Friction stir welding tool

Abstract

A friction stir welding tool has a probe 2 of a superhard material (e.g. diamond or PCBN). The probe 2 has a height Hp extending along the longitudinal axis of rotation. The surface of the probe 2 has

Description

FIELD OF THE INVENTION This disclosure relates to a friction stir welding (FSW) tool. In particular, it relates to a FSW tool comprising a superhard material, in particular, diamond, and a method of its use in friction stir welding non-ferrous alloys, in particular aluminium. BACKGROUND Friction stir welding of aluminium is a technique which has been adopted in various sectors of the manufacturing industry, such as in the shipbuilding, automotive, aerospace and railway industries. However, one of the challenges associated with friction stir welding of aluminium is that of productivity. Two key productivity challenges relate to the attainable welding speed and stoppages required for tool changes as a result of tool wear or failure. First, the incumbent steel friction stir welding tools wear rapidly (within less than 700 m of welding), which results in deterioration of the weld quality, and second, as a result of the rapid wear, the incumbent steel friction stir welding tools are practically limited to use at lower welding speeds. Superior tools are therefore required to replace the incumbent steel tools and provide an increase in productivity. It is an aim of the invention to provide a friction stir welding tool that addresses the abovementioned problem. SUMMARY OF THE INVENTION In accordance with the invention, there is provided a friction stir welding tool comprising: a probe with a height Hp extending along a longitudinal axis of rotation, wherein the probe comprises a base portion and an apex; wherein at least the probe comprises a superhard material, for example, diamond; wherein a surface of the probe comprises a helical path with a width W, wherein the helical path extends from the base portion towards the apex; and wherein a ratio between the height H, (mm) of the probe and the width W (mm) is 15 or less. As an option, the helical path extends substantially to the apex. As an option, the helical path extends to the apex. As an option, the friction stir welding tool further comprises a shoulder, the probe extends from the shoulder and the base portion is adjacent, for example, directly adjacent, to the shoulder. As an option, the friction stir welding tool comprises a plurality of helical paths with a width W extending from the base portion towards the apex, wherein the ratio between the height Hp (mm) of the probe and the width W (mm) of at least one of the helical paths is 15 or less. As an option, the ratio between the height Hp (mm) of the probe and the width W (mm) of at least one of the helical paths is 12 or less, or 10 or less, or 5 or less. As an option, the ratio between the height Hp (mm) of the probe and the width W (mm) of at least one of the helical paths is 1 or more, or 2 or more, or 5 or more. As an option, the ratio between the height Hp (mm) of the probe and the width W (mm) of at least one of the helical paths is from approximately 1 to approximately 15, for example from approximately 2 to approximately 15, for example from approximately 5 to approximately 15. As an option, the ratio between the height Hp (mm) of the probe and the width W (mm) of at least one of the helical paths is from approximately 1 to approximately 12, for example from approximately 2 to approximately 12, for example from approximately 5 to approximately 12. As an option, the probe has a cone angle of up to 15°, and the helical path has a corresponding cone angle. As an option, the probe has a cone angle of up to 10°, for example up to 5°. As an option, the probe has a cone angle of at least 0.1 °, for example at least 0.5°, for example at least 1° As an option, the probe has a cone angle of from approximately 0° to approximately 15°, for example, from approximately 0.1 ° to approximately 15°, for example from approximately 0.1 ° to approximately 10°, for example from approximately 0.1 ° to approximately 5°. As an option, the helical path is a peripheral helical recess. As an option, the helical path is a peripheral helical recess and the ratio between the height Hp (mm) of the probe and the width W (mm) of at least one of the helical paths is 12 or less, for example from approximately 1 to approximately 12, for example from approximately 2 to approximately 12, for example from approximately 5 to approximately 12. As an option, the helical path is formed by a conical helix. As an option, a floor projection of the conical helix is a plane spiral. As an option, the plane spiral is an Archimedean spiral or a Fermat's spiral. As an option, the helix pitch is from approximately 15% of the probe height to approximately 50% of the probe height. As an option, the helix pitch is from approximately 25% of the probe height to approximately 50% of the probe height. As an option, the helix comprises at most four crests along the longitudinal axis of the tool. As an option, the helix comprises at most three crests along the longitudinal axis of the tool, for example at most two crests along the longitudinal axis of the tool. As an option, the helix comprises at least one crest along the longitudinal axis of the tool, for example at least two crests along the longitudinal axis of the tool, for example at least three crests along the longitudinal axis of the tool. As an option, the helix comprises from one to four crests along the longitudinal axis of the tool, for example from one to three crests along the longitudinal axis of the tool, for example from two to four