GB2173570A

GB2173570A - Fibre-reinforced metal pistons

Info

Publication number: GB2173570A
Application number: GB08606996A
Authority: GB
Inventors: R A Day; G M Coldrick; M C Jones
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1985-04-04
Filing date: 1986-03-20
Publication date: 1986-10-15
Also published as: GB2173570B; GB8606996D0

Abstract

A piston having fibre reinforcement is made by a method comprising placing in the crown region of one part of a squeeze casting piston mould a porous composite insert of fibres of whiskers or both, the insert having at least two components 32-35 and comprising a first insert component 32 of relatively high density fibres or whiskers this first insert component forming at least part of the crown area of the resulting piston, a second insert component 33 having a lower density than that of the first component and being substantially inwardly of the crown and the first component, introducing a quantity of molten metal into the mould, closing the mould and applying a pressure by a second part of the squeeze casting piston mould to the molten metal to impregnate the pores in the composite insert with the molten metal and maintaining pressure until solidification of the molten metal. Other configurations of the insert components are described. <IMAGE>

Description

SPECIFICATION Improvements in or relating to pistons The present invention relates to pistons and particularly to pistons having regions of fibre or whisker reinforcement and methods for manufacturing such pistons.

Where pistons are to be used in particularly harsh environments such as, for example, in diesel engines fibre reinforcements are frequently employed in the crown and piston ring-groove areas. Fibre reinforcement increases both the strength of the piston material in those regions and also the temperatures which may be withstood without failure. Generally speaking, the geometries of fibre inserts which have been used have necessarily been relatively simple. Because of the methods are available for the manufacture of the fibre inserts which are used for incprporation into pistons the inserts possess certain inherent disadvantages. Where, for example, variations in the thickness of the insert section is required this results in variations in the density of the fibre insert.

The thinnest sections have the highest density and conversely the thickests sections have the lowest density. These density variations are often disadvantageous when the insert is actually incorporated into the piston in that the low density sections are often in the regions where highest density is required and vice versa.

Fibre inserts are often incorporated into pistons by the technique of squeeze casting which results in an alloy having a porosity-free, homogeneous structure and superior mechanical properties, However, difficulties may arise with inserts having different thicknesses of section as described above in that the residual stresses generated on solidification and thermal stresses generated in operation may tend to cause cracking of the piston at the junction of low and high density fibre sections. Furthermore, problems are often experienced at the junction of the piston alloy and the fibre insert in that the differential thermal expansion in this region may also cause cracking problems. This is especially so where the fibre insert is of relatively high density.

In the piston of the present invention it is an object of the invention to reduce potential cracking problems due to varying densities of unitary fibre inserts and also due to gross differences in thermal expansion coefficients.

It is a further object of the present invention to provide a piston having more advantageous fibre densities and properties in the piston regions where they are most desired.

According the a first aspect of the present invention a method of producing a piston comprises the steps of: placing in the crown region of one part of a squeeze casting piston mould a porous composite insert of fibres or whiskers or both, the insert having at least two components and comprising a first insert component of relatively high density fibres or whiskers this first insert component forming at least part of the crown area of the resulting piston, a second insert component having a lower density than that of the first component and is substantially inwardly of the crown and the first component, introducing a quantity of molten metal into the mould, closing the mould and applying a pressure by a second part of the squeeze casting piston mould to the molten metal to impregnate the pores in the composite insert with the molten metal and maintaining pressure uritil solidification of the molten metal.

The components of the composite insert may be placed in the mould part either as an assembled unit or as the individual camponents of the composite insert.

The components of the composite insert may be of different material. For example, the first component may be of zirconia and the second component may be of alumina.

The composite insert may comprise more than two component parts, for example, the insert may comprise a third insert component of intermediate density between those of the first and second component and extend axially between said first and second components.

According to a second aspect of the present invention where is provided a piston for use in an internal combustion engine when made by the method of the first aspect of the invention.

In order that the invention may be more fully understood examples will now be described by way of ilustration only with reference to the accompanying drawings of which: Figure 1 shows a prior art piston having a crown insert of unitary construction; Figure 2 shows a first embodiment of a piston according to the present invention having a composite insert; Figure 3 shows a second embodiment of a piston according to the present invention having a composite insert; Figure 4 shows a third embodiment of a piston according to the present invention having a composite insert.

Referring not to Figure 1 which shows a piston 10 produced by squeeze casting. The piston 10 comprises an alumina fibre crown reinforcing member 11, a cast iron top ring-groove reinforcing member 12, a body and skirt portion 13 having gudgeon pin boss holes 14.

The piston 10 has formed in the crown region 15 part of the combustiori"chamber 16 in the centre of which is a projection 17. The methods available for manufacturing single piece fibre inserts such as depicted by 11 mean that the portions 18 delineated by the dotted lines have a relatively low density and the portions 19 and 20 have a relatively high density.

