GB2614889A - Friction stir processing method - Google Patents

Abstract

A friction stir welding (FSW) tool, having a filler material feed, may have a shoulder in contact with a first workpiece, such that filler material is stirred into the bulk of the workpiece, deforming the edge of the workpiece laterally. Downward pressure from the shoulder can prevent filler material from rising above the workpiece, creating a distal edge protruding tab 13, which has been pushed out sideways. This can shape the workpiece’s distal edge, e.g. in preparation for joining to a second workpiece. The edge may be part of a fracture in the first workpiece, which is subsequently closed/repaired when the edge is forced sideways. A second workpiece, adjacent a distal edge, may form a template or guide, to shape the lateral deformation of the first workpiece (possibly closing the gap between workpieces before subsequent joining). The shoulder may have an L-shaped overhang (19, fig 3) to restrict expansion.

Description

FRICTION STIR PROCESSING METHOD FIELD OF THE INVENTION

The present invention relates to a method of modifying a workpiece in which an edge of the workpiece is deformed. The method can be used for creating tabs, filling gaps, voids, cracks or similar defects, and for improved joining of

BACKGROUND TO THE INVENTION

Friction stir welding is a method known for joining two or more workpieces together. In a friction stir welding process, a probe of material harder than the workpiece material is caused to enter the region of the joint to be formed 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 of the workpiece material to take up a plasticised condition. Optionally the technique can involve causing relative movement between the workpieces and the probe in the direction of the joint region (resulting in an extended joint). After applying the probe as required, it is removed and the plasticised portions are allowed to consolidate and join the workpieces together. Examples of friction stir welding are described in EP0615480A 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 particular need for consumables or fillers, low distortion in long welds, little preparation, solid phase (which avoids forming fumes, porosity and splatter, requires lower heat input, and avoids solidification of a molten weld pool), excellent mechanical properties and forming characteristics of joints.

Since its invention, friction stir welding has seen particular utility in the joining of aluminium extrusions, such as for the manufacture of rail cars. When joining long length extrusions, it is often the case that tolerance deviations and improper fit up can lead to the occurrence of gaps between the extrusions. To deal with this using friction stir welding, several approaches have been considered.

EP0797043A and EP0893189A disclose the local thickening of extrusions around the joint region, so that the friction stir welding tool can be ioverplungedi with displaced material used to fill any gaps between the extrusions. Although successful in dealing with relatively small gaps of a few millimetres, it is not possible to deal with wider or long lengths of gapped regions, or thicker plate sections.

EP1762327A discloses a method whereby the abutting faces of workpieces are cut straight in advance of insertion of a strip of filler material before being friction stir welded together. In practice, this is not a particularly effective technique for long lengths, as jigging, cutting and fixture operations are not easily implemented.

Whilst friction stir welding does not specifically require filler materials, the addition of these has been employed for closing pin exit holes at the end of welds or for other purposes. Various methods and apparatus have been developed to deliver, deposit and process filler materials: EP10212708 relates to an apparatus for joining workpieces using friction stir welding. The technique described involves the use of a pin and body (the lowermost part of which is the shoulder, as is provided in some typical friction stir tools) that are mutually movable, allowing the pin and body to perform different movement patterns relative to one another. This also discloses the supply of additional material during joining, for example to fill exit holes and deal with material thickness differences along the joint where the shoulder may not properly pressurise and consolidate the joint.

EP1790425B discloses a deposition process whereby a filler material is placed on or about a joint line and friction stir welded to deposit additional material standing proud of the surface. The filler material can additionally be contoured by a non-rotating head/die through which a shouldered friction stir welding tool extends. Whilst this process is useful for adding deposited/contoured features to workpieces or potentially filling exit holes, it is limited in that the filler material itself is directly forming features, so it cannot be applied to either narrow or irregular gaps.

US7784667B discloses a welding process that utilises a claimed friction stir-like approach, but using a consumable probe (pin) in a non-autogenous process which can be used to fill gaps or account for shape mismatches. It is not clear how this process would operate successfully in practice, as friction stir welding requires a non-consumable tool, and provision of a consumable tool would not be expected to sufficiently plasticise or consolidate workpiece material, and would require a whole tool-feeding apparatus linked to the tool rotating mechanism and continuous provision of the 'tool' over long processing runs.

