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Title:
A METHOD OF HOT CREEP FORMING AND SUPER PLASTIC FORMING AN ARTICLE AND A DIE FOR THE SAME
Document Type and Number:
WIPO Patent Application WO/2010/003513
Kind Code:
A1
Abstract:
A die (68) for forming an inflated aerofoil comprising an upper (70) and a lower jaw (72) that define a convex and a concave surface of the inflated aerofoil. Also a method of using the die (68) comprising the steps of: a. Forming a layered, planar pre-form (34), b. Separating the jaws (70, 72) of the die (68) and inserting the pre-form (34) therebetween, c. Simultaneously applying heat and closing the jaws (70, 72) of the die (68) towards each other to hot creep form the pre-form (34) until the lower jaw (72) abuts and the upper jaw (70) is spaced from a central portion (80) of the preform (34), and both jaws (70, 72) clamp an edge portion (74) of the pre-form (34), and d. Inflating the pre-form (34) until its external surfaces conform to the jaw surface profiles to form the inflated aerofoil.

Inventors:
BRENNAND PHILLIP (GB)
Application Number:
PCT/EP2009/004315
Publication Date:
January 14, 2010
Filing Date:
June 16, 2009
Export Citation:
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Assignee:
ROLLS ROYCE PLC (GB)
BRENNAND PHILLIP (GB)
International Classes:
B21D26/055; B21D53/78
Foreign References:
EP1092485A22001-04-18
US5457884A1995-10-17
US5253419A1993-10-19
EP1338353A12003-08-27
Attorney, Agent or Firm:
GUNN, Michael Alan (Intellectual Property Dept. WH 20P.O. Box 3, Filton, Bristol BS34 7QE, GB)
Download PDF:
Claims:
CLAIMS

A method of forming an inflated aerofoil using a die (68), the die (68) comprising an upper (70) and a lower jaw (72) having a surface profile defining, respectively, a convex and a concave surface of the inflated aerofoil; the method comprising the steps of:

a) Forming a layered, planar pre-form (34) having external surfaces,

b) Separating the jaws (70, 72) of the die (68) and inserting the pre-form (34) therebetween,

c) Simultaneously applying heat and closing the jaws (70, 72) of the die (68) towards each other to hot creep form the pre-form (34) until the lower jaw (72) abuts and the upper jaw (70) is spaced from a central portion (80) of the pre-form (34), and both jaws (70, 72) clamp an edge portion (74) of the pre-form (34), and

d) Inflating the pre-form (34) until its external surfaces conform to the jaw surface profiles to form the inflated aerofoil.

A method as claimed in claim 1 wherein there are additional steps between steps c) and d) comprising:

1 Separating the jaws (70, 72) of the die (68) and removing the pre-form (34),

2 Processing the pre-form (34), and

3 Inserting the pre-form (34) between the jaws (70, 72) of the die (68) and closing the jaws (70, 72). A method as claimed in claim 2 wherein the processing comprises surface coating.

A method as claimed in claim 1 wherein step d) further comprises simultaneously applying heat whilst inflating the pre-form (34).

A method as claimed in claim 1 wherein the aerofoil is a blade or a vane for a gas turbine engine (10).

A method as claimed in any preceding claim wherein the aerofoil comprises titanium or aluminium.

A method of forming an inflated article using a die (68), the die (68) comprising two jaws (70, 72) each having a surface profile defining a surface of the inflated article; the method comprising the steps of:

a) Forming a layered, planar pre-form (34) having external surfaces,

b) Separating the jaws (70, 72) of the die (68) and inserting the pre-form (34) therebetween,

c) Simultaneously applying heat and closing the jaws (70, 72) of the die (68) towards each other to hot creep form the pre-form (34) until one jaw (72) abuts and the other jaw (70) is spaced from a central portion (80) of the pre-form (34), and both jaws (70, 72) clamp an edge portion of the preform (34), and

d) Inflating the pre-form (34) until its external surfaces conform to the jaw surface profiles to form the inflated article. A method as claimed in claim 7 wherein there are additional steps between steps c) and d) comprising:

1 Separating the jaws (70, 72) of the die (68) and removing the pre-form (34),

2 Processing the pre-form (34), and

3 Inserting the pre-form (34) between the jaws (70, 72) of the die (68) and closing the jaws (70, 72).

A method as claimed in claim 8 wherein the processing comprises surface coating.

