Abstract:
A method for the production of a piston for an internal combustion engine, composed of at least two components, each of which has at least one corresponding joining surface, has the following steps: a) pre-working the at least two components, at least in the region of the joining surfaces; b) coating at least a part of the surface of at least one component with a covering medium containing at least one phyllosilicate; c) assembling the at least two components; d) connecting the at least two components along their corresponding joining surfaces, by means of beam welding, to produce a piston blank; e) removing the covering medium and any excess weld material adhering to it; and f) machining the piston blank to produce a finished piston.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Applicant claims priority under 35 U.S.C. §119 of German Application No. 10 2011 107 656.9 filed Jul. 12, 2011, 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 at least two components, each of which has at least one corresponding joining surface. The present invention furthermore relates to a piston that can be produced using such a method. 
     2. The Prior Art 
     In beam welding, excess weld material regularly occurs, generally in the form of weld beads or weld splashes. In the following, the term “weld beads” is used to refer to all forms of excess weld material. 
     In the production of a piston by beam welding, there is the risk that weld beads adhere to the piston. It is particularly disadvantageous if the weld beads get into the cooling channel and take hold there. During engine operation, the weld beads can come loose again and enter the cooling oil and thus enter into the cooling oil circuit and the lubrication oil circuit. In this case, the internal combustion engine would suffer irreparable harm. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for the production of a piston that prevents the exit of weld beads into the oil circuit during engine operation. 
     This object is achieved by a method having the following steps: 
     a) pre-working the components, at least in the region of the joining surfaces; 
     b) covering at least a part of the surface of at least one component with a covering medium containing at least one phyllosilicate; 
     c) assembling the components; 
     d) connecting the components along their corresponding joining surfaces, by means of beam welding, to produce a piston blank; 
     e) removing the covering medium and any excess weld material adhering to it; and 
     f) machining the piston to finish it. 
     The object of the present invention is furthermore a piston for an internal combustion engine that can be produced according to the method according to the invention and thus, in the end result, has at least two components connected with one another by means of beam welding and, at the same time, is free of weld material adhering to them. 
     Using the method according to the invention, when the at least two components are connected, weld beads due to the beam welding do not remain adhered to the components. In particular, walls of a cooling channel that might be present can be kept free of weld beads. The weld beads either remain adhered to the covering medium or do not adhere at all. The covering medium is removed from the components again after beam welding. In the end result, a beam-welded piston is obtained that is free of weld beads. 
     The weld beads do not occur with the same frequency or thickness everywhere. For example, the weld beads occur more frequently in those zones of the components that lie opposite the weld seams. These regions are supposed to be particularly protected. 
     The known pre-working of the components to be connected also includes cleaning and degreasing, in order to obtain a firm weld seam in step d). 
     Preferably, bentonite is used in step b); this is a stone material that contains montmorillonite, a clay mineral from the class of phyllosilicates, as its major component. Bentonite has the advantage that it behaves in thixotropic manner, in other words it can be applied easily and then solidifies. 
     The covering medium can furthermore contain disodium tetraborate (borax). Borax is soluble in water, acts as a binder or flux agent, and thereby contributes to allowing the covering medium to be particularly easily removed by washing with warm or cold water, after the welding process. Borax is generally very fine-grained, with a particle diameter of less than 1 μm, and, because of its layer-like crystal structure and its low Mohs hardness of 2 to 2.5, does not have any abrasive properties, so that the risk of damage to the internal combustion engine during engine operation is further reduced. Borax furthermore withstands temperatures above 500° C. and does not enter into any reaction with the material of the components to be connected. 
     It is practical if, in step b), the covering medium is applied removed at least 1 mm from the edge of each joining surface, in order to prevent it from being damaged during beam welding or from impairing the quality, particularly the strength, of the weld seam. The joining surfaces themselves remain metallically shiny and uncoated. 
