Tool

Abstract

A friction stir welding tool has an interlayer 22 between a substrate 12 and a tip 14 / 20. The tip 14 is formed from a superhard material, e.g. PCBN or diamond. Interlayer 22 may comprise discrete layers 22a, 22b, 22c; having a compositional gradient from the substrate end 12 to the tip end 14. The interlayer 22 may comprise a bonded mass of particles of a second superhard material, a refractory material and a bonding phase; or a gradient across the interlayer formed by varying superhard grain size. A layer adjacent the substrate 12 may be predominantly formed of a refractory material while the layer adjacent the tip 14 may be predominantly formed of a superhard material. The flexural strength of the interlayer 22 may be more than 1000 Mpa; and greater than the flexural strength of the tip 14. The tool may join non-ferrous metals such as aluminium, magnesium, titanium and copper alloys.

Description

FIELD OF THE INVENTION This disclosure relates to a friction stir welding (FSW) tool assembly. In particular, it relates to a FSW tool assembly for friction stir welding non-ferrous metals. BACKGROUND FSW is a technique whereby a rotating tool is brought into forcible contact with two adjacent workpieces to be joined and the rotation of the tool creates frictional and viscous heating of the workpieces. Extensive deformation as mixing occurs along a plastic zone. Upon cooling of the plastic zone, the workpieces are joined along a welding joint. FSW tool assemblies typically comprise a stirring pin (often referred to as a probe) and a shoulder. The stirring pin protrudes from the shoulder and is inserted into the softened workpieces to effect mixing of the workpiece materials. As the stirring pin is inserted into the workpieces at high rotation speeds, significant bending forces act on the stirring pin. When the stirring pin is formed of a brittle material, such as a superhard material like polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN), these bending forces can lead to catastrophic failure of the tool at the base of the stirring pin. The high cost of superhard materials such as PCD and PCBN relative to other tool materials, such as steel, makes failure even more undesirable than usual. It is an aim of the invention to provide a friction stir welding tool assembly that addresses the above-mentioned problem. SUMMARY OF THE INVENTION In accordance with the invention, there is provided a friction stir welding tool assembly comprising: a substrate having a substrate end; a tip having a tip end; and an interlayer between the substrate end and the tip end; wherein the tip further comprises a stirring pin, the stirring pin being located at a distal end to the tip end; and wherein the tip is formed of a superhard material (i.e. a first superhard material). As an option, the first superhard material comprises diamond. As an option, the first superhard material is a diamond-based composite material. As an option, the first superhard material comprises or consists of polycrystalline diamond material, silicon carbide-bonded diamond material, or diamond enhanced carbide. As an option, the first superhard material comprises or consists of polycrystalline cubic boron nitride. As an option, the substrate comprises a (first) refractory material. As an option, the interlayer comprises a bonded mass of particles of a second superhard material and particles of a second refractory material. As an option, the interlayer further comprises a bonding phase. As an option, the bonding phase comprises nickel, cobalt, iron, and / or alloys containing one or more of these metals. As an option, the size of the particles of the second superhard material is the same as or more than that of the particles of the second refractory material. As an option, the interlayer comprises at least 85 wt.% of combined second refractory material and second superhard material and at most 15 wt.% bonding phase, wherein of the combined second refractory material and second superhard material, 30 vol.% to 70 vol.% is second refractory material and the remainder is second superhard material. As an option, the interlayer has a compositional gradient from the substrate end to the tip end. As an option, the interlayer comprises superhard grains, and there is a gradient across the interlayer formed by varying the grain size. As an option, the gradient is continuous. As an option, the compositional gradient is formed by at least two discrete layers of differing composition. As an option, the compositional gradient is formed by at least three, forexample three, discrete layers of differing composition. As an option, the first and second superhard materials are the same, and the first and second refractory materials are the same, and each layer is formed of a mixture comprising the first / second superhard material, the first / second refractory material, and a bonding phase. As an option, the first and second superhard materials each have a predominant component, and the first and second refractory materials each have a predominant component, and the predominant component of the first and second refractory materials is the same and the predominant component of the first and second superhard materials is the same. As an option, the second superhard material, for example diamond or cubic boron nitride, is the predominant