GB2411662A

GB2411662A - A method of creating residual compressive stresses

Info

Publication number: GB2411662A
Application number: GB0404769A
Authority: GB
Inventors: Michael Stewart Hodgson; Derek John Foster
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2004-03-02
Filing date: 2004-03-02
Publication date: 2005-09-07
Also published as: GB0404769D0

Abstract

A surface of a component 2, for example a blade of a gas turbine engine, is locally heated by a travelling column of hot plasma 24 to a temperature below the melting point of the material of the component 2. The plasma 24 may be generated by a plasma gun operated in a pilot mode, in which a pilot arc 22 is formed between a nozzle body 6 and an inner electrode 12. The component 2 may be cooled, for example at its surface opposite the surface being treated. Cooling may be achieved, for example, by means of an ice chuck. The process creates residual compressive stresses in the treated surface of the component, which improve the resistance of the component to fatigue failure.

Description

2411 662

A METHOD OF CREATING RESIDUAL COMPRESSIVE STRESSES

This invention relates to a method of creating residual compressive stresses in a surface layer of a component. The present invention is particularly, although not exclusively, concerned with the creation of residual compressive stresses in metallic components such as fan or turbine blades of gas turbine engines.

It is known that the fatigue life of components such as gas turbine fan blades can be increased by treating surfaces of the components in vulnerable regions in such a way as to induce residual compressive stresses at those surfaces. Such compressive stresses can be induced by mechanical processes such as shot peening or laser shock peening. In such processes, the surface to be treated is subjected to impacts, either directly by high-velocity particles such as shot, or by using a laser to ablate a coating applied to the surface to generate a shockwave which propagates into the material of the component. Such processes require specialised equipment. In the case of shot peening, a shot peening gun is required, and the process requires the handling and recovery of the particles. Also, it is not always possible to control accurately the position and intensity of the peening operation. In the case of laser shock peening, the ablating coating must be applied to the surface to be treated, and the process needs to take place in the presence of a reaction component, which may take the form of a film of water dispersed over the coated surface. These features increase the complexity of the peening process.

According to the present invention there is provided a method of creating residual compressive stresses in a surface layer of a component, the method comprising: applying heat to successive spots on the surface to be treated to heat the surface at each spot to a temperature below the melting point of the material of the surface layer; and - 2 causing or allowing those regions of the surface to which heat is not being applied to cool.

In the context of the present invention, references to the "surface layer" of a component refer to the region of the material of the component just its below its surface. The surface layer is not necessarily a separate layer applied to the component, and in many cases will, before treatment by a method in accordance with the present invention, have the same composition as the rest of the component.

In a preferred method in accordance with the present invention, the heat is applied by heating means which is traversed over the surface to be treated so that the required area is covered. Preferably, the application of heat is maintained during the traversing of the heat-applying means, so that the successive spots merge into one another to form a continuous line.

The heat may be applied to the surface to be treated by directing a hot gas at the surface. The gas may be an ionised gas in the form of a plasma. The plasma may be generated by a plasma gun in which an electric arc is maintained in a flowpath of plasma gas, whereby the plasma gas is ionised by the arc.

The plasma gun may comprise a nozzle body defining a cavity having an opening at one end, and an inner electrode disposed within the cavity, the arc being established between the nozzle body and the inner electrode.

In practice, a conventional plasma welding gun may be used to form the plasma, the welding gun being operated in a pilot mode in which no arc is established between the welding gun and the component. In performing a method in accordance with the present invention using a conventional plasma welding gun, it is desirable for the gap between the nozzle tip of the welding gun and the surface to be treated to be smaller than that conventionally employed in a plasma welding process. For example, the gap may be not greater than 2mm. Additionally, the flow rate of plasma gas through the gun may be substantially higher than the flow rate in a conventional plasma welding - 3 process, for example the flow rate may be in excess of 5 litres per minute and may be approximately 10 litres per minute.

The component may be allowed to cool in the ambient atmosphere, but preferably specific measures are taken to accelerate cooling in the regions away from the spot at which heat is applied. For example, cooling means may be applied to a surface of the component on a side of the component opposite the surface being treated. The cooling means may comprise a coolant fluid, but preferably refrigeration means is used to generate a temperature below that of the ambient surroundings. In a preferred method in accordance with the present invention, an ice chuck is employed to support the component at one surface of the component, while the opposite surface of the component is treated. Ice chucks are known, for example from EP 0811457.

Preferably, the ice chuck has a surface which is complementary to that of the supported surface of the component.