crests along the longitudinal axis of the tool, for example from two to three crests along the longitudinal axis of the tool. As an option, the helical path is interrupted by one or more interruptions. As an option, the interruption is a flat or a flute. As an option, the shoulder and the probe are integrally formed. As an option, the shoulder comprises protrusions in a whorled or scrolled pattern and said protrusions have a maximum height G, and the ratio between the height Hp (mm) of the probe and the maximum height G (mm) of the protrusions is 10 or less. As an option, the scroll has a scroll pitch of from approximately 5% to approximately 15% of the diameter of the shoulder. As an option, the scroll has a scroll thickness of from approximately 2.5% to approximately 5% of the diameter of the shoulder. As an option, the whorled pattern is formed of concentric circles or a spiral. As an option, the diamond is polycrystalline diamond (PCD). As an option, the tool further comprises a body which is joined to the probe and / or shoulder. The present disclosure further provides a method of friction stir welding a workpiece which comprises aluminium, the method comprising: providing a friction stir welding tool comprising a probe for contacting the workpiece, wherein the probe of the friction stir welding tool comprises a superhard material, for example, diamond; rotating the friction stir welding tool at a rotation speed of from approximately 1000 rpm to approximately 4000 rpm; urging the friction stir welding tool against the workpiece; and traversing the rotating friction stir welding tool across the workpiece at a traverse speed of from approximately 500 mm / min to approximately 8000 mm / min; wherein the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 2 rev / mm or less. As an option, the rotation speed is from approximately 1000 rpm to approximately 3000 rpm. As an option, the rotation speed is from approximately 2500 rpm to approximately 4000 rpm. As an option, the rotation speed is from approximately 3000 rpm to approximately 4000 rpm. As an option, the traverse speed is from approximately 1000 mm / min to approximately 8000 mm / min. As an option, the traverse speed is from approximately 500 mm / min to approximately 6000 mm / min. As an option, the traverse speed is from approximately 1000 mm / min to approximately 6000 mm / min. As an option, the traverse speed is from approximately 2000 mm / min to approximately 8000 mm / min, for example from approximately 3000 mm / min to approximately 8000 mm / min, for example from approximately 4000 mm / min to approximately 8000 mm / min, for example from approximately 5000 mm / min to approximately 8000 mm / min, for example from approximately 6000 mm / min to approximately 8000 mm / min, for example from approximately 7000 mm / min to approximately 8000 mm / min. As an option, the traverse speed is from approximately 2000 mm / min to approximately 6000 mm / min, for example from approximately 3000 mm / min to approximately 6000 mm / min, for example from approximately 4000 mm / min to approximately 6000 mm / min, for example from approximately 5000 mm / min to approximately 6000 mm / min. As an option, the ratio of the rotation speed ofthe friction stir welding tool to the traverse speed of the friction stir welding tool is 1.9 rev / mm or less, or 1.8 rev / mm or less, or 1.7 rev / mm or less, or 1.6 rev / mm or less, or 1.5 rev / mm or less, or 1.4 rev / mm or less, or 1.3 rev / mm or less, or 1.2 rev / mm or less, or 1.1 rev / mm or less. As an option, the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 1 rev / mm or less. As an option, the traverse speed is from approximately 1000 mm / min to approximately 8000 mm / min, and the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 1 rev / mm or less. As an option, the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is from approximately 0.16 rev / mm to approximately 2 rev / mm, for example from approximately 0.25 rev / mm to approximately 2 rev / mm, for example from approximately 0.50 rev / mm to approximately 2 rev / mm, for example from approximately 0.75 rev / mm to approximately 2 rev / mm. As an option, the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is from approximately 0.16 rev / mm to approximately 1.5 rev / mm, for example from approximately 0.25 rev / mm to approximately 1.5 rev / mm, for example from approximately 0.50 rev / mm to approximately 1.5 rev / mm, for example from approximately 0.75 rev / mm to approximately 1.5 rev / mm. As an option, the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is from approximately 0.16 rev / mm to approximately 1 rev / mm, for example from approximately 0.25 rev / mm to approximately 1 rev / mm, for example from approximately 0.50 rev / mm to approximately 1 rev / mm, for example from approximately 0.75 rev / mm to approximately 1 rev / mm. As an option, the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is from approximately 0.16 rev / mm to approximately 0.75 rev / mm, for example from approximately 0.25 rev / mm to approximately 0.75 rev / mm, for example from approximately 0.50 rev / mm to approximately 0.75 rev / mm. As an option, the minimum ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is at least 0.16 rev / mm, or at least 0.25 rev / mm, or at least 0.50 rev / mm, or at least 0.75 rev / mm. As an option, the torque acting on the friction stir welding tool is from approximately 15 Nm to 100 Nm. As an option, the diamond is polycrystalline diamond (PCD). As an option, the friction stir welding tool is as disclosed herein. The present disclosure further provides a use of the friction stir welding