The present invention relates to a friction stir welding method and apparatus used to join multiple workpieces, for example for joining workpieces such as cables in an end-to-end fashion.

Background to the Invention

Friction stir welding is a method in which a probe of material harder than the workpiece material is caused to enter the joint region and opposed portions of the workpieces on either side of the joint region while causing relative cyclic movement (for example rotational or reciprocal) between the probe and the workpieces whereby frictional heat is generated to cause the opposed portions to take up a plasticised condition; optionally causing relative movement between the workpieces and the probe in the direction of the joint region; removing the probe; and allowing the plasticised portions to consolidate and join the workpieces together. Examples of friction stir welding are described in EP0615480 and W095/26254. The benefits of friction stir welding have been widely reported in the prior art, especially in comparison to conventional fusion welding techniques. These benefits include no need for consumables or fillers, low distortion in long welds, little preparation, solid phase (no fumes, porosity or splatter, lower heat input, and the avoidance of solidification of a molten weld pool), excellent mechanical properties and forming characteristics of joints.

Friction stir welding is commonly executed using a simple cylindrical or slightly tapered probe or "pin" protruding from a larger diameter flat, domed or tapered shoulder, although in some instances a 'shoulderless' tool has been used comprising of a probe with no appreciable shoulder feature. Typical examples of the common pin tool are described in GB2306366. Many modifications of the simple pin tool are known in the prior-art. Typical workpiece materials commonly joined using friction stir welding are of a relatively low melting temperature and are in this context generally termed as being low temperature metals or materials, that is materials with a melting temperature below 1 100 degrees centigrade. The most commonly friction stir weldable of these materials are metals based upon aluminium, magnesium, copper, lead and other similar materials. It is also possible to join high temperature metals based upon iron, titanium, nickel and others, but often more traditional joining methods using melting/fusion are preferred due to cost issues, particularly in relation to the tool materials required.

The configuration and clamping of workpieces joined by friction stir welding is numerous, but often dictated by the vagaries of the joining process itself. Workpieces must be securely clamped to prevent separation due to the high process forces and a backing member, such as an anvil, C-clamp or reaction tool is also normally required. Joint design also has to be carefully considered. For example, when joining relatively thin or delicately walled workpiece, such as aluminium extrusions, the joint interfaces are often designed to react the force from the tool and also prevent the escape of softened material. As well as clamping/support issues, the escape of material is important when considering pipes/tubes, stranded workpieces (cables), the joining of more than two workpieces at once (node joints), t-joints and other complex joint interfaces.

The end-to-end joining of cables or wires using friction stir welding, particularly those which are more difficult to join using fusion methods such as aluminium or copper cables, is of interest but has so far proven difficult due to factors such as material containment, formation of voids due to stranded nature of workpieces (reduced material volume for a given cross-section when compared to solid material), clamping difficulties, insufficient material mixing, coatings on wires &c. Various methods have been disclosed in the prior-art for the joining of cables and wires using friction stir welding:

JP2003-071576 describes a method of joining metal wires or foils, whereby their ends are buried in a terminal plate in a side-by-side fashion and a friction stir weld tool is traversed across each wire through the plate to join the wires and plate together. It is not possible to join workpieces end to end in any fashion using this method. JP2003-126972 describes a method of joining metal wires, plates or foils arranged in parallel (e.g. side-by-side or stacked) using a friction stir welding tool, either with or without a covering plate, whereby the tool is traversed through or across the workpieces. It is not possible to join workpieces end to end in any fashion using this method.

JP2004-160477 discloses a method of friction stir welding (typically square section) aluminium sleeves to (typically circular section) aluminium wires. A joined assembly can potential be combined with another, again using friction stir, either side-by-side or end-to-end. Although this allows end-to-end joining of wires, it will inevitably leave a relatively large local increase in cross-section in the form of the sleeve which, especially when scaling up in thickness, may prove excessive and need considerably machining if reduction to original cross-section is required. WO2005/082567 discloses a method of joining (insulation coated) wires by aligning their ends side-by-side, clamping and shaping/crimping them in a die and inserting a friction stir welding tool from the end to join the ends together. This allows joining of the ends without stripping the insulation, but it is not possible to join workpieces end to end in this fashion.

