WO2013076472A1

WO2013076472A1 - Friction stir welding tool with two contacting shoulders

Info

Publication number: WO2013076472A1
Application number: PCT/GB2012/052870
Authority: WO
Inventors: Jonathan Peter MARTIN
Original assignee: The Welding Institute
Priority date: 2011-11-24
Filing date: 2012-11-20
Publication date: 2013-05-30
Also published as: GB201120274D0; TW201325790A

Abstract

A friction stir welding tool comprises opposed shoulders (1a, 2a) which taper towards each other (1b, 2b) and contact each other at their narrow ends.

Description

FRICTION STIR WELDING TOOL WITH TWO CONTACTING SHOULDERS

Field of Invention

The present invention relates to friction stir welding and, more particularly, relates to a tool for use during friction stir welding.

Background / Prior-art

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 piasticised condition; optionally causing relative movement between the workpieces and the probe in the direction of the joint region; removing the probe; and allowing the piasticised portions to consolidate and join the workpieces together. Examples of friction stir welding are described in EP0615480, W095/26254 and US-B-7686202.

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. Typical examples of this type of tool are described in GB2306366. Many modifications of the simple pin tool are known in the prior-art.

The primary role of the tool shoulder is to prevent material from being ejected during formation of a joint (or processed) region, consolidate the region and avoid the formation of voids and other defects. This is achieved by providing a restraining action or compressive force to the surface of the workpieces. If the tool shoulder is rotating, a substantial frictional heating action will also occur.

The tool pin primarily acts as a mechanism for moving and mixing workpiece material, but also provides frictional heating. When joining workpieces, the pin acts to eliminate the material interface, breaking up and dispersing oxides and impurities from the joint line as the tool is traversed through the material. Complex forging and extrusion of hot softened material is achieved by the features that are typically machined onto the probe body. In the case of a bobbin tool, heat and forging effects are provided from both sides of a material simultaneously due to the action of the opposed shoulders. In conventional bobbin tool types, the differentiation can clearly be made between the pin portion, which provides the majority of the workpiece material mixing/moving, and the shoulder, which provides containment and the majority of the heating.

The rules governing tool design depend quite strongly on the thickness of material being processed or joined. For bobbin tools, joining of medium to thick materials (e.g. 8mm to 15mm) can be considered relatively easy when compared to thin materials (especially below 5mm), see for instance 'Development of the bobbin tool technique on various aluminium alloy', Marie et al, 5th International FSW Symposium, Metz, France, 14-16 September 2004 and WO-A-2010061094.

The effect of different material types and structural feature types on the pin and shoulder affect workpiece material behaviour in different ways, see for instance 'Relationship between the features on an FSW tool and weld microstructure', Beamish et al, 8th International Friction Stir Welding Symposium, 18-20 May 2010, Timmendorfer Strand, Germany and 'Design and properties of FSW tools: a literature review', Duborg et al, 6th International Symposium on Friction Stir Welding which was held at St Sauveur, Canada on 10-13 October 2006. Another common type of tooling known from the prior art is known as the "bobbin tool", as described in EP-B-0615480. This type of tooling overcomes the need for a backing member, often required to react the force created by the action of the tool on the workpieces. Several variants of the bobbin tool have been introduced, including fixed geometry tools, tools with adjustable pin/shoulders that adjust to variations in workpiece thickness (RPT bobbin tools), tools where the shoulders rotate in different directions or at different rates and stacked tools consisting of many shoulders for joining complex extrusions. Examples of these can be found in JP4043005, US6199745, US6908690 and others.

Various physical features have been applied to both the simple pin tool and the bobbin tool. Examples are known including those with shaped & textured shoulders/pins, threaded and fluted pins, those consisting of interchangeable pins and shoulders, and of differing combinations of materials depending upon the application. Examples of these can be found in W095/26254, WO02/092273, US6277430, W099/52669, EP1361014, US6676004 and many others.

