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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. 10 2013 004 576.2, filed Mar. 18, 2013, German Patent Application No. 10 2013 014 346.2, filed Aug. 28, 2013, and International Patent Application No. PCT/DE2014/000137, 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 upper part and a piston lower part, wherein the piston upper part has a piston crown with a combustion depression with a dome, has an encircling fire land and has an encircling ring section, wherein the piston lower part has piston bosses equipped with boss bores, which piston bosses are connected to one another by way of running surfaces, having the following method steps: a) producing a blank of the piston upper part by way of a deformation process, b) producing a blank of the piston lower part by way of a deformation process or a casting process, c) connecting the blanks of piston upper part and piston lower part to form a piston body by means of a welding process, d) performing secondary machining and/or finish machining of the piston body to form the finished piston. The present invention also relates to a piston producible by way of said method. 
       BACKGROUND 
       [0003]    A generic production method and a piston produced thereby are known from the German patent applications 10 2011 013 141 A1 and DE 10 2011 013 067 A1. According thereto, the blank of the piston upper part is finished by forging in the entire region of the combustion depression, such that the contour of the combustion depression is 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 upper part, 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 d), 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 upper part 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]    Is advantageous if, in step a), if desired, at least one valve niche is fully produced in the piston crown and/or in the piston depression by way of the deformation process, such that secondary machining of the at least one valve niche is also omitted. 
         [0011]    In step a), the piston crown can also be fully produced by way of the deformation process, in order that secondary machining of said piston crown is also eliminated. In this case, it is expedient for excess material to additionally be formed in the region of the fire land, as said material can be easily removed in said region, for example by simply being removed by turning. It must merely be ensured that, in the region of the dome of the combustion depression, an adequate amount of excess material is formed in order that the predetermined volume of the combustion depression can be accurately set. 
         [0012]    A refinement of the method according to the invention provides that, in step d), for the inspection of the volume of the combustion depression, a reference point at the lowest point of the combustion depression and a reference point at the level of the fully produced piston crown are selected, and thus the present depth of the combustion depression is determined. 
         [0013]    Instead, it may be provided that, in step a), excess material is additionally formed in the region of the piston crown. This has the advantage that secondary machining of the piston crown permits flexible setting of the compression height. 
         [0014]    A refinement of said method provides that, in step d), 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 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. 
         [0015]    It is expediently provided that, in steps a) and b), welding surfaces and cooling duct regions are formed into the blanks and are finish-machined. Accurately fitting welding surfaces are required for the welding process in step c). The cooling duct regions are no longer accessible after the welding of the blanks. 
         [0016]    It is furthermore advantageous if, between step b) and step c), in the blank of the piston lower part, the interior space is finish-machined and inlet and outlet openings for cooling oil are formed into the cooling duct region. The piston lower part is easier to handle for this purpose than the welded piston body. 
         [0017]    Furthermore, based on the same considerations, it is recommended that, between step a) and step c), on the blank of the piston upper part and/or of the piston lower part, the outer diameter be pre-machined, and/or that, on the blank of the piston lower part, the piston bosses be pre-machined. 
         [0018]    A preferred refinement provides that, in step d), the boss bores are formed into the piston bosses after the piston crown has been finish-machined. Since the compression height of a piston is defined by the distance between the piston crown and the central axis of the boss bore, the predetermined compression height of the finished piston can be obtained in a particularly simple manner. 
         [0019]    Various deformation methods may be selected for the production of the blank of the piston upper part. 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. 
         [0020]    It is expediently provided that, in step c), the blanks are connected to one another by way of a friction welding process. The use of a friction welding process is however not imperative. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    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: 
           [0022]      FIG. 1  shows an exemplary embodiment of a piston produced by way of the method according to the invention; 
           [0023]      FIG. 2  shows a first exemplary embodiment of a blank of a piston upper part for a piston as per  FIG. 1 ; 
           [0024]      FIG. 3  shows a second exemplary embodiment of a blank of a piston upper part for a piston as per  FIG. 1 ; 
           [0025]      FIG. 4  shows a third exemplary embodiment of a blank of a piston upper part for a piston as per  FIG. 1 ; 
           [0026]      FIG. 5  shows an exemplary embodiment of a combustion depression with valve niche for a piston as per  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  shows an exemplary embodiment of a piston  10  according to the invention. The piston  10  has a piston upper part  11  with a piston crown  12 . In the piston crown  12  there is provided a combustion depression  13  which has a central dome  14 . The piston upper part  11  furthermore has an encircling fire land  15  and an encircling ring section  16  with annular grooves for receiving piston rings (not illustrated). An encircling cooling duct  17  is provided at the level of the ring section  16 . 
         [0028]    The piston  10  furthermore has a piston lower part  18  with piston bosses  19  and boss bores  21  for receiving a piston pin (not illustrated). The piston bosses  19  are connected to one another by way of running surfaces  22 ,  23 . 
