Field of the Invention

The present invention relates to the manufacture of joints, generally between dissimilar materials, using power beams, particularly the joining of fibre- reinforced composites to metals using lasers to form fasteners from melted filler material in-situ.

Background to the Invention

In various industries, it is often desirable to design and manufacture structures that make use of more than one material type. For instance, in automotive and aerospace engineering there is a desire to reduce the weight of vehicles for improving fuel efficiency whilst maintaining safety and performance requirements. Whilst it would be ideal to construct structures from a single material that meets all requirements, it is normally the case that multiple materials are combined to make use of the different properties of said materials, such as combinations of metals with fibre-reinforced polymer composites. To make use of such structures, it is necessary to provide efficient joints between the different materials. Whilst some materials, such as certain metal combinations, can form joints by welding or mixing, this is not the case for composites to metals. Composites and metals are often joined using adhesives or mechanical means such as strapping, bolting and screwing as well as more intricate methods such as those outlined in EP1551590, or by metal z-pins inserted into the composite during manufacture of the composite material.

The provision of interlock joints is a favourable method of joining composites to metals, however a truly satisfactory method for achieving this has so far not been found. Some examples of the creation of interlock joints exist in the art: EP2698224 describes a method of making an overlap joint between plastic composites and metals using a cylindrical metal rivet inserted into a through-hole present in both metal and composite, where the rivet is laser welded to the metal panel. The rivet is of a top-hat or conical shape, where the narrower diameter section fits through the hole in the metal and the composite, while the larger diameter section is positioned on the open side of the plastic part and mechanically secures the plastic part to the metal part. The part of the rivet passing through to the open side of the metal part is laser welded to the metal part from that open side. Whilst this method may provide a secure joint, it is very inconvenient from a processing perspective to require access to perform a laser weld on one side of a workpiece whilst having access to the other side for insertion of a rivet. This means that either open-sided access is required to the open sides of both parts that form the finished workpiece, or additional manipulation and clamping of the parts is required during processing. Additionally, the component cannot easily be used for exterior (e.g. aesthetic) surfaces without considerable extra finishing operations, or pre-coated surfaces.

JP57-2051 16 describes a method of overlap joining a synthetic resin plate to a metal plate by drilling holes in the resin plate, inserting stud bolts (with a wide head and narrow shaft) that contact the undrilled metal plate via the narrow shaft which fits through the drilled holes and stud-welding (arc welding via capacitive discharge) the studs to the metal. Again, the wider head secures the plastic to the metal. Although only single-sided access is required, the use of stud welding requires provision of a moveable stud welding gun and often the use of flux in addition to the stud material. Manual or automatic position of the stud gun can lead to slow processing times in many circumstances, some metals may not lend themselves to attachment via metal studs, which may not be made of ideally compatible materials, and it may also be undesirable to have protruding stud- heads. Again, the component cannot easily be used for exterior (e.g. aesthetic) surfaces without considerable extra finishing operations.

GB567418A describes a method of joining materials, that cannot be united by welding, by drilling holes through one or both of the materials, placing metal inserts in the holes and electric arc welding either to an undrilled surface of the other material, or another insert placed in a hole in the other workpiece. The use of a manually or automatically positioned electric arc welding device would undoubtedly lead to very slow processing times. In light of the prior-art, it is therefore an objective of the invention to provide a new method for joining of materials, particularly dissimilar materials of the types discussed above.

Summary of the Invention

In accordance with the invention we provide a method for joining first and second workpieces, the method comprises:

a. Providing a first workpiece and a second workpiece, the first workpiece having a through-hole passing from a proximal side to a distal side of the first workpiece;

b. Arranging respective parts of the first and second workpieces to be mutually adjacent so as to provide a joining region, wherein the end of the through-hole at the distal side of the first workpiece is adjacent the part of the second workpiece in the joining region;

c. Providing a filler material in the through-hole, the filler material being capable of forming a welded joint with the second workpiece;

d. Directing a power beam, in the form of a laser or electron beam, into the through-hole from the proximal side of the first workpiece, thereby melting the filler to form an interlocking structure in the through-hole which is welded to the second workpiece and which mechanically secures the first workpiece to the second workpiece.

The method provides significant processing advantages over known techniques by enabling the joining of the first and second workpieces by processing performed from one side of the arrangement, in particular, the power beam. In comparison to known methods, the present invention allows the use of a fast, non-contact/remote welding method which can be implemented without unnecessarily complicated clamping, fixtures or manipulation for access to both sides of the workpiece during processing.