     The covering medium is preferably applied, in step b), in the form of a suspension, which is preferably based on water. In this form, the covering medium can be applied particularly easily, for example by brushing it on, spraying it on, rolling it on, or imprinting it. It is practical if the coated components are dried, after step b), by heating them to 80° C. to 180° C. 
     In step b), the covering medium can also be applied to the coated components in the form of a powder, by means of thermal spraying. A covering medium that contains disodium tetraborate is particularly well suited for this, because disodium tetraborate loses its water of crystallization at temperatures above 400° C., and its melting point in the anhydrous phase amounts to 848° C. This melt serves as a carrier for the at least one phyllosilicate. 
     The covering medium should be applied with a layer thickness of at least 100 μm, in order to guarantee effective protection of the at least one component and to reliably prevent weld beads from remaining adhering to the surface of the at least one component. 
     The components to be coated with the covering medium can be preheated to 50° C. to 80° C. before the covering medium is applied, in order to guarantee good adhesion of the covering medium. 
     In step e), the covering medium, together with the weld beads adhering to it if applicable, is removed from the piston blank, particularly preferably by means of washing it with warm or cold water. This is particularly practical if the covering medium contains disodium tetraborate. Borax is water-soluble in every form (11 g/l at 20° C. and 88 g/l at 80° C.), so that the complete coating, together with the phyllosilicate distributed in it, and, if applicable, together with the weld material adhering to the coating, is removed from the piston blank without leaving any residue. 
     The at least two components to be connected can be tacked together before beam welding. Furthermore, at least one component can be shrunk-fit onto another component. In this way, the components are fixed in place relative to one another, in terms of their position. 
     The at least two components can be connected by means of electron beam welding or laser welding. The use of a CO 2  laser is preferred, because comparatively small amounts of weld beads are formed with it. 
     Before beam welding, the components to be connected can be preheated to 400° C. to 550° C., in known manner, in order to obtain a particularly strong and reliable weld connection and to avoid cracks. 
     It is practical to protect the piston blank against corrosion, in known manner, after removal of the covering medium and, if applicable, after the drying process. 
     The piston blank should furthermore be inspected for complete removal of weld beads. The inspection of a cooling channel that might be present can be undertaken using an endoscope, for example. 
     The machine finishing of the piston blank comprises a heat post-treatment known to a person skilled in the art, depending on the material used for the components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention. 
       In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
         FIG. 1  shows a first embodiment of a piston according to the invention, in section; 
         FIG. 2  shows another embodiment of a piston according to the invention, in section; 
         FIG. 3  shows another embodiment of a piston according to the invention, in section; 
         FIG. 4  shows an exploded view of the embodiment according to  FIG. 1 , before the components to be connected are assembled; 
         FIG. 5  shows the embodiment according to  FIG. 1  after beam welding. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawings,  FIG. 1  shows a first embodiment of a piston  10  according to the invention. Piston  10  has a component  11  configured as a piston base body, which is produced, for example, from an annealed steel such as 42CrMo4 or an AFP steel such as 38MnSiVS6, for example, or a bainitic AFP steel alloyed with 0.4 wt.-% molybdenum. Component  11  has a part of a piston crown  12 , a circumferential top land  13 , as well as a circumferential ring belt  14  having ring grooves for accommodating piston rings (not shown). Component  11  furthermore has the bottom  15   a  of a combustion bowl  15 . Component  11  thus forms an essential part of piston head  16  of piston  10 . Component  11  furthermore forms piston skirt  17  of piston  10  according to the invention, in known manner. 
     The piston according to the invention furthermore has a component  18  configured as an insert that forms the entire bowl wall  15   b  as well as the bowl edge region  15   c  of combustion bowl  15 , and furthermore part of piston crown  12 . Component  18  preferably consists of a particularly high-strength material. For this purpose, an annealed steel or AFP steel can be used for piston base body  11 . Furthermore, a steel that is resistant to high elevated temperatures, corrosion-resistant, and heat-resistant is suitable. Valve steels such as, for example, CrSi steel (X45CrSi93), Chromo193 steel (X85CrMoV182), 21-4 N steel (X53CrMnNiN219), 21-2 steel (X55CrMnNiN208), and materials such as Nimonic80A (NiCr20TiAl), ResisTEL, or VMS-513 are particularly suitable. 
