encircling cooling duct  22  with inlet and outlet openings for cooling oil (not illustrated) is provided at the level of the ring section  21 . 
         [0025]    In the present exemplary embodiment, the piston main body  11  and the piston ring part  17  are connected to one another by way of a welding process, for example electron beam welding or laser welding. 
         [0026]    The piston main body  11  is produced from a material which can be subjected to a deformation process. This is typically a tempering steel, for example 42CrMo4 or an AFP steel such as 38MnVS6. 
         [0027]    According to the invention, it is the intention to produce a piston  10  whose combustion depression  12  has a volume within a predetermined tolerance range. It is the intention to achieve this aim regardless of the geometry of the combustion depression, such that even a combustion depression of complex geometry has a volume within the predetermined tolerance range after the completion of the production process. It is the intention that, by means of the method according to the invention, the aim according to the invention can be achieved even for combustion depressions which are radially offset with respect to the piston central axis or which are in an inclined arrangement. 
         [0028]    For this purpose, as illustrated in  FIG. 2 , it is firstly the case that blanks for the piston main body  11  and the piston ring part  17  are produced. The blank  11 ′ for the piston main body  11  has, in the exemplary embodiment, been further processed by way of hot forming at 1200° to 1300° and subsequent cold calibration (pressing of the surfaces of the blank  11 ′ at room temperature). 
         [0029]    In the exemplary embodiment, the geometry of the combustion depression  12 , with the exception of the dome region  13   a  around the dome  13 , is finished by forging. This means that, for the production of the finished piston  10 , no secondary machining of the combustion depression  12  is necessary, other than in the dome region  13   a.    
         [0030]    It is essential that, after the forging process, the dome region  13   a  has an adequate amount of excess material  23  in order that the volume of the combustion depression  12  can be accurately set in accordance with the invention (in this regard, see further below). 
         [0031]    A blank  17 ′ of the piston ring part  17  may be produced from any suitable material, for example by deformation or casting. 
         [0032]    As illustrated in  FIG. 3 , it is possible, after the forging process, for the blanks  11 ′,  17 ′ to be pre-machined to form machined blanks  11 ″,  17 ″. On both blanks  11 ′,  17 ′, it is for example possible for the outer diameter to be pre-machined. 
         [0033]    On the blank  11 ′ for the piston main body  11 , it is also possible for the boss region to be pre-machined. Finally, the interior space  24  can be finish-machined. Furthermore, a cooling duct region  22   a , which in the finished piston  10  forms a part of the cooling duct  22 , is formed into the blank  11 ′. The cooling duct region  22   a  may also be formed in during the forging process, and in this case is finish-machined after the forging process. The inlet and outlet openings for cooling oil are formed into the cooling duct region  22   a.    
         [0034]    On the blank  17 ′ for the piston ring part  17 , it is for example the case that a cooling duct region  22   b , which in the finished piston  10  forms a part of the cooling duct  22 , is formed into the blank  17 ′. 
         [0035]    Finally, after the forging process, in both blanks  11 ′,  17 ′, the welding surfaces  26 ,  27  and  28 ,  29 , respectively, by way of which the blanks  11 ′,  17 ′ are to be connected to one another, are finish-machined. 
         [0036]    Then, the blanks  11 ″,  17 ″ are, by way of their welding surfaces  26  and  28 , and  27  and  29 , respectively, connected to one another in a manner known per se by way of a suitable welding process to form a piston body  30 , such as is illustrated in  FIG. 4 . In this case, the welding process leads to local heating of the material in the region of the welding surfaces  26  and  28 , and  27  and  29 , respectively. Said local heating effects a change in the microstructure, and the dissipation of stresses, in the material. This generally has the effect that the geometry and thus the volume of the combustion depression  12  then deviate considerably from the values predefined for the finished piston  10 . 
         [0037]    Then, in a subsequent method step, on the piston body  30  that results from the welding process, the dome region  13   a  is finish-machined by virtue of the excess material  23  being removed. This is performed to such an extent that, as a result, the predetermined volume of the piston depression  12  is accurately set after the welding of the blanks  11 ″,  17 ″, without the need to manipulate the geometry of the combustion depression  11  outside the dome region  13   a . The remaining region of the combustion depression  12  has already been fully produced by way of the deformation process, that is to say requires no further secondary machining 
         [0038]    Since it is only after the removal of the excess material  23  in the dome region  13   a  that the piston crown  18  is finish-machined, it is the case that, for the inspection of the volume of the combustion depression  12 , during the removal of the excess material  23  in the dome region  13   a , the present lowest point P 1  of the combustion depression  12  is detected, and a plane E running perpendicular to the piston central axis is applied to said lowest point, said plane being used as a starting point for the finish machining of the piston crown  18 . 
         [0039]    To complete the method according to the invention, the piston body  30  is finish-machined by virtue, for example, of the final fine contour being produced and the annular grooves being formed into the ring section  21  and the boss bores  15  being formed into the piston bosses  14 . The boss bores  15  are formed in such that the predetermined compression height of the finished piston is determined by the central axis of said boss bores in relation to the piston crown  18 . A piston as per  FIG. 1  is obtained as a result. 
         [0040]      FIG. 5  shows a piston body  130  composed of a machined blank  17 ″ for the piston ring part  17  as per the first exemplary embodiment according to  FIGS. 1 to 4  and of a machined blank  111 ′ for the piston main body  11 , which machined blank differs from the blank  11 ′ as per the first exemplary embodiment according to  FIGS. 1 to 4 . The same reference signs are used for common structures of the two exemplary embodiments of the piston body  30 ,  130 , and in this regard, reference is made to the description relating to  FIGS. 1 to 4 . 
         [0041]    The machined blank  111 ″ for the piston main body  11  is produced in the manner described for the exemplary embodiment as per  FIGS. 1 to 4 . The connection of the machined blanks  111 ″,  17 ″ is performed in the manner described for the exemplary embodiment as per  FIGS. 1 to 4 . 
         [0042]    In this exemplary embodiment, too, the geometry of the combustion depression  12 , with the exception of the dome region  113   a , is finished by forging. The main difference in relation to the exemplary embodiment as per  FIGS. 1 to 4  consists in that the dome region  113   a  extends from the tip of the dome  13  to the lowest point P 11  of the combustion depression  13 . This means that, for the production of the finished piston  10 , no secondary machining of the combustion depression  12  is necessary, other than in the dome region  113   a.    
         [0043]    In this exemplary embodiment, too, on the piston body  130  that results from the welding process, the dome region  113   a  is finish-machined by virtue of the excess material  123  being removed. This is performed to such an extent that, as a result, the predetermined volume of the combustion depression  12  is accurately set after the welding of the blanks  111 ″,  17 ″, without the need to manipulate the geometry of the combustion depression  11  outside the dome region  113   a . The remaining region of the combustion depression  12  has already been fully produced by way of the deformation process, that is to say requires no further secondary machining. 
         [0044]    Since it is only after the removal of the excess material  123  in the dome region  113   a  that the piston crown  18  is finish-machined, it is the case that, for the inspection of the volume of the combustion depression  12 , during the removal of the excess material  123  in the dome region  113   a , the present lowest point P 11  of the combustion depression  12  is detected, and a plane E running perpendicularly to the piston central axis is applied to said lowest point, aid plane being used as a starting point for the finish machining of the piston crown  18 . 
         [0045]    To complete the method according to the invention, the piston body  130  is finish-machined as has been described for the exemplary embodiment as per  FIGS. 1 to 4 . A piston as per  FIG. 1  is obtained as a result.