Preparation of a component for use in a joint

A method of preparing a component (60) to be joined to another component (55). The method comprises growing an array of projections (56, 57) on a bond region of the component (55) in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region. The joint may be used to join a pair of structural components, for instance in an aerospace application. For instance the joint may be used to join a reinforcing plate, floating rib foot, or stringer to a panel such as a wing or fuselage cover. Alternatively the joint may be used to join adjacent layers in a laminate structure.

This application is the U.S. national phase of International Application No. PCT/GB2008/050152 filed 4 Mar. 2008 which designated the U.S. and claims priority to British Patent Application No. 0704753.3 filed 13 Mar. 2007, the entire contents of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a method of preparing a component to be joined to another component; a component prepared by such a method; and a joint including such a component.

BACKGROUND OF THE INVENTION

Joining between composite and metallic or thermoplastic components is currently approached in a number of ways, each with its own limitations.

The use of fasteners is commonplace but tends to result in de-lamination around fastener holes, as well as the associated difficulties of drilling holes in composites such as Carbon Fibre and Aramids such as Kevlar.

The bearing strength of laminated composites tends to be low, as does the inter-laminar shear strength. This results in a requirement for significant reinforcement around fastener holes, leading to a large weight increase, which is particularly undesirable in aerospace applications.

Fastened joints tend to be particularly weak in the pull-through direction (that is, the direction of axial load through the fastener) and as such are not well suited to aerospace applications such as fastening ribs to covers, where air and fuel pressure loads tend to result in significant axial component of load through the fastener.

Adhesive bonds are an increasingly common means of joining metallic components to composite laminates, however these perform poorly in peel, tension and cleavage, and tend to fail with little or no warning. Their weakness in peel and in tension makes bonded joints similarly limited in their application within conventional aerospace structures. Any mitigation for the poor performance in peel or tension tends to result in large bond surface areas, with the associated weight penalties that go with this.

Existing research into the use of ‘surface’ features to improve the strength of metallic/composite joints is limited.

WO 2004/028731 A1 describes a method by which surface features are generated by using a ‘power-beam’ such as an electron beam, in order to ‘flick-up’ surface material on a metallic component to sculpt protruding features that are intended to increase bond surface area and improve bond strength when incorporated into the matrix of a co-cured laminate.

This process has certain limitations.

Firstly, the process displaces surface material to create the protruding features. This could lead to surface damage of the component, and is likely to generate crack initiators that will adversely affect the fatigue life of the parent part.

Secondly, the process does not provide scope for optimising the profile and shape of the protrusions, which could produce significant improvements in the performance of the joint—particularly in tension and peel.

Thirdly, the process is an additional step in the production of a component and as such has an adverse time and cost impact.

Fourthly, the method can only form protruding features with a relatively low aspect ratio.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of preparing a component to be joined to another component, the method comprising growing an array of projections on a bond region of the component in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region.

A second aspect of the invention provides a method of joining a pair of components, the method comprising growing an array of projections on a bond region of a first component in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region; and joining the bond region of the first component to a second component such that the projections are embedded in the second component.

A third aspect of the invention provides a joint comprising a first component having a bond region with an array of projections; and a second component joined to the bond region of the first component, wherein the projections have been grown in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region.

The invention employs an additive fabrication technique to form the projections, instead of forming the projections by displacing surface material as in WO 2004/028731 A1. This reduces the risk of damaging the component. Also, by growing the projections in a series of layers, the shape of each layer can be selected to enable the profile of the protrusions to be optimised.

The layered fabrication method also permits the projections to be formed with a relatively high aspect ratio, which we define herein as the ratio between the height of the projection perpendicular to the bond region and its average width parallel to the bond region. Typically the aspect ratio is greater than 1, preferably it is greater than 2, and most preferably it is greater than 3. This can be contrasted with the projections described in WO 2004/028732 which have aspect ratios less than 1.

The head and the bond region may remain stationary during the growth process: for example the head may have a fixed array of lasers and/or nozzles which extend over the entire bond region and are modulated as required to directing energy and/or material to selected parts of the bond region. However more preferably the method further comprises causing relative movement between the head and the bond region. Preferably this relative movement is caused by moving the head, but it will appreciated that the relative movement may be caused by moving the component or by a combined movement of both parts.

Various additive fabrication techniques may be used, including techniques in which the head directs material to selected parts of the bond region, and techniques in which a series of beds of material are deposited on the bond region and the head directs energy to selected parts of each bed.