     Components  11 ,  18  form a circumferential outer cooling channel  19 . Cooling channel  19  runs at the level of ring belt  14 , and at the level of bowl wall  15   b  of combustion bowl  15 . 
     Component  18  has a lower circumferential joining surface  24   a  (see  FIG. 4 ) that forms a lower weld seam  21  with a circumferential joining surface  23   a  (see  FIG. 4 ) on component  11  that encloses bottom  15   a  of the combustion bowl  15 . Lower weld seam  21  has a length of 3.5% to 5.5% of piston diameter D, and encloses an acute angle a with the piston center axis M. Lower weld seam  21  therefore runs radially toward the outside, proceeding from the bowl wall  15   b,  and downward (in the direction of the piston skirt  17 ), and ends in cooling channel  19 , in the region of the cooling channel bottom. 
     Component  18  furthermore has an upper circumferential joining surface  24   b  (see  FIG. 4 ) that forms an upper weld seam  22  with a circumferential joining surface  23   b  (see  FIG. 4 ) on component  11 , in the region of top land  13 . Upper weld seam  22  has a length of 4.5% to 6.0% of piston diameter D. Upper weld seam  22  runs from the cooling channel ceiling to the piston crown  12  and parallel to piston center axis M, and encloses an acute angle β with lower weld seam  21 . 
     Lower weld seam  21  and upper weld seam  22  are produced by beam welding and are disposed in such a manner that they are accessible to a tool for beam welding. During beam welding, excess weld material enters cooling channel  19 , for example in the form of weld splashes, and usually collects, for example in the form of weld beads, in a region of cooling channel  19  that lies opposite weld seams  21 ,  22 . 
       FIG. 2  shows another embodiment of a piston  110  according to the invention. Piston  110  has a component  111  configured as a piston base body, which can consist of a material such as that described for component  11  according to  FIG. 1 , for example. Component  111  has a bottom  115   a  of a combustion bowl  115 . Component  111  furthermore forms piston skirt  117  of piston  110  according to the invention, in known manner. 
     Piston  110  according to the invention furthermore has a component  118  that forms the entire bowl wall  115   b  as well as bowl edge region  115   c  of combustion bowl  115 , and furthermore piston crown  112 , top land  113 , and ring belt  114 , in the embodiment shown. Component  118  preferably consists of a particularly high-strength material, such as that described for component  18  according to  FIG. 1 . 
     Components  111 ,  118  form a circumferential outer cooling channel  119 . Cooling channel  119  runs at the level of the ring belt  114 , and at the level of bowl wall  115   b  of combustion bowl  115 . 
     Component  118  has an inner circumferential joining surface that forms an inner weld seam  121  with a circumferential joining surface on component  111 , which surface encloses bottom  115   a  of combustion bowl  115 . Inner weld seam  121  has a length of 3.5% to 5.5% of piston diameter D, and encloses an acute angle with the piston center axis M. Inner weld seam  121  therefore runs radially toward the outside, proceeding from bowl wall  115   b,  and downward (in the direction of the piston skirt  117 ), and ends in cooling channel  119 , in a region of the cooling channel bottom. 
     Component  118  furthermore has an outer circumferential joining surface that forms an outer weld seam  122  with a circumferential joining surface  111  below ring belt  114 . 
     Inner weld seam  121  and outer weld seam  122  are produced by beam welding and are disposed in such a manner that they are accessible to a tool for beam welding. During beam welding, excess weld material enters cooling channel  119 , for example in the form of weld splashes, and preferentially collects, for example in the form of weld beads, in a region of cooling channel  119  that lies opposite weld seams  121 ,  122 . 