JP2014-057994 describes a method of joining (metal coated) wires by aligning portions of their length side-by-side and creating an elongate joint by traversing a tool for a distance along the butted lengths. Although this no doubt creates a joint, it is not convenient nor often feasible to align lengths of workpieces in this manner, or bend them to shape. It is not possible to join workpieces end to end in any fashion using this method. EP1046453A2 describes a friction agitation joining apparatus for joining a plurality of abutted members which includes rotatable chucking portions for chucking opposite ends of the abutted members. This apparatus is suitable for joining abutted pipes, but it is not possible to join other types of workpieces end to end in any fashion using this apparatus.

JPH10180467A describes a similar friction agitation joining apparatus to EP1046453A2. The apparatus is suitable for joining pipes together and includes support rollers, but it is not possible to join other types of workpieces end to end in any fashion using this apparatus.

JPH10166165A describes a method and device for friction welding capable of dealing with long and large joining faces, as well as vertical, horizontal, or inclined joining faces. However, it is not possible to join workpieces end to end in any fashion using either the method or the device described.

WO2015/045490A1 describes a friction stirring tool, a friction stir welding device, and a friction stir welding method with which the parts of the metal materials that are to be welded can be friction stir welded in a suitable manner, while limiting the load applied to the tool, even if the thickness of the parts to be welded varies. However, it is not possible to join workpieces end to end in any fashion using any of the tool, the device, or the method described.

None of the above solutions presented in the art provide a complete solution to the end-to-end joining of wires, or intersections of other workpieces, nor do they appear to address the issues apparent when joining cables or other stranded workpieces, where an enhanced mixing action and method of dealing with the reduced material volume for a given cross-section (when compared to solid material) is required. Summary of the Invention

In accordance with a first aspect of the present invention we provide a method of joining workpieces, the method comprising: a. locating respective portions of the workpieces in a joining body;

b. providing a friction stir welding probe, plunging the probe into the joining body and cyclically moving the probe to stir the material of the joining body and portions of the workpieces material; and

c. removing the probe and allowing the stirred material to consolidate and join the workpieces together.

We have found that the above inventive method is particularly useful for joining workpieces which are not conducive to the more traditional clamping methods used in existing friction stir processes. For example, the inventive method enables the end-to-end joining of cables, where the cable strands cannot be entirely stirred together and do not provide enough material to form a satisfactory joint using existing friction stir processes. The insert or joining body used in methods according to the invention provides the necessary extra material. The cyclic motion of the probe may comprise rotation of the probe, reciprocal motion of the probe, or both. In most cases, the probe has an accompanying shoulder, where the probe extends from the shoulder. The shoulder can be used to confine displacement of the stirred material, forcibly displace material and optionally provide heating by rotation. In an embodiment of the invention the probe and the shoulder (collectively the 'tool') are able to move relative to one another, although it is possible to provide a tool in which the probe and the shoulder form a single element. The use of a probe and shoulder which move relative to one another, typically along an axis parallel to the axis along which the probe extends from the shoulder, known in the art as 'retractable pin', provides a method of controlling the placement of stirred material to form a properly consolidated joint, whereby when the probe retracts into the shoulder at least some of the displaced material moves into the area voided by the probe. It is often the case that the joint produced protrudes outside the axis of the original workpiece to leave a thick or bulbous section. This can be machined post-process to form the joint nearer, or indeed conforming, to the shape of the original workpiece sections. Although methods according to the invention can be used to virtually refill the hole left when removing the probe from the workpiece, it is not usually necessary to do so as long as enough material is forced back in to create a satisfactory joint, especially when a post-weld machining step is carried out to remove excess material around the joint, as outlined. As such, in an embodiment of the invention, the probe and the shoulder of the tool are able to move relative to one another and removing the probe comprises causing the probe to retract into the shoulder whilst at least some of the displaced material moves into the area voided by the probe and further comprises using the shoulder to force further displaced material into the area voided by the probe.

In order to allow the workpiece and joining body materials to be stirred together without escaping, in an embodiment of the invention the workpieces and joining body are held within a die. In a further embodiment the stirred material is confined within a space defined by the die. In a further embodiment the stirred materials is confined within a space defined by the die and the shoulder of the tool.

Although the die is conveniently a removable item, in some cases the die could be integral to the joining body and as such remain in place after joining is completed. This could be enabled by containing, coating or partially containing or coating the joining body within a higher melting point material which acts as the die.

