GB2297285A - Investment casting mould - Google Patents

Description

INVESTMENT CASTING MOULD This invention is concerned with investment casting moulds and, in particular but not exclusively, with investment casting moulds for use in casting directionallysolidified, columnar grain structure (DS), or single crystal structure (SX) components.

Investment casting involves forming a wax pattern in the shape of the component required, forming a mould around the pattern, removing the wax, using the mould in a casting process, and breaking the mould away from the casting. The moulds are formed by dipping the pattern in a slurry, dusting with a stucco, and drying. This procedure is repeated several times to give a mould made up of layers.

The first one or two layers have a fine texture to form a face coating of the mould which takes up the fine detail of the pattern, and the remaining layers are coarser to form a back-up coating of the mould which gives it strength.

Many DS and SX investment casting moulds comprise alumina or alumino-silicate particles within, for example, a zircon or alumina flour matrix bonded with silica.

However, such moulds have disadvantages because the mould experiences high temperatures for an extended period and the strength and stiffness of the mould may be inadequate.

In addition, such moulds have a low thermal conductivity and low emissivity so that the casting process is slow because heat cannot be extracted rapidly.

It is an object of the present invention to provide an investment casting mould which is suitable for use with DS and SX components and which has higher thermal conductivity than the existing moulds described above.

The invention provides an investment casting mould comprising a face coating and a back-up coating, wherein at least the back-up coating comprises silicon carbide bonded by mullite.

An investment casting mould according to the invention is found to have a considerably greater thermal conductivity than typical existing moulds (at least 7 times greater). It has an emissivity of about 0.7 compared with about 0.2 for typical existing moulds. Furthermore, it is surprisingly found to exhibit less creep (about half as much) and to have greater strength at temperatures up to about 15000C.

Preferably, the back-up coating also comprises fine ground reactive alumina, since reactive alumina reacts with silica to form the mullite and an excess of alumina is desirable to ensure that substantially all of the silica is reacted. The term "fine ground" is used herein to denote a particle size of about 1 micron.

The face coating may have a different structure to that of the back-up coating. For example, the face coating may comprise mullite bonded by mullite which structure can have its coefficient of thermal expansion matched with that of the back-up coating. Alternatively, a face coating formed from a slurry comprising silica, alumina and/or zircon may be used.

The invention also provides a method of forming an investment casting mould comprising making a slurry comprising colloidal silica, fine ground reactive alumina, and silicon carbide, and coating a combination comprising a pattern and a face coating formed on the pattern with the slurry, dusting the dipped combination with silicon carbide particles, drying the coated and dusted combination, and repeating the coating, dusting, and drying steps a plurality of times to build up a back-up coating of the mould.

Preferably, the slurry contains at least enough fine ground reactive alumina to react to form mullite with substantially all of the silica present in the slurry, including silica present on the surfaces of the silicon carbide, ie there is at least enough alumina to achieve a stoichiometric balance. For example, for every 100 gs of colloidal silica, the slurry may contain at least 57.4 g of fine ground reactive alumina.

Preferably, the slurry comprises silicon carbide in the form of a flour and also large grain silicon carbide.

The flour may contain two or more grain sizes to improve packing.

The method may also comprise removing the pattern and firing the mould at a temperature above 11000c in an oxidizing atmosphere. The firing may, for example, take place at between 11000c and 12000c for about half an hour in a 5% oxidizing atmosphere. It is conventional to use a firing step at about 9250c in order to strengthen the mould 4.

but, sui-prisingly, firing at above 11000c is found to give at least a further twofold increase in strength to a mould according to the invention, ie a fourfold increase compared to omitting the firing.

There now follows an example which is illustrative of the invention.

In the illustrative example, an investment casting mould was formed which comprised a face coating comprising mullite bonded by mullite, and a back-up coating comprising silicon carbide bonded by mullite.

Firstly, in the illustrative example, a combination of a wax pattern and a face coating formed on the pattern was formed. To form the face coating, the pattern was dipped into an aqueous slurry containing: 550 g of colloidal silica (30% solids); 312 g of fine ground reactive alumina; and 1.4 kg of dense fused mullite (325 mesh).

The slurry also contained conventional wetting and deairing agents.

The dipped pattern was then dusted with a prime stucco of fused alumina (80 mesh). The dipped and dusted pattern was then dried and the dipping, dusting, and drying steps were repeated so that a face coating of two layers was built up on the pattern.

To form the back-up coating, the combination of the pattern and the face coating formed thereon was dipped into an aqueous slurry containing: 600 g of colloidal silica (25% solids); 550 g of silicon carbide flour (600 grit); 550 g of silicon carbide flour (220 grit); 550 g of large grain silicon carbide (46 grit); and 345 g of fine ground reactive alumina.

The slurry was designed to have a small excess of reactive alumina above that required to react with the silica (including the silica present on the surfaces of the silicon carbide). The slurry had a density of 2.5 gcm, a pH value of 9 to 10, and a viscosity as measured by a No.

5 Zahn Cup of about 10 seconds.

The dipped combination was dusted with silicon carbide powder (46 grit) and dried. The dipping, dusting and drying steps were repeated to build up a back-up coating of 5 layers. The final dipping was only followed by drying.

Next, in the illustrative method, the wax pattern was removed in a conventional way and the mould was fired. The firing took place in an oxidizing atmosphere at about 11500c. The mould was then heated and used in an investment casting process.

Heating the mould caused the silica and reactive alumina in the face coating to react to form mullite which bound together the mullite already present. The heating also caused the reactive alumina in the back-up coating to react with the silica to form mullite which bound together the silicon carbide. The reactive alumina reacted with the colloidal silica and also with silica present on the surfaces of the silicon carbide thereby avoiding the possibility of the formation of the glassy phase of silicon carbide.

Moulds formed in the illustrative example were found to have a thermal conductivity of about 10 w.m/k which is about 7 times that of typical moulds, an emissivity of about 0.7, a creep of about half that of typical moulds, a green strength of about 10 MPa (compared to about 5 MPa for typical moulds), a strength of about 20 MPa at 13000c (about 6 MPa), and a strength of about 5 MPa at 15000c (about 5 MPa).

Claims (10)

1 An investment casting mould comprising a face coating and a back-up coating, wherein at least the back-up coating comprises silicon carbide bonded by mullite.

2 An investment casting mould according to claim 1, wherein the back-up coating also comprises reactive alumina.

3 An investment casting mould according to either one of claims 1 and 2, wherein the face coating comprises mullite bounded by mullite.

4 An investment casting mould substantially as hereinbefore described with reference to the illustrative example.

5 A method of forming an investment casting mould comprising making a slurry comprising colloidal silica, fine ground reactive alumina, and silicon carbide, and coating a combination comprising a pattern and a face coating formed on the pattern with the slurry, dusting the dipped combination with silicon carbide particles, drying the coated and dusted combination, and repeating the coating, dusting, and drying steps a plurality of times to build up a back-up coating of the mould.

6 A method according to claim 5, wherein the slurry contains at least enough reactive alumina to react to form mullite with substantially all of the silica present in the slurry, including silica present on the surfaces of the silicon carbide.

7 A method according to either one of claims 5 and 6, wherein the slurry comprises silicon carbide in the form of a flour and also large grain silicon carbide.

8 A method according to claim 7, wherein the flour contains two or more grain sizes.

9 A method according to any one of claims 5 to 8, wherein the method also comprises removing the pattern and firing the mould at a temperature above 11000c in an oxidizing atmosphere.

10 A method of forming an investment casting mould substantially as hereinbefore described with reference to the illustrative example.