Patent Publication Number: US-2002005265-A1

Title: Crystal selector pattern

Description:
[0001] This invention relates to a crystal selector pattern for use in the preparation of investment casting moulds for single crystal casting processes.  
       BACKGROUND OF THE INVENTION  
       [0002] 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.  
       [0003] 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.  
       [0004] 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.  
       [0005] It is an object of the present invention to provide a crystal selector pattern which is sufficiently robust for reliable use in an investment casting process.  
       [0006] It is a further object of the present invention to provide a crystal selector pattern which is easily flushed from a mould cavity.  
       SUMMARY OF THE INVENTION  
       [0007] 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.  
       [0008] Preferably, the water soluble organic composition is mouldable and so may be thermoplastic.  
       [0009] Preferably, the solubility of the composition is not less than 500 g/l, and more preferably it is not less than 900 g/l.  
       [0010] 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%.  
       [0011] 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%.  
       [0012] 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%. 
     
    
    
     SUMMARY OF THE DRAWINGS  
     [0013]FIG. 1 represents an investment casting process; and  
     [0014]FIG. 2 shows a crystal selector pattern for use in the process represented in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
     [0015]FIG. 1 shows mould cavities  2  for casting turbine blades of a gas turbine engine. The mould cavities  2  are present in a shell of ceramic material (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 .  
     [0016] 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.  
     [0017] 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.  
     [0018] 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.  
     [0019] 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.  
     [0020] 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.