Examples of the former include fused deposition modelling (in which the head extrudes hot plastic through a nozzle) and powder feed fabrication (in which a laser beam directs energy to fuse a powdered material as it is delivered to the bond region). Advantages of these methods are that:the amount of wastage of material in the fabrication process is minimized;the projections can be made from a different material to the component; andthe component can be rotated relative to the head during the fabrication process in order to form a complex shape.

Examples of the latter include stereolithography (in which a laser is used to cure selected parts of a bed of liquid photopolymer) and powder bed fabrication (in which a series of beds of powder are deposited on the bond region and selected parts of each bed are fused by a laser). Advantages of using the head to deliver energy to the selected parts of a previously deposited bed of material are that:it enables the component and the array of projections to be formed together; andunconsolidated parts of each bed can support successive beds, enabling relatively complex shapes to be formed.

Typically the projections are formed by fusing a powder, for instance in a powder bed process or a powder feed process.

The components may be joined by penetrating the second component with the array of projections. This may be achieved by moving the first component, the second component, or both.

The joint may be secured by hardening the second component after the array of projections has penetrated into it, or by using an intermediate adhesive layer between the two components. In the latter case the second component may also have a bond region with an array of projections formed by an additive fabrication method.

The projections may be symmetrical (for instance cylindrical or conical, and extending at right angles to the component) or at least one of the projections may be asymmetrical (for instance the projection(s) may lean to one side and/or may have a non-circular cross-section). Asymmetrical projections can be used to improve properties in a particular load direction.

The first component may be an interfacing element between two components. In this case the joint further comprises a third component joined to a second bond region of the first component, the second bond region having an array of projections formed by the method of the first aspect of the invention. The bond regions may be on opposite faces of the first component, on separate parts of a single face, or on adjacent faces.

The second component may be a laminar component formed in a series of layers. In this case, the projections reduce the risk of de-lamination compared with conventional fastener joints. Preferably the second component is a composite component such as a fibre-reinforced composite component.

The projections may be formed from a metallic material (for instance Titanium or stainless steel); a thermoplastic material such as polyetheretherketone (PEEK); or any other material which can be grown by the additive fabrication technique of the first aspect of the invention.

The projections may be formed from the same material as the first component and/or the second component, or they may be formed from a different material.

The joint may be used to join a pair of structural components, for instance in an aerospace application. For instance the joint may be used to join a reinforcing plate, floating rib foot, or stringer to a panel such as a wing or fuselage cover. Alternatively the joint may be used to join adjacent layers in a laminate structure.

DETAILED DESCRIPTION OF EMBODIMENTS

A metallic floating rib foot1shown inFIG. 1comprises a web portion3and a pair of flanges2. The web portion3has a pair of fastener holes4. An array of projections5extend from the underside of the flanges2. As can be seen inFIG. 1, the projections5are distributed evenly over a bond region which extends around the periphery of the flanges2and surrounds a central region6with no projections.

The floating rib foot1is integrated into a mould tool10as shown inFIG. 2. The mould tool10has a mould surface12with a recess11which receives the flanges2as shown inFIG. 3. Web portion3extends into a channel between a pair of plates13,14, and is secured in place by a fastener17passing through a pair of holes15,16in the plates13,14as shown inFIG. 3. In the example ofFIG. 3only one fastener17is shown, but in alternative arrangements two or more fasteners may be used to secure the floating rib foot to the mould tool. In the case where two fasteners are used, then they may be passed through the holes4in the web portion3.

After the floating rib foot1has been integrated into the mould tool10, a composite lay-up18is laid onto the mould tool. The composite lay-up18comprises a series of plies of uni-axial carbon fibre, pre-impregnated with uncured epoxy resin. Each ply is conventionally known as a “prepreg”. The initial prepregs are penetrated by the projections5as shown inFIG. 3.

After the lay-up18has been formed as shown inFIG. 3, it is cured and consolidated by a so-called “vacuum bagging” process. That is, the lay-up is covered by a vacuum membrane (and optionally various other layers such as a breather layer or peel ply); the vacuum membrane is evacuated to apply consolidation pressure and extract moisture and volatiles; and the lay-up is heated (optionally in an autoclave) to cure the epoxy resin matrix. As the epoxy resin matrix melts prior to cure, it flows into intimate contact with the projections5. The projections5mechanically engage with the matrix, while also increasing the surface area of the bond.

The components are then removed from the mould and assembled with various other wing box components as shown inFIG. 4. In this example the cured lay-up18is a wing cover, and the floating rib foot1secures a rib to the wing cover18. The rib comprises a rib web20and a fixed rib foot21extending downwardly from the rib web20. Fasteners (not shown) are passed through the fastening holes4in the web portion3of the floating rib foot1to secure the floating rib foot1to the fixed rib foot20.