Typically the materials of the first and second workpieces are dissimilar so as to be incompatible for welding together. Preferably the dissimilar materials are polymer or polymer composites (as the first workpiece) and metals (as the lower workpiece), although other material combinations are of course possible. Although the invention is described primarily with reference to composite-to- metal joints, the invention is applicable to other materials in place of the composite such as ceramics, polymers, wood or even combinations of metals which cannot normally be welded together. Whilst the second workpiece is preferably metallic it will be appreciated that a polymeric workpiece may alternatively be used since the welding of polymers is readily achievable, in this case using a polymeric filler material for example. A range of different thicknesses of the workpieces may be used, from millimetres down to tens of microns, with the use of thin section and thicker sections possible, both for the first and second workpieces. Thus there is no requirement that the two workpieces have similar thicknesses.

The first workpiece may be in direct physical contact with the second workpiece. Other arrangements are contemplated which include a small gap between the workpieces or an intervening structure such as a surface layer applied to one or each of the workpieces. In most cases the first and second workpieces are mutually adjacent such that the second workpiece acts as an end surface closure of the through-thickness hole (with or without a gap or intervening layer). One or each of the workpieces may take a number of different geometrical forms although they may be conveniently provided as generally planar sheets. Thus in this case, at least the part of the first workpiece in the joining region has parallel distal and proximal surfaces which are generally planar. The through-hole can be of a regular geometric shape, such as of circular, square or star-shaped cross section. The through-hole is also typically linear having a principal or central axis, although in principle the through-hole may be formed to as to have curved or other non-linear geometry. In each case an axis defining the through-hole may be at a non-orthogonal angle to the end surface or surfaces onto which it opens. The through-hole may be provided as a bore having a cylindrical, smoothly tapering or stepped tapering geometry. In other arrangements the through-hole has an inconstant width along its length wherein a narrowest width of the through-thickness hole is located at a position separate from the proximal surface. This provides a convenient method of interlocking the workpieces. The through-hole adjoining the proximal surface of the first workpiece may be rebated, countersunk or counterbored. More generally the through-hole may be provided with a regular or irregular cross section. For example it may be formed to provide superior mechanical interlocking using irregular cross sections or recessed regions along the length of the hole (formed for instance during moulding of a part, or via keyhole machining).

The through-hole is preferably formed prior to arranging or "lay-up" of the workpieces, however it can be formed with the workpieces already arranged (such as whilst lapped) by drilling, power beam or other technique. Similar techniques for forming the through-hole in the first workpiece may be used to form a blind hole in the second workpiece to act effectively as an extension of the through-hole in the first workpiece. In the case of ceramic-to-metal joints, it is likely that the through-holes will be formed in production of the ceramic or via a specialised cutting/drilling process.

In principle the filler material may be compatible with the first workpiece in terms of weldability. However the invention finds greatest application in circumstances where the material of the first workpiece and the filler material are mutually incompatible for welding. This may be due to lack of wettability factors, metallurgical differences, melting temperature difference or other factors. The interlocking structure attaches the first and second workpieces together mechanically. Accordingly, upon forming the interlocking structure a peripheral gap may be formed between at least part of the interlocking structure and the first workpiece, typically within the through-hole. Any such gap is preferably significantly smaller than the dimensions of the interlocking structure so as to avoid any appreciable relative movement between the first and second workpieces. The filler material may be provided in the form of a wire or powder, or in a preformed geometry suitable for insertion into the through-hole (such as a solid block matching the shape of the hole). When provided as a block or wire the filler can be left free-standing prior to application of the power beam or it may be secured to the second workpiece prior to the application of the power beam (e.g. by a process such as stud welding, adhesive flux, friction joining). The use of a filler wire is generally preferred since this may be provided from a spool and therefore lends itself to use in an automated or semi-automated process. When the filler is provided as a unitary piece of filler material, it may be convenient to initially spot weld the filler material to the second workpiece in the through-hole prior to the step of melting the filler material with the power beam.

It is advantageous to provide the filler from the proximal side of the first workpiece, that is the same side as the power beam application, meaning that the process as a whole can be one-sided in nature. It is preferred that the first workpiece is positioned above the second workpiece such that the proximal side is positioned vertically above the distal side. Thus a notional plane defining the proximal and distal sides is arranged horizontally. This assists in the filling of the through-hole with filler material and also the containment of any molten material in larger scale through-holes. However such a plane may be arranged vertically so as to provide generally horizontal application of the filler and power beam. Other arrangements are also contemplated depending upon the application in question. In general the power beam is provided along an incident axis which is either normal to or at an angle (generally less than 45 degrees, but could be greater depending upon workpiece or hole/filler geometry) to the normal to the above described plane defining the proximal and distal sides.

