Abstract:
A cooling channel piston for an internal combustion engine which includes a piston bottom and a piston shaft that are joined thereto of a friction welding process. The piston bottom and the piston shaft jointly form a cooling channel. An annular wall which radially delimits the cooling channel towards the outside is formed by the piston bottom and/or the piston shaft. The annular wall can be sealed by a welding process once the piston bottom and the piston shaft have been joined together.

Description:
CROSS-REFERENCE TO CO-PENDING APPLICATION 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 11/576,302 filed Sep. 8, 2009, for a SIMPLE FRICTIONAL WELD, the entire contents of which are incorporated herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a one-piece, steel welded cooling-channel piston of forged steel. 
         [0003]    It is desirable to develop a piston surpassing the prior art in which the cooling channel or cooling cavity is tightly sealed. A piston of this type is described, for example, in the unpublished German patent application DE 10 2004 038 465.7. 
         [0004]    A cooling channel piston is known from the previously published German patent application DE 102 44 512 A1 having the features which constitute the genre. The upper part of the piston has circumferential radial webs running coaxially behind the ring belt which are joined to corresponding webs on the piston skirt by means of friction welding. After the joining of piston upper part and piston lower part, the lower face of the ring belt is located adjacent the upper circumferential radial face of the piston skirt. In the state when piston upper and lower parts are joined, the outer piston wall areas between both parts of the piston form a contact surface which is characterized by a gap a few tenths of a millimeter wide. It can remain open or be sealed by means of a temperature-resistant sealing ring which is positioned before the joining of both parts of the piston on one of the contact surfaces, for example that of the lower part of the piston. A sealing ring of this kind has the advantage that the cooling channel is then closed to form a seal, but in a disadvantageous manner it represents an additional component which has to be produced and correctly positioned when the two parts of the piston are joined. In addition, the sealing ring, just like the few tenths of a millimeter wide gap without a sealing ring, has the disadvantage that the upper part of the piston (piston crown) cannot be supported on the lower part of the piston (piston skirt) while the piston is operating. While operating, the piston crown is deformed in a disadvantageous manner by the combustion pressures acting on it so that the strength and durability of the piston are compromised. 
         [0005]    A cooling channel piston for a combustion engine is known from EP-A-1 061 249, having a piston crown and a piston skirt joined by means of friction welding, which together form a cooling channel, where a ring wall delimiting the cooling channel radially outward is formed by the piston crown and/or the piston skirt and the ring wall can be closed to form a seal by an interference fit and/or positive fit after piston crown and piston skirt have been joined. As shown in  FIGS. 2 to 5 , the surfaces of the ring belt and of the piston skirt facing each other are radially circumferential and coplanar with each other. The mutually coplanar facing surfaces on the lower edge of the ring belt and the upper edge of the piston skirt do not have any type of shoulder so that when the cooling channel piston is operating, there is no adequate support for the piston crown on the piston skirt. Only in  FIG. 6  is it shown that the upwardly facing termination of the piston skirt has a single shoulder with which the coplanar aligned lower part of the ring belt can come into contact. This certainly improves the supporting effect, but not optimally. In addition, the inwardly directed L-shape of the shoulder at the upper end of the piston skirt is extremely difficult and expensive to manufacture with the required precision, so that no practical solution can be derived from EP-A-1 061 249. 
         [0006]    Generic cooling channel pistons are also known from US 2004/144247 A1 and US-B1-6698391 in which, however, the circumferential radial lower edge of the ring belt and the upper edge of the piston skirt coming into contact therewith are also only configured coplanar so that the supporting effect is not disclosed. 
         [0007]    It is therefore desirable to prepare a cooling channel piston which effectively avoids the disadvantages described above. 