As well as carrying projections5, the lower surface of the flanges2may also be formed with resin bleed channels26shown inFIG. 5. Resin flows through the channels26during the curing process.

Each projection5is grown in a series of layers by an additive manufacturing process: either a powder bed process as shown inFIG. 6, or a powder feed process as shown inFIG. 7.

In the powder bed process shown inFIG. 6, the array of projections is formed by scanning a laser head laterally across a powder bed and directing the laser to selected parts of the powder bed. More specifically, the system comprises a pair of feed containers30,31containing powdered metallic material such as powdered Titanium. A roller32picks up powder from one of the feed containers (in the example ofFIG. 6, the roller32is picking up powder from the right hand feed container) and rolls a continuous bed of powder over a support member33. A laser head34then scans over the powder bed, and a laser beam from the head is turned on and off to melt the powder in a desired pattern. The support member33then moves down by a small distance (typically of the order of 0.1 mm) to prepare for growth of the next layer. After a pause for the melted powder to solidify, the roller32proceeds to roll another layer of powder over support member33in preparation for sintering. Thus as the process proceeds, a sintered part35is constructed, supported by unconsolidated powder parts36. After the part has been completed, it is removed from support member33and the unconsolidated powder36is recycled before being returned to the feed containers30,31.

The powder bed system ofFIG. 6can be used to construct the entire floating rib foot1, including the web portion3, flanges2and projections5. Movement of the laser head34and modulation of the laser beam is determined by a Computer Aided Design (CAD) model of the desired profile and layout of the part.

The powder feed fabrication system shown inFIG. 7can be used to build up the projections5on a previously manufactured floating rib foot. That is, the web portion3and flanges2have been previously manufactured before being mounted in the powder feed fabrication mechanism.

A projection5is shown being built up on the underside of one of the flanges2inFIG. 7. The powder feed fabrication system comprises a movable head40with a laser41and an annular channel42around the laser41. Un-sintered powder flows through the channel42into the focus of the laser beam43. As the powder is deposited, it melts to form a bead44which becomes consolidated with the existing material.

The powder feed system may be used to grow the projections in series, or in parallel. More specifically, the projections may be grown in parallel by the following sequence:

or in series by the following sequence:

where P(X) L(Y) represents the growth of a layer X of a projection Y.

This can be contrasted with the powder bed system which can only grow the projections in parallel.

In contrast to the powder bed system ofFIG. 7, the powder feed system ofFIG. 6directs powder to only the selected parts of the bond region, and fuses the powder as it is delivered. Therefore the powder feed mechanism produces structures that are unsupported by powder, and so supports (not shown) may need to be built integrally into the part and machined off later, in particular where the projections have large overhanging parts.

The head40may be the only moving feature in the process, or the part may be rotated during fabrication. In other words, the head40directs powder to selected parts of the bond region with the part in a first orientation relative to the head40; the part is rotated so that it adopts a second orientation relative to the head40; and the head then directs material to selected parts of the bond region with the part in the second orientation. This facilitates the manufacturing of complex shapes without the need for removable supports. For instance overhanging features can be formed by rotating the part between layers in order to always ensure that the element being built is at no more than 30 degrees from the vertical. As the build area is at a temperature significantly below the melting point of the material, the area being built will only need to maintain a supportable angle for a brief time after the laser energy is removed in order for it to solidify enough to become self supporting. If the projections are built in a parallel sequence then it is possible to re-orientate the part between each layer to enable unsupported overhanging features to be built.

FIG. 8shows an interfacing strip50with an upper face carrying an array of projections51for joining the interfacing strip to an upper workpiece, and an opposite lower face with an array of projections52for joining the interfacing strip to a lower workpiece. The interfacing strip and projections may be manufactured by the powder bed process ofFIG. 6, or the projections may be built onto a prefabricated strip using the powder feed process ofFIG. 7.

FIG. 9shows an interfacing strip55with upper and lower arrays of projections56,57. The interfacing strip55is shown inFIG. 10joining an upper workpiece58to a lower workpiece59. The interfacing strip55is particularly useful where significant through-thickness stresses are acting to pull the workpieces58,59apart. The penetration of the projections into the workpieces58,59provides a significant increase in the strength of the joint when it is subject to tensile, peel or cleavage loads.

