Field of the Invention

[0001 ] The present invention relates to a method of forming a fibre metal composite component.

[0002] The invention has been primarily developed for use in forming turbine blades for a hydro-powered electricity generator. Such generators are used to convert kinetic energy from flowing fluids, such as water and wind, to electrical power. However, the invention is not limited to this particular field of use and is also suitable for forming many other types of components, including gas turbine fan blades, hydrofoils, yacht masts and stays, ship and boat propellers, bone replacements, aircraft wings, rotor blades for lift fans, archery bows, and structural beams in bridges and buildings.

Background of the Invention

[0003] Fibre composite components, particularly carbon fibre composites, are used in applications that require high stiffness, high strength and low weight. It is known to encapsulate metal components into fibre composite components, primarily to provide for fixing points for bolts etc,

[0004] The typical known method of manufacture of fibre composite components requires a reference form such as a machined tool or a solid casting. The method of construction involves either a manual fibre layup, which constitutes the greater percentage of the cost, or an automated fibre winding process into the reference form. The components are usually constructed with a thin woven fibre skin over an open core or a foam/honeycomb core. The fibre skin is hardened, and attach ed to the core, by binding resin or hardened in the contact presence of the foam with a binder. The shape of the part dictates the suitability of adoption to composite construction.

[0005] Fibre composite components have several disadvantages. Firstly, they are relatively expensive and have a relatively long production time, both of these issues arising from the relatively high manual labor content required in the fibre layup. Secondly, the fibres are only unidirectionally stress resistant requiring a woven cloth arrangement to provide multi directional stress resistance. This woven arrangement reduces the pure directional strength. Thirdly, they have relatively low corrosion, impact and abrasion resistance, the binding resin becomes the weakest component. Fourthly, they are not suitable for use in a salt water environment, as the salt water penetrates and delaminates the fibre skin, and an electrolytic reaction between the fibres and the salt degrades the resin to fibre bond.

Object of the Invention

[0006] It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages.

Summary of the Invention

[0007] Accordingly, in a first aspect, the present invention a method of forming a fibre metal composite component, the method including the steps of:

a. forming a substantially elongate metal skin with a hollow interior of a first volume and open opposed first and second ends;

b. sealing the second end;

c. inserting fibres through the first end and into the hollow interior in a direction substantially towards the second end;

d. sealing the first end;

e. injecting resin into the hollow interior until the hollow interior is full and the fibres are saturated;

f. compressing the exterior of the metal skin in a forming die or dies and reducing the hollow interior to a second volume which is smaller than the first volume, such that pressure within the metal skin force excess resin out through the second end; and g. maintaining the compression of the metal skin in the forming die or dies until the resin has cured.

[0008] The method preferably includes sealing the first end with an inlet valve arrangement. The resin is preferably injected into the hollow interior through the inlet valve arrangement.

[0009] The method preferably includes sealing the second end with an outlet valve arrangement having a pressure relief val ve. The metal skin is preferably compressed until the pressure within the interior is high enough to open the pressure relief valve. The method preferably includes applying a vacuum to the outlet valve arrangement during the injection of the resin.

[0010] The metal skin is preferably formed from a continuous pipe section or a plurality of cut panels.

[001 1] The cut panels are preferably joined together by TIG, Laser, Plasma or compression welding. The cut panels are preferably formed into a curved shape before welding.

Alternatively, the cut panels are welded together flat and then expanded to form the hollow interior, for example by hydroforming.

[0012] The fibres are carbon fibre, most preferably in the form of a dry single bundles of fibres (known as a tow). The dry tow is preferably cut to a length slightly shorter than the length of the metal skin. Alternatively, the fibres are resin-impregnated (known as prepreg) tow or prepreg tape.

[0013] The fibres are preferably inserted into the hollow interior in a bundle having a volume smaller than the first volume.

[0014] The method preferably includes blowing air into the hollow interior from the first end during or after the fibre insertion step for the purpose of assisting fibre insertion and fibre alignment.

[0015] The method preferably includes applying heat to the metal skin during the curing step.

[0016] The method preferably includes cutting the inlet valve arrangement and the outlet valve arrangement off from the first end and the second end respectively, after the resin has cured. The method preferably includes placing a first cap and a second cap on the first end and the second end respectively, after the cutting steps. The first cap may be incorporated into a base mounting system.

