GB2095589A - Method of making a blade for a gas turbine engine - Google Patents

Abstract

A blade for a gas turbine engine is made from two halves which are metallurgically joined together by diffusion brazing. In order to produce the very accurately conforming matching faces (22, 23) necessary for the joining, one half which may be investment cast or forged, is used as an electrodes in an electrodischarge machine to machine matching surfaces on the other half. The polarity can be reversed during machining to give equal metal removal from each half. The two halves may define cooling passages within the blade. <IMAGE>

Description

SPECIFICATION Method of making a blade for a gas turbine engine This invention relates to a method of making a blade for a gas turbine engine.

It has been proposed in the past, for instance in British Patent 1224521, that such a blade may be made in two halves which are subsequently metallurgically joined together.

This process has a number of advantages over the commonly-used lost wax casting of the complete blade, amongst which are the avoidance of the need for ceramic cores and of the difficulty of determining and inspecting the wall thickness and passage widths within the blade.

Although this split blade technique is accepted as being advantageous, it has proved difficult to operate in practice. The main problem has lain in the difficulty of ensuring that the join between the halves is sufficiently good, which in turn reflects the difficulty of making the conformity between the joint faces sufficiently accurate to enable diffusion brazing or similar techniques.

The present invention provides a method in une stage of which the blade halves may be produced with accurately conforming mating surfaces.

According to the present invention a method of making a blade for a gas turbine engine comprises forming two halves of the blade using one blade half as an electrode in an electro-discharge machining process to machine a matching face on the other blade half and metallurgically joining the two halves at their matching faces.

It may be necessary in some instances to clean the matching faces before the joining step takes place.

The invention is particularly suitable for the manufacture of cooled blades having hollows or passages within the aerofoil, in which case the aerofoil is advantageously split substantially along a mid-chord line.

Figures 1-6 illustrate various stages of the method of the invention, and Figure 7 shows part of the completed blade made by the method of Figs. 1-6.

In Fig. 1 there is shown a section through a split die comprising two die pieces 10 and 11. Between them the pieces 10 and 11 define a cavity 1 2 having the shape of the blade half which it is desired to make. Molten wax is injected into the cavity 1 2 via a sprue passage 1 3 to fill the cavity, and the wax is then allowed to solidify. The die pieces 10 and 11 are then separated to leave the wax pattern 14.

The pattern 14 is shown in perspective in Fig. 2 and it will be seen that the shape of the pattern comprises half of a complete blade, the split between the two halves being made approximately down the mid-chord line of the blade. It will be noted that the split is not on a plane surface, nor even a regularly curved surface, but that it consists of an approximate plane in the region of the root 1 5 and shank 1 6 of the blade, the plane surface smoothly joining with an approximately mid-chordal surface in the region of the platform 1 7 and maintaining this mid-chordal shape in the aerofoil 18.

It will also be noted that because the aerofoil is split down its mid-chord region, the complex interior of the aerofoil, necessary for cooling purposes, becomes exposed to view and can be formed without the need for reentrant cavities or separate cores.

The wax pattern 1 4 thus formed is invested with a ceramic shell 1 9 as shown in Fig. 3.

This shell is formed by a repeated process of dipping the pattern in a ceramic slurry followed by causing dry ceramic material to adhere to the dipped pattern by a stuccoing or raining process. The shell mould is then used in such a process, and molten metal is poured into it and aliowed to solidify.

Once the metal has solidified, the ceramic shell may be removed by a mechanical fettling operation or equivalent process. The blade half thus formed is shown at 20 in Fig. 4, as is the other half 21 of the blade which has been formed by an identical process which is not separately illustrated.

If it is desired to effect any further machining of the internal features of the aerofoil 18, they are now exposed to view and can be machined as necessary. It will be appreciated that the two halves 20 and 21 are shaped so that their matching surfaces 22 and 23 respectively are as close as possible to being in exact conformity with each other. However, the lost wax casting process is not inherently sufficiently accurate to allow the surfaces to match to the extremely high degree of accuracy necessary to allow a satisfactory joint to be achieved repeatably.

Therefore, in accordance with the present invention, the two blade halves 20 and 21 are mounted in an electro-discharge machine as anode and cathode, with their matching surfaces 22 and 23 confronting one another.

Fig. 5 illustrates how the half 21 is mounted in a static frame 24 and is electrically connected to one output of a machining power supply 25, while the other half 20 is mounted in a moveable frame 26 and is electrically connected to the other output of the power supply 25. The mounting is such as to cause the faces 22 and 23 to face one another but spaced a small distance apart, and the motion of the frame 26 and hence of the blade half 20 is controlled by a control unit 27 acting through a servo 28 to move this half towards the other half.