       SUMMARY 
       [0008]    In one aspect, the upper and lower parts of the cooling channel steel piston are first joined by means of friction welding or resistance press welding where a ring wall delimiting the cooling channel in an outward direction does not initially form a material bond and the tight sealing of the ring wall of the cooling channel is achieved by means of an interference and/or positive fit, and specifically, a subsequent welding process. An advantageous joining and closing of the cooling channel is now possible through the design of the cooling channel piston in accordance with the invention, wherein the joined parts, that is, piston crown and piston skirt, are joined by means of friction welding or resistance press welding. The subsequent external welding (not friction welding), or the corresponding shaping of the ring wall respectively, affects the cooling channel to the extent that no flashes from friction welding extend into the cooling channel in the area of the ring wall. Through the subsequent welding of the outer ring wall, or generally through the interference and/or positive fit, the piston is supported in the area of the ring belt, that is, the forces acting on the piston crown can be transferred to the piston skirt, which considerably increases the strength and durability of the cooling channel piston. At the same time, the welding also counteracts deformation in the skirt area, particularly in its upper area. In addition, manufacturing tolerances between the upper and lower part are compensated. In addition, provision is made in accordance with the invention for both the face of the ring wall below the ring belt and also the adjacent face of the ring side above the piston skirt to have a matching shoulder. As a result, both in the case of the interference fit and in the case of the positive fit connection the cooling channel is closed to from a seal and the support of the piston crown on the piston skirt is optimized in the contact or joint area. 
         [0009]    Additional welded-in parts can be dispensed with due to the piston being formed only of piston crown and piston skirt, which considerably simplifies the manufacture of a cooling channel piston in accordance with the invention. Additionally, one advantage of the invention is that, depending on the configuration of the joint area around the ring wall, this joint area can serve to lend rigidity to and position the joined parts during the friction welding. A cost saving results at the same time because welded-in parts can be dispensed with in the area of the cooling channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The friction weld will be described hereinafter with reference to an aspect of a friction weld which is not restricted and explained by the drawings in which: 
           [0011]      FIG. 1  shows the section through a one-piece, steel welded cooling channel piston, specifically of forged steel; 
           [0012]      FIG. 2  is an enlarged view of one aspect of the ring belt face; 
           [0013]      FIG. 2 a    is the view in  FIG. 2  showing the gap closed; 
           [0014]      FIG. 3  is an enlarged view of another aspect of the ring belt face; 
           [0015]      FIG. 3 a    is the view of  FIG. 3  showing the gap closed; 
           [0016]      FIG. 4  is an enlarged view of another aspect of the ring wall face; and 
           [0017]      FIG. 4 a    is the view of  FIG. 4  showing the gap closed. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    One half of a sectioned view of a piston and specifically of a cooling channel piston for a combustion engine is shown In  FIG. 1 . The cooling channel piston  1  shown here consists of a piston crown  2  and a piston skirt  3  which are joined by means of friction welding which results in a friction welding seam  4 . The piston crown  2  is formed from a combustion bowl  5 , an upper land  6 , a ring belt  7  and one part of a cooling channel  8 . The area of the piston skirt  3  comprises an outer skirt surface  9  of the cooling channel piston  1 , a piston pin bore  10  and a lower part  11  of the cooling channel. The cooling channel  8  is consequently formed of an upper and a lower area  11  of the piston crown  2  and the piston skirt  3  and a recessed, circumferential web of the piston crown  2  and the piston skirt  3 . 
         [0019]    In this aspect, an additional groove  12  is introduced into the outer surface of the piston  1  below the ring belt  7 . After the piston crown  2  and the piston skirt  3  have been joined by friction welding, the two parts of the piston  2 ,  3  merely abut each other in the area of the ring wall  13  without the existence of a material bond and/or tight connection in the area of this ring wall  13 . The joining of the ring wall  13  is carried out in a subsequent, supplementary welding process. Here, the widest variety of welding processes (but not friction welding), such as for example, electron beam welding, WIG welding, MIG welding, MAG welding, laser welding, etc.  20  is conceivable. 
