Friction Stir Welding Tool Assembly

Abstract

The present disclosure relates to a friction stir welding tool assembly (10) comprising a base (12) having a base end (12a), the base end (12a) comprising a convex portion, and a tip end (14) having a tip end (14a), the tip end (14a) comprising a concave portion, the tip end further comprising a stir pin (20), the stir pin (20) being located distally relative to the tip end (14a). The tip end (14) is formed of a superhard material, and the base end (12a) is joined to the tip end (14a) at corresponding convex portions and concave portions.

Description

[Technical Field]

[0001] The present disclosure relates to a friction stir welding (FSW) tool assembly, and more particularly to an FSW tool assembly for friction stir welding non-ferrous alloys, such as aluminum-silicon alloys. [Background technology]

[0002] FSW is a technique in which a rotating tool is forced into contact with two adjacent workpieces to be joined, generating frictional and viscous heat in the workpieces as the tool rotates. Extensive deformation by mixing occurs along the plastic zone. When the plastic zone cools, the workpieces are joined along the weld joint. Because the workpieces remain in a solid phase, the process is technically a forging process rather than a welding process, but by convention it is referred to as welding or friction stir welding, and that convention will be followed in this specification. For FSW of low-temperature metals such as aluminum, magnesium, titanium, and copper and their alloys, polymers such as thermoplastic polymers, and polymer composites such as fiber-reinforced polymer composites, the entire tool / tool ​​holder is typically made of a single, molded tool steel, often referred to as a "probe." Aluminum-silicon alloys are a particularly important class of aluminum alloys. FSW of low-silicon aluminum alloys is commercially practiced using high-strength steels. However, as the silicon content of aluminum-silicon alloys increases, the abrasiveness of the alloy increases, resulting in insufficient tool life for steel tools. Therefore, there is a need for FSW tools that can withstand more abrasive alloys and have sufficient tool life. For this reason, the use of superhard materials such as PCBN and PCD for FSW tools has been proposed. However, using PCD in FSW tools presents certain challenges: for example, the thickness of the tip must be slightly greater than the weld depth, which for aluminum alloys is typically around 2.5 mm, but creating such a layer is difficult because PCD layers thicker than 2.5 mm are prone to cracking. It is therefore an object of the present invention to provide a friction stir welding tool assembly that addresses the above-mentioned problems. Summary of the Invention

[0003] In accordance with the present invention, there is provided a friction stir welding tool assembly comprising: a base having a base end, the base end comprising a convex portion; a tip portion having a tip end, the tip end comprising a concave portion; the tip portion further comprising a stir pin, the stir pin being located at an end distal to the tip end; the tip portion formed of a superhard material; and the base end joined to the tip end at corresponding convex portions and concave portions. There is further provided in accordance with the present invention a method of forming a friction stir welding assembly as described herein, the method comprising the steps of providing a base having a base end, the base end comprising a convex portion; and a tip having a tip end, the tip end comprising a concave portion, the tip portion formed of a superhard material, and the base end being joined to the tip end at the corresponding convex portion and concave portion; and forming a stir pin at a distal end of the tip portion. There is further provided in accordance with the present invention the use of a friction stir welding tool assembly as described herein in a friction stir welding method. Optional and / or preferred features of the invention are set out in claims 2 to 19, 21 and 23 to 25. In one option, the base end comprises an array of protrusions and the tip end comprises a corresponding array of recesses. Alternatively, the array is discontinuous. The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0004] [Figure 1] FIG. 1 is a perspective view of an FSW tool assembly according to the present invention. [Figure 2] FIG. 1 is a schematic diagram of one embodiment of an FSW tool assembly according to the present invention, showing a means for attaching a substrate to a tool holder. [Figure 3] FIG. 1 is a schematic diagram of one embodiment of an FSW tool assembly according to the present invention. [Figure 4] FIG. 1 is a schematic diagram of another embodiment of an FSW tool assembly in accordance with the present invention. [Figure 5] FIG. 1 is a schematic diagram of another embodiment of an FSW tool assembly in accordance with the present invention. [Figure 6] FIG. 1 is a schematic diagram of another embodiment of an FSW tool assembly in accordance with the present invention. [Figure 7A] FIG. 1 is a schematic diagram of another embodiment of an FSW tool assembly in accordance with the present invention. [Figure 7B] FIG. 7B is an enlarged view of region E in FIG. 7A. [Figure 8] FIG. 1 is a perspective view of one embodiment of a substrate for an FSW tool assembly according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0005] The diagram is not to scale. Throughout this specification, like parts are assigned the same reference numbers.