The use of a bobbin tool has the advantage of giving a processed zone in the workpiece which is more or less rectangular in cross section, as opposed to the triangular zone which is more typically found when conventional friction stir welding tool designs are used. In addition, when using a fixed bobbin tool, the net axial force on the workpiece is almost zero, which has significant beneficial implications in machine design, fixturing and cost. During operation, a bobbin tool typically penetrates the joint line either directly, or from a milled recess, in the material's edge. A specially formed hole can also be used, where, in the case of multi-piece tools, the lower shoulder is attached to the upper shoulder through the hole. The process lends itself to joining closed sections where conventional use of a well-supported backing bar is difficult or impossible. Since, by definition, the bobbin creates a full penetration weld, the potential for creating kissing bonds, usually associated with lack of tool penetration, is eliminated. As the tool is traversed the material passes between the shoulders of the bobbin. So the process involves zero vertical force, unlike single shoulder adaptations of the friction stir welding process. No backing bar is required because the bottom shoulder supports the underside of the weld. Laboratory trials have shown that the bobbin variant of the friction stir weldingprocess creates very little distortion. This is because the rotating tool creates a symmetrical heat input throughout the weld section. Although the use of bobbin tools with automatically adjustable shoulder/pin combinations helps compensate for workpiece variation, the cost of such apparatus is prohibitive and operation can be complex and troublesome. Fixed geometry tools that move in response to variations in the workpiece position via a splined tool holder are a simple alternative, known in the art as 'floating bobbin tools'. Since the bobbin tool is free to move up and down when using the floating variant, it naturally takes up a vertical position where the top and bottom shoulder forces are equal. This feature also compensates for any misalignment between the tool and the material. However, only limited workpiece variations can be reliably dealt with using existing fixed geometry tool designs. An especial weakness of known fixed bobbin tool technology is the inability to deal with very thin materials, particularly materials of less than 5mm thickness.

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. The most commonly friction stir weldable of these materials are metals based upon aluminium, magnesium, copper, lead and other similar materials.

The first applications of friction stir welding were carried out on aluminium using tools composed of readily available tool steels. For joining harder, stronger and higher temperature materials, a harder, higher temperature tool material is generally required.

High (melting point) temperature materials (also known as high strength materials or high softening temperature materials) and having melting points above that of aluminium can be used in a variety of applications and can be joined using a variety of techniques. Example materials include steel, nickel and titanium. One of the most common ways of joining high temperature metals is by fusion methods, although fusion methods have many drawbacks.

Much work has been carried out on joining of high temperature materials by friction stir welding, with varied success. One of the largest problems when friction stir welding high temperature materials is selecting the correct tool material, which has conventionally been by use of refractory metals or ceramic materials.

W099/52669 considers the use of pure tungsten, tungsten rhenium alloy and tungsten carbide for ferrous materials; and cobalt materials, ceramic or cermet materials for other high temperature applications.

WO01/85385 relates to the friction stir welding of MMCs, ferrous alloys, non-ferrous alloys, and superalloys using a tool wherein the pin and the shoulder at least include a coating comprised of a superabrasive material. This is typically polycrystalline cubic boron nitride (PCBN). GB2402905 describes a tool fabricated from a tungsten-based refractory material, useful for welding of high strength materials like nickel and titanium alloys.

Various other tool materials for joining high temperature materials are also described in EP1982001 , EP2067564, EP2076352 among others.

Major issues exist when deploying the abovementioned materials in friction stir welding tools. Namely, the various ceramics, PCBN and other such very hard materials display a lack of fracture toughness and have a tendency to crack during the extreme conditions presented during friction stir welding of high temperature materials and metallic tools, while being tougher, tend to lack wear resistance. PCBN and the more exotic ceramic tools are very expensive to manufacture, can only be made in a relatively small size and there is limited scope for including features on the tool surfaces, both from a machining point of view and due to the introduction of crack initiators. Hybrid tools, comprising of metallic/PCBN/ceramic combinations do show promise but are not entirely satisfactory from a cost or performance perspective.

W02008/109649 describes the use of a tensioning/decoupling member for inducing compressive stresses in the pins of friction stir welding tools to improve resistance to fracture caused by bending stresses, and other tool features to reduce stress concentrators. However, inclusion of this feature does not help overcome the fractures caused in the pin by repeated plunge cycles, especially prevalent when using ceramic tools, and does not at all protect the tool shoulder.

Summary

In accordance with a first aspect of the present invention, a friction tool welding tool comprises opposed shoulders which taper towards each other and contact each other at their narrow ends.

