GB2124116A

GB2124116A - A coating on a foundry core or mould

Info

Publication number: GB2124116A
Application number: GB08219932A
Authority: GB
Inventors: Gordon John Spenc Higginbotham; Peter Anthony Crooke
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-07-09
Filing date: 1982-07-09
Publication date: 1984-02-15

Abstract

An alumina rich coating on a silica core or mould used for casting single crystal, or directionally solidified castings in nickel based super alloy.

Description

SPECIFICATION Improvements in or relating to foundry cores and moulds This invention relates to foundry cores and moulds, and more particularly to such cores and moulds suitable for use in the manufacture of nickel super alloy castings by the "single crystal" or "directional solidification" techniques.

It is well known to cast gas turbine blades from nickel super alloys by the "single crystal" or "directional solidification" techniques, and to produce such castings usually necessitates the use of highly accurate shell moulds and cores to define features of the casting. An essential component of both cores and shells is silica which is used because of its refractory nature and may be present as either binder or solid particles or a mixture of both. Zircon is often also used as one of the other constituents of the refractory mixture. Large proportions of silica are used also because it produces a strong and stable core and is also relatively easily removed from the completed casting.

It is known however that the surface layers of such cores or moulds react with the hafnium in the nickel super alloy and this results in a glassy type material being formed upon the surface of the core or mould.

In normal conventional casting this glassy layer has an advantage in that it can actually improve the surface finish of the completed casting by smoothing out irregularities on the core surface.

However in the case of a "directionally solidified" or "single crystal" cast product the reaction between the hafnium within the metal and the silica containing material within the surface layer of the core or mould has a distinct disadvantage, in that it can cause grooves or similar such irregularities appearing at the cast surface if an excessive amount of hafnium oxide is produced. This is known as "canaling".

It is believed that such canaling is caused as the solidification front of the metal passes along the casting during its solidification. During this phase of the casting process the glassy layer appears not to remain evenly deposited upon the surface of the core or mould as in conventional casting. The glassy layer appears to migrate over the surface and is more likely to occur in locations where changes in either shape or cross-section of the casting occur.

The displaced glassy material is therefore no longer evenly disposed over the surface but tends to lie in ridges which are formed transversely of the solidification front of the metal within the casting. The glassy ridges therefore form grooves or canals within the surface of the completed casting.

An object of the present invention is to provide a ceramic core or mould consisting predominantly of silica in which the aforementioned disadvantages are substantially eliminated.

According to the present invention a foundry core or mould consisting predominantly of silica for use in the manufacture of directionally solidified or single crystal castings includes an alumina rich layer over at least a portion of its surface.

Preferably the alumina rich layer is applied to only a portion of the core or mould where it changes either cross-sectional area or shape. Alternatively the alumina rich layer may be applied by plasma spraying.

Preferably the completed core or mould is suitable for defining at least a portion of a cast single crystal or directionally solidified turbine blade manufactured from a nickel super alloy.

For better understanding thereof an embodiment of the present invention will be more particularly described by way of example only.

Silica foundry cores, or moulds are made by introducing a fluid slip into a mould and therefore allowing the material to dry out, and subsequently firing the moulded material to produce the finished core.

Alternatively the core material may consist of a mixture of ceramic material and thermosetting or thermoplastic resin which is introduced into a die to form the core or mould shape. The shaped component is thereafter fired to burn off the resin, and cure the core or mould. Alternatively shell moulds may be formed by dipping and stuccoing onto a wax pattern.

In accordance with the present invention after the core or mould has been manufactured by any of the well known conventional methods at least a portion of the surface of the core or mould is provided with an alumina rich layer. The layer is provided at least to some of the areas of the core where there are changes in either cross-section or shape and which require protection.

A particular alumina rich mixture which has been found to be suitable contains 72 parts of 5 micron fused alumina, 8 parts of 1 micron activated alumina, 1 part methyl cellulose and 22 part of binder, which contains of aluminium poly-hydroxy nitrate, and 1% chromium oxide added to the alumina to suppress a reaction leading to a loss of chromium in the surface of the casting.

The mixture is ball milled and dispersed in either water or alcohol and thereafter applied to either a selected portion or portions of the core or mould surface or in some circumstancesforthe sake or convenience etc. the surface may be completely covered with the coating. The coating may be accomplished by any of the conventional processes for applying a liquid covering for example spraying, painting or dipping. The coated core is thereafter allowed to dry and is then ready for use within a mould. Alternatively the mixture may be applied in dry form by vapour spraying.

It will be appreciated by those skilled in the art that although one alumina coating material has been particularly described the invention is not restricted to such a formulation as it would be relatively easy to formulate many different alumina rich coating materials which would function satisfactorily.