[0006] As shown in FIG. 1 , a friction stir welding tool assembly 10 of the present invention includes a base 12 and a tip 14. The base 12 may be attached to or attachable to a tool holder body 16. For example, the base 12 may be shrink-fit or press-fit into a hole in the tool holder body 16, and / or the base 12 may be bonded to the tool holder body 16 by brazing or the like. Alternatively, the base 12 may be attached to the tool holder body 16 by threads S, as shown in FIG. 2 . This is particularly advantageous because temperatures exceeding 500°C are reached during FSW of aluminum alloys. At such temperatures, conventional brazed joints would soften and potentially shear under the torque applied to the tool as it is rotated during use. Attaching the base 12 to the tool holder body 16 using threads S avoids this. The threads S can be machined into the base 12 by electrical discharge machining (EDM) or laser ablation. The tool assembly 10 may further include a retention mechanism (not shown) that mechanically locks the base 12 and tool holder together, thereby preventing separation during FSW. The tool holder may further include a body member 18, which may be solid and cylindrical. The purpose of the body member 18 is to facilitate connection of the FSW tool assembly 10 to an FSW machine. The toolholder body 16 and / or the body member 18 (if present) may comprise steel, such as stainless steel. In certain arrangements, the toolholder body 16 and / or the body member 18 (if present) comprise H13 steel. Alternatively or additionally, the toolholder body 16 and / or the body member 18 (if present) may comprise a high-temperature, high-strength alloy. For example, the toolholder body 16 and / or the body member 18 (if present) may comprise any one or more of the following materials: a Ni-Cr alloy, such as NIMONIC® 80A, having a 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. % Ni, 0-3.0 wt. % Ni, 0-4.0 wt. % Ni, 0-4.0 wt. % Ni, 0-5.0 wt. % Ni, 0-5.0 wt. % Ni, 0-6.0 wt. % Ni, 0-6.0 wt. % Ni, 0-7.0 wt. % Ni, 0-8.0 wt. % Ni, 0-9.0 wt. % Ni, 0-10 ... % Fe, 0-0.1% Mn, 0-2.0% Co, 0-0.008% B, 0-0.15% Zr, 0-0.015% S, and the balance being Ni and trace impurities; Inconel alloys (a class of nickel-chromium based superalloys); W-Ni (tungsten-nickel) alloys; TZM (molybdenum-titanium-zirconium) alloys; and high-entropy alloys. Generally, these alloys are characterized by excellent strength at high temperatures.

[0007] The base 12 has a base end 12a with a protrusion, and the tip 14 has a tip end 14a with a recess. The base end 12a is joined to the tip end 14a at a corresponding protrusion and recess. As defined herein, the corresponding recess and protrusion extend along a plane defined by the longitudinal axis L and the diameter d of the base. The tip 14 further includes an agitating pin 20 located distal to the tip end 14a. The shape of the stirring pin 20 is not particularly limited as long as it is suitable for FSW. The stirring pin 20 may have a conical or cylindrical profile. In a specific example, the stirring pin 20 may have a generally conical profile that gradually widens outward from a rounded apex toward the tip end 14a. The stirring pin 20 has a cone angle θ of about 15 to about 75 degrees, for example, about 30 to about 45 degrees. cAs shown in FIG. 3 , the cone angle is the angle between a line a drawn along the apex of the conical profile of the stirring pin 20 and a line b parallel to the longitudinal axis L. The tip 14 may include a shoulder adjacent the stirring pin 20. This shoulder extends circumferentially from the base of the stirring pin 20 and forms a generally flat surface opposite the tip end 14a. The shoulder is disk-shaped and has a larger diameter than the circular base of the stirring pin 20. The stirring pin 20 may include a spiral groove extending downward from the apex toward the shoulder. In use, rotation of the tool assembly 10 causes this spiral to direct the flow of workpiece material from the edge of the shoulder toward the center and then down the length of the stirring pin 20. This causes the workpiece material to circulate within the stirring zone, filling the void created by the stirring pin 20 as the tip 14 moves in a known manner.

