Abstract:
A method for the production of a piston made of steel, for an internal combustion engine, in which the upper piston part is produced using the forging method, and the lower piston part is produced using the forging or casting method, and they are subsequently welded to one another. To simplify the production method and make it cheaper, the upper piston part is forged using the method of hot forming and of cold calibration, to finish it to such an extent that further processing of the combustion bowl and of the upper cooling channel regions can be eliminated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional of U.S. patent application Ser. No. 13/066,559, filed on Apr. 8, 2011, which claims priority under 35 USC  119  from DE 10 2011 013 141.8, filed on Mar. 4, 2011, the disclosures of which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for the production of a piston for an internal combustion engine. 
         [0003]    From the state of the art, it is generally known to produce pistons from steel for an internal combustion engine, in that first an upper piston part is produced using the forging method, and a lower piston part is produced using the forging method or by means of casting, and then the upper piston part is welded to the lower piston part. In this regard, reference should be made to the patent documents DE 195 01 416 A1, DE-OS 29 19 638, DE 196 03 589 A1, and DE 198 46 152 A1. In this connection, the method of hot forming, in other words hot forging, at a steel temperature of 950° C. to 1300° C., is used. 
         [0004]    In this connection, an uncontrollable oxide layer forms on the surface of the forged blank, and in order to remove it, the surface of the forged blank must be blasted with coarse blasting material. This results in great variations in the forged contour, so that as a consequence of this, complicated reworking of the forged blank, by means of a chip-cutting processing method, is required. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, it is the task of the present invention to avoid the aforementioned disadvantages of the state of the art, whereby in particular, complicated reworking of the combustion bowl and of the cooling channel is supposed to be avoided. 
         [0006]    It is furthermore the task of the present invention to indicate a method with which pistons having combustion chamber bowls and cooling channels that are not configured with rotation symmetry or in centered manner can be produced in cost-advantageous manner. 
         [0007]    Finally, it is the task of the present invention to indicate a method with which pistons can be produced, in which the wall between the edge of the combustion bowl and the upper part of the cooling channel has a constant thickness over the circumference. 
         [0008]    These tasks are accomplished with the characteristics that stand in the characterizing part of the main claim and of the dependent claims. Advantageous embodiments of the invention are the object of the dependent claims. 
         [0009]    In this connection, the result is achieved, by means of cold calibration or cold forming of the forged blank, that the combustion bowl and the cooling channel are formed in finished manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Some exemplary embodiments of the invention will be explained in the following, using the drawings. These show: 
           [0011]      FIG. 1  a sectional diagram of a piston produced according to the method according to the invention, in a section plane that lies perpendicular to the pin bore axis, 
           [0012]      FIG. 2  a section through the piston, in a section plane that lies on the pin bore axis, 
           [0013]      FIG. 3  a section through the upper piston part after semi-hot forming, 
           [0014]      FIG. 4  a section through the upper piston part after over-lathing of the outer contour and of the contact regions intended for friction welding, 
           [0015]      FIG. 5  a top view of a configuration of the upper piston part having an asymmetrically configured and eccentrically disposed combustion bowl, 
           [0016]      FIG. 6  a section through the upper piston part along the line VI-VI in  FIG. 5 , 
           [0017]      FIG. 7  the upper piston part and the lower piston part before joining by means of friction welding, 
           [0018]      FIG. 8  a top view of another embodiment of the upper piston part having an asymmetrically configured and eccentrically disposed combustion bowl and having a valve niche, and 
           [0019]      FIG. 9  a section through the upper piston part along the line IX-IX in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0020]      FIG. 1  shows an embodiment of a piston  1  produced according to the method according to the invention, in section, perpendicular to the pin axis  2 , consisting of an upper piston part  3  and a lower piston part  4 , which are connected with one another by way of a friction-welding seam  5 . 
