GB2379628A

GB2379628A - Investment casting

Info

Publication number: GB2379628A
Application number: GB0119356A
Authority: GB
Inventors: Christopher William Shaw
Original assignee: Micro Metalsmiths Ltd
Current assignee: Sylatech Ltd
Priority date: 2001-08-08
Filing date: 2001-08-08
Publication date: 2003-03-19
Also published as: GB0119356D0

Abstract

A method of investment casting by forming a model from a meltable material, submerging the model in slurry of a refractory material, hardening the slurry, heating to remove the model and form a mould cavity, casting metal into the cavity and washing the refractory material from the casting. The slurry preferably comprises 20-30 % of beta hemi-hydrate plaster, with the balance being silica. The silica is preferably 25-30 % fine sand, 15-20 % of less than 300 mesh quartz and 25-30 % of less than 300 mesh Cristobalite. The slurry may be applied in vacuum around the model. The method may be used to make components comprising an external skin 7 internally reinforced by a three dimensional latticework of interjoined struts 2.

Description

CASTING METAL STRUCTURES OF COMPLEX DESIGN The present invention relates to the manufacture of cast metal structures of complex shape or design and, more particularly, although not exclusively, to the manufacture of cast components which have complex internal reinforcing structures and are strong but of lightweight.

Strong, lightweight components are demanded by the aerospace and other industries where weight saving is important. The reinforcement of such components is typically made from aluminium sections, known as ribs or stringers, which are welded, riveted or otherwise bonded together. Such components are also made by laminating a fibrous material, such as carbon fibre, Kevlar or glass fibre, over a core which may be a foam or honeycomb or other lightweight material.

The disadvantage associated with components constructed from joined sections of metal is their cost. The disadvantage associated with laminated structures is their susceptibility to damage.

It would therefore be desirable to produce components of the type referred to above as one-piece castings. Hitherto, however, the design of such components as castings would have been practically impossible and the tool-making would have been completely impossible. Such castings would create almost insuperable difficulties. For these reasons, lightweight reinforced components, such as those required by the aerospace industry, simply did not exist as cast components.

It is an object of the present invention to provide a method of casting complex designs or shapes from metal. It is another object to provide a method of casting hollow lightweight, metal components having an external skin reinforced by an internal strut structure.

To this end, the invention consists in a method of investment casting a metal component, comprising the steps of producing a solid model of the

A component to be cast from wax or other heat liquifiable material suitabie for use in an investment casting process, submerging the model in a liquid slurry of a refractory material which, subsequent to cooling of the cast component, can readily be washed from the resulting metal casting, enabling the slurry to

harden about the model to produce a solid mould about the model, heating the mould to remove the heat liquifiable material therefrom and produce mould cavities defining the component, filling the mould cavities with liquid metal, enabling the metal to solidify and removing the refractory material forming the mould from the cast component including the step of washing the refractory material from the casting.

The present invention enables the casting of lightweight structures with inherent reductions in cost and increases in performance values, such as strength for a given weight or stiffness for a given weight. In this regard, it is particularly suitable for manufacturing castings where an external skin is reinforced by an internal structure having an extremely complex geometry, such as, for manufacturing cast components reinforced internally with an octet strut structure. An octet strut structure in the context of this invention is a symmetrical three-dimensional latticework of interjoined struts forming a stable configuration for supporting an external skin and in which each junction is reinforced by a multiplicity of struts.

The liquid slurry used with the invention preferably comprises plaster and a ceramic refractory material. The ceramic particles must be held together by the plaster with a sufficiently weak bond so as to be capable of being washed out from the casting with a jet of water. However, the bond must be such as not to be unduly disturbed by fast heating or cooling of the mould at any time in its handling during the casting process.

The wax model for use in the invention may be produced by a rapid prototyping machine which is a computerised machine able to build up the solid model in successive slices or layers of a wax or other suitable material by deriving information on the configuration of the desired model from a drawing programme loaded onto the machine.