component of the first superhard material, for example polycrystalline diamond or polycrystalline cubic boron nitride, respectively. As an option, the second refractory material, for example tungsten carbide, is the predominant component of the first refractory material, for example cemented tungsten carbide. As an option, the predominant component of the first and second refractory materials is tungsten carbide and the predominant component of the first and second superhard materials is diamond or cubic boron nitride. As an option, the layer adjacent to the substrate end is predominantly formed of the second refractory material and the layer adjacent to the tip end is predominantly formed of the second superhard material. As an option, the layer adjacent to the substrate end comprises at least 60 vol.% second refractory material and at most 40 vol.% second superhard material with respect to the combined amount of second refractory material and second superhard material in the layer, and the layer adjacent to the tip end comprises at least 60 vol.% second superhard material and at most 40 vol.% second refractory material with respect to the combined amount of second refractory material and second superhard material in the layer. As an option, the second refractory material comprises a carbide and the second superhard material comprises diamond. As an option, the carbide is tungsten carbide. As an option, the substrate comprises cemented tungsten carbide (i.e. the first refractory material), the tip comprises polycrystalline diamond (i.e. the first superhard material), and each layer is formed of a mixture comprising diamond (i.e. the second superhard material), tungsten carbide (i.e. the second refractory material), and a bonding phase. As an option, the interlayer comprises a metal. As an option, the interlayer has a flexural strength of 1000 MPa or greater, for example 1100 MPa or greater, for example 1200 MPa or greater, for example 1300 MPa or greater, for example of 1400 MPa or greater, for example of 1500 MPa or greater, for example of 1600 MPa or greater, for example of 1700 MPa or greater, for example of 1800 MPa or greater, for example of 1900 MPa or greater, for example of 2000 MPa or greater, for example of 2100 MPa or greater, for example of 2200 MPa or greater, for example of 2500 MPa or greater, for example of 2700 MPa or greater, for example of 3000 MPa or greater, for example of 3200 MPa or greater, for example of 3500 MPa or greater, for example 3700 MPa or greater. As an option, the interlayer has a flexural strength of from 1000 MPa to 4000 MPa, for example from 1300 MPa to 3700 MPa, for example from 1500 MPa to 3500 MPa, for example from 2000 MPa to 3000 MPa. The flexural strength can be measured by a 3-point bending test, for example following a method according to ASTM C1161-13. As an option, the flexural strength of the interlayer is greater than the flexural strength of the tip. As an option, the flexural strength of the interlayer is greater than the flexural strength of the first superhard material. In accordance with the invention, there is further provided a use of the friction stir welding tool assembly of as disclosed herein in a friction stir welding process. As an option, the friction stir welding process joins non-ferrous metals. As an option, the non-ferrous metals are aluminium, magnesium, titanium or copper or alloys thereof. As an option, the aluminium alloy is a silicon-aluminium alloy. 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 a perspective view of a FSWtool assembly in accordance with the invention; Figure 2 is a schematic cross-sectional view of an embodiment of the FSWtool assembly in accordance with the invention; Figure 3 is a labelled version of the schematic cross-sectional view of the embodiment of Figure 2 to show how certain parameters are measured; Figure 4 is a schematic cross-sectional view of another embodiment of the FSWtool assembly in accordance with the invention; Figure 5 is a schematic cross-sectional view of another embodiment of the FSWtool assembly in accordance with the invention; Figure 6 is a schematic cross-sectional view of another embodiment of the FSWtool assembly in accordance with the invention; and Figure 7 is a schematic cross-sectional view of another embodiment of the FSWtool assembly in accordance with the invention. The Figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals. DETAILED DESCRIPTION As shown in Fig. 1, the friction stir welding tool assembly 10 of the present invention comprises a substrate 12 and a tip 14. The substrate 12 may be mounted or mountable onto or into the body 16 of a tool holder. For example, the substrate 12 may be shrink or press fitted into a bore provided in the body 16 of the tool holder, and / orthe substrate 12 may be bonded to the body 16 of the tool holder, such as by brazing. Alternatively, the substrate 12 may be attached to the body 16 of the tool holder by means of a screw thread (not shown). This may be particularly advantageous as the temperatures reached during FSW of, for example, aluminium alloys are greater than 500°C. At these temperatures conventional braze joints can soften and shear under the torque applied to the tool when it is rotated in use. This is avoided by use of a screw thread to attach the substrate 12 to the body 16 of the tool