tool as disclosed herein in a process of friction stir welding a non-ferrous metal. As an option, the non-ferrous metals are aluminium, magnesium, titanium or copper or alloys thereof. As an option, the non-ferrous metal is aluminium or alloys thereof. BRIEF DESCIPTION OF THE DRAWINGS The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an isometric projection of a FSW tool in accordance with the invention; Figure 2 is a front view of the FSWtool of Fig. 1; Figure 3 is a top view of the FSWtool of Fig. 1; Figure 4 is a cross-sectional view taken through the line A-A of Figure 3; Figures Sa and Sc are an enlarged view of zone B of Figure 4; Figure Sb is an enlarged view of zone C of Figure Sa; Figure 6 is an isometric projection of another FSWtool in accordance with the invention; Figure 7 is a front view of the FSWtool of Fig. 6; Figure 8 is a top view of the FSWtool of Fig. 6; Figure 9 is a cross-sectional view taken through the line D-D of Figure 8; Figure 10 is an isometric projection of another FSW tool in accordance with the invention; Figure 11 is a front view of the FSWtool of Fig. 10; Figure 12 is a top view of the FSWtool of Fig. 10; Figure 13 is a cross-sectional view taken through the line E-E of Figure 12; Figure 14 is an enlarged view of zone F of Figure 13; Figure 15 is a flow diagram showing a method according to the invention of friction stir welding an aluminium workpiece; and Figure 16 is a flow diagram showing a further method according to the invention of friction stir welding an aluminium workpiece. The Figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals. DETAILED DESCRIPTION The above-mentioned problem is solved herein by the provision of a friction stir welding tool comprising a superhard material, preferably diamond, more preferably polycrystalline diamond, the surface of which is shaped so as to have a high surface area. Diamond tools have far greater wear resistance than the incumbent steel tools. However, like-for-like replacement of an incumbent steel tool with a diamond tool results in poor performance, because diamond has a lower friction coefficient than steel. As a result of diamond's low friction coefficient, surface slippage can be generated between the surface of the probe and the workpiece material during the friction stir welding process. This slippage is reduced or eliminated by the provision of a friction stir welding tool containing a macroscale surface texture feature, specifically a helical path, as described herein. The result of this reduction in surface slippage is that the diamond tool can be used at higher traverse speeds than the incumbent steel tool while maintaining high rotation speeds, and without detriment to the weld quality. A first embodiment of a friction stir welding tool 1 according to the invention is depicted in Figures 1-5. As shown in Figures 1 and 2, the friction stir welding tool 1 of the present invention comprises a probe 2, a shoulder 3 and a body 4. The probe 2 of the tool 1 shown in Figures 1-5 has a height Hp extending from the shoulder 3 along a longitudinal axis of rotation L and a width or diameter Op measured perpendicular to the longitudinal axis, as shown in Fig. Sa. The probe comprises a base portion 2a adjacent to the shoulder 3 and an apex 2b. The probe 2 comprises a superhard material, in this example, diamond, specifically, polycrystalline diamond (PCD). The probe further comprises a helical path 2c in the form of a peripheral helical recess. Specifically, here the probe comprises four helical recesses, each of which extends from the base portion towards the apex. The four helical recesses here are similar in form to a multi-start screw thread made up of four threads. The helical recesses may be formed in the probe using electrical discharge machining or laser ablation, or a combination thereof. As shown in Figure Sb, the recesses of the depicted embodiment have a semicircular cross section. However, the shape is not particularly limited. For example, the recess could have the form of a V-type thread, a truncated V-type thread, or a square thread. The width W of the helical path in this embodiment is equal to the depth of the helical recesses. The depth is measured as shown in Figure Sb, and in an analogous way to the depth of a screw thread. In this case, the height of the probe Hp is 4 mm and the width W (i.e. the depth of the helical recesses) is 0.35 mm, giving a ratio of 11 .4. It should be noted that, although in this embodiment the depth of all four helical recesses is the same, embodiments in which there are multiple helical recesses of differing depths also form part of the disclosure, as long as at least one of the helical recesses satisfies the above-mentioned ratio criterion (15 or less). A ratio of 15 or less means that the width of the helical path is relatively deep with respect to the height of the probe. This depth results in a tool with a high surface area, but which also has robust surface features which are not rapidly worn away or broken off. The minimum ratio is limited by the strength of the probe after formation of the recess -- if the recess is too deep, the probe will lack structural integrity and will readily snap. A suitable minimum ratio of the height of the probe Hp and the width W may be 5, 6, 7, 8, 9 or 10, for example 5 or 10. The tool's resistance to surface slippage can be further enhanced by the generation using laser ablation or another suitable technique of a textured surface layer comprising a predefined repeating pattern on the probe and / or shoulder, in the manner detailed in WO 2021 / 123398 A 1, the entire contents of which are hereby incorporated by reference. This