[0008] The tip 14 is formed of an ultra-hard material. The ultra-hard material may include or consist of diamond. For example, the ultra-hard material may include or consist of a diamond-based composite material. Examples of diamond-based composite materials include polycrystalline diamond (PCD) material, silicon carbide-bonded diamond (SCD) material, and diamond-reinforced carbide (DEC) material, all of which are described in more detail below. The ultra-hard material may include or consist of a sintered polycrystalline ultra-hard material, such as polycrystalline diamond (PCD) material, polycrystalline cubic boron nitride (PCBN) material (as used herein, PCBN material has cubic boron nitride (cBN) particles dispersed within a substrate comprising a metal or ceramic material), or silicon carbide-bonded diamond (SCD) material (as used herein, the term "diamond" includes both natural and synthetic diamonds unless otherwise specified). The tip 14 may comprise or consist of a diamond-reinforced carbide (DEC) material as described in GB 2459272, which is incorporated herein by reference in its entirety. Diamond-reinforced carbide refers to any composite material containing particulates of diamond or another ultra-hard phase such as cubic boron nitride (cBN) and at least one other hard phase (usually comprising a carbide such as WC), which are held together by a binder phase, preferably a metallic binder phase, which is usually a transition metal (e.g., Co). Preferably, the ultra-hard material may comprise or consist of a PCD material.

[0009] As used herein, man-made diamond, also referred to as synthetic diamond, is a manufactured diamond material. As used herein, polycrystalline diamond (PCD) material comprises a collection of diamond particles, the majority of which are directly interconnected, with the diamond content being approximately 80% or more by volume of the material. The gaps between the diamond particles may be at least partially filled with a filler material, which may include a catalyst material for synthetic diamond, or may be substantially empty. As used herein, a catalyst material for synthetic diamond (sometimes referred to as a solvent / catalyst material) can promote the growth of synthetic diamond particles and / or the direct intergrowth of synthetic or natural diamond particles at temperatures and pressures at which synthetic or natural diamond is thermodynamically stable. Examples of catalyst materials for diamond include Fe, Ni, Co, and Mn, as well as certain alloys containing these. A body containing PCD material may include at least regions where the catalyst material has been removed from the gaps, leaving voids between the diamond particles. The catalyst material and / or solvent may have been removed by leaching with a strongly acidic aqueous solution, for example by methods detailed in GB 2465175, GB 2499092, or WO 2021136833, the contents of which are incorporated herein by reference in their entirety.

[0010] In one embodiment, the PCD material comprises about 82% by weight of substantially inter-grown diamond grains and about 18% by weight of a filler material disposed in the interstitial regions between the diamond grains, the filler material comprising cobalt. The diamond grains may have an average grain size of about 1 to about 50 microns, for example about 20 microns. It is understood that when measuring the mass or volume percent content of components of a polycrystalline or composite material, the volume of material for which the content is measured must be large enough so that the measurement substantially represents the bulk properties of the material. For example, if a PCD material comprises intergrown diamond grains and cobalt filler located in the interstices between the diamond grains, the volume or mass percent filler content of the PCD material should be measured over a volume of PCD material that is at least several times the volume of the diamond grains, so that the average ratio of filler to diamond material is substantially accurately representative of that in a bulk sample of PCD material (of the same grade).

[0011] In some arrangements, the tip 14 may be composed of or consist essentially of a single grade of PCD, or may include multiple PCD grades arranged in various ways, such as in a layered or laminated arrangement. The tip 14 may comprise multiple layers arranged such that adjacent layers include different PCD grades, with adjacent layers bonded directly to one another by intergrowth of the diamond grains. As used herein, a PCD grade is a type of PCD material characterized by the volume fraction and / or grain size of diamond particles, the volume fraction of interstitial regions between the diamond particles, and the composition of materials that may be present in the interstitial regions. Different PCD grades may have different microstructures and may also have different mechanical properties, such as the modulus of elasticity (or Young's modulus) E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K1C toughness), hardness, density, and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in service. For example, different PCD grades may have different wear resistance and fracture resistance. In other arrangements, the volume of tip 14 may be between 70% and 150% of the volume of base 12 .

