WO2025045755A1

WO2025045755A1 - Stationary shoulder friction stir welding

Info

Publication number: WO2025045755A1
Application number: PCT/EP2024/073658
Authority: WO
Inventors: Santonu GHOSH; Ali KHAGHANI; Branislav DZEPINA; Teresa Rodriguez Suarez
Original assignee: Element Six UK Ltd
Current assignee: Element Six UK Ltd
Priority date: 2023-08-31
Filing date: 2024-08-23
Publication date: 2025-03-06
Anticipated expiration: 2026-02-28
Also published as: GB2635440A; GB202412410D0

Abstract

The present disclosure provides a stationary shoulder for friction stir welding, comprising: a first body (3) comprising a first surface, a second surface, and a throughhole extending from the first surface of the first body to the second surface of the first body; and a second body (9) comprising a first surface which is configured to abut the second surface of the first body, a second surface, and a throughhole extending from the first surface of the second body to the second surface of the second body; wherein the throughholes of the first and second bodies are coaxial about a longitudinal axis (L) and are configured to receive a friction stir welding tool assembly; wherein a rolling-element bearing (4) is mounted on the first body in the throughhole; wherein the lower surface of the first body comprises a protrusion and the upper surface of the second body comprises a corresponding recess, or vice versa; and wherein the first body is connected to the second body by means of one or more compressible fasteners (11). The present disclosure further provides an apparatus for stationary shoulder friction stir welding, and use of the same in a stationary shoulder friction stir welding process. The present disclosure further provides a protective bushing (8) for stationary shoulder friction stir welding, an apparatus for stationary shoulder friction stir welding comprising said protective bushing, and use of the protective bushing or the apparatus in a method of stationary shoulder friction stir welding.

Description

TOOL

FIELD OF THE INVENTION

This disclosure relates to a stationary shoulder for friction stir welding (FSW), an apparatus for stationary shoulder friction stir welding, and use of the stationary shoulder or the apparatus in a stationary shoulder friction stir welding process. This disclosure further relates to a protective bushing for stationary shoulder friction stir welding, an apparatus for stationary shoulder friction stir welding comprising said protective bushing, and use of the protective bushing or the apparatus in a method of stationary shoulder friction stir welding.

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. Since the workpiece remains in the solid phase, this process is technically a forging process rather than a welding process, none the less by convention, it is referred to as welding or friction stir welding and that convention is followed here.

In the originally developed friction stir welding process, the rotating tool comprises a pin (sometimes referred to as a probe) and a shoulder. The pin and shoulder are attached to one another or integrally formed, and both rotate while the tool is in use.

In general, FSW operations comprise a number of steps, for example: a) an insertion step (also known as the plunge step), from the point when the tool comes into contact with the workpieces to the point where the pin is fully embedded up to the shoulder in the heated and softened workpieces, b) a traversing step, when the tool moves laterally along the line in between the workpieces to be joined, and c) an extraction step, when the tool is lifted or traversed out of the workpieces.

Stationary shoulder friction stir welding (SSFSW) was developed by TWI in 2004. Unlike the originally developed friction stir welding processes, in SSFSW the pin rotates but the shoulder is separate and does not rotate relative to the pin. Hence the shoulder is stationary with respect to the pin during the friction stir welding process. However, while the term “stationary shoulder” is that used in the art, the stationary shoulder does in fact move during the friction stir welding process. Specifically, the stationary shoulder moves laterally along the workpiece(s), and is extracted at the end of the welding process. Nevertheless, the term of the art “stationary shoulder” is used herein.

In industry, SSFSW is used most in the production of T-joints between aluminium-based workpieces. There is, however, a need for improved SSFSW apparatus. One of the problems particular to SSFSW is that, because the shoulder is separate from the pin, there is a gap between the stationary shoulder and the rotating pin. During FSW, the workpiece material softens and, as a result of the large downward pressure from the stirring pin, flows upwards into the gap between the shoulder and the pin. The displacement of softened material is particularly significant on the first plunge of the pin into the material. Once the pin is retracted, the softened material hardens and fouls the pin, thereby preventing rotation of the tool assembly. While some materials can be relatively easily removed from the pin, the likes of steel cannot be. Thus, SSFSW of steel would be problematic using conventional SSFSW apparatus as the lifetime of the pin would be compromised.

