GB2326363A

GB2326363A - Casting using a wax model produced with an auxiliary casting die

Info

Publication number: GB2326363A
Application number: GB9813346A
Authority: GB
Inventors: Axel Rosmann; Juergen Wortmann
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1997-06-20
Filing date: 1998-06-19
Publication date: 1998-12-23
Anticipated expiration: 2018-06-19
Also published as: FR2764829B1; FR2764829A1; DE19726111C1; GB2326363B; GB9813346D0

Abstract

A method for casting a turbomachine blade 1 having a blade plate 2 and a blade base 3 for the locking the blade 1 into a rotor comprises the following steps: a) making a wax model of the blade 1, with the blade base 3 being produced by filling an auxiliary casting die 4a which has the blade base contour; b) constructing a shell mould 5 on the wax model of the blade 1 with the base casting die 4a; c) melting out the wax model after the solidification of the shell mould 5; d) casting the blade 1; and e) removing the shell mould 5 after the solidification of the casting. This allows turbomachine blades to be manufactured whose bases have great accuracy and surface quality and require no or only slight finishing, since they are defined by the auxiliary die and not by the shell mould. The auxiliary die can either be a mould post or it can remain on the base and itself constitute the outer surface.

Description

CASTING A TURBINE BLADE is 2326363 The invention relates to a method for

manufacturing a two part rotary blade, in particular a turbomachine or turbine blade, by means of casting techniques; such a blade has a blade plate for interacting with the fluid and a blade base which is constructed for the positive-locking positioning of the blade plate in a rotor.

The blades for rotors of turbomachines, for example gas turbines or jet engines, are generally produced by the so-called lost-wax process. For this purpose a wax model of the blade is made and a shell mould is constructed on the wax model. After the solidification of the shell mould the wax model is melted out to form a cavity in the shell mould which corresponds to the desired contour of the blade. This cavity is filled up with molten metal to cast the blade. After the solidification of the casting the shell mould is removed.

Adequate accuracy and surface quality of the plate section of the blade can be achieved with this method; however, the shrouds and the blade base must be machined, by grinding for instance, in consideration of the greater accuracy demands which are made on the bearing surfaces than on the blade. This additional processing constitutes a large portion of the new-part costs. If it were possible to make this additional processing superfluous it would signify a decisive advantage in terms of costs.

In the seventies it was possible to prove, at least in terms of experimental technology, that adequately great accuracy can be attained if the blades are cast into the Christmas-tree grooves for accommodating the blade base in the rotor of the turbine. However, a problem here was the unfavourable is columnar grain structure resulting from the rapid cooling starting at the relatively cold rotor disc.

One feature was that after cooling, the blades could be removed from the rotor: there was no metallic connection because of the oxide skin that forms on the surface of the blade base.

It is an object of the invention to create a method for manufacturing a turbomachine blade by means of casting techniques without a costly finishing of the blade base being necessary.

The invention provides a method for manufacturing a turbomachine blade by means of casting techniques, the blade having a blade plate and a blade base designed for the form-locking positioning of the blade plate in a rotor, including the following procedural steps:

a) making a wax model of the blade, the blade base being produced by filling a casting die which has the blade base contour, b) constructing a shell mould on the wax model of the blade and on the base casting die, melting out the wax model after the solidification of the shell mould, d) casting the blade, and removing the shell mould after the solidification of the casting.

An advantage of methods in accordance with the invention is that after the casting the blades, and in particular the blade base, have the necessary accuracy and surface quality so that an additional finishing is not necessary or, at most, is only necessary to a small extent. This is because the blade base form is defined by the blade base casting die, which is a form of auxiliary mould, and not by the main (shell) mould.

Another advantage is that the casting dies provided for manufacturing the blade base can be manufactured c) e) economically by using reasonably priced materials and favourable manufacturing methods.

The casting die will generally have an inner contour which corresponds to the intended outer contour of the blade base and, after the solidification of the casting, is preferably removed with the shell mould.

The casting die can be reusable so as to be used repeatedly for casting blade bases; advantageously it is manufactured from a heat-resistant, adequately oxidation-resistant alloy.

As an alternative to using a reusable casting die, the casting die can be a throw-away part used only once for casting a blade base; here it can be manufactured from a low-alloy steel or from a ceramic material.

The casting die can be manufactured by means of injection moulding, embossing or extrusion moulding.

Advantageously, the casting die is removed at a raised mould-release temperature, above room temperature. Moreover, the blade may be cast in a slightly oxidizing atmosphere. This has the advantage that an oxide skin forms on the surface of the cast part, preventing a material closure by welding between the cast part and the casting die and facilitating release.

According to another embodiment of the invention the casting die is manufactured as a thin-walled mould part of a heat-resistant material and has an outer contour corresponding to the intended outer contour of the blade base; after the solidification of the casting it remains on the blade base in order to form its surface zone. The advantage of this is that the outer contour of the blade base is already accurately defined by the outer contour of the casting die before the casting, whereby even greater accuracy and better surface quality of the blade base can be achieved.