would provide the probe and / or shoulder with a textured surface layer, as determined by optical microscopy or scanning electron microscopy, the textured surface layer comprising a pre-defined repeating pattern. The textured surface layer would have a depth of between 0.5 pm and 50 pm. The pre-defined repeating pattern may be defined as the repetitive deviation from the nominal surface that forms the three-dimensional topography of the surface and includes roughness as nano- and microroughness, waviness as macroroughness, lay, and flaws. The probe 2 of this embodiment has a substantially truncated conical profile with a modest taper. Specifically, the probe 2 has a cone angle 8c of 10°. As shown in Fig. Sc, the cone angle is the included angle between a line a drawn along a vertex of the profile of the probe 2 and a line b parallel to the longitudinal axis L. The helical recesses taper in a corresponding fashion. This modest taper aids in the insertion of the probe into the workpiece. The helix pitch Pp is the distance between two crests, as shown in Fig. Sc. In this embodiment, each of the four helical recesses have a pitch of 1.25 mm, and thus the helical recesses have a lead of5 mm. The helix pitch is therefore 31% of the probe height Ha. Larger macroscale surface texture features are advantageous as they are easier to machine. Hence, it is preferred that the helix pitch Pp is from approximately 15% of the probe height to approximately 50% of the probe height, for example, from approximately 25% of the probe height to approximately 50% of the probe height. The consequence of this relatively large helix pitch Pp is that the helix will comprise relatively few crests along the longitudinal axis of the tool. The helical recesses on the probe can thus be compared to a coarse screw thread. For example, the friction stir welding tool shown in Figs. Sa and Sc has only two crests. Four crests are fewer is optimum. As best seen in Figure 3, the shoulder 3 of the friction stir welding tool of the first embodiment comprises protrusions 3a in a scrolled pattern. The scrolled pattern in the first embodiment is made up of four scrolls 3a. The present inventors have found that two, three or four scrolls are optimal for friction stir welding of aluminium workpieces using PCD tools as this ensures there are sufficient scrolls to retain definition and thereby push material into the weld pool, but not so many scrolls that the weld pool overheats. The scroll pitch P. is measured analogously to the pitch of a screw thread, i.e. it is the distance between two adjacent crests, as shown in Fig. 5c. As also shown in Fig. 5c, the scrolls have a scroll height H, and a scroll thickness T, The height of the scrolls is measured along the longitudinal axis L from the surface of the shoulder 3 to the tip of the scroll. The thickness of the scrolls is measured perpendicular to the longitudinal axis L from one axial extremity of the scroll to the other, again as shown in Fig. Sc. The diameter D of the shoulder 3 is measured perpendicular to the longitudinal axis. In this embodiment, the scrolls have a pitch of 5 / 3 mm, a thickness of 0.5 mm and a height of 0.5 mm, and the diameter D. of the shoulder is 15 mm. Accordingly, the ratio between the height H, of the probe 2 and the maximum height H. of the scrolls (i.e. the protrusions) is 8. When the ratio between the height H, of the probe 2 and the maximum height H, of the scrolls is 10 or less, this provides macroscale surface texture features on the shoulder which work in combination with those on the probe to provide a further enhancement to the reduction in surface slippage. The scroll pitch P, is 11% of the diameter D. of the shoulder 3, and the scroll thickness T, is 3.33% of the diameter D. of the shoulder 3. As with the helix pitch P,, it is beneficial for the scroll pitch to be relatively large with respect to the shoulder diameter so as to provide large macroscale surface texture features. For example, the scroll pitch may be from approximately 5% to approximately 15% of the diameter of the shoulder. The scrolls should be sufficiently thick to ensure they are resistant to wear. For example, the scrolls may have a scroll thickness of from approximately 2.5% to approximately 5% of the diameter of the shoulder. While in this embodiment the protrusions on the shoulder are scrolls, the protrusions may instead form a whorled pattern, such as concentric circles or a spiral. Similar considerations apply to the pitch and thickness of protrusions in a whorled pattern as to those explained above in the context of the scrolled pattern. A second embodiment of a friction stir welding tool 11 according to the invention is depicted in Figures 6-9. This differs from the first embodiment only in that a) the scrolled pattern is made up of three scrolls rather than 4 and b) the helical recesses are interrupted by three flats. Flats can help to improve material flow around the probe. Instead of flats, flutes may also be provided to interrupt the helical recesses. Flutes increase the surface area of the tool, thereby increasing the contact area between the tool and the workpiece during the welding process. 