Although a method in accordance with the present invention has wide application in different industries, it is particularly appropriate to the treatment of metallic components, and particularly components made from alloys commonly used in the aerospace industry, such as titanium alloys. Methods in accordance with the present invention are particularly useful in the treatment of components, such as fan and turbine blades, of gas turbine engines.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 represents the application of heat to a component by means of a plasma welding gun; and Figure 2 represents a component supported by an ice chuck. - 4

Figure 1 shows a component 2 subjected to treatment by a plasma welding gun 4. The welding gun 4 comprises a nozzle 6 defining a cavity 8 terminating at an outlet opening 10. An inner electrode 12 is provided within the cavity 8 leaving a flow passage for plasma gas, as indicated by arrows 14. A shield 16 surrounds the nozzle 6 providing an annular passage 18 for a shield gas as indicated by arrows 20.

Plasma guns such as that shown in Figure 1 are conventionally used for metal cutting and welding. Such guns operate in two modes, namely a pilot mode and an operating mode. In the pilot mode, a high frequency AC voltage (commonly in the range 3000 to 10,000 volts) is applied between the nozzle 6 and the inner electrode 12. This creates an arc 22 between the electrode 12 and the nozzle 6. A plasma gas such as argon is supplied to the interior of the nozzle 6 and flows, as indicated by the arrows 14, to emerge at the outlet 10. As the gas 14 passes the arc 22, it is ionised and so becomes conductive, allowing current to flow between the electrode 12 and the nozzle 6.

In the operating condition in a welding process, the component 2 is electrically connected to the nozzle 6. Consequently, as the welding gun 4, in the pilot mode, is brought close to the component 2, the arc 22 will transfer from the nozzle 6 to the component 2, at which point the high frequency AC supply is automatically terminated and replaced by a DC supply, maintaining a flow of very hot ionised gas, or plasma, from the outlet 10 to the component 2.

In accordance with the present invention, the welding gun 4 is maintained in the pilot mode, and the component 2 is electrically isolated from the nozzle 6. Consequently, the arc 22 does not transfer to the component 2, but remains established between the electrode 12 and the body 6. A flow of ionised gas at high temperature is ejected from the opening 10 to impinge on the component 2. As illustrated in Figure 1, the flow of gas is in the form of a column 24. In practice, the column 24 will tend to diverge or splay in the direction away from the nozzle 6 and it is therefore preferable for the nozzle 6 to be positioned closer to the component 2 than would be the case in a conventional welding operation. Thus, the distance from the nozzle tip to the component 2 is - 5 preferably less than 2 mm in a method in accordance with the present invention, whereas a gap greater than 3 mm is common in a welding or cutting operation.

Furthermore, the flow rate of plasma gas is increased by comparison with the flow rate in a welding operation. For example, the flow rate in a method in accordance with the present invention may be greater than 5 litres per minute and possibly approximately 10 litres per minute, whereas a flow rate of approximately 0.5 litres per minute is typical in a normal welding operation.

A shielding gas may be supplied, as indicated by the arrows 20 through the passage 18 to shield the region of the component 2 around the column of hot plasma 24, in order to prevent oxidation of the component 2.

The plasma 24 creates a hot spot at the surface of the component 2, causing that region of material to expand. The gun 4 is traversed along the component 2, as indicated by an arrow 26, so that the hot spot moves along a path 28, along which the material of the component 2 is successively heated by the plasma 24 and subsequently cooled. Several passes of the gun 4 may be made in order to cover an entire surface area of the component 2 which is to be treated.

The plasma gun 4 is controlled so that the temperature at the surface of the component 2 remains below the melting point of the material from which it is made, and is preferably not less than 75% and not greater than 95% of the melting point. For example, if the component 2 is made from a titanium alloy, having a melting point in the range 1 500 C to 1 700 C, the maximum temperature reached at the hot spot formed by the plasma column 24 is preferably not less than 1200 C and not more than 1600 C.

The localised abrupt heating and subsequent cooling of the material of the component 2 and the associated thermal expansion and contraction of the material causes residual compressive stresses to build up in the surface layer of the component 2, so increasing the resistance of the component, in the treated area, to fatigue failure. - 6

Cooling of the component 2 will occur under ambient conditions, and this may be sufficient to create adequate stress levels in the material. However, the properties of the component 2 may be enhanced if a substantial thermal gradient is created across the component from the position of the plasma column 24 to the opposite surface of the component 2. It is therefore preferable for the underside of the component (as shown in Figure 1) to be subjected to a greater cooling effect than occurs under ambient conditions. As shown in Figure 2, one means by which this may be achieved is by the use of an ice chuck 30.