The casting die in such an embodiment can be is manufactured by means of injection moulding, or alternatively as an extruded section or rolled section. It is advantageously manufactured from a fine-grained casting die material.

According to a variant the casting die which forms the surface zone of the blade base can be fused with the cast part. However, in an alternative variant the casting die which forms the surface zone of the blade base is held on the cast part by means of form or positive-locking closure, without being fused therewith.

Advantageously, the shell mould with the base casting die is brought to an increased temperature before the blade is cast. This avoids the formation of an undesirable grain structure in the cast part.

According to an embodiment of the invention the blade base is constructed as a Christmas-tree base, but it can also be constructed as a dovetail base. This has the advantage that the great accuracy necessary for constructing the blade base as a Christmas-tree base because the multiplicity of simultaneously acting bearing surfaces is avoided, so that the maintenance of tolerances is less critical. Also, the dovetail base can be constructed in such a way that it is supported by a circumferential groove in the rotor disc which supports the blade plates.

For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows two sectional views a) and b) through a turbine blade manufactured by means of casting techniques in accordance with an embodiment of the invention, the blade being shown cast into the shell mould and the base casting die; Figure 2 shows a sectional view through a base casting die, according to Figure 1; and Figure 3 shows a sectional view through a turbine blade manufactured according to a second embodiment of the invention, the blade being shown already mounted on a rotor.

Figures la) and b) show a longitudinal section, section A, and a crosssection, section B, of a turbomachine blade 1 which is manufactured by means of casting techniques according to a first method in accordance with the invention. The complete blade 1 comprises a blade plate 2 and a blade base 3 which is used for the form- or positive-locking of the blade plate 2 in a rotor of a turbine. The turbine can be, for example, a turbine, a gas turbine or a jet engine. In the embodiment shown, the blade base 3 is constructed as a Christmas-tree base; the multiple bearing surfaces of the blade base rest on corresponding bearing surfaces of a recess formed in the rotor of the turbine, and in this way secure the blade 1.

The manufacture of the blade 1 takes place as follows. First of all a wax model of the blade 1 has to be made. This is done in two parts. The model of the plate blade is made in the usual way. The wax model of the blade base 3 is made using a casting die 4a having internally the negative contours of the blade base 3, and this is filled with wax. Such a casting die is shown in cross section in Figure 2. This will be explained in more detail later. It is important for the casting die 4a to be an accurate negative mould of the blade base 3, so as to impart to the outer contour of the base the necessary accuracy and surface quality. In the manufacture of the wax model of the blade 1 the accurate mutual positioning of the wax model of the blade plate 2 and the casting die 4a, or of the wax model of the blade base 3 which is formed by the is casting die 4a, is of considerable importance.

After making the wax model of the blade 1 a shell mould 5 is formed on the entire wax model including the base casting die 4a. The shell mould 5 is produced in a suitable manner from a suitable material, as is usual with the lost-wax process. After the solidification of the shell mould 5 the wax model of the blade 1 is melted out and in this way a cavity in the shell mould 5 is formed, which corresponds precisely to the negative mould of the blade 1 to be manufactured. The blade 1 is then cast. The casting in itself takes place in turn in the usual way, as is common with lostwax casting. After the solidification of the casting the shell mould 5, with the base casting die 4a, is removed. In this way the turbomachine blade 1 is manufactured practically to its final dimensions with adequate accuracy and surface quality, so that no finishing, or at most only slight finishing, is necessary.

The base casting die 4a shown in Figure 2, which is used for manufacturing a turbomachine blade 1 by means of casting techniques, as is shown in Figures la) and ib), has an inner contour which corresponds to the intended outer contour of the blade base 3. After the solidification of the casting the base casting die 4a is removed from the casting together with the shell mould 5. The casting die 4a can be reusable and can therefore be used repeatedly to cast blade bases 3, or it can be a throw-away part and is used only once for casting a single blade base 3. For the case where the casting die 4a is to be reusable, it must consist of a material which is thermally and mechanically adequately resistant to a repeated casting. An adequately heatresistant and oxidation-resistant alloy is suitable as material for the casting die 4a. on the other hand, if the casting die 4a is only intended for a single use, a is cheaper material can be used which is economical when manufactured on an appropriately large scale but nevertheless provides adequate strength, accuracy and surface quality for a single casting process. Lowalloy steels or ceramics are suitable for this purpose. The casting die 4a can be manufactured by means of injection moulding, embossing or extrusion moulding, with the accuracy and surface quality of the part being significant.

Before the casting process the shell mould 5 and the base casting die 4a are brought to a suitable raised temperature, which avoids the formation of an undesirable crystalline structure, and are then cast on in the usual way. After the solidification of the casting the casting die 4a together with the shell mould 5 is removed at a suitable mould-release temperature; here the base casting die 4a, in the case of a reusable casting die, is to be removed without being destroyed, whereas, in the case of a base casting die 4a which is to be used only once, it can also be destroyed during the mould-release process. The ability of the casting die to be released from the mould depends on suitable temperature control and atmosphere before and during the casting process. A slightly oxidizing atmosphere makes the removal of the casting from the casting die possible because an oxide skin which forms on the casting avoids a material fusion due to welding.

