WO1996022849A1 - Investment casting mould - Google Patents

Investment casting mould Download PDF

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Publication number
WO1996022849A1
WO1996022849A1 PCT/GB1996/000095 GB9600095W WO9622849A1 WO 1996022849 A1 WO1996022849 A1 WO 1996022849A1 GB 9600095 W GB9600095 W GB 9600095W WO 9622849 A1 WO9622849 A1 WO 9622849A1
Authority
WO
WIPO (PCT)
Prior art keywords
coating
silicon carbide
slurry
investment casting
mullite
Prior art date
Application number
PCT/GB1996/000095
Other languages
French (fr)
Inventor
Christopher Patrick Hyndman
Steven Raymond Irwin
David Thornton Pindar
Original Assignee
Aetc Limited
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 Aetc Limited filed Critical Aetc Limited
Priority to AU43971/96A priority Critical patent/AU4397196A/en
Publication of WO1996022849A1 publication Critical patent/WO1996022849A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
    • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
    • B22C1/18—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
    • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
    • B22C1/165—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents in the manufacture of multilayered shell moulds

Definitions

  • This invention is concerned with investment casting moulds and, in particular but not exclusively, with investment casting moulds for use in casting directionally- solidified, columnar grain structure (DS) , or single crystal structure (SX) components.
  • DS columnar grain structure
  • SX single crystal structure
  • 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.
  • moulds comprise alumina or alumino-silicate particles within, for example, a zircon flour matrix bonded with silica.
  • alumina or alumino-silicate particles within, for example, a zircon flour matrix bonded with silica.
  • Such moulds have disadvantages when DS or SX components are involved because the mould experiences high temperatures for an extended period and the strength and stiffness of the mould may be inadequate.
  • the invention provides an investment casting mould comprising a face coating and a back-up coating, characterised in that 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 said existing moulds (at least 7 times greater) . Furthermore, it is surprisingly found to exhibit less creep (about half as much) and to have greater strength at temperatures up to about 1500°C.
  • the back-up coating also comprises 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 face coating may have a different structure to that of the back-up coating.
  • 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.
  • a conventional face coating formed from a slurry comprising silica, alumina and zircon may be used.
  • the invention also provides a method of forming an investment casting mould, characterised in that the method comprises making a slurry comprising colloidal silica, 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.
  • 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, ie there is at least enough alumina to achieve a stoichiometric balance.
  • the slurry may contain at least 57.4 gs of reactive alumina.
  • 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.
  • an investment casting mould which comprised a face coating comprising mullite bonded by mullite, and a back-up coating comprising silicon carbide bonded by mullite.
  • a combination of a wax pattern and a face coating formed on the pattern was formed.
  • the pattern was dipped into an aqueous slurry containing:
  • colloidal silica (25% solids) ; 312 gs of fine ground reactive alumina; and 1.4 kgs of dense fused mullite (325 mesh).
  • the slurry also contained conventional wetting and de- airing agents.
  • the dipped pattern was then dusted with a stucco of white fused mullite having a grain dimension of 0.25 to 0.5mm.
  • 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.
  • the combination of the pattern and the face coating formed thereon was dipped into an aqueous slurry containing:
  • 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 gem" 3 , 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 at least 5 layers.
  • the final dipping was only followed by drying.
  • the wax pattern was removed in a conventional way and the mould was fired.
  • 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 7 w.m/°k which is about 7 times that of existing moulds, a creep of about half that of existing moulds, a green strength of about 10 MPa (compared to about 5 MPa for existing moulds) , a strength of about 20 MPa at 1300°c (about 6 MPa) , and a strength of about 5 MPa at 1500°c (about 5 MPa) .

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Compositions Of Oxide Ceramics (AREA)
  • Mold Materials And Core Materials (AREA)

Abstract

An investment casting mould having a face coating and a back-up coating. The back-up coating comprises silicon carbide bonded by mullite. The back-up coating is formed from a slurry containing silica, reactive alumina, and silicon carbide.

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 directionally- solidified, 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 investment casting moulds comprise alumina or alumino-silicate particles within, for example, a zircon flour matrix bonded with silica. However, such moulds have disadvantages when DS or SX components are involved because the mould experiences high temperatures for an extended period and the strength and stiffness of the mould may be inadequate.
Existing moulds for DS and SX components are formed from slurries containing mullite (3Al203.2Si02) , zircon flour, reactive alumina, and colloidal silica. However, 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, characterised in that 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 said existing moulds (at least 7 times greater) . Furthermore, it is surprisingly found to exhibit less creep (about half as much) and to have greater strength at temperatures up to about 1500°C.
Preferably, the back-up coating also comprises 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 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 conventional face coating formed from a slurry comprising silica, alumina and zircon may be used.
The invention also provides a method of forming an investment casting mould, characterised in that the method comprises making a slurry comprising colloidal silica, 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 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 gs of 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.
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 gs of colloidal silica (25% solids) ; 312 gs of fine ground reactive alumina; and 1.4 kgs of dense fused mullite (325 mesh). The slurry also contained conventional wetting and de- airing agents.
The dipped pattern was then dusted with a stucco of white fused mullite having a grain dimension of 0.25 to 0.5mm. 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 gs of colloidal silica (25% solids) ;
550 gs of silicon carbide flour (600 grit) ;
550 gs of silicon carbide flour (220 grit) ;
550 gs of large grain silicon carbide (46 grit) ; and
345 gs of 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 gem"3, 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 at least 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 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 7 w.m/°k which is about 7 times that of existing moulds, a creep of about half that of existing moulds, a green strength of about 10 MPa (compared to about 5 MPa for existing moulds) , a strength of about 20 MPa at 1300°c (about 6 MPa) , and a strength of about 5 MPa at 1500°c (about 5 MPa) .