[0012] In some arrangements, the substrate 12 can comprise bonded tungsten carbide, such as cobalt-bonded tungsten carbide, a metal such as steel, a ceramic material, a silicon carbide-bonded diamond material, or a superhard material, such as any of the superhard materials detailed above in the context of the tip 14. In some embodiments, the superhard material of the tip 14 can be bonded to the substrate 12, meaning that the superhard material of the tip 14 is manufactured (e.g., sintered) in the same general process by which the superhard tip 14 is bonded to the substrate 12. In some embodiments, the substrate 12 can comprise a bonded tungsten carbide material that includes at least about 5% and at most about 10% or at most about 8% by weight of a binder material, which can include cobalt (measured before the substrate 12 is subjected to high pressure and high temperature conditions that may occur in the manufacture of the superhard tip 14; the actual binder content after such processing will likely be somewhat lower). For example, the substrate 12 may comprise a cobalt-bonded tungsten carbide material comprising about 92% by weight tungsten carbide (WC) particles and about 8% by weight cobalt (Co). The tungsten carbide particles may have an average particle size of about 6 microns or less, about 5 microns or less, or about 3 microns or less. The tungsten carbide particles may have an average particle size of about 1 micron or more, or about 2 microns or more. The cemented carbide material may have a Rockwell hardness "A" of about 88 HRa or more, e.g., about 88.7 HRa or about 90 HRa or more; a transverse rupture strength of about 2,500 megapascals or more, e.g., about 2,800 megapascals (MPa); and / or a magnetic saturation of about 8 G·cm 3 / g (gauss·cubic centimeter per gram) or more and approximately 16 G·cm 3 / g (Gauss·cubic centimeter per gram) or less, or approximately 13 G·cm 3 / g (Gauss·cubic centimeter per gram) or less, for example, about 10.5 to about 12.8 G·cm 3 / g (gauss·cubic centimeter per gram) or approximately 7 G·cm 3 / g (gauss·cubic centimeter per gram) and approximately 11 G·cm 3 / g (gauss·cubic centimeter per gram) or less; and a magnetic coercivity of about 6 kA / m (kiloamperes per meter) or more and about 14 kA / m (kiloamperes per meter) or less, for example, about 7.2 to about 8.8 kiloamperes per meter (kA / m). The fracture toughness may be about 14.6 megapascals (MPa), and the Young's modulus may be about 600 megapascals (MPa). Cemented carbides with relatively low binder content are likely to provide increased stiffness and support to the tip 14 during use, which may help reduce the risk of fracture, and are likely to exhibit excellent wear resistance.

[0013] In some arrangements, the substrate 12 can include an intermediate volume and a core volume comprising a cemented carbide material, the intermediate volume being adjacent to the substrate end portion 12a and the core volume, the intermediate volume being larger than the volume of the tip portion 14, and including an intermediate material having an average Young's modulus in the range of about 60-90% of the Young's modulus of the cemented carbide material. The shape of the base end 12a is not particularly limited, as long as it has a convex portion. Similarly, the shape of the tip end 14a is not particularly limited, as long as it has a concave portion corresponding to the convex portion of the base end 12a. If the tip end 14a were flat, the thickness of the PCD layer would need to be correspondingly increased, which would make the PCD layer more susceptible to cracking. However, this shape of the tip end 14a allows the stir pin 20 to be elongated (e.g., to a length suitable for FSW of non-ferrous metals including aluminum, magnesium, copper, titanium, and their alloys, polymers such as thermoplastic polymers, and polymer composites such as fiber-reinforced polymer composites) without this need. This facilitates the manufacture of a more robust PCD-tip FSW tool assembly. In one embodiment, the shape of the tip end 14a and base end 12a is as shown in Figure 3, where a generally dome-shaped base end 12a is provided with a corresponding tip end 14a. In some arrangements, the base end 12a may include a generally dome-shaped central region having a cross-sectional radius of curvature of 1 mm or more, 2 mm or more, or 5 mm or more. In some examples, the radius of curvature of the base end 12a may be about 20 mm or less.