A method as claimed in claim 7 wherein step d) further comprises simultaneously applying heat whilst inflating the pre-form (34).

A method as claimed in claim 7 wherein the inflated article has a convex and a concave surface profile.

A method as claimed in claim 11 wherein the one jaw (72) abuts the concave surface and the other jaw (70) is spaced from the convex surface.

A die (68) for hot creep and super plastic forming an article, the die (68) comprising two jaws (70, 72), each having a surface profile to define a profile of the article to be formed therein; the die (68) having an open and a closed position wherein in the open position the jaws (70, 72) receive therebetween, in use, a planar pre-form (34) of the article to be formed and in the closed position, in use, the jaws (70, 72) clamp an edge portion (74) of the pre-form (34) whilst one jaw (72) abuts and the other jaw (70) is spaced from a central portion (80) of the pre-form (34). A die (68) as claimed in claim 13 wherein the article is an aerofoil.

A die (68) as claimed in claim 14 wherein the aerofoil is a blade or vane for a gas turbine engine.

A die (68) as claimed in any of claims 13 to 15 wherein the die (68) has an upper (70) and a lower jaw (72), the lower jaw (72) abuts and the upper jaw (70) is spaced from the central portion (80) of the pre-form (34).

Description:
A METHOD OF HOT CREEP FORMING AND SUPER PLASTIC FORMING AN

ARTICLE AND A DIE FOR THE SAME

The present invention relates to a method of hot creep forming and super plastic forming an article and a die for performing the method. More particularly, but not exclusively, it relates to a method of hot creep forming and super plastic forming an aerofoil for a gas turbine engine.

A gas turbine engine comprises compressor and turbine arrangements having alternating stages of rotating aerofoil blades and stationary aerofoil vanes. In order to reduce engine weight, particularly for gas turbine engines used to power aircraft, it is conventional to form these blades and vanes by inflating them from planar pre-forms using the process known as super plastic forming. Super plastic forming of aerofoils is described, for example, in US 5,729,901.

First three planar sheets of material, eg titanium, are laid on top of each other, with a stop-off such as yttria coating interior regions of the inside of the outer skin sheets. The sheets are then diffusion bonded together to form a pre-form having the edges and portions of the interior of the three sheets bonded, whilst those interior regions coated with yttria simply abut without bonding. The pre-form is then placed between the jaws of a hot creep form die where it is subjected to heat and pressure to deform it. The preform is then cooled and transferred to a second die for super plastic forming. Fluid, preferably an inert gas, is pumped between the layers of the pre-form to inflate the regions coated with yttria to produce the inflated aerofoil.

One disadvantage of this method is that the inflated aerofoil usually exhibits 'quilting' on one or both of its externa! surfaces after super plastic forming, particularly on the concave surface. This is due to the uneven inflating causing variations in the thickness of the skin sheets. The quilting leads to potential weak places in the aerofoil skin, which may fail after some time in service. It may also alter the aerodynamic properties of the aerofoil surface profile by disrupting boundary layer airflow in use. Although the quilting can be removed from the outside surface of the aerofoil by subsequent machining this requires the skin sheets to be thicker initially than their desired final thickness thereby causing material to be wasted. The material also cannot be removed by machining on the inside of the aerofoil, which means there may be excess weight carried and / or localised thinning of the skin sheet, which reduces the reliability and strength of the aerofoil.

A further disadvantage of this method is the wear on each of the dies, since the preform is heated and formed in each of the hot creep form and super plastic form dies. Hence portions of the dies and pre-form wear against each other during the hot creep form and super plastic form processes.

The present invention seeks to provide a method of hot creep forming and super plastic forming an aerofoil that seeks to address the aforementioned problems. It also seeks to provide a die for hot creep forming and super plastic forming an aerofoil.