1. Afoundry core or mould consisting predominantly of silica for use in the manufacture of directionally solidified or single crystal castings, including an alumina rich layer over at least a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to foundry cores and moulds This invention relates to foundry cores and moulds, and more particularly to such cores and moulds suitable for use in the manufacture of nickel super alloy castings by the "single crystal" or "directional solidification" techniques. It is well known to cast gas turbine blades from nickel super alloys by the "single crystal" or "directional solidification" techniques, and to produce such castings usually necessitates the use of highly accurate shell moulds and cores to define features of the casting. An essential component of both cores and shells is silica which is used because of its refractory nature and may be present as either binder or solid particles or a mixture of both. Zircon is often also used as one of the other constituents of the refractory mixture. Large proportions of silica are used also because it produces a strong and stable core and is also relatively easily removed from the completed casting. It is known however that the surface layers of such cores or moulds react with the hafnium in the nickel super alloy and this results in a glassy type material being formed upon the surface of the core or mould. In normal conventional casting this glassy layer has an advantage in that it can actually improve the surface finish of the completed casting by smoothing out irregularities on the core surface. However in the case of a "directionally solidified" or "single crystal" cast product the reaction between the hafnium within the metal and the silica containing material within the surface layer of the core or mould has a distinct disadvantage, in that it can cause grooves or similar such irregularities appearing at the cast surface if an excessive amount of hafnium oxide is produced. This is known as "canaling". It is believed that such canaling is caused as the solidification front of the metal passes along the casting during its solidification. During this phase of the casting process the glassy layer appears not to remain evenly deposited upon the surface of the core or mould as in conventional casting. The glassy layer appears to migrate over the surface and is more likely to occur in locations where changes in either shape or cross-section of the casting occur. The displaced glassy material is therefore no longer evenly disposed over the surface but tends to lie in ridges which are formed transversely of the solidification front of the metal within the casting. The glassy ridges therefore form grooves or canals within the surface of the completed casting. An object of the present invention is to provide a ceramic core or mould consisting predominantly of silica in which the aforementioned disadvantages are substantially eliminated. According to the present invention a foundry core or mould consisting predominantly of silica for use in the manufacture of directionally solidified or single crystal castings includes an alumina rich layer over at least a portion of its surface. Preferably the alumina rich layer is applied to only a portion of the core or mould where it changes either cross-sectional area or shape. Alternatively the alumina rich layer may be applied by plasma spraying. Preferably the completed core or mould is suitable for defining at least a portion of a cast single crystal or directionally solidified turbine blade manufactured from a nickel super alloy. For better understanding thereof an embodiment of the present invention will be more particularly described by way of example only. Silica foundry cores, or moulds are made by introducing a fluid slip into a mould and therefore allowing the material to dry out, and subsequently firing the moulded material to produce the finished core. Alternatively the core material may consist of a mixture of ceramic material and thermosetting or thermoplastic resin which is introduced into a die to form the core or mould shape. The shaped component is thereafter fired to burn off the resin, and cure the core or mould. Alternatively shell moulds may be formed by dipping and stuccoing onto a wax pattern. In accordance with the present invention after the core or mould has been manufactured by any of the well known conventional methods at least a portion of the surface of the core or mould is provided with an alumina rich layer. The layer is provided at least to some of the areas of the core where there are changes in either cross-section or shape and which require protection. A particular alumina rich mixture which has been found to be suitable contains 72 parts of 5 micron fused alumina, 8 parts of 1 micron activated alumina, 1 part methyl cellulose and 22 part of binder, which contains of aluminium poly-hydroxy nitrate, and 1% chromium oxide added to the alumina to suppress a reaction leading to a loss of chromium in the surface of the casting. The mixture is ball milled and dispersed in either water or alcohol and thereafter applied to either a selected portion or portions of the core or mould surface or in some circumstancesforthe sake or convenience etc. the surface may be completely covered with the coating. The coating may be accomplished by any of the conventional processes for applying a liquid covering for example spraying, painting or dipping. The coated core is thereafter allowed to dry and is then ready for use within a mould. Alternatively the mixture may be applied in dry form by vapour spraying. It will be appreciated by those skilled in the art that although one alumina coating material has been particularly described the invention is not restricted to such a formulation as it would be relatively easy to formulate many different alumina rich coating materials which would function satisfactorily. CLAIMS

1. Afoundry core or mould consisting predominantly of silica for use in the manufacture of directionally solidified or single crystal castings, including an alumina rich layer over at least a portion of its surface.

2. A foundry core or mould as claimed in claim 1 in which the alumina rich layer is applied to only a portion of the core or mould where it changes either cross-sectional area or shape.

3. A foundry core or mould as claimed in claim 1 in which the alumina rich layer is applied to the entire core or mould surface.

4. Afoundry core or mould as claimed in any preceding claim in which the material constituting the alumina rich layer consists of 72 parts fused alumina, 8 parts activated alumina, 1 part methyl cellulose and 200 parts binder and 2 parts chromium oxide.

5. Afoundry core or mould as claimed in any preceding claim in which the materials constituting the alumina rich layer are disposed within a liquid carrier material and are applied to at least a portion ofthe core or mould by spraying, dipping or painting.

6. Afoundry core or mould as claimed in any of claims 1 to 4 in which the alumina rich layer is applied by plasma spraying.

7. Afoundry core or mould as claimed in any preceding claim suitable for defining at least a portion of a cast single crystal or directionally solidified turbine blade manufactured from nickel super alloy.

8. A method of making a foundry core or mould consisting predominantly of silica and including an alumina rich layer over at least a portion of its surface.

9. A foundry core or mould substantially as hereinbefore described by way of example only.