GB2246972A - Method of producing a composite casting - Google Patents

Abstract

A method of producing a composite casting, such as a self-lubricating bush, comprises the steps of providing a mould (15A; 15B) having a plurality of recesses in a surface (23) of a cavity (16) of the mould and locating components (22), for example graphite plugs, in the recesses so that portions of the components project from the cavity surface. A body is then cast in the cavity (16) from a material having a melting point lower than that of the components with the result that the projecting portions of the components (22) are embedded in the material of the body. Subsequently the mould is removed and the components machined to be flush with a surface of the body shaped by the mould cavity surface (16). Formation of the recesses in the mould cavity surface can be carried out with the mould separated into parts to expose the surface. <IMAGE>

Description

METHOD OF PRODUCING A COMPOSITE CASTING The present invention relates to a method of producing a composite casting.

Composite casting processes are widely employed to produce articles with two different metals intimately and permanently connected together, for example a soft metal bearing surface layered on a hard metal carrier. Such processes are particularly useful when it is desired to combine metals with different properties, such as hardness, temperature-resistance and corrosion-resistance.

Known composite casting processes involving moulds conventionally entail formation of some form of coating or shell on a surface of a preformed body and the mould is utilised to define the shape and size, for example thickness, of the coating or shell. The body is the principal part of the resultant composite casting and the mould has only a subsidiary function in determining the main features of shape of the casting. It is consequently commonplace, when a casting is the principal part of an article consisting of different materials, to produce the casting first and to incorporate preformed components of another material in the casting by welding, gluing, swaging, upsetting or other form of mechanical attachment.

Thus, for example, in the production of a self-lubricating bush consisting of a metal sleeve with graphite zones in the bearing surface, it is usual to drill bores completely through the wall of the sleeve and mount graphite plugs in the bores. A secure mechanical interconnection of plugs and sleeve is difficult to achieve and drilling locations are confined to relatively thin-wall areas of the bush; the presence of a flange or other departure from the external cylindrical form of the sleeve prohibits drilling and thus restricts the layout of the graphite zones. Problems of this kind are frequently encountered in composite material articles and may oblige partial redesign of the principal part of its article.

It is accordingly the main object of the present invention to avoid the disadvantages of welded, glued and other such forms of interconnection of different-material parts of an article in which one of the parts is a casting.

Other objects and advantages of the invention will be apparent from the following description.

According to a first aspect of the present invention there is provided a method of producing a composite casting, the method comprising the steps of providing a mould having a plurality of recesses in a surface of a cavity of the mould, locating components in the recesses so that portions of the components project from said cavity surface, casting a body in the mould cavity from a material having a melting point lower than that of the components such that the projecting portions of the components are surrounded by and anchored in the material of the body, and removing the mould from the composite casting of body and components.

Preferably the method comprises the further step, after removal of the mould, of processing the components to have a predetermined relationship to a surface of the body defined by the cavity surface with the recesses. The processing may involve, for example, machining the components to be substantially flush with that surface of the body. For preference the step of providing the mould comprises machining the recesses, such as by drilling blind bores, into the mould.

Machining of the recesses can be carried out with a portion of the mould separated into parts to expose the mentioned surface of the mould cavity, the parts being subsequently joined together before casting of the body. It may then be convenient to also locate the components in the recesses before the mould parts are joined together.

The accuracy of the cast body may be enhanced if the mould portion is originally produced as an integral member, for example of resincoated sand, which defines part of the cavity and which is separated into parts by being broken. A relatively precise reconstitution of the mould member can be achieved by joining at a break line in a resinsand structure.

The components themselves can be lubricating elements, such as graphite plugs, and the recesses can be so arranged at spacings from each other that the lubricating elements will be disposed to provide discrete lubricating zones in the body. The spacings are preferably substantially uniform. Enhanced anchorage of the components in the body can be obtained if the projecting portions of the components include undercut regions. In general, however, shrinkage of the casting material around the projecting portions ensures firm retention of the components in the finished casting.

The method particularly lends itself to a gravity casting process, but pressure feed, immersion and other casting processes are equally usable.

According to a second aspect of the invention there is produced a composite casting when produced by the method of the first aspect of the invention. The casting can be, for example, a bearing element such as a bush with the components disposed in the bore of the bush, or the casting can have a threaded portion with the components disposed in the threaded portion.

