WO1995033586A1 - Improvements in and relating to bearing surfaces - Google Patents

Abstract

The invention provides a method for forming components, such as cam-rings, having improved load bearing capacities. By using a thixoformable material, introducing it to a die (1) of the desired configuration and machining the article (6), a surface having a grain structure substantially normally aligned relative to the surface can be produced. The alignment of the grains gives enhanced load bearing capabilities.

Description

IMPROVEMENTS IN AND RELATING TO BEARING SURFACES

This invention relates to improving properties of bearing surfaces, in particular, but not exclusively, in relation to cam rings.

Cam rings such as those employed in injector pumps for diesel systems experience very high Hertzian stresses. Such stresses can give rise to rolling contact fatigue failures. Similar problems are encountered in many components which must withstand high bearing loads.

It is known that by processing a metal such that, its microstructure is aligned normal to the bearing surface, the mechanical properties of that surface are optimised in relation to anything pressed against that surface.

The production of articles with radially aligned grain flow, for instance, is known to be possible by more conventional routes, but the conventional method involved is both complicated and expensive.

The conventional method consists basically of the steps of casting an article, with a subsequent forming process to produce directional alignment of the grain structure. Under these conditions the method develops a texture and the best mechanical properties are found along the longest axis of each elongated grain.

In the case of cam ring production, rolling is used to give a grain structure which is aligned normal to the forging load. This material is then slit into "rashers" to expose the grains aligned normal to the surface. Sections of material having this alignment are then taken and formed into a semi-circle by heating to 800-900°C and forging in a die. Two such halves are then held in place by a backing ring and electron beam welded to produce the structure. The alignment is due mainly to the extension in the direction normal to the forging load of carbon inclusions and/or areas rich in carbide.

In addition it is known to use further techniques such as Hot Isostatic Pressing to reduce internal cavities resulting from the forming process. This technique involves high temperatures and high argon gas pressures to remove voids in the material.

These techniques are complicated, expensive, time consuming and produce a product which is prone to flaws or irregularities in its structure.

According to our invention an article is produced with a surface having a grain structure aligned normal to the surface, comprising the steps of: introducing a thixoformable material into a die, the thixoformable material being in a semi-solid state; forming the material in the die to produce an article; and removing the article from the die and treating or machining the article to expose the surface having the grain structure aligned substantially normal to the surface.

This enables us to produce bearing surfaces with improved high fatigue strength and at lower cost compared with known conventional methods . The grain structure may be elongated and substantially normally aligned relative to the surface.

Alternatively if isotropic on a micro structural scale, the surface produces properties which are substantially equal to the exposed cross-section of longitudinally orientated grains in a conventionally produced material, for example similar to the properties of longitudinally forged grains.

In this way the exposed surface has an enhanced capacity for bearing loads over conventional arrangements, making the article particularly suitable for use as a cam ring.

The die may be so shaped as to produce an article having radial aligned elongate grains or microstructural isotropy.

The grains may be aligned at greater than 80° to the surface in question or a substantial portion of them may be so aligned, for instance 80% of grains at 80° or more to the surface.

Preferably the surface produced is radially arranged relative to a centre of alignment, or centre of spreading.

The die may be so shaped as to produce an article having a disc shaped portion. The disc may be treated or machined to produce one or two toroidal surfaces, i.e. inner or outer, each having radially aligned elongate grains. The thixoformable material introduced is preferably at such a temperature as to flow in a laminar manner.

The thixoformable material may have anisotropic properties upon introduction to the die. These properties may be present on a macro or micro scale.

On a macro scale the properties may be generated by producing thixoformable material having different layers having different properties. The layers may be of different materials, different thicknesses or have different materials, such as reinforcement, added to them.

By the use of the method in accordance with our invention we are able to produce a component having a grain structure which is elongate and substantially normally aligned relative to a surface, and which is formed from a thixoformable material. This may be enhanced by a final solid state forging step if an increase in grain elongation is required.

Preferably the component has a toroidal surface with radially aligned elongated grains.

Various embodiments and variations of the invention are illustrated, by way of example only, in the following drawings in which:-

Figure 1 is a cross sectional view through a die block immediately prior to the forming stage;

Figure 2 shows the cross-section through the die block of Figure 1 after forming; Figure 3 shows a cross-section through an article indicating the grain alignment; and

Figure 4 shows an alternative shape of die block.

Figure 1 illustrates a die block 1 made of two components 2, 3 and which is suitable for forming an article of a desired shape in a cavity defined between the two components and including a disc portion 6. A slug 4 of thixoformable material, which has been heated to a required extent, is placed in the mouth of the die block 1.

