Abstract:
A method for producing a piston for an internal combustion engine, composed of first and second piston components, has the following steps: (a) providing a blank of the first piston component composed of a tempered or precipitation-hardened steel, having at least one joining surface, (b) providing a blank of the second piston component composed of a tempered or precipitation-hardened steel, having at least one joining surface, (c) tempering or precipitation-hardening the blanks, (d) friction-welding the joining surfaces of the blanks to produce a piston blank, with the formation of at least one friction-welding seam and a heat influence zone in the region of the at least one friction-welding seam, (e) annealing or low-stress annealing of the piston blank, thereby obtaining the heat influence zone(s), (f) re-machining and/or finishing the piston blank to produce a piston. A piston so produced is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Applicants claim priority under 35 U.S.C. §119 of German Application No. 10 2010 033 881.8 filed on Aug. 10, 2010, the disclosure of which is 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, composed of a first piston component and a second piston component. The present invention furthermore relates to such a piston for an internal combustion engine. The present invention furthermore relates to a piston for an internal combustion engine, composed of a first piston component and a second piston component, whereby the first piston component has a piston crown, a circumferential top land, and a circumferential ring belt provided with ring grooves, as well as a piston skirt. 
     2. The Prior Art 
     Pistons for modern internal combustion engines are increasingly designed for reduced oil consumption. Since the thermal and mechanical stress on the pistons is increasingly rising at the same time, in modern internal combustion engines, there is the risk that the piston will not be sufficiently supplied with oil during engine operation. This results in greater tribological stresses. This in turn results in increased wear and thus in a reduction in the useful lifetime of the piston. For this reason, attempts are made to harden partial regions or partial structures of the piston that are subject to this increased wear, such as, for example, the lower groove flank of the first ring groove, for example by means of nitriding (see EP 0 985 739 A1) or by means of laser beams (see DE 10 2007 006 948 A1). These measures are connected with increased effort and thus with increased costs. 
     SUMMARY OF THE INVENTION 
     The task of the present invention consists in further developing a method for the production of a piston, in such a manner that hardening of a partial region or a partial structure is made possible, with reduced effort. 
     The solution consists in a method having the following steps: a) making available a blank of the first piston component composed of a tempered steel or a precipitation-hardened steel, having at least one joining surface; (b) making available a blank of the second piston component composed of a tempered steel or a precipitation-hardened steel, having at least one joining surface; (c) tempering the blanks; (d) connecting the blank of the first piston component with the blank of the second piston component by way of their joining surfaces, by means of friction welding, to produce a piston blank, with the formation of at least one friction-welding seam as well as a heat influence zone in the region of the at least one friction-welding seam; (e) annealing or low-stress annealing of the piston blank, thereby obtaining the heat influence zone; (f) re-machining and/or finishing the piston blank to produce a piston. The piston according to the invention is characterized in that the first piston component and/or the second piston component consist(s) of a tempered steel or a precipitation-hardened steel, and are connected with one another by means of friction welding, and that the at least one friction-welding seam is surrounded by a heat influence zone. 
     Tempering is understood to mean a heat treatment of steel that consists of hardening and annealing, by means of which treatment the desired hardness and strength of the steel are adjusted. In this connection, hardening means a heat treatment of steel, consisting of austenitization and rapid cooling, with the goal of martensite and/or bainite formation. Annealing is any heating of a work piece made of steel to temperatures below the lower conversion point Ac 1  in the iron/carbon diagram, holding, and subsequent cooling. Low-stress annealing is understood to be a heat treatment by means of which internal stresses that occur during cooling of a work piece are reduced, without any significant change in structure. 
     The terms used in this patent application, such as hardening, annealing, tempering, low-stress annealing, etc., refer to DIN EN 10052. 
     In the case of tempered steel or precipitation-hardened steel (AFP steel), hardening takes place in the immediate vicinity of the friction-welding seam, by means of the friction welding. In the state of the art, blanks composed of such steels might be pre-machined, connected with one another in the soft state, by means of friction welding, and only the resulting piston blank is subsequently tempered. In this way, the soft material is hardened, but the hardening is cancelled out again in the immediate vicinity of the friction-welding seam. 