5 In combination with the helical recesses, flats or flutes can help to further improve the performance of the tool. A third embodiment of a friction stir welding tool 21 according to the invention is depicted in Figures 10-14. In this embodiment, instead of the helical path being formed by a peripheral 10 helical recess, the helical path is formed by a conical helix. As can be seen from Figure 12, the floor projection of the conical helix is a plane spiral, specifically an Archimedean spiral. This produces a helical path which has a precipice to the side closest to the periphery of the probe and a wall furthest from the periphery of the probe. In a particular embodiment, the width W of the helical path, i.e. from precipice to wall, is 0.35 mm. 15 Alternatively, the floor projection of the conical helix could be a Fermat's spiral. As the width of the helical path for a Fermat's spiral varies along its length, the relevant width W would be the maximum width of the path. 20 The probe 2 in this embodiment has a broadly conical profile, tapering outwardly from a rounded apex towards the shoulder. The probe 2 may have a cone angle 8c of from about 15 degrees to about 75 degrees, for example from about 30 degrees to about 45 degrees. In this particular embodiment, the cone angle 8c is 30 degrees. As shown in Fig. 14, the cone angle is the included angle between a line a drawn along a vertex of the conical profile of the probe 25 2 and a line b parallel to the longitudinal axis L. The height of the probe H is 5 mm and the width Wis 0.35 mm, giving a ratio of 14.3. The pitch Pp of the helical path is the distance between two corresponding path edges, as shown in Figure 14. The helical path has a pitch Pp of 1 mm. Here, there is only one helix, and the 30 lead is therefore also 1 mm. The helix pitch is therefore 20% of the probe height. As detailed above in the context of the first embodiment, the macroscale surface texture features provided by the helical path of this embodiment result in an improved friction stir welding tool. In this embodiment, the shoulder is concave. However, in place of a concave shoulder a 35 convex shoulder could instead be used. Such a shoulder could be decorated with protrusions in scrolled or whorled patterns, as detailed in the context of the first and second embodiments. Similarly, the helical path could be interrupted by flats or flutes, as detailed in the context of the second embodiment. In all of the above embodiments, the body 4 may be mounted or mountable onto or into a tool holder (not shown), and the bottom of the body may be shaped accordingly, as seen in Figures 2, 7 and 11. For example, the body 4 may be shrink or press fitted into a bore provided in the tool holder, and / or the body 4 may be bonded to the tool holder, such as by brazing. Alternatively, and as shown above in the context of the third embodiment, the body 4 may be attached to the tool holder by means of a screw thread. The screw thread can be machined into the body 4 by electrical discharge machining or laser ablation. The tool 1, 11, 21 may further comprise a retention mechanism (not shown) to mechanically lock the body 4 and the tool holder together, thereby preventing separation during FSW. The tool holder may further comprise a trunk member, which may be solid and cylindrical. The purpose of the trunk member is to facilitate connection of the FSWtool 1, 11, 21 to FSW machinery. The tool holder may comprise steel, for example stainless steel. In a particular arrangement, the tool holder comprise(s) H13 steel. Alternatively or additionally, the tool holder may comprise a high temperature high strength alloy. For example, the tool holder may comprise any one or more of the following materials: Ni-Cr alloys, such as NIMON IC® 80A, with the general composition of 18.0-21.0 wt.% Cr, 1.8-2.7 wt.% Ti, 1.0-1.8 wt.% Al, 0-0.10 wt.% C, 01.0 wt.% Si, 0-0.2 wt.% Cu, 0-3.0 wt.% Fe, 0-0.1 wt.% Mn, 0-2.0 wt.% Co, 0-0.008 wt.% B, 00.15 wt.% Zr, 0-0.015 wt.% S, and balance Ni and trace impurities; lnconel alloys (a class of nickel-chromium based super alloys); W-Ni (tungsten-nickel) alloys; TZM (molybdenum-titanium-zirconium) alloys; and high entropy alloys. In general, these alloys are characterised by good strength at elevated temperatures. The tool may comprise a shoulder. The shoulder may be integrally formed with the probe or it may be separate, as in a stationary shoulder friction stir welding assembly. In the embodiments shown in Figures 1 to 14, the shoulder is integrally formed with the probe. In an embodiment where the shoulder is separate, the height Hp of the probe is measured along the longitudinal axis of rotation from the end of the helical path which opposes the apex -- in other words, the base portion of the probe coincides with the beginning of the helical path, and the height of the probe is measured along the longitudinal axis between the apex and the base portion. As stated above, the probe may comprise diamond. For example, the probe may comprise polycrystalline diamond (PCD) material or silicon carbide-bonded diamond (SCD) material (as used herein, unless otherwise specified, the term "diamond" will include both natural and fabricated diamond). The probe 2 may comprise or consist of diamond enhanced carbide (DEC) material, such as that described in GB2459272A, the entirety of which is incorporated herein by reference. Diamond enhanced carbide refers to any composite material that comprises particulates of diamond and at least one other hard phase (typically including a carbide, such as WC), wherein these particles are held together by means of a binder phase, preferably a metallic binder phase which is typically a transition metal (for example Co). DEC material may comprise further super-hard phases in addition to diamond, such as cubic boron nitride (cBN). The probe may comprise polycrystalline cubic boron nitride (PCBN). Preferably, the probe comprises or consists of PCD material. As used herein, fabricated diamond, which is also called man-made or synthetic diamond, is diamond material that has been manufactured. As used herein, polycrystalline diamond (PCD) material