GB2105624A - Turbine blade manufacture - Google Patents

Abstract

Manufacture of turbine blades (1) involves the use of ceramic core members in investment casting moulds to define coolant flow passages (A, B, C) within the blades together with small heat transfer members or "pedestals" (39) integral with the walls of the flow passages. In order to produce the pedestals accurately in a way which facilitates production of the ceramic core, laser energy is used to form small holes in the ceramic core after it has been produced in its final shape, these small holes producing the pedestals in the cast blade. <IMAGE>

Description

SPECIFICATION Manufacture of cast turbine blades This invention relates to the manufacture of cast turbine blades or guide vanes for gas turbine engines, and in particular is concerned with the production of heat exchanging features in coolant flow passages within the blades or vanes.

References to blades in this specification and claims should be taken to include vanes.

Efficient exchange of heat between the walls of a gas turbine blade and coolant fluid circulating through coolant flow passages within the blade is required in modern gas turbines in order to keep the walls of the blade at acceptable temperatures.

One means developed to facilitate rapid transfer of heat to the coolant fluid (generally air passing through the blade under pressure) is to provide the flow passages with arrays of so-called "pedestals", which are small-sized heat transfer members integral with the walls of the passages; they increase the surface area of the passages and also the degree of turbulence in the coolant flow, both these factors increasing the rate of heat transfer relative to a smooth-walled passage. The pedestals may either extend completely across the passage, being integral at both ends with opposing walls of the passage, or they may extend only part-way across the passage, being integral with only one wall thereof. For ease of manufacture and heat-transfer efficiency the pedestals are generally circular in cross-section.

The only practical way of producing these pedestals is to cast them together with the rest of the blade using the investment ("lost wax") casting process. The coolant flow passages within the blade are produced by utilising one or more ceramic cores within the mould, these then being removed from the casting by a suitable process such as leaching with a strong alkali.

If the pedestals are to extend completely across a passage, then a corresponding hole is required in the ceramic core, the hole extending from one side of the core to the other. If the pedestals extend only part-way across the passage, then the corresponding hole in the ceramic core will be blind.

In existing methods of manufacture, any holes, gaps, slots, etc., in the ceramic core are mouldedin during manufacture of the core, involving injection of a self-setting slurry into a split die incorporating all the necessary features to produce the finished core. However, problems arise in the production of the small holes required for pedestals, these problems including a) expense and difficulty in producing the pedestal features in the die, since they are machined into the die surface by electro-discharge machining and are of sufficiently small diameter to present machining problems -- further the lower limits of the diameter of the pedestals and pitch of the spacing between them in the die depend upon the capabilities of this machining process; b) the pedestal features in the die are easily broken off during handling;; c) the holes in the ceramic core tend to bind with the corresponding pedestal features in the die as the die sections are separated, causing chipping of the holes or breakage of the core; d) straight-through holes in the ceramic core must be formed by pedestal features in the die which are in two halves, the halves being part of opposed portions of the split die - the problems here are the required accuracy in machining and location of the die portions to eliminate mis-match or misalignment between the two halves of the pedestal features, and the allied difficulty of ensuring that the free end faces of all the pedestal features meet at the split-line between the opposed die portions without any gaps between them, leading to flash in the holes which must be removed by hand in a time-consuming and expensive operation;; e) the holes in the ceramic core reduce the strength of the core, leading to further accidental breakages especially when it is still in the "green" state after removal from the die and before curing; f) a change in the size or spacing of the pedestals requires that a new die be made.

Hitherto, drilling of holes in ceramic cores has not been practicable because of the fragility of the cores, tool wear problems, and the high expense and low production rate involved in forming the holes individually or even in groups.

The present invention provides a method of manufacture for a cast turbine blade, the turbine blade being the type having at least one internal coolant flow passage provided with small heat transfer members integral with the walls of said passage, the method of manufacture involving the use of a ceramic core in an investment casting mould to define said flow passage and said heat transfer members and including the steps of: producing the ceramic core; forming small holes in the ceramic core by means of laser energy, said holes being for the purpose of defining said heat transfer members in the cast blade; incorporating the ceramic core in the mould for investment casting of the blade; casting the blade; and removing the ceramic core from the casting to leave said coolant flow passage and said heat transfer members within the blade.

The laser energy is preferably applied in the form of pulses.

The present invention also comprises a method of manufacturing a ceramic core for use in an investment casting mould, the method including the steps of moulding the ceramic core in a die, removing the ceramic core from the die, and thereafter forming a desired number of holes in the ceramic core by means of laser energy.

The holes may be formed either before or after curing of the ceramic core, and may either be blind or extend completely through the ceramic core.