         [0029]    In the present exemplary embodiment, the piston upper part  11  and the piston lower part  18  are connected to one another by way of a friction welding process. 
         [0030]    The piston upper part  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. 
         [0031]    According to the invention, it is the intention to produce a piston  10  whose combustion depression  13  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, such as is illustrated for example in  FIG. 5 , has a volume within the predetermined tolerance range after the completion of the production process.  FIG. 5  shows a plan view of a combustion depression  13   a  in a piston upper part Ila. The combustion depression  13   a  has a cloverleaf-shaped contour. A valve niche  24  is additionally formed into the piston crown  12 . 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. 
         [0032]    For this purpose, a blank for the piston upper part  11  is firstly produced by deformation. A first exemplary embodiment of a blank  11 ′ of said type is illustrated in  FIG. 2 . The blank  11 ′ has, in the exemplary embodiment, been forged by warm working at 600° to 900° and subsequent cold working at at most 150° C. The blank  11 ′ is illustrated in section by hatching, whereas the final contours of the piston upper part  11  in the finished piston  10  are indicated by dash-dotted lines. In the exemplary embodiment, the geometry of the combustion depression  13 , with the exception of the dome region  14 ′ around the dome  14 , and the piston crown  12  are finished by forging. This means that, for the production of the finished piston  10 , no secondary machining of the piston crown  12  and of the combustion depression  13  is necessary, other than in the dome region  14 ′. 
         [0033]    It is essential that, after the forging process, the dome region  14 ′ has an adequate amount of excess material  25  in order that the volume of the combustion depression can be accurately set in accordance with the invention (in this regard, see further below). Further excess material  26  that is not required for the setting of the volume of the combustion depression  13  may be formed outside the piston crown  12  in the region of what will later become the fire land  15 . 
         [0034]    After the forging process, the outer diameter of the blank  11 ′ may be pre-machined, wherein in particular, the excess material  26  may be removed. Furthermore, a cooling duct region  17 ′, which in the finished piston  10  forms a part of the cooling duct  17 , is formed into the blank  11 ′. The cooling duct region  17 ′ may also be formed in during the forging process, and in this case is finish-machined after the forging process. Finally, after the forging process, the welding surfaces  27 ,  28 , by way of which the blank is to be connected to the blank of the piston lower part  18  (cf.  FIG. 1 ), are finish-machined. 
         [0035]    A blank of the piston lower part  18  may be produced from any suitable material by deformation or casting. In a manner known per se, the outer diameter and the hub region of the blank may be pre-machined. Furthermore, the interior space  29  and a cooling duct region  17 ″, which in the finished piston  10  forms a part of the cooling duct  17 , are finish-machined. Inlet and outlet openings  31  for cooling oil are formed into the cooling duct region  17 ″, (cf.  FIG. 1 ). Finally, after the forging process, the welding surfaces  32 ,  33 , by way of which the blank  11 ′ is to be connected to the blank of the piston upper part  11  (cf.  FIG. 1 ), are finish-machined. 
         [0036]    Then, the blanks are, by way of their welding surfaces  27 ,  28 ,  32 ,  33 , connected to one another in a manner known per se by way of a friction welding process. In this case, the friction welding process leads to local heating of the material in the region of the welding surfaces  27 ,  28 ,  32 ,  33 . 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  13  then deviate considerably from the values predefined for the finished piston ( 10 ). 
         [0037]    Then, in a subsequent method step, on the piston body that results from the welding process, the dome region  14 ′ is finish-machined by virtue of the excess material being removed. This is performed to such an extent that, as a result, the predetermined volume is accurately set after the welding of the blanks, without the need to manipulate the geometry of the combustion depression  13  outside the dome region  14 ′. The remaining region of the combustion depression has already been fully produced by way of the deformation process, that is to say requires no further secondary machining. 
         [0038]    During said process, the volume of the combustion depression  13  is inspected. For this purpose, a reference point P 1  at the lowest point of the combustion depression  13  and a reference point P 2  at the height of the fully produced piston crown  12  are selected. The present depth of the combustion depression  13  is thus determined. If, for example, the depth of the combustion depression  13  lies at the upper tolerance limit, the machining in the dome region  14 ′ must lie substantially at the lower tolerance limit, such that, as a result, the actual volume of the combustion depression  13  lies as close as possible to the middle of the tolerance range of the predetermined volume. 
         [0039]    To complete the method according to the invention, the piston body is finish-machined by virtue, for example, of the final fine contour being produced and the annular grooves being formed into the ring section  16  and the boss bores  21  being formed into the piston boss  19 . The boss bores  21  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  12 . A piston as per  FIG. 1  is obtained as a result. 
