PISTON MANUFACTURE

Abstract: The manufacture is of a piston (20) of aluminium or aluminium alloy for an internal combustion engine. The piston ring grooves (21) of such pistons tend to be worn by the associated piston rings. Before a piston ring groove is machined on the piston an alloying material is applied to the piston and treated by an electron beam or laser beam to form an alloyed region extending around the piston. The alloy in the region is more wear resistant than the material of the piston. A piston ring groove is then machined in the alloyed region to provide a groove with side walls of wear-resistant alloy.

Description: PISTON MANUFACTURE The invention relates to the manufacture of pistons of aluminium or aluminium alloy for combustion engines, or for reciprocating compressors. Such pistons are formed with one or more piston ring grooves extending circumferentially around the piston and receiving a piston ring for sealing contact with a cylinder within which the piston reciprocates. During such reciprocating movement in highly rated applications, the piston ring tends to wear the side walls of the piston ring groove to an unacceptable extent over a period of time, so that the seal deteriorates. This is due to the fact that although the aluminium or aluminium alloys used for pistons have the advantages of lightness and ease of casting, they are not highly resistant to wear of this type. To reduce the effects of this wear, it has previously been proposed to form the grooves in an annular ring carrier of iron material cast into the piston. The cast iron is resistant to this form of wear. This has the disadvantages however, that it is expensive to produce, that is difficult to ensure secure bonding between the ring carrier and the piston and that it is heavy, thus requiring strengthening of the associated reciprocating engine parts. It is an ob ect of the invention to miti ate thes, disadvantages. According to a first aspect of the invention, there is provided a method of manufacturing a piston of an aluminium or aluminium alloy material for a combustion engine or a reciprocating compressor, comprising, before machining at least one piston ring groove in the piston, applying around the piston an alloying material which forms, with the material of the piston, an alloy which is more wear-resistant than the material of the piston, heating with an electron beam or laser beam, only the region to which the alloying material is applied to melt the alloying material and the piston material radially inwardly of said alloying material to form an annular region of said alloy extending both circumferentially around the piston and radially into the piston, and then, after the alloy has cooled, machining a piston ring groove in said region so that at least a portion of radially extending side walls of the piston ring groove are formed of said alloy. Preferably, the radial depth of the alloyed region is at least substantially equal to the required radial depth of the piston ring groove. Alternatively, the radial depth of the alloyed region may be only the radially outer part of the required radial depth of the piston ring groove. The alloying material may be applied around two spaced circumferential regions of the piston separated by a width of the finished piston ring groove so that, on heating, two of said annular regions of alloy are formed, each forming one wall of the machined piston ring groove. In this case, the heating step may comprise heating both regions * of the alloying material simultaneously by oscillating the electron beam or laser beam. Alternatively, the alloying material may be applied around a single circumferential portion of the piston, the width of the single portion being at least as great as the required width of the piston ring groove so that, on heating, a single said annular region of alloy is formed in which the piston ring groove is machined. The method may comprise applying the alloying material to the piston in the form of a powder of said alloying material and performing the heating step as the powdered alloying material is applied. The powder may be a silicon powder. The method may, however, comprise applying the alloying material to the piston in the form of a wire of said alloying material. The wire may be mild steel or stainless steel or nickel or copper. The wire may be laid around the piston before the heating step or may be fed continuously to the piston as the piston is heated. The rate of feed may be varied to alter the composition of the alloy region. In either of these embodiments, a shallow groove may, before the alloying material is applied, be cut around the piston, at the region to which the alloying material is to be applied, the alloying material, in the form either of powder or a wire, then being applied into the shallow groove. Where two regions of alloying material are used for a piston ring groove, two shallow grooves will be cut at the required separation. The alloying material may also be applied to the piston in the form of a slurry of a liquid and a powder of the alloying material, the slurry being allowed to dry before the heating step. The powder in the slurry may be of silicon or copper oxide or pure aluminium or a mixture of aluminium and carbon. Alternatively, the alloying material may be applied in the form of a foil of alloying material applied by electroplating the alloying material onto the piston. The. electroplated alloy material may be copper . According to a second aspect of the invention there is provided a piston when made by the method of the first aspect of the invention. The following is a more detailed description of two embodiment of the invention, by way of example, reference being made to the accompanying drawings, in which: Fig. 1 is a cross-section through part of a piston casting in a plane including the piston axis and showing two shallow annular grooves cut in the piston during a first method of piston manufacture, Fig. 2 is a similar view to Fig. 1 but showing wires placed in the shallow annular grooves in said method, Fig. 3 is a similar view to Fig. 2 but showing the piston after a heating step of said method, Fig.