     The idea according to the invention consists in first tempering the blanks or bringing them to the desired strength by means of another targeted heat treatment after the forging process (precipitation hardening), and then connecting them with one another in this state, by means of friction welding. In this case, too, hardening takes place in the immediate vicinity of the friction-welding seam. The hardness increases by up to 400 HV (Vickers) in this region. This hardened region is referred to as “heat influence zone” in this patent application. The heat influence zone is harder than the tempered or precipitation-hardened material of the piston blank outside of the heat influence zone. 
     Tempering after friction welding is no longer necessary, according to the method according to the invention. Instead, the piston blank that results from the friction welding is only subjected to annealing or low-stress annealing afterwards, in order to reduce any stresses that might be present. In this connection, the hardness in the heat influence zone drops slightly, but greater hardening with a hardness of up to 250 HV (Vickers) remains. The heat influence zone in the finished piston is therefore a region around the friction-welding seam that has a greater hardness than the remaining material of the piston. 
     This heat influence zone is utilized, according to the invention, for hardening of partial regions or partial structures of the piston that are subject to greater wear. For this purpose, the friction-welding seam is positioned, or the joining surfaces of the blanks of the piston components to be connected by means of friction welding are positioned in such a manner that the partial regions or partial structures of the piston to be produced, which are subject to greater wear and therefore are supposed to be hardened, lie in the heat influence zone after friction welding. In this way, it is no longer necessary to subject these partial regions or partial structures to a separate hardening process, such as nitriding or laser-beam treatment. 
     The present invention is suitable for all piston construction variants according to the claims, as well as for all piston components that consist of a steel material that is accessible to tempering. 
     Advantageous further developments are evident from the dependent claims. 
     The blanks can be forged or cast, and it is practical if they are pre-machined before friction welding, in order to shape structures such as pin bores, combustion chamber bowl, and cooling channel parts, for example, more precisely than is possible by means of forging or casting. 
     A preferred embodiment consists in that the at least one friction-welding seam is configured in such a manner that it runs at an acute angle or at a obtuse angle relative to the piston center axis, in the radial direction. In this way, supported centering of the two piston components is possible at the beginning of the friction-welding process. Additional guide surfaces, guide edges, or the like are therefore not required. 
     A particularly preferred further development provides that at least one blank is provided with at least one ring groove, and at least one pair of corresponding joining surfaces of the blanks are positioned in such a manner that after step (d), the distance of a lower groove flank of a ring groove from the center of the outer friction-welding seam is less than the axial height of the heat influence zone of the friction-welding seam. An alternative to this consists in that in step (f), the piston blank is provided with at least one ring groove, in such a manner that the distance of a lower groove flank of a ring groove from the center of the at least one friction-welding seam is less than the axial height of the heat influence zone of this friction-welding seam. In this manner, the ring grooves, which are subject to greater wear, can be hardened. This particularly relates to the lower groove flank of the first ring groove. 
     The method according to the invention is suitable, for example, for pistons that have a piston base body having a piston skirt as the first piston component, and a piston ring element having a piston crown, a circumferential top land, and a circumferential ring belt provided with ring grooves as the second piston component, whereby the piston base body and the piston ring element form a circumferential cooling channel. Particularly for such pistons, in step (a) a blank of the piston base body is made available, having an outer joining surface and an inner joining surface as well as a circumferential lower cooling channel part between the two joining surfaces, and in step (b), a blank of the piston ring element is made available, having an outer joining surface and an inner joining surface, as well as a circumferential upper cooling channel part between the two joining surfaces. The blank of the piston ring element can have a combustion chamber bowl. Instead, the blank of the piston ring element can have at least one wall region of a combustion chamber bowl. In this case, the blank of the piston base body has at least one crown region of a combustion chamber bowl. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be explained in greater detail below, using the attached drawings. These show, in a schematic representation, not true to scale: 
         FIG. 1   a  an exemplary embodiment of a piston according to the invention, in section; 
         FIG. 1   b  an enlarged detail view of the piston according to  FIG. 1   a;    
         FIG. 2   a  another exemplary embodiment of a piston according to the invention, in section; 
         FIG. 2   b  an enlarged detail view of the piston according to  FIG. 2   a;    
         FIG. 3  an 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. 4  the blanks according to  FIG. 3  after pre-machining; 
         FIG. 5  the piston blank produced from the components according to  FIG. 4 , for a piston according to the invention, in section; 
         FIG. 6  the piston according to the invention, produced from the piston blank according to  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1   a  and  1   b  show a first exemplary embodiment of a piston  10  according to the invention. The piston  10  consists of a piston base body  11  and a piston ring element  12 . Both components can consist of any steel material, particularly according to DIN EN 10083 or DIN EN 10267, which can be subjected to tempering and is suitable for friction welding. 