US10279422B discloses a friction stir welding tool that uses a stationary (non-rotating) shoulder and rotating probe, where filler material is fed into a screw thread ('conveyor worm structure') around the probe from the vicinity of the stationary shoulder The filler material can then be used to compensate for gaps between workpieces or deal with friction stir welding 'exit holes' which are often left when a probe is withdrawn from a workpiece. In this case the filler material is described as always being fed directly into the gaps or regions that requiring extra material, which is difficult to control whilst filling irregular gaps or practically impossible with wide or unrestrained gaps.

US7762447B discloses a method of filling large grooves between workpieces (preforms) using layers of material inserted into the grooves and (optionally) multi-pass friction stir welding to gradually build up a joint.

JP2004025296A discloses a friction stir welding tool, whereby filler wire is fed through the tip of the tool and can be used to fill gaps or deal with exit holes. In this case the filler material is described as always being fed directly into the gaps or regions that require extra material, which is difficult to control whilst filling irregular gaps or practically impossible with wide or unrestrained gaps.

US20120279441A discloses a friction stir tool through which (typically powder) filler material is delivered via screw-type auger disposed in a central throat in the tool and then softened and deposited in an additive fashion proud of the workpiece surface or directly in gaps, akin to a friction deposition process which 'smears' material on a surface. Whilst sound deposits can be made, bonding/mixing of the filler material with the workpiece is difficult to achieve in this fashion, and it is difficult to see how narrow or unrestrained gaps could be reliable and soundly be filled with sufficient mechanical strength for e.g. subsequent joining.

In light of the drawbacks associated with known friction stir welding and friction stir additive techniques, there is a need for a method of producing consistent, high quality joints and repairs where voids and gaps occur.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of modifying a first workpiece, the method comprising: causing a rotating probe of a friction stir welding tool to engage a surface of the first workpiece, at a location on the surface that is laterally spaced from an edge of the surface, and to enter the first workpiece at that location, wherein the friction stir welding tool comprises a shoulder arranged to contact the surface while the rotating probe is received in the first workpiece, and wherein the rotating probe extends from the shoulder; while maintaining contact between the shoulder and the surface of the first workpiece, causing a filler material to be stirred into the bulk of the workpiece by the rotating probe such that the edge of the workpiece deforms laterally with respect to the axis of the rotating probe.

In some embodiments, the shoulder and probe may be machined from a single piece of material or the shoulder and probe may be made of separate pieces of material, where, for example, the probe might extend from a bore of the shoulder.

The shoulder can be brought into contact with the workpiece before or after the rotating probe enters the workpiece, wherein in the latter case the probe can be extended from a bore of the shoulder whilst the shoulder is already in contact with the workpiece. As will be discussed in detail below, in some embodiments the shoulder may be rotating or non-rotating, or a portion of the shoulder may rotate, for example a radially inner portion, with a radially outer portion not rotating while the probe is received in the workpiece.

In the method of the invention, a friction stir welding tool is employed to shape an edge of a first workpiece. The rotating probe of the friction stir welding tool enters the first workpiece at a position spaced from the edge to be shaped and filler material is stirred into the first workpiece at that location. The addition of filler material in this way causes the total volume of the first workpiece to increase. As the filler material is being stirred into the first workpiece, the shoulder from which the rotating probe extends is in contact with the surface in which the probe is receiving and thus prevents the added filler material from extending out of the plane of the surface. Consequently, the edge of the workpiece (which is not constrained by the shoulder) expands in a lateral direction (relative to the axis of rotation of the rotating probe) in response to this increase in the total quantity of material in the first workpiece. It will be appreciated that the direction in which the edge expands is not necessarily strictly perpendicular to the axis of rotation of the probe since the probe may be received by the workpiece at an angle non-perpendicular to the direction in which the shoulder is free to expand. The position at which the rotating probe is applied to the first workpiece should be sufficiently spaced from the edge that the material at the edge is not plasticised by the rotating probe. However, the location at which probe is applied should be sufficiently close to the edge that the workpiece material between the plasticised region and the edge can deform laterally under the influence of the force with which the friction stir welding tool is applied to the workpiece.