[0014] As described above, the tip 14 includes the stirring pin 20 located at its distal end relative to the tip end 14a. As depicted in FIG. 3, a longitudinal axis L can be defined that passes through the center of the stirring pin 20. The tip 14 can include a sloped surface, e.g., a substantially flat sloped surface, that slopes toward the stirring pin 20 and is disposed at an angle θ relative to the longitudinal axis L. The angle θ can be in the range of 30 to 60 degrees. Alternatively, instead of a substantially flat sloped surface as depicted in FIGS. 2 to 7, the tip 14 can include a curved surface having a radius of about 15 to about 30 mm, e.g., about 20 to about 25 mm. The height P of the stirring pin 20, measured along the longitudinal axis L, can be about 0.5 to about 10 mm, e.g., about 1 to about 5 mm, e.g., about 2 to about 3 mm, e.g., about 2.5 mm. The height H1 of the superhard material platform, i.e., the distance from tip end 14a to the distal end of stirring pin 20, is measured along longitudinal axis L, as is the height H2 of the combined assembly of base 12 and tip end 14. The height H1 of the superhard material platform can be about 0.5 to about 20 mm, e.g., about 1 to about 10 mm, e.g., about 2 to about 5 mm, e.g., about 5 mm. The ratio of the height H1 of the superhard material platform to the height P of the stirring pin can be about 40:1 to about 1:1, e.g., about 20:1 to about 1:1, e.g., about 10:1 to about 1:1, e.g., about 8:1 to about 1:1, e.g., about 20:3 to about 1:1, e.g., about 4:1 to about 1:1, e.g., about 2:1 to about 1:1. The ratio of the height H1 of the super-hard material to the height P of the stirring pins may be 40:1 or less, or 20:1, or 10:1, or 8:1, or 20:3, or 4:1, or 2:1. The ratio of the height H1 of the super-hard material to the height P of the stirring pins may be 1:1 or more, or 2:1, or 4:1, or 20:3, or 8:1, or 10:1, or 20:1. The height H2 of the combined base and tip assembly may be about 8 mm or more, such as about 10 mm or more, for example about 15 mm or more. Additionally or alternatively, the height H2 of the combined base 12 and tip 14 assembly may be about 50 mm or less, such as about 45 mm or less, for example about 40 mm or less, for example about 35 mm or less, for example about 30 mm or less, for example about 25 mm or less. For example, the height H2 of the assembly of the base 12 and tip 14 can be in the range of about 15 to about 30 mm, for example, about 20 to about 25 mm.The ratio of the height H2 of the assembly of the base 12 and the tip portion 14 to the height P of the stirring pins can be about 100:1 to about 5:1, for example, about 50:1 to about 5:1, for example, about 25:1 to about 5:1, for example, about 20:1 to 5:1, for example, about 50:3 to about 5:1, for example, about 10:1 to about 5:1. The ratio of the height H2 of the assembly of the base 12 and the tip portion 14 to the height P of the stirring pins can be 100:1 or less, or 50:1, or 25:1, or 20:1, or 50:3, or 10:1. The ratio of the height H2 of the assembly of the base 12 and the tip portion 14 to the height P of the stirring pins can be 5:1 or more, or 10:1, or 50:3, or 20:1, or 25:1, or 50:1, or 100:1. The ratio of the height H2 of the combined assembly of the base 12 and tip 14 to the height H1 of the superhard material can be about 100:1 to about 2.5:1, such as about 50:1 to about 2.5:1, for example, about 25:1 to about 2.5:1, for example, about 10:1 to about 2.5:1, for example, about 5:1 to about 2.5:1. The ratio of the height H2 of the combined assembly of the base 12 and tip 14 to the height H1 of the superhard material can be 100:1 or less, or 50:1, or 25:1, or 10:1, or 5:1. The ratio of the height H2 of the combined assembly of the base 12 and tip 14 to the height H1 of the superhard material can be 2.5:1 or more, or 5:1, or 10:1, or 25:1, or 50:1, or 100:1. The diameter d of the substrate, measured perpendicular to the longitudinal axis L, can be from about 8 to about 30 mm, for example, from about 10 to about 20 mm, for example, about 15 mm.

[0015] The base end 12a may further include depressions and / or protrusions, and the tip end 14a may have corresponding depressions and / or protrusions. For example, the base end 12a shown in FIG. 4 has protrusions 22b, and the tip end 14a has corresponding depressions 22a. The number of depressions and / or protrusions is not particularly limited and is applicable to all embodiments shown herein. The protrusions and / or depressions may be generally hemispherical. Such depressions and protrusions can promote bonding between the tip end 14a and the base end 12a by increasing the effective interfacial surface area between the two. 5, the convex portion of the base end 12a may have a generally dome-shaped central region at least partially surrounded by a peripheral shelf 24. The dome-shaped region may further include a depression in the center of the base end 12a, and the tip end 14a may be provided with a corresponding central protrusion, or vice versa. In some arrangements, as shown in FIG. 6, the convex portion of the base end 12a may have a generally flattened dome shape. That is, the base end 12a has a tapered surface formed thereon, beginning at the cylindrical rim of the base 12 and terminating in a raised, substantially flat central portion. The flat central portion may have a diameter of about 3.2 to about 6 mm. In some arrangements, the base end 12 may have a generally flattened dome-shaped central region at least partially surrounded by a peripheral shelf. The flattened dome-shaped region may include the depressions or protrusions described above. The tip end 14a has a corresponding shape.