comprises an aggregation of a plurality of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume per cent of the material. Interstices between the diamond grains may be at least partly filled with a filler material that may comprise catalyst material for synthetic diamond, or they may be substantially empty. As used herein, a catalyst material (which may also be referred to as a solvent / catalyst material) for synthetic diamond is capable of promoting the growth of synthetic diamond grains and or the direct inter-growth of synthetic or natural diamond grains at a temperature and pressure at which synthetic or natural diamond is thermodynamically stable. Examples of catalyst materials for diamond are Fe, Ni, Co and Mn, and certain alloys including these. Bodies comprising PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains. The catalyst material and / or solvent may have been removed by leaching with a strong aqueous acid, for example, by a method as detailed in GB2465175A, GB2499092A or WO2021136833A 1, the contents of which are incorporated herein by reference in their entirety. The body 4 may comprise cemented tungsten carbide, for example, cobalt-cemented tungsten carbide, metal, for example, steel, ceramic material, silicon carbide cemented diamond material or diamond enhanced carbide material (as detailed above in the context of the probe 2). In some examples, the probe 2 and / or shoulder 3 may be formed joined to the body 4, by which is meant that the diamond of the probe 2 is produced (for example sintered) in the same general step in which the probe 2 becomes joined to the shoulder 3 and the body 4. As a result of its macroscale surface texture features and the probe comprising a superhard material, preferably diamond, the friction stir welding tool disclosed herein is particularly suited for use in a friction stir welding method for aluminium. Two such methods are outlined in the flow diagrams shown in Figures 15 and 16. The friction stir welding tool disclosed herein may also be successfully deployed in methods of friction stir welding other non-ferrous metals. The method of friction stir welding to a workpiece comprising aluminium is as follows. S1. Provide a friction stir welding tool comprising a probe for contacting a workpiece, wherein the probe comprises a superhard material, for example, diamond, and the workpiece comprises aluminium. The probe may be any of those described above. The workpiece may comprise an aluminium alloy, for example an alloy of aluminium and one or more of copper, manganese, silicon, magnesium, zinc and lithium. In an embodiment, the workpiece comprises an aluminium-magnesium-silicon alloy, also known as a 6000 series aluminium alloy. S2. Rotating the friction stir welding tool at a rotation speed of from approximately 1000 rpm to approximately 4000 rpm. As it is the rotation speed of the tool that is primarily responsible for frictional heating of the workpiece, it is desirable to use relatively high rotation speeds. However, it is important not to use too high a rotation speed, as this could result in melting of the metal or damage to the weld. The inventors have found that a rotation speed of from approximately 1000 rpm to approximately 4000 rpm, in particular from approximately 1000 rpm to approximately 3000 rpm, is optimal in friction stir welding of aluminium using a probe comprising diamond, in particular PCD. S3. Urging the friction stir welding tool against the workpiece. The friction stir welding tool must be urged against at least one workpiece, but it is common to urge the friction stir welding tool against two workpieces simultaneously to prepare a butt or lap weld between two workpieces. S4. Traversing the rotating friction stir welding tool across the workpiece at a traverse speed of from approximately 500 mm / min to approximately 8000 mm / min, wherein the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 2 rev / mm or less. Advantageously, using a diamond-containing tool as disclosed herein allows friction stir welding of aluminium to be performed at high welding speeds. In conjunction with the enhanced wear resistance offered by the diamond of the probe, the tool can be traversed at high speeds without being detrimental to the weld quality. This combination provides a significant increase in productivity compared to the incumbent steel tools. As depicted in the flow diagram shown in Figure 16, in a preferred embodiment, the superhard material is diamond, the traverse speed is from approximately 1000 mm / min to 8000 mm / min, and the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 1 rev / mm or less. As an option, the traverse speed is from approximately 2000 mm / min to approximately 8000 mm / min, for example from approximately 3000 mm / min to approximately 8000 mm / min, for example from approximately 4000 mm / min to approximately 8000 mm / min, for example from approximately 5000 mm / min to approximately 8000 mm / min, for example from approximately 6000 mm / min to approximately 8000 mm / min, for example from approximately 7000 mm / min to approximately 8000 mm / min. As an option, the traverse speed is from approximately 2000 mm / min to approximately 6000 mm / min, for example from approximately 3000 mm / min to approximately 6000 mm / min, for example from approximately 4000 mm / min to approximately 6000 mm / min, for example from approximately 5000 mm / min to approximately 6000 mm / min. Such traverse speeds cannot be sustainably achieved using conventional steel tools. As an option, the minimum ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is at least 0.16 rev / mm, or at least 0.25 rev / mm, or at least 0.50 rev / mm, or at least 0.75 rev / mm. When using a tool according to the above-mentioned design, as less energy is wasted in surface slippage events, more energy is transferred to plasticise the workpiece. The energy input to the system is via the torque applied by the spindle to which the tool is attached. As less of the energy input from the spindle is wasted, the torque applied to the spindle can be reduced. Alternatively, and perhaps more advantageously, the same torque can be applied, and the "saved" energy instead used to enhance the traverse speed, thereby improving productivity. The torque may be from 15 Nm to 100 Nm. While the above-mentioned friction stir welding tool has been particularly designed for friction stir welding of aluminium-based materials, it can also be used in friction stir welding processes of other non-ferrous metals, such as magnesium, titanium or copper or alloys thereof. While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims. Certain embodiments of the present disclosure include: 1. A friction stir welding tool comprising: a probe with a height Hp extending along a longitudinal axis of rotation, wherein the probe comprises a base portion and an apex; wherein the probe comprises diamond; wherein a surface of the probe comprises a helical path with a width W, wherein the helical path extends from the base portion towards the apex; and wherein a ratio between the height H, of the probe and the width W is 15 or less. 2. The friction stir welding tool of embodiment 1, further comprising a shoulder, wherein the probe extends from the shoulder, and wherein the base portion is adjacent to the shoulder. 3. The friction stir welding tool of embodiment 1 or embodiment 2, wherein the friction stir welding tool comprises a plurality of helical paths with a width W extending from the base portion towards the apex, wherein the ratio between the height H, of the probe and the width W of at least one of the helical paths is 15 or less. 4. The friction stir welding tool of embodiment 3, wherein the probe has a cone angle of up to 15°, and the helical path has a corresponding cone angle. 5. The friction stir welding tool of embodiment 3 or embodiment 4, wherein the helical path is a peripheral helical recess. 6. The friction stir welding tool of embodiment 1 or embodiment 2, wherein the helical path is formed by a conical helix. 7. The friction stir welding tool of embodiment 6, wherein a floor projection of the conical helix is a plane spiral. 8. The friction stir welding tool of embodiment 7, wherein the plane spiral is an Archimedean spiral or a Fermat's spiral. 9. The friction stir welding tool of any one of the preceding embodiments, wherein the helix pitch is from approximately 15% of the probe height to approximately 50% of the probe height. 10. The friction stir welding tool of any one of the preceding embodiments, wherein the helix comprises at most four crests along the longitudinal axis of the tool. 11. The friction stir welding tool of any one of the preceding embodiments, wherein the helical path is interrupted by one or more interruptions. 12. The friction stir welding tool of embodiment 11, wherein the interruption is a flat or a flute. 13. The friction stir welding tool of any one of embodiments 2 to 12, wherein the shoulder and the probe are integrally formed. 14. The friction stir welding tool of any one of embodiments 2 to 13, wherein the shoulder comprises protrusions in a whorled or scrolled pattern and said protrusions have a maximum height G, and wherein the ratio between the height H, of the probe and the maximum height G of the protrusions is 10 or less. 15. The friction stir welding tool of embodiment 14, wherein the scroll has a scroll pitch of from approximately 5% to approximately 15% of the diameter of the shoulder. 16. The friction stir welding tool of embodiment 14 or embodiment 15, wherein the scroll has a scroll thickness of from approximately 2.5% to approximately 5% of the diameter of the shoulder. 17. The friction stir welding tool of embodiment 14, wherein the whorled pattern is formed of concentric circles or a spiral. 18. The friction stir welding tool of any one of embodiments 1 to 17, wherein the diamond is polycrystalline diamond (PCD). 19. A method of friction stir welding a workpiece which comprises aluminium, the method comprising: providing a friction stir welding tool comprising a probe for contacting the workpiece, wherein the probe of the friction stir welding tool comprises diamond; rotating the friction stir welding tool at a rotation speed of from approximately 1000 rpm to approximately 4000 rpm; urging the friction stir welding tool against the workpiece; and traversing the rotating friction stir welding tool across the workpiece at a traverse speed of from approximately 1000 mm / min to approximately 8000 mm / min; wherein the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 1 rev / mm or less. 20. The method of embodiment 19, wherein the torque acting on the friction stir welding tool is from approximately 15 Nm to 100 Nm. 21. The method of embodiment 19 or embodiment 20, wherein the diamond is polycrystalline diamond (PCD). 22. The method of any one of embodiments 19 to 21, wherein the friction stir welding tool is according to any one of claims 1 to 18. 23. Use of the friction stir welding tool of any one of embodiments 1 to 18 in a process of friction stir welding a non-ferrous metal. 24. The use of embodiment 23, wherein the non-ferrous metals are aluminium, magnesium, titanium or copper or alloys thereof. 25. The use of embodiment 24, where the non-ferrous metal is aluminium or alloys thereof.