Although methods according to the invention are particularly useful for joining cables, other workpieces and joint geometries which are not particularly well served by traditional friction stir processes can also benefit. These include T- joints, where clamping is difficult and material can escape between the cross piece and leg; pipes/tubes, which suffer from similar problems; wires, which tend not to be stiff enough to be self-supporting during friction stir without complex clamping; extrusions which otherwise require thickening, integral support and specialist joint designs; and numerous others. An advantage of the 'spot' -type welding approach, where the probe is only plunged and not traversed is that it allows the use of relatively simple equipment and creation of a rapid joint (shorter weld cycle), with reduced likelihood of tool material damage. As well as joining elongate workpieces such as cables, tubes or beams, the invention is equally applicable to joining of workpieces comprising of multiple branches, such as extruded units used to create parts of a larger branched structure or extended structure. As such, the joining body may comprise two or more openings for receiving respective workpieces, and at least one of said openings may pass through the joining body. Coated, uncoated or composite workpieces can all equally be joined and, although reference is made primarily to metals, there is no reason why polymer workpieces cannot be joined. In the case of insulated cables, it is particularly useful that the invention can be applied without stripping insulation or removing any present moisture-blocking compound.

In most cases, the joining body will be composed of the same material as the workpieces, however in some cases the joining body material may differ. This could be in the case where heat treatment or mechanical working of the joint/workpieces is required (e.g. heat-treatable aluminium alloys), or when dissimilar workpieces are to be joined. The joining body could be made of a combination of materials in itself, or multiple pieces which could be temporarily attached to the ends of the workpieces. Material combinations of joining body and workpiece are likely to follow the conventional rules of materials, particularly metallurgy as laid down in the art of both friction welding, fusion welding and heat treatment in general. Although multiple workpieces are described, it is possible for the workpieces to be different ends of a long workpiece, such as a cable. In accordance with a second aspect of the present invention we provide an apparatus for use in joining workpieces, the apparatus comprising: a. a die configured to receive two or more workpieces and a joining body;

b. the joining body defining in use a joint region between the workpieces; and

c. a friction stir welding probe, and means for causing cyclic motion of the probe when it is plunged into the joining body to carry out a friction stir welding process.

In an embodiment of this aspect of the invention, the probe has an accompanying shoulder, the probe extending from the shoulder

In an embodiment of this aspect of the invention the probe and the shoulder are able to move relative to one another, for example the probe is operable to retract into the shoulder whilst the shoulder forces displaced material into the area voided by the probe.

In an embodiment of the invention the die comprises a lower assembly and a detachable upper assembly, whereby the upper assembly is removable to allow the die to receive the workpieces and joining body. Brief Description of the Drawings

Examples of the invention are illustrated with reference to the accompanying drawings, in which:

Figures 1 a-1 c show a schematic illustration of the assembly steps of workpieces, joining body and die.

Figures 2a-2d show a schematic illustration of a welding procedure according to an example of the invention.

Figure 3 shows a schematic illustration of disassembly of the die and a product made using an example of a method according to the invention. Figure 4 shows a photograph of a disassembled die.

Figure 5 shows a joining body for use in examples of methods according to the invention.

Figure 6 shows a product made by an example of a method according to the invention.

Figure 7 shows a possible configuration of workpieces and joining body for application of the invention.

Figure 8 shows a possible configuration of workpieces and joining body for application of the invention.

Figure 9 shows a possible configuration of workpieces and joining body for application of the invention.

Description of Preferred Embodiments Figures 1 a-1 c illustrate the procedure for setting up (Figure 1 a) the die 20, composed of upper die assembly 1 and lower die assembly 3, placing (Figure 1 b) of insert or joining body 4 (can be a single piece or multiple pieces) in the die 20 and locating (Figure 1 c) workpieces 5 in apertures 22 in the joining body 4 aligned with die holes 2. Notably, the die may be made of multiple sub- assemblies depending upon the requirements of the joint, workpieces and joining body to be used.

