# Patent

## Title
Method for the production of a piston for an internal combustion engine

## Abstract

A method for the production of a piston (1) made of steel, for an internal combustion engine, in which the upper piston part (3) is produced using the forging method, and the lower piston part (4) is produced using the forging or casting method, and they are subsequently welded to one another. To simplify the production method and make it cheaper, the upper piston part is forged using the method of semi-hot forming, to finish it to such an extent that further processing of the combustion bowl and of the upper cooling channel regions can be eliminated.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
Applicants claim priority under 35 U.S.C. §119 of German Application No. 10 2011 013 067.5 filed on Mar. 4, 2011, the disclosure of which is incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for the production of a piston for an internal combustion engine, in accordance with the preamble of claim 1.
2. The Prior Art
From the state of the art, it is generally known to produce pistons from steel for an internal combustion engine, in that first an upper piston part is produced using the forging method, and a lower piston part is produced using the forging method or by means of casting, and then the upper piston part is welded to the lower piston part. In this regard, reference should be made to the patent documents DE 195 01 416 A1, DE-OS 29 19 638, DE 196 03 589 A1, and DE 198 46 152 A1. In this connection, the method of hot forming, in other words hot forging, at a steel temperature of 950° C. to 1300° C., is used.
This method has the disadvantages that a great expenditure of energy is required for heating the forged blank. Furthermore, an uncontrollable oxide layer forms on the surface of the forged blank, and in order to remove it, the surface of the forged blank must be blasted with coarse blasting material. This results in great variations in the forged contour, so that as a consequence of this, complicated reworking of the forged blank, by means of a chip-cutting processing method, is required.
SUMMARY OF THE INVENTION
Accordingly, it is the task of the present invention to avoid the aforementioned disadvantages of the state of the art, whereby in particular, complicated reworking of the combustion bowl and of the cooling channel is supposed to be avoided.
It is furthermore the task of the present invention to indicate a method with which pistons having combustion chamber bowls and cooling channels that are not configured with rotation symmetry or in centered manner can be produced in cost-advantageous manner.
Finally, it is the task of the present invention to indicate a method with which pistons can be produced, in which the wall between the edge of the combustion bowl and the upper part of the cooling channel has a constant thickness over the circumference.
These tasks are accomplished with the characteristics that stand in the characterizing part of the main claim. Advantageous embodiments of the invention are the object of the dependent claims.
Because the upper piston part is produced using the method of semi-hot forming, the upper piston part can be produced with greater measurement accuracy and improved surface quality, thereby eliminating complicated reworking of the forged blank, particularly in the region of the combustion bowl and the upper cooling channel. In this connection, because of the low forming temperature, the scale formation on the surface of the piston blank is clearly reduced, so that a blasting method that is gentle on the surface can be used, or it is actually possible to do without blasting entirely. Furthermore, a material having a lower heat resistance but a greater strength and hardness can be used for the forging die. As a result, deeper contours can be produced, as required for the cooling channel. Finally, in this connection, a lower expenditure of energy is required for heating the forged blank than in the case of hot forging.



