Piston for an internal combustion engine and method for its production

A piston for an internal combustion engine has a piston head that has a circumferential cooling channel as well as a combustion bowl having a circumferential bowl wall that makes a transition into a piston crown by way of a bowl edge region. The combustion bowl is formed at least in part by a piston base body and the bowl wall is formed at least in part from an insert. The insert is connected with the piston base body by means of beam welding. A lower weld seam is configured in the bowl wall, which seam encloses an acute angle with the piston center axis (M) and ends in the lower half of the cooling channel. An upper weld seam runs from the cooling channel ceiling to the piston crown and is disposed centered or radially offset toward the outside, with reference to the clear width of the cooling channel.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of German Application No. 10 2011 107 655.0 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 piston for an internal combustion engine, having a piston head that has a circumferential cooling channel as well as a combustion bowl having a circumferential bowl wall that makes a transition into a piston crown by way of a bowl edge region. The combustion bowl is formed at least in part by a piston base body, and the bowl wall is formed at least in part from an insert. The present invention furthermore relates to a method for the production of such a piston.

2. The Prior Art

A piston of this type is described in German Patent Application No. DE 30 32 671 A1. It has a piston base body made of a flow-pressed steel and an insert welded or soldered to the piston base body, made of a highly heat-resistant material. The piston base body is provided with a combustion bowl in the region of the piston head, which bowl is delimited by a circumferential bowl wall that makes a transition into the piston crown by way of the bowl edge region. The piston base body and the insert form a circumferential cooling channel.

The bowl wall and, in particular, the bowl edge region are subject to particularly great mechanical and thermal stresses during operation, which stresses lead to material fatigue over the course of time, which in turn can cause cracks to form. For this reason, inserts structured in numerous ways have been proposed, in order to reduce or neutralize the stresses that act on the bowl edge region. Because of the further development of engine technology, these suggestions are no longer able to keep pace with the increased mechanical and thermal stresses on pistons during engine operation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a piston of this type and a method for its production, in such a manner that it withstands the increased mechanical and thermal stresses.

This object is accomplished in that an insert is connected with the piston base body by means of beam welding, and a lower weld seam is configured in the bowl wall. This seam encloses an acute angle with the piston center axis and ends in the lower half of the cooling channel. An upper weld seam runs from the cooling channel ceiling to the piston crown and is disposed centered or radially offset toward the outside, with reference to the clear width of the cooling channel.

The method according to the invention has the following method steps: a) pre-working the piston base body and the insert, at least in the region of the joining surfaces; b) assembling piston base body and insert; c) connecting piston base body and insert along their corresponding joining surfaces, by means of beam welding; d) machining the piston to finish it.

The piston according to the invention withstands great thermal and mechanical stresses very well, because the connection of piston base body and insert by means of beam welding is particularly strong and resistant. Furthermore, the weld seams are disposed in those regions of the piston head in which only slight stresses occur in the material, on the basis of the thermal and mechanical stresses during engine operation, so that the risk of crack formation in the region of the weld seams is minimized. The location of the joining surfaces of piston base body and insert is furthermore selected in such a manner that they are easily accessible for beam welding, so that the location of the resulting weld seams is optimized.

Advantageous further developments are evident from the dependent claims.

Particularly preferably, the piston base body and the insert are connected with one another by means of laser welding, in order to obtain a particularly strong and reliable connection between these components.

The lower weld seam and the upper weld seam preferably enclose an acute angle, so that the corresponding joining surfaces are particularly easily accessible before piston base body and insert are connected.

In one embodiment, the lower weld seam ends in the cooling channel in the region of the cooling channel bottom. In this region, the lowest stresses in the material occur during engine operation, so that the risk of crack formation in the region of the lower weld seam is minimized.

The lower weld seam preferably has a length of 3.5% to 5.5% of the piston diameter. The upper weld seam preferably runs parallel to the piston center axis, and preferably has a length of 4.5% to 6.0% of the piston diameter.

In a preferred embodiment of the present invention, the thickness of the piston crown increases radially toward the outside, above the cooling channel. As a result, heat dispersion by way of the first piston ring is promoted.

In one embodiment which is particularly effective, the insert forms the entire bowl wall. The piston base body and the insert preferably consist of different materials. Then, identical piston base bodies can be processed for use for different piston types for a great number of different internal combustion engines, simply by the selection of the suitable material for the insert.

It is practical if the insert consists of a steel that is resistant to high elevated temperatures, corrosion-resistant, and heat-resistant, particularly of a valve steel. In contrast to this, the piston base body can consist of an annealed steel, for example.

In one embodiment of the method, the joining surfaces of piston base body and insert are cleaned and smoothed in step a), in order to obtain particularly strong and reliable weld seams. Subsequent to this, in step b), the piston base body and the insert can be assembled by a press fit and/or by tacking, in order to fix the components in place, relative to one another, in a particularly reliable manner during the actual welding process.