[0017] The method preferably includes adding nano fibres to the hollow interior during the resin injection step. [0018] In one form, the metal skin is formed by hydroformimg. In another form, the metal skin is formed by cold spraying.

Brief Description of Drawings

[0019] A preferred embodiment of the invention will now be described, by way of an example only, with reference to the accompanying drawings in which:

[0020] Fig. 1 is a perspective view of a pair of curved metal skin panels;

[0021 ] Fig. 2 shows the panels of Fi g. 1 after joining together to form an elongated metal skin with a hollow interior;

[0022] Fig. 3 shows a series of carbon fibre dry tows;

[0023] Fig. 4 is a perspective view of the skin shown in Fig. 2 after the addition of an inlet valve arrangement to a first end and an outlet valve arrangement with pressure release valve to a second end;

[0024] Fig. 5 is a perspective detailed view of the first end showing the components of the inlet valve arrangement;

[0025] Fig. 6 is a side view of the components shown in Fig. 5;

[0026] Fig. 7 is a cross sectional end view of the metal skin with a carbon fibre bundle therein; [0027] Fig. 8 shows the metal skin of Fig. 7 after resin injection;

[0028] Fig. 9 shows the metal skin of Fig. 8 after in placement between a pair of fom ing dies and;

[0029] Fig. 10 shows the formation of a fibre metal composite component after compression of the metal skin by the forming dies. Detailed Description of Preferred Embodiment

[0030] Fig 1 shows a pair of titanium elongate cut panels 20 that are generally elongate and outwardly convex. The panels 20 are formed by hydroforming. The panels are about 0.5 to 1 mm thick, 3000 mm long, 550 mm wide at a first end 20a and 250 mm wide at a second end 20b. In the context of a turbine blade, the first end 22a and the second end 22b are known as a root and a tip respectively.

[0031] Fig. 2 shows the panels 20 after they have been welded together along their side edges to form an elongated skin 22 with a hollow interior of a first volume, which is about 100mm deep near the first end 22a and about 60 mm deep near the second end 22b.

[0032] Fig. 3 shows three lengths of dry carbon fibre tow 24. The fibres 24 are cut to a length slightly shorter than that of the skin 22 using a winding machine with an indexed plate or put a computer controlled robot. The fibres 24 are cut at one end and draped over a set of retaining rings, bobbins, pins or plates (bobbins are shown). The length of the fibres 24 is also such that there combined compacted volume substantially matches that of the skin after a compression step (which is discussed in more detailed below) has been undertaken. The dry tow 24 is cut to varying lengths commensurate with a taper variation in the longitudinal direction of the skin's final (i.e. compressed) volume.

[0033] Fig. 4 shows the skin 22 after an inlet valve arrangement 26 has been mounted to the first end 22a or root and an outlet valve arrangement 28, which incorporates a pressure release valve, has been mounted to the second end 22b or tip.

[0034] Figs. 5 and 6 show the components of the inlet valve arrangement 26, being an inlet tube 26a, a sealing plate 26b and a sealing clamp 26c.

[0035] Fig. 7 shows the skin 22 after a bundle 30 of the carbon fibre dry tows 24 has been inserted into its holl ow interior. As can be seen, the interior of the skin 22 is sufficiently wide enough for all of the fibres 24 to pass freely into the interior. If desired, air can be blown in through the inlet valve arrangement 26 towards the outlet valve arrangement 28. This air blowing process will both align the fibres 24 to the contour of the interior of the skin 22 and also add a degree of cross tangling. The cross tangling will increase the final component's resistance to sheer stress failure and reduce the possibility of delamination of the unidirectional fibres.

[0036] Fig. 8 shows the skin 22 after resin 32 has been injected through the inlet valve arrangement 26. The resin 32 flows towards the outlet valve arrangement 28. If desired, a vacuum can be applied at the outlet val ve arrangement 28 to reduce the possibility of air bubbles and to ensure full saturation of the carbon fibres 24.

[0037] Fig. 9 shows the skin 22 being placed between a pair of forming dies 34, which each have a forming surface 34a. The dies 34 are then brought together in the direction of arrows 36, to the position shown in Fig. 10. The dies 34 exert a pressure of about 10 mPa. This action

compresses the skin 22 and its contents, such that the initial first volume of its hollow interior is reduced to a second smaller volume. During this process, pressure within the skin 22, as indicated by arrows 38, forces the skin 22 outwardly into conformity with the abutting forming surfaces 34a of the dies 34 of the dies 34. More particularly, the metal skin 22 captures and contains the pressurized resin 32 during this process.