     In the exemplary embodiment, the piston base body consists of 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 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 in known manner, by means of friction welding. Accordingly, the piston  10  has an outer friction-welding seam  25  in the region of the ring belt  22  and an inner friction-welding seam  26  in the region of the combustion chamber bowl  24 . In the exemplary embodiment, the outer friction-welding seam  25  runs perpendicular to the center axis A of the piston  10 , while the inner friction-welding seam  26  runs at an acute angle α relative to the center axis A. The course of the inner friction-welding seam  26  allows unsupported centering of the piston ring element  12  on the piston base body  11 . Of course, any other desired position and angle combinations of the friction-welding seams are possible with regard to the course of the friction-welding seams. 
       FIGS. 2   a  and  2   b  show another exemplary embodiment of a piston  110  according to the invention, composed of a piston base body  111  and a piston ring element  112 . The piston  110  essentially corresponds to the piston  10  according to  FIGS. 1   a  and  1   b , so that structural elements that agree with one another are provided with the same reference symbols, and reference is made, in this regard, to the above description of  FIGS. 1   a  and  1   b . The only difference consists in that the outer friction-welding seam  125  runs at an obtuse angle β relative to the center axis A of the piston  110 . In combination with the friction-welding seam  26 , which runs at an acute angle α relative to the center axis A, these position and angle combinations of the friction-welding seams allow particularly reliable supportive centering of the piston ring element  12  on the piston base body  11 . 
       FIGS. 3 to 6  show another exemplary embodiment of a piston  210  according to the invention, as well as an exemplary embodiment of a production method according to the invention, for all the exemplary embodiments of the piston  10 ,  110 ,  210  according to the invention. The piston  210  essentially corresponds to the piston  10  according to  FIGS. 1   a  and  1   b , so that structural elements that agree with one another are provided with the same reference symbols, and reference is made, in this regard, to the above description of  FIGS. 1   a  and  1   b . The only difference consists in that the inner friction-welding seam  226  runs perpendicular β relative to the center axis A of the piston  210 . 
     The pistons  1 ,  110 ,  210  according to the invention are produced in the manner described below. The production method is described for the piston  210 , but the description applies analogously for the pistons  10 ,  110 . 
     According to  FIG. 3 , first a blank  211 ′ of the piston base body  211  as well as a blank  212 ′ of the piston ring element  212  are made available, for example by means of forging or casting or sintering. In the exemplary embodiment, ring belt, cooling channel, combustion chamber bowl, pin bosses and pin bores are not machined, or not machined completely. The upper piston part can also be produced by means of ring rolling or by means of being cut from a pipe. 
     The blanks  211 ′,  212 ′ are adjusted to the desired strength after casting or forging, in known manner, by means of tempering or precipitation hardening. 
     The guideline data of DIN EN 10083 apply for tempering: For 42CrMo4: austenitization at 850° C., hardening/quenching in oil, annealing at 600° C.; 
     Precipitation hardening 38MnVS6: solution annealing at approximately 1280° C., forming up to approximately 1000° C., then controlled air cooling to &lt;600° C. After the heat treatment, the blanks  211 ′,  212 ′ have a hardness of 240-360 HV (Vickers). 
     In the exemplary embodiment, the blanks  211 ′,  212 ′ are pre-machined according to  FIG. 4  after the heat treatment. 
