METHOD FOR PRODUCING A PISTON

Abstract

A method for producing a piston may include forming a piston blank in a first forming tool such that the piston blank surrounds a ring carrier configured to receive a piston ring via positive engagement after producing the ring carrier by a sintering process. The piston blank, at least in a circumferential region disposed at a piston head, may be composed of a light metal alloy suitable for forging. The method may also include removing the piston blank from the first forming tool and placing the piston blank in a second forming tool, and inserting a holding-down tool into the second forming tool to hold the ring carrier down. The method may further include pressing a final forming punch into the second forming tool to deform the piston blank and form a piston.

Claims

1. A method for producing a piston, comprising: forming a piston blank in a first forming tool such that the piston blank surrounds a ring carrier configured to receive a piston ring via positive engagement after producing the ring carrier by a sintering process, wherein the piston blank, at least in a circumferential region disposed at a piston head, is composed of a light metal alloy suitable for forging; removing the piston blank from the first forming tool and placing the piston blank in a second forming tool, and inserting a holding-down tool into the second forming tool to hold the ring carrier down; and pressing a final forming punch into the second forming tool to deform the piston blank and form a piston.

2. The method as claimed in claim 1, wherein forming the piston blank includes: placing the piston blank in a bottom die of the first forming tool such that the piston head rests on an upsetting base of the bottom die, wherein the piston blank has a first outside diameter at the piston head that narrows via a step to a second, smaller outside diameter, and wherein the piston blank is held positively via the first outside diameter in the bottom die; placing the ring carrier on the step, the ring carrier having an outside diameter substantially corresponding to an inside diameter of the bottom die; placing a top die on the bottom die, wherein an inside diameter of the top die is smaller than the inside diameter of the bottom die; upsetting the piston blank via an upsetting punch, wherein upsetting the piston blank includes connecting the ring carrier to the piston blank via infiltration; wherein removing the piston blank from the first forming tool and placing the piston blank in the second forming tool includes resting the piston blank on an inside diameter of the second forming tool that corresponds to the inside diameter of the bottom die of the first forming tool such that the piston blank in a region of the piston head on a final forming base of the second forming tool; the holding-down tool having an inside diameter corresponding to the inside diameter of the top die of the first forming tool; and removing the piston from the second forming tool and finish-machining the piston after pressing the final forming punch into the second forming tool.

3. The method as claimed in claim 1, further comprising heating the ring carrier under a protective gas atmosphere before forming the piston blank in the first forming tool, the ring carrier composed of a corroding material.

4. The method as claimed in claim 1, further comprising heating the ring carrier under a protective gas atmosphere before forming the piston blank in the first forming tool, a heating time of the ring carrier being shorter than a cycle time for forging.

5. The method as claimed in claim 1, wherein the piston blank is composed of one of an aluminum-silicon alloy and an aluminum-copper alloy.

6. The method as claimed in claim 2, wherein: the piston blank is composed of an alloy including two main alloying components; the alloy is one of an aluminum-silicon alloy and an aluminum-copper alloy; and upsetting the piston blank includes upsetting the piston blank while a temperature of the piston blank is no more than 220 K below a solidus temperature of the two main alloying components.

7. The method as claimed in claim 1, wherein the ring carrier is composed of one of an iron alloy and a nickel alloy.

8. The method as claimed in claim 1, further comprising sintering the ring carrier from a sintering powder before forming the piston blank, wherein the sintering power includes more than 28% by volume of particles with a diameter d>150 m.

9. A piston produced according to the method as claimed in claim 1.

10. The piston as claimed in claim 9, wherein a porosity of the ring carrier is between 20% by volume to 80% by volume.

11. The piston as claimed in claim 9, wherein the piston is composed of one of an aluminum-silicon alloy and an aluminum-copper alloy.

12. The method as claimed in claim 6, wherein the temperature of the piston blank during upsetting is no more than 50 to 80 K below the solidus temperature of the two main alloying components.

13. The method as claimed in claim 4, wherein the ring carrier is sintered by a sintering powder containing more than 28% by volume of particles with a diameter d>150 m.

14. The method as claimed in claim 2, further comprising heating the ring carrier under a protective gas atmosphere before forming the piston blank in the first forming tool, the ring carrier composed of a corroding material.

15. The method as claimed in claim 2, wherein the piston blank is composed of one of an aluminum-silicon alloy and an aluminum-copper alloy.