         [0040]    A second exemplary embodiment of a blank  111 ′ of said type for a piston upper part  11  is illustrated in  FIG. 3 . In the exemplary embodiment, the blank  111 ′ has been forged by hot working at 1200° to 1300° and has subsequently been further processed by cold calibration (pressing of the upper surfaces of the blank  111 ′ at room temperature). The blank  111 ′ is illustrated in section by way of hatching, whereas the final contours of the piston upper part  11  in the finished piston  10  are indicated by dash-dotted lines. In the exemplary embodiment, the geometry of the combustion depression  13 , with the exception of the dome region  114 ′ around the dome  14 , is finished by forging. This means that, for the production of the finished piston  10 , no secondary machining of the combustion depression  13  is necessary, other than in the dome region  114 ′. 
         [0041]    It is essential that, after the forging process, the dome region  114 ′ has an adequate amount of excess material  125  in order that the volume of the combustion depression can be accurately set in accordance with the invention (in this regard, see further below). Further excess material  126  that is not required for the setting of the volume of the combustion depression  13  may be formed outside the piston crown  12  in the region of what will later become the piston crown. 
         [0042]    After the forging process, the outer diameter of the blank  111 ′ may be pre-machined. Furthermore, a cooling duct region  117 ′, which in the finished piston  10  forms a part of the cooling duct  17 , is formed into the blank  111 ′. The cooling duct region  117 ′ may also be formed in during the forging process, and in this case is finish-machined after the forging process. Finally, after the forging process, the welding surfaces  127 ,  128 , by way of which the blank  111 ′ is to be connected to the blank of the piston lower part  18  (cf.  FIG. 1 ), are finish-machined. 
         [0043]    The blank  111 ′ is, as described above, connected together, in a manner known per se, with a blank for a piston main body  18 , by way of the welding surfaces  27 ,  28 ,  32 ,  33  of said blanks (cf.  FIG. 1 ). In this case, the friction welding process leads to local heating of the material in the region of the welding surfaces  27 ,  28 ,  32 ,  33 . 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  13  then deviate considerably from the values predefined for the finished piston  10 . 
         [0044]    Then, in a subsequent method step, on the piston body that results from the welding process, the dome region  114 ′ is finish-machined by virtue of the excess material  125  being removed. This is performed to such an extent that, as a result, the predetermined volume is accurately set after the welding of the blanks, without the need to manipulate the geometry of the combustion depression  13  outside the dome region  114 ′. The remaining region of the combustion depression has already been fully produced by way of the deformation process, that is to say requires no further secondary machining. 
         [0045]    For the inspection of the volume of the combustion depression ( 13 ), during the removal of the excess material  125  in the dome region  114 ′, the present lowest point P 11  of the combustion depression  13  is detected, and a plane E 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  12  (cf.  FIG. 3 ). 
         [0046]    To complete the method according to the invention, the piston body is finish-machined by virtue, for example, of the final fine contour being produced and the annular grooves being formed into the ring section  16  and the boss bores  21  being formed into the piston boss  19 . Furthermore, the piston crown  12  is finish-machined by virtue of the excess material  126  being removed. The boss bores  21  are subsequently 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  12 . In this case, the distance between the piston crown  12  and central axis of the boss bores  21  can be varied by way of the amount of material removed. A piston as per  FIG. 1  is obtained as a result. 
         [0047]    A third exemplary embodiment of a blank  211 ′ of said type for a piston upper part  11  is illustrated in  FIG. 4 . The blank  211 ′ substantially corresponds to the blank  111  as per  FIG. 3 , such that the same reference signs are used for common structures, and in this regard, reference is made to the description relating to  FIG. 3 . The blank  211 ′ is illustrated in section by way of hatching, whereas the final contours of the piston upper part  11  in the finished piston  10  are indicated by dash-dotted lines. In the exemplary embodiment, the geometry of the combustion depression  13 , with the exception of the dome region  214 ′, is finished by forging. The dome region  214 ′ extends from the tip of the dome  14  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  13  is necessary, other than in the dome region  214 ′. 
         [0048]    It is essential that, after the forging process, the dome region  214 ′ has an adequate amount of excess material  225  in order that the volume of the combustion depression can be accurately set in accordance with the invention (in this regard, see further below). Further excess material  126  that is not required for the setting of the volume of the combustion depression  13  may be formed in the region of what will later become the piston crown  12 . 
         [0049]    In the case of the production method described for the exemplary embodiment as per  FIG. 3 , on the piston body that results from the welding process, the dome region  214 ′ is finish-machined by virtue of the excess material  225  being removed. This is performed to such an extent that, as a result, the predetermined volume is accurately set after the welding of the blanks, without the need to manipulate the geometry of the combustion depression  13  outside the dome region  214 ′. The remaining region of the combustion depression has already been fully produced by way of the deformation process, that is to say requires no further secondary machining. 
         [0050]    For the inspection of the volume of the combustion depression ( 13 ), during the removal of the excess material  225  in the dome region  214 ′, the present lowest point P 11  of the combustion depression  13  is detected, and a plane E 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  12  (cf.  FIG. 4 ). 
         [0051]    The finish machining is performed as described for the exemplary embodiment as per  FIG. 3 . A piston as per  FIG. 1  is obtained as a result.