GB2270718A

GB2270718A - Single crystal turbine blades having pedestals.

Info

Publication number: GB2270718A
Application number: GB9220000A
Authority: GB
Inventors: Rajesh Thakorbhai Patel
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1992-09-22
Filing date: 1992-09-22
Publication date: 1994-03-23
Also published as: GB9220000D0

Abstract

In a single crystal turbine blade having cooling passages therein, integral pedestals 48 are provided, extending between opposed walls of the passages. The pedestals are arranged so that they are angled (A, B) obliquely to the opposing walls 52, 50 of the passages, and, in particular, the axes of the pedestals are directed towards the epicentre of crystal growth of the blade. The arrangement is stated to minimise recrystallisation during casting of the blade, particularly where the pedestals join on to the inner surfaces of the blade, and adjacent their mid-points. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO TURBINE BLADES This invention concerns improvements in or relating to turbine blades for aircraft gas turbine engines, and in particular concerns improvements to cooled blades.

It is necessary in the interests of performance and efficiency that modern aircraft gas turbine engines operate at high temperatures, frequently close to or even above the melting points of the alloys of which engine components such as turbine blades are made. It is accordingly necessary to cool these components, and this is usually achieved in the case of turbine blades by passing cooling air through passages within the blades.

Turbine blades for high performance engines are usually cast from single crystal high temperature nickel-based alloys by the investment casting or "lost wax" technique, and the cooling passages in such blades have transverse structures known as "pedestals" connecting opposed surfaces of the passages in each blade. The pedestals of the prior art are essentially for cooling purposes; strengthening characteristics are of secondary importance. Blades made from single crystal alloys are known as "single crystal blades". In the context of the present invention, "blades" will be understood to include vanes, where appropriate. Hence a reference to a cooled single crystal blade will also be a reference to a cooled single crystal vane.

In the casting of blades with cooling passages the passages are provided by casting the metal around ceramic cores which are shaped not only to provide the specific passage shape but also to provide the pedestals. The ceramic cores are removed after the cast alloy has solidified.

A current design of blade incorporating pedestals within its cooling passages is illustrated in Figures 1-3 of the accompanying diagrammatic drawings in which, Figure 1 is a perspective view of a sectioned turbine blade; Figure 2 is a perspective view of the blade of Figure 1 cut away on line Il-Il; Figure 3 is a section on line I-m of Figure 1.

In Figures 1 and 2 there is shown a turbine blade 10 made of a cast single crystal high temperature nickel-based alloy, having opposed blade surfaces 12,14 and provided with core cooling passages 16. The passages are provided with transverse strengthening pedestals 18 which, as shown in Figure 3, lie substantially at right angles to the opposed blade surfaces 12,14.

It is found that in some blades recrystallisation of the single crystal alloy can occur, and this will generally necessitate scrapping the blade so affected.

Recrystallisation, if it takes place, is found to occur typically at one or more specific locations within the blade. With reference to Figure 3 these locations are commonly at the junction 20 of a pedestal 18 with an inner surface 22 of a core passage 16, and halfway along a pedestal at 24. Arrow 34 indicates direction of crystal growth.

It is believed that recrystallisation of a pedestal may be due to excessive plastic deformation in the pedestal during cooling of the solidified casting prior to removal of the ceramic core used to form the cooling passages.

The natural cooling contraction of the alloy is resisted by the solid core between pressure and suction surfaces, and load is thereby carried by the pedestals.

Construction of the ceramic core often results in a double tapered pedestal hole in the core with the smallest section on the neutral axis of the core. Hence this section is the one most likely to be stressed.

During solution heat treatment of the blade after core removal, the excessive plastic deformation is believed to be the driving force for recrystallisation.

It is an objective of the present invention to minimise the occurrence of recrystallisation in a single crystal alloy turbine blade provided with a cooling passage having at least one integral pedestal extending between opposing walls of the passage.

