GB2078596A

GB2078596A - Method of Making a Blade

Info

Publication number: GB2078596A
Application number: GB8020145A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1980-06-19
Filing date: 1980-06-19
Publication date: 1982-01-13

Abstract

The invention relates to a method of making a cast metal blade for a gas turbine engine with a closure for an internal cooling passage, and to the blade made by this method. According to the method a composite casting core is formed from two materials, the first of which (17) defines a cavity 31 within the metal blade Fig. 5 while the second of which (15) forms a closure for this cavity. The core is used in the conventional lost-wax coating process to provide a shell mould 29, Fig. 4 but only the part (17) of the core made of the first material is leached out after casting. This produces a cavity (31) closed by the piece (15) of the core left in situ. <IMAGE>

Description

SPECIFICATION Method of Making a Blade This invention relates to a method of making a cast blade for a gas turbine engine and to the blade made by this method. 4 In the manufacture of cast blades foxs gas turbine engines the formation of the various cooling passages within the aerofoil has always provided problems which have in general been solved by various methods. One such problem which has proved relatively intractable lies in the sealing of the tip extremities of cooling air cavities and the provision of so-called 'dust holes' which extend from the tip extremities of cooling passages through the end cap of the blade.These passages, as their name implies, are provided to enable a continuous small flow of cooling fluid which prevents the build-up of dust or other forms of dirt which may block the cooling passages with deleterious effect.

In the past a variety of methods have been used to seal the extremities and simultaneously to form these holes, ranging from the use of brazed or otherwise fastened blanking plates to the use of casting cores with very thin sections which define the dust holes. However, none of these methods has proved entirely satisfactory, since for instance the attached plates have tended to crack or to fly off the blade when operatiiig, while the use of a very thin core portion makes the casting process difficult and prone to a high scrap rate.

The present invention provides a method of making a cast blade which enables the closure of the tip extremity incorporating if required the dust-holes to be formed relatively simply and with good mechanical integrity.

According to the present invention a method of making a cast blade for a gas turbine engine comprises forming a composite casting core of two materials, the first of which defines the shape of a cavity within the blade and the second of which forms a closure for the tip extremity of said cavity in the finished blade, forming a wax pattern of the blade which incorporates the composite core, investing the wax pattern with ceramic to form a mould, removing the wax pattern from the mould to leave the core within the mould cavity, casting metal into the mould cavity, removing the mould from the cast metal, and removing that part of the core formed from the first material while leaving the remainder of the core in situ in the blade.

Preferably the part of the core made of the second material has a passage therethrough 'which forms the dust hole in the finished blade.

This part may be provided with a waisted portion which gives a mechanical retention of the part in the finished blade, and may also extend beyond the tip of the as-cast blade to provide support for the casting core during the casting operation.

In a preferred embodiment the first material comprises essentially silica while the second comprises essentially alumina.

The invention will now be particularly described merely by way of example with reference to the accompanying drawings in which: Figures 1 to 4 illustrate stages in the method of the invention the scale of which varies from Figure to Figure and, Figure 5 is a section through the blade made by the method of Figures 1-4.

In the manufacture of a blade in accordance with the invention the first step is to make a composite core. The core is made in two parts, one of which comprises a silica tube while the second comprises an injection moulded alumina piece. The silica tube is made by a conventional glass tube-forming process such as extrusion or the like, while Figure 1 illustrates the method of making the alumina piece.

In this method a split die made up of halves 10 and 11 is used, the die halves together defining a die cavity 12 and an entry passage 1 3 for the alumina material. The die halves also provide location for a pin 14. To make the piece, the die halves 10 and 11 and the pin 14 are assembled together as shown in Figure 1 and the alumina in the form of an aqueous suspension is injected under pressure through the entry passage 13 by means not shown.

The alumina fills the cavity 12 and is allowed to solidify. The pin 14 is then withdrawn and the die halves supported to leave a 'green' alumina piece 1 5 having a form of 'bobbin' shape and a central passage 1 6. This piece is then fired to sinter the material.

The piece 1 5 is then attached to the tip of a silica tube 1 7 made as discussed above, by a conventional method. As can best be seen from Figure 2 the piece 15 in addition to the passage 16 has a pair of annular flange parts 18 and 19 which between them define a waisted portion and a spigot 20 which surrounds one end of the central passage. The spigot 20 is sized to fit within the tip of the tube 1 7 and gives some mechanical retention between the two parts, and this retention is completed by cementing or otherwise fastening the alumina piece to the tube.

