GB1600325A

GB1600325A - Pistons

Info

Publication number: GB1600325A
Application number: GB2029778A
Authority: GB
Original assignee: Wellworthy Ltd
Current assignee: Wellworthy Ltd
Priority date: 1978-05-17
Filing date: 1978-05-17
Publication date: 1981-10-14
Also published as: DE2919897A1

Description

(54) IMPROVEMENTS IN PISTONS (71) We, WELLWORTHY LIMITED, a British Company of Lymington, Hampshire, S04 9YE, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to pistons for internal combustion engines and in particular to pistons forged from light metal alloys such as aluminium alloys.

Aluminium alloy pistons tend to suffer from excessive ring groove wear and it is accordingly known to provide ferrous inserts which reinforce the ring grooves. This presents particular problems in forged aluminium alloy pistons since it is not easy to forge the piston around a ferrous insert in the position where the top ring groove is usually machined due to the considerable radial depth required for the insert.

In particular there is now a requirement to provide a forged light metal alloy piston having reinforced ring grooves for special "fire rings" which operate very near to the top of a piston.

Accordingly in one aspect the present invention consists in a forged light alloy metal piston having a groove reinforcing insert extending to the top surface of the piston, wherein the insert is both mechanical ally keyed and metallurgically bonded to the piston. The mechanical key may be provided by one or more projections which extend radially inwardly into the piston.

Advantageously the metallurgical bond is formed by a material or a constituent of a material which is deposited prior to the forging operation, on to the surface or surfaces of the insert which are in contact with the forged alloy of the piston.

In a second aspect the invention consists in a method of manufacturing a forged light metal alloy piston which comprises providing a groove reinforcing insert having one or more regions to serve as a mechanical key, forming a layer of a bonding alloy on the surface or surfaces of the insert to be contacted by the forging, and forging the piston around the insert at a temperature sufficient to cause diffusion of at least one constituent of the bonding layer into the metal of the piston thereby to form a metallurgical bond between the insert and the piston.

In order that the invention may be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a cross-section through a preformed billet and ring insert, and Figure 2 is a similar view through part of a piston manufactured in accordance with the present invention.

Referring to Figure 1 of the drawings there is shown a billet 1 of aluminium alloy which has been preformed to provide a shoulder 2 on which is positioned to a solid ferrous ring insert 3. The insert 3 is intended to extend to the top surface of the composite piston be forged from the billet 1. As the insert 3 will extend to the top surface of the piston1 the insert can only be mechanically keyed to the piston at a region below the top of the piston crown and to this end is provided with a radially inwardly projecting ledge 4. Since the radial projection of this ledge 4 is relatively small, no difficulty is encountered in forging the aluminium alloy around this form.However, in simple mechanical terms the key thus provided may prove insufficiently strong to retain the insert 3 against the considerable thermal and mechanical stresses which arise when the piston is in service.

Accordingly, prior to the forging of the piston the insert 3 is first spray-coated with a layer of a material known to achieve a high strength bond upon spraying and which possesses good properties of atomic diffusion into aluminium alloys in the temperature range 300 - 500"C. Such a material is aluminium bronze. It is then ensured that the temperature at which the forging is carried out is sufficiently high (400 C 500"C) to initiate diffusion of the copper atoms from the sprayed coating into the aluminium alloy of the piston. The result of this is that a metallurgical bond is formed between the sprayed insert 3 and the piston, which bond gives considerable added security to the location of the insert in the piston.

To extend the degree of diffusion and, at the same time, improve the properties of the piston material, the composite piston may then be reheated to 4500 - 520"C (the upper temperature being determined by the type of alloy used) for us to one hour and subsequently cooled at a rate that achieves an adequate degree of solution treatment.

The cooling may be carried out by sprayquenching.

Referring now to Figure 2 of the drawings which shows the composite piston after forging the bond between insert 3 and the piston alloy is shown diagrammatically at 5.

The piston can be seen to have a pair of fire ring grooves 6 located close to the top surface of crown 9 of the piston, the grooves 6 being intended to receive "fire rings".

It will be appreciated that the ferrous insert 3 in the position shown in Figure 2 i.e.

surrounding the piston crown, is subjected in service to considerable heat and also to high inertia forces so that the bond will be subjected to considerable stress. It is thus always necessary to shape the inserts so as to obtain a very good mechanical key. Furthermore, the forces acting on the bond can be reduced by providing additional anchorage of the insert to the piston body. The insert 3 is thus provided with a number of axial leg-like projections 7 having undercut portions 7a, or alternatively being of irregular shape. The legs 7 are each screw-threaded into a threaded bore in the underside of the insert 3 as shown at 8. Thes legs extend into bores drilled in the billet 1 (see Figure 1) and are securely gripped during the forging operation.Such mechanical devices do not detract from the importance of the metallurgical bond between the insert and the piston as this ensures effective heat transfer from the ferrous insert to the aluminium of the piston and also prevents fretting at the joint faces which may result if only mechanical attachment of the insert is provided. It will be appreciated that the piston can be made from alternative light metal alloys such as magnesium, and that bronze is not the only material with which the insert can be coated to provide the required metallurgical bond on forging. Furthermore although the drawing shows a continuous, radially inwardly projecting ledge 4, it will be appreciated that this ledge might be replaced by a plurality of radially inwardly extending projections.