In operation, a dielectric liquid is pumped by a pump 28 through a nozzle 29 into the gap between the faces 22 and 23 to fill the gap. The liquid then falis into a collecting sump 30 to be returned to the pump 28 and to be recirculated. The power supply unit 25 provides pulses of high voltage potential across the two halves 20 and 21, and the servo 28 drives the half 20 downwards closer to the other half 21. The gap between the surfaces 22 and 23 therefore decreases, until arising or electro-discharge occurs between the surfaces, eroding the surface 23. The control system 27 will then reduce or halt the rate of advance of the half 20 to match the rate at which this arcing erodes away the surface of the half 21.

It will be appreciated that the electro-discharge machining operation will occur selectively over the extent of the surfaces 22 and 23, initially occurring only where the gap is at its smallest. The machining action will increase this gap in relation to the gap between the remainder of the surfaces, and in this way the machining action will eventually result in a gap of very accurate constant thickness.

Clearly this will imply that the machined surfaces 22 and 23 have become very accurately conformed to one another, which is the desired situation to enable a good joint to be made between the blade halves.

It is desirable, but not essential, that the plurality of the discharged may be reversed during this process to give an approximately equal removal of metal from each half, to reduce and equalise the cost stock needed at the joint line.

Once the machining of the blade halves has reached this stage, the halves may be removed from the electro-deposition machine and the matching surfaces 22 and 23 cleaned as necessary. The two halves may then be joined together by a brazing or diffusion brazing process such for instance as is described in our British Patent 1224521 (US Patent 3656222). In this process the halves are held together, with a thin foil of brazing alloy trapped between the matching surfaces 22 and 23. The brazing alloy may comprise one of a number of alternatives, but it will normally be broadly similar in constitution to that of the halves but with added amounts of melting point depressants such as boron and silicon.

The 'sandwich' thus formed is then heated to a temperature at which the brazing foil will melt, but the parent material remains solid.

The molten braze material infiltrates adjacent parent metal, depressing its melting point below that of the assembly and causing local melting and intermingling of the alloy of the two halves. Maintaining this high temperature for a longer time allows the melting point depressants to diffuse away into the bulk of the parent material, causing the molten interface to solidify and producing a joint whose strength at least approximates to that of the parent material.

Of course it would be possible to use other joining processes, but most of these depend upon the existence of a close conformity between the matching surfaces, and this is provided by the method of the invention.

Fig. 6 illustrates the completed blade and shows how the relieved parts of the halves join to form cooling air passages. In Fig. 7 a portion of the joint between the halves is shown enlarged, so that it can be seen that using the method of the invention it is not necessary for the matching surfaces 22 and 23 to be of a particular nominal shape. Thus in the illustrated portion the projection at 31 in the face has machined an equivalent depression at 32 in the face 23, and although the joint face departs from the nominal the two faces still conform accurately with one another.

It will be understood that although the method described above is a preferred embodiment, it will be possible to use other methods of manufacturing the blade halves and alternative methods of joining the halves once they have been machined to have conforming faces in accordance with the invention. In particular it may be desirable to use an injected shell mould in which to cast the blade halves, if necessary incorporating a lost wax step. Alternatively the blade halves could be machined or forged.

Claims (10)

1. A method of manufacturing a blade or vane for a gas turbine engine comprising forming two halves of the blade, using one blade half as an electrode in an electro-discharge machine to machine a matching face on the other blade half, and metallurgically joining the two halves at their matching faces.

2. A method as claimed in claim 1 and comprising cleaning said matches faces after they are machined.

3. A method as claimed in claim 1 or claim 2 and in which said blade or vane is a cooled blade or vane having hollows and/or passages within the aerofoil, and said blade halves include portions of the aerofoil split substantially along the mid-chord line of the aerofoil.

4. A method as claimed in claim 3 and in which said blade or vane comprises a rotor blade, said halves being split substantially along a mid-chord line of the aerofoil and substantially along a central plane of the root and shank.

5. A method as claimed in any one of the preceding claims in which the polarity of the electro-discharge machining is reversed during the machining process.

6. A method as claimed in any one of the preceding claims and in which said halves are metallurgically joined by a diffusion brazing process.

7. A method as claimed in claim 6 and in which said diffusion brazing process involves phasing and brazing foil between the halves, pressing the halves together into constant with the foil, and heating at a temperature above the melting point of the foil but below that of the parent metal for a sufficient time to enable the foil to melt and for its constituents to diffuse into the parent metal.

8. A method as claimed in any one of the preceding claims and comprising making said halves by a lost-wax casting process.

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

10. A blade or vane as made by the method of any one of the preceding claims.