The invention can be used to push out an edge of a first workpiece such as a plate-shaped workpiece or long extruded workpiece in place of joining material (e.g. filler or strips of material) to the side edge, which can be especially advantageous where the side edge is not suitable for adding material to directly, for example possess an uneven surface, or is inaccessible directly, or is unrestrained by another surface. This technique can be used to shape the edge of the first workpiece for various applications. For example, the shape produced by performing the method of the invention may be the shape required of the finished workpiece. As will be described in more detail below, the edge could also be shaped in order to prepare it for joining to a second workpiece. It should be appreciated that the "edge" in question is not necessarily an outermost edge of the first workpiece: for example, the edge could be defined by a fracture in the first workpiece, in which case the edge could be shaped in accordance with the method of the invention in order to close the fracture, possibly in preparation for subsequently sealing the fracture (e.g. by welding together the sealed edges of the fracture).

Whereas the background techniques discussed previously deal with gaps in friction stir welding, all directly fill the gap or deposit filler material on a surface, that is, the filler material is softened and plasticised by action of the tool and joined with or deposited in the region of the free faces of the workpiece(s) to be joined or additively built upon. In this respect, it is only possible to feed in a certain amount of material for a given workpiece and friction stir welding tool size, so gaps above a few millimetres are not able to be reliably filled, and consolidation with the workpiece material is questionable. Although additive processes can build material up on existing structures, these configurations are not suitable for dealing with inconsistent gaps between butted workpieces, as the tool must build up on top of the workpiece surface. By contrast, the present invention provides a way of shaping the edge of a workpiece which does not require the addition of filler material directly to that edge.

The invention can be used to form 'spot' type bulges on the edge of the first workpiece, either without traversing the probe across the workpiece On other words, insertion and removal of probe without moving the probe laterally relative to the position at which it first entered the first workpiece) or via intermittent feed pressure, for the production of 'tabs'. However, in some particularly preferred embodiments, the probe is advantageously traversed and filler material is fed in at a proportional rate to the traverse, for example over long workpiece lengths with irregular or wide gaps. Hence, in some preferred embodiments, the method further comprises moving the rotating probe along a predetermined path while supplying the filler material to the rotating probe so as to deform an extended section of the edge. Contact between the rotating probe and the workpiece is maintained as the rotating probe moves along the predetermined path. Filler material is stirred into the workpiece along the predetermined path, resulting in an extended section of the edge being deformed as the parts of the edge proximate to the predetermined path expand laterally due to the added filler material. In particularly preferred implementations, the predetermined path extends along the surface at a substantially constant distance from the edge. For example, if the edge were a straight edge, the predetermined path would lie parallel to the edge at a non-zero distance from it. This ensures that the edge deforms in a consistent manner since the thickness of workpiece material between the edge and the plasticised region created by the rotating probe will be constant along the extent of the path. Where the probe is moved along a predetermined path in the manner described above, preferably the filler material is supplied to the rotating probe by feeding the filler material to the rotating probe from upstream of the rotating probe.

"Upstream" here means from the direction of the part of the predetermined path which has not yet been traversed by the rotating probe.

The invention is especially useful when applied toward the joining of butted workpieces, for example long extruded panels used in the manufacture of railcars.

Often, the extruded panels are not perfectly flat or uniform and, especially when butted over long lengths (e.g. tens of metres), they exhibit intermittent gapping which cannot be bridged using conventional friction stir welding methods without surface preparation (cutting or milling) or direct addition of filler material, which is difficult in uneven gaps and over long lengths. In this regard, a single pass using the inventive method can be used to fill in an uneven gap prior to joining the workpieces using friction stir welding. Using existing processes, it is particularly difficult to bridge even consistent gaps between butted workpieces that are of width greater than 25% of processing depth, and even more difficult when those gaps reach 50% of processing depth. However, the inventive method is not limited in terms of the gap widths that can be bridged, as multiple passes can be used to push out bridging material. For simple plate workpieces, the processing depth is typically the thickness of the workpieces, although it is sometimes the case that, when considering workpieces that are of more complex construction (e.g. hollow extrusions) or large thickness, that the processing depth will only be partially penetrating into the workpiece. For partial penetration processing where the workpiece needs joining from both sides (e.g. hollow extrusions), it is possible to carry out the invention simultaneously on both sides of the workpiece.

