GB2364006A

GB2364006A - Crystal selector pattern

Info

Publication number: GB2364006A
Application number: GB0115752A
Authority: GB
Inventors: David Alan Ford
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2000-06-27
Filing date: 2001-06-27
Publication date: 2002-01-16
Also published as: GB0115752D0

Abstract

A pattern for forming a crystal selector in an investment casting mould, the pattern being formed from a water soluble organic composition. The water soluble organic composition may be a thermoplastic, which may comprise hydroxypropylcellulose and which may have a solubility of at least 900 g/l. The water soluble composition ideally comprises urea in a proportion of 70-90%, polyvinyl acetate in a proportion of 10-25% and 1 to 5% stearic acid. The crystal selector pattern may be formed in an injection moulding process.

Description

2364006 CRYSTAL SELECTOR PATTERN This invention relates to a crystal

selector pattern for use in the preparation of investment casting moulds for single crystal casting processes.

In a conventional investment casting process, wax patterns of the article to be cast are assembled with wax patterns of other mould components, such as runner systems, and dipped in a ceramic slurry composition repeatedly, with each layer of slurry being dried before the next dipping The wax is then removed from the hardened slurry mould, for example by steam autoclaving, leaving a mould cavity of the desired structure After firing in a kiln, molten metal is poured into the mould cavity to form the components.

The mould is then broken to release the formed components.

In a single crystal casting process, the mould is placed over a chill plate, so that the molten metal solidifies first at the chill plate and the component is formed by solidification as a single grain in the direction away from the chill plate However, in practice, the molten metal cooled by the chill plate forms a large number of columnar grains, and so provision needs to be made to ensure that only one of these grains propagates into the mould cavity of the component itself so as to grow in the mould cavity as a single crystal To achieve this a crystal selector is provided in the mould to annihilate most of the columnar grains generated from the furnace chill plate, leaving only one grain to penetrate into the mould cavity Such selectors comprise a helical passage.

The columnar grains growing into the selector passage must branch in order to grow within the helix and consequently they quickly annihilate until a single grain remains which grows along the remainder of the selector passage and into the mould cavity.

The selector works efficiently if the helical passage is narrow However, a pattern to form such a narrow helical passage is very delicate and the use of wax for this purpose is unsatisfactory because the wax is not strong enough to support the weight of the component patterns Instead of wax, polystyrene has been used, but this has the significant disadvantage that it is not removed during the steam autoclave wax pattern removal process Instead, the polystyrene is removed during firing of the mould at high temperature, but the problem here is that the polystyrene pattern expands more than the mould material and so has a tendency to crack the mould The disadvantages of wax and polystyrene selector patterns have inhibited the development of single crystal casting using crystal selectors.

According to the present invention there is provided a crystal selector pattern for forming a crystal selector in an investment casting mould, the crystal selector pattern being formed from a water- soluble organic composition.

Preferably, the water soluble organic composition is mouldable and so may be thermoplastic.

Preferably, the solubility of the composition is not less than 500 g/l, and more preferably it is not less than 900 g/l.

The water-soluble organic composition may comprise hydroxypropylcellulose, but in a particular preferred embodiment, the composition comprises urea.

Preferably, the urea comprises not less than 50 % of the composition, and more preferably not less than 70 %.

The composition may also comprise a hydrophilic soluble polymer such as polyvinyl acetate, which may be present in the composition in the proportion not less than 10 % and preferably not less than 15 %.

The composition may also comprise an additive to enhance the dimensional stability of the crystal selector Such an additive may, for example, comprise a lubricant such as stearic acid, which may be present in proportions of not less than 1 %.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 represents an investment casting process; and Figure 2 shows a crystal selector pattern for use in the process represented in Figure 1.

Figure 1 shows mould cavities 2 for casting turbine blades of a gas turbine engine The mould cavities 2 are present in a ceramic mould (not shown) which also includes a runner system 4 and a pouring basin 6 for introducing molten alloy into the mould cavity 2 At the lower end of each mould cavity 2, there is a crystal selector passage 8 which is of helical form The mould is positioned over a water cooled chill plate 10, with a starter block or seed crystal 12 positioned between the chill plate 10 and the lower end of each crystal selector passage 8.

In use, molten alloy is poured into the pouring basin to fill the mould cavities 2 and other interior parts of the mould The molten alloy reaching the cooled seed crystals 12 is itself rapidly cooled and begins to solidify in the form of columnar grains extending upwardly from the seed crystals 12 These grains grow upwardly into the crystal selector passages 8 To propagate along the passages 8, the grains need to branch successively and, in this process, they rapidly annihilate leaving only a single grain growing from the upper end of the crystal selector passage 8.