The discussion herein is generally provided in terms of a first and second workpiece. However, it is conceivable that one or more additional workpieces are provided between the first workpiece and the second workpiece, each additional workpiece having a through-hole in communication with the through- hole of the first workpiece. Functionally, the first workpiece can therefore be thought of as comprising a number of separate stacked workpieces, such as in a sandwiched or other arrangement. For example three or four overlaid workpieces may be readily joined. It is not however essential that the additional workpieces take a similar form to the first workpiece, or indeed are formed from a similar material as the first workpiece. The method described enables the simplification of any clamping arrangements utilised to hold the first and second workpieces in their desired relative positions. It is possible that the workpieces could be glued, tacked or secured together by some other means to either aid clamping whilst forming the interlock joint or to supplement the interlock connection.

In the case of incompatible metals, it is probable that an additional layer of material, such as a sealant, glue, protective or mutually compatible layer, will be provided between the two metals both between the laying surfaces and disposed in the hole drilled in one of the workpieces (or both, if alternating holes are to be used). In some cases, this can be a layer of polymer (e.g. laminated, sprayed, adhesive, or other coating method), ceramic (e.g. monolithic, layered or powdered), compatible metal (e.g. thermal/cold spray) or even a layer formed by treatment of one of the metal layers (e.g. anodising, carburising, nitro- carburising). An additional layer of material may provided for non-metal first workpieces, for similar (protective) reasons.

The power beam used in the invention is typically a laser, as these are particularly ubiquitous in manufacturing industry, or it could be an electron beam. This depends upon various processing factors including the material of the second workpiece, the geometry of the joining region, the speed required, the heat input required and so on.

For example, when using a laser, the laser parameters used in accordance with the invention can vary greatly depending upon the degree of processing required by a particular substrate and include variations in laser power, spot size, traverse speed, laser-on time and laser pulse length (if applicable). A balance is usually struck between the necessary energy for melting the filler and underlying lower workpiece to the required degree, avoiding damage to the upper workpiece and economical rates of processing.

Whilst in principle the primary function of the power beam is to cause a weld between the filler and the second workpiece, such that only part of the filler material is melted, it is generally the case that the power beam is used to cause the melting of all of the filler material in the through-hole. In such a case the through-hole in particular is filled with the filler in fluid form which, upon solidification produces the interlocking structure. However, it is also contemplated that, whilst all parts of the filler material undergo melting, the pool of melt may travel along the hole. Thus, the part of the filler which is first melted may solidify prior to the final amount of filler material being provided. Accordingly whilst the provision of the filler material in step (c) may occur prior to the initiation of the power beam in step (d), alternatively the power beam may be initiated first (for example to begin melting the second workpiece) prior to the provision of the filler, or indeed each step may be initiated at the same time.

The interlocking structure mechanically securing the first and second workpieces may be described as an in-situ formed fastener. This may be arranged to form a smooth surface that is level or flush with the upper workpiece top surface upon solidification. However it may be left with some degree of protrusion either to aid bonding of a coating or the bonding of a further workpiece on the proximal surface of the first workpiece, or, if it does not affect either final finishing or performance. It is usual that the distal side (from the fastener) of the second or lowermost workpiece is relatively unaffected by the joining process and hence is eminently suitable for surfaces with this requirement, e.g. aesthetic surfaces, smooth surfaces, coated or other sensitive surfaces etc.

Although it is preferable for speed of processing that the power beam simply melts the material, it is possible for the power beam to perform a treatment such as the power beam modification described in EP1551590, known as the Surfi- Sculpt® process. Apart from just melting the filler material, the material can be redistributed by multiple swipes of the power beam to provide additional mechanical interlocking or to smooth the surface of the melted filler material. It is also contemplated that the method may be applied at multiple locations in the joining region so as to form multiple points of interlock between the first and second workpieces. These may be formed sequentially or simultaneously using either multiple power beams or a rapidly traversing beam. Brief Description of the Drawings

Some examples of the method according to the invention are now described with reference to the accompanying drawings, in which:

Figures 1 to 4 show schematic illustrations of stages in an example method; Figure 5 shows a pair of lapped workpieces joined in accordance with the invention;

Figure 6 shows an example of a micro-ring spot weld pattern used to fix a filler wire in accordance with the example; and,

Figure 7 is a flow diagram illustrating the example method.