For implementing methods according to the present invention, it is possible to use modified versions of existing equipment that can add filler materials either via feed into the probe through a shoulder bore, through the shoulder itself into the probe, or fed underneath the shoulder, such as via a feeding wheel and shaped non-rotating shoulder 'bearing' surface (the part of the shoulder which bears upon the workpiece). Using a feeding wheel, wire can be held in position and rolled in to the probe at the rate of processing head traverse, but ideally active feed control can be used to control the pressure of fed material, along with active control of probe and shoulder position (e.g. via force control). The various methods of monitoring and control of the friction stir head which may be used to implement the method of the invention are typically in common with those typically used for existing friction stir processes.

In general, the shoulder of the friction stir welding tool may be any feature of the tool from which the probe extends in the manner described above (i.e. from a bore in the shoulder) and at least a part of which is arranged to contact the surface of the workpiece while the probe is received in the first workpiece. However, in some preferred embodiments, either the shoulder or a portion thereof rotates at least while the rotating probe is received in the first workpiece. For example, the shoulder may comprise a rotating portion which is the radially innermost part of the shoulder and a non-rotating portion which is radially outward with respect to the rotating portion. Although the rotating probe can be in the form of a simple 'pin', cylinder or other design, the probe can take the form of a pin and radially inner shoulder portion, in the case where the shoulder comprises a rotating portion (which may rotate either with or independently from the pin), with the remainder or radially outer non-rotating portion of the shoulder being disposed at least partially around the circumference of the inner shoulder portion and at least partially in contact with the workpiece(s) while the rotating probe is received in the workpiece. The face of the rotating shoulder portion could be co-planar with the remainder of the shoulder, or have any number of angular configurations, including concave, convex or variable taper sections. It is useful in the case of a co-rotating probe pin and inner shoulder portion for the features on the probe pin and shoulder portion to cooperate, e.g. flutes may run along the length of the pin and continue onto the shoulder portion, sometimes extending all the way to the part of the rotating shoulder (radially inner) most proximate to the non-rotating part of the shoulder (radially outer), if present.

The workpiece materials upon which the invention is practiced are typically those which can be friction stir processed. Most commonly, this is aluminium, but other higher melting temperature materials can be processed, and the invention can be used to enhance joining of dissimilar workpiece materials, for instance by applying the process to an aluminium workpiece in an aluminium-steel butted joint, where the aluminium workpiece material is tightly forced into the steel surface prior to friction stir joining at the aluminium-steel interface.

The filler material used can be of the same type as the workpiece to be processed, or could be of a different grade or mixture of materials. If using a wire-based filler material, this could be sourced from conventional wire forming routes or by using wire obtained from the process disclosed in W02018083438, which uses a friction stir process to extract material from the bulk of workpieces to create channels with extracted material typically extruded as a wire. It is possible that the same apparatus can be used to perform both the process in W02018083438, with some modification of parameters and potential modular additions (e.g. one or more of the non-rotating shoulder surface, rotating shoulder and probe) to swap between processes. It is also possible to use wire sourced from swan f from a friction stir extrusion process.

For controlling the apparatus used in applying the invention, the use of sensors, probes and feelers to monitor gap distances can be used. In some embodiments, the shoulder will have a specially-shaped surface. Particularly, the shoulder may have an edge portion which overhangs the second surface and partially restrains the surface so as to control and shape the bulging material, for the purposes of retaining first surface shape and overall integrity of the intended geometry of bulged material. In some cases, the overhanging feature may be provided with sensors for measuring force of bulging material, gap measurements and such like.

It is possible to use mulfiprobe heads (e.g. Twinstir) for pre-softening (multiple heads in-line of processing direction) and also simultaneous insertion (staggered heads) for bridging gaps faster. In terms of apparatus, processing heads for carrying out the invention can be mounted upon conventional devices used for friction stir, including machining centres, gantry systems and various robots for both 2D and 3D processing.

