Abstract:
A piston for an internal combustion engine has a lower piston part and an upper piston part disposed on the lower piston part. The upper piston part has a top land that runs around its circumference, and a ring belt that runs around its circumference. At least the upper piston part consists of a sintered material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional under 35 U.S.C. §120 of U.S. patent application Ser. No. 12/315,968, filed on Dec. 8, 2008. Applicants also claim priority under 35 U.S.C. §119 of German Application No. 10 2007 061 601.7 filed Dec. 20, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a piston for an internal combustion engine, having a lower piston part and an upper piston part disposed on the lower piston part, which has a top land that runs around its circumference, and a ring belt that runs around its circumference. 
         [0004]    2. The Prior Art 
         [0005]    German Patent Application No. DE 103 40 292 A1 describes a piston having an essentially cylindrical basic body that has a ring element in the radially outer region of the piston crown, which element forms a cooling channel together with the basic body. The ring element accommodates a ring insert for a compression ring. 
         [0006]    Because of the many different demands on pistons for modern internal combustion engines, new production methods are sought, with which pistons having a variable structure, and which are adapted as well as possible to the requirements in engine operation, can be obtained with the least possible effort. 
       SUMMARY OF THE INVENTION 
       [0007]    The solution consists in a piston according to the invention, in which at least the upper piston part consists of a sintered material. In the method according to the invention, at least the upper piston part is produced by means of pressing and sintering, the lower piston part is produced by means of pressing and sintering or casting or recasting, and the lower piston part and the upper piston part are joined together by means of a solder material. 
         [0008]    Therefore, with the piston according to the invention, the screw connection between the upper piston part and lower piston part is eliminated. The configuration of at least the upper piston part as a sintered component makes it possible to make the structures and properties of the piston according to the invention, such as weight, construction height, cooling, etc., for example, significantly more variable than before. In particular, powdered sintered materials having a composition that can be chosen as desired can be used which are pressed to produce a molded part and then sintered to produce the finished upper piston part, or to produce the finished upper piston parts and lower piston parts. In this manner, extremely varied microstructure structures can be implemented, in a particularly simple manner, for example from ferritic to austenitic states and mixtures of them (duplex). The method according to the invention is furthermore characterized by particular economic efficiency. 
         [0009]    In a preferred embodiment, the upper piston part is produced from a forged or cast material, particularly a steel material, while the lower piston part is preferably produced from a sintered steel material. Such materials have particularly great thermal resistance, which is particularly advantageous for use in diesel engines. The sintered material of the upper piston part and, if applicable, a sintered lower piston part, can be infiltrated with a metallic material in order to increase its heat conductivity. In this way, heat conduction out of the piston is improved, and the component temperature is lowered. 
         [0010]    A particularly preferred further development provides that the lower piston part and the upper piston part are connected with one another by a solder material. In this connection, the solder material penetrates both into the interstices between the lower piston part and the upper piston part, and into the pores, at least of the sintered upper piston part, by means of the capillary effect. In this way, a particularly strong connection, able to withstand great mechanical stress, is produced between the lower piston part and the upper piston part. Particularly suitable solder materials are, for example, copper, copper alloys, nickel, or nickel alloys. To optimize the connection between lower piston part and upper piston part, inner and outer joining surfaces that correspond to one another are preferably provided. It is practical if the solder material is provided in the region of the joining surfaces. 
         [0011]    In a particularly practical manner, the sintered material used in an individual case can be infiltrated with the solder material. In this connection, sintering of the sintered material and joining of lower piston part and upper piston part can take place in a single production step. It can be practical, particularly in the case of different capillary effects of the pores of the sintered material, on the one hand, and the interstices between lower piston part and upper piston part, on the other hand, to use a metallic material whose melting temperature is lower than the melting temperature of the solder material to infiltrate the sintered material, in order to ensure reliable and complete infiltration of the sintered material. Infiltration of the sintered material and joining of upper piston part and lower piston part then take place at different temperatures during heating. 
         [0012]    The piston crown can be provided with a combustion bowl that is configured as desired, depending on the engine design, in known manner. This combustion bowl can be formed either only by the upper piston part or by both the upper piston part and the lower piston part, depending on the requirements of the individual case. 
         [0013]    To improve the cooling effect, the upper piston part and the lower piston part can enclose an outer circumferential cooling channel. In addition, an inner cooling chamber or an inner circumferential cooling channel can be provided. Conducting heat away then takes place out of the piston, particularly out of the piston crown region, in the direction of the cooling channel or cooling channels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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. 
           [0015]    In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
           [0016]      FIG. 1  shows a first embodiment of a piston according to the invention, in section; and 
           [0017]      FIG. 2  shows another embodiment of a piston according to the invention, in section. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring now in detail to the drawings,  FIG. 1  shows a firs embodiment of a piston  10  according to the invention. Piston  10  has a lower piston part  11 , which is produced from a forged or cast metallic material. For example, forging steels such as AFP steels, for example 38MnVS6, or annealing steels such as 42CrMo4, for example, are suitable. Piston  10  furthermore has an upper piston part  12 , which is produced from a sintered material, particularly a sintered steel material. For example, alloys of iron and carbon or alloys of iron, carbon, and molybdenum are suitable. Using these alloys, it is particularly possible to produce ferritic microstructure structures. The carbon content is preferably 0.4-0.8%, the molybdenum content is preferably 0.0-2.0%, particularly 0.8-1.6%. 
