WO2001056722A1

WO2001056722A1 - Investment casting method and apparatus

Info

Publication number: WO2001056722A1
Application number: PCT/GB2001/000401
Authority: WO
Inventors: John Cliff; Roger Morey; Neil Watts
Original assignee: Trucast Limited
Priority date: 2000-02-02
Filing date: 2001-01-31
Publication date: 2001-08-09
Also published as: AU3037901A

Abstract

The invention provides a hollow body for use for casting apparatus, the hollow body being in the form of a tube (6) made of a good thermally insulating material, such as ceramic fibre. The tube has a first opening (6a) at one end to receive molten material from a crucible (1) and a plurality of further openings as side bores (7) in communication with the first opening (6a), each said further opening (7) being arranged to mount a respective wax pattern (3).

Description

INVESMENT CASTING METHOD AND APPARATUS

This invention relates to a method of. and apparatus for, casting metallic articles.

The invention is particularly concerned with an improvement to the well-known investment casting process for manufacturing castings with precise dimensions and good surfaces.

In investment casting (lost wax or precision casting), a pattern of wax or other fusible material is die-cast, and a mould or a shell is formed around the wax pattern. In the latter case, typically the shell is formed by applying a number of layers (usually ten to twelve) of ceramic slurry and stucco (sand and grit). Once the requisite number of layers have been applied, the shell is "fired" to harden it, thereby causing the wax to evaporate and burn away leaving a clean cavity for the metal casting. Although the investment casting process is expensive, it is used extensively to produce castings of precise dimensions and good surfaces. It finds particular use in the casting of refractory alloys for gas turbine blades, and for jewellery, and ornamental and dental castings.

Recently, there has been perceived a need to produce large numbers of small turbine wheels to meet the demand of the next generation of passenger car turbo chargers. In order to manufacture these wheels economically, it is necessary to produce the wheels on very large assemblies with minimum subsequent handling. This gave rise to two problems, the first of which was how to feed sufficient metal to the casting apparatus to produce large assemblies, and the second of which was how to produce large assemblies (of say 50+ articles) of precision castings.

According to a first aspect, the invention provides a hollow body for use for casting apparatus, the hollow body being made of a good thermally insulating material, the hollow body having a first opening to receive molten material from a crucible and a plurality of further openings in communication with the first opening, each said further opening being arranged to mount a respective wax pattern.

Preferably, the density of the hollow body is at least 600 kg/m3 and preferably no more than 950 kg/m3.

Preferably the ratio of the internal volume of the hollow body to the number of said further openings is less than a predetermined value.

The hollow body may be, for example, a riser. Preferably the hollow body is an integral body. The hollow body may take any suitable form and may comprise a tube which may be made by extrusion. The first opening may then conveniently comprise an open end of the tube. The end of the tube is conveniently adapted to fit into the end of a crucible. The further openings are suitably bores in the side wall of the tube and may be dimensioned to receive a stem portion of a wax pattern.

Advantageously, the tube is made of a suitable ceramic fibre, such as, for example, alumino silicate fibre.

According to a second aspect, the invention provides an assembly for use in the simultaneous casting of a plurality of metallic articles, the assembly comprising a crucible, a hollow body, such as a riser, and a plurality of wax patterns, the crucible having an inlet for receiving molten metal, and an outlet for directing molten metal into the hollow body, the hollow body being as defined above, and mounting the plurality of wax patterns.

Preferably, the components making up the assembly are dimensioned such that the casting yield obtainable is at least 70%. J In one embodiment, the hollow body is fitted into the outlet of the crucible.

In a preferred embodiment, the assembly further comprises a shell surrounding the wax patterns and the hollow body, the shell being formed by a plurality of layers of ceramic slurry or stucco.

According to a third aspect, the invention provides a method of simultaneously casting a pluralit}" of articles, the method comprising the steps of positioning a hollow body of a good thermally insulating material to receive molten material from a crucible through a first opening into the hollow body, the hollow body being formed with a plurality of further openings communicating with the first opening, mounting a respective wax pattern at each of the further openings, forming a protective shell around the wax patterns and the hollow body, removing the wax to define a plurality of moulds, and pouring molten material from the crucible until molten material fills all of the moulds.

Preferably, the hollow body is a hollow body according to the first aspect of the invention.

Preferably, the hollow body and moulds are dimensioned such that the casting yield obtainable is at least 70%.