Claims

An investment casting mould comprising a face coating and a back-up coating, characterised in that at least the back-up coating comprises silicon carbide bonded by mullite.
An investment casting mould according to claim 1, characterised in that the back-up coating also comprises reactive alumina.
An investment casting mould according to either one of claims 1 and 2, characterised in that the face coating comprises mullite bounded by mullite.
A method of forming an investment casting mould, characterised in that the method comprises making a slurry comprising colloidal silica, 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.
A method according to claim 4, characterised in that 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.
A method according to either one of claims 4 and 5, characterised in that the slurry comprises silicon carbide in the form of a flour and also large grain silicon carbide. A method according to claim 6, characterised in that the flour contains two or more grain sizes.
PCT/GB1996/000095 1995-01-25 1996-01-19 Investment casting mould WO1996022849A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU43971/96A AU4397196A (en) 1995-01-25 1996-01-19 Investment casting mould

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9501448A GB2297285A (en) 1995-01-25 1995-01-25 Investment casting mould
GB9501448.6 1995-01-25

Publications (1)

Publication Number Publication Date
WO1996022849A1 true WO1996022849A1 (en) 1996-08-01

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ID=10768546

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1996/000095 WO1996022849A1 (en) 1995-01-25 1996-01-19 Investment casting mould

Country Status (3)

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AU (1) AU4397196A (en)
GB (1) GB2297285A (en)
WO (1) WO1996022849A1 (en)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998042460A2 (en) * 1997-03-25 1998-10-01 Komtek, Inc. Producing a metal article by casting and forging
CN101633031A (en) * 2008-07-25 2010-01-27 通用电气公司 High emittance shell molds for directional casting
WO2015112232A3 (en) * 2013-11-26 2015-10-08 General Electric Comapny Silicon carbide-containing mold and facecoat compositions and methods for casting titanium and titanium aluminide alloys
CN105268906A (en) * 2014-06-04 2016-01-27 通用电气公司 Casting mold with grading of silicon carbide
US9511417B2 (en) 2013-11-26 2016-12-06 General Electric Company Silicon carbide-containing mold and facecoat compositions and methods for casting titanium and titanium aluminide alloys
US9802243B2 (en) 2012-02-29 2017-10-31 General Electric Company Methods for casting titanium and titanium aluminide alloys

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4664172A (en) * 1984-08-09 1987-05-12 Agency Of Industrial Science And Technology Method for production of investment shell mold for grain-oriented casting of super alloy
EP0343401A2 (en) * 1988-05-19 1989-11-29 Ae Turbine Components Limited Investment casting mould
WO1994003410A1 (en) * 1992-07-31 1994-02-17 Lonza A.G. Refractory moulded articles made of silicon carbide with mullite bonding, a method of producing such articles, a moulding compound for use as an intermediate in the method, and the use of such articles as kiln furniture
JPH06219828A (en) * 1993-01-27 1994-08-09 Chichibu Cement Co Ltd Production of mullite-silicon carbide combined ceramics

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1342599A (en) * 1970-03-17 1974-01-03 Doulton & Co Ltd Investment casting cores and their method of manufacture
DE9307468U1 (en) * 1993-05-17 1994-09-29 Hüttenes-Albertus Chemische Werke GmbH, 40549 Düsseldorf Sizing for the production of form coatings

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4664172A (en) * 1984-08-09 1987-05-12 Agency Of Industrial Science And Technology Method for production of investment shell mold for grain-oriented casting of super alloy
EP0343401A2 (en) * 1988-05-19 1989-11-29 Ae Turbine Components Limited Investment casting mould
WO1994003410A1 (en) * 1992-07-31 1994-02-17 Lonza A.G. Refractory moulded articles made of silicon carbide with mullite bonding, a method of producing such articles, a moulding compound for use as an intermediate in the method, and the use of such articles as kiln furniture
JPH06219828A (en) * 1993-01-27 1994-08-09 Chichibu Cement Co Ltd Production of mullite-silicon carbide combined ceramics

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 018, no. 588 (C - 1271) 10 November 1994 (1994-11-10) *

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998042460A2 (en) * 1997-03-25 1998-10-01 Komtek, Inc. Producing a metal article by casting and forging
WO1998042460A3 (en) * 1997-03-25 1998-10-29 Komtek Inc Producing a metal article by casting and forging
CN101633031A (en) * 2008-07-25 2010-01-27 通用电气公司 High emittance shell molds for directional casting
US9802243B2 (en) 2012-02-29 2017-10-31 General Electric Company Methods for casting titanium and titanium aluminide alloys
WO2015112232A3 (en) * 2013-11-26 2015-10-08 General Electric Comapny Silicon carbide-containing mold and facecoat compositions and methods for casting titanium and titanium aluminide alloys
US9192983B2 (en) 2013-11-26 2015-11-24 General Electric Company Silicon carbide-containing mold and facecoat compositions and methods for casting titanium and titanium aluminide alloys
US9511417B2 (en) 2013-11-26 2016-12-06 General Electric Company Silicon carbide-containing mold and facecoat compositions and methods for casting titanium and titanium aluminide alloys
CN105268906A (en) * 2014-06-04 2016-01-27 通用电气公司 Casting mold with grading of silicon carbide
CN105268906B (en) * 2014-06-04 2020-01-21 通用电气公司 Casting mold with silicon carbide classification

Also Published As

Publication number Publication date
GB9501448D0 (en) 1995-03-15
AU4397196A (en) 1996-08-14
GB2297285A (en) 1996-07-31

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