Accordingly the present invention provides a method of forming an inflated aerofoil using a die, the die comprising an upper and a lower jaw having a surface profile defining, respectively, a convex and a concave surface of the inflated aerofoil; the method comprising the steps of:

a) Forming a layered, planar pre-form having external surfaces,

b) Separating the jaws of the die and inserting the pre-form therebetween,

c) Simultaneously applying heat and closing the jaws of the die towards each other to hot creep form the pre-form until the lower jaw abuts and the upper jaw is spaced from a central portion of the pre-form, and both jaws clamp an edge portion of the pre-form, and d) Inflating the pre-form until its external surfaces conform to the jaw surface profiles to form the inflated aerofoil.

This method has the advantage of a more streamlined process that requires only one die profile and not the two die profiles used in the prior art.

There may be additional steps between steps c) and d) above comprising:

1 Separating the jaws of the die and removing the pre-form,

2 Processing the pre-form, and

3 Inserting the pre-form between the jaws of the die and closing the jaws.

The processing may comprise surface coating. These additional steps enable the preform surfaces to be coated in a non-stick material prior to inflation in the super plastic forming process so that the inflated aerofoil can be easily removed from the die following inflation.

Step d) may further comprise simultaneously applying heat whilst inflating the pre-form.

Preferably the aerofoil is a blade or a vane for a gas turbine engine. More preferably the aerofoil comprises titanium or aluminium.

Another aspect of the present invention provides a method of forming an inflated article using a die, the die comprising two jaws each having a surface profile defining a surface of the inflated article; the method comprising the steps of:

a) Forming a layered, planar pre-form having external surfaces,

b) Separating the jaws of the die and inserting the pre-form therebetween, c) Simultaneously applying heat and closing the jaws of the die towards each other to hot creep form the pre-form until one jaw abuts and the other jaw is spaced from a central portion of the pre-form, and both jaws clamp an edge portion of the pre-form, and

d) Inflating the pre-form until its external surfaces conform to the jaw surface profiles to form the inflated aerofoil.

This method has the advantage of a more streamlined process that requires only one die profile and not the two die profiles used in the prior art.

There may be additional steps between steps c) and d) above comprising:

1 Separating the jaws of the die and removing the pre-form,

2 Processing the pre-form, and

3 Inserting the pre-form between the jaws of the die and closing the jaws.

The processing may comprise surface coating. These additional steps enable the preform surfaces to be coated in a non-stick material prior to inflation in the super plastic forming process so that the inflated aerofoil can be easily removed from the die following inflation.

Step d) may further comprise simultaneously applying heat whilst inflating the pre-form.

The inflated article may have a convex and a concave surface profile. The one jaw may abut the concave surface and the other jaw may be spaced from the convex surface. A third aspect of the present invention provides a die for hot creep and super plastic forming an article, the die comprising two jaws, each having a surface profile to define a profile of the article to be formed therein; the die having an open and a closed position wherein the open position the jaws receive therebetween, in use, a planar pre-form of the article to be formed and in the closed position, in use, the jaws clamp an edge portion of the pre-form whilst one jaw abuts and the other jaw is spaced from a central portion of the pre-form. The die is suitable for both hot creep forming and super plastic forming an article and therefore obviates the need for two different dies, one for each process.

The article may be an aerofoil. Preferably the aerofoil is a blade or vane for a gas turbine engine. Preferably the die has an upper and a lower jaw, the lower jaw abuts and the upper jaw is spaced from the central portion of the pre-form.

The present invention will be more fully described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a sectional side view of a gas turbine engine;

Figure 2 is a schematic side view of an uninflated pre-form;

Figure 3 is a schematic section of a prior art hot creep form die with an uninflated, creep formed pre-form;

Figure 4 is a schematic section of a prior art super plastic form die with the uninflated, creep formed pre-form;

Figure 5 is a schematic section of a prior art super plastic form die with an inflated article formed therein and illustrates quilting; Figure 6 is a schematic section of a die according to the present invention with an uninflated pre-form therein; and

Figure 7 is a schematic section of the die according to the present invention with an inflated article therein.