         [0020]    In this aspect, a shoulder  14  is incorporated in the joint area of the ring wall  13  which has the advantage that when welding the ring wall  13 , the weld seam root does not extend into the cooling channel  8  and in addition, a thick weld is made possible by welding up the shoulder and a clean, pore-free root is created. 
         [0021]    The design of the shoulder  14  is not restricted to this aspect, but rather the creation of other shoulder shapes  14  in the area of the ring wall  13  is conceivable which either assist the subsequent welding process or have a positive effect on the friction welding. 
         [0022]    An additional advantage of welding the ring wall  13  afterward is that until immediately before the final joining of the piston crown  2  and the piston skirt  3 , inert gases can penetrate through the gap in the ring wall  13  into the area of the friction weld seam which in turn have a positive effect on the material structure in the friction weld seam  4 . The location of the shoulder  14  in the area of the ring wall  13  is placed exactly in the center of the groove  12  only as an example. This shoulder  14  lies advantageously below the ring belt  7  and above the lower end  15  of the cooling piston channel. It is also conceivable in accordance with the invention not to introduce a groove  12  into the piston  1  and to form the ring wall  13  only with the surface of the skirt  9 . 
         [0023]    With a view to  FIG. 1 , it should be pointed out once more that the ring wall  13  is formed by the lower face of the ring belt  7  and the upper shoulder of the piston skirt  3 . Because of the geometry in the area of the ring wall  13  shown in  FIG. 1 , the welding procedure can be performed particularly conveniently. The matching shoulders of the opposing faces, as already explained, prevent the weld seam from forming towards the cooling channel  8 . Besides that, the advantage of the welding process to be performed radially on the outside remains, specifically that the weld seam projecting radially outward can be removed, for example by machining. In addition to this ring wall  13  geometry shown in  FIG. 1 , it is also conceivable that the upper part of the piston skirt  3  has a plateau which the lower face of the ring belt  7  contacts after the friction welding process. It is also conceivable that the ring belt  7  is located on the piston skirt and comes into contact with a lower face below the upper land  6 . The advantages described initially remain with these geometries as well. The basic principle of the present invention is important and advantageous, namely that the piston crown  2  and the piston skirt  3  have webs recessed from the outer surface running radially circumferential which are joined by means of friction welding while the ring wall  13 , which is shaped so that the faces of the ring wall  13  bordering each other (of the piston crown  2  and/or of the piston skirt  3 ) do not yet form a material joint in the friction welding process, is directly on the outer surface of the cooling channel piston  1 . The welding of the adjacent faces of the ring wall  13  takes place in an additional step from the outside only after piston crown  2  and piston skirt  3  have been joined through their internal webs by means of friction welding. 
         [0024]      FIG. 2  shows the area of the ring wall  13  where the part of the face of the ring belt  7  projecting downward is configured in a grooved shape into which a radially circumferential shoulder  14  facing upwards engages above the face of the piston skirt  9 . Here too, welding the adjacent faces of the ring wall  13  is subsequently carried out from the outside. This closes the cooling channel  8  completely as shown in  FIG. 2 a   , the ring belt  7  is supported against deformation when the cooling channel piston  1  is operating, and the piston skirt is supported against deformation, in particular in the upper area. 