Figure 3 shows in cross section a turbomachine blade which has been manufactured according to another embodiment of the casting method in accordance with the invention. In place of a casting die which has an inner contour corresponding to the intended outer contour of the blade base 3 of the turbomachine blade 1 and which, after the solidification of the casting, is removed together with the shell mould 5, a casting die is 4b is used here which has an outer contour corresponding to the intended outer contour of the blade base 3 and which, after the solidification of the casting, remains on the blade base 3 in order to form its surface zone. For this purpose the casting die 4b is manufactured as a thinwalled mould part from a heat-resistant material and its outer surface is processed with appropriate accuracy and surface quality in order to satisfy the demands which are made of the accuracy and the surface quality of the blade base 3. By contrast to the previous embodiment, in the embodiment shown this blade base 3 is manufactured as a dovetail base, which is inserted into a corresponding recess on the circumference of the rotor 6 of the turbomachine. The casting die 4b which forms the surface zone of the blade base 3 can be manufactured as a section part by means of injection moulding or by means of extrusion moulding or rolling. Advantageously, the casting die 4b consists of a finegrained casting die material. The casting process of the blade 1 can be carried out in such a way that the base body of the casting is fused with the casting die 4b which forms the surface zone of the base 3. Alternatively, the casting process can also be carried out in such a way that the casting die 4b which forms the surface zone of the blade base 3 is not fused with the cast body. In this case the casting die 4b is held positively on the cast part by means of form locking.

In a construction of the blade base 3 as a dovetail base it would seem appropriate to construct the recesses on the rotor 6 which accommodate the dovetail bases of the turbine blades 1 as a single circumferential groove, with manufacturing expenditure being further reduced as a result.

Claims

Claims

A method for manufacturing a turbomachine blade having a blade plate (2) and a blade base (3) for positioning the blade plate (2) in a rotor, including the following steps:

a) making a wax model of the blade (1), with the blade base (3) being produced by filling into a casting die (4a; 4b) which has the blade base contour, constructing a shell mould (5) on the wax model of the blade (1) with the base casting die (4a; 4b), melting out the wax model after the solidification of the shell mould (5), casting the blade (1), removing the shell mould (5) after the solidification of the casting.

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2. A method according to claim 1, in which the casting die (4a) has an inner contour which corresponds to the intended outer contour of the blade base (3), and, after the solidification of the casting, the casting die (4a) is removed with the shell mould (5).
3. A method according to claim 2, in which the casting die (4a) is reusable and is removed intact for repeated use in casting blade bases (3).
4. A method according to claim 3, in which the reusable casting die (4a) is manufactured from a heatresistant and oxidation-resistant alloy.
5. A method according to claim 2, in which the casting die (4a) is a throw-away part and is used only once for casting a blade base (3).
6. A method according to claim 5, in which the throw-away casting die (4a) is manufactured from a lowalloy steel.
7. A method according to claim 5, in which the throw-away casting die (4a) is manufactured from a ceramic material.
8. A method according to any of claims 5 to 7, in which the casting die (4a) is manufactured by means of injection moulding, embossing or extrusion moulding.
9. A method according to any of claims 2 to 8, in which the casting die (4a) is removed at an increased mould-release temperature.
10. A method according to one of claims 2 to 9, in which the blade (1) is cast in a slightly oxidizing atmosphere.
11. A method according to claim 1, in which the casting die (4b) is manufactured as a thin-walled mould insert of a heat-resistant material and has an outer contour corresponding to the intended outer contour of the blade base (3) and, after the solidification of the casting, remains on the blade base (3) in order to form at least part of its surface zone.
12. A method according to claim 11, in which the casting die (4b) is manufactured by means of injection moulding.
A method according to claim 11, in which the casting die (4b) is manufactured as an extruded section or rolled section.
14. A method according to claim 11, 12 or 13, in which the casting die (4b) is manufactured from a finegrained casting die material.

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15. A method according to any of claims 11 to 14, n which the casting die (4b) is fused with the blade cast.
16. A method according to any of claims 11 to 14, in which the casting die (4b) is located positively to the base part, without being fused with it.
17. A method according to any preceding claim, in which the shell mould (5) with the base casting die (4a; 4b) is brought to an increased temperature before the blade (1) is cast.
18. A method according to any preceding claim, in which the blade base (3) is constructed as a Christmastree base.
19. A method according to any of claims 1 to 17, in which the blade base (3) is constructed as a dovetail base.
20. A method according to claim 19, in which the dovetail base is constructed in such a way that it is held in the rotor disc by a circumferential groove.
21. A method substantially as described herein with reference to the accompanying drawings.
22. A cast turbine blade having a blade plate (2) and a blade base (3), in which the surface part of the base is a separate part fixed to the remainder of the blade base.