       FIG. 3  shows another exemplary embodiment of a piston  210  according to the invention. Piston  210  has a component  211  configured as a piston base body, which is produced from a material such as that described for component  11  according to  FIG. 1 , for example. Component  211  has a part of piston crown  212  as well as a combustion bowl  215 . Component  211  furthermore forms piston skirt  217  of piston  210  according to the invention, in known manner. 
     The piston according to the invention furthermore has a component  218 , configured in ring shape, that forms part of piston crown  212 , a circumferential top land  213 , as well as a circumferential ring belt  214  having ring grooves for accommodating piston rings (not shown). Component  218  preferably consists of a particularly high-strength material, such as that already described for component  18 . 
     Components  211 ,  218  form a circumferential outer cooling channel  219 . Cooling channel  219  runs at the level of ring belt  214 , on the one hand, and at the level of the bowl wall of combustion bowl  215 , on the other hand. 
     Component  218  has a lower circumferential joining surface below ring belt  214 , that forms a lower weld seam  221  with a lower circumferential joining surface on component  211 . Component  218  furthermore has an upper circumferential joining surface in the region of top land  213 , which surface forms an upper weld seam  222  with an upper circumferential joining surface in the region of the combustion bowl  215  on component  211 . Upper weld seam  222  runs from the cooling channel ceiling to piston crown  212 , as well as parallel to piston center axis M. 
     Lower weld seam  221  and upper weld seam  222  are produced by beam welding and are disposed in such a manner that they are accessible to a tool for beam welding. During beam welding, excess weld material enters cooling channel  219 , for example in the form of weld splashes, and preferentially collects, for example in the form of weld beads, in a region of cooling channel  219  that lies opposite weld seams  221 ,  222 . 
     An exemplary embodiment of the method according to the invention, for production of a piston according to the invention, for example piston  10 ,  110 ,  210 , will be described in greater detail in the following, using a piston  10  according to  FIG. 1  as well as using  FIGS. 4 and 5 . Of course, the method described in the following applies analogously for the production of the pistons  110 ,  210  according to  FIGS. 2 and 3 , respectively. 
     First, components  11 ,  18  to be connected are pre-worked. In particular, circumferential joining surfaces  23   a,    23   b  of component  11  as well as corresponding circumferential joining surfaces  24   a,    24   b  of component  18 , the regions of cooling channel  19  (see  FIG. 5 ), piston crown  12 , and the outer contour are pre-lathed. If necessary, a one-pass can be lathed in, in order to securely fix in place components  11 ,  18  that are to be connected, against one another. Making available cleanly lathed joining surfaces  23   a,    23   b;    24   a,    24   b  as well as inner and outer contours serves to prepare for weld seams  21 ,  22  (see  FIG. 5 ), in order to obtain a firm and reliable weld connection. Furthermore, joining surfaces  23   a,    23   b;    24   a,    24   b  should be cleaned and degreased, for example with acetone. 
     In the embodiments shown in  FIGS. 1 to 3 , covering medium  25  provided according to the invention is applied in the region of cooling channel  19 , because the joining surfaces  23   a,    23   b ;  24   a,    24   b  are positioned in such a manner that weld beads  26  enter into the region of cooling channel  19  during the welding process (see  FIG. 5 ). Covering medium  25  should be applied so that it is removed from each edge of joining surfaces  23   a,    23   b;    24   a,    24   b  at a distance of at least 1 mm, so that it is not damaged during the later welding process, and that the quality, particularly the strength, of weld seams  21 ,  22  (see  FIG. 5 ) is not impaired. Covering medium  25  can be applied in thickened form in those regions that lie opposite joining surfaces  23   a,    23   b;    24   a,    24   b , because the most weld beads impact in these regions during the subsequent welding process. 
     The components to be connected can be preheated to 50° C. to 80° C., in advance, in order to achieve good adhesion of covering medium  25  on the components. 
