US20020005265A1 - Crystal selector pattern - Google Patents

Crystal selector pattern Download PDF

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Publication number
US20020005265A1
US20020005265A1 US09/887,053 US88705301A US2002005265A1 US 20020005265 A1 US20020005265 A1 US 20020005265A1 US 88705301 A US88705301 A US 88705301A US 2002005265 A1 US2002005265 A1 US 2002005265A1
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United States
Prior art keywords
crystal selector
pattern
water
soluble organic
organic composition
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Abandoned
Application number
US09/887,053
Inventor
David Ford
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Rolls Royce PLC
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Rolls Royce PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Rolls Royce PLC filed Critical Rolls Royce PLC
Assigned to ROLLS-ROYCE PLC reassignment ROLLS-ROYCE PLC ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: FORD, DAVID ALAN
Publication of US20020005265A1 publication Critical patent/US20020005265A1/en
Abandoned legal-status Critical Current

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C9/00—Moulds or cores; Moulding processes
    • B22C9/02—Sand moulds or like moulds for shaped castings
    • B22C9/04—Use of lost patterns
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C7/00—Patterns; Manufacture thereof so far as not provided for in other classes
    • B22C7/02—Lost patterns
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C30B11/14—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method characterised by the seed, e.g. its crystallographic orientation

Definitions

  • This invention relates to a crystal selector pattern for use in the preparation of investment casting moulds for single crystal casting processes.
  • 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.
  • molten metal is poured into the mould cavity to form the components.
  • the mould is then broken to release the formed components.
  • the mould 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.
  • 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.
  • 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.
  • 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.
  • polystyrene 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.
  • 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.
  • the water soluble organic composition is mouldable and so may be thermoplastic.
  • 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.
  • 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%.
  • a hydrophilic soluble polymer such as polyvinyl acetate
  • the composition may also comprise an additive to enhance the dimensional stability of the crystal selector.
  • an additive may, for example, comprise a lubricant such as stearic acid, which may be present in proportions of not less than 1%.
  • FIG. 1 represents an investment casting process
  • FIG. 2 shows a crystal selector pattern for use in the process represented in FIG. 1.
  • 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 .
  • 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 .
  • 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 .
  • 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.
  • 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 .
  • 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.
  • 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.
  • urea for the crystal selector pattern results in a composition of very high solubility, urea having a solubility in excess of 1000 g/l.
  • 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.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Molds, Cores, And Manufacturing Methods Thereof (AREA)

Abstract

A crystal selector pattern for use in forming a mould in an investment casting process, which is formed from a water-soluble organic composition which, in a preferred embodiment, comprises 80% urea.

Description

  • This invention relates to a crystal selector pattern for use in the preparation of investment casting moulds for single crystal casting processes. [0001]
    • BACKGROUND OF THE INVENTION
  • 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. [0002]
  • 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. [0003]
  • 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. [0004]
  • 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. [0005]
  • It is a further object of the present invention to provide a crystal selector pattern which is easily flushed from a mould cavity. [0006]
    • SUMMARY OF THE INVENTION
  • 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. [0007]
  • Preferably, the water soluble organic composition is mouldable and so may be thermoplastic. [0008]
  • Preferably, the solubility of the composition is not less than 500 g/l, and more preferably it is not less than 900 g/l. [0009]
  • 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%. [0010]
  • 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%. [0011]
  • 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%.[0012]
    • SUMMARY OF THE DRAWINGS
  • FIG. 1 represents an investment casting process; and [0013]
  • FIG. 2 shows a crystal selector pattern for use in the process represented in FIG. 1.[0014]
    • DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
  • FIG. 1 shows [0015] 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.
  • In use, molten alloy is poured into the pouring basin to fill the [0016] 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 [0017] 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 [0018] 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. [0019]
  • 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. [0020]

Claims (23)