The piece 1 5 and the tube 1 7 together make up a composite casting core. This core is mounted in the die 21 of a wax injection machine, the die 21 being shown in Figure 3. The die consists of die halves 22 and 23 defining between them a cavity 24 whose shape is that of the desired cast blade but which also has depressions at 25 and 26 within which may be located the two ends of the composite core, and a wax entry passage at 27.

In use, the die 21 is assembled with the core 15/17 in place and molten wax is injected via the passage 27 to fill the cavity 24. The wax then solidifies, entrapping the composite core 15/17.

and the die 21 is split to allow the solid wax pattern 28 with embedded core to be removed.

The pattern 28 is then invested with ceramic by a conventional technique, which will normally involve the dipping of the pattern into ceramic slurries followed by raining fragments of dry ceramic to stucco the slurry coated pattern. In this way the shell mould 29 (Figure 4) is built up to have an internal form which is the same as that of the wax pattern 28. It will be appreciated that depending on the casting process to be used, a variety of ingates, runners and risers may be formed by attaching separately formed wax pieces to the pattern before the ceramic coating step. For illustrative purposes only one ingate 30 is shown as being formed as part of the shell mould 29.

It will be noted that because the ends of the composite core 15/17 project beyond the wax pattern 28, these ends will become firmly embedded in the shell mould 29 firmly locating the composite core within the mould. When the shell and wax are heated, the wax will melt and be allowed to run out of the mould, leaving the core embedded within the empty mould.

Molten metal is next poured into the mould 29 through the ingate 30 and is allowed to solidify.

The mould 29 is then destroyed, usually by a mechanical operation, and the silica tube 1 7 leached from the interior of the blade thus formed by the use of a chemical reagent. Because the piece 1 5 has a different composition from that of the tube 17, it is possible to choose a reagent which will remove the tube 1 7 but will not affect the piece 1 5.

Therefore the finished casting has a cooling air passage 31 formed by the first part 1 7 of the composite core, and a closure for the end of this passage provided by the piece 1 5 which is embedded within the solid metal of the tip of the blade. The passage 1 6 forms a dust hole as described above, while the waisted part between the flanges 1 8 and 1 9 provides a mechanical key between the alumina piece and the surrounding metal which will ensure that the alumina will be retained against centrifugal and other loads.

It will be seen that the piece 5 extends from the tip of the blade, and this may require to be dressed down to be flush with the rest of the blade tip in a separate machining operation.

It should be noted that the example described above is a relatively simple application of the present invention and that a variety of modifications are possible. Thus in particular the silica tube 17 could be replaced by more complex structures made of different materials, while the piece 1 5 could also be made of different material and each blade could embody more than one said piece.

Of course, the method of the invention could also be applied to specialised casting processes such as directional solidification and single crystal casting methods.

Claims

1. A method of making a cast blade for a gas turbine engine comprising forming a composite casting core of two materials, the first of which defines the shape of a cavity within the blade and the second of which forms a closure for the tip extremity of said cavity in the finished blade, forming a wax pattern of the blade which incorporates the composite core, investing the wax pattern with ceramic to form a mould, removing the wax pattern from the mould to leave the core within the mould cavity, casting metal into the mould cavity, removing the mould from the cast metal, and removing that part of the core formed from the first material while leaving the remainder of the core in situ in the blade.

2. A method as claimed in claim 1 and in which that part of the composite core made of the second material is formed with a passage therethrough which forms a dust hole in the cast blade.

3. A method as claimed in claim 1 or claim 2 in which that part of the composite core made of the second material is formed with a waisted portion which provides mechanical retention of the part within the cast blade.

4. A method as claimed in any one of the preceding claims and in which at least one end of the composite core is arranged to extend beyond the boundaries of the as-cast blade to provide support for the casting core during the cast operation.

5. A method as claimed in any one of the preceding claims and in which said first material comprises essentially silica while said second material comprises essentially alumina.

6. A method substantially as hereinbefore particularly described with reference to the accompanying drawings.

7. A blade made by the method of any one of the preceding claims.