Figures 2a-2d illustrate the steps of carrying out a friction stir welding operation in accordance with the invention using a tool 6, comprising a shoulder 7 and probe 8. This figure is referred to in terms of the sub-figures a, b, c & d, for clarity. In figure 2a, the co-rotating probe 8 and shoulder 7 are moved toward the assembled insert or joining body 4 and workpieces 5, into the interior of the die 20. The plunging of the probe causes material of the insert or joining body 4 and workpieces 5 to soften, mix together 1 1 and be displaced by the probe to partially or completely fill the die 20. In figure 2b, the displaced material meets the shoulder 7, which compacts the material within the die 20 and provides a heating effect due to frictional contact which maintains the displaced material in a plasticised state. A process dwell may occur at this point to allow proper mixing of the material of insert or joining body 4 and workpieces 5, especially in the case of a stranded or coated workpiece such as a cable. Such a dwell may include adjustment of rotational speed of the shoulder/probe (together, or separately if applicable) and pressing force, depending upon material properties and machine feedback. In figure 2c, the probe 8 is withdrawn whilst the shoulder 7 pushes the material back into the space vacated by the probe. Material mixing 1 1 is shown purely for illustration and is not to scale nor or a true representation of material mixing that occurs throughout the die 20, workpiece 5, and insert/joining body 4 regions. In figure 2d, the probe 8 is completely withdrawn and consolidation of the joint occurs. The existence of an exit hole in the joined part depends upon the amount of material available from the insert or joining body 4 and workpieces 5 when compared to the die 20 size and plunge depth of the probe/shoulder, and whether the rotation/pressing of the retracted probe/shoulder is sufficient to fully refill the space voided by the probe 8. In most cases an exit hole occurs to some extent, but not always.

Figure 3a shows the completed joint 9, with the upper die assembly 1 removed, followed by complete ejection of the part 10 (in figure 3b). Following ejection, it is likely that some post-processing will be required, such as by machining, heat treating or finishing the joined parts.

Figure 4 shows a photograph of the dissembled die 20, comprising upper die assembly 1 in the form of several sub-assemblies, and lower die assembly 3, also made up of several sub-assemblies.

Figure 5 shows a photograph of an insert or joining body 4, as used in some methods according to the invention, with opening 23.

Figure 6 shows a photograph of a completed joint 9, made between two cable workpieces 5 comprising aluminium strands. The exit hole 12 of the probe 8 is clearly visible, however the deepest point of this can be seen to be outside of the primary joint line (axis/intersection of the joined workpiece). In some cases, the hole can be completely filled but it is not generally necessary, especially when post-process machining of the joint occurs - it is of more importance that the joint is correctly consolidated.

Figure 7 shows a configuration where the insert or joining body 41 and workpieces 51 form a node. In this example the joining body 41 has three openings 24 for receiving respective workpieces.

Figure 8 shows a T-joint configuration with the insert or joining body 42 and two workpieces 52. Such a configuration may be achieved by providing an insert or joining body 42 with a first opening 25 which passes through the joining body and a second opening 26 which may open into the first opening 25.

The configurations of figures 7 and 8 may be achieved either with a joining body which is a single piece or from a joining body comprising multiple pieces.

Figure 9 shows multiple connections used to form an extended structure, where each of the workpieces 53 is made up of identical (or non-identical) pieces, which could be extrusions. In a flat lattice, the joints 91 could be made from either the top, bottom or side, whilst in a three-dimensional lattice they would be made from a convenient open side. Further pieces can be added once a joint is completed and (optionally) post-processed to form a large branched structure. Many different configurations of workpiece are obviously possible.

Although a co-rotating probe and shoulder are used for simplicity of operation and equipment, it is possible if necessary to use differing rates of rotation for the shoulder and probe, different directions of rotation for the shoulder and probe, a probe which performs reciprocal motion, or even a probe which both rotates and performs reciprocal motion. It is also possible to use a stationary shoulder either alone or with an additional source of energy, such as that provided by resistance heating, induction heating, laser heating etc. It is also possible to use a friction stir probe without a shoulder, optionally using other methods to compact/force/move material, such as a multi-piece die where parts of the die are movable in relation to each other, enabling 'squeezing' of the material within the die. In a production environment it is also envisaged that cooling (or possibly heating, dependent upon materials and processing environment) of the workpieces, die and/or tool will be required, either directly or indirectly by cooled fixtures. Cooled fixtures are likely useful in reducing Heat Affected Zone (HAZ) effects and minimising changes/damage to parent material or components. Certain materials may also require an inert or protective atmosphere.

Although the invention is typically carried out by plunging with the probe and shoulder moving toward the workpieces (into the die) at roughly the same rate, with the probe extended as shown, in some circumstances the shoulder may be moved to contact the joining body before the probe is plunged and then moved away as the probe plunges or the probe may be made to contact extending relative to the shoulder before the shoulder follows. Embodiment / Worked Example

Workpieces

22mm diameter cables made up of stranded aluminium wires. Machine

TWI Powerstir FSW machine. The machine was specifically designed for welding thick section materials and therefore has the capability of delivering high torque and forces potentially required during methods according to the invention when applied to thick workpieces.