     In the exemplary embodiment, the crown region  27  as well as a part of a wall region  28  of the combustion chamber bowl  24  is worked into the blank  211 ′ of the piston base body  211 , for example lathed in. Furthermore, the pin bosses  16  and pin bores  17  as well as the skirt regions  18  with the working surfaces are worked in. Finally, a circumferential lower cooling channel part  23   a  of the cooling channel  23  is worked in. 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  24  is worked into, for example lathed into the blank  212 ′ of the piston ring element  212 . Furthermore, a circumferential upper cooling channel part  23   b  of the cooling channel  23  is worked in. This results in an outer joining surface  32  and an inner joining surface  33 . 
     The outer joining surface  29  of the blank  211 ′ corresponds with the outer joining surface  32  of the blank  212 ′. In corresponding manner, the inner joining surface  31  of the blank  211 ′ corresponds with the inner joining surface  33  of the blank  212 ′. This means that the two blanks  211 ′,  212 ′ can be connected along their joining surfaces  29 ,  31  and  32 ,  33 , respectively, to form a piston blank  210 ′. To connect the two blanks  211 ′,  212 ′, these are braced in alignment, in known manner, whereby supportive centering can take place by means of the weld seam positions chosen. Welding of the parts then takes place by way of the known flywheel friction-welding method, which is sufficiently known from the state of the art. 
     By means of the friction-welding process, a heat influence zone  30 ,  30 ′ as shown in  FIGS. 1   b  and  2   b  forms around the friction-welding seams  25 ,  226  that have formed. The heat influence zones extend above and below the friction-welding seams  25 ,  26 ,  125 ,  226 , over about 1 to 3 mm, in each instance. In the region of the heat influence zones  30 ,  30 ′, the hardness is increased by up to 400 HV (Vickers) as compared with the tempered material of the blanks  211 ′,  212 ′ outside of the heat influence zones  30 ,  30 ′. This means that maximal hardness values of approximately 600 to 800 HV (Vickers) occur in the region of the heat influence zones  30 ,  30 ′. 
     In the exemplary embodiment, the resulting piston blank  210 ′ is subjected to heat treatment, namely annealing or low-stress annealing, preferably for 1 h at 550° C., after friction welding. This heat treatment leads to the result that the hardness of the material is reduced by about 200 HV (Vickers) in the heat influence zones  30 ,  30 ′. The remaining regions experience no change in hardness by means of the heat treatment. Thus, a permanent difference in hardness remains in existence between the harder heat influence zones  30 ,  30 ′ and the remaining material of the blanks  211 ′,  212 ′. 
     The piston blank  210 ′ according to  FIG. 5  has friction-welding beads  35  along the outer friction-welding seam  25  and along the inner friction-welding seam  226  as the result of the friction-welding process described above. The piston blank  210 ′ is re-machined or finished in known manner, depending on the configuration of the blanks  211 ′,  212 ′. For example, the external shape, surfaces, combustion chamber bowl, pin bores, etc. can be finished, and the accessible friction-welding beads  35  are removed. According to the invention, the ring belt  22  with ring grooves  34 ,  36 ,  37  (see  FIGS. 1   b ,  2   b ) are worked in, in such a manner that the lower groove flank  34 ′ of the first ring groove  34  is positioned in the region of the heat influence zone  30 , and the distance of the lower groove flank  34 ′ of the ring groove  34  from the center of the outer friction-welding seam  25 ,  125  is less than the axial height of the heat influence zone  30  (see  FIGS. 1   b  and  2   b ). In the end result, the lower groove flank  34 ′ is hardened, particularly in the region of its outer edge  34 ″. The hardness of the groove flank now lies at approximately 400 to 600 HV (Vickers), which corresponds to an increase in hardness, according to the invention, by approximately 100 to 200 HV (Vickers), and leads to an improvement in the wear resistance. In the final analysis, a finished piston  210  according to  FIG. 6  is obtained. Of course, any other ring groove  36 ,  37  can also be hardened in this manner. The same holds true for other partial regions and partial structures of the piston  10 ,  110 ,  210  according to the invention that are supposed to be subjected to hardening. Such partial regions and partial structures should lie in the region of the heat influence zones  30 ,  30 ′ after friction welding.