Another problem encountered in SSFSW is that of the exit hole. The exit hole is formed when the pin is removed from the workpiece at the end of the welding process. A processing step is then required to remove the exit hole, thereby decreasing the overall efficiency of the welding process. The exit hole can be eliminated by gradually removing the pin from the workpiece while continuing to traverse the workpiece. However, typically in prior art SSFSW designs it has been necessary to use bespoke spindles in combination with a stationary shoulder in order to provide a gradual removal mechanism. This limits flexibility and adoption of SSFSW.

It is therefore an aim of the invention to provide a stationary shoulder and apparatus that addresses the above-mentioned problem.

SUMMARY OF THE INVENTION

The present disclosure provides a stationary shoulder for friction stir welding, comprising: a first body comprising a first surface, a second surface, and a throughhole extending from the first surface of the first body to the second surface of the first body; and a second body comprising a first surface which is configured to abut the second surface of the first body, a second surface, and a throughhole extending from the first surface of the second body to the second surface of the second body; wherein the throughholes of the first and second bodies are coaxial about a longitudinal axis and are configured to receive a friction stir welding tool assembly; wherein a rolling-element bearing is mounted on the first body in the throughhole; and wherein the lower surface of the first body comprises a protrusion and the upper surface of the second body comprises a corresponding recess, or vice versa.

As an option, the protrusion and recess are annular.

As an option, the protrusion and recess are adjacent to the throughhole.

As an option, the rolling-element bearing is a ball bearing or a roller bearing.

As an option, the roller bearing is a cylindrical roller bearing, a spherical roller bearing, a tapered roller bearing or a needle roller bearing.

As an option, the rolling-element bearing is a cylindrical roller bearing.

As an option, the first body is connected to the second body by means of one or more compressible fasteners.

As an option, the compressible fasteners are spring-loaded pins.

As an option, the stationary shoulder further comprises one or more guide pins inserted through the first body and received by the second body.

As an option, the stationary shoulder further comprises a sleeve adjacent to or abutting the surface of the bearing which faces the throughhole.

As an option, the sleeve comprises adjusting means configured to adjust the displacement of the friction stir welding tool assembly along the longitudinal axis.

As an option, the adjusting means are one or more axial pins.

As an option, the stationary shoulder further comprises a cap.

As an option, the cap has a first surface and a second surface, wherein the second surface of the cap abuts or is adjacent to the first surface of the first body, and wherein the cap comprises a throughhole extending from the first surface to the second surface, wherein the throughhole is coaxial about the longitudinal axis and is configured to receive the friction stir welding tool assembly.

The present disclosure also provides an apparatus for stationary shoulder friction stir welding, comprising: the stationary shoulder as described herein; and a friction stir welding tool assembly comprising a tool insert and a tool holder. As an option, the apparatus further comprises a protective bushing.

As an option, the protective bushing comprises: a connection member configured to connect the protective bushing to the stationary shoulder; and a contact member configured to contact a workpiece to be welded; wherein the protective bushing has a first surface, a second surface and a throughhole extending therebetween.

As an option, the friction stir welding tool assembly further comprises a collar.

As an option, the friction stir welding tool assembly further comprises a retention means to attach the collar to the tool holder.

As an option, the tool insert comprises a body portion and a connection member; wherein the tool insert has a longitudinal axis of rotation; wherein the body portion has first and second boundaries spaced apart along the longitudinal axis; wherein the longest linear dimension of the first boundary is less than the longest linear dimension of the second boundary such that the body portion tapers along the longitudinal axis between the first boundary and the second boundary; wherein the second boundary of the body portion abuts or is integral with the connection member; and wherein the connection member is configured to mate with a tool holder so as to prevent rotation of the tool insert relative to the tool holder when in use.

As an option, the taper of the body portion of the tool insert is a non-linear taper or a linear taper.