The piston base body and the insert are preferably connected with one another by electron beam welding, particularly preferably by laser welding. Before the welding process, the piston base body and/or the insert can be preheated to 400° C. to 550° C., in order to reduce the risk of stresses in the material due to thermal stress during the welding process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings,FIGS. 1 and 2show a first embodiment of a piston10according to the invention. Piston10has a piston base body11, which is produced, for example, from an annealed steel such as 42CrMo4, for example, or an AFP steel or a bainitic AFP steel doped with 0.4 wt.-% molybdenum. The piston base body11has a part of a piston crown12, a circumferential top land13, as well as a circumferential ring belt14having ring grooves for accommodating piston rings (not shown). Piston base body11furthermore has the bottom15aof a combustion bowl15. Piston base body11thus forms an essential part of piston head16of piston10. Piston base body11furthermore forms piston skirt17of piston10according to the invention, in known manner. The compression height KH of piston10, defined as the ratio of the height H of the piston10(measured from the center of the pin bore to the piston crown) to diameter D of the piston10, amounts to 38% to 45% in this embodiment.

The piston according to the invention furthermore has an insert18that forms the entire bowl wall15bas well as the bowl edge region15cof combustion bowl15, and furthermore part of the piston crown12, in the embodiment shown. Insert18preferably consists of a particularly high-strength material. For this purpose, a steel that is resistant to high elevated temperatures, corrosion-resistant, and heat-resistant is particularly suitable. Valve steels such as, for example, CrSi steel (X45CrSi93), Chromo193 steel (X85CrMoV182), 21-4 N steel (X53CrMnNiN219), 21-2 steel (X55CrMnNiN208), Nimonic80A steel (NiCr20TiA1), ResisTEL steel, or VMS-513 steel, are particularly suitable.

Piston base body11and insert18form a circumferential outer cooling channel19. Cooling channel19runs at the level of the ring belt14, and at the level of the bowl wall15bof the combustion bowl15. Cooling channel19has a cooling channel bottom19a, a cooling channel ceiling19b, an outer side wall19c, and an inner side wall19d. Cooling channel bottom19aand outer side wall19care formed by piston base body11in this exemplary embodiment. Inner side wall19dis formed by insert18, while cooling channel ceiling19bis formed jointly by piston base body11and by insert18.

Insert18has a lower circumferential joining surface that forms a lower weld seam21with a circumferential joining surface on piston base body11that encloses bottom15aof the combustion bowl15. Lower weld seam21has a length of 3.5% to 5.5% of piston diameter D, and encloses an acute angle α with a piston center axis M. The lower weld seam21therefore runs radially toward the outside, proceeding from bowl wall15b, and downward (in the direction of the piston skirt17), and ends in cooling channel19, in the region of cooling channel bottom19a. In this region of piston10, the lowest stresses in the material occur during engine operation, so that the risk of crack formation in the region of weld seam21is minimized.

Insert18furthermore has an upper circumferential joining surface that forms an upper weld seam22with a circumferential joining surface on piston base body11, in the region of top land13. Upper weld seam22has a length of 4.5% to 6.0% of the piston diameter D. Upper weld seam22runs from the cooling channel ceiling19bto piston crown12and parallel to piston center axis M, and encloses an acute angle β with lower weld seam21. Upper weld seam22is disposed centered with reference to the clear width W of cooling channel19(seeFIG. 3). According to the invention, upper weld seam22is therefore disposed far enough away from bowl edge region15cso that the risk of crack formation due to thermal and mechanical stresses in the region of weld seam22is reduced. In this region of piston crown12, in which upper weld seam22is disposed, comparatively low stresses in the material occur during engine operation.

FIG. 3, in a detail view, shows another embodiment of a piston110according to the invention. Piston110differs from piston10according toFIGS. 1 and 2only in that the thickness of piston crown112increases radially toward the outside in the region of cooling channel119. The cooling channel ceiling119bof cooling channel119therefore extends radially outward and downward (in the direction of the piston skirt17).

FIG. 4, in a detail view comparable toFIG. 3, shows another embodiment of a piston210according to the invention. Piston210differs from piston10according toFIGS. 1 and 2only in that upper weld seam222is disposed in the region of top land13, and runs at a slant, namely proceeding from the outer edge of piston crown12radially toward the inside and downward (in the direction of the piston skirt17). Upper weld seam222ends in cooling channel19, between cooling channel ceiling19band outer side wall19c. Upper weld seam222and lower weld seam21enclose an acute angle γ.

Piston base body11and insert18are connected with one another in known manner, by means of beam welding, particularly preferably by means of laser welding. Typically, piston base body11and insert18are pre-worked in known manner. In particular, the joining surfaces are cleaned and smoothed. Then, piston base body11and insert18are joined together, for example by means of press fit. In a next step, weld seams21,22,222can be tacked, at certain points or circumferentially at a low welding depth. Then piston10,110,210is heated to a temperature of 400° C. to 550° C. Thereupon weld seams21,22,222are welded through. Finally, piston10,110,210is finished, by machining it after it has cooled, in known manner.