[0016] 7A and 7B, the base end 12a may have a depression 26 with substantially the same structural features and dimensions as those described above with reference to FIG. 2, except that the base end 12a has a bottom generally opposite the stirring pin 20 of the tip end 14a and is otherwise defined by a generally convex recess in the base end 12a. The base end 12a may be described as a dome with a recess at its tip, with the depression 26 at least partially surrounded by a ridge 28. The depression 26 has a radius of curvature R in the longitudinal direction (i.e., in a plane parallel to L). d is about 0.5 mm or more and about 10 mm or less, and the depth D from the surrounding protuberance 28 d In one particular embodiment, the depth R d is about 0.3 mm. In some arrangements, base end 12a includes one or more generally hemispherical protrusions and tip end 14a includes one or more corresponding generally hemispherical depressions, or vice versa. In the embodiments described above, the convex portions of the substrate end were generally dome-shaped. However, other types of convex structures are also contemplated as part of the present invention. For example, the substrate end may include an array of convex portions, e.g., a discontinuous array, and the tip end may include a corresponding array of concave portions. An example of a substrate end 12a including a discontinuous array of convex portions is shown in FIG. 8. FIG. 8 is a perspective view of a substrate 12 including a discontinuous array of convex portions at the substrate end 12a. The tip end 14a (not shown) has a corresponding discontinuous array of concave portions.

[0017] One or more intermediate layers may be interposed between the base end 12a and the tip end 14a. The intermediate layer may comprise a bonded mass of ultrahard abrasive grains and refractory particles, with the ultrahard abrasive grain size being the same as or smaller than the refractory grain size. In the intermediate layer, the ultrahard abrasive grains and refractory particles generally exist as separate entities with little, no, or substantially no intergrowth or direct bonding between the grains. A binder phase may also be present. This binder phase may comprise or consist of nickel, cobalt, iron, or an alloy containing one or more of these metals. The intermediate layer may comprise a composite formed from non-interbonded grains of ultrahard material, preferably diamond grains, and one or more oxides, nitrides, carbides, silicides, carbonitrides, and / or oxycarbides of any one or more transition metals, including, for example, titanium, zirconium, vanadium, hafnium, tantalum, niobium, chromium, molybdenum, tungsten, copper, manganese, and / or rhenium, or alloys thereof. The amount of ultra-hard abrasive grains in the intermediate layer can generally range from about 10 to about 90 volume percent. The superhard abrasive grains can be diamond or cubic boron nitride. Generally, if the tip contains PCD, the superhard abrasive grains will be diamond, and if the tip contains PCBN, the superhard abrasive grains will be cubic boron nitride. A mixture of superhard abrasive grains can be present in the intermediate layer. The refractory particles may be carbide, nitride, or boride. Carbide particles are preferred. In some arrangements, the intermediate layer is formed of a diamond-reinforced carbide material, as detailed above for tip 14.

[0018] The particle size of the ultra-hard abrasive grains may be the same as or smaller than the particle size of the refractory grains. If the particle size of the ultra-hard abrasive grains is smaller than the particle size of the refractory grains, the particle size of the ultra-hard abrasive grains is generally about 10 microns, preferably about 5 microns, or smaller than the particle size of the refractory grains. The thickness of the intermediate layer can range from about 100 to about 2000 microns, typically from about 200 to about 500 microns. As noted above, the intermediate layer, if present, is between tip end 14a and base end 12a. The intermediate layer generally has an area bonded to and in contact with tip end 14a and an area bonded to and in contact with the surface of base end 12a. One or more additional intermediate layers may be provided between the superabrasive / carbide interlayer and base end 12a and / or between the superabrasive / carbide interlayer and tip end 14a. The inclusion of an intermediate layer is advantageous because it reduces peak stresses between tip end 14a and base end 12a.

[0019] A method of forming a friction stir welding assembly 10 described herein includes the steps of: providing a base 12 having a base end 12 a, where the base end 12 a comprises a convex portion; and a tip 14 having a tip end 14 a, where the tip end 14 a comprises a concave portion, where the tip 14 is formed of a superhard material, and where the base end 12 a is joined to the tip end 14 a at the corresponding convex portion and concave portion; and forming a stir pin 20 at a distal end of the tip 14. The agitator pin 20 may be formed by machining the distal end of the tip 14 using, for example, laser ablation or electrical discharge machining (EDM). The distal end of tip 14 before machining can be substantially planar, or generally dome-shaped, pointed, rounded conical, pointed conical, or frusto-conical.