Description of Embodiments

We now describe an example method according to the invention. The principal stages in the example method are illustrated in Figures 1 to 4. The method is also described in accordance with Figure 7 which is a flow diagram of the method.

A first stage 100 in the method, as illustrated in Figure 7, is the initial preparation of the first and second workpieces to be joined. The first workpiece, denoted workpiece 1 , is formed from Carbon Fibre Reinforced Plastic (CFRP) of 1.2mm thickness. The second workpiece, denoted workpiece 2, is formed from aluminium alloy, in this case unclad AA5754 of 1 mm thickness. For clarity, the nomenclature used herein to represent aluminium alloy types is in accordance with the International Alloy Designation System, as those skilled in the art will appreciate. The first digit represents the 'series', whilst the next three represent the specific alloy.

A hole 3 of 3mm diameter is formed in workpiece 1 , either as part of the workpiece manufacture (e.g. as a preform), or pre-drilled in the workpiece 1 using a prior-art technique such as mechanical piercing, forming or drilling, laser drilling, water-jet cutting etc. Both workpieces are cleaned with acetone prior to arranging, to remove grease or other contaminants.

Having prepared the workpieces, at step 102, the workpieces 1 ,2 are arranged with respect to one another. In this case they are lapped with the first workpiece being placed vertically over and in physical contact with the second workpiece. The areas of the workpieces that overlap define a joining region in which the workpieces may be joined together. At step 104, the workpieces are firmly clamped together, for example by applying force from the top surface, with the lower workpiece 2 pressed against a backing plate or fixture. This allows the clamping to be effected from a single side, in this case the upper side. A G or C-clamping robot or other device could also be used to achieve the clamping effect. The arrangement of the lapped workpieces is illustrated in Figure 1 (the clamping being omitted for clarity).

At step 106 a filler material 6 is placed within the hole 3. In the present case this takes the form of wire supplied from a spool 5. An optional step 108 may then be provided in which the filler material is attached to the surface of the workpiece 2 in the base of the hole 3. This is discussed further below in association with Figure 6.

At step 1 10, a power beam 4, in this case a laser, is impinged upon the lower workpiece 2 in the hole 3 and the filler material 6 is simultaneously fed from a spool 5 into the hole. This is illustrated in Figure 2. The power beam 4 is provided from above the lapped workpieces at a low angle to the normal to the surface of the first workpiece (this normal being parallel to the principal axis of the through-hole 3).

The laser system used in this example is an IPG Continuous Wave (CW), single mode Yb-fibre laser (YLS-1000-SM), operating at 400-500 W beam average power, 300-1200 ms beam on time. The beam is delivered through an optical fibre of ~15pm core diameter to an Arges 3D galvanometer driven beam focusing and scanning system. The scanning system has the capability to move the focused beam at very high speeds (> 1 m/s linear speed). It can thus be made to direct the beam on a fixed location, along a simple straight line or be programmed to describe a wide range of scan patterns. The CW laser system operates at 1070±10nm wavelength (infra-red) with a maximum output power of 1 kW. The optics used enable a minimum spot size of 47 m diameter to be achieved, although greater spot size diameters (at the workpiece top-surface) can also be used. Manufacturer specifications of the CW IPG YLS 1000 Yb-fibre laser source are summarized below.

The filler material in this example is AA5356 wire of 0.9mm diameter, cleaned to minimise oxidation and contamination, which is metallurgically compatible with the AA5754 lower workpiece 2 and may be fed in on-the-fly or pre-joined (adhesive, micro-ring spot welding, tacking) to the lower workpiece (as mentioned earlier). In this case it was micro-ring spot tacked at step 108. A description of the micro-ring spot welding is provided below with reference to Figure 6. Returning to step 1 10, the laser causes melting of the lower workpiece 2 and filler material 6, forming a melt pool into which the filler wire is fed. Notably the surface of the workpiece 2 in the base of the hole 3 is melted by the power beam and the melt from filler material 6 and workpiece 2 mix together to form a weld on cooling. Gradual feed in of wire, either manually, driven or via gravity, and melting by the laser causes a build-up of fused material in the hole, in a similar fashion to an additive process. The filling of the hole causes the formation of an fastener structure (interlocking structure) which is joined by the weld to the lower workpiece 2.