The invention includes a turbine blade or a ceramic core when manufactured by the above methods.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a part-sectional side elevation of a cast turbine blade manufactured in accordance with the invention; Figure 2 is an enlarged section along line II--II in Figure 1; and Figure 3 shows an enlarged side elevation of a ceramic core for use in the manufacture of the turbine blade.

The drawings are not to scale.

Referring to Figure 1, there is shown a side elevation of a complete gas turbine blade whose aerofoil portion 1 is shown sectioned along the chord line to reveal internal details of coolant flow passages 3, 5 and 7.

Besides the aerofoil portion 1, the blade comprises the outer shroud portion 1 8 (also sectioned), the inner platform/gas-sealing portion 2 and the root portion 4, which locates in the rim of a turbine disc (not shown). Coolant passages in portions 2 and 4 are shown in broken lines.

Coolant flow passages 3, 5 and 7 in the aerofoil portion 1 branch off from coolant feed passage 9 in the platform portion 2, which also extends through root portion 4 to a supply opening 11, where most of the coolant, which is pressurised air, enters the blade.

Passages 3 and 5 in aerofoil 1 receive air from passage 9, and this air passes directly up passages 3 and 5 in the direction shown by the arrows, some being exhaused from the outer shroud 1 8 of the blade through small bleed holes 13 and 14 respectively. However, most of the air in passages 3 and 5 is utilised for film -- cooling the leading edge 17 and the two flanks 19 and 21 of the blade (see Figure 2), and for this purpose it exits from passages 3 and 5 through arrays of small drillings 23 and 25 respectively in the walls of the aerofoil. Although not shown in Figure 1, drillings 23 and 25 are evenly distributed over the span of the aerofoil portion.

The cooling air received by passage 7 from feed passage 9 is augmented by additional air received through a supply opening 27 in the side of blade portion 2.

In distinction from passages 3 and 5, passage 7 comprises three "legs" A, B and C. These are in series with each other and are contained within aerofoil portion 1.

Cooling air from feed passage 9 is supplied to the inner end of leg A, which is closest to the trailing edge 29 of the aerofoil, and then successively flows out along leg A, inwards along leg B, and outwards along leg C. Some cooling air is expelled from bleed hole 1 6 as it comes out of leg A, and much air from leg C is used to film cool the concave flank of the aerofoil via small drillings 31, the remainder being exhaused from the blade through bleed hole 1 5.

In order to cool efficiently the most downstream portions of the flanks of the aerofoil (i.e. those parts of the walls of the aerofoil numbered 33, 34 and 35 on the convex side and 36 and 37 on the concave side) without the need to use large amounts of cooling air for film-cooling the flanks, passage 7 incorporates heat transfer members, known as "pedestals".

In leg A of passage 7, the pedestals 39 extend right across the passage from one flank of the aerofoil to the other, being integral with wall areas 33 and 36. In leg B, the pedestals 41,42 extend only part-way across the passage, being provided on opposing walls of the passage so that pedestals 41 are integral with wall area 37 on the concave flank of the aerofoil and pedestals 42 are integral with wall area 34 on the convex flank. In legs A and B the pedestals therefore take heat from both flanks of the blade and transfer it to the cooling air. In leg C, the pedestals 43 again extend only part-way across the passage, but are only provided on one wall thereof, being integral with wall area 35 on the convex flank so as to take heat from it.

Coolant flow passage 7 is formed, complete with its pedestals, during manufacture of the blade by the "lost wax" investment casting process, which is well known as a principle and will not be described in detail.

In order to form passage 7 and its pedestals, a ceramic core is first produced and incorporated in the casting mould, along with a further ceramic core or cores by means of which the other passages 3, 5 and 9 are formed.

After casting of the blade, the cores are leached out of the casting using a strong alkali, leaving the interior of the blade finished apart from the drillings for film cooling and bleed holes.

A ceramic core suitable for producing passage 7 is illustrated in Figure 3. It comprises three legs A', B' and C' corresponding to legs A, B and C of passage 7 in the blade. It is manufactured in a split die (not shown) by injecting a self-setting ceramic slurry into the die under pressure, letting the slurry set, and extracting the "green" ceramic core from the die. Subsequently the core is fired to render it strong enough for incorporation in the casting mould.

Leg A' of the core has small holes 39' formed in it to extend completely through its thickness; these holes correspond to pedestals 39 in the finished blade.

Leg B' of the core has two sets of small blindended holes 41' and 42' formed in respective flanks of the core; these holes correspond to pedestals 41 and 42 respectively in the finished blade (holes 41' are on the far side of the core and are shown as broken circles).

Leg C' of the core has small blind-ended holes 43' formed in one of its flanks; these holes correspond to pedestals 43 in the finished blade.