An example of the method of the invention and an embodiment of a casting produced by the method will now be more particularly described with reference to the accompanying drawing in which: Fig.1 is a schematic sectional elevation of casting equipment for use in carrying out a method exemplifying the invention; and Fig.2 is an axial section of a composite casting produced by such a method.

Referring now to the drawings, there is shown in Fig.1 gravity casting equipment incorporating a mould for production of a composite casting in the form of a self lubricating flanged bush. The equipment essentially comprises a pouring spout 10 for molten metal connecting with a feed pipe 11 of fibreglass or resin-coated sand. The feed pipe 11 rests on a circular base 12 which has a central inlet opening 13 communicating with the bore of the pipe and a number of outlet openings 14 distributed around the central opening and incorporating filters.

Also forming part of the castinq equipment is a mould of resincoated zircon sand consisting of an upper mould half 15A and a lower mould half 15B. Apart from being shaped to interfit and jointly define a mould cavity 16, the upper mould half 15A closes off with the spout 10 and pipe 11 and rests on the periphery of the top face of the base 12 and lower mould half 15A abuts the peripheral edge of the base 12. The upper mould half 15A defines with the pipe 11 and base 12 a reservoir 17 which communicates with the outlet openings 14 in the base and which is of sufficient height to create a pressure head for assisting feed afmolten metal to extremities of the mould cavity 16. The mould half 15B is recessed below the base 12 and defines therewith a sump 18 through which molten metal flows between the pipe 11 and reservoir 17 and serves to collect dross. The upper mould half 15A is provided, in a core portion thereof, with a plurality of equally spaced feeder runners 19 (only two of which are shown in Fig.l) extending between and communicating with the reservoir 17 and the mould cavity 16. The upper mould half 15A isialso provided at the top with a plurality of vents 20 communicating with the mould cavity 16 and with the ambient atmosphere. Adjacent to the vents 20 is a repaired break line 21 at which the upper mould half 15A was broken into two parts and subsequently reconstituted, for reasons which are explained further below.

The mould cavity 16 defined by the two interfitted mould halves is shaped in accordance with the article to be produced, which, as previously mentioned, is a bush with an external flange. The bush is to be provided at the surface of its bore with discrete lubricating zones, for which purpose graphite plugs 22 are to be anchored in the wall of the bush sleeve at the intended locations of the lubricating zones. The graphite plugs are preformed components oversized in length and, for the casting process, are located in recesses in a cylindrical surface 23 of the mould cavity, in particular a surface intended to define the surface of the bore of the bush. The disposition of the recesses is thus coincident with the locations of the lubricating zones and, in the illustrated example, four rows of recesses are provided, with the recesses in each row being uniformly spaced.Any desired disposition of lubricating zones and thus plugs/recesses is possible, including in the region of flange of the bush.

For formation of the recesses, the upper mould half 15A is intentionally broken into two parts at the line 21 so that the surface 23 is freely accessible for machining of the recesses, which is accomplished by drilling an appropriate number of blind bores of depth equal to about half the plug length. The construction of the mould halves from resin-coated sand means that breakage along the line 21 can be readily achieved and the coarse interface at the line 21 when the parts are rejoined provides keying of the parts and thus accurate reassembly to the original form. Prior to reassembly of the two parts, the graphite plugs 22 are inserted into the drilled recesses.

Following repair of the upper mould half 15A with incorporated graphite plugs 22, the mould halves are interfitted in association with the remaining parts of the casting equipment and casting of the bush can be carried out. For this purpose molten metal, for example aluminium-bronze at about 12500C, is slowly poured into the spout 10 in sufficient quantity to fill the pipe 11, sump 18, reservoir 17 and ultimately the mould cavity 16, air in the cavity being displaced through the vents 21. Communication between the pipe, sump, reservoir and cavity is by way of the openings 13 and 14 in the base 12 and the runners 19 in the core portion of the upper mould half 15A.Filtration of metal entering the reservoir is carried out by the filters in the openings 14 and the pressure head developed in the reservoir ensures filling of the mould cavity 16 in the extremities represented by the ends of the bush sleeve and the flange. Due to the slow pouring of the aluminium-bronze and internal feeding of the mould cavity 16 by way of the runners 19, the heat build-up in the proximity of the surface 23 remains high for an extended period of time and the metal thus cools relatively slowly in the region that ultimately forms the surface of the bore of the bush. A greater Brunell hardness of the bore surface is thereby achieved.