I claim:
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, which is formed in an injection moulding process.
4. A crystal selector pattern as claimed in claim 1, in which the solubility of the water-soluble organic composition is not less than 500 g/l.
5. 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 claim 1, in which the water-soluble organic composition comprises hydroxypropylcellulose.
7. A crystal selector pattern as claimed in claim 1, in which the water-soluble organic composition comprises urea.
8. A crystal selector pattern as claimed in claim 1, in which the water-soluble organic composition comprises polyvinyl acetate.
9. A crystal selector pattern as claimed in claim 1, in which the water-soluble organic composition comprises stearic acid.
10. 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 which contains urea.
11. A crystal selector pattern as claimed in claim 10, in which urea is present in the water-soluble organic composition in a proportion not less than 50%.
12. A crystal selector pattern as claimed in claim 11, in which the urea is present in the water-soluble organic composition in a proportion not less than 70%.
13. A crystal selector pattern as claimed in claim 10, in which the water-soluble organic composition comprises polyvinyl acetate.
14. A crystal selector pattern as claimed in claim 13, in which the polyvinyl acetate is present in the water-soluble organic composition in a proportion not less than 10%.
15. A crystal selector pattern as claimed in claim 14, in which the polyvinyl acetate is present in the water-soluble organic composition in a proportion not less than 15%.
16. A crystal selector pattern as claimed in claim 10, in which the water-soluble organic composition comprises stearic acid.
17. A crystal selector pattern as claimed in claim 16, in which the stearic acid is present in the water-soluble organic composition in a proportion not less than 1.
18. 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 which comprises:
70 to 90% urea,
10-25% polyvinyl acetate, and
1 to 5% stearic acid.
19. A crystal selector pattern in accordance with claim 18, in which the water-soluble organic composition comprises:
80% urea,
18% polyvinyl acetate, and
2% stearic acids.
20. An investment casting process comprising:
making a pattern corresponding to the shape of a desired mould cavity, which mould cavity includes a crystal selector region;
investing the pattern with a hardenable coating of a ceramic material thereby to form a mould cavity about the pattern;
hardening the coating;
removing the pattern from the mould cavity; and
casting an article in the mould cavity,
wherein the pattern corresponding to the crystal selector region is made form a water-soluble organic composition, and wherein the step of removing the pattern corresponding to the crystal selector region comprises steam autoclaving the moulding cavity.
21. An investment casting process as claimed in claim 20, wherein the water-soluble organic composition contains urea.
22. An investment casting process as claimed in claim 20, wherein the water-soluble organic composition comprises:
70 to 90% urea,
10-25% polyvinyl acetate, and
1 to 5% stearic acid.
23. An investment casting process as claimed in claim 20, wherein the water-soluble organic composition comprises:
80% urea,
18% polyvinyl acetate, and
2% stearic acid.
US09/887,053 2000-06-27 2001-06-25 Crystal selector pattern Abandoned US20020005265A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0015673.7 2000-06-27
GBGB0015673.7A GB0015673D0 (en) 2000-06-27 2000-06-27 Crystal selector pattern

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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1452251A1 (en) * 2003-02-26 2004-09-01 ROLLS-ROYCE plc Method and mould for component casting using a directional solidification process
US20050045301A1 (en) * 2003-08-28 2005-03-03 Bullied Steven J. Investment casting
US20060175034A1 (en) * 2005-02-10 2006-08-10 Jorge Okhuysen-Caredenas Fluid-Soluble Pattern Material for Investment Casting Process, and Methods for Using Same
US20100051142A1 (en) * 2008-08-26 2010-03-04 Rolls-Royce Plc Directional solidification mould
CN102380577A (en) * 2011-11-02 2012-03-21 宁波万冠精密铸造厂 Multi-station automatic shell manufacturing machine for investing casting
WO2015080854A1 (en) * 2013-11-27 2015-06-04 United Technologies Corporation Method and apparatus for manufacturing a multi-alloy cast structure
CN109351951A (en) * 2018-11-29 2019-02-19 中国科学院金属研究所 A kind of process reducing single crystal blade platform rarefaction defect
CN110788279A (en) * 2019-11-01 2020-02-14 泰州市金鹰精密铸造有限公司 Preparation method of ceramic mould shell of single crystal high-temperature alloy turbine blade

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1452251A1 (en) * 2003-02-26 2004-09-01 ROLLS-ROYCE plc Method and mould for component casting using a directional solidification process
US20060157220A1 (en) * 2003-02-26 2006-07-20 Rolls-Royce Plc Component casting
US7152659B2 (en) 2003-02-26 2006-12-26 Rolls-Royce, Plc Component casting
US20050045301A1 (en) * 2003-08-28 2005-03-03 Bullied Steven J. Investment casting
US7201212B2 (en) * 2003-08-28 2007-04-10 United Technologies Corporation Investment casting
US20060175034A1 (en) * 2005-02-10 2006-08-10 Jorge Okhuysen-Caredenas Fluid-Soluble Pattern Material for Investment Casting Process, and Methods for Using Same
US20100051142A1 (en) * 2008-08-26 2010-03-04 Rolls-Royce Plc Directional solidification mould
CN102380577A (en) * 2011-11-02 2012-03-21 宁波万冠精密铸造厂 Multi-station automatic shell manufacturing machine for investing casting
WO2015080854A1 (en) * 2013-11-27 2015-06-04 United Technologies Corporation Method and apparatus for manufacturing a multi-alloy cast structure
US10449605B2 (en) 2013-11-27 2019-10-22 United Technologies Corporation Method and apparatus for manufacturing a multi-alloy cast structure
CN109351951A (en) * 2018-11-29 2019-02-19 中国科学院金属研究所 A kind of process reducing single crystal blade platform rarefaction defect
CN110788279A (en) * 2019-11-01 2020-02-14 泰州市金鹰精密铸造有限公司 Preparation method of ceramic mould shell of single crystal high-temperature alloy turbine blade

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Publication number Publication date
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Owner name: ROLLS-ROYCE PLC, ENGLAND

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:FORD, DAVID ALAN;REEL/FRAME:012118/0297

Effective date: 20010506

STCB Information on status: application discontinuation

Free format text: ABANDONED -- FAILURE TO RESPOND TO AN OFFICE ACTION