         [0021]    The piston  1  has a piston crown  6  into which a combustion bowl  7  is formed. Radially on the outside, a ring wall  8  directed downward, having a ring belt  9  for piston rings not shown in the figure, is formed onto the piston crown  6 . Radially within the ring wall  8 , the piston  1  has a ring-shaped support  10  formed onto the underside of the piston crown  6 . 
         [0022]    The lower piston part  4  consists of two skirt elements  11  and  12  that lie opposite one another, which are connected with one another by way of two pin bosses  13  and  14  that lie opposite one another, each having a pin bore  15  and  16 . In  FIG. 1 , only the pin boss  13  having the pin bore  15  can be seen, because of the position of the section plane. 
         [0023]    A ring-shaped contact part  17  connected with the pin bosses  13 ,  14  is disposed on the top of the lower piston part  4 . 
         [0024]    Furthermore, the lower piston part  4  has a circumferential ring rib  18  on its top, which rib is disposed radially outside of the contact part  17  and connected with the skirt elements  11 ,  12 . A radially oriented ring element  19  extends between the contact part  17  and the ring rib  18 . 
         [0025]    In this connection, the support  10  and the contact part  17  are disposed in such a manner that the underside of the support  10  and the top of the contact part  17  have contact with one another and form a first contact region  20 . Furthermore, the ring wall  8  and the ring rib  18  are disposed in such a manner that the lower face side of the ring wall  8  and the top of the ring rib  18  also have contact with one another and form a second contact region  21 . The first and the second contact region  20  and  21  form friction-welding surfaces during the production of the piston  1 . 
         [0026]    In this way, the result is achieved that a circumferential cooling channel  22  disposed close to the piston crown  6 , radially on the outside, is delimited, at the top, by the piston crown  6 , radially on the inside partly by the piston crown  6 , partly by the support  10 , and partly by the contact part  17 , at the bottom by the ring element  19 , and radially on the outside partly by the ring wall  8  and partly by the ring rib  18 . The cooling channel  22  has an inflow opening for introduction of cooling oil and an outflow opening for discharge of cooling oil, but these are not shown in the figure. 
         [0027]    In  FIG. 2 , the piston  1  is shown in section along the pin bore axis  2 . Here, the two pin bosses  13 ,  14  can be seen, with the contact part  17  formed onto them, as can the ring element  19  that is connected with the contact part  17  and the pin bosses  13 ,  14 , respectively. 
         [0028]    The piston  1  is produced from AFP steel, in other words from precipitation-hardened ferritic-pearlitic steel, such as case-hardened steel 38MnVS6, for example. However, any other suitable steel can be used, such as tempered steel 42CrMo4, for example. In this connection, production of the lower piston part  4  takes place in conventional manner, by means of casting or hot forging. 
         [0029]    The upper piston part  3  is produced by means of the method of hot forming. In this connection, a piece of AFP steel that is shaped to fit into the drop-forging machine intended for the upper piston part  3  is heated to 1200° C. to 1300° C., and subsequently formed or pre-formed in multiple forming stages, in other words forging processes, in the same drop-forging machine. The scale that forms during forging is removed by means of blasting. 
         [0030]    Subsequently, the finished forged upper part blank is cold-calibrated at room temperature, whereby all the surfaces of the upper piston part  3  are pressed at room temperature, in order to achieve the final dimensions. 
         [0031]    Alternatively to this, the pre-formed upper part blank can also be brought into its final shape by means of cold-forming at room temperature. It is advantageous, in this case, if an annealing process is still carried out before blasting, in order to reduce the tendency to form cracks during cold forming. 
         [0032]    Furthermore, other processes can also be used for production of the pre-form, such as the method of cold forming, of semi-hot forming, or of milling, for example. Thus, the pre-form can also be produced by means of a precision casting method. In order to avoid scale formation, the latter method should be used under an inert gas atmosphere. 