This new friction stir welding tool is effectively 'pinless' or "probeless" in shape and with respect to workpiece contact, since no pin is involved in processing of workpiece material. This structure leads to improved mixing of the joint line due to increased material flow path length. When joining a pair of workpieces, this leads to a greatly reduced joint line remnant and is especially beneficial when applied to thin section workpieces. In tools and methods according to the invention, the actions of heating, mixing and containing are all achieved by the shoulder member.

By thin section we mean having a thickness less than 5mm.

Tools can be single-piece (in which case the component shoulders will not be maintained in compression) or multi-piece or multi-section. In one embodiment, when a multi-piece tool is used, the invention comprises a bobbin-type friction stir welding tool where the component shoulders are in contact (or contiguous), maintained in compression and forming the narrowest profile of the tool at their meeting point. For multi-piece tools, this means that no surface of a tool used in the inventive method that is in contact with workpiece material is held in tension.

In the case of a multi-section tool, the opposing shoulders are typically held together using a fastening such as a securing pin or bolt, passing through a hole in the centre of both shoulders and providing compression to the shoulder materials during the friction stir operation. The securing bolt does not come into contact with the workpiece materials and therefore does not have to be particularly hard and wear resistant. Primarily, the bolt must be tough and of appreciable tensile strength and stiffness to maintain the shoulders in compression. The material must display low strain and creep behaviour, so as not to stretch in operation. Where the securing pin passes through the top most shoulder and into a tool holder or collet, a set compressive force may be automatically maintained by the use of springs, washers, or controlled by a draw bar system either manually or automatically. Suitable materials for the securing pin included H13 tool steel, MP159 or any of a wide variety of known materials that display high strengths, especially at elevated temperatures.

Within the scope of the invention, a taper is defined as being one of a straight taper, variable taper, curved taper (convex or domed), and combinations of the aforementioned taper types.

Typically, the angular transition between surfaces for tools of the invention is always less than 90 degrees, and typically less than 45 degrees. For straight tapers, this can be measured as the included angle between the tapered surfaces. Where the taper is curved, this can be measured as the included angle between imaginary lines drawn from the inner to outer edges of the working diameter of the shoulder, the working diameter being the portion of the shoulder in contact with or embedded in workpiece material.

Typically, each shoulder is similarly tapered but in some cases different tapers could be used.

The present invention overcomes the abovementioned issues in the prior-art, namely dealing with variations in workpiece position and thickness, tool material weakness and overall equipment complexity and therefore equipment cost.

According to another aspect of the invention, the tool shoulders are composed of materials exhibiting low wear rates during friction stir processes, which help maintain a consistent gap between the shoulder surfaces ('pinch gap'). It is important to maintain a pinch gap within relatively narrow tolerances to ensure process stability. Such low wear rate materials include various ceramics (nitrides, carbides and oxides, such as alumina, zirconia, silicon nitride and sialon), hard wearing metals (for example, refractory metals such as molybdenum, tungsten &c) and superabrasives such as PCBN and composites of the aforementioned materials. The coupling behaviour of the shoulder material with the workpiece should be accounted for in selecting the shoulders, and in some cases shoulders of the same tool could be made from different materials. A single shoulder can also be made of different materials with, for example, a harder material closer to the meeting point with the opposing shoulder (the narrowest point of the taper) or a graded shoulder, changing in composition from the narrowest point to the widest point of the taper, although a single materials shoulder is often most advantageous from a tool fabrication perspective.

A variety of surface features or patterns can be provided on the tool shoulders, including scrolls (spirals, rings), flats, ridges and other commonly found features generally disclosed in the cited prior-art. Such patterns need not be present along the entirety of a shoulder surface, for example a spiral scroll could be present from the narrowest point of a taper to the three-quarter thickness of the taper, with a differing pattern or smooth surface present for the remainder of the shoulder to the outer edge where the shoulder is widest to improve surface finish of a joint. Equally, features could be presented in different directions on each shoulder or different across one shoulder, such as scrolls with different 'hand' spirals for causing plasticised material to flow in different directions. Tools according to the invention are especially suited for use as floating bobbin tools.

In accordance with a second aspect of the present invention, a method of friction stir welding uses a rotating friction stir welding tool according to the first aspect of the invention, the friction stir welding tool being rotated in the joint region between a pair of workpieces. In the case of a multi-section tool, the shoulders are maintained in compression.

The inventive tools are extremely tolerant to component thickness variations. Tools used in the invention provide a variable effective shoulder diameter which changes depending upon the material thickness.