As shown in Figure 2, the ice chuck 30 comprises a base 32 on which a refrigeration unit 34 is supported. The refrigeration unit 34 carries a replaceable chill block 36 having a profile matching that of the surface of the component 2 which is to be supported on the ice chuck 30. Thus, in the present case, the chill block 36 has a convex profile matching the concave profile of the component 2.

In use, a film of ice or other freezing agent is applied to the surface of the chill block 36, the component 2 is placed against the wetted chill block 36 and the refrigeration unit 34 is activated to cause the water or freezing agent to freeze, so bonding the component 2 to the chill block 36. The exposed surface of the component 2 can then be treated using the plasma gun 4 as described with reference to Figure 1. As well as holding the component 2 firmly in place, the ice chuck 30 maintains the lower surface of the component 2 at a low temperature, thus establishing a steep temperature gradient between the spot heated by the plasma column 24 and the opposite surface of the component 2. The temperature differences created by the thermal gradient enhance the induced compressive stresses in the component 2.

As an alternative to the use of the ice chuck 30, cooling of the component 2 may be achieved by other means, for example by flowing a stream of cryogenic gas (ie gas at a temperature below approximately - 70 C) over the parts of the component 2 to be cooled. Thus, cryogenic gas may be applied to the upper surface of the component as shown in Figure 2, immediately following the path of the plasma column 24, to ensure rapid temperature change. Alternatively, cryogenic gas may be directed to the 7 underside of the component 2, to create a steep thermal gradient across the thickness of the component. Such cooling measures may reduce the ductility of the material of the component, so increasing the residual stress levels.

Although the ice chuck 30 of Figure 2 and other cooling techniques have been described in association with localised heating using the plasma gun shown in Figure 1, such cooling techniques may also be used to advantage in conventional shot peening or laser peening techniques.

It will be appreciated that means other than the plasma welding gun shown in Figure 1 may be employed to heat the surface of the component 2. For example, a de-focussed laser may be used. - 8

Claims

1 A method of creating residual compressive stresses in a surface layer of a component, the method comprising: applying heat to successive spots on the surface to be treated to heat the surface at each spot to a temperature below the melting point of the material of the surface layer; and causing or allowing those regions of the surface to which heat is not being applied to cool.

2 A method as claimed in claim 1, in which the heat is applied by means which traverses the surface of the component.

3 A method as claimed in claim 2, in which the application of heat is maintained during traverse of the heat-applying means, so that the heat is applied along a line on the surface of the component.

4 A method as claimed in any one of the preceding claims, in which the heat is applied to the component by directing a stream of hot gas at the surface to be treated.

A method as claimed in claim 4, in which the hot gas comprises plasma.

6 A method as claimed in claim 5, in which the plasma is generated by a plasma gun in which a gas is ionised before discharge from the gun to the surface to be treated.

7 A method as claimed in claim 6, in which the gas is ionised by means of an arc generated within the gun by means of a high frequency AC supply. 9-

8 A method as claimed in claim 7, in which the high frequency AC supply has a voltage not less than 3,000 and not more than 10,000 volts.

9 A method as claimed in any one of claims 6 to 8, in which the arc is generated between a nozzle body and an inner electrode, the nozzle body defining a cavity within which the inner electrode is situated.

A method as claimed in any one of claims 6 to 9, in which a shield gas is supplied from the plasma gun to shield the plasma gas from the ambient atmosphere.

11 A method as claimed in any one of claims 6 to 10, in which the plasma gun is a plasma welding gun which is operated in a pilot mode.

12 A method as claimed in any one of claims 6 to 11, in which the spacing between the plasma gun and the surface to be treated is not greater than 2mm.

13 A method as claimed in any one of the preceding claims, in which cooling means is provided for cooling the component.

14 A method as claimed in claim 13, in which the cooling means is applied to a surface of the component opposite the surface being treated.

A method as claimed in claim 14, in which the cooling means comprises refrigerating means.

16 A method as claimed in claim 15, in which the cooling means comprises an ice chuck by which the component is supported. -

17 A method as claimed in claim 16, in which the ice chuck comprises a chill block having a surface shape which is complementary to the shape of the surface of the component engaged by the ice chuck.

18 A method as claimed in any one of the preceding claims, in which the component is metallic.

19 A method as claimed in claim 18, in which the component is a component of a gas turbine engine.

A method as claimed in claim 19, in which the component is a blade for a gas turbine engine.

21 A method of inducing compressive stresses as claimed in claim 1 and substantially as described herein.