A gas turbine engine 10 is shown in Figure 1 and comprises an air intake 12 and a propulsive fan 14 that generates two airflows A and B. The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28. A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32. Within the bypass duct 32 are outlet guide vanes (OGVs) 31 that comprise aerofoils that may be formed by the method of the present invention. The fan 14, intermediate and high pressure compressors 16, 18 and the high, intermediate and low pressure turbines 22, 24, 26 all comprises aerofoil blades that can be formed by the method of the present invention. The compressor and turbine stages 16, 18, 22, 24, 26 also comprise stator vanes that are aerofoils that may be formed by the method of the present invention.

A prior art method of forming a super plastically formed aerofoil is shown in Figures 3 to 5. Firstly a pre-form 34 is constructed as shown in Figure 2. The pre-form comprises a pair of outer skins 36 with an inner membrane 38 sandwiched between them. The inside surfaces of the outer skins 36 and / or the surfaces of the inner membrane 38 are coated with yttria or another suitable stop-off material. The pre-form 34 is then diffusion bonded by applying heat and pressure to bond each outer skin 36 to a surface of the inner membrane 38 except where the yttria coating is present. Typically, one or more inflation orifices (not shown) are provided to enable gas to be pumped between the outer skins 36 and inner membrane 38 to inflate the pre-form to the desired inflated profile. The pre-form 34 is shown in Figure 3 encapsulated by a hot creep form die 40 according to the prior art. The hot creep form die 40 comprises an upper and a lower jaw 42, 44 that are relatively moveable between open and closed positions. In the open position the jaws 42, 44 are spaced apart to allow the pre-form 34 to be inserted therebetween and to lie unstressed and supported by at least portions of the lower jaw 44. The die 40 is shown in Figure 3 in the closed position with the pre-form 34 having been hot creep formed so that its outer surfaces conform to substantially the inner surface profiles of the jaws 42, 44. At each lateral end 46 of the die 40 the jaws 42, 44 are arranged to be substantially parallel to each other and to be spaced apart by a distance equalling or slightly thinner than the thickness of the pre-form 34. In this way, the lateral ends 46 of the jaws 42, 44 clamp the ends of the pre-form 34 once the die 40 is in the closed position.

The laterally inner portion of the pre-form 34 is stretched and thinned, relative to its unstressed thickness, by the hot creep forming process. There is a reduced distance between the upper and lower jaws 42, 44 in a central portion 48 thereof so that the preform 34 is substantially encapsulated and supported by the surfaces of the jaws 42, 44. Radiused portions 50 connect the lateral ends 46 and central portion 48 of the lower jaw 44 to provide a smooth but steep gradient joining the discontinuity between the relative heights of the jaw 44 surface. At a similar location on the upper jaw 42 to the radiused portions 50 on the lower jaw 44 are bulbous portions 52, which have a more pronounced curve than the radiused portions 50 and break an otherwise substantially smooth curve from end to end through the central portion 48 of the upper jaw 42. This is in contrast to the lower jaw 44, which has a distinct discontinuity between the height of the lateral ends 46 and central portion 48. As shown in the figure by pale lines, the pre-form 34 exhibits a smoother transition between the thickness in its central portion and the thickness at its lateral ends. The voids formed by radiused portions 50 and bulbous portions 52 help to relieve stresses in the aerofoil during hot creep forming. It is to be understood that the shape of the die cavity, defined by the surface profiles of the die jaws 42, 44, changes along its length (into the plane of the page) to provide an approximation to the desired shape of the inflated aerofoil. Thus the figures show a typical section through an aerofoil towards the root of the blade or vane but the cavity between the jaws 42, 44 may have a different shape if viewed on a different section.

Following hot creep forming in the hot creep form die 40 and cooling in the die 40 or after removal therefrom, the creep-formed, uninflated pre-form 34 is transferred into a super plastic form die 54, as shown in Figure 4 and Figure 5. The die 54 comprises upper and lower jaws 56, 58, which are relatively moveable between an open and a closed position. The lower jaw 58 is a continuous smooth curve with the lateral ends approximately matching the curve of the lateral ends 46 of the hot creep form die 40. The upper jaw 56 similarly has curved lateral ends 60 that approximately match the curve of the lateral ends of the hot creep form die 40. However, the central portion 62 of the upper jaw 56 is curved on a tighter radius than the lateral ends 60 so that it stands proud of the lateral ends 60.