As an option, the non-linear taper is a stepped taper or a curved taper.

As an option, the taper angle is from 5 degrees to 45 degrees.

As an option, the connection member comprises one or more cut-out portions or protrusions which are configured to mate with the tool holder so as to prevent rotation of the tool insert relative to the tool holder when in use.

As an option, the connection member comprises two cut-out portions or protrusions, and said cut-out portions or protrusions are opposing circular segments when viewed in the axial projection.

As an option, the connection member comprises a central cut-out portion or a central protrusion.

As an option, the central cut-out portion or protrusion has a polygonal profile when viewed in the axial projection.

As an option, the polygonal profile is triangular, quadrilateral, pentagonal, hexagonal, heptagonal, octagonal, nonagonal, decagonal, hendecagonal or dodecagonal. As an option, the distance between the first and second boundaries of the body portion as measured along the longitudinal axis is from approximately 3 mm to approximately 30 mm.

As an option, the tool insert further comprises a stirring pin, wherein the first boundary of the body portion abuts or is integral with the stirring pin.

As an option, the stirring pin comprises a superhard material.

As an option, the body portion comprises a superhard material.

As an option, the superhard material comprises or consists of polycrystalline diamond, polycrystalline cubic boron nitride, silicon carbide-bonded diamond or diamond enhanced carbide.

As an option, the stirring pin, the body portion and the connection member are integrally formed.

As an option, the body portion and the connection member are integrally formed.

As an option, the stirring pin and the body portion are integrally formed.

The tool assembly has a longitudinal axis of rotation and the tool insert, the tool holder and the collar are coaxially aligned around the axis of rotation. The collar has an inner surface and an outer surface, and at least a portion of the inner surface of the collar substantially conforms to the taper of the body portion of the tool insert. The collar is mounted about at least a portion of the body portion of the tool insert and at least a portion of the tool holder.

As an option, when the tool insert comprises one or more cut-out portions, the tool holder comprises one or more corresponding protrusions, and/or when the tool insert comprises one or more protrusions, the tool holder comprises one or more corresponding cut-out portions.

As an option, where the inner surface of the collar is adjacent to the body portion of the tool insert, it (i.e. the inner surface of the collar) substantially conforms to the taper of the body portion of the tool insert.

As an option, the retention means comprises a screw thread on the tool holder and a corresponding screw thread on the collar.

As an option, the retention means comprises a circumferentially extending groove in the tool holder and a corresponding circumferentially extending flange in the collar, or vice versa.

As an option, the retention means comprises at least one locking pin that couples with a locking aperture in the collar and the tool holder.

As an option, there are at least two diametrically opposed locking pins. As an option, the friction stir welding tool assembly comprises a stirring pin, and the stirring pin and the stationary shoulder are formed of different materials.

The present disclosure further provides the use of the stationary shoulder for friction stir welding as described herein or the apparatus as described herein in a friction stir welding process.

The present disclosure further provides a protective bushing for stationary shoulder friction stir welding, comprising a connection member configured to connect the protective bushing to a stationary shoulder; and a contact member configured to contact a workpiece to be welded; wherein the protective bushing has a first surface, a second surface and a throughhole extending therebetween.

As an option, the connection member comprises an annulus which is configured to connect the protective bushing to the stationary shoulder.

As an option, the connection member and/or the contact member comprises, consists of, or consists essentially of a refractory metal or a refractory metal alloy.

As an option, the refractory metal is one or more selected from the group consisting of: Ti, V, Cr, Mn, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os and Ir.

As an option, the refractory metal is one or more selected from the group consisting of: Nb, Mo, Ta, W and Re.

As an option, the connection member and/or the contact member comprises W or a W alloy.

As an option, the connection member and/or the contact member comprises an alloy of W and one or more of the following: Ni, Fe, Re, Cu, Co, Nb, Ti and Mo.

As an option, the connection member and/or the contact member comprises an alloy of W and one or more of the following: Ni, Fe, Re, Cu, Co and Mo.

As an option, the W alloy comprises at least 90 wt.% W, for example at least 92.5 wt.% W, for example at least 95 wt.% W, for example 98.5 wt.% W.