In terms of determining parameters for carrying out the invention, these are similar to those developed during other friction stir processes, in that there is an interrelationship between workpiece material strength, thickness, tool design and materials, required properties and processing parameters. Atypical from usual friction stir processes, consideration needs to be given to filler parameters (e.g. wire grade and thickness, filler insertion method), required gap bridging rate, stationary shoulder position (which may be positioned slightly above the workpiece surface, or in forced contact) and processing distance from the workpiece edge / second surface, which is mostly determined by workpiece and filler properties. For example, bridging gaps between 4mm aluminium alloy AA6082-T6 plate workpieces can be achieved using the simple apparatus shown in the accompanying figures equipped with 3mm diameter filler wire of the same grade as the workpiece, 1600rpm probe rotation speed and 9kN axial downforce (force control).

Prime applications for the invention include manufacture of joints between extruded panels (e.g. long panels for rail and aerospace vehicles), high-value, difficult to process workpieces and for the creation of tabs for use during other processes, including joining. The benefits of the invention are felt strongly in these applications since they are particularly susceptible to the drawbacks associated with known techniques discussed previously.

The use of a non-consumable tool (i.e. a friction stir welding tool) in mixing the filler material with the first workpiece provides superior incorporation of filler material when compared with techniques which use consumable tools. Also, when compared to other friction stir 'filler' techniques, which leave filler as a surface or gap-filling deposit, bonding is not likely to be as intimate as when filler is mixed into the bulk of the workpiece and mechanical properties of the bulged material will likely be comparable with those of the first workpiece in its original form and far superior to additive features such as those formed by adding material directly onto the edge.

As noted previously, the method may be performed while a second workpiece is arranged adjacent to the edge of the first workpiece that is to be deformed by the application of the rotating probe. Thus, in some preferred embodiments, the method further comprises: positioning a second workpiece adjacent to the edge of the first workpiece before bringing the rotating probe into contact with the surface of the first workpiece; and maintaining the second workpiece in position while applying the rotating probe to the first workpiece such that the deformation of the edge of the first workpiece is constrained by the part of the second work piece adjacent to the edge of the first workpiece. The "second workpiece" referred to here could be any object or structure suitable for constraining the expansion of the edge of the first workpiece when placed adjacent to it. The filler material can only be stirred into the bulk of the first workpiece while the edge remains able to expand laterally, so the presence of a second workpiece which constrains the lateral expansion of the edge will prevent additional filler material being stirred into the first workpiece at any given location on the first workpiece once the part of the edge proximate to that location has expanded to the point of being in contact with the second workpiece. While it should have sufficient mechanical strength to resist the expansion of the edge of the first workpiece when in contact with it, the second workpiece is not itself necessarily made of a material that is suitable for being subject to friction stir welding techniques or for joining to the first workpiece. For example, the second workpiece could be a template or guide shaped in accordance with a desired profile to be imparted to the edge of the first workpiece.

However, as discussed above, the method of the invention finds application in closing the gap between two adjacent workpiece before joining the workpieces along the gap. Therefore, in some preferred embodiments, the method further comprises, after deforming the edge of the first workpiece, joining the first workpiece to the second workpiece along the deformed edge of the first workpiece. The joining could be performed by friction stir welding (for example using the same rotating probe or a different rotating probe to that which was used to deform the edge of the first workpiece) or a different technique such as fusion welding. In particularly preferred implementations, however, joining the first workpiece to the second workpiece comprises friction stir welding the first workpiece to the second workpiece along the deformed edge of the first workpiece.

Where the second workpiece is not intended to be joined to the first workpiece, the method may further comprise, after deforming the edge of the first workpiece, removing the second workpiece from the position adjacent to the edge of the first workpiece. The second workpiece could for example be a removable template which is re-used for shaping multiple edges (or parts of the same edge) of the first workpiece and/or for shaping other workpieces.

In some embodiments, the shoulder, or a portion thereof, rotates at least while the rotating probe is received in the first workpiece. Typically the axis of rotation of the shoulder (or rotating portion thereof) will be the same as that of the rotating probe. The rotating shoulder (or the rotating part thereof) could be integral or otherwise constrained to move with the rotating probe, in which case the rotating shoulder (or the rotating part thereof) will co-rotate with the rotating probe. In the case where only a portion of the shoulder rotates, the rotating portion could alternatively be provided by an insert mounted inside a non-rotating outer part.