As shown in Figure 4, the pre-form 34 is inserted into the cavity between the jaws 56, 58 of the super plastic form die 54 so that the central portion of the pre-form is spaced from both jaws 56, 58. Typically, the hot creep form die 40 profile and the super plastic form die 54 profile are arranged so that the inner membrane 38 of the creep formed pre-form 34 is spaced from the upper and lower jaws 56, 58 respectively in the ratio 3:2. The pre-form 34 is shown in Figure 5 after inflation by pumping fluid, preferably inert gas, between the outer skins 36 and inner membrane 38. Thus, both the convex and concave outer skins 36 are inflated by an approximately equal amount. Thus the outer skins 36 are stretched and thinned to substantially conform to the surface profile of the die jaws 56, 58. The concave outer skin 36a substantially conforms to the profile of the lower jaw 58 whilst the convex outer skin 36b substantially conforms to the profile of the upper jaw 56. The inner membrane 38 is bonded to the outer skins 36 at attachment points 64 and is stretched between such points to form a Warren girder structure in the hollow cavity of the aerofoil. Due to the slight thickening at the attachment points 64, the concave surface of the inflated aerofoil exhibits quilting 66. Thus the concave surface of the inflated aerofoil is rippled such that the outer skin 36 thickness is reduced; the outer skin 36 is also retracted towards the inner membrane 38 at these attachment points 64. The concave outer skin 36a may be machined to remove the quilting from the outer surface of the skin 36a but this requires the skin 36a to be thicker than ultimately required when initially bonded to the inner membrane 38, which is wasteful of material and hence expensive. However, it is not possible to remove the quilting from the inner surface of the outer skin 36a (facing the aerofoil cavity). Quilting may also be apparent on the convex outer skin 36b in a similar fashion.

A further disadvantage of this prior art method is that there is a large contact area between the jaws 42, 44 of the hot creep form die 40 and the pre-form 34. This results in relatively rapid wear of the die surfaces. Due to the close contact, there is a limit to how often the surfaces can be redressed, so that replacement of the die 40 is needed relatively frequently. This is both costly and time consuming.

An exemplary embodiment of the present invention is shown in Figures 6 and 7. As with the prior art method, an uninflated pre-form 34 is provided as shown in Figure 2 from which an inflated aerofoil is produced using the method and die of the present invention. A single die 68 is provided comprising an upper and a lower jaw 70, 72 that are relatively moveable between an open and a closed position. In the open position the jaws 70, 72 are sufficiently spaced from one another to received therebetween the planar pre-form 34. In the closed position, as shown in Figure 6 and 7, the lateral ends 74 of the jaws 70, 72 clamp the ends of the pre-form 34 in a similar manner to that described with respect to Figure 3 thereby holding the pre-form 34 in its hot creep formed shape for super plastic forming by inflation. The die 68 is substantially the same shape as the prior art super plastic form die 54 with the lower jaw 72 describing a smooth curve and the upper jaw 70 having a central portion 76 with a tighter radius than the lateral ends 78. In accordance with the method of the present invention the uninflated, planar pre-form 34 is placed between the open jaws 70, 72 of the die 68 so that a central portion 80 of the pre-form 34 is supported by a central part of the lower jaw 72. The die 68 and pre-form 34 are heated whilst pressure is applied to the jaws 70, 72 to move them to the closed position and consequently to hot creep form the pre-form 34.

As shown in Figure 6, the central portion 80 of the aerofoil pre-form 34 is stretched and thinned to compensate for the relative widening in the lateral direction as the pre-form 34 is hot creep formed. Since the lower jaw 72 does not exhibit a central portion of raised height compared to the lateral ends, unlike the prior art hot creep form die 40, the central portion 80 of the pre-form 34 is able to retain its substantially planar shape. A centre part is supported by the lower jaw 72 and small voids 82 are apparent between the pre-form 34 and the lower jaw 72. The central portion 76 of the upper jaw 70 is substantially spaced apart from the convex outer skin 36b.