holder. The screw thread can be machined into the substrate 12 by electrical discharge machining (EDM) or laser ablation. The tool assembly 10 may further comprise a retention mechanism (not shown) to mechanically lock the substrate 12 and the tool holder together, thereby preventing separation during FSW. The tool holder may further comprise a trunk member 18, which may be solid and cylindrical. The purpose of the trunk member 18 is to facilitate connection of the FSW tool assembly 10 to FSW machinery. The body 16 and / or the trunk member 18 (if present) of the tool holder may comprise steel, for example stainless steel. In a particular arrangement, the body 16 and / orthe trunk member 18 (if present) of the tool holder comprise(s) H13 steel. Alternatively or additionally, the body 16 and / orthe trunk member 18 (if present) of the tool holder may comprise a high temperature high strength alloy. For example, the body 16 and / orthe trunk member 18 (if present) of the tool holder may comprise any one or more of the following materials: Ni-Cr alloys, such as NIMONIC® 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, 0-1.0 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, 0-0.15 wt.% Zr, 0-0.015 wt.% S, and balance Ni and trace impurities; Inconel 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. A schematic representation of an embodiment of a friction stir welding tool assembly according to the invention is shown in Figures 2 and 3. The substrate 12 has a substrate end 12a and the tip 14 has a tip end 14a. The tip 14 further comprises a stirring pin 20, the stirring pin 20 being located at a distal end to the tip end 14a. The tool assembly further comprises an interlayer 22 between the substrate 12 and the tip 14. The shape of the stirring pin 20 is not particularly limited, so long as it is suitable for FSW. The stirring pin 20 may have a conical or cylindrical profile. In the example shown in Figures 2 and 3, the stirring pin 20 has a broadly conical profile, tapering outwardly from a rounded apex towards the tip end 14a. The stirring pin 20 may have a cone angle 0C of from about 10 degrees to about 75 degrees, for example from about 15 degrees to about 75 degrees, for example from about 30 degrees to about 45 degrees. As shown in Fig. 3, the cone angle is the included angle between a line a drawn along a vertex of the conical profile of the stirring pin 20 and a line b parallel to the longitudinal axis L. The tip 14 may comprise a shoulder portion 24 proximate the stirring pin 20. This shoulder portion 24 extends circumferentially from the base of the stirring pin 20 to form a broadly planar surface in opposition to the tip end 14a. In this embodiment, the shoulder portion 24 is disc-like and has a larger diameter than a circular base of the stirring pin 20. In this embodiment, the stirring pin 20 comprises an inscribed spiral feature running from the apex down towards and onto the shoulder portion 24. In use, rotation of the tool assembly 10 is such that the spiral drives workpiece material flow from the edge of the shoulder portion 24 to the centre and then down the length of the stirring pin 20. This forces workpiece material to circulate within the stirred zone and to fill the void formed by the stirring pin 20 as the tip 14 traverses in a known manner. In the embodiment shown in Figures 2 and 3, the interlayer 22 is below the shoulder portion 24 and stirring pin 20. The tip 14 comprises a superhard material (a first superhard material). The first superhard material may comprise or consist of diamond (as used herein, unless otherwise specified, the term “diamond” will include both natural and fabricated diamond). For example, the first superhard material may comprise or consist of a diamond-based composite material. Examples of diamond-based composite materials are polycrystalline diamond (PCD) material, silicon carbide-bonded diamond (SCD) material and diamond enhanced carbide (DEC) material, all of which are described in more detail below. The first superhard material may comprise polycrystalline cubic boron nitride (PCBN) material (as used herein, PCBN material comprises grains of cubic boron nitride (cBN)). The tip 14 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 or other super-hard phase, such as cubic boron nitride (cBN) 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). Preferably, the first superhard material 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 WO2021136833A1, the contents of which are incorporated herein by reference in their entirety. The substrate 12 may comprise a refractory material, for example a carbide, for example tungsten carbide, for example a cemented tungsten carbide, for example, cobalt-cemented tungsten carbide, metal, for example, steel, ceramic material, silicon carbide cemented diamond material or superhard material, for example any superhard material detailed above in the context of the tip 14. As noted above, one or more interlayers 22 are inserted between the substrate end 12a and the tip end 14a. The interlayer 22 may comprise a bonded mass of superhard particles (i.e. particles of a second superhard material) and refractory particles (i.e. particles of a second refractory material) wherein the size of the superhard particles is the same as or more than that of the refractory particles. In the