PISTON AND PISTON RING

Abstract: A method of reducing wear between the radial faces of a piston ring and the radial faces of the co-operating piston ring groove in an aluminium alloy piston is described. The method comprises the steps of locally alloying the region in which the piston ring groove radial faces are located to increase the concentration of at least one strengthening element and coating at least the co-operating radial faces of a piston ring with a compatible material. Nickel has been found to be particularly advantageous and an example is described of a piston (10) which is locally alloyed by electron-beam (25) melting wherein the nickel (16) content is raised to about 12%. The co-operating piston ring was electroless plated with a nickel-phosphorus alloy.

Description: The present invention relates to pistons and piston rings. It is well-known to provide piston rings with coatings which improve the wear-resistance or compatability of the working face against an associated cylinder wall and also the wear-resistance of the piston ring faces which co-operate with the faces of the piston ring groove in the piston. Coatings such as chromium and molybdenum are frequently used on the face which rubs against the cylinder wall whilst coatings such as those produced by nitro-carburising are frequently used on the other working faces. Similarly it is also well-known in the case of aluminium alloy pistons to include inserts cast into the piston such as, for example, Ni-resist cast iron inserts incorporated by the Alfin process. Other techniques which have been used include local alloying of the basic piston alloy to enhance wear-resistance or the incorporation of porous inserts by various pressure casting techniques. A technique for local alloying is described by Hiller in GB 2 026 649 whilst an example of the inclusion of porous inserts is described in US 4 334 507. Other methods of enhancing piston ring-groove performance include surface treatments such as anodising. Heretofore judged on a performance basis, the best performing technique of improving wear-resistance of the piston ring groove has been the incorporation of solid Ni-resist cast iron inserts by the Alfin technique. Unfortunately this is also the most expensive technique and adds most weight. The surface coating techniques such as anodising give good wear-resistance for a limited period but the coating eventually wears away allowing the wear rate to rise dramatically. The porous insert technique, whilst giving good results, is inherently expensive as it necessitates the use of a pressure casting technique such as squeeze-casting, for example. Local alloying is economic and gives a significant improvement over the wear rate of the untreated aluminium alloy but has not proved to be as wear-resistant as an Alfin bonded insert. It has now been discovered that unexpectedly superior wear-resistance of both the piston ring faces and the piston ring groove may be achieved by a combination of local alloying of the piston ring groove region and coating of the piston ring. According to a first aspect of the present invention there is provided a method of reducing wear between the radial faces of a piston ring groove in an aluminium alloy piston and the radial faces of a co-operating piston ring, the method comprising the steps of locally alloying at least the region of the piston, in which the piston ring groove radial faces are located, to increase the concentration of at least one strengthening element selected from the group comprising nickel and cobalt characterised in that the locally alloyed region contains from 5 to 25 wt% of the at least one strengthening element and in that the radial side faces of the co-operating piston ring are coated with a nickel-based or cobalt-based material. In one embodiment of the present invention it has been found that the local alloying step is advantageously carried out by an energy beam melting and alloying process. Preferably an electron-beam melting process may be used as this has been found to produce substantially homoseneously alloyed material with a minimum of defects such as, for example, oxide inclusions and porosity. Other methods such as laser beam melting or TIG welding, for example, may be employed. Nickel and nickel alloys have been found to be particularly advantageous with aluminium alloy piston materials as they readily alloy therewith to form hard, wear-resistance intermetallic compounds, are readily available in suitable forms such as wire and powder and are economic. Cobalt and cobalt alloys may alternatively be employed as similar properties are produced but, however, the cost of cobalt is significantly greater than nickel. A nickel content in the range of 5 to 25wt% has been found to have excellent properties. A preferred range is 8 to 20 wt% whilst the most preferred range is 10 to 17 wt%. It has surprisingly been found that a piston ring coating also comprising a relatively high proportion of nickel has produced the best wear results when tested in combination with an aluminium alloy piston having a locally alloyed piston ring groove region containing nickel as described above. A particularly good coating has been found to be one of nickel which also contains phosphorus. The phosphorus may be present in the range from 3 to 15wt%. A preferred content, however, may lie in the range from 8 to 13 wt% of phosphorus. The piston ring may be coated by any known technique such as, for example, by electro-plating, composite electroplating (eg nickel-alumina, nickel-silicon carbide, cobalt-chromium carbide), physical vapour deposition, chemical vapour deposition. It has been found that the well-known electroless plating technique may advantageously be used. Coatings applied by this process have been found to give superior wear-resistance in conjunction with a piston locally alloyed as described above and the technique is very economic to perform. Piston rings so coated may be heat treated as a temperature between 300 and 600°C. Preferably the heat treatment