When the mould cavity 15 is filled, the metal is allowed to completely cool and solidify and in the course of solidifying shrinks around the portions of the graphite plugs 22 projecting out of the recesses in the mould half 15A. This shrinkage provides firm anchorage of the projecting portions of the plugs 22 in the part of the casting that forms the wall of the bush sleeve. In addition, the graphite plugs 22 chill the surrounding metal so as to provide hardening of the adjacent faces in the wall of the bush sleeve and regions of the sleeve bore. This hardening, which tightens the granular structure of the metal, enhances the wear resistance of the bush.

When the casting is fully coated, the mould is removed by separation of the mould halves and breakage of the upper half 15A.

The exposed bush contains the graphite plugs 22 and the portions of the plugs formerly located in the upper mould half now project from the surface of the bore of the bush sleeve. Machining of the bore is then carried out to reduce the plugs until flush with the bore surface.

A finished self-lubricating bush 25 produced by the method explained above in connection with Fig.1 is shown in Fig.2. The bush consists of a sleeve 26 with an external flange 27. Embedded in the wall of the sleeve are lubricating elements 28 represented by the machined-down graphite plugs 22. The elements are equidistantly spaced in four rows and are present in the part of the sleeve encircled by the flange 27. Drilling of the sleeve by the prior art method in order to locate elements 28 in line with the flange would not be practicable.

The elements 28 are securely retained in the sleeve by virtue of the shrinkage process previously mentioned, and can be additionally anchored by undercutting of the elements.

The self-lubricating bush shown in Fig.2 is merely one example of a composite casting producible by the method of the invention.

Other forms of casting with incorporated components of graphite, metal or other materials can also be produced by the method.

Claims (22)

1. A method of producing a composite casting, the method comprising the steps of providing a mould having a plurality of recesses in a surface of a cavity of the mould, locating components in the recesses so that portions of the components project from said cavity surface, casting a body in the mould cavity from a material having a melting point lower than that of the components such that the projecting portions of the components are surrounded by and anchored in the material of the body, and removing the mould from the composite casting of body and components.

2. A method as claimed in claim 1, comprising the further step, after removal of the mould, of processing the components to have a predetermined relationship to a surface of the body shaped by said cavity surface.

3. A method as claimed in claim 2, wherein the step of processing comprises machining the components to be substantially flush with said surface of the body.

4. A method as claimed in any one of the preceding claims, wherein the step of providing the mould comprises machining the recesses into the mould.

5. A method as claimed in claim 4, wherein the machining of the recesses comprises drilling blind bores in the mould.

6. A method as claimed in either claim 4 or claim 5, wherein the machining of the recesses is carried out with a portion of the mould separated into parts to expose said surface of the cavity, the parts being joined together before casting of the body.

7. A method as claimed in claim 6, wherein the step of locating the components in the recesses is carried out before the mould parts are joined together.

8. A method as claimed in either claim 6 or claim 7, wherein said mould portion is produced as an integral member defining part of the cavity and is separated into parts by breaking the member.

9. A method as claimed in any one of the preceding claims, wherein the mould comprises resin-coated sand.

10. A method as claimed in any one of the preceding claims, wherein the components are lubricating elements.

11. A method as claimed in claim 10, wherein the lubricating elements are graphite plugs.

12. A method as claimed in either claim 10 or claim 11, wherein the recesses are so arranged at spacings from each other that the lubricating elements will be disposed to provide discrete lubricating zones in the cast body.

13. A method as claimed in claim 12, wherein the recesses are arranged at substantially uniform spacings.

14. A method as claimed in any one of the preceding claims, wherein the projecting portions of the components include undercut regions.

15. A method as claimed in any one of the preceding claims, wherein the material for casting of the body is aluminium bronze alloy.

16. A method as claimed in any one of the preceding claims, wherein the step of casting is carried out by a gravity casting process.

17. A method as claimed in claim 1 and substantially as hereinbefore described with reference to Fig.1 of the accompanying drawings.

18. A composite casting when produced by a method as claimed in any one of the preceding claims.

19. A casting as claimed in claim 18, wherein the casting is a bearing element.

20. A casting as claimed in claim 19, wherein the bearing element is a bush and the components are disposed in the bore of the bush.

21. A casting as claimed in claim 18, wherein the casting has a threaded portion and the components are disposed in the threaded portion.

22. A casting as claimed in claim 18 and substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.