         [0025]      FIG. 3  shows a section of the area of the ring wall  13  where the lower circumferential section of the ring belt  7  and the upper circumferential section of the piston skirt  9  are shown. In the design in accordance with  FIG. 3 , the face of the ring wall  13  below the ring belt  7  and the adjacent face of the ring side  13  above the piston skirt  9  are configured in the manner of a groove, similar to  FIG. 2 , but with the particular feature that the circumferential tongue  17  has a height X which is greater than the depth Y of the groove  16 . The procedure during manufacture of the cooling channel piston  1  is that piston crown  2  and piston skirt  3 , as already described, are joined by means of friction welding or resistance press welding and thereby the two faces of the ring wall  13  from  FIG. 3  are at a distance (with a gap) and do not contact each other as shown in  FIG. 3 . Subsequently the cooling channel piston  1  is pressed together from above and from below, in the axial direction of the piston stroke. The opposed faces of the underside of the ring belt  7  and of the upper side of the piston skirt  9  come into contact, where simultaneously the radially circumferential tongue  17  is deformed inside the radially circumferential groove  16  so that an interference fit and a positive fit result in the area of the ring wall  13  as shown in  FIG. 3 a   , which ensures that the cooling channel  8  is closed to form a seal and at the same time the piston crown  2  can rest on the piston skirt  9 . In one aspect, this interference and positive fit is sufficient, although it is also conceivable to reinforce the contact area additionally in the area of the ring wall  13  through a material bond by means of a welding procedure. 
         [0026]    Something similar applies in the aspect in accordance with  FIG. 4 , where the face of the ring wall  13  below the ring belt  7  and the adjacent face of the ring side  13  above the piston skirt  9  both have a matching shoulder. In this instance also, as shown in  FIG. 4 , the opposing faces are not in initial contact after the piston crown  2  and piston skirt  3  have been joined, but instead a gap has formed. To eliminate the gap to achieve sealing of the cooling channel  8  and absorption of force for the piston crown  2  on the piston skirt  3 , the cooling channel piston  1  is again exposed to a force from above or from below so that in the area of the contact surfaces an interference or a positive fit results as shown in  FIG. 4 a   . The cooling channel  8  is sealed and absorption of force is provided so that in contrast to the aspect from  FIG. 1 , a welding procedure in the area of the ring wall  13  can be omitted. 
         [0027]    In summary, it must be pointed out once again that initially the piston crown  2  is joined to the piston skirt  3  by means of friction welding or resistance press welding. This joining takes place in an area clearly set back from the ring belt  7  which is located coaxially between the back side of the ring belt  7  and the outer circumference of the combustion bowl  5  to achieve sufficient rigidity. Following this joining of piston crown  2  and piston skirt  3 , the ring wall  13  (the area below the ring belt  7  and above the piston skirt  9 ) is closed by means of an interference and/or positive fit such that on the one hand the cooling channel  8  is sealed and on the other hand the piston crown  2  can rest on the piston skirt  3  in such a way that deformation of the piston crown  2  when the cooling channel piston  1  is operating is clearly reduced since the combustion pressures or forces acting on the piston crown  2  can be transferred to the piston skirt  9 . In this case the contact area in the region of the ring wall  13  can still be materially joined by means of a welding process  20 , but this can normally be dispensed with. 
         [0028]    Alternatively, or additionally, the geometric relationships after the joining of piston crown  2  and piston skirt  3  are such that the faces below the ring belt  7  and above the piston skirt  9  are not yet in contact, but a gap has formed. In order to seal the cooling channel  8  and to ensure the absorption of forces from the piston crown  2  to the piston skirt  3 , this area of the ring wall  13  is closed as a positive fit by means of a welding procedure, as already described. The opposing faces can be configured planar or be configured from other geometric shapes (as for example step-shaped in accordance with  FIG. 4  or tongue-and-groove as in  FIG. 3 . 
         [0029]    With reference to  FIG. 1 , it must be pointed out supplementaly that in the internal region of the cooling channel piston  1  there may be, but does not have to be, an additional, radially circumferential web  18 . This web  18  which can be manufactured with the cooling channel piston  1  or as a separate component which can be joined to the cooling channel piston  1  then forms at least one further cooling channel  19 , where in addition to the two cooling channels  8 ,  19 , which can also be described as cooling chambers, a cooling medium can be introduced centrally in the interior area of the cooling channel piston  1 , permanently or circulating. Not shown, but present, are on the one hand means with which the cooling medium can be sprayed into the cooling channel or cooling channels, and on the other hand the necessary orifices, as for example holes on the underside of the interior area or also inside the webs separating the cooling channels.