As an option, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, from 0 wt.% to 5 wt.% Mo, and unavoidable impurities.

As an option, the W alloy is a W base, high-density metal as defined in ASTM B777-07. As an option, the connection member and/or the contact member comprises, consists of, or consists essentially of a superhard material.

As an option, the superhard material is polycrystalline boron nitride (PCBN) or polycrystalline diamond (PCD).

As an option, the contact member has a substantially square or rectangular cross-section.

As an option, the contact member has a substantially V-shaped cross-section.

The present disclosure further provides an apparatus for stationary shoulder friction stir welding, comprising: a stationary shoulder which comprises first and second surfaces and a throughhole therebetween; and the protective bushing as disclosed herein; wherein the protective bushing is connected to the stationary shoulder by means of the connection member.

As an option, the apparatus further comprises a friction stir welding tool assembly which is inserted through the throughholes of the stationary shoulder and the protective bushing, the friction stir welding tool assembly comprising a stirring pin.

As an option, the distance between at least a portion of the inner surface of the throughhole of the protective bushing and at least a portion of the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole is less than or equal to 1% of the diameter of the throughhole of the protective bushing.

As an option, the distance between the inner surface of the throughhole of the protective bushing and the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole is less than or equal to 1% of the diameter of the throughhole of the protective bushing.

The present disclosure also provides a use of the protective bushing as disclosed herein or the apparatus as disclosed herein in a method of stationary shoulder friction stir welding.

As an option, the use of the protective bushing or the apparatus is in a method of stationary shoulder friction stir welding of steel.

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 cutaway perspective view of an apparatus for stationary shoulder friction stir welding in accordance with the invention;

Figure 2 is an exploded cutaway perspective view of the apparatus for stationary shoulder friction stir welding of Fig. 1 ;

Figure 3 is a perspective view of the first body of the stationary shoulder;

Figure 4 is a perspective view of the second body of the stationary shoulder;

Figure 5 shows a top view of the first body of the stationary shoulder in accordance with the invention and a cross-sectional view of the first body along the line A-A;

Figure 6 is a cutaway perspective view of an embodiment of the invention in which the friction stir welding tool assembly is mounted in a sleeve which is mounted in the rolling-element bearing of the first body of the stationary shoulder;

Figure 7 is a perspective view of a sleeve as shown in Figure 6;

Figure 8 is a perspective view of an embodiment of a protective bushing in accordance with the invention;

Figure 9 shows a top view of the protective bushing of Figure 8 and cross-sectional views of the protective bushing along the lines A-A and B-B;

Figure 10 is a perspective view of another embodiment of a protective bushing in accordance with the invention;

Figure 11 shows a top view of the protective bushing of Figure 10 and cross-sectional views of the protective bushing along the lines A-A and B-B;

Figure 12 shows a bottom view of the tool insert of Figures 1 and 2 and a cross-sectional view of the tool insert along the line A-A;

Figure 13 shows a perspective view of the tool holder of Figures 1 and 2;

Figure 14 shows a perspective view of the collar of Figures 1 and 2; and

Figure 15 is a cutaway perspective view of an apparatus for stationary shoulder friction stir welding 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

The term “stationary shoulder” is one with which the person skilled in the art is familiar. As is widely understood in the art, the term “stationary” here is used to refer to the shoulder with respect to the friction stir welding tool assembly. A stationary shoulder in friction stir welding is understood to be a shoulder which is separate from a friction stir welding tool assembly comprising a pin or a probe, where the friction stir welding tool assembly is inserted through the stationary shoulder, and the friction stir welding tool assembly rotates while the stationary shoulder does not. The word “stationary” is used to indicate that the shoulder does not rotate with the tool assembly. The stationary shoulder does traverse the workpiece to be welded, and, as described below, it can be retractable, meaning that parts of it may be displaced longitudinally with respect to the friction stir welding tool assembly. Thus, and as would be understood by any practitioner in this field, a literal interpretation of “stationary shoulder”, i.e. a shoulder which does not move, is not correct in this context.