[0038] The pressure release valve incorporated into the outlet valve arrangement 28 controls the internal pressure during this compression stage. Excess resin 32 within the skin 22, in combination with the carbon fibres 24, pushes against the skin 22 forcing the skin 22 to take the shape of the pressed dies 34. The fibres 24 will also flow in directions orthogonal to the

(longitudinal) fibre direction. The resin 32 flows with the fibres 24 and out along the length of the fibres 24 towards the outlet valve arrangement 28. Once the pressure within the interior of the skin 22 is high enough to open the pressure release valve (e.g 10 mPa), the excess resin 34 flows out of the outlet valve arrangement 28. This outflow does two things. Firstly, it provides an even flow of fibres 24 into a final compressed state. Secondly, it provides another level of alignment with a degree of pre-tension to the fibres 24.

[0039] The dies 34 are then heated in order to heat cure the resin 32. As the resin 32 heats up its viscosity will drop with temperature then begin to rise as it cures. The increasing viscosity also causes a pre-tensioning of the fibres 24 from the root, where the fibres 24 are constrained to the inlet valve arrangement 26. After the dies 34 have completely compressed, the skin 22 remains therein until the resin 34 has cured. After the curing cycle, the completed component (i.e. the skin 22 with the fibre/resin 24/32 core bonded thereto) is removed from the dies 34. The first and second ends and associated valve components are then removed. This completes the forming of the fibre metal composite component, which in this embodiment is a turbine blade for a hydro-powered electricity generator. A sealed end cap and base cap can be fitted to the cut ends to prevent moisture from penetrating the fibre/resin core 24/32.

[0040] The above method, and resulting component, have several advantages. Firstly, the component is very lightweight, very strong in bending, very stiff and relatively very inexpensive to produce. The latter is due to: the use of low cost carbon fibre tow material; the reduction in manual labour content and processing time, particularly as manual fibre layup is avoided completely; the fact that there is no need for a mould release and preparation, because the resin does not make contact with the tooling; and the fibres being aligned during the compression step, which eliminates any manual work in terms of aligning the fibres to the principle stress direction. Thirdly, the method is suitable to produce components with varying shapes and cross sectional areas. Fourthly, the method allows for fixing points to be easily incorporated into the metal skin and the fibre anchors for ease of production and also superior strength .

[0041 ] Further, the metal of the skin is isotropic in stress loading, as opposed to a fibre which is unidirectional in stress resistance. A high tensile steel has an ultimate tensile strength in the region of 1480 Mpa. Titanium alloy Ti6A14V has an ultimate tensile strength of 1 170 Mpa. A high tensile strength Carbon Fibre has an ultimate tensile strength of up to 6000 MPa. A 3000 MPa Cai'bon Fibre will have, in the woven state, a reduced ultimate tensile strength of 1 120 MPa due to the compaction and weave angles of the fibres. This latter value is similar to the value for Titanium. Accordingly, the Titanium skin provides similar skin delamination resistance to a woven cloth outer skin layer.

[0042] The metal skin can also be tailored for different applications where corrosion and abrasion resistance of varying degrees are required.

[0043] The metal skin also provides the required containment of the fibres by resisting delamination. The fibres are aligned to resist the dominate bending induced loads with their superior ultimate tensile strength while the metal skin's properties serve a double duty of shear resistance and protection. [0044] The metal skin also has further advantages over a fibre composite component, such as corrosion resistance, wear and impact resistance and UV light protection.

[0045] In the context of the embodiment (i.e. a turbine blade for a hydro-powered electricity generator), the shape of the first end (root) can be tail ored into the forming dies to produce a mount capable of rotation for the purpose of pitch control. The orientation and splaying of the fibres in this region is important for reducing the local stress concentrations therein. The flexibility of the forming method with the fibre anchor arrangement lends itself to a superior structure.

[0046] In the context of using the method to produce gas turbine fan blades, the metal skin advantageously resists bird strikes. Fibre composites are not usable in this application as they can not resist the bird strike test.

[0047] Although the invention has been described with reference to a preferred embodiment, it will be appreciated by those person skilled in the art of the invention may be embodied in many other forms. For example, the panels 20 can alternatively be formed by cold spraying, in which metal powder is sprayed at supersonic velocity onto a tool die.