- 4 is a similar view to Fig. 3 but showing the piston after a piston ring groove machining step of said method, Fig. 5 shows a microsection (x 400) of an alloyed region produced by the method, Fig. 6 is an elevation of a partly finished piston of aluminium or aluminium alloy and including a shallow annular groove cut during a second method of piston manufacture, Fig. 7 is a diagrammatic view of an apparatus for applying an alloying powder and for melting the powder and part of the piston in the method, Fig.8 is a cross-sectional view of the shallow groove of Fig. 6 to a larger scale, Fi .9 is a similar view to Fi . 6 but showin application of a powdered alloying material to the shallow groove in the method, Fig. 10 is a similar view to Fig. 9 but showing the piston after a heating step of the method, Fig. 11 is a similar view to Fig. 10 but showing the piston after a piston ring groove machining step of the method. Fig.12 shows a microsection (x 12) of an alloyed region produced by the method of Figs. 6 to 12. Referring first to Figs. 1 to 5, the first method is for use in the manufacture of a piston ring groove in a piston 10 which is for internal combustion engines and which is made from aluminium or aluminium alloy such as the aluminium alloy sold under the trade name • Lo-Ex 1 . The piston 10 is cast or forged from the aluminium or aluminium . alloy and is then proof-turned to give the piston 10 a smooth annular outer surface. Two shallow grooves 11a., Ilk of serai-circular cross- section are then machined around the annular outer surface of the piston 10 (see Fig. 1) and each shallow groove 11a,, lib. has the centre of its semi-circular cross-section lying in a plane normal to the piston axis. The distance separating these two planes is equal or substantially equal to the required width of the finished piston ring roove. The diameter of each shallow roove ma fo example, be 1.0mm to 1.5mm. An alloying wire 12a., 12b. is then placed in each shallow groove 11a., life to extend around the whole circumference of the piston 10 (see Fig. 2). The diameter of each wire 12a., 12k is substantially the same as the diameter of the associated shallow groove 11a. or life and each wire 12a., 1212 may be held in place by locally deforming the sides of the associated shallow groove 11a., Ilk to pinch the wire. The wire 12&, 12k may be, for example, of stainless steel or maraging steel and may, where the grooves are 1.5mm in diameter, be 1.2mm in diameter. It will, however, be appreciated that any other suitable material may be used. The wire-bearing piston 10 is then placed in a vacuum chamber of an electron beam welding machine and the beam is directed at one of the wires 12a., 12k and at right angles to the piston. axis as the piston is rotated about its longitudinal axis. The other wire is then similarly treated. Alternatively, the electron beam can be rastered at 300Hz to treat both wires 12 simultaneously. The electron beam melts the wires 12a., 12k ≥s well as the metal of the piston in the region radially inwardly of the shallow grooves 11a., Ilk. The material of the wires 12a., 12k is dispersed through the melted piston material to form generally triangular cross-section alloyed regions 13a., 13k- Since the unmelted part of the piston provides an almost infinite heat sink, the melted alloy is rapidly the piston at the radially outer end of each alloyed region 13a., 13k. The electron beam is preferably directed at each alloy region 13a., 13k a second time to achieve the desired alloy structure. The second pass is at the same power and speed as the first pass but starts from a diametrically opposite point to the first pass. The piston 10 is then finish machined to provide an annular piston ring groove 14 (see Fig. 4) extending around the piston. Each side wall 15a., 15k of the piston ring groove 14, being aligned with the position of the centre of the grooves 11a., Ilk / is formed by the alloy material. As seen in Fig. 5, which is a micrograph showing the microstructure of legions 13a., 13k, the alloy material has the steel dispersed throughout the aluminium or aluminium alloy piston material to give an alloy which is much harder and more wear resistant than the piston material. Thus, the side walls 15a., 15k of the piston ring groove 14 are better able than the piston material to withstand the wear imposed on them in use by a piston ring, particularly the wear which occurs as the piston changes its direction of reciprocation at top dead centre and bottom dead centre. Although the alloying material has, in the foregoing example, been in the form of a wire 10a., 10k it will also be appreciated that it may be applied to the piston in the form of a foil of alloying material, or by spraying the alloying material onto the piston in the region at which material onto the region. As shown above in Figs. 1 and 2, a separate shallow groove 11a., Ilk and wire 12^. 12k is used for each side wall of the piston ring groove. It will be appreciated that a single groove could be cut, having a width slightly greater than the desired final width of the piston ring groove and a wire of corresponding width placed in the groove and melted to form a wider alloyed region. The piston ring groove is then cut wholly or substantially wholly within this wider alloyed region, so that at least the radially outer part of the side walls 15a., 15k are formed of the wear-resistant material. In the alloyed regions 13a., 13k described above and shown in Fig. 5, the alloying material represents about 8% of the total volume of the alloyed region 13a., 13k. This may be greater than required and by reducing the cross- sectional area of the wires 12a., 12k/ the concentration may be reduced to between 1 and 3%. In general, pistons are provided with two or more piston ring grooves. The problem of piston ring groove wear is most acute in the piston ring groove closest to the crown of the piston and so the above described method is in general only applied to that groove. It will be appreciated, however, that the method nay be applied to more or all of the piston ring grooves. In the embodiment described above with reference to the drawings, the piston may be subjected to further treatments which modify the structure of the alloy region. Referring next to Figs. 6 to 12, the second method is for use in the manufacture of a piston ring groove in a piston 20 which is for internal combustion engines and which is made from aluminium or aluminium alloy such as the aluminium alloy sold under the trade name 'Lo-Ex'. The piston 20 is cast or forged from the aluminium or aluminium alloy and is proof turned to give the piston 20 a smooth annular outer surface. A single shallow annular groove 21 is then machined around the annular outer surface of the piston 20 (see