         [0033]    The resulting blank of the upper piston part  3  is shown in  FIG. 3 . In this connection, the combustion bowl  7 , the upper cooling channel region, and the inner mandrel region  29  are already formed in their final form, so that no further processing steps are any longer required in these regions. In this connection, the result is also achieved that the wall thickness between the bowl edge and the upper cooling channel region is almost constant over the circumference. The upper piston part  3  as it looks after finishing is shown in  FIG. 3  with broken lines. 
         [0034]    In the subsequent method step, the radially outer region  23  of the piston crown  6 , the radially outer region  24  of the upper piston part  3  intended for the ring belt  9 , the lower face surface  25  of the ring wall  8 , the lower region  26  of the inner surface  27  of the ring wall  8 , and the contact surface  28  of the support  10  are machined by means of lathing, so that the upper piston part  3  as shown in  FIG. 4  is obtained. The lower region of the cooling channel  22 , the lower face surface  25  of the ring wall  8 , and the contact surface  28  of the support  10  are formed in finished form after this latter method step. Here again, the upper piston part  3 , as it looks after finishing, is shown with broken lines. 
         [0035]    The production method of hot forming in combination with cold calibration or cold forming, respectively, particularly allows production of upper piston parts  3 ′ having combustion bowls  7 ′ that are configured asymmetrically and disposed eccentrically, as shown in  FIGS. 5 and 6 . Here, again, no further processing of the combustion bowl  7 ′ is required any longer, once the process of hot forming and of cold calibration or cold forming, respectively, of the upper piston part  3 ′ has been completed. 
         [0036]    In the present exemplary embodiment according to  FIGS. 5 and 6 , the combustion bowl  7 ′ has approximately the shape of a four-leafed clover. However, any desired shape of a combustion bowl can be implemented with the method of hot forming in combination with cold calibration or cold forming, respectively. 
         [0037]      FIGS. 8 and 9  show the upper piston part according to  FIGS. 5 and 6 , produced in this manner, whereby in addition, a valve niche  30  has been formed into the piston crown  6  of the upper piston part  3 ″. 
         [0038]    The upper piston part  3 ,  3 ′,  3 ″ according to  FIG. 4 ,  5 ,  6 ,  8 ,  9  is braced into a friction-welding device (not shown in the figure) together with the lower piston part  4 , and, as shown in  FIG. 7 , they are brought into position, relative to one another, so that they can be put into rotation, moved toward one another with force, and friction-welded to one another when the upper piston part  3 ,  3 ′,  3 ″ makes contact with the lower piston part  4  in the region of the contact regions  20  and  21 . If the combustion bowl  7 ′ is configured asymmetrically or eccentrically, care must be taken during friction welding to ensure that after completion of the welding process, the combustion bowl  7 ′ assumes a clearly defined rotation position relative to the pin axis  2 , for example. 
         [0039]    In this connection, the piston  1  shown in  FIGS. 1 and 2  is obtained. 
         [0040]    Within the scope of the last method step, the grooves of the ring belt  9  are lathed into the outer piston wall and the piston crown  6  is lathed flat, as indicated in  FIGS. 3 and 4 . Furthermore, the precision piston contour and the pin bores are worked in. 
       REFERENCE SYMBOL LIST 
       [0000]    
       
           1  piston 
           2  pin axis 
           3 ,  3 ′,  3 ″ upper piston part 
           4  lower piston part 
           5  friction-welding seam 
           6  piston crown 
           7 ,  7 ′ combustion bowl 
           8  ring wall 
           9  ring belt 
           10  support 
           11 ,  12  switch element 
           13 ,  14  pin boss 
           15 ,  16  pin bore 
           17  contact part 
           18  ring rib 
           19  ring element 
           20  first contact region 
           21  second contact region 
           22  cooling channel 
           23  outer region of piston crown  6   
           24  outer region of upper piston part 
           25  lower face surface of ring wall  8   
           26  lower region of inner surface  27  of ring wall  8   
           27  inner surface of ring wall  8   
           28  contact surface of support  10   
           29  inner mandrel region 
           30  valve niche