     For the production of the covering medium provided according to the invention, 50 g to 100 g bentonite as well as 5 g to 10 g borax (Na2[B4O5(OH)4)]×8 H2O) are dissolved in 100 ml hot water and stirred intensively for about 10 min. The resulting aqueous suspension is applied to the components to be coated, in the region of cooling channel  19 , as a closed layer having a layer thickness of 100 μm, by means of a conventional paint spray gun. The resulting coating is subsequently dried at room temperature. Joining surfaces  23   a,    23   b;    24   a,    24   b  are not coated. 
     After application of covering medium  25 , component  18  is shrunk-fit onto component  11  in known manner, in that component  11  is heated to 180° C. to 200° C., component  18  is set on, and component  11  is subsequently cooled. Shrink-fitting should take place without a gap, as much as possible, in other words joining surfaces  23   a,    23   b;    24   a,    24   b  should lie firm and flat on one another, so that during the later welding process, smooth, firm weld seams  21 ,  22  are obtained. In addition, components  11 ,  18  to be connected can be tacked together along their joining surfaces  23   a,    23   b;    24   a,    24   b,  at points or circumferentially, at a low welding depth. 
     Components  11 ,  18  are connected by means of laser welding, using at least one commercially available CO 2  laser  27   a,    27   b . For this purpose, the components are heated, in advance, to 400° C. to 550° C. In this connection, the borax contained in covering medium  25  loses its water of crystallization and makes a transition into the anhydrous form Na 2 B 4 O 7 . Aside from this, covering medium  25  remains stable at these temperatures. 
     When using a CO 2  laser  27   a,    27   b,  particularly few weld beads  26  occur. Of course, other lasers, such as solid body lasers, are also suitable. Components  11 ,  18  can also be connected with one another by electron beam welding. The required power of the welding tool is dependent on the materials used for components  11 ,  18  and the length of weld seams  21 ,  22  to be formed. The required parameters can be set in known manner by a person skilled in the art. No additional welding material is required. 
     Joining surfaces  23   a,    23   b;    24   a,    24   b  should be laid in such a manner that weld seams  21 ,  22  in finished piston  10  are disposed in those regions in which as little stress as possible occurs during engine operation, in order to reduce the risk of crack formation in the region of weld seams  21 ,  22 . Of course, joining surfaces  23   a,    23   b;    24   a,    24   b  must also be laid in such a manner that they are accessible for the weld beams, which are the laser beams  28   a,    28   b.  The position of joining surfaces  23   a,    23   b;    24   a ,  24   b  therefore generally represents a compromise between the stability of the finished piston  10  and the requirements of the production method. Slanted joining surfaces  23   a,    24   a  and weld seams  21 , respectively, automatically center components  11 ,  18  relative to one another, in known manner. 
     Corresponding deliberations apply analogously, of course, also for pistons  110 ,  210  according to  FIGS. 2 and 3 , respectively. 
     In the exemplary embodiment, component  18  was laser-welded to component  11  by means two CO 2  lasers  27   a,    27   b,  using two butt seams  21 ,  22 . 
     After the welding process, covering medium  25 , together with the weld beads adhering to it, is removed from the resulting piston blank  10 ′. For this purpose, cooling channel  19  is washed with warm water. In this connection, the anhydrous disodium tetraborate Na 2 B 4 O 7  dissolves in the water, so that the bentonite is slurried up again and washed out together with the weld beads that might be present. Washing is continued until only clear water exits from piston blank  10 ′. 
     Subsequently, piston blank  10 ′ is dried and immediately protected against corrosion. It is subsequently recommended to inspect cooling channel  19  by means of an endoscope, to check for complete removal of the weld beads. 
     The piston blank is finally machined, in known manner, to produce finished piston  10 ,  110 ,  210 . This includes, depending on the materials used, a heat post-treatment known to a person skilled in the art. Thus, a piston is achieved that lacks any excess weld material. 
     Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.