Claims

1. A friction stir welding tool comprising:a probe with a height H, extending along a longitudinal axis of rotation, wherein the probe comprises a base portion and an apex;wherein the probe comprises a superhard material;wherein a surface of the probe comprises a helical path with a width W, wherein the helical path extends from the base portion towards the apex; andwherein a ratio between the height H, of the probe and the width W is 15 or less.

2. The friction stir welding tool of claim 1, further comprising a shoulder, wherein the probe extends from the shoulder, and wherein the base portion is adjacent to the shoulder.

3. The friction stir welding tool of claim 1 or claim 2, wherein the friction stir welding tool comprises a plurality of helical paths with a width W extending from the base portion towards the apex, wherein the ratio between the height H, of the probe and the width W of at least one of the helical paths is 15 or less.

4. The friction stir welding tool of claim 3, wherein the probe has a cone angle of up to 15°, and the helical path has a corresponding cone angle.

5. The friction stir welding tool of claim 3 or claim 4, wherein the helical path is a peripheral helical recess.

6. The friction stir welding tool of claim 1 or claim 2, wherein the helical path is formed by a conical helix.

7. The friction stir welding tool of claim 6, wherein a floor projection of the conical helix is a plane spiral.

8. The friction stir welding tool of claim 7, wherein the plane spiral is an Archimedean spiral or a Fermat's spiral.

9. The friction stir welding tool of any one of the preceding claims, wherein the helix pitch is from approximately 15% of the probe height to approximately 50% of the probe height.

10. The friction stir welding tool of any one of the preceding claims, wherein the helix comprises at most four crests along the longitudinal axis of the tool.

11. The friction stir welding tool of any one of the preceding claims, wherein the helical path is interrupted by one or more interruptions.

12. The friction stir welding tool of claim 11, wherein the interruption is a flat or a flute.

13. The friction stir welding tool of any one of claims 2 to 12, wherein the shoulder and theprobe are integrally formed.

14. The friction stir welding tool of any one of claims 2 to 13, wherein the shoulder comprises protrusions in a whorled or scrolled pattern and said protrusions have a maximum height G, and wherein the ratio between the height H, of the probe and the maximum height G of the protrusions is 10 or less.

15. The friction stir welding tool of claim 14, wherein the scroll has a scroll pitch of from approximately 5% to approximately 15% of the diameter of the shoulder.

16. The friction stir welding tool of claim 14 or claim 15, wherein the scroll has a scroll thickness of from approximately 2.5% to approximately 5% of the diameter of the shoulder.

17. The friction stir welding tool of claim 14, wherein the whorled pattern is formed ofconcentric circles or a spiral.

18. The friction stir welding tool of any one of claims 1 to 17, wherein the superhardmaterial is diamond, preferably wherein the diamond is polycrystalline diamond (PCD).

19. A method of friction stir welding a workpiece which comprises aluminium, the method comprising:providing a friction stir welding tool comprising a probe for contacting the workpiece, wherein the probe of the friction stir welding tool comprises a superhard material;rotating the friction stir welding tool at a rotation speed of from approximately 1000 rpm to approximately 4000 rpm;urging the friction stir welding tool against the workpiece; andtraversing the rotating friction stir welding tool across the workpiece at a traverse speed of from approximately 500 mm / min to approximately 8000 mm / min;wherein the ratio of the rotation speed of the friction stir welding tool to the traverse speed of the friction stir welding tool is 2 rev / mm or less.

20. The method of claim 19, wherein the torque acting on the friction stir welding tool is from approximately 15 Nm to 100 Nm.

21. The method of claim 19 or claim 20, wherein the superhard material is diamond, preferably wherein the diamond is polycrystalline diamond (PCD).

22. The method of any one of claims 19 to 21, wherein the friction stir welding tool is according to any one of claims 1 to 18.

23. Use of the friction stir welding tool of any one of claims 1 to 18 in a process of friction stir welding a non-ferrous metal.

24. The use of claim 23, wherein the non-ferrous metals are aluminium, magnesium, titanium or copper or alloys thereof.

25. The use of claim 24, where the non-ferrous metal is aluminium or alloys thereof.IntellectualPropertyOfficeApplication GB2501345.9Search report under Section 17 of the Patents Act 1977Date search completed: 30 July 2025Claims searched: 1 to 18 &23 to 25; and, in part, 22International classificationSubclass and subgroup Valid from B23K103 / 08 01 / 01 / 2006 B23K103 / 10 01 / 01 / 2006 B23K103 / 12 01 / 01 / 2006 B23K103 / 14 01 / 01 / 2006 B23K20 / 12 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:B23KDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant claims Document of relevanceIntellectual Property Office is an operating name of the Patent Office www.gov.uk / ipoX 1 to 10, 13 to 18 &23 to 25 WO 2024132845 A1 (ELEMENT 1), see esp diamond tool with 0.03 to 0.6 mm groove depth X 1 to 18 & 23 to 25 US 2014 / 0263574 A1 (MEGASTIR 1), see esp diamond tool with groove of fig 17 X 1 to 18 & 23 to 25 US 2019 / 0061046 A1 (MEGASTIR 2), see esp PCD tool with groove of figs 6 &7 X 1 to 18 & 23 to 25 WO 2022 / 200585 A1 (ELEMENT 2), see esp figs X 1 to 18 & 23 to 25 US 2022 / 0364209 A1 (ELEMENT 3), see esp figs X 1 to 18 & 23 to 25 WO 2021 / 123398 A1 (ELEMENT 4), see esp figs X 1 to 18 & 23 to 25 JP 2012086267 A (FURUKAWA), see esp groove depths of 0.5 and 1.0 mm Non-patent Category iterature Relevant claims Document of relevance Categories Letter or DescriptionsymbolX Document indicating lack of novelty or inventive step.Letter or symbol Description Y Document indicating lack of inventive step, if combined with another document of the same category. & Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application. E Earlier application published on or after the filing date of the present application.

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