In the example described here, the calculated volume of filler wire melted to produce the fastener structure is approximately 18mm ³ . An air-knife (not shown) can be used to protect the process head from spatter and fume produced during the process, and shielding gas can be used to prevent oxidation of the workpieces and filler. In this case, 99.998% purity argon gas shielding (gas type 11 to BS EN ISO 14175) is used supplied at a flow rate of 10£/min and delivered through a 10 mm internal diameter copper tube (not shown), positioned 45° from the vertical direction and oriented to deliver shielding gas on the area being processed.

Figure 3 shows the build-up of the incomplete fastener structure 7 which then occurs at step 1 12, the fastener being made from filler material and fused to the lower workpiece. The geometry of the fastener once formed provides mechanical interlocking with the upper workpiece. As can be seen, in this case the top of the fastener protrudes out of the hole both vertically and laterally. The lateral spread is important since this provides the interlocking function which prevents separation of the first and second workpieces. It is sometimes the case that the power beam (or additional detachment apparatus) is used to detach the formed fastener from the filler material, if it is fed by wire or in another continuous form, for example by laser cutting or mechanical cutting. If very accurate pre-dosed amounts of filler are provided, this could alternatively be accomplished by filler with self-detaching (e.g. frangible) regions.

Figure 4 shows the final in-situ formed fastener structure 8, mechanically interlocking the upper and lower workpieces. As can be seen, the lower end of the fastener 8 is embedded (fused by welding) within the workpiece 2 whereas the upper part projects above the workpiece 1 as a shallow dome, the width of this being greater than the diameter of the through-hole 3. At step 1 14 the structure is allowed to cool and any relevant finishing processes are then applied, which may include cleaning and other mechanical processes applied to one or each of the fastener 8 and surface of the workpiece 1. A further step 116 is shown in Figure 7. This represents the possibility that more than one interlocking structure is formed within the joining region. Thus the earlier steps of the process may be repeated sequentially for each instance of further fasteners 8 which are desired to be formed. It is also contemplated that the process may be modified to provide "parallel" processing of the fasteners such that a number of fasteners are formed simultaneously at the same time within the joining region. This is achievable if multiple power beams are used or if a single power beam is capable of being moved at speed between different through-hole locations in the joining region. Multiple instances of spooled filler wires could be provided, or alternatively the filler material could be provided in powdered form for injection/spraying into the through-holes using an injector mounted to the power beam apparatus. Such powder deposition could be performed prior-to or ideally simultaneously to melting of the filler (and workpiece, if applicable, for formation of a melt pool). Figure 5 shows a pair of dissimilar lapped workpieces 1 , 2 with a series of interlocking structures 9 within the joining region formed as per the method described. Although roughly dome capped structures are shown, these can be formed flush with the upper workpiece top surface by appropriate shaping of the holes, for example with a "rebated" edge or inverse triangular cross-section, which allows the in-situ formed structure to adequately interlock the lower workpiece to the upper workpiece. Alternatively, the interlock structures may be machined flush or even formed below the plane of the upper workpiece top surface and filled with an appropriate resin, sealant or paint, which may be applied to the entire top surface during the finishing step 114 (e.g. for providing an automotive body component).

Figure 6 shows an example of a micro-ring spot welding pattern used to tack weld the filler wire to the lower workpiece 2, prior to processing with the laser. In Figure 6 the plane of the figure can be thought of as the region of the surface of the workpiece 2 within the hole 3. This approach, of joining the filler wire to the lower workpiece, is used in certain cases to aid flow of molten filler wire, once the melt pool is formed, to flow through the hole in the upper workpiece. This figure shows an example of the laser beam path forming overlapping rings combined with a linear movement, where the radius of the rings is shown by line 10. Beam peak power in this example is typically between 500 - 900Watts, with a beam on time of 100ms and ring radius of 0.75mm.

Although described using a metal workpiece, the lower workpiece could be any melt-bondable material for which compatible filler can be deposited and joined, for instance a thermoplastic polymer such as Acrylonitrile Butadiene Styrene (ABS), which is used in laser-driven Additive Manufacturing (AM) systems, with filler also of an ABS-based (or other compatible) material. Equally, the upper workpiece material can be varied, generally without limitation since it is not required that the interlocking structure provides anything other than mechanical interlocking with the workpiece 1 once the interlocking structure is formed. The process used to produce the interlocking structure should merely be sufficient to avoid damaging the workpiece 1.