interlayer 22 the superhard particles and the refractory particles will generally be present as discrete entities with little or no or substantially no intergrowth or direct particle-to-particle bonding. A bonding phase may also be present. This bonding phase may comprise or consist of nickel, cobalt, iron and / or alloys containing one or more of these metals. The interlayer 22 may comprise a composite material formed of noninterbonded grains of super hard material, preferably diamond grains with, for example, any one or more of oxides, nitrides, carbides, silicides, carbonitrides, and / or oxycarbides of any one or more transition metals including titanium, zirconium, vanadium, hafnium, tantalum, niobium, chromium, molybdenum, tungsten, copper, manganese, and / or rhenium or an alloy thereof. The amount of superhard particles in the interlayer 22 may generally be in the range of about 10 vol.% to about 90 vol.%. The material of the superhard particles, i.e. the second superhard material, may be diamond or cubic boron nitride. Generally, when the tip comprises PCD (i.e. as the first superhard material), the superhard material of the interlayer (i.e. the second superhard material) will be diamond and when the tip comprises PCBN (i.e. as the first superhard material), the superhard material of the interlayer (i.e. the second superhard material) will be cubic boron nitride. A mixture of superhard particles may be present in the interlayer 22. The refractory particles may be or comprise carbide, nitride, or boride. Carbide particles are preferred. In a particular example, the interlayer 22 is formed of a diamond enhanced carbide material as detailed above in the context of the tip 14. This is particularly effective when the tip comprises PCD, as diamond enhanced carbide material is tougher than PCD. The size of the superhard particles (i.e. of the interlayer) may be the same as or more than that of the refractory particles (i.e. of the interlayer). When the size of the superhard particles is more than that of the refractory particles, they will generally have a size of about 20 microns. Preferably, the size of the superhard particles is about 5 microns or more than that of the refractory particles. The thickness of the interlayer 22 may be in the range from about 100 to about 2000 microns, typically from about 200 to about 500 microns. As noted above, the interlayer 22, is between the tip end 14a and the substrate end 12a. The interlayer 22 will generally have a region in contact with and bonded to the tip end 14a and a region in contact with and bonded to a surface of the substrate end 12a. As described below, the interlayer 22 may comprise more than one layer. In an embodiment, the interlayer may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase (also known as a binder), preferably cobalt, wherein of that combined refractory material and superhard material, 30 vol.% to 70 vol.% is refractory material and 70 vol.% to 30 vol.% is superhard material, for example 40 vol.% to 60 vol.% is refractory material and 60 vol.% to 40 vol.% is superhard material, for example 45 vol.% to 55 vol.% is refractory material and 55 vol.% to 45 vol.% is superhard material. In a particular example, the refractory material of the interlayer is tungsten carbide and the superhard material of the interlayer is diamond. As noted above, the refractory material and / or the superhard material may be particulate. The interlayer comprises less refractory material, e.g. tungsten carbide, by weight than the substrate and less superhard material, e.g. diamond, by weight than the tip, thus enabling it to act as a transition layer between the substrate and the tip. For example, if the substrate is a cemented tungsten carbide which contains 90 wt.% tungsten carbide and the tip is polycrystalline diamond which comprises 90 wt.% diamond, then the interlayer would have to contain less than 90 wt.% tungsten carbide and less than 90 wt.% diamond. The interlayer 22 may have a compositional gradient from the substrate end 12a to the tip end 14a. This may be a continuous compositional gradient. Alternatively, the compositional gradient may be formed by at least two layers of differing composition. In a particular embodiment depicted schematically in Figure 4, the substrate 12 comprises a cemented tungsten carbide, the tip 14 comprises polycrystalline diamond, and the interlayer 22 comprises two layers 22a, 22b which are each formed from a mixture comprising tungsten carbide and diamond. The layer 22a adjacent to the substrate end 12a is predominantly formed of tungsten carbide and the layer 22b adjacent to the tip end 14a is predominantly formed of diamond. For example, the layer 22a adjacent to the substrate end 12a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 60 vol.% is tungsten carbide and at most 40 vol.% is diamond, and the layer 22b adjacent to the tip end 14a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 60 vol.% is diamond and at most 40 vol.% is tungsten carbide. As another example, the layer 22a adjacent to the substrate end 12a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 65 vol.% is tungsten carbide and at most 35 vol.% is diamond, and the layer 22b adjacent to the tip end 14a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 65 vol.% is diamond and at most 35 vol.