A single grain continues to grow into the mould cavity 2, with the result that the eventual blade is formed from a single crystal.

The mould is formed by assembling together patterns of the various cavities within the mould.

Thus, wax patterns of the blades to be formed in the mould cavities 2 are assembled with wax patterns corresponding to the runner system 4 and pouring basin 6 However, if wax is used to form the crystal selector passages 8, the resulting patterns have proved to be inadequately robust to withstand the stresses applied to the patterns during the manufacture of the mould itself Consequently, in accordance with the present invention, the patterns used for forming the crystal selector passages are made from a thermoplastic injection mouldable water-soluble organic composition comprising, in a preferred embodiment, 80 % urea, 18 % polyvinyl acetate and 2 % stearic acid.

The assembled patterns are dipped in a ceramic slurry material so that a layer of slurry is formed over the patterns Once this layer has dried, the assembly is dipped again into the ceramic slurry, and so on, so that a series of layers is built up over the patterns When the total slurry layer is of adequate thickness, and is dried, the patterns are removed from the interior by steam autoclaving This raises the temperature of the wax used for the patterns for the mould cavities 2, the runner system 4 and the pouring basin 6, so the wax melts and flows from the mould cavities The crystal selector patterns 8, being made from a water soluble material, are dissolved in the autoclaving steam and are similarly removed from the mould cavity When the mould cavity is clean, the mould itself is fired in a kiln before use in the casting process.

The use of urea for the crystal selector pattern results in a composition of very high solubility, urea having a solubility in excess of 1000 g/l However, urea is brittle and the addition of polyvinyl acetate serves to improve the toughness of the composition.

Stearic acid serves to improve the dimensional stability of the resulting moulding, as well as improving the injection moulding properties of the composition Dimensional stability is important in order to avoid cracking of the mould by dimensional changes in the crystal selector pattern which result, for example, from temperature changes.

The present invention thus provides a crystal selector pattern which is suitable for use in an investment casting process, the selector pattern being made from a composition which can readily be formed into the desired shape by injection moulding, and which is easily removed from the moulding cavity by conventional processes used for removing wax patterns.

Claims

1 A crystal selector pattern for forming a crystal selector in an investment casting mould, the crystal selector pattern being formed from a water- soluble organic composition.

2 A crystal selector pattern as claimed in claim 1, in which the water-soluble organic composition is thermoplastic.

3 A crystal selector pattern as claimed in claim 1 or 2, which is formed in an injection moulding process.

4 A crystal selector pattern as claimed in any one of the preceding claims, in which the solubility of the water-soluble organic composition is not less than 500 g/l.

A crystal selector pattern as claimed in claim 4, in which the solubility of the water-soluble organic composition is not less than 900 g/l.

6 A crystal selector pattern as claimed in any one of the preceding claims, in which the water-soluble organic composition comprises hydroxypropylcellulose.

7 A crystal selector pattern as claimed in any one of claims 1 to 5, in which the water-soluble organic composition comprises urea.

8 A crystal selector pattern as claimed in claim 7, in which urea is present in the water-soluble organic composition in a proportion not less than 50 %.

9 A crystal selector pattern as claimed in claim 8, in which the urea is present in the water- soluble organic composition in a proportion not less than 70 %.

A crystal selector pattern as claimed in any one of the preceding claims, in which the water-soluble organic composition comprises polyvinyl acetate.

11 A crystal selector pattern as claimed in claim 10, in which the polyvinyl acetate is present in the water-soluble organic composition in a proportion not less than 10 %.

12 A crystal selector pattern as claimed in claim 11, in which the polyvinyl acetate in present in the water-soluble organic composition in a proportion not less than 15 %.

13 A crystal selector pattern as claimed in any one of the preceding claims, in which the water-soluble organic composition comprises stearic acid.

14 A crystal selector pattern as claimed in claim 13, in which the stearic acid is present in the water-soluble organic composition in a proportion not less than 1 %.

A crystal selector pattern in accordance with any one of the preceding claims, in which the water- soluble organic composition comprises 70 to 90 % urea, 10-25 % polyvinyl acetate and 1 to 5 % stearic acid.

16 A crystal selector pattern in accordance with claim 6, in which the water-soluble organic composition comprises 80 % urea, 18 % polyvinyl acetate and 2 % stearic acid.

17 A crystal selector pattern substantially as described herein with reference to, and as shown in, the accompanying drawings.

18 An investment casting process in which a crystal selector passage is formed in the mould by use of a crystal selector pattern in accordance with any one of the preceding claims.