A cutaway perspective view of a stationary shoulder friction stir welding apparatus according to the invention is shown in Figure 1 , and an exploded cutaway perspective view of the apparatus is shown in Figure 2.

The apparatus comprises a retractable stationary shoulder formed of two bodies, first body 3 and second body 9. First body 3 and second body 9 comprise coaxial throughholes through which a friction stir welding tool assembly is inserted. In this embodiment, the friction stir welding tool assembly comprises a tool insert 7, a collar 6, a tool holder 2 and a retaining nut 5; however, any suitable friction stir welding tool assembly may be used. By suitable it is meant that the tool assembly has a pin or probe at one end which can be used to perform a friction stir welding process and has some means distal to the pin or probe which permits it to be attached to a spindle so the tool assembly can be rotated. The friction stir welding tool assembly, or more specifically, the tool holder of the friction stir welding tool assembly, is attached to the first body 3 by means of a roller-element bearing 4 which sits in a suitably shaped cavity adjacent the throughhole. The roller-element bearing 4 comprises an outer race which sits in the bore of the first body 3 and an inner race which contacts the friction stir welding tool assembly. The top surface of the roller-element bearing 4 is preferably flush with the top surface of the first body 3 - in other words, the outer race of the roller-element bearing 4 preferably fits snugly in the cavity. In this embodiment, the roller-element bearing 4 is a cylindrical roller bearing. The advantage of using a roller-element bearing such as the cylindrical roller bearing as depicted in Figures 1 and 2 is that it prevents both axial and radial displacement of the friction stir welding tool assembly. This is in contrast to the prior art, in which the bearings used only prevent radial displacement. This is important to prevent run- out of the tool assembly when in use. The roller-element bearing 4 also offers smooth reaction force and torque transaction throughout the stationary shoulder structure and enhances transverse linear motions.

The apparatus further comprises a cap 12 which can be adapted to attach the assembly to the spindle of a friction stir welding machine, though such means of adaptation are not shown here. Advantageously, this means that the stationary shoulder of the present application can be attached to any spindle simply by adapting the cap design. This facilitates deployment of the stationary shoulder in industrial applications as it can be mounted to existing spindles. This is in contrast to existing retractable stationary shoulder designs, which require specifically adapted machines to provide the retraction mechanism. The combination of the bearing, protrusion and recess, compressible fastener, cap and guide pins ensures that the tool assembly sits concentric to the axis and can only be displaced axially with respect to the stationary shoulder.

The stationary shoulder is formed of two bodies, first body 3 and second body 9. A perspective view of an embodiment of first body 3 is shown in Figure 3, while a perspective view of an embodiment of second body 9 is shown in Figure 4. First body 3 is connected to second body 9 by means of a compressible fastener, for example a spring integrated compressible fastener, which in this embodiment is a spring-loaded pin formed from connection pin 10 and spring 11 . The spring-loaded pin is inserted through a throughole 19 in the first body 3 and received into a suitably sized recess 20 in the second body 9 (see Figures 3 and 4). This two-body design in combination with the compressible fastener provides a retractable stationary shoulder. When the apparatus is not in use, the spring is not compressed and the second body 9 is separated from the first body 3 such that the tool insert 7 of the friction stir welding tool assembly is “retracted”, that is to say, it does not protrude from the base of the apparatus. When the apparatus is brought into use, a force is applied which pushes the apparatus downwards towards a workpiece to be welded. As the apparatus contacts the workpiece, the spring is compressed until the first body 3 and the second body 9 are brought into contact with one another. This compression also exposes the end of the tool insert 7. The stationary shoulder can thus be described as a retractable stationary shoulder. Once the welding process is completed, the force is gradually removed from the apparatus, and the end of the tool insert 7 is gradually covered by the second body 9 as the spring decompresses. This has the advantage of preventing the formation of an exit hole on the workpiece, which eliminates the requirement for an additional processing step to remove said exit hole from the workpiece.