% is tungsten carbide. As another example, the layer 22a adjacent to the substrate end 12a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 70 vol.% is tungsten carbide and at most 30 vol.% is diamond, and the layer 22b adjacent to the tip end 14a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 70 vol.% is diamond material and at most 30 vol.% is tungsten carbide. As another example, the layer 22a adjacent to the substrate end 12a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 75 vol.% is tungsten carbide and at most 25 vol.% is diamond, and the layer 22b adjacent to the tip end 14a may comprise at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, preferably cobalt, wherein of that combined refractory material and superhard material, at least 75 vol.% is diamond and at most 25 vol.% is tungsten carbide. As a result of this, the compositional gradient provided by the two layers 22a, 22b of the interlayer 22 allows a gradual transition from the predominant component of the first refractory material of the substrate 12 (in this case, tungsten carbide) to the predominant component of the first superhard material of the tip 14 (in this case, diamond) along the longitudinal axis of the tool assembly. Although in this particular example, the second refractory material is tungsten carbide and the second superhard material is diamond, any suitable pair of refractory material and superhard material as detailed above may instead be used in the interlayer, for example tungsten carbide and cubic boron nitride, depending on the predominant components of the materials of the tip and substrate. It is also contemplated that a mixture of refractory materials and / or a mixture of superhard materials (such as diamond and cubic boron nitride) can be used. In a specific example, layer 22b (adjacent to the tip end) comprises 10 wt.% cobalt and 90 wt.% combined refractory material and superhard material, wherein 80 vol.% of the combined refractory material and superhard material is diamond and 20 vol.% of the combined refractory material and superhard material is tungsten carbide and layer 22a (adjacent to the substrate end) comprises 10 wt.% cobalt and 90 wt.% combined refractory material and superhard material, wherein 20 vol.% of the combined refractory material and superhard material is diamond and 80 vol.% of the combined refractory material and superhard material is tungsten carbide. As noted above, the refractory material and / or the superhard material may be particulate. In a particular example, and as shown schematically in Figure 5, the compositional gradient is formed by an interlayer 22 which comprises three discrete layers 22a, 22b, 22c of differing composition. Layers 22a and 22b may have the same composition as described above in the context of the two-layer interlayer. The third layer 22c is between layers 22a and 22b, and typically comprises at least 85 wt.% of combined refractory material and superhard material and at most 15 wt.% bonding phase, wherein of that combined refractory material and superhard material, approximately 30 vol.% to approximately 70 vol.%, for example approximately 40 vol.% to approximately 60 vol.%, for example approximately 50 vol.% is tungsten carbide and the remainder is diamond. In a specific example, layer 22b (adjacent to the tip end) comprises 10 wt.% cobalt and 90 wt.% combined refractory material and superhard material, wherein 80 vol.% of the combined refractory material and superhard material is diamond and 20 vol.% of the combined refractory material and superhard material is tungsten carbide, layer 22a (adjacent to the substrate end) comprises 10 wt.% cobalt and 90 wt.% combined refractory material and superhard material, wherein 20 vol.% of the combined refractory material and superhard material is diamond and 80 vol.% of the combined refractory material and superhard material is tungsten carbide, and layer 22c (between layers 22a and 22b) comprises 10 wt.% cobalt and 90 wt.% combined refractory material and superhard material, wherein 50 vol.% of the combined refractory material and superhard material is diamond and 50 vol.% of the combined refractory material and superhard material is tungsten carbide. As a result of this, the compositional gradient provided by the three layers 22a, 22b, 22c of the interlayer 22 allows a gradual transition from the predominant component of the first refractory material of the substrate 12 (in this case, tungsten carbide) to the predominant component of the first superhard material of the tip 14 (in this case, diamond) along the longitudinal axis of the tool assembly. As noted above, the refractory material and / or the superhard material may be particulate, and mixtures of refractory materials and / or superhard materials may be used. Where there are two or more layers in the interlayer, each layer may have approximately the same thickness, or the layers may be of varying thicknesses. For example, each layer may have a thickness of from approximately 100 microns to approximately 1000 microns, for example from approximately 200 microns to approximately 500 microns, for example, approximately 250 microns. The advantage of having a compositional gradient is that it reduces peak residual stresses between the tip end 14a and the substrate end 12a by providing a transition between a tip 14 predominantly formed of superhard