BRIEF DESCRIPTION OF THE DRAWINGS
Some exemplary embodiments of the invention will be explained in the following, using the drawings. These show:
FIG. 1 a sectional diagram of a piston produced according to the method according to the invention, in a section plane that lies perpendicular to the pin bore axis,
FIG. 2 a section through the piston, in a section plane that lies on the pin bore axis,
FIG. 3 a section through the upper piston part after semi-hot forming,
FIG. 4 a section through the upper piston part after over-lathing of the outer contour and of the contact regions intended for friction welding,
FIG. 5 a top view of a configuration of the upper piston part having an asymmetrically configured and eccentrically disposed combustion bowl,
FIG. 6 a section through the upper piston part along the line VI-VI in FIG. 5,
FIG. 7 the upper piston part and the lower piston part before joining by means of friction welding,
FIG. 8 the top view of an embodiment of the upper piston part having an asymmetrically configured and eccentrically disposed combustion bowl and having a valve niche, and
FIG. 9 a section through the upper piston part along the line IX-IX in FIG. 8.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an embodiment of a piston 1 produced according to the method according to the invention, in section, perpendicular to the pin axis 2, consisting of an upper piston part 3 and a lower piston part 4, which are connected with one another by way of a friction-welding seam 5.
The piston 1 has a piston crown 6 into which a combustion bowl 7 is formed. Radially on the outside, a ring wall 8 directed downward, having a ring belt 9 for piston rings not shown in the figure, is formed onto the piston crown 6. Radially within the ring wall 8, the piston 1 has a ring-shaped support 10 formed onto the underside of the piston crown 6.
The lower piston part 4 consists of two skirt elements 11 and 12 that lie opposite one another, which are connected with one another by way of two pin bosses 13 and 14 that lie opposite one another, each having a pin bore 15 and 16. In FIG. 1, only the pin boss 13 having the pin bore 15 can be seen, because of the position of the section plane.
A ring-shaped contact part 17 connected with the pin bosses 13, 14 is disposed on the top of the lower piston part 4. Furthermore, the lower piston part 4 has a circumferential ring rib 18 on its top, which rib is disposed radially outside of the contact part 17 and connected with the skirt elements 11, 12. A radially oriented ring element 19 extends between the contact part 17 and the ring rib 18.
In this connection, the support 10 and the contact part 17 are disposed in such a manner that the underside of the support 10 and the top of the contact part 17 have contact with one another and form a first contact region 20. Furthermore, the ring wall 8 and the ring rib 18 are disposed in such a manner that the lower face side of the ring wall 8 and the top of the ring rib 18 also have contact with one another and form a second contact region 21. The first and the second contact region 20 and 21 form friction-welding surfaces during the production of the piston 1.
In this way, the result is achieved that a circumferential cooling channel 22 disposed close to the piston crown 6, radially on the outside, is delimited, at the top, by the piston crown 6, radially on the inside partly by the piston crown 6, partly by the support 10, and partly by the contact part 17, at the bottom by the ring element 19, and radially on the outside partly by the ring wall 8 and partly by the ring rib 18. The cooling channel 22 has an inflow opening for introduction of cooling oil and an outflow opening for discharge of cooling oil, but these are not shown in the figure.
In FIG. 2, the piston 1 is shown in section along the pin bore axis 2. Here, the two pin bosses 13, 14 can be seen, with the contact part 17 formed onto them, as can the ring element 19 that is connected with the contact part 17 and the pin bosses 13, 14, respectively.
The piston 1 is produced from tempered steel, such as chromium steel 42CrMo4, for example. In this connection, production of the lower piston part 4 takes place in conventional manner, by means of casting or hot forging.
The upper piston part 3 is produced by means of the method of semi-hot forming, thereby giving the upper piston part 3 a high surface quality and, in particular, making it possible for the part to be produced with great dimensional accuracy, particularly in the regions of the combustion bowl 7 and the upper cooling channel 22 and in the inner mandrel region 29.
In this connection, a piece of chromium steel that has been shaped to fit the die of the drop-forging machine intended for the upper piston part 3 is heated to 600° C. to 900° C., and subsequently formed in multiple forming steps, in other words forging processes, in the same drop-forging machine. The slight scale that forms during forging is removed by means of fine blasting, for example with walnut granulate. Subsequently, the blank of the upper piston part 3 that results from this is tempered in accordance with the material requirements. This means that the blank is heated to approximately 800° C. to 900° C., quenched, and then annealed at approximately 550° C. to 650° C. In order to avoid scale formation, tempering takes place under an inert gas atmosphere. The blank of the upper piston part 3 that results from this is shown in FIG. 3. In this connection, the combustion bowl 7, the upper cooling channel region, and the inner mandrel region 29 are already formed in their final form, so that no further processing steps are any longer required in these regions. In this connection, the result is also achieved that the wall thickness between the bowl edge and the upper cooling channel region is almost constant over the circumference. The upper piston part 3 as it looks after finishing is shown in FIG. 3 with broken lines.
In the subsequent method step, the radially outer region 23 of the piston crown 6, the radially outer region 24 of the upper piston part 3 intended for the ring belt 9, the lower face surface 25 of the ring wall 8, the lower region 26 of the inner surface 27 of the ring wall 8, and the contact surface 28 of the support 10 are machined by means of lathing, so that the upper piston part 3 as shown in FIG. 4 is obtained. The lower region of the cooling channel 22, the lower face surface 25 of the ring wall 8, and the contact surface 28 of the support 10 are formed in finished form after this latter method step. Here again, the upper piston part 3, as it looks after finishing, is shown with broken lines.
The production method of semi-hot forming particularly allows production of upper piston parts 3′ having combustion bowls 7′ that are configured asymmetrically and disposed eccentrically, as shown in FIGS. 5 and 6. Here, again, no further processing of the combustion bowl 7′ is required any longer, once the process of semi-hot forming for production of the upper piston part 3′ has been completed.
Alternatively to this, the upper piston part can also be produced by means of a fine-casting method. In order to avoid scale formation, this should be done under an inert gas atmosphere.
In the present exemplary embodiment according to FIGS. 5 and 6, the combustion bowl 7′ has approximately the shape of a four-leafed clover. However, any desired shape of a combustion bowl can be implemented with the method of semi-hot forming.
FIGS. 8 and 9 show the upper piston part according to FIGS. 5 and 6, whereby in addition, a valve niche 30 has been formed into the piston crown 6 of the upper piston part 3′′.
The upper piston part 3, 3′, 3′′ according to FIG. 4, 5, 6, 8, 9 is braced into a friction-welding device (not shown in the figure) together with the lower piston part 4, and, as shown in FIG. 7, they are brought into position, relative to one another, so that they can be put into rotation, moved toward one another with force, and friction-welded to one another when the upper piston part 3, 3′, 3′′ makes contact with the lower piston part 4 in the region of the contact regions 20 and 21. If the combustion bowl 7′ is configured asymmetrically or eccentrically, care must be taken during friction welding to ensure that after completion of the welding process, the combustion bowl 7′ assumes a clearly defined rotation position relative to the pin axis 2, for example.
In this connection, the piston 1 shown in FIGS. 1 and 2 is obtained.
Within the scope of the last method step, the grooves of the ring belt 9 are lathed into the outer piston wall and the piston crown 6 is lathed flat, as indicated in FIGS. 3 and 4. Furthermore, the precision piston contour and the pin bores are worked in.
REFERENCE SYMBOL LIST