Preferably each wax pattern is mounted by pushing a stem of the respective wax pattern into a respective one of the further openings. The hollow body is preferably an integral body and may be a tube made for example by extrusion from a suitable ceramic material. In a preferred embodiment the tube is positioned upright to receive the molten material from the crucible. The crucible may be fitted onto the end of the tube

Embodiments of the invention will now be described in greater detail, by way of example, with reference to the drawings, in which:-

Figure 1 is a perspective view of a conventional casting apparatus; Figure 2 is a cross-section taken on the line II-II of Figure 1 ; Figure 3 is a cross-section, similar to that of Figure 2, showing a casting apparatus constructed in accordance with the invention;

Referring to the drawings, Figure 1 shows a conventional casting apparatus for casting eight turbine wheels. The apparatus comprises a crucible 1 made of ceramic fibre, and a riser 2 made of wax. A respective wax wheel pattern 3 is positioned at the free end of each of the arms of the riser 2, the wheel patterns 3 being made in a conventional manner. Prior to casting, a ceramic shell 4 is built-up, in layers, in a conventional manner using a ceramic slurry and stucco.

Figure 3 shows casting apparatus constructed in accordance with the invention, this apparatus being a modified and improved version of the apparatus of Figure 2, so that like reference numerals are used for like parts. Figure 3 shows a ceramic fibre tube 6 having a first, open end 6a which fits into the snouted end 5 of a custom shaped crucible 1. The fibre tube 6 is extruded from a high density ceramic fibre (alumino silicate fibre), and is provided with four diametrical through bores 7. The ceramic fibre of the tube 6 has a density of 650 kg/m3

A respective wax wheel pattern 3 (not shown in Figure 3) is then positioned with its stem 9 being pushed into each end of each of the bores 7 to provide an eight-off assembly. As before, a shell 4 (not shown in Figure 3) is then formed around the eight wax wheel patterns 3 using layers of ceramic slurry and stucco. After firing of the shell 4, and hence the removal of the wax by evaporation and/or burning off, eight wheels are cast by pouring molten metal, for example a nickel based superalloy such as GMR 235, into the crucible 1. When the castings cool, the fibre tube 6 is removed by sand blasting, and the eight wheels are separated from redundant solidified metal feed. These wheels have a good internal structure, with no evidence of fibre inclusions, and no porosity.

It will be apparent that the casting apparatus described above has a number of advantages. In particular, the use of the fibre tube 6 provides a self-insulating riser which insulates the molten metal in the riser and feed area of the casting. This gives a good temperature gradient during solidification of the components, producing sound castings. Moreover, because of the insulating properties and the inherent mechanical strength of the fibre tube 6, the dimensions of the riser can be greatly reduced. This obviously means that less metal has to be poured, and less metal is wasted forming the riser system. In this connection, it should be noted that conventional casting techniques give yields of 55 to 60%, whereas yields of 70 to 80 or even 85% can be achieved using the casting apparatus of the invention. In this connection, percentage yield is defined as : weight of castings (wheels) produced x 100 weight of metal poured.

The apparatus of the invention also has the advantage of ease and speed of assembly, in that the wax patterns are pushed into pre-bored holes in the fibre tube 6. The present invention also results in the elimination of injection of risers, the fibre riser (the tube 6) being pushed into the end of the snouted crucible 1. so that no wax is involved in the formation of the assembly, apart from the wax wheel patterns themselves. A further advantage is reduced shell weight, which is partly as a result of the reduced dimensions of the fibre tube riser, and partly as a result of the density difference between wax and fibre, giving a lighter, less fragile shell. Finally, the reduced diameter of the fibre riser 6 facilitates separation of the finished wheels, and smaller cutting losses.

It will be apparent that modifications could be made to the apparatus described above. Thus, as the fibre tube 6 is formed, for example, by a simple extrusion process, any combination of wall thicknesses and internal diameters can be made. Consequently, the method of the invention could be applied to a range of component sizes, and also to the high vacuum and air melt casting processes. Hybrid assemblies of wax and fibre expand the potential, giving assembly designs that would be impossible in wax alone.

Claims

1. A hollow body for use for casting apparatus, the hollow body being made of a good thermally insulating material, the hollow body having a first opening to receive molten material from a crucible and a plurality of further openings in communication with the first opening, each said further opening being arranged to mount a respective wax pattern, the density of the hollow body being at least 600 kg/m3.