Suitably the slug 4 has been heated to such an extent that it is in a semi-liquid state.

The extent of heating required is a reflection of the forming process to be used. Liquid fractions of 30-40% are commonly employed for thixoforging. The higher the liquid content the lower the force required to cause the material to flow. Unloaded, the material behaves as a solid, but once subjected to a high shear rate it flows as a liquid.

A number of established techniques exist for the production of thixoformable materials, such as the mechanical stirring of the original MIT process, induction electromagnetic stirring and the atomised spraying on to a former of the Osprey process.

All these techniques are suitable for producing the material, which may be used direct from the production process or which may be allowed to cool, stored and reheated. During forming a ram 5 is forced down upon the semi-liquid slug 4 which deforms in response to the pressure to a shape defined by that of the cavity. The die is of such a shape that the prime area for expansion of the slug is by flowing into the disc portion 6.

The thixoformable properties of the slug 4 ensure laminar flow as the material is semi-liquid. During the flow, which originates due to the shear forces generated by the ram, the roughly spherical grains may be elongated in the direction of flow due to the forces applied, in any case the effect on properties is to produce best or maximum load bearing in this direction.

Due to the consistent flow direction in the disc portion 6, i.e. radially outwards from the centre-line X-X, the grains are aligned in the same consistent direction. This is achieved by a regularly shaped die and a regularly shaped blank.

Upon cooling the article can be withdrawn from the die and, where necessary, machined to expose the surface required. In this way substantially the minimum cross-sectional area of each grain is exposed.

In the article illustrated in Figure 3 of the accompanying drawings, the removal of a core 7 and a circular part 8 of the disc 6 produces an inner trochoidal running surface 9. A proportion 10 of the outer surface is also removed to produce a further radially aligned surface 11.

By using a different shaped die as illustrated in Figure 4 of the accompanying drawings, other shapes of normally aligned grain flow surfaces can be produced, such as a linear shape. The use of an elongate slug is useful in this regard to improve the consistency of flow during thixoforming. Linear flow away from the X-Y plane produces such alignment. Elliptical surfaces with normally aligned elongate grain structures are also possible by using dies or blanks of the correct shape.

This method clearly produces articles having optimal stress resistant properties in their bearing surfaces. Furthermore, the method represents savings in terms of the reduced number of production steps and the simpler equipment required when compared with existing techniques.

Furthermore, the semi-liquid nature of the material ensure that the material produced in is metallurgically sound, without requiring further steps to remove entrained gas filled voids in the metal.

Claims

1. A method of producing an article with a surface having a grain structure aligned normal to the surface, characterised by comprising: introducing a thixoformable material (4) into a die (1), the thixoformable material being in a semi-solid state; forming the material in the die to produce an article; and removing the article from the die and treating or machining the article to expose the surface having the grain structure aligned substantially normal to the surface.

2. A method according to claim 1, comprising introducing the thixoformable material (4) into the die at such a temperature as to flow in a laminar manner.

3. A method according to claim 1 or claim 2, comprising applying a final solid state forging step to the article to achieve an increase in grain elongation.

4. A method according to claim 3, including rolling to achieve the forging step.

5. A method according to any preceding claim comprising removing a core (7) and a circular part (8) of a disc (6) to produce an inner trochoidal running surface (9) .

6. An article produced by the method of any preceding claim in which the article has a grain structure which is elongate and substantially normally aligned relative to a surface.

7. An article according to claim 6, in which the grains are aligned at greater than 80° to the surface.

8. An article according to claim 6, in which a substantial portion of the grains is aligned at 80° or more of the surface.

9. An article according to claim 8, in which substantially 80% of grains are elongate at 80° or more to the surface.

10. An article according to any of claims 6-9, in which the thixoformable material has an isotropic properties upon introduction to the die.

11. An article according to claim 10, in which the properties are present on a macro or micro scale.

12. An article according to claim 11, in which on the macro scale the properties are generated by producing thixoformable material having different layers having different properties.

13. Apparatus for producing an article having a surface which has a grain structure aligned normal to the surface, characterised by comprising a die (1) having die cavity into which a slug of thixoformable material in a semi-state is introduced, a ram (5) adapted to deform the slug to conform to the profile of the cavity to produce the article, whereafter, after cooling, the article can be removed from the die for further treatment.

14. Apparatus as claimed in claim 13, in which the die is so shaped as to produce an article having a disc shaped portion.