In the inventive method, improved mixing of material is achieved due to the path length that the majority of the material is forced to traverse when the tool has passed, when compared to prior-art tools. Typically, for an equivalent workpiece material thickness, prior-art tools have a relatively small diameter pin for mixing material, while tools according to the invention have on average a far larger diameter mixing surface provided by the tapered shoulder. This larger average diameter means that during traversal of the tool through a workpiece, material is on average transferred over a larger circumferential distance before the tool has passed and is mixed more strongly than would be the case for a smaller diameter pin as taught by the prior art. This leads to more effective plasticisation of material and dissipation/break up of inhomogenities, contaminants and oxides &c.

When joining workpieces, the inventive method exhibits more tolerance to changes in joint line than prior-art methods. In the prior-art, the position of the pin in relation to the joint line greatly influences the joint quality. The enhanced plasticisation and mixing provided by the invention means the central axis of the tool can deviate further from the joint line without terminally undermining the quality of the formed joint.

Tools designed for use in certain aspects of the inventive method can utilise the materials in their construction far more effectively than possible with previous tools. For example, relatively simple shapes can be used to construct the shoulder portions. This is especially advantageous when considering the use of ceramic/PCBN type tool materials, where the fabrication methods, cost or material properties limit the size and shape of constructions. PCBN, for example, is made using a ultra-high temperature and pressure fabrication process and is available in relatively limited sizes and shapes.

In aspects of the inventive method utilising multi-piece compression-induced tools, when compared to existing friction stir welding processes, display reduced tool breakage. Using prior-art tools and methods, the wear properties of the pin material were often a deciding factor in tool performance. Since the pin/bolt has no contact with the workpieces, a tough, high-tensile strength material can be used for the bolt without having to consider the high temperature wear properties of the bolt material.

The compression-induced aspect of the invention is especially beneficial in prolonging the service life of tools made using crack-sensitive materials such as ceramics and PCBN. Maintaining these hard materials in compression is highly advantageous, as these materials are very sensitive to crack initiation and failure in tension. Apart from the compressive forces induced in these methods the inherently pinless design means that potential crack-initiator sites, such as pin-to-shoulder interfaces subject to repeated bending (tensile stresses) are not present at all.

The method can be implemented using a variety of existing apparatus including standard milling machines and machine centres, bespoke friction stir welding machines, articulated robots and in manually-controlled or hand-held machines for thin-section materials. The invention is of particular utility in lower force machine applications, since very little reactive force is necessary to operate bobbin tools when compared to typical friction stir welding tools, and also when applied to three-dimensional work paths due to the extra clearance and flexibility for dealing with workpiece variation inherent in the inventive tool designs. Although the rotating tool typically traverses in relation to the workpiece, equally the rotating tool could be fixed while the workpiece was made to traverse. Some examples of friction stir welding tools and methods according to the invention will now be described and contrasted with known examples with reference to the accompanying drawings in which:-

Figure 1 A -1 F show several examples of prior-art bobbin tools; Figure 2 shows a side view of an example of an assembled tool according to the invention;

Figure 3 shows the separate components of the tool shown in Figure 2;

Figure 4 is a schematic section showing the key components in another example of a tool according to the invention;

Figure 5 illustrates a floating tool embodiment of the invention;

Figure 6A-6D show a variety of taper profiles that can be applied to tools of the invention;

Figure 7 shows a simplified schematic of a welding assembly using a tool according to an example of the invention; and.

Figure 8 shows a macrosection of a joint formed by an example of a method in accordance with the invention.

Description of Drawings

Figures 1A-F illustrate various prior-art bobbin tools, all of which incorporate shoulders interconnected by a pin, where the pin plays a key role in mixing of workpiece material Figures 1A and 1 B illustrate examples of bobbin tools disclosed in EP-A-0615480. Figure 1A illustrates a fixed bobbin tool formed of a single piece and having opposed shoulder members 200, 202 separated by a pin or probe 204. The shoulder members, 200, 202 are formed with domed shoulders 206,208 respectively.

Figure 1 B illustrates a multi-piece bobbin tool with an upper shoulder member 210 formed with a downwardly tapering tool body 212 terminating in a cylindrical shoulder 214. This component or piece is secured to an opposed shoulder member 216 by a cotter pin 218 forming a probe section.