2. A hollow body as claimed in claim 1, wherein the density of the hollow body is no more than 950 kg/m3.

3. A hollow body as claimed in claim 2, wherein the hollow body is a riser.

4. A hollow body as claimed in claim 1, 2 or claim 3, wherein the hollow body is an integral body.

5. A hollow body as claimed in any preceding claim, wherein the hollow body comprises a tube.

6. A hollow body as claimed in claim 5, wherein the tube is made by extrusion.

7. A hollow body as claimed in claim 5 or claim 6, wherein the first opening comprises an open end of the tube.

8. A hollow body as claimed in claim 5, 6 or 7, wherein the end of the tube is adapted to fit into the end of a crucible.

9. A hollow body as claimed in any of claims 5 to 8, wherein the further openings are bores in the side wall of the tube.

10. A hollow body as claimed in any of claims 5 to 9, wherein each further opening is dimensioned to receive a stem portion of a wax pattern.

11. A hollow body as claimed in any preceding claim, wherein the tube is made of ceramic fibre.

12. A hollow body as claimed in any preceding claim, wherein the tube is made of alumino silicate fibre.

13. An assembly for use in the simultaneous casting of a plurality of metallic articles, the assembly comprising a crucible, a hollow body, such as a riser, and a plurality of wax patterns, the crucible having an inlet for receiving molten metal, and an outlet for directing molten metal into the hollow body, the hollow body being as claimed in any preceding claim, each said further opening of the hollow body mounting a respective wax pattern, the density of the hollow body being at least 600 kg/m3.

14. An assembly as claimed in claim 13, wherein the density of the hollow body is no more than 950 kg/m3.

15. An assembly for use in the simultaneous casting of a plurality of metallic articles, the assembly comprising a crucible, a hollow body, such as a riser, and a plurality of wax patterns, the crucible having an inlet for receiving molten metal, and an outlet for directing molten metal into the hollow body, the hollow body being as claimed in any preceding claim, each said further opening of the hollow body mounting a respective wax pattern, the components making up the assembly being dimensioned such that the casting yield obtainable is at least 70%.

16. An assembly as claimed in claim 13, 14 or 15. wherein the hollow body is fitted into the outlet of the crucible.

17. An assembly as claimed in any of claims 13 to 16. wherein the assembly further comprises a shell surrounding the wax patterns and the hollow body, the shell being formed by a plurality of layers of ceramic slum' or stucco.

18. A method of simultaneously casting a pluralit)' of articles, the method comprising the steps of positioning a hollow body as claimed in any of claims 1 to 12 to receive molten material from a crucible through a first opening into the hollow body, mounting a respective wax pattern at each of the further openings of the hollow bod}-, forming a protective shell around the wax patterns and the hollow body, removing the wax to define a plurality of moulds, and pouring molten material from the crucible until molten material fills all of the moulds.

19. A method of simultaneously casting a plurality of articles, the method comprising the steps of positioning a hollow body of a good thermally insulating material to receive molten material from a crucible through a first opening into the hollow body, the hollow body being formed with a plurality of further openings communicating with the first opening, mounting a respective wax pattern at each of the further openings, forming a protective shell around the wax patterns and the hollow body, removing the wax to define a plurality of moulds, and pouring molten material from the crucible until molten material fills all of the moulds, the hollow body and moulds being dimensioned such that the casting yield obtainable is at least 70%.

20. A method as claimed in claim 18 or claim 19, wherein each wax pattern is mounted by pushing a stem of the respective wax pattern into a respective one of the further openings.

21. A method as claimed in claim 18, 19 or 20, wherein the tube is positioned upright to receive the molten material from the crucible.

22. A method as claimed in any of claims 18 to 21, wherein the crucible is fitted onto the end of the tube.

23. A method as claimed in any of claims 18 to 22, wherein the molten material is metal.

24. A hollow body for use for casting apparatus, the hollow body being substantially as described herein with reference to Figs 2 and 3 of the accompanying drawings.

25. An assembly for use in the simultaneous casting of a plurality of articles, the assembly being substantially as described herein with reference to Figs 2 and 3 of the accompanying drawings.

26. A method of simultaneously casting a plurality of articles , the method being substantially as described herein with reference to Figs 2 and 3 of the accompanying drawings.