The spring may have a stiffness k of 10-120 N/mm, for example 20-100 N/mm, for example 30-70 N/mm. The stiffness k is calculated according to Hooke’s law. In addition to the spring-loaded pins, guide pins 16 are inserted via a throughhole 21 in the first body 3 and received in a recess 22 of the second body 9. These help to ensure that the first body 3 and the second body 9 remain in alignment. As depicted in Figures 1 and 2, the guide pins 16 are received into connectors 15 which are themselves received into guide pin receivers 14 which are embedded in the second body 9. However, the connectors 15 and the guide pin receivers 14 are not essential and can be omitted, with the guide pins 16 received directly into the recesses 22. Alternatively, the connectors 15 can be eliminated, and the guide pins 16 received directly into guide pin receivers 14.

In the depicted embodiment, the first body 3 comprises four throughholes 19 through which spring-loaded pins can be inserted and the second body 9 comprises four corresponding recesses 20 into which the spring-loaded pins are received. The first body 3 further comprises two throughholes 21 through which guide pins 16 can be inserted and the second body 9 comprises two corresponding recesses 22 into which the guide pins are received. However, different numbers of spring-loaded pins and guide pins are contemplated within the scope of the invention. For example, in some embodiments there are four guide pins, and four corresponding throughholes and recesses to accommodate them.

In this embodiment, the lower surface of the first body 3 comprises a protrusion 23 and the upper surface of the second body 9 comprises a corresponding recess 24. The protrusion 23 is annular and surrounds the throughhole. This helps to ensure a good connection between the first body 3 and the second body 9 when in use. When the tool is in use, significant bending forces impact on the pin as it traverses the workpiece. The presence of the protrusion 23 and the recess 24 ensures that this bending moment is transferred from the pin to the shoulder structure, thereby reducing the bending forces acting on the pin. This reduces the likelihood of pin failure. The protrusion and recess further work in combination with the above- mentioned bearing and compressible fastener to reduce bending of the pin and hold the tool assembly in place, thereby reducing any lateral or transverse movement with respect to the stationary shoulder during friction stir welding applications.

While in this embodiment the first body 3 comprises the protrusion 23 and the second body 9 comprises the recess 24, an arrangement in which the first body 3 comprises the recess and the second body 9 comprises the protrusion is also contemplated within the scope of the invention. It is also noted that the shape of the protrusion and corresponding recess is not particularly limited, so long as the bearing force between the protrusion and the recess is maintained. A cylindrical protrusion is particularly effective.

To ensure a secure fit, the recess 24 should be configured to snugly receive the protrusion 23. For example, a tolerance of 0.1 mm to 1 mm is suitable. A significant problem faced by prior art retractable stationary shoulder designs is misalignment between the moving and stationary parts during the FSW process. Such misalignment can cause the friction stir welding tool assembly to collide with the stationary shoulder, damaging one or both parts and necessitating equipment downtime for repair or at the least realignment. This problem is solved by the combination of the protrusion 23, the recess 24, the spring- loaded pins and the guide pins 16 of the present design, which provide multiple points of contact between the first body 3 and the second body 9, thereby preventing misalignment.

A top view of the first body 3 is depicted in Figure 5, along with cross-sectional view A-A. First body 3 may have a longest linear dimension, length or diameter D1 , measured perpendicular to the longitudinal axis L, of from approximately 10 mm to approximately 150 mm, for example from approximately 40 mm to approximately 150 mm, for example from approximately 10 mm to approximately 140 mm, for example from approximately 10 mm to approximately 130 mm, for example from approximately 10 mm to approximately 120 mm, for example from approximately 10 mm to approximately 110 mm, for example from approximately 10 mm to approximately 100 mm, for example from approximately 15 mm to approximately 95 mm, for example from approximately 20 mm to approximately 90 mm, for example from approximately 25 mm to approximately 85 mm, for example from approximately 30 mm to approximately 80 mm, for example from approximately 35 mm to approximately 75 mm, for example from approximately 40 mm to approximately 70 mm, for example from approximately 45 mm to approximately 65 mm, for example from approximately 50 mm to approximately 60 mm, for example from approximately 55 mm to approximately 65 mm, for example from approximately 70 mm to approximately 100 mm, for example from approximately 75 mm to approximately 95 mm, for example from approximately 75 mm to approximately 90 mm. First body 3 may have a longest linear dimension, length or diameter D1 , measured perpendicular to the longitudinal axis L of at least 10 mm, or at least 15 mm, or at least 20 mm, or at least 25 mm, or at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 45 mm, or at least 50 mm, or at least 55 mm, or at least 60 mm, or at least 65 mm, or at least 70 mm, or at least 75 mm, or at least 80 mm, or at least 85 mm, or at least 95 mm, or at least 100 mm, or at least 110 mm, or at least 120 mm, or at least 130 mm, or at least 140 mm. First body 3 may have a longest linear dimension, length or diameter D1 , measured perpendicular to the longitudinal axis L of at most 15 mm, or at most 20 mm, or at most 25 mm, or at most 30 mm, or at most 35 mm, or at most