Figure 1 C shows another example of a bobbin tool described in WO-A-2010/061094. This tool comprises a mixing pin 230 which is mounted on a welding head in use, a lower shoulder member 232 defining a shoulder 234 and an upper shoulder member 236 defines a shoulder 238 facing the shoulder 234. The mixing pin 230 has a working portion 240 formed of two frusto-conical portions 242,244 arranged such that the smaller diameter cross-section (d) of one portion 242 is directed towards the smaller diameter cross-section (d) of the other portion 244, and vice versa. The length of the working portion 240 is denoted H and it can be seen that each portion 242,244 has a length H/2. This tool is particularly effective for the welding of thick plate materials (with thickness typically in excess of 10mm) where the high risk of breakage of the pin normally necessitates a large pin diameter - this publication states that a large pin diameter for a wholly cylindrical pin gives rise to higher rates of deformation and swept volume of material at equivalent rates of traverse and rotation when compared to the use of a pin with two frusto-conical portions (in the example presented in WO20100061094, the purely cylindrical pin was 15mm, while the frusto-conical shapes were 15mm at their widest and 12mm at their narrowest). The lower overall swept volume achieved by the frusto-conical pin is stated as offering the advantage of reduced traverse forces and improved weld quality when used for thick materials. However, if scaled down for use in thinner materials, the reduction in swept volume provided by this tool will lead to unacceptable defects when welding. In addition, the tool design incorporates stress raising features upon the pin which will inevitably be placed in tension by the action of the workpieces upon the relative wide shoulders 234,238 between which the pin 230 is fixed. The magnitude of this extra stress can usually be managed for thick workpieces, where the pin is relatively thick. However, if this design was applied to thin materials, the pin would not support the stresses imposed upon it by the contact of the much wider shoulders with the workpiece.

Figure 1 D shows an example of a bobbin tool described in JP-A-2005007466 having a pair of opposed shoulder members 101 , 102 separated by a grooved pin 103. The facing surfaces 108 of the shoulders are provided with a scroll groove 1 10 which generates material flow patterns with the intention of reducing the degree of warping of joined workpieces. Figure 1 E illustrates a bobbin tool described in US-B-6669075. This tool includes an upper support body 132 rotatable about an axis 134, and a non-consumable pin 136 attached to the support body 132 and extending from an end 138 of the support body 132. The end 138 of the support body 132 defines a shoulder 140, with the pin 136 extending from the end 138 of the support body 132 downward and away from the shoulder 14. Typically, the support body 132 is circular in cross-section and the pin 136 is centred therein, such that the pin 136 also rotates about the axis 134. As shown in Fig. 1 E, the shoulder 140 is tapered, with the taper extending from an outer edge 142 of the support body 132 downward toward the pin 136 at an angle Θ referenced from a plane 144 perpendicular to the axis 134. Additionally, the tapered shoulder 140 includes a plurality of grooves 146 machined into a face of the shoulder 140. The grooves 146 are commonly known in the friction stir welding art as a scroll shoulder. The grooves 146 may be machined into the face of the shoulder 140 as a spiral formed groove or as a plurality of concentric grooves and, additionally, may be machined normal to the face of the shoulder 140. The tool further includes a bottom support member 154 connected to a distal end of the pin 136. The bottom support member 154 is spaced from the support body 132 and includes a bottom shoulder 156 facing the shoulder 140 of the support body 132, such that the pin 136 is disposed between the support body 132 and the bottom support member 154. The bottom shoulder 156 is tapered, with the taper extending from an outer edge 158 of the bottom support member 154 upward in the direction of arrow 159 toward the pin 136 at an angle Θ from the plane 144 perpendicular to the axis 134. Additionally, the bottom shoulder 156 includes grooves 160 machined in a face thereof similar to the grooves machined in the shoulder 140 of the upper body 132. A pair of workpieces 150, 152 is shown defining a joint line along which the tool is traversed. While this type of tool is useful for dealing with material variations it is has a limited ability in joining thin materials and it is not possible to maintain the working surfaces of the tool in compression.

