GB2326876A - Ceramic cores - Google Patents

Description

IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF CERAMIC COMPONENTS This invention concems improvements in or relating to the manufacture of ceramic components. In particular the invention concerns the manufacture of ceramic cores for forming complex intemal passages in cast metal aerofoils, such as blades and vanes.

"Metal" in the context of the present invention will be understood to include both elemental metals and metal alloys.

The manufacture of cast turbine aerofoils with internal air cooling requires the use of ceramic cores to create the required internal passages for the air cooling. Essential requirements of the cores are that they should be soluble in a solvent which does not attack the alloy of which the aerofoil is composed and that they should have sufficient high temperature strength to resist deformation during casting and solidification processes.

Ceramic cores in general use are manufactured by injection moulding using materials based on silica, silica plus zircon or alumina. In practice, over 90% of cores produced are based on silica with additions of zirconia, alumina or magnesia to aid sintering and high temperature strength. Silica is the material of choice since it can be easily removed by dissolution in caustic soda or potash using high pressure autoclaves.

However, silica as a core base has significant disadvantages when used for castings produced by directional solidification and single crystal processes. These processes require the core to be held at temperatures in excess of 1 5000C for several hours during the solidification process. The disadvantage of silica arises from the fact that the manufacturing process for cores involves a firing at high temperatures to achieve a degree of sintered strength. In the case of silica the firing temperature is limited to 1200"C due to a crystal structure change which forms a compound known as cristoballite. The volume changes associated with the formation of this compound precludes the use of firing temperatures at the casting process conditions and as a consequence the material does not achieve its full strength at the casting temperature.

To compensate for this lack of strength the cores are usually held in position using platinum pins.

A further problem with silica cores lies in their chemical reactivity with single crystal alloys containing yttrium and lanthanum. Hence, for these alloys it is usual to use alumina as a core material. However, alumina is very difficult to dissolve, and extended processing times are required to remove cores from castings, which renders the process expensive.

Yttria as a potential core material has been known for some time and has the useful property that it can be readily dissolved in dilute nitric or organic acids without attacking the metal. However, attempts to produce cores from commercially available yttria have hitherto been unsuccessful due to a very high shrinkage during the firing operation. Such shrinkage is in excess of 10% and dimensional control is not possible. This is clearly unacceptable and has hitherto prevented yttria from being a useful core material.

It is believed that this shrinkage effect of cornmercial yttria under firing is due to its preparation from amorphous precipitated yttria and, as such, has little or no crystalline structure, the average particle size being about 10,um. Firing of this material at 1500"C causes a sintering of the fine particles with consequent marked volumetric shrinkage.

It is an object of the present invention to provide an yttria ceramic core which has high refractory strength, has an acceptable fired shrinkage, can be readily dissolved from within a casting, and is suitable for use with single crystal alloys containing yttrium and lanthanum.

In its most general aspect, the invention uses fused yttria as a material for manufacturing ceramic cores for use in making internal passages in cast components.

Fused yttria can be fired to the casting temperature without crystal structure change and can be dissolved out in dilute nitric or organic acid without damage to the casting mould.

According to a more specific aspect of the present invention there is provided a ceramic core for providing internal passages in cast metal components, the core being characterised in that it is made from comminuted crystalline fused yttria.

The fused yttria is preferably characterised in that it has previously been prepared by firing precipitated yttria at a temperature of at least 20000C to provide a crystalline structure.

The core may be manufactured from.the comminuted fused yttria by mixing the comminuted yttria with a liquid carrier to form a slurry, injecting the slurry into a mould, and firing at approximately 1500"C.

The core may be characterised in that it has a fired shrinkage of less than 3% by volume.

The invention and how it may be carried into practice will now be described, by way of example, with reference to a particular example. In the example, yttria that had previously been prepared by firing and fusing amorphous precipitated yttria at 2000 C to provide a crystalline fused product and subsequently comminuting the fused yttria to form relatively coarse crystalline particles of a size suitable for injection moulding, was formed into a slurry, injected into a mould, and fired at 15009C to form a ceramic core. In practice, the green ceramic mould is fired at a temperature in the range 12000C to 16500C depending on the fired characteristics required by the application of the fired core. On removal from the mould the ceramic core was found to have high refractory strength and a fired shrinkage of less than 3% by volume. On using the core in casting a single crystal titanium alloy containing yttrium and lanthanum it was found that the core had the required degree of strength at the casting temperature and did not require to be supported by platinum pins as is well known in the art. After casting and cooling, it was found that the core was easily removed by dissolving it in 50% nitric acid. It was also found that the single crystal alloy casting was unharmed by the yttria core and by removal ofthe core using nitric acid.

Nitric acid of strength as low as 25% (or lower) may be used for leaching out the yttria core of the invention. Alternatively, leaching of the yttria may be carried out using organic acids such as formic acid or other acids from the lower end of the aliphatic acid series, or acids such as tartaric acid.

The fused yttria core of the invention has the advantage that cores of extremely complex shapes for providing cooling passages in single crystal alloy components that are designed for use in extreme high temperatures and under severe loading conditions may be used in the knowledge that the cores have a fired shrinkage of less than 3% by volume, a high degree of dimensional stability, are strong enough to withstand high casting temperatures, and are easily leached from the components on completion of the casting process.

Claims (5)

1 A ceramic core for providing intemal passages in cast metal components, the core being characterised in that it is made from comminuted crystalline fused yttria.

2 A ceramic core as claimed in claim 1 wherein the fused yttria is characterised in that it has previously been prepared by firing precipitated yttria at a temperature of at least 20000C to provide a crystalline structure.

3 A ceramic core as claimed in claim 1 or 2 wherein the core is manufactured from the comminuted fused yttria by mixing the comminuted yttria with a fugative carrier to form a paste, injecting the paste into a mould, and firing at a temperature in the range 1200-1650"C.

4 A ceramic core as claimed in any preceding claim wherein the core is characterised in that it has a fired shrinkage of less than 3% by volume.

5 A ceramic core for providing internal passages in cast metal components, substantially as hereinbefore described with reference to the or each example.