The joint shown inFIG. 10may be manufactured in a number of ways, including:pressing the interfacing strip55into one of the workpieces (using a vibrating hammer or roller); then pressing the other workpiece onto the exposed projections of the interfacing strip (using the vibrating hammer or roller); orjoining the interfacing strip with a first one of the workpieces using a method similar to that shown inFIGS. 2 and 3; co-curing the interfacing strip and first workpiece; pressing the second (uncured) workpiece onto the exposed projections of the interfacing strip (using a vibrating hammer or roller); and then curing the second workpiece; orjoining the interfacing strip with a first one of the workpieces using a method similar to that shown inFIGS. 2 and 3; co-curing the interfacing strip and first workpiece; integrating the co-cured interfacing strip and first workpiece into a second mould; laying up the second workpiece onto the second mould using a method similar to that shown inFIG. 2; and curing the second workpiece.

An example of the use of the interfacing strip55is shown as an exploded view inFIG. 11. Note that inFIG. 11the bond region carrying the projections56,57is at one end of the interfacing strip55only. In this case, the lower workpiece is a wing cover61and the upper workpiece is a stringer run-out60comprising a pair of flanges63,64and a blade portion62.FIG. 12shows the stringer run-out60joined to the cover61by the interfacing strip (which is not visible inFIG. 12).

A fibre-metal laminate70is shown inFIG. 13in cross-section. The laminate70comprises a series of layers of carbon-fibre reinforced polymer (CFRP)71interleaved with layers of Titanium72. Each Titanium layer72carries an array of projections73on its lower surface and an array of projections78on its upper surface, each array of projections being embedded in the adjacent CFRP layer.

The laminate70is fabricated using the process shown inFIGS. 14-16. In an initial step, a first CFRP layer71is laid up (for instance as a stack of prepregs) on a mould tool (not shown). A first Titanium layer72is then laid onto the CFRP layer71with its lower projections73engaging the upper surface of the CFRP layer71as shown inFIG. 15. A roller74carrying a series of annular ridges75with the same spacing as the upper projections78is then applied to the upper surface of the layer72. The upper projections78are each received in a channel between an adjacent pair of ridges75as shown inFIG. 14. The roller74is then rolled over the interfacing strip, and vibrated to agitate the projections73as they penetrate into the uncured CFRP layer71as shown inFIG. 14. The roller74may be rolled back and forth in a number of passes to fully embed the projections73in the CFRP layer71.

A second CFRP layer76is then laid on top of the layer72as shown inFIG. 16, and a second roller77(without ridges) is rolled over the second CFRP layer76and vibrated to press the upper projections78into the second CFRP layer76. The process is then repeated to form a series of pairs of layers as shown inFIG. 13. Note that the projections for each Titanium layer72are offset from the previous layer.

A metal-metal joint is shown inFIG. 17. A first workpiece80is formed with a series of projections81. A second workpiece82is formed with a set of complementary projections83which interlock with the projections81as shown. A thin layer of adhesive (not shown) is provided in the gap between the work pieces, sealing them together in the manner of a tongue-and-groove joint.

The workpieces80,82and projections81,83may be manufactured by the powder bed process ofFIG. 6, or the projections81,83may be built onto prefabricated workpieces using the powder feed process ofFIG. 7.

Various alternative projection profiles are shown inFIG. 18which can be manufactured by additive layer manufacturing and used in any of the joints described above. Projection90comprises a conical spike. Projection91comprises a frustoconical base92and a conical tip93with an overhanging edge94. Projection95comprises a cone which leans by an angle of θ to the vertical. Projection96comprising a cone leaning at an angle of θ to the vertical, with a pair of ridges97,98on its overhanging side. Projection100comprises a cylindrical base101and a conical tip102. Projection103comprises a frustoconical base104, a frustoconical part105with an overhanging edge, and a conical tip106with an overhanging edge.

Note that the aspect ratios of the projections are relatively high, giving firm mechanical engagement and a high surface area. If we define the aspect ratio as H/W, where H is the height perpendicular to the bond region of the component and W is the average width parallel to the bond region, then the aspect ratio varies between approximately 3.5 (for the projection100) and 5 (for the projections90and95). The aspect ratio of the projections may be increased or decreased to give the desired properties.

The various geometries shown inFIG. 18may be selected to maximise the performance of the joint, and tailored to the specific loading that it is subjected to. Thus for example:projections91,96and103(all of which include a part with an overhanging edge) may be used in a joint (or a selected part of a joint) that requires enhanced pull-off (tensile) strength;an asymmetrical projection95or96may be used to improve properties in a particular load direction; andeach of the projections have pointed tips to enable them to penetrate easily into the composite material.