material, such as diamond, and a substrate predominantly formed of a refractory material, such as tungsten carbide. Instead of having a compositional gradient, the interlayer may instead have a gradient formed by varying the grain size of the superhard material across the interlayer. The grain size may be varied to provide a gradient comprising discrete layers, or the grain size gradient may be continuous. The interlayer may comprise a metal. This is particularly advantageous where the superhard material of the tip is or comprises PCBN. For example, the metal may be a high temperature high strength alloy. For example, the interlayer may comprise anyone or more of the following materials: Ni-Cr alloys, such as NIMONIC® 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, 0-1.0 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, 0-0.15 wt.% Zr, 0-0.015 wt.% S, and balance Ni and trace impurities; nickel-chromium alloys, such as Inconel® alloys (a class of nickel-chromium based super alloys), such as nickel-chronium-molybdenum alloys, such as Inconel® 718 alloy; W-Ni (tungsten-nickel) alloys; W-Re (tungsten-rhenium) alloys; TZM (molybdenum-titanium-zirconium) alloys; and high entropy alloys. In general, these alloys are characterised by good strength at elevated temperatures. For example, the interlayer may comprise a refractory metal, such as any one or combination of the following: Ti, V, Cr, Mn, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os and Ir, preferably any one or combination of the following: Nb, Mo, Ta, W and Re. The metal may comprise one or more superalloys, such as the Inconel alloys and / or the TZM alloys mentioned above. The interlayer is strongerthan the material of the tip. This makes the tool less prone to failure at the base of the stirring pin. For example, the flexural strength of the interlayer may be greater than the flexural strength of the superhard material of the tip. For example, the interlayer may have a flexural strength of 1000 MPa or greater, for example of 1300 MPa or greater. The most common point of failure on the tool is at the base of the stirring pin where it connects to the shoulder. It can therefore be advantageous to provide the interlayer, which is stronger than the tip material, such that it coincides with or overlaps the base of the stirring pin, i.e. the position on the tool which is most susceptible to failure. An embodiment in which the tip end 14a coincides with the base of the stirring pin 20 is shown in Figure 6, while an embodiment in which the interlayer 22 coincides with the base of the stirring pin 20 is shown in Figure 7. These embodiments are in other respects identical to the embodiment depicted in Figures 2 and 3. The embodiment in which the interlayer 22 coincides with the base of the stirring pin 20 is most advantageous. A method of forming a friction stir welding assembly 10 as described herein comprises: providing a substrate 12 having a substrate end 12a, an interlayer 22, and a tip 14 having a tip end 14a, wherein the tip 14 is formed of a first superhard material; and forming a stirring pin 20 at a distal end of the tip 14. The stirring pin 20 may be formed by machining the distal end of the tip 14, for example using laser ablation or electrical discharge machining (EDM). Prior to machining, the distal end of the tip 14 may be substantially planar, orgenerally domed, pointed, rounded conical, blunted conical orfrusto-conical in profile. The friction stir welding tool assembly as described herein may be used in a friction stir welding process. In particular, the friction stir welding assembly as described herein may be used for friction stir welding processes involving joining non-ferrous metals, such as aluminium, magnesium, titanium and copper or alloys thereof. The friction stir welding tool assembly as described herein is particularly suitable for use in a friction stir welding process of aluminiumsilicon alloys. The increasingly abrasive nature of these alloys as the silicon content is increased results in poor lifetime of conventional steel probes. Typical aluminium-silicon alloys include: AI-50wt.% Si-50wt.%, Al-64wt.% Si-36wt.%, Al-65wt.% Si-35wt.%, Al-75wt.% Si-25wt.%, Al-88wt.% Si-12wt.%, AI-90wt.% Si-10wt.%, and Al-98wt.% Si-2wt.%. The friction stir welding tool assembly as described herein is particularly suitable for use in a friction stir welding process of aluminium-silicon alloys which comprise 9-20 wt.% silicon, with the balance aluminium and inevitable impurities. The friction stir welding tool assembly as described herein may also be used for friction stir welding processes involving polymers, for example thermoplastic polymers. The friction stir welding tool assembly as described herein may also be used for friction stir welding processes involving composite materials, for example polymer composites, such as fibre reinforced polymer composites, or metal matrix composites. The friction stir welding process may involve joining one or more articles comprising one or more non-ferrous metals, polymers or polymer composites as described above. The friction stir welding process may involve joining at least two articles, for example two articles, comprising one or more non-ferrous metals, polymers or polymer composites as described above. 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.