The holes 39',41 42' and 43' are formed by means of pulsed laser energy the total amount of energy delivered per hole being controlled according to the depths of the hole being produced: clearly, holes 39' require the expenditure of more laser energy than holes 41', 42' and 43', since holes 39' extend right through the thickness of the core, whilst the other holes extend only part-way through.

The user of a laser drilling machine to form the holes in the ceramic core, instead of the conventional technique of moulding, has the following advantages: a) The dies for moulding the cores are much cheaper to produce, since they do not have to incorporate the pedestal features for forming the corresponding holes in the cores; further, the lower limits of the diameter of the pedestals in the finished blade are set only by capabilities of the casting process, since very small diameter laser beams can be produced; b) The dies are less easily damaged, since they do not contain pedestal features; c) Damage to the cores during separation of the dies after moulding is reduced, since absence of pedestal features in the dies mean that there is no binding between the pedestal features and corresponding holes in the cores; d) Absence of pedestal features in the dies mean that cores provided with straight-through holes do not suffer from problems of mis-match or misalignment of the holes, or ceramic flash in the holes from the moulding process; e) Holes can be formed in the cores after curing of the cores, thus reducing the risk of breakage of the core whilst it is in its "green" state;; f) A modification in the desired size or spacing of the pedestals in the finished blade does not require that new dies be made.

Clearly, laser drilling of the required hole patterns is readily amenable to automation, the machine being controlled to drill the holes according to a program stored on punched tape or in a microprocessor.

Only a change of program is necessary if the size or spacing of the holes is modified.

The time taken to form each hole and move to the next one using a laser drilling machine is only of the order of a fraction of a second and therefore a hole pattern such as the one shown in Figure 3 can be produced quickly; the core can be turned over automatically in order to drill holes 41'.

Furthermore, the holes are produced without the application of any appreciable mechanical stress to the core, so that no cracking or fracturing of the core occurs during drilling.

It may be found that laser drilling of ceramic cores produces holes with sharp edges. This is undesirable because in the finished blade it would give rise to an abrupt change of section between the pedestal and the part of the coolant flow passage wall with which it is integral, which would be a stress-raising feature. If sharp-edged holes are produced, suitable fillets can be produced at the junction of pedestal and wall by radiussing or chamfering the edges of the holes after drilling. This can easily be done by e.g.

defocussing the laser beam and briefly illuminating each hole again with a pulse (which may be of lower power than the drilling pulses) so that only the material in and near the sharp edge of the hole is vapourised because of its low thermal mass. This additional phase of the cycle of laser drilling for each hole would of course be part of the program for the automatic control of the laser drilling machine, and would occur immediately after the drilling pulse or pulses for each hole, adding only a fraction of a second to the cycle time for each hole.

Claims (12)

1. A method of manufacture for a cast turbine blade, the turbine blade being of the type having at least one internal coolant flow passage provided with small heat transfer members integral with the walls of said flow passage, the method of manufacture involving the use of a ceramic core in an investment casting mould to define said flow passage and said heat transfer members and including the steps of: producing the ceramic core; forming small holes in the ceramic core by means of laser energy, said holes being for the purpose of defining said heat transfer members in the cast blade; incorporating the ceramic core in the mould for investment casting of the blade; casting the blade; and removing the ceramic core from the casting to leave said coolant flow passage and said heat transfer members within the blade.

2. A method of manufacturing a ceramic core for use in an investment casting mould, the method including the steps of moulding the ceramic core in a die, removing the ceramic core from the die, and thereafter forming a desired number of holes in the ceramic core by means of laser energy.

3. A method of manufacture according to Claim 1 or Claim 2 in which after the drilling of each hole, the laser beam is defocussed and the drilling machine used to illuminate the hole again such that only the material in and near the edge of the hole is vapourised, thereby to produce a hole having a radiussed or chamfered edge.

4. A method according to any one of Claims 1 to 3 in which the ceramic core is cured before forming of the hole occurs.

5. A method according to any one of Claims 1 to 3 in which the ceramic core is cured after forming of the hole occurs.

6. A method according to any one of Claims 1 to 5 in which the laser energy is applied in the form of pulses of energy.

7. A method according to any one of Claims 1 to 6 in which the holes in the ceramic core are formed as blind holes.

8. A method according to any one of Claims 1 to 6 in which the holes in the ceramic core are formed to extend through the ceramic core from side to the other side.

9. A method of manufacture for a turbine blade substantially as hereinbefore described.

10. A method of manufacture for a ceramic core substantially as hereinbefore described.

11. A turbine blade manufactured by a method according to Claim 1 or any one of Claims 3 to 9.

12. A ceramic core manufactured by a method according to Claim 10 or any one of Claims 2 to 8.