         [0019]    The lower piston part  11  has a piston skirt  20  as well as a central or inner region  13  of a piston crown  14 , which is provided, in known manner, with a combustion bowl  15 . Below piston crown  14 , pin bosses  16  are provided, which are provided with pin bores  17  for allowing a piston pin, not shown, to pass through. 
         [0020]    Upper piston part  12  has a circumferential, essentially cylindrical ring element  24 , which is provided on its mantle surface, in known manner, with a top land  25  and a ring belt  26  having multiple ring grooves for accommodating piston rings, not shown. The lower, free end of ring element  24  forms an outer joining surface  27 , which supports itself on a corresponding joining surface  28  of lower piston part  11 . 
         [0021]    Ring element  24  furthermore has a circumferential edge  29  that extends radially inward, which forms outer ring-shaped region of piston crown  14 . The lower free end of edge  29  is formed by an inner joining surface  31 , which supports itself on a corresponding joining surface  32  of lower piston part  11 . 
         [0022]    Lower piston part  11  and upper piston part  12  are joined together by means of a solder material that is provided along joining surfaces  27 ,  28  or  31 ,  32 , respectively. Copper or copper alloys, or nickel or nickel alloys, are suitable, for example. The melting point of the solder material is lower than the melting point of the material of lower piston part  11  and lower than the melting point of the material of upper piston part  12 . At the same time, the melting point of the solder material is higher than the maximal operating temperature that occurs at piston  10 . 
         [0023]    Ring element  24  as well as circumferential edge  29  of upper piston part  12 , or a circumferential recess  33  made in lower piston part  11 , respectively, form an outer circumferential cooling channel  34 . 
         [0024]      FIG. 2  shows another exemplary embodiment of a piston  110  according to the invention. Piston  110  has a lower piston part  111  that consists of the same material as lower piston part  11  of piston  10  from  FIG. 1 . Piston  110  furthermore has an upper piston part  112  that also consists of the same material as upper piston part  12  of piston  10  from  FIG. 1 . Lower piston part  111  furthermore also has a piston skirt  120  as well as pin bosses  116  provided with pin bores  117 . 
         [0025]    Upper piston part  112  has a piston crown  114  that is provided, in known manner, with a combustion bowl  115 . In this embodiment, combustion bowl  115  is formed solely in the upper piston part  112 . Piston crown  114  is delimited by a circumferential, essentially cylindrical ring element  124 . On its mantle surface, ring element  124  is provided, in known manner, with a top land  125  and a ring belt  126  having multiple ring grooves for accommodating piston rings, not shown. The lower free end of ring element  124  forms a joining surface  127 , which supports itself on a corresponding joining surface  128  of lower piston part  111 . 
         [0026]    Upper piston part  112  has two additional joining surfaces below combustion bowl  115 . For one thing, an inner circumferential joining surface  131  is provided, which supports itself on a corresponding inner circumferential joining surface  132  of lower piston part  11 . Furthermore, a central joining surface  135  is provided, which supports itself on a corresponding joining surface  136  of lower piston part  111 . 
         [0027]    Lower piston part  111  and upper piston part  112  are joined together by means of a solder material that is provided along joining surfaces  127 ,  128  or  131 ,  132 , respectively, as well as  135 ,  136 . For example, copper or copper alloys, or nickel or nickel alloys are suitable. The melting point of the solder material is lower than the melting point of the material of lower piston part  111  and lower than the melting point of the material of upper piston part  112 . At the same time, the melting point of the solder material is higher than the maximal operating temperature that occurs at piston  110 . 
         [0028]    A circumferential recess  133   a  provided in upper piston part  112 , between ring element  124  and combustion bowl  115 , and a corresponding circumferential recess  113   b  provided in lower piston part  111 , respectively, form an outer circumferential cooling channel  134 . Furthermore, an inner circumferential cooling channel  137  is configured between inner circumferential joining surfaces  131 ,  132  and central joining surfaces  135 ,  136 . If joining surfaces  135 ,  136  are omitted, a central cooling chamber (not shown) is formed instead of the inner circumferential cooling channel. 
         [0029]    To assemble piston  10 ,  110  according to the invention, lower piston part  11 ,  111  and upper piston part  12 ,  112  are joined together by means of the solder material, in known manner. For this purpose, the solder material is brought into contact with the joining surfaces and heated, together with lower piston part  11 ,  111  and upper piston part  12 ,  112 , until the solder material melts. In this connection, because of the capillary effect, the solder material penetrates both into the interstices between the joining surfaces, and into the pores of the sintered material of upper piston part  12 ,  112  or the sintered materials of the two parts of piston  10 ,  110 , respectively. In this connection, sintering of at least upper piston part  12 ,  112  and joining of lower piston part  11 ,  111  and upper piston part  12 ,  112  can take place in one and the same production step, for example during the same oven pass. First, the powdered material is pressed into molded parts that have only a low strength. These parts result in upper piston part  12 ,  112  or the two components  10 ,  110 . This pressing precedes the combined sintering and joining process here. This results in a particularly cost-advantageous production method for piston  10 ,  110  according to the invention. 
         [0030]    After cooling, a firm connection between lower piston part  11 ,  111  and upper piston part  12 ,  112  is obtained, which is able to withstand great mechanical stress. 
         [0031]    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.