Finally, Figure 1 F illustrates a multi-shouldered friction stir welding tool as described in WO-A-2009/1 14861. This comprises an integral shank-pin unit (411 ) having a plurality of pin portions on the shank-pin unit, the plurality of pin portions for driving into a plurality of joints to perform a friction stir welding operation on the corresponding plurality of joints and, where a shank portion of the shank-pin unit is for attachment to an optional axial tension rod (419). Each of the friction stir welding modules comprises a pair of shoulders (413,414) that is connected to the shank-pin unit where each shoulder has a distal end and a proximal end, where the proximal end of each shoulder faces the pin portion of the shank-pin unit, whereby the shoulder and pin(s) rotate in unison, and a pair of split collars or a pair of nuts that is connected to the shank-pin unit and faces the distal end of each shoulder, where the plurality of friction stir welding modules are connected to each other whereby the modules rotate in unison to simultaneously make a plurality of parallel welds. In all these cases, in terms of tool profile, there is always a gradual angular transition between shoulders in the prior-art, via the pin, typically equal to or greater than 90 degrees. The inclusion of a pin feature and such gradual changes in angular transition between shoulders proves disadvantageous in several ways. Any attempts to keep the shoulder material in compression is in practice nearly impossible when a pin feature or gradual angular transition between shoulders is present, and neither is it possible to make a complete bobbin tool from materials such as ceramics or PCBN with prior-art designs; any tool features, such as the traditional bobbin tool pin, that lie perpendicular to the workpiece surfaces are inevitably placed in tension. Compression could only be induced in two facing shoulders constructed of such material by independently actuating two separate tools using complex and unwieldy equipment.

Figure 2 shows a side view of a first example of an assembled tool according to the invention. The tool comprises of two separate sections joining together by a centrally located threaded bolt (not shown). The first section comprises the main body of the first shoulder 1a, the first shoulder face 1 b and threaded bolt running through a central bore (not shown). The threaded bolt connects to a nut located in a mating bore running through the central axis of the second section comprising the main body of the second shoulder 2a, the second shoulder face 2b and an externally threaded portion 3 for connection to a tool holder of a welding machine (not shown). The shoulder faces and bodies shown are each integrally formed from the same material piece but could be formed from separate materials. Treatments that could be performed on the tool shoulder faces and/or bodies to enhance their performance include carburising, nitriding and carbo-nitriding. Alternatively, functional coatings could be deposited on their surfaces, such as Titanium Nitride or diamond. Many treatments for modifying tool performance are known in the art. These coatings and treatments could exhibit properties of low friction, wear resistance, temperature resistance, diffusion resistance, low reactivity and solid state lubrication, among others. Figure 3 shows the dissembled tool from figure 2 and shows clearly the fixing bolt 4 and mating bore 5, in which is located the threaded nut.

Figure 4 shows a schematic of an example of another tool according to the invention similar to that of Figure 2 and also showing a workpiece 100. The two shoulders 1a,1 b have a similar construction with the shoulder faces 1 b,2b being mirror images of each other. The workpiece thickness t and shoulder taper angle Θ give rise to an effective shoulder contact diameter of d. The minimum shoulder taper width is w1 , while the maximum is w2. Calculation of dimensions for tool components and operational parameters are dependent upon the materials used to construct the tool and also the workpiece material (type, thickness, geometry) being processed. For instance, the minimum taper width of the shoulders is influenced by the choice of bolt material; a stronger bolt can be of a narrow diameter, and therefore for a given maximum shoulder taper width the minimum taper width can be smaller and the taper angle can be greater than with a weaker, larger bolt. A wide variety of dimensions for the tool are envisaged, with a typical example given below.

Figure 5 illustrates a floating tool embodiment of the invention. Main tool holder 6 connects to a welding machine (not shown) and comprises of a bore 6¹ for holding a floating tool holder 7 and keyway 9 for accommodating a spline/key component 8 of the floating tool holder. The bore 6¹ of the main tool holder or outer surface 7' of the floating tool holder can be coated or surface treated to improve wear or frictional properties. A simple coating could be a molybdenum-based high temperature grease, which lubricates and reduces corrosion. The spline/key is free to move the entire length of the keyway between points 9a and 9b, allowing the floating tool holder to move along the same length. The floating tool holder holds the tool 10 according to an embodiment of the invention via an externally screw threaded spigot 10A received in a screw threaded portion 7A of the floating tool holder 7. The floating tool holder 7 will move automatically in response to forces applied to the tool 10. Since the tool 10 is force-balanced in normal operation, this means that the tool is able to adjust to variations in workpiece height, as the tool will always seek to operate at a force-balanced position.