Where the shoulder or a portion thereof rotates, preferably the shoulder or rotating portion thereof rotates in the same direction and/or with the same frequency as the rotating probe.

Advantageously, the shoulder of the friction stir welding tool may comprise a feature adapted to overhang the edge of the first workpiece when the rotating probe is received in the first workpiece so as to constrain the expansion of the edge of the first workpiece. For example, the shoulder could be formed with an 12-shaped profile and arranged such that one arm of the L' can overhangs the edge of the first workpiece when the probe is received in the first workpiece. This feature may be present in apparatus in accordance with the invention.

As noted previously, friction stir welding techniques are particularly suitable for application to aluminium and alloys thereof. Thus, in some preferred embodiments, one or more of the first workpiece, the filler material and, if provided, the second workpiece, comprises aluminium or an alloy thereof. The disclosed techniques are however suitable for application to other materials.

The invention also provides an apparatus comprising: a friction stir welding tool; and a control unit configured to operate the friction stir welding tool so as to perform the method of any preceding claim. The control unit could comprise for example a processor in communication with the friction stir welding tool and configured to issue instructions to move and operate the rotating probe so as to treat the first workpiece in the required manner.

The invention also provides a workpiece modified by the method described above. This workpiece corresponds to the first workpiece defined above in the context of the method. In terms of workpieces processed using the invention, cross sections typically exhibit very particular striping and reduction of grain size with multiple overlapping passes. The process can also be identified by comparing parent material with processed material by examining microstructures and crystallographic textures. Microstructures will also exhibit a narrow weld profile and heat affected zone when using a non-rotating shoulder when compared with 'conventional' friction stir tools with a co-rotating pin and shoulder.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of methods and apparatus in accordance with embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure la and lb show views of an example of a friction stir welding tool suitable for carrying out methods in accordance with the invention; Figure 2a-2h show illustrations of process steps in carrying out a method in accordance with an embodiment of the invention; Figure 3 shows a schematic of a shoulder assembly with a specific edge profile, engaging in a process in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

In Figures la (side view) and lb (bottom! plan view) there is shown an example of a simple friction stir end effector 1, which is a kind of friction stir welding tool, suitable for use in carrying out methods according to the invention, which will typically be mounted upon a head as part of a larger machine for carrying out friction stir operations, typically a gantry or robot (not shown), and engaged to rotate with a spindle drive (not shown) by spindle taper 2. A machine that incorporates the friction stir end effector 1 may also include a control unit such as a processor configured to control the tool in order to perform methods in accordance with embodiments the invention, examples of which will be discussed below. The friction stir welding tool comprises a main body 3, which may include various bearings, temperature controls, monitoring sensors and electronics, as typical in the art (not shown). In this case, upon the main body is mounted a non-rotating shoulder assembly 4 which will, in operation, press against and slide along the surface of a workpiece via a shoulder bearing surface 4i, which may or may not be slightly protruding from the shoulder assembly as a whole, where the shoulder bearing surface may be formed as an integral part of the shoulder assembly (e.g. single machined object) or as a form of insert/attachment, together forming a radially outer, non-rotating shoulder. An insert/attachment may be used, for example, when the shoulder bearing surface is formed of an exotic material/coating (e.g. for processing high temperature or high wearing workpieces), needs to be changed frequently without the need for replacing the whole shoulder assembly, or has a special or modifiable profile. A friction stir probe 5, is mounted upon a probe holder (not shown) which passes through a bore 4a in the shoulder bearing surface/shoulder assembly (radially outer shoulder) and is coupled to the spindle taper 2 to transmit rotational force, and also to allow extension and retraction of the friction stir probe relative to the radially outer, non-rotating shoulder. The profile of the bore 4a through the shoulder 4 is indicated by dashed lines in this drawing. It will be appreciated that in other embodiments such as those where the probe 5 and shoulder 4 are integral with one another, the bore will not be present. The friction stir probe 5 in this case is constructed of a nonconsumable material as a relatively plain cylindrical shape with a thread that rotates and is pushed into workpieces to generate plasticised workpiece material for processing workpieces. In some cases the probe 5 may have a co-rotating or independently rotating shoulder (i.e. radially inner shoulder) or stepped features on the probe. In the case of methods in accordance with the invention, the friction stir probe also acts to draw in, plasticise and force filler material, shown in the form of a filler wire 6, into the bulk of workpieces. Filler control wheel 7 and filler guiding channel 8 act to align the filler wire into the desired position for feeding to the tool, and the filler control wheel may be powered (actively forcing material into the tool) or unpowered and free to rotate as the apparatus is traversed along workpieces in the direction shown by arrow 9. Typically, the control wheel will have a ridged or knurled surface for gripping the wire.