Figs. 6 and 8). The width of the groove is generally somewhat greater than the required width of the finished piston ring groove. For example, the shallow groove 21 may be 4mm in width and 0.5mm in depth. The grooved piston 20 is then placed in the apparatus which is shown diagrammatically in Fig. 7 and which comprises a rotation jig {not shown) for rotating the piston 20 about its longitudinal axis, a powder feed line 22 and a laser 23. The powder feed line 22 is for conveying an alloying material from a hopper (not shown) to a point adjacent the shallow groove 21. ' The hopper is of known type for providing a steady flow of powder at a controlled rate. The angle of the powder feed line at its outlet is preferably 30° to 45° relatively to a vertical The laser 23 may be a continuous wave carbon dioxide laser having a maximum output of 5kW and a wavelength of 10.6 m. The beam may either be defocussed with a beam width of 5mm or focussed. The apparatus is arranged so that the laser 23 is directed vertically downwardly towards the shallow groove 21 and the feed line 22 arranged before the laser in the direction of rotation of the piston 20 as shown by arrow 24. The powder in the hopper is preferably a silicon powder or a powder which is an aluminium/silicon alloy with a high proportion of silicon, for example 30% by weight although other suitable powders may be used. The piston 20 is then rotated so that its surface speed is, for example, 10-30mm/sec, and the silicon powder fed at about 4 grammes/minute. The laser beam is defocussed and at full power although, alternatively, the beam may be focussed and rastered. The silicon powder 25 is fed into the shallow groove 21 (see Fig. 9) and immediately passes into the laser beam which melts the silicon powder and also the piston material in the region beneath the groove 21. The molten powder mixes with the molten piston material to produce an alloyed region 26 (see Fig. 10). As the molten alloyed material passes away from the laser beam it is rapidly quenched since the remaining piston material forms a substantially infinite heat sink. A second laser treatment may be required to consolidate- When the alloyed region is fully treated, the piston 20 is removed from the apparatus of Fig. 7 and finish machined to produce, within the region 26, an annular piston ring groove 27 (see Fig. 11), such that each side wall 28a., 28k / or a substantial part of the side wall, of the piston ring groove 27 is formed of the alloy material. The alloy material of the walls 28a., 28k forms an aluminium-silicon alloy which is much harder and more wear-resistant than the piston material. The microstructure of the alloy is shown in Fig. 12. Thus the side walls 28a., 28k of the piston ring groove 27 are better able than the piston material to withstand the wear on them imposed by a piston ring, particularly the wear which occurs as the piston changes its direction of reciprocation at top and bottom dead centre. It will be appreciated that the characteristics of the alloyed region 26 may be altered by altering the feed rate of the silicon powder, by controlling the characteristics of the laser beam, or by altering the geometry of the shallow groove 21. In the ring groove seen in Fig.11, the side walls 28a., 28k extend beyond the radially inner end of the alloyed region. While this is not a particular problem, because the majority of the wear takes place at the radially outer ends of the walls 28a., 28k / the characteristics mentioned above may be altered to increase the depth of the alloyed region so that the walls 28., 28k are formed wholly of alloyed material. Alternatively, two shallow grooves may be machined for each piston ring groove. Either two powder feed lines 22 are then provided with two laser beams being focussed to melt the two regions simultaneously or a single feed line 22 is provided as described above and the regions treated consecutively. The alloying material need not be applied in the form of a powder. Firstly, a slurry may be made up using a powdered alloying material and a binder such as water or matt black paint. The slurry is then applied to the shallow groove or grooves 21 and allowed to dry before being treated by the laser beam in one of the ways described above. Suitable alloying materials for use in the slurry are silicon or copper oxide or pure aluminium or an aluminium and graphite mixture. Secondly, the alloying; material may be electroplated onto the piston at the required region. Copper may be used as the alloying material in this case and may be placed in a furnace after electroplating to form copper oxide, which increases the absorption of the laser beam by the alloying material. The piston is then treated by the laser beam in one of the ways described above. Thirdly, the alloying material may be in the form of a wire cr wires laid in the shallow groove 21. The wire or wires is or are then treated by the laser beam in one of the ways described above. Suitable materials for such copper or manganese/nickel. Alternatively, the wire could be fed to the piston 10 in the same way as the powder is fed in the apparatus of Fig.7, the wire being surrounded by an inert gas shield. Such an arrangement would allow variation of the alloying proportions by a variation in the rate of feed of the wire, a reduction in the rate of wire feed reducing the proportion of alloying material in the alloyed region. In general, pistons are provided with two or more piston ring grooves. The problem of piston ring groove wear is most acute in the piston ring groove closest to the crown of the piston and so the above described method is in general only applied to that groove. It will be appreciated, however, that the method may be applied to more or all of the piston ring grooves. In the second embodiment described above with reference to the drawings, the piston may be subjected to further treatments which modify the struction of the alloy region. The piston ring grooves 1427, provided by the first and second methods described above with reference to the drawings are cheap and easy to produce. In addition, they are integrally formed with the remainder of the piston so that there is no possibility of the portion of the piston around the piston ring groove becoming detached from the remainder of the piston. Further, the weight of the piston is not unduly increased by the methods so that the need to increase the strength of the reciprocating engine parts associated with the piston.