    1 piston
    2 pin axis
    3, 3′, 3′′ upper piston part
    4 lower piston part
    5 friction-welding seam
    6 piston crown
    7, 7′ combustion bowl
    8 ring wall
    9 ring belt
    10 support
    11, 12 switch element
    13, 14 pin boss
    15, 16 pin bore
    17 contact part
    18 ring rib
    19 ring element
    20 first contact region
    21 second contact region
    22 cooling channel
    23 outer region of piston crown 6
    24 outer region of upper piston part
    25 lower face surface of ring wall 8
    26 lower region of inner surface 27 of ring wall 8
    27 inner surface of ring wall 8
    28 contact surface of support 10
    29 inner mandrel region
    30 valve niche

## Claims

1. A method for a production of a piston for an internal combustion engine, comprising the following steps:
forging an upper piston part made of tempered steel, which wherein the upper piston part comprises:
a piston crown having a combustion bowl,
a ring wall formed onto the piston crown radially on the outside, directed downward, and
a ring-shaped support disposed radially within the ring wall formed onto an underside of the piston crown,

forming an upper part of a cooling channel between the ring wall and the ring-shaped support,
producing a lower piston part made of steel, using a forging or casting method, wherein said lower piston part comprises:
two skirt elements that lie opposite one another, which are connected with one another by way of two pin bosses that lie opposite one another,
a ring-shaped contact part disposed on the top of the lower piston part and connected with at least one of said at least two pin bosses, and a circumferential ring rib disposed radially outside of the ring-shaped contact part and connected with the two skirt elements, whereby the lower part of the cooling channel is formed between the contact part and the ring rib,

welding of the upper piston part to the lower piston part by way of contact surfaces that enter into contact with one another, of the ring wall and the ring rib, and of the support and the contact part, whereby the cooling channel formed by the upper piston part and by the lower piston part is closed,
finishing the piston using a chip-cutting production method,
wherein for production of the upper piston part, an upper piston part blank is forged using a semihot-forming method, at 600° C. to 900° C., after which the combustion bowl or the upper part of the cooling channel undergo no further processing, and after which the radially outer region of the piston crown, the radially outer region of the ring wall, the lower region of the inner surface of the ring wall, and the contact surface of the support of the upper piston blank are finished to produce the upper piston part.

2. The method for the production of a piston for an internal combustion engine according to claim 1, wherein the upper piston part is forged with a constant thickness in the region of the piston crown between a bowl edge of the combustion bowl and the cooling channel, over its circumference.

3. The method for the production of a piston for an internal combustion engine according to claim 1, wherein in said step of semi-hot forging, the upper piston part blank is tempered in an inert gas atmosphere.

4. The method for the production of a piston for an internal combustion engine according to claim 1, wherein an asymmetrically configured and eccentrically disposed combustion bowl is formed into the upper piston part.

5. The method for the production of a piston for an internal combustion engine according to claim 1, further comprising the step of forming at least one valve niche into the upper piston part.