may be for more than 15 minutes and more preferably be for between 30 minutes and 2 hours. The wear-resistance of anodised aluminium alloy piston material is very good but due to the coating being thin the life is relatively short. It is considered, however, that anodised coatings, either hard or soft, may be applied to the locally alloyed piston ring region. Thus even after the anodised coating has been penetrated there is still the wear-resistant locally alloyed material below. There is also provided according to a second aspect of the present invention a piston and piston ring combination when made by the method of the first aspect. In order that the present invention may be more fully understood an example will now be described by way of illustration only with reference to the accompanying drawings of which: Figure 1 shows a schematic representation of an apparatus for the local alloying of the piston ring groove region of a piston; Figure 2 shows a schematic representation of a section through the ring groove region of a locally alloyed piston; and Figure 3 which shows a histogram of wear test results of different material combinations. Referring now to Figure 1. A piston 10 to be reinforced is mounted on a suitable support apparatus 11 with the piston axis lying in the horizontal plane. Means are provided by a motor and gearbox unit 12 to rotate the piston 10 about its axis, speed control being provided by the control unit 13. The piston 10, support 11 and motor/gearbox unit 12 are all mounted on an X-Y table 14 having control means 26, to allow precise positioning of the piston 10. A wire-­ feed unit 15 feeds Nickel 61 (trade mark) wire 16 from a spool 17 via a flexible feed conduit 18 to the piston 10. The wire-feed unit 15 is controlled by a control unit 19. The piston and drive apparatus and wire-feed system are all contained within a vacuum chamber 20 having a pumping port 21 connected to a pump (not shown). In the top of the chamber 20 is a port 22 having a valve 23, the port 22 being positioned vertically above the area occupied by the piston 10. A known electron- beam welding apparatus 24 is associated with the chamber 20 via the port 22 such that when the valve 23 is displaced, an electron-beam 25 may impinge on the desired area of the piston 10. In operation, a cast 78mm diameter piston blank 10 of a known Lo-ex (trade mark) alloy and in the proof turned, solution treated and aged condition and having a nominal composition in wt% of: is mounted on the support 11. Nickel 61 wire 16 of 0.8mm diameter and nominal composition in wt% of: is fed from the spool 17 via the feed unit 15 and the conduit 18 to the point on the piston 10 where the molten metal pool 27 will be formed. The chamber 20 is pumped down to 10⁻³ Torr via the port 21. The valve 23 is displaced and a low power electron-beam 25 from the apparatus 24 is generated to sight the piston 10 with respect to the beam 25. Any necessary positional adjustment is made to the table 14 by the remote control means 26. The piston 10 is rotated at 2 rev./min. and the beam power increased to 50KV and 50 mA to produce a molten pool 27. The wire 16 is fed into the pool 27 at approximately 2 m/min. for one complete revolution, whereupon the wire feed is stopped. The electron-beam power is maintained for one further complete revolution to promote further mixing and alloying and hence homogeneity. Locally alloyed pistons produced as described typically possess as indicated in Figure 2 a generally V-shaped section 30 of between 35 and 45 sq/mm of alloyed material. The section is substantially free of porosity and also substantially free of gross intermetallic particules or undissolved pieces of wire. The resulting structure is substantially uniform showing a distribution of fine needles of nickel aluminides. Composition analyses on typical areas taken from around the treated periphery of a piston gave the results shown in the Table. Hardness of the alloyed area produced in the example lies within the range 150 to 180 VPN. Piston rings of S.G. iron material were electroless plated using the following procedure and materials. (1) Vapour degrease (2) Electroclean - anodic alkaline cleaner 4-6V at 4-10 A/dm² 1-3 mins. at 60-95°C (3) Acid dip - immerse in 20-30% hydrochloric acid-rinse (4) Ni strike - rinse (5) Electroless Ni plate (85-95°C) 30-60 mins. Temperature and pH control vary composition of coating. The coating produced had a composition in wt% of Ni 91% P 9%. Coating carried out at 90°C pH 5.0 in Nifoss 3000 (trade mark). The plated rings were heat treated at 400°C for 1 hour. The samples of the prepared materials were then tested under the conditions described below to determine the wear-resistance of the mating surfaces. Other material combinations were also tested under the same conditions. The results of these wear tests are given in the histogram of Figure 3. The pin was 6 mm diameter having a rounded end of 25 mm radius of curvature. The different piston groove materials were: (1) Alfin = Ni-rest austenitic cast iron. (2) Hyp.Al. = Hypereutectic aluminium-silicon piston alloy. (3) Hyp.Al.+ Ni = As (2) plus local alloying with Ni by electron-beam melting. (4) Lo-ex = Eutectic aluminium-silicon piston alloy. (5) Lo-ex + Ni = As (4) plus local alloying with Ni by electron-beam melting. (6) Hard Anodised = As (4) treated by hard anodising. (7) Soft Anodised = As (4) treated by soft anodising. The different piston ring materials were: (8) Electroless Nickel = Electroless plated nickel/phosphorus as described. (9) Nitro-carburised = Nitro-carburised S.G. iron. (1O) Standard = S.G. iron piston ring material - as commonly used in many piston ring applications. As may be seen from Figure 3 the combination of locally alloyed Lo-ex (5) with the electroless nickel plated ring material (8) gave wear results which were superior to all other combinations except electroless nickel against soft anodised Lo-ex (7). The latter materials, however, is only a thin coating and the life of such a coating is relatively short.