Figures 2a to 2h show steps in a typical processing procedure in accordance with the invention when applied to joining workpieces in a butt arrangement with a gap therebetween. Figure 2a shows a first workpiece 10 and second workpiece 11 with a gap between their adjacent faces, which will be referred to as butted for the purposes of this example even though a gap exists. Figure 2b shows the probe of an apparatus of similar construction to that shown in figures la and 1 b. The apparatus (not shown) will be brought toward the workpiece, with first contact being between the filler control wheel, which may have a spring action, filler and workpiece. The non-rotating shoulder of the apparatus (not shown in Figures 2a-2h) is brought into position just above the first workpiece, with the edge of the shoulder contacting the first workpiece surface facing the tool and extending laterally at least to the butted edge of the first workpiece. The friction stir probe 5 is then rotated and plunged gradually, using for example a position control strategy, into the first workpiece until a desired depth is reached. Material displaced by the probe will push against the shoulder, at which point axial force control may be used to maintain a predetermined downforce on the probe and shoulder In this embodiment, the apparatus is then caused to traverse the first workpiece along a predetermined path whilst filler material feeds into the region of the friction stir probe, typically ramping up in traverse to a maximum welding speed over a set distance. Figure 2c shows the friction stir probe during its traverse along the workpiece (shown from travelling from the top downward) with a processed and bulk filled region 12, which will typically show a relatively smooth surface due to action of the polishing action of the non-rotating portion of the shoulder During the traverse, the filler material is plasticised by the action of the tool and provides extra material into the bulk of the first workpiece which, due to the constraining action of the shoulder forces the first workpiece butted edge to bulge outward (bulge shown as 13) toward the second workpiece by an amount proportional to the volume of added material, in this case shown substantially traversing the gap. Note that this does not show the start position, which would show a gap in the processed region and gap in the bulged material. Figure 2d shows the friction stir tool traversing further, with the first workpiece butted edge now closer to the second workpiece along more of the required length of the workpiece. Once the desired length is required (i.e. once the probe 5 has been traversed along the entire predetermined path), the apparatus is halted, the probe retracted and the force removed. Figure 2e shows the two workpieces with a complete first processed and bulk filled region and no or a greatly reduced gap, with the facing surfaces 14 being in sufficient contact. Although this shows an example where the gap is traversed by workpiece material in one pass, it could be the case that multiple repeated passes are required e.g. to fill particularly large gaps. Figure 2f shows a friction stir probe 15, which could be part of the same apparatus as used for closing the gap (and indeed could in some embodiments be the same probe 5 as that which was used to deform the edge of the first workpiece as shown in Figures 2a-2d) or could be part of a different apparatus, being inserted into the region of the butted first and second workpiece surfaces and progressing through figure 2g to form a weld 16, shown completed in Figure 2h, which shows the friction stir bulk filled region 12 and friction stir weld region 16. The probe need not be in centre of the joint region, as shown, so long as the probe sufficiently stirs the interface of the workpieces to form a sound joint. For dissimilar materials it may be the case that the probe is deliberately offset, e.g. where the first workpiece is aluminium and the second workpiece steel, where in practical terms the probe may be offset into the aluminium and just rubbing the steel surface. Although the friction stir filled region and friction stir weld region are shown with a degree of separation, it the many cases it is likely that these will overlap, especially where friction stir bulk fill passes are required closer to the workpiece edges (e.g. thicker workpieces).

