# Patent

## Title
Piston for an internal combustion engine and method for its production

## 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.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
Applicants 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
1. Field of the Invention
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.
2. The Prior Art
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.
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
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.
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.
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.
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.
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.
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.
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
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; and
FIG. 2 shows another embodiment of a piston according to the invention, in section.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
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%.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A circumferential recess 133a provided in upper piston part 112, between ring element 124 and combustion bowl 115, and a corresponding circumferential recess 113b 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.
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.
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.
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.

## Claims

1. A method for the production of a piston for an internal combustion engine, having a lower piston part and an upper piston part disposed on the lower piston part, the upper piston part having a circumferential top land, and a circumferential ring belt, the method comprising the following steps:
producing the upper piston part by means of pressing and sintering;
producing the lower piston part by means of pressing and sintering or casting or recasting or forging, and
subsequently joining the lower piston part and the upper piston part together with a solder material, wherein the piston parts produced by sintering are infiltrated with a metallic material whose melting point is lower than a melting point of the solder material.

2. The method according to claim 1, wherein the lower piston part consists of a forged or cast steel material, and the upper piston part consists of a sintered steel material.

3. The method according to claim 1, wherein the solder material is copper, a copper alloy, nickel or a nickel alloy.

4. The method according to claim 1, wherein the piston parts produced by sintering are infiltrated with the solder material.

5. A method for the production of a piston for an internal combustion engine, having a lower piston part and an upper piston part disposed on the lower piston part, the upper piston part having a circumferential top land, and a circumferential ring belt, the method comprising the following steps:
producing the upper piston part by means of pressing and sintering;
producing the lower piston part by means of pressing and sintering or casting or recasting, and
joining the lower piston part and the upper piston part together,
wherein the steps of producing by sintering, and joining of lower piston part and upper piston part are carried out at the same time.