Claims: 1. A method of reducing wear between the radial faces of a piston ring groove in an aluminium alloy piston (10) and the radial faces of a co-operating piston ring , the method comprising the steps of locally alloying at least the region (30) of the piston in which the piston ring groove radial faces are located to increase the concentration of at least one strengthening element selected from the group comprising nickel and cobalt characterised in that the locally alloyed region (30) contains from 5 to 25 wt% of the at least one strengthening element and the radial side faces of the co-operating piston ring are coated with a nickel-based or cobalt-based material. 2. A method according to Claim 1 characterised in that the content of the at least one strengthening element lies in the range from 8 to 20 wt%. 3. A method according to Claim 1 characterised in that the content of the at least one strengthening element lies in the range from 10 to 17 wt%. 4. A method according to any one preceding claim characterised in that a nickel-based material piston ring coating contains from 3 to 15 wt% of phosphorus. 5. A method according to claim 4 characterised in that a nickel-based material piston ring coating contains from 8 to 13 wt% of phosphorus. 6. A method according to any one preceding claim characterised in that the piston ring coating is deposited by the technique of electroless plating. 7. A method according to any one preceding claim characterised in that the piston ring is heat treated after coating. 8. A method according to Claim 7 characterised in that the heat treatment lies in the temperature range from 300°C to 600°C for more than 15 minutes 9. A method according to any one preceding claim from 1 to 3 characterised in that the coating on the piston ring is selected from the group comprising nickel-aluminium oxide, nickel-silicon carbide and cobalt-chromium carbide. 10. An aluminium alloy piston and a piston ring, the piston having a locally alloyed region (30) in which at least one piston ring groove is situated and a piston ring received in the piston ring groove, the locally alloyed region containing at least one strengthening element selected from the group comprising nickel and cobalt characterised in that the locally alloyed region contains from 5 to 25 wt% of the at least one strengthening element and the piston ring has its radially directed side faces coated with a nickel-based or cobalt-based material. 11. An aluminium alloy piston and a piston ring as claimed in claim 10 characterised in that the locally alloyed region contains from 8 to 20 wt% of the at least one strengthening element. 12. An aluminium alloy piston and a piston ring as claimed in claim 10 characterised in that the locally alloyed region contains from 10 to 17 wt% of the at least one strengthening element. 13. An aluminium alloy piston and a piston ring as claimed in any one preceding claim from 10 to 12 characterised in that a nickel-based material coating on the piston ring side faces contains from 3 to 15 wt% phosphorus. 14. An aluminium alloy piston and a piston ring as claimed in claim 13 characterised in that the nickel-based material coating on the piston ring side faces contains from 8 to 13 wt% phosphorus. 15. An aluminium alloy piston and a piston ring as claimed in any one preceding claim from 10 to 12 characterised in that the coating of the piston ring side faces is selected from the group comprising nickel-aluminium oxide, nickel-silicon carbide and cobalt-chromium carbide.

Inventors: ADAMS, DAVID ROBERT, PHILBY, JOHNATHAN DAVID
Applicants: AE PLC
Application year: 1988
Main classification: F16J9
IPC codes: B23K35, C21D9, F16J9, C23C18