This situation arises because the fibre insert 11 is made by pressing to approximate shape of a mat of wet fibres prior to drying. The liquid wetting medium in the fibre mat also contains a binder which remains after drying to hold the fibre mat in the pressed shape. On casting the binder is dispersed by the molten metal. The problem is that on pressing on the wet fibre mat which is initially of substantially uniform section, the densities of the neighbouring sections 18, 19 and 20 then become different perhaps by as much as a factor of two or more.Quite apart from the fact that it would be more advantageous to have the portions 18 at a relatively higher density, the interface regions between portions 18 and 19 and 18 and 20 are subject two high thermally induced stresses generated both during solidification of the piston alloy and during operation in service. Such stresses may result in cracking of the piston in the region of the high and low density interface regions.

Furthermore, higher stresses also result at the interface between the alloy of the piston body 13 and the relatively high density portions 19 and 20 than between the alloy and the lower density portions 18. This also may result in an interface prone to cracking.

Figure 2 shows a piston 30 where the combustion chamber configuration of the piston of Figure 1 is substantially maintained but with more advantageous physical properties. The insert 31 comprises four main parts; and upper annular first component 32, a lower annular second component 33, a third annular component 34 below component 32 and outwardly of component 33 and a fourth axial component 35. Component 32 may be of zirconia fibres and of relatively high density, such as about 30% of the full density. Component 33 may be of alumina fibres and have a density around 10% of the full density. Component 34 also includes the upper ring groove 36 and may be around between 1 0 and 20% of full density the fibre orientation and type being chosen for wear-resistance rather than for thermal properties.In this instance the fibres of component 34 are aligned axially with respect to the piston. The density of component 35 may also be around 20% of full density. It will be noted in this embodiment that the component 34 extends into a rebate formed in the lower face of component 32 to enable the constituent components to be mutually interlocking and self-registering.

It may be seen from Figure 2that the desired configurations may be achieved and that both type of material and density may be chosen to reach an optimum compromise between properties required and undesirable expansion effects at interfaces. The components of the composite insert may be partially pre-shaped as shown by components 32 and 35. The combustion chamber cavity 36 may be formed in situ by appropriate shape of the base of the mould or by use of a salt core placed in the squeeze casting mould prior to introducing the molten metal.

The piston 40 shown in Figure 3 demonstrates a composite insert 41 composed of three fibre inserts which may be pre-assembled and placed in a squeeze casting mould as a unit. The insert 41 comprises a first component 42 of density around 30%, a second component 43 of density around 10% and third axial component 44 of density around 20%. After casting of the piston alloy the required features may be machined on the cast blank.

Figure 4 shows a piston 50 having an insert 51 comprising four main parts; some of which are preshaped to minimise post casting machining as in Figure 2. In this embodiment the insert 51 comprises an upper annular component 52 forming the periphery and part ofthe piston crown and being of a relatively high density of around 30%; a lower annular component 53 of about 20% density and which also forms a reinforcement for the upper ring groove; an axial component 54 also of around 20% density and a lower flat disc component 55 of about 10% density. The piston of this embodiment possesses a graded change of thermal properties from the alloy of the piston body 56 to the relatively high density fibre reinforced portion 52 of the crown.

Since the various component parts of the composite insert may be chosen for their particular properties this may be done to favour the application of particular types of thermal barrier coatings which may be subsequently applied to the piston crown. For example, plasma sprayed zirconia may be aplied to a zirconia fibre or whisker reinforced crown thereby improving thermal compatibility between the barrier coating and the substrate.

Claims

1. A method of producing a piston and comprising the steps of: placing in the crown region of one part of a squeeze casting piston mould a porous composite insert of fibres or whiskers or both,the insert having at least two components and comprising a first insert component of relatively high density fibres or whiskers this first insert component forming at least part of the crown area of the resulting piston, a second insert component having a lower density than that of the first component and is substantially inwardly of the crown and the first component, introducing a quantity of molten metal into the mould, closing the mould and applying a pressure by a second part of the squeeze casting piston mould to the molten metal to impregnate the pores in the composite insert with the molten metal and maintaining pressure until solidification of the molten metal.

2. A method according to Claim 1 and wherein the composite insert is placed in the mould part as an assembled unit.

3. A method according to Claim 1 or Claim 2 and wherein the components of the composite insert comprise alumina fibres or whiskers.

4. A method according to Claim 1 or Claim 2 and wherein the component parts of composite insert comprise more than one material.

5. A method according to Claim 4and wherein the composite insert comprises alumina and zirconia components.

6. A method according to any one preceding claim and which further includes the step of coating the piston crown surface with a thermal barrier material.

7. A piston having a composite insert, the composite insert comprising at least two components; a first component of fibres or whiskers having a relatively high density and forming at least part of the piston crown area and a second component of fibres or whiskers having a relatively lower density than the first component and which second component lies substantially inwardly of the first component, the first and second components being substantially completely impregnated with the metal of which the piston is made.

8. A piston according to Claim 7 and wherein the first insert component has a density of about 30% of the full density of its constituent material and the second insert component has a density of about 20% or less of the full density of its constituent material.

9. A piston according to either Claim 7 of Claim 8 and wherein the composite insert comprises more thah two components:

10. A piston according to any one preceding claim from 7 to 9 and wherein the first component comprises zirconia.

11. A piston according to any one preceding claim from 7 to 10 and wherein the piston crown area is at least partially coated with a thermal barrier material.

12. A piston substantially as hereinbefore described with reference to the accompanying specification and any one of Figures 2,3 or 4 of the drawings.