Claims: 822.- 15P 3 CLAIMS 1. A method of manufacturing a piston of an aluminium or an aluminium alloy material for a combustion engine or a reciprocating compressor, characterised by the steps of, before machining at least one piston ring groove in the piston, applying around the piston an alloying material which forms , with the material of the piston, an alloy which is more wear-resistant than the material of the piston, heating with an electron beam or laser beam , only the region to which the alloying material is applied to melt the alloying material and the piston material radially inwardly of said alloying material to form an annul a r r eg i on of sa i d al l oy ext e nd ing bo th circumferentially around the piston and radially into the piston, and then, after the alloy has cooled, machining a piston ring groove in said region so that at least a portion of radially extending side walls of the piston ring groove are formed of said alloy. 2. A method according to claim 1, characterised in that the radi al depth of the all oyed reg ion is at least piston ring groove. 3. A method according to claim 1, characterised in that the radial depth of the alloyed region is only the radially outer part of the required radial depth of the piston ring groove. 4. A method according to any one of claims 1 to 3 characterised in that the alloying material is applied around two spaced circumferential regions of the piston separated by a distance equal or substantially equal to the required width of the finished piston ring groove so that, on heating, two of said annular regions of alloy are formed, each forming one wall of the machined piston ring groove. 5. A method according to claim 4 characterised in that the heating step comprises heating both regions of the alloying material simultaneously by oscillating the electron beam or laser beam. 6. A method according to any one of claims 1 to 3 characterised in that the alloying material is applied around a single circumferential portion of the piston, the width of the single portion being at least as great as the required width of the piston ring groove so that, on heating, a single said annular region of alloy is formed in which the piston ring groove is machined. 7 A method ccordin to an on o c aims 1 characterised by the step of applying the alloying material to the piston in the form of a powder of said alloying material, in particular a silicon powder, and performing the heating step as the powdered alloying material is applied. 8. A method according to any one of claims 1 to 6 characterised by the step of applying the alloying material to the piston in the form of a ' wire of said alloying material. 9. A method according to claim 8 characterised in that the wire is mild steel wire or stainless steel wire or nickel wire or copper wire. 10. A method according to claim 8 or claim 9 characterised in that the wire is laid around the piston before the heating step or is fed continuously to the piston as the piston is heated. 11. A method according to any one of claims 7 to 10 characterised in that a shallow groove is cut around the piston, before the alloying material is applied, at the region to which the alloying material, in .the form either of powder or a wire, then being applied into the shallow groove. 12. A method according to any one of claims 1 to 6 characterised in that the alloying material is applied to the iston in the form of a slur of a l i a powder of the alloying material , the slurry being allowed to dry before the heating step. 13. A method according to claim 12 characterised in that the powder in the slurry is of silicon or copper oxide or pure aluminium or a mixture of aluminium and carbon. 14. A method according to any one of claims 1 to 6 characterised in that the alloying material applied in the fo rm of a f oil of all oying mate r i al appl i ed by electroplating or electroless plating of the alloying material onto the piston. 15. A piston when made by the method of any of claims 1 to 14. 822 : 15PM3

Inventors: AVEZOU JEAN CLAUDE, GROGAN ANTHONY FREDERICK, PHILBY JONATHAN DAVID, TRAVAILLE JACQUES
Applicants: AE PLC
Application year: 1982
Main classification: B23P15
IPC codes: B23K35, B23P15, C22F1, C22F3