16. The method as claimed in claim 2, wherein the ring carrier is composed of one of an iron alloy and a nickel alloy.

17. The method as claimed in claim 2, further comprising sintering the ring carrier from a sintering powder before forming the piston blank, wherein the sintering power includes more than 28% by volume of particles with a diameter d>150 m.

18. A method for producing a piston, comprising: producing a ring carrier configured to receive a piston ring from a sintering powder by a sintering process, the sintering power includes more than 28% by volume of particles with a diameter d>150 m, and the ring carrier composed of a corroding material; heating the ring carrier under a protective gas atmosphere; forming a piston blank in a first forming tool such that the piston blank surrounds the ring carrier via positive engagement, wherein the piston blank, at least in a circumferential region disposed at a piston head, is composed of a light metal alloy suitable for forging; removing the piston blank from the first forming tool and placing the piston blank in a second forming tool, and inserting a holding-down tool into the second forming tool to hold the ring carrier down; and pressing a final forming punch into the second forming tool to deform the piston blank and form a piston.

19. The method as claimed in claim 18, wherein the piston blank is composed of one of an aluminum-silicon alloy and an aluminum-copper alloy.

20. The method as claimed in claim 18, wherein the ring carrier is composed of one of an iron alloy and a nickel alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The figures are in each case schematic, wherein

[0022] FIG. 1 shows a section through a first forming tool in the region of a bottom die for carrying out a method according to the invention for the production of a piston,

[0023] FIG. 2 shows an illustration like that in FIG. 1 but with the top die placed on top,

[0024] FIG. 3 shows an illustration like that in FIG. 2 but with the upsetting punch in contact,

[0025] FIG. 4 shows a piston blank in a partially upset state,

[0026] FIG. 5 shows an illustration like that in FIG. 4 but in the fully upset state,

[0027] FIG. 6 shows the preforged piston blank during retraction from the bottom die and with the top die removed,

[0028] FIG. 7 shows a section through a second forming tool with the holding-down tool inserted,

[0029] FIG. 8 shows an illustration like that in FIG. 7 but with a piston blank upset in a different way and a holding-down tool lowered completely onto a ring carrier,

[0030] FIG. 9 shows an illustration like that in FIGS. 7 and 8 but with the final forming punch lowered.

DETAILED DESCRIPTION

[0031] FIGS. 1 to 9 show a method according to the invention for producing a piston 1, which, at least in a circumferential region close to a piston head 2, is composed of a light metal alloy which is suitable for forging and has at least one ring carrier 3 to receive a piston ring (not shown).

[0032] In general, the method according to the invention is divided into three production steps, wherein, in a first production step, which is illustrated in FIGS. 1 to 6, a piston blank 4 is formed in such a way in a first forming tool 5 that it surrounds the ring carrier 3 produced by a sintering process by positive engagement. FIGS. 7 and 8 then show an intermediate step in a second forming tool 7, in which a holding-down tool 6 is inserted into a die of the second forming tool 7 and holds the ring carrier 3 down directly or indirectly during a forging step, which follows on according to FIG. 9. The third and last method step is thus shown in FIG. 9, in which a final forming punch 8 is pressed into the die of the second forming tool 7 and the piston blank 4 is thereby formed to give the piston 1. By means of the method according to the invention, it is possible to relieve the load on that region of the piston 1 at the ring carrier 3 which was hitherto subject to extremely high loads and, at the same time, to produce a forged piston 1 having a forged-in ring carrier 3 in a dependable process and in an economical manner and, at the same time, with a high quality. In the method according to the invention, in contrast to the previously known method, the internal shape comprising the case and bosses is produced only after the forging in of the ring carrier 3, which is a complete departure from the previous procedure.

[0033] If the method according to the invention is then considered, the piston blank 4 will, according to the method step illustrated in FIG. 1, first of all be placed in a bottom die 9 of the first forming tool 5 in such a way that a subsequent piston head 2 rests on an upsetting base 10 of the bottom die 9, wherein, at the piston head 2, the piston blank 4 has a first diameter d.sub.1, which narrows via a step 11 to a second, smaller diameter d.sub.2, and wherein the piston blank 4 is held positively by means of its first diameter d.sub.1 in the bottom die 9 of the first forming tool 5. The piston blank 4 thus rests by means of an outer lateral surface on an inner lateral surface of the bottom die 9. According to FIG. 1, the ring carrier 3, which is produced by a sintering process, is then placed on the step 11, wherein the outside diameter d.sub.R of the ring carrier 3 corresponds to an inside diameter d.sub.1 of the bottom die 9 and thus substantially to the diameter d.sub.1.