Tooling

The general design of the tooling was based on the following principles:

• The joining body to be large enough to house the 22mm diameter cable but small enough to be fully plasticised during welding. In this case, it was manufactured from aluminium AA6082-T6 and the bore adapted to provide easy fit of the cables.

• The joined cable assembly must be easily released from the die. This was achieved by an ejector screw and a tapered bottom die. • The probe assembly must fully cover and mix the cable diameter.

• The die must contain the displaced material (consumable joining body and cable mixed) during the plunging of the probe. Friction stir welding tool

The FSW tool set comprises of a probe and shoulder. They were designed to be used with an existing FSW welding head.

Both the probe and shoulder were manufactured from H13 tool steel, machined to shape and then heat treated to RC 44.

Typical weld cycle

Welds can be carried out using position and/or force control of the tool components. A strategy biased toward force control can be employed to avoid errors in material volume calculation, help deal with gaps or inconsistencies in parent material, allow for environmental variability, compensate for material loss (e.g. plasticised material forced into parent workpiece or out of die) and provide more consistency with regard to joint properties, However, a position control strategy will likely require less complicated equipment set-up, which may be beneficial for field equipment, in terms of size, robustness etc. Of course, a mixture of the two can conceivably be used. Briefly, position control is where the tool is positioned/moved relative to a datum point along the plunge axis, typically parallel to the axis along which the probe extends from the shoulder. This is an open loop control strategy. In position control, for example, the tool shoulder could be programmed to move to a set position along the plunge axis with respect to the joining body/workpieces, irrespective of the imparted force. Force control is where the tool is positioned along the plunge axis relative to the force it imparts. This is closed loop control strategy, using force feedback from sensors/transducers etc. In force control, for example, the tool shoulder would be programmed to push on/into the joining body/workpieces, along the plunge axis, with a programmed force. The shoulder will move along the plunge axis into the workpieces until the set force is reached. If the force imparted by the shoulder as measured by the sensors is seen to decrease below the programmed force, then the shoulder will plunge deeper into the workpieces. Conversely, if the force imparted by the shoulder is seen to increase above the programmed force then the shoulder will reduce the plunge depth. Programmed positions or forces can of course be altered, either manually or automatically, as part of the joining process. Force or position control can be applied to either the probe or shoulder, the probe and shoulder combined or by using a mixture of control methods.

(refering to table below in relation to A,B,C,Y and process parameters)

· A typical weld cycle consisted of the following sequence:

• Set the probe "stick out" from the shoulder. This was initially calculated to be equal to the plunge depth (A-Y) plus the distance that the expelled material was expected to rise above the consumable joining body face, but may need adjustment following trial runs.

· Move shoulder and probe to stand off position set above the consumable joining body top surface.

• Rotate the shoulder and probe and plunge them together a distance A.

• The expelled material rises and contacts the shoulder.

• Continue to plunge the shoulder a distance B whilst lifting the probe a distance C. The rate of probe retraction is automatically set so the probe and shoulder complete their moves at the same time.

• Dwell the shoulder under force control prior to extraction of the tool assembly.

The strategy above focuses on calculating the amount of added material, due to the 'voids' in the stranded cable and material being extruded into the clamp area,

SUBSTITUTE SHEET RULE 26 and estimating where the expelled material would meet the advancing shoulder at the end of the 1st stage plunge.

In practice this is can sometimes be difficult to do, as there are many factors which affect the amount of expelled materials, such as:

• Amount of material being forged around the workpiece and being extruded into the clamp area.

• How much material was adhered to the tool from the previous weld cycle. · The gap between the workpiece ends when securing in the fixture prior to welding.

A technique that uses a more force control bias is possible, where the probe and shoulder are positioned on the top of the consumable joining body initially before the probe is extended under position control whilst the shoulder is lifted under force control. This allows constant contact to be maintained with the expelled material. Once the probe plunge is completed the probe is withdrawn and the shoulder pushed under force control to complete the joint. This technique is not as sensitive to the factors detailed above which affect the amount of expelled material, but requires more complex equipment.

SUBSTITUTE SHEET RULE 26