According to the present invention there is provided, for a gas turbine engine, a single crystal turbine blade having a cooling passage therein, the cooling passage having at least one pedestal integral with the blade extending between opposed walls of the passage, characterised in that the pedestal is angled acutely to said opposing walls.

Preferably, a plane in which the axis of the pedestal lies is directed towards the epicentre of crystal growth of the blade.

Preferably, there is provided a plurality of pedestals within the passage. The pedestals may be arranged in rows. Alternate rows of pedestals may be angled in opposite directions.

The invention will now be described by way of example only with reference to Figures 4,5,6 of the accompanying diagrammatic non-scale drawings in which, Figure 1 is a perspective view of a sectioned turbine blade of the prior art; Figure 2 is a perspective view of the blade of Figure 1 cut away on line II-II; Figure 3 is a section on line III-III of Figure 1; Figure 4 is a perspective view of a sectioned turbine blade according to the invention; Figure 5 is a perspective view of the blade of Figure 4 cut away on line V-V; and Figure 6 is a section on line VI-VI of Figure 4.

Referring to Figures 4-6 of the drawings, there is shown a gas turbine blade 40 made of a cast single crystal high temperature nickel-based alloy, having opposed blade surfaces 42,44 and provided with a core cooling passage 46.

The passage 46 is provided with transverse strengthening pedestals 48 which, as shown in Figure 6, lie at in a plane which is at an acute angle A to opposed inner surfaces 50,52 of the passage. The angle A is chosen so that the plane in which the pedestals lie is directed towards the epicentre of crystal growth of the blade. The direction itself of crystal growth is shown in Figure 6 by arrow 54.

Within the aforesaid plane the pedestals 48 may be angled diagonally as shown by angle B in Figure 5.

The angled nature of the pedestals allows the single crystal to grow more easily along the pedestals during the casting process because growing upwards and diagonally is easier than growing upwards and sideways at right angles as in the prior art. The angling of the pedestals enables the pedestals to join to the internal surface of the blade over a wider area, thereby increasing its strength, improving heat flow between the surface of the blade and the pedestals, and so assisting the cooling effect of the air flowing through the blade.

Because the pedestals are angled and no longer at right angles to the surface of the blade (as in the prior art), and therefore more radially inclined than in the prior art, they play a greater role in transferring centrifugal loading down through the blade during operational conditions.

During manufacture of the blade, when the cast alloy cools it creates residual tensile loads in the pedestals.

Because, in the present invention, the pedestals are angled to the surface of the blade, bending moments in the pedestals are also created. These combine with the tensile loads to create a combined shearing effect on the ceramic core during cooling, thus breaking it up. This means that lower stress concentrations occur, thereby greatly reducing recrystallisation during heat treatment.

A further effect of the angling of the pedestals is that each pedestal is longer and has an increased surface area. This improves the cooling and fewer pedestals are required to effect the cooling rate of the prior art pedestals, assuming the same pitch. This further reduces the risk of recrystallisation.

If alternate rows of pedestals are angled in opposite directions this will create a more turbulent environment for the cooling air within the blade, thus enhancing the cooling effect.

The filleting of the ends of the pedestals to the interior of the blade will be optimised for the manufacturing process, cooling effect, and structural integrity.

Claims

1 For a gas turbine engine, a single crystal turbine blade having a cooling passage therein, the cooling passage having at least one pedestal integral with the blade extending between opposed walls of the passage, characterised in that the pedestal is angled acutely to said opposing walls.

2 A blade as claimed in claim 1 wherein a plane in which the axis of the pedestal lies is directed towards the epicentre of crystal growth of the blade.

3 A blade as claimed in claim 1 wherein there is provided a plurality of pedestals within the passage.

4 A blade as claimed in claim 3 wherein the pedestals are arranged in rows.

5 A blade as claimed in claim 4 wherein alternate rows of pedestals are angled in opposite directions.

6 For a gas turbine engine, a single crystal turbine blade substantially as hereinbefore described with reference to Figures 4,5,6 of the accompanying drawings.