WHAT WE CLAIM IS: 1. A forged light metal alloy piston having a groove reinforcing insert extending to the top surface of the piston, wherein the insert is both mechanically keyed and metallurgically bonded to the piston.

2. A piston as claimed in claim 1, wherein the mechanical key is provided by one or more projections which extend radially inwardly into the piston crown.

3. A piston as claimed in claim 1, wherein the mechanical key is a continuous ledge which extends substantially radially into the piston crown.

4. A piston as claimed in any one of the preceding claims, wherein the insert is provided with one or more axial projections which extend into the piston body to provide an additional mechanical connection between the insert and the piston.

5. A piston as claimed in any one of the preceding claims, wherein the metallurgical bond is formed by at least one constituent of a material which is deposited prior to the forging operation on to the surface or surfaces of the insert which are in contact with the forged alloy of the piston.

6. A piston as claimed in any one of the preceding claims, wherein the insert is ferrous and the piston is made from an aluminium alloy.

7. A piston as claimed in claim 6, wherein the metallurgical bond is formed by a material having at least one constituent which possesses good atomic diffusion properties into aluminium alloys in the temperature range 300-500"C and which, prior to the forging operation, is sprayed on to the surface, or surfaces of the insert which come into contact with the forged aluminium alloy of the piston.

8. A piston as claimed in claim 7, wherein the material is bronze.

9. A method of manufacturing a forged light metal alloy piston comprising providing a groove reinforcing insert having one or more regions to serve as a mechanical key, forming a layer of a bonding alloy on the surface or surfaces of the insert to be contacted by the forging, and forging the piston around the insert at a temperature sufficient to cause diffusion of at least one constituent of the bonding layer into the metal of the piston to form a metallurgical bond between the insert and the piston.

10. A method as claimed in claim 9, wherein the bonding alloy is bronze.

11. A method as claimed in claim 10, wherein the light metal alloy of the piston is an aluminium alloy.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. strength bond upon spraying and which possesses good properties of atomic diffusion into aluminium alloys in the temperature range 300 - 500"C. Such a material is aluminium bronze. It is then ensured that the temperature at which the forging is carried out is sufficiently high (400 C 500"C) to initiate diffusion of the copper atoms from the sprayed coating into the aluminium alloy of the piston. The result of this is that a metallurgical bond is formed between the sprayed insert 3 and the piston, which bond gives considerable added security to the location of the insert in the piston. To extend the degree of diffusion and, at the same time, improve the properties of the piston material, the composite piston may then be reheated to 4500 - 520"C (the upper temperature being determined by the type of alloy used) for us to one hour and subsequently cooled at a rate that achieves an adequate degree of solution treatment. The cooling may be carried out by sprayquenching. Referring now to Figure 2 of the drawings which shows the composite piston after forging the bond between insert 3 and the piston alloy is shown diagrammatically at 5. The piston can be seen to have a pair of fire ring grooves 6 located close to the top surface of crown 9 of the piston, the grooves 6 being intended to receive "fire rings". It will be appreciated that the ferrous insert 3 in the position shown in Figure 2 i.e. surrounding the piston crown, is subjected in service to considerable heat and also to high inertia forces so that the bond will be subjected to considerable stress. It is thus always necessary to shape the inserts so as to obtain a very good mechanical key. Furthermore, the forces acting on the bond can be reduced by providing additional anchorage of the insert to the piston body. The insert 3 is thus provided with a number of axial leg-like projections 7 having undercut portions 7a, or alternatively being of irregular shape. The legs 7 are each screw-threaded into a threaded bore in the underside of the insert 3 as shown at 8. Thes legs extend into bores drilled in the billet 1 (see Figure 1) and are securely gripped during the forging operation.Such mechanical devices do not detract from the importance of the metallurgical bond between the insert and the piston as this ensures effective heat transfer from the ferrous insert to the aluminium of the piston and also prevents fretting at the joint faces which may result if only mechanical attachment of the insert is provided. It will be appreciated that the piston can be made from alternative light metal alloys such as magnesium, and that bronze is not the only material with which the insert can be coated to provide the required metallurgical bond on forging. Furthermore although the drawing shows a continuous, radially inwardly projecting ledge 4, it will be appreciated that this ledge might be replaced by a plurality of radially inwardly extending projections. WHAT WE CLAIM IS:

1. A forged light metal alloy piston having a groove reinforcing insert extending to the top surface of the piston, wherein the insert is both mechanically keyed and metallurgically bonded to the piston.

8. A piston as claimed in claim 7, wherein the material is bronze.

10. A method as claimed in claim 9, wherein the bonding alloy is bronze.

12. A method as claimed in any one of

claims 9 to 11, wherein the insert is ferrous.

13. A method as claimed in any one of clairns 9 to 12, wherein the one or more regions serving as a mechanical key are formed by one or more projections which extend radially inwardly into the piston crown.

14. A method as claimed in claim 13, wherein the mechanical key is provided by a continuous ledge which extends substantially radially into the piston crown.

15. A method as claimed in any one of claims 9 to 14 wherein the insert is provided with one or more axially extending projections which provide an additional mechanical connection between the inserts and the piston.

16. A method of forging a piston substantially as hereinbefore described with reference to the accompanying drawings.

17. A light metal alloy piston when manufactured by the method as claimed in any one of claims 9 to 16.

18. A light metal alloy piston substantially as hereinbefore described with reference to the accompanying drawings.