[0020] The friction stir welding tool assembly 10 described herein may be used in friction stir welding processes. In particular, the friction stir welding assembly 10 described herein may be used in friction stir welding processes involving joining non-ferrous metals, such as aluminum, magnesium, titanium, and copper, or alloys thereof. The friction stir welding tool assembly described herein is particularly suitable for use in friction stir welding processes of aluminum-silicon alloys. These alloys become increasingly abrasive with increasing silicon content, resulting in insufficient life for conventional steel probes. Exemplary aluminum-silicon alloys include Al-50% / Si-50%, Al-64% / Si-36%, Al-65% / Si-35%, Al-75% / Si-25%, Al-88% / Si-12%, Al-90% / Si-10%, and Al-98% / Si-2%, by weight. The friction stir welding tool assembly 10 described herein is particularly suitable for use in friction stir welding methods of aluminum-silicon alloys containing 9-20% silicon by weight, with the remainder being aluminum and unavoidable impurities. The friction stir welding tool assembly 10 described herein may also be used in friction stir welding methods involving polymers, e.g., thermoplastic polymers. The friction stir welding tool assembly 10 described herein may also be used in friction stir welding methods involving composite materials, e.g., polymer composites, such as fiber-reinforced polymer composites, or metal matrix composites. The friction stir welding method may include joining one or more articles comprising one or more non-ferrous metals, polymers, or polymer composites, as described above. The friction stir welding method may include joining at least two articles, e.g., two articles, comprising one or more non-ferrous metals, polymers, or polymer composites, as described above.

[0021] Although the present invention has been shown and described with particular reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims.

[0022] Specific embodiments of the present invention are as follows. 1. A substrate having a substrate end portion, the substrate end portion having a convex portion; 1. A friction stir welding tool assembly comprising: a tip portion having a tip end, the tip end comprising a recess; the tip further comprises an agitation pin, the agitation pin located distally relative to the tip end; The tip is made of ultra-hard material; the base end is joined to the tip end with corresponding protrusions and recesses; Friction stir welding tool assembly. 2. A friction stir welding tool assembly as described in embodiment 1, wherein the super-hard material comprises or consists of a polycrystalline diamond material, a polycrystalline cubic boron nitride silicon carbide bonded diamond material, or a diamond reinforced carbide material. 3. The friction stir welding tool assembly according to embodiment 1 or embodiment 2, wherein the convex portion of the base end is substantially dome-shaped. 4. A friction stir welding tool assembly as described in embodiment 3, wherein the substantially dome-shaped convex portion of the base end is at least partially surrounded by a peripheral shelf. 5. The friction stir welding tool assembly of embodiment 1 or embodiment 2, wherein the convex portion of the base end is a flattened dome-shaped central region having a flat central portion. 6. A friction stir welding tool assembly according to embodiment 1 or embodiment 2, wherein the convex portion of the base end comprises a depression, the bottom of the depression facing the stir pin of the tip end, and the other portion being defined by the recess of the base end which is the convex portion. 7. A friction stir welding tool assembly according to any one of embodiments 1 to 6, wherein the base end includes one or more protrusions and the tip end includes one or more corresponding depressions, and / or the tip end includes one or more protrusions and the base end includes one or more corresponding depressions. 8. A friction stir welding tool assembly as described in embodiment 7, wherein the protrusions and depressions are generally hemispherical. 9. The friction stir welding tool assembly of any one of embodiments 1-8, further comprising one or more intermediate layers between the base end and the tip end. 10. The friction stir welding tool assembly of embodiment 9, wherein the one or more intermediate layers comprise a bonded mass of ultra-hard abrasive grains and refractory particles, wherein the grain size of the ultra-hard abrasive grains is the same as or smaller than the grain size of the refractory particles. 11. The friction stir welding tool assembly of embodiment 10, wherein the intermediate layer further comprises a bonding phase. 12. A friction stir welding tool assembly according to embodiment 11, wherein the bonding phase comprises nickel, cobalt, iron, or an alloy containing one or more of these metals. 13. The friction stir welding tool assembly according to any one of embodiments 1 to 12, wherein the height H1 of the superhard material support is about 1 to about 10 mm. 14. The friction stir welding tool assembly of any one of the preceding embodiments, further comprising a tool holder, the substrate being attached to the tool holder by a screw thread. 15. A method of forming a friction stir welding assembly according to any one of embodiments 1-14, comprising: providing a base having a base end, the base end having a convex portion; and a tip portion having a tip end, the tip end having a concave portion, the tip portion being formed of a superhard material and the base end being joined to the tip end at the corresponding convex portion and concave portion; forming an agitation pin at the distal end of the tip; and a method comprising: 16. The method of embodiment 15, wherein forming the agitator pins comprises using laser ablation and / or electrical discharge machining. 17. Use of the friction stir welding tool assembly according to any one of embodiments 1 to 14 in a friction stir welding method. 18. The use of embodiment 17, wherein the friction stir welding method joins non-ferrous metals. 19. The use according to embodiment 18, wherein the non-ferrous metal is aluminum, magnesium, titanium, or copper, or an alloy thereof.