[0034] FIG. 2 then illustrates the subsequent method step, in which a top die 12 is placed on the bottom die 9, wherein an inside diameter d.sub.IO of the top die 12 is smaller than an inside diameter d.sub.IU of the bottom die 9. As shown in FIGS. 3 to 5, the piston blank 4 is then upset by means of an upsetting punch 13, wherein the ring carrier 3 is firmly connected to the piston blank 4, in particular infiltrated by the alloy of the piston blank 4 and thus interlocked.

[0035] In the method step illustrated in FIG. 6, the preforged piston blank 4 is then removed from the first forming tool 5 and, as shown in FIG. 7, placed in a second forming tool 7, the inside diameter d.sub.IZ of which corresponds to the inside diameter d.sub.IU of the bottom die 9 of the first forming tool 5. Here, a subsequent piston head 2 of the piston blank 4 rests on a final forming base 14 of the second forming tool 7. According to FIG. 7, the holding-down tool 6 is then inserted, which holds down the ring carrier 3 during the subsequent forging step illustrated in FIG. 9 and the inside diameter d.sub.IN of which corresponds to the inside diameter d.sub.IO of the top die 12 of the first forming tool 5 (cf. FIG. 2).

[0036] FIG. 7 shows an embodiment in which the holding-down tool 6 is lowered only into the region of the ring carrier 3 but is not in direct contact with the ring carrier. As an alternative, it is, of course, also possible to make provision for the top die 12 to rest directly on the ring carrier 3, as shown in FIG. 2, thereby creating the possibility that the holding-down tool 6 can likewise be lowered directly onto the ring carrier 3, as shown in FIG. 8.

[0037] In the subsequent method step shown in FIG. 9, the final forming punch 8 is then pressed in, and the piston blank 4 is thereby formed to give the piston 1. In a further production step (not shown), the piston 1 can then be removed from the second forming tool 7 and finish-machined, being ground or turned for example.

[0038] Before being placed in the first forming tool 5, the ring carrier 3 is usually heated under a protective gas atmosphere, thereby largely avoiding possible corrosion or scale formation on the ring carrier independently of the preheating time. Here, the ring carrier 3 is heated so rapidly that the heat-up time of the ring carrier 3 is shorter than the cycle time for forging.

[0039] An aluminum-silicon alloy or an aluminum-copper alloy is usually used for the piston 1 or piston blank 4, offering advantages in respect of corrosion resistance and workability and, in particular, suitability for forging. During upsetting, a temperature of the piston blank 4 is no more than 220 K, preferably 50 to 80 K, below the solidus temperature of the two main alloying components of the piston blank 4, which has the major advantage that stresses under thermal loads at the interface between the ring carrier and the piston alloy, of the kind which occur during operation of the engine owing to the differing thermal expansion coefficients of the materials of the piston blank and the ring carrier, are minimized.

[0040] For the ring carrier 3, in turn, an iron alloy or a nickel alloy is used and, in particular, sintering powder containing more than 28% by volume of particles with a diameter of d<150 m is used for sintering the ring carrier 3, thereby making it possible to achieve a porosity of the ring carrier 3 of between 20 and 80% by volume. Such a high open porosity makes it possible for the alloy of the piston ring 4 to infiltrate the ring carrier 3 during the upsetting of the piston blank 4 and thereby to interlock extremely well with the carrier.

[0041] The method according to the invention thus represents a complete departure from previous forging methods, wherein, in the method according to the invention, the ring carrier 3 is first of all forged in and only then is the piston blank 4 finally formed to give the piston 1. With the forming of the internal shape taking place first, as previously, very different forces were applied to the ring carrier 3 in the direction of a piston longitudinal axis over the circumference of the ring carrier owing to the lack of rotational symmetry of the case and bosses, producing high internal transverse forces which, in the worst case, could lead to shaft runout.

[0042] By the reversal of the method steps and the introduction of the intermediate step as well as by holding down the initially forged-in ring carrier 3 during the final forging of the piston 1, the internal shape can be formed or forged without damaging the previously forged-in ring carrier 3 since said ring carrier is held by means of the holding-down tool 6, at least over partial areas or over the entire circumference, and is thereby relieved of loads.