As noted previously, in some embodiments, the second workpiece 11 is not intended to be joined to the first workpiece but rather is employed as a guide or template for constraining the expansion of the edge of the first workpiece 10 being treated. In such an embodiment, the second workpiece 11 could be removed after removing the probe 5 as shown in Figure 2e. While in the example illustrated here the edge of the second workpiece that contacts the edge of the first workpiece to be treated is shown as generally straight, the second workpiece could in principle be shaped in accordance with any profile to be imparted to the edge of the first workpiece.

Figure 3 shows a workpiece 17 being processed in accordance with the invention.

A non-rotating shoulder assembly 18 with non-rotating shoulder bearing surface 19 is shown whilst traversing the workpiece (traverse direction into the figure), with the workpiece mounted on a backing bar (not shown) opposite the shoulder assembly. The shoulder assembly 18 in this example is engaged in processing workpiece where the shoulder bearing surface has an I' shaped profile, with the short length of the 1' positioned to overhang the edge of the workpiece so as to restrain the workpiece edge material 17i which has been displaced in the direction shown by arrow 20 by the action of the filler material 21 added to the bulk of the workpiece via insertion/processing via friction stir probe 22. This arrangement can be employed to achieve a consistent level of expansion of the edge at each point and/or along the extent of each path to which the probe 22 is applied.

Claims (16)

1. CLAIMS1. A method of modifying a first workpiece, the method comprising: causing a rotating probe of a friction stir welding tool to engage a surface of the first workpiece, at a location on the surface that is laterally spaced from an edge of the surface, and to enter the first workpiece at that location, wherein the friction stir welding tool comprises a shoulder arranged to contact the surface while the rotating probe is received in the first workpiece, and wherein the rotating probe extends from the shoulder; while maintaining contact between the shoulder and the surface of the first workpiece, causing a filler material to be stirred into the bulk of the workpiece by the rotating probe such that the edge of the workpiece deforms laterally with respect to the axis of the rotating probe.
2. 2. The method of claim 1, further comprising moving the rotating probe along a predetermined path while supplying the filler material to the rotating probe so as 15 to deform an extended section of the edge.
3. 3. The method of claim 2, wherein the path extends along the surface at a substantially constant distance from the edge.
4. 4. The method of claim 2 or 3, wherein the filler material is supplied to the rotating probe by feeding the filler material to the rotating probe from upstream of the rotating probe.
5. 5. The method of any preceding claim, further comprising: positioning a second workpiece adjacent to the edge of the first workpiece before bringing the rotating probe into contact with the surface of the first workpiece; and maintaining the second workpiece in position while applying the rotating probe to the first workpiece such that the deformation of the edge of the first workpiece is constrained by the part of the second work piece adjacent to the edge of the first workpiece.
6. 6. The method of claim 5, further comprising, after deforming the edge of the first workpiece, joining the first workpiece to the second workpiece along the deformed edge of the first workpiece.
7. 7. The method of claim 6, wherein joining the first workpiece to the second workpiece comprises friction stir welding the first workpiece to the second workpiece along the deformed edge of the first workpiece.
8. 8. The method of claim 5, further comprising, after deforming the edge of the first workpiece, removing the second workpiece from the position adjacent to the edge of the first workpiece.
9. 9. The method of claim 8, wherein the second workpiece is a template.
10. 10. The method of any preceding claim, wherein the shoulder, or a portion thereof, rotates at least while the rotating probe is received in the first workpiece.
11. 11. The method of claim 10, wherein the shoulder or rotating portion thereof rotates in the same direction and/or with the same frequency as the rotating probe.
12. 12. The method of any of claims 1 to 9, wherein the shoulder does not rotate relative to the surface of the first workpiece while in contact with the surface of the first workpiece.
13. 13. The method of any preceding claim, wherein the shoulder comprises a feature adapted to overhang the edge of the first workpiece the rotating probe is received in the first workpiece so as to constrain the expansion of the edge of the first workpiece.
14. 14. The method of any preceding claim, wherein one or more of the first workpiece, the filler material and, if provided, the second workpiece, comprises aluminium or an alloy thereof.
15. 15. An apparatus comprising: a friction stir welding tool; and a control unit configured to operate the friction stir welding tool so as to perform the method of any preceding claim.
16. 16. A workpiece modified by the method of any of claims 1 to 14.