[0023] 20. The use according to embodiment 19, wherein the aluminum alloy is a silicon-aluminum alloy.

Claims

1. a substrate having a substrate end portion, the substrate end portion having a protrusion; a tip having a tip end, the tip end comprising a recess; 1. A friction stir welding tool assembly comprising: the tip further comprising an agitation pin, the agitation pin being distally disposed relative to the tip end; the tip is formed of an ultra-hard material; the base end is joined to the tip end at the corresponding protrusions and recesses; Friction stir welding tool assembly.

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

3. The friction stir welding tool assembly of claim 1 , wherein the superhard material is a diamond-based composite material.

4. The friction stir welding tool assembly of claim 1 , wherein the superhard material comprises or consists of a polycrystalline diamond material.

5. The friction stir welding tool assembly of claim 1 , wherein the superhard material comprises or consists of a silicon carbide bonded diamond material.

6. The friction stir welding tool assembly of claim 1 , wherein the super-hard material comprises or consists of diamond reinforced carbide.

7. The friction stir welding tool assembly of claim 1 , wherein the super-hard material comprises or consists of polycrystalline cubic boron nitride.

8. The friction stir welding tool assembly according to any one of claims 1 to 7, wherein the convex portion of the base end is substantially dome-shaped.

9. The friction stir welding tool assembly of claim 8 , wherein the substantially dome-shaped convex portion of the base end is at least partially surrounded by a peripheral shelf.

10. 8. The friction stir welding tool assembly according to any one of claims 1 to 7, wherein the convex portion of the base end is a flattened dome-shaped central region having a flat central portion.

11. 8. The friction stir welding tool assembly according to claim 1, wherein the convex portion of the base end comprises a depression, the bottom of the depression faces the stir pin of the tip end, and the other portion is defined by a recess in the base end which is a convex portion.

12. 12. The friction stir welding tool assembly of claim 1, wherein the base end includes one or more protrusions and the tip end includes one or more corresponding depressions, and / or the tip end includes one or more protrusions and the base end includes one or more corresponding depressions.

13. The friction stir welding tool assembly of claim 12 , wherein the protrusions and depressions are generally hemispherical.

14. 14. The friction stir welding tool assembly of claim 1, further comprising one or more intermediate layers between the base end and the tip end, the one or more intermediate layers comprising a bonded mass of ultra-hard abrasive grains and refractory particles, the grain size of the ultra-hard abrasive grains being the same as or smaller than the grain size of the refractory particles.

15. The friction stir welding tool assembly of claim 14 , wherein the intermediate layer further comprises a bonding phase.

16. The friction stir welding tool assembly of claim 15, wherein the bonding phase comprises nickel, cobalt, iron, or an alloy containing one or more of these metals.

17. The friction stir welding tool assembly according to any one of claims 1 to 16, wherein the height H1 of the super-hard material support is from about 1 to about 10 mm.

18. Friction stir welding tool assembly according to any one of claims 1 to 17, further comprising a tool holder, the base body being attached to the tool holder by a screw thread.

19. 19. A friction stir welding tool assembly according to any preceding claim, wherein the base end comprises an array of discontinuous protrusions and the tip end comprises a corresponding array of recesses.

20. A method of forming a friction stir welding assembly according to any one of claims 1 to 19, comprising the steps of: providing a base having a base end, the base end comprising a convex portion; and a tip portion having a tip end, the tip end comprising a concave portion, the tip portion being formed of a superhard material and the base end being joined to the tip end at the corresponding convex portion and concave portion; forming an agitation pin at the distal end of the tip and a method comprising:

21. 21. The method of claim 20, wherein forming the agitator pins comprises using laser ablation and / or electrical discharge machining.

22. Use of a friction stir welding tool assembly according to any one of claims 1 to 19 in a friction stir welding method.

23. 23. The use according to claim 22, wherein the friction stir welding method joins non-ferrous metals.

24. 24. The use according to claim 23, wherein the non-ferrous metal is aluminium, magnesium, titanium or copper, or an alloy thereof.

25. 25. The use according to claim 24, wherein the aluminum alloy is a silicon-aluminum alloy.

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