Figure 6 shows a number of tapers that can be applied to tool shoulders used in the invention - a straight taper 11 (Fig. 6A), variable straight taper 12 (Fig. 6B), domed/curved taper 14 (Fig. 6D) and a combined domed/straight taper 13 (Fig. 6C). Figure 7 illustrates an embodiment of the invention where the tool shoulders are composed of a relatively brittle tool material such as PCBN or ceramic. First outer collar 16 is typically connected to a main tool holder (not shown) to impart rotational torque. Bolt 15 passes through a bore in the first outer collar and connects to threaded nut 16a. The bolt is fixed to or integral with lower outer collar 21 to transmit rotational torque thereto. First and second inner collars 17 & 20 hold first and second shoulders 18 & 19 which act to friction stir workpiece material 22. The first and second inner collars 17,20 can be made of the same material as the first and second outer collars or, alternatively, could be made of a material with a higher coefficient of thermal expansion, whereby upon heating (either from heat generated by friction action of the first and second shoulders, or externally supplied), additional, mostly radial, compressive forces are imparted on the shoulder materials to prevent relative rotation between the shoulders and inner collars. In one embodiment, either the collars or tool holder include a thermal barrier material to control heat flow, and prevent damage to the driving machine (not shown) and various tool holder cooling means are also envisaged. Embodiment of Invention / Worked Example

A tool manufactured from hardened H13 tool steel with a MP159 bolt holding the two shoulders together (shown in Figure 2 and 3) was used to join several butted 3mm thick plates of aluminium alloy designation AA6082-T6. All trials were performed with the tool being driven directly onto the workpiece without the aid of a starting notch. Results of the trials are shown in the table below:

Samples for a macrosection, tensile and root and face bend tests were taken from weld 4. A macrograph of this weld is shown in Figure 8. The weld was fully consolidated, with a deformed jointline remnant. The remnant had no effect on the successful 180° root and face bend tests and a tensile test result of 229MPa was achieved (79% joint efficiency), with failure being in the heat affected zone on the advancing side of the weld.

In the example above, the bolt was 4mm in diameter, the minimum taper width was 5mm and the taper angle of the convex scroll shoulder was 23 degrees, giving a target effective shoulder contact diameter of 12mm. The scroll was a helix feature with 1.9mm radial pitch by 0.8mm axial pitch machined using a 1 mm radius ball-nose cutter.

Claims

1. A friction stir welding tool comprising opposed shoulders which taper towards each other and contact each other at their narrow ends.

2. A tool according to claim 1 , wherein the tool is made of a single piece.

3. A tool according to claim 1 , wherein the tool is made of multiple sections secured together.

4. A tool according to claim 3, wherein the multiple sections are secured together by a fastening, such as a bolt, extending through the sections.

5. A tool according to any of the preceding claims, wherein each taper is one of a straight taper, variable taper, curved taper (convex or domed) or a combination.

6. A tool according to any of the preceding claims, wherein each shoulder is similarly tapered.

7. A tool according to any of the preceding claims, wherein the angle between the tapers is less than 90^°, preferably less than 45^°.

8. A tool according to any of the preceding claims, the tool being made of a low wear rate material such as a ceramic, hardwearing material, or super abrasive.

9. A tool according to any of the preceding claims, wherein the shoulders are made of different materials.

10. A tool according to any of the preceding claims, wherein one shoulder is made of different materials.

1 1 . A tool according to claim 10, wherein the one shoulder is made of at least two materials, a first material adjacent the opposite shoulder and a second material, the first material being harder than the second material.

12. A tool according to any of the preceding claims, wherein one or both of the shoulders is provided with a surface feature such as a scroll, flat, ridge and the like.

13. A method of friction stir welding using a rotating friction stir welding (FSW) tool according to any of the preceding claims, in which the FSW tool is rotated in a joint region between a pair of workpieces.

14. A method according to claim 13, when dependent on at least claim 3, wherein the shoulders are maintained in compression.

15. A method according to claim 13 or claim 14, wherein the thickness of the workpieces is less than the distance between the radially outer diameters of the shoulders.

16. A method according to any of claims 13 to 15, wherein the thickness of the workpieces is no greater than 5mm.

17. A method according to any of claims 13 to 16, wherein the rotating FSW tool is traversed along the joint region between the workpieces.

18. A pair of workpieces that have been friction stir welded using a method according to any of claims 13 to 17.