Claims

1. A friction stir welding tool assembly comprising:a substrate having a substrate end;a tip having a tip end; andan interlayer between the substrate end and the tip end;wherein the tip further comprises a stirring pin, the stirring pin being located at a distal end to the tip end; andwherein the tip is formed of a first superhard material.

2. The friction stir welding tool assembly of claim 1, wherein the first superhard material comprises diamond.

3. The friction stir welding tool assembly of claim 2, wherein the first superhard material is a diamond-based composite material.

4. The friction stir welding tool assembly of claim 3, wherein the first superhard material comprises or consists of polycrystalline diamond material, silicon carbide-bonded diamond material, or diamond enhanced carbide.

5. The friction stir welding tool assembly of claim 1, wherein the first superhard material comprises or consists of polycrystalline cubic boron nitride.

6. The friction stir welding tool assembly of any one of claims 1 to 5, where the substrate comprises a first refractory material.

7. The friction stir welding tool assembly of any one of the preceding claims, wherein the interlayer comprises a bonded mass of particles of a second superhard material and particles of a second refractory material.

8. The friction stir welding tool assembly of claim 7, wherein the interlayer further comprises a bonding phase.

9. The friction stir welding tool assembly of claim 8, wherein the bonding phase comprises nickel, cobalt, iron, or alloys containing one or more of these metals.

10. The friction stir welding tool assembly of any one of claims 7 to 9, wherein the size of the particles of the second superhard material is the same as or more than that ofthe particles of the second refractory material.

11. The friction stir welding tool assembly of any one of claims 8 to 10, wherein the interlayer comprises at least 85 wt.% of combined second refractory material and second superhard material and at most 15 wt.% bonding phase, wherein ofthe combined second refractory material and second superhard material, 30 vol.% to 70 vol.% is second refractory material and the remainder is second superhard material.

12. The friction stir welding tool assembly of any one ofthe preceding claims, wherein the interlayer has a compositional gradient from the substrate end to the tip end.

13. The friction stir welding tool assembly of any one of claims 1 to 11, wherein the interlayer comprises superhard grains, and wherein there is a gradient across the interlayer formed by varying the grain size.

14. The friction stir welding tool assembly of claim 12 or claim 13, wherein the gradient is continuous.

15. The friction stir welding tool assembly of claim 12, wherein the compositional gradient is formed by at least two discrete layers of differing composition.

16. The friction stir welding tool assembly of claim 15, wherein the compositional gradient is formed by three discrete layers of differing composition.

17. The friction stir welding tool assembly of claim 15 or claim 16, wherein the layer adjacent to the substrate end is predominantly formed ofthe second refractory material and the layer adjacent to the tip end is predominantly formed ofthe second superhard material.

18. The friction stir welding tool assembly of claim 17, wherein the layer adjacent to the substrate end comprises at least 60 vol.% second refractory material and at most 40 vol.% second superhard material with respect to the combined amount of second refractory material and second superhard material in the layer, and the layer adjacent to the tip end comprises at least 60 vol.% second superhard material and at most 40 vol.% second refractory material with respect to the combined amount of second refractory material and second superhard material in the layer.

19. The friction stir welding tool assembly of any one of claims 16 to 18, wherein the second refractory material comprises a carbide and the second superhard material comprises diamond.

20. The friction stir welding tool assembly of any one of the preceding claims, wherein the interlayer comprises a metal.

21. The friction stir welding tool assembly of any one of the preceding claims, wherein the interlayer has a flexural strength of 1000 MPa or greater.

22. The friction stir welding tool assembly of any one of the preceding claims, wherein the flexural strength of the interlayer is greater than the flexural strength of the tip.

23. The friction stir welding tool assembly of any one of claims 7 to 22, wherein the first and second superhard materials each have a predominant component, and the first and second refractory materials each have a predominant component, and the predominant component of the first and second refractory materials is the same and the predominant component of the first and second superhard materials is the same.

24. Use of the friction stir welding tool assembly of any one of claims 1 to 23 in a friction stir welding process.

25. The use of claim 24, wherein the friction stir welding process joins non-ferrous metals.18

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