In order that the invention may be more readily understood, an embodiment thereof will now be described with reference to the accompanying drawings, in which :-

Figure 1 is a perspective view of a hollow aerofoil component reinforced by an internal structure of octet struts and manufactured as a onepiece casting by the method of the invention, and Figure 2 is a perspective view of a half section of the component shown in Fig. 1, and illustrates the complex nature of the internal reinforcement.

The drawings illustrate a typical component which may be manufactured as a one-piece casting by the present invention and which is a small aluminium aerofoil or fin 1 for an aeronautical or hydronautical vehicle, for example, for an aerospace vehicle such as a missile or aircraft. The fin 1 has a complex integral, internal reinforcing structure 2 constructed with a latticework of octet struts, as more clearly shown in Fig. 2. The aerofoil fin is open at one end 3 and closed at the opposite end 4. It has a cylindrical housing 5 for a shaft extending longitudinally of the fin and supported in position by the octet struts and longitudinal ribs 6 integral with the skin 7 of the fin.

The fin 1 is manufactured in one-piece by an investment casting process in accordance with the present invention. The aerofoil section is designed, firstly, by developing a lattice of octet struts on a solid modeller in a three-dimensional solid model drawing programme. This latticework is then "inserted"within a shell which forms the external skin 7 of the fin and is trimmed by the computer to the inside edge of the shell. Further details, such as the housing 5 for the shaft, are inserted in a similar manner and, by this means, a computer model of the exact requirements is built up. Such a computer model may also provide weight information and help in stress analysis. The computer model is used to produce files, known as stereo lithography files or STL files, which are then used in a selected rapid prototyping machine to produce a solid model of the fin structure.

The rapid prototyping machine produces a solid model from the STL files by building up successive layers of wax, as controlled by the files, to produce the model. This solid wax model can readily be coated or immersed in a refractory material which hardens around the wax and from which the

wax IS subsequently removed. Preferably, the wax model is submerged in a liquid slurry comprising plaster and ceramic refractory material. Such a slurry may comprise a beta hemi-hydrate plaster in the amount of 25 to 30% with the remainder being various forms of silica. Preferably, it has been found that the silica should be divided into fine sand in an amount of 25 to 30%, less than 300 mesh quartz in an amount of 15 to 20% and less than 300 mesh Cristobalite in an amount of 25 to 30%. Such a refractory material, when suitably applied in a vacuum around the wax model, not only sets very closely to the model, so conforming to all details, but will also, at the end of the casting process, readily wash out of the metal casting to give a simple, clean and undamaged internal finish.

The wax model encapsulated in the solidified ceramic material is placed in an oven which cycles to a high temperature to remove all traces of wax from within the ceramic material mould. Preferably, this involves heating up the mould to about 700 C in the presence of oxygen to remove any remaining carbonaceous material. The heating should be slow to avoid cracks in the ceramic refractory material. A typical cycle requires heating from room temperature to 7000C over a period of about 10 to 20 hours.

Prior to heating, the wax model and solidified refractory material are surrounded by a steel canister having an outlet which is positioned downwardly when the canister is placed in the oven so that the wax material of the wax model can flow downwardly from the mould and so that, when the canister is removed from the oven and inverted, molten aluminium can be poured in to fill the mould cavities left by removal of the wax. Preferably, a vacuum is drawn through the base of the mould as it is filled with liquid metal so that all interstices of the mould cavities are filled with the liquid metal.

Upon solidifying of the molten metal, the resulting casting is removed from the mould, the refractory material remaining inside the casting being washed out with a water jet. This is easily achieved with the refractory material described as the ceramic particles are bonded together with a weak bond, although not sufficiently weak so as to be disturbed by the heating and

cooling of the mould. The cleaned casting is the fin 1 which is a strong, but lightweight component.

Whilst a particular embodiment has been described, it will be understood that modifications can be made without departing from the scope of the invention. Hence, whilst the particular embodiment described is concerned with the manufacture of an aerofoil fin, for example, for a missile application, the invention is not restricted to the production of such components but may be used in numerous components where lightweight is of importance. Hence, there are a number of pressure vessels in aircraft hydraulic systems which have to be sufficiently strong to withstand strong internal pressures. Hitherto, these have either been made from thick-wall metal material or have been produced by a kind of fibre-glass system in which a thin strand of fibre is spun round a mandrel and embeds itself in resin so that the wall of the pressure vessel is actually manufactured from a reinforced filament material. Such pressure vessels can conveniently be cast as a hollow, reinforced one-piece casting by the method of this invention.