In the method of the present invention, the same die 68 is used for both hot creep forming and super plastic forming of the pre-form 34 to form the inflated aerofoil. Therefore, once the jaws 70, 72 of the die 68 have been moved to the closed position, the pre-form 34 is held clamped between the jaws 70, 72 and fluid, preferably inert gas, is pumped between the outer skins 36 and inner membrane 38 to inflate the pre-form 34 as shown in Figure 7. Optionally the gas may be heated as it is pumped into the cavity. Since the hot creep forming occurs within the die 68, the pre-form 34 creeps to a shape that sits asymmetrically within the cavity of the die 68. This means that the concave outer skin 36a requires little inflation to conform to the surface profile of the lower jaw 72 whereas the convex outer skin 36b requires a larger amount of inflation to conform to the surface profile of the upper jaw 70. However, the convex and concave outer skins 36 experience different reactive resistances and so, surprisingly, the unequal inflation required results in a better conformation of the inflated aerofoil profile to the surface profile of the jaws 70, 72. Thus the method of the present invention results in an inflated aerofoil that overcomes the problem of quilting exhibited by aerofoils produced by prior art methods.

Although the method and die of the present invention have been described in relation to the specific embodiment of producing an inflated aerofoil for use in a gas turbine engine, the method and die have equal utility in producing other articles which required hot creep forming to a complex shape and then inflating. For example, an aircraft wing or other aircraft structures may be made using the method and die of the present invention.

Although the die 68 of the present invention has been described as substantially the same shape as the prior art super plastic form die 54, minor modifications and refinements of the shape may be appropriate in putting the invention into effect. Such modifications and refinements are considered to fall within the scope of the present invention and to result merely from detailed design choices.

Usually the planar pre-form 34 is twisted in a separate method step prior to hot creep forming in the die 68 of the present invention. Thus the twisting operation applies the majority of the deformation to the pre-form 34 and the hot creep forming moves the preform from nearly the correct shape to exactly the correct shape for subsequent inflation. However, it may be possible for the shape of the die 68 of the present invention to cause the planar pre-form 34 to be bent and twisted at the same time under the application of heat and the pressure of the jaws 70, 72 of the die 68 closing. This may be put into effect, for example, by the use of a segmented die 68 so that the hot creep forming process is separated into tasks of bending and then twisting the aerofoil into the desired shape ready for inflation. This also means that different shaped aerofoils or other articles could be formed using the same die 68 with the segments arranged to set a different profile. The twist step is required where a substantial degree of twist is required in the inflated article. If a substantially straight pre-form 34 is inflated to produced the inflated article, for example an aircraft wing, the twist step may be omitted from the method.

Since only a single die 68 is used in the method of the present invention there are half the number of die components to experience wear than in the prior art method. Furthermore, the reduced contact area of the single die 68, particularly in comparison with the prior art hot creep form die 40, means less wear occurs in each use of the die 68. The main wear surfaces are at the lateral ends 74 of the jaws 70, 72 where it is less important to have an accurate profile since the majority of this part of the aerofoil is removed during finishing operations before fitting in an engine for service. This further prolongs the life of the die jaws 70, 72 since they can be redressed more times before requiring replacement. As the upper jaw 70 is spaced from the pre-form 34 except at the lateral ends 74 it is likely to need replacement less frequently than the lower jaw 72, which has an additional wear surface where it supports the central portion 80 of the preform 34.

Some of the benefits of the present invention may be obtained by use of a hybrid method and die wherein the lower jaw 72 of the die 68 of the present invention is used in conjunction with a pair of upper jaws. Thus, for hot creep forming of the pre-form 34 the upper jaw 42 of the prior art hot creep form die 40 is used in conjunction with the lower jaw 72 of the single die 68. Then the upper jaw 42 is removed and replaced by the upper jaw 70 of the single die 68 before the pre-from 34 is inflated. Thus the main advantage of the die 68 of the present invention is maintained in that the pre-form 34 is hot creep formed to be supported by the lower jaw 72 of the die 68 whilst the upper jaw is spaced from the pre-form 34 to provide an unequal inflation requirement. This hybrid arrangement enables old die upper jaws to be used until such time as they are worn irreparably, thereby reducing or delaying expenditure on new dies. Alternatively there may be a pair of lower jaws 72 that are substantially identical, one used for hot creep . forming and one used for super plastic forming.