Another application is the kind of hydraulic struts that are found in undercarriages, or in wing flap operating devices where a thick wall tube can be replaced by a tube formed from two thin-walls, held apart by an octet strut or similar structure, and produced as a casting by the method of the invention. Moreover, less important aircraft components, such as components for seats, panels for flooring, engine panels and inspection hatches may also be produced by the method described.

Claims

CLAIMS 1. A method of investment casting a metal component, comprising the steps of producing a solid model of the component cast from wax or other heat liquifiable material suitable for use in an investment casting process, immersing the model in a slurry of a refractory material which, subsequent to cooling of the cast component, is readily washed from the resulting cast component, enabling the slurry to harden about the model to produce a solid mould about the model, heating the mould to remove the heat liquifiable material therefrom and produce mould cavities defining the component, filling the mould cavities with liquid metal, enabling the metal to solidify, and removing the refractory material forming the mould from the cast component including the step of washing the refractory material from the cast component.
2. The method claimed in claim 1, wherein the slurry of the refractory material comprises plaster and a ceramic refractory material, the slurry being such that, in the resulting mould, the ceramic particles are held together by the plaster with a sufficiently weak bond so as to be capable of being washed from the cast component with water, the bond not being unduly disrupted by heating or cooling of the mould during the casting process.
3. The method claimed in claim 2, wherein the slurry of refractory material comprises a beta hemi-hydrate plaster in the amount of 25% to 30% by weight with the remainder being a silica material.
4. The method claimed in claim 3, wherein the silica material comprises fine sand in the amount of 25% to 30% by weight, less than 300 mesh quartz in an amount of 15% to 20% by weight and less than 300 mesh Cristobalite in an amount of 25% to 30% by weight.
5. The method claimed in claim 1,2, 3 or 4, wherein the slurry of refractory material is applied in a vacuum about the wax model so that the refractory material sets in close conformity to the model and reproduces its detail.

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6. The method claimed in any one of the proceeding claims, wherein the heat liquifiable material is removed from the mould by heating the mould to a temperature of about of 7000C in the presence of oxygen.
7. The method claimed in any one of proceeding claims, wherein the heating step is performed slowly so as to avoid cracks in the ceramic refractory material, for example, over a period of from 10-20 hours.
8. The method claimed in any one of the proceeding claims, wherein the wax model and solidified refractory material are surrounded by a steel canister having an outlet which is positioned downwardly when the canister is heated so that the max material of the wax mould flows downwardly from the mould, whereafter the canister is inverted and molten metal is poured into the inlet to fill the mould cavities left by the removal of the wax.
9. The method claimed in any one of the proceeding claims, wherein the metal is poured into the mould whilst the mould is subjected to a vacuum so that all interstices of the mould cavities are filled with the liquid metal.
10. The method claimed in any one of the proceeding claims, wherein the wax model is produced by a computer controlled machine capable of building up the solid model in the heat liquifiable material by deriving information on the configuration of the model from a drawing program included in the computer controlled machine.
11. The method claimed in any one of the proceeding claims, including the step of developing the design of the component as a three dimensional solid model drawing program.
12. The method claimed in Claim 11, wherein the component comprises a shell reinforced by a latticework of octet struts, and wherein a computer is used to develop the latticework as a three dimensional solid model drawing program, insert the latticework within the shell, trim the latticework to the inside of the shell, and insert further details of the component as required, thereby to complete a computer model of the exact component.

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13. The method claimed in Claim 12, including the step of using the computer model to produce files to control the computerised machine for building up the solid model from the heat liquifiable material.
14. The method of investment casting a metal component substantially as hereinbefore described with reference to the accompanying drawings.
15. A metal component formed as a one piece casting and comprising an outer skin or shell reinforced by an internal latticework structure of octet struts.
16. A metal component constructed substantially as hereinbefore described with reference to the accompanying drawings.