40 mm, or at most 45 mm, of at most 50 mm, or at most 55 mm, or at most 60 mm, or at most

65 mm, or at most 70 mm, or at most 75 mm, or at most 80 mm, or at most 85 mm, or at most

95 mm, or at most 100 mm, or at most 110 mm, or at most 120 mm, or at most 130 mm, or at most 140 mm, or at most 150 mm. As depicted in the cross-sectional view, in this embodiment, first body 3 comprises three sections, namely upper section 3a, middle section 3b and lower section 3c. Upper section 3a comprises a first surface and a second surface. An inner annular surface which defines a throughhole extends between the first surface and the second surface. Upper section 3a comprises a cavity which is configured to receive the roller-bearing element 4. It is preferable that the upper surface of the roller-bearing element 4 is flush with the upper surface of upper section 3a. In other words, it is preferable that height H3 of upper section 3a is equal to the height of the roller-bearing element 4, in particular the outer race of the roller-bearing element 4. Longitudinal movement of the roller-bearing element 4 will then be prevented once a cap as described herein has been installed. Upper section 3a has a height H3 (measured parallel to the longitudinal axis L as shown in Figure 5) of from approximately 5 mm to approximately 50 mm, for example from approximately 10 mm to approximately 45 mm, for example from approximately 15 mm to approximately 40 mm, for example from approximately 20 mm to approximately 35 mm, for example from approximately 25 mm to approximately 30 mm, for example from approximately 10 mm to approximately 25 mm, for example from approximately 10 mm to approximately 20 mm, for example from approximately 10 mm to approximately 15 mm. Upper section 3a has a height H3 (measured parallel to the longitudinal axis L as shown in Figure 5) of at least 5 mm, or at least 10 mm, or at least 15 mm, or at least 20 mm, or at least 25 mm, or at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 45 mm. Upper section 3a has a height H3 (measured parallel to the longitudinal axis L as shown in Figure 5) of at most 10 mm, or at most 15 mm, or at most 20 mm, or at most 25 mm, or at most 30 mm, or at most 35 mm, or at most 40 mm, or at most 45 mm. The height of the upper section 3a is not particularly limited so long as it is sufficient to fully house the roller-bearing element 4. As shown in Figure 5, the upper section 3a may comprise an annular bevel extending from its upper surface towards its lower surface.

First body 3 further comprises middle section 3b. Middle section 3b comprises an upper surface and a lower surface with a throughole extending therebetween. At least a portion of the upper surface of the middle section 3b is configured to engage the lower surface of the roller-bearing element 4. In this way, roller-bearing element 4 is received into the cavity in upper section 3a and supported by the at least a portion of the upper surface of the middle section 3b. The portion of the upper surface of the middle section 3b which is configured to engage the lower surface of the roller-bearing element may be an annular shelf around the periphery of the cavity. The annular shelf may have a width D4 of from approximately 1 mm to approximately 15 mm, for example from approximately 2 mm to approximately 14 mm, for example from approximately 3 mm to approximately 13 mm, for example from approximately 4 mm to approximately 12 mm, for example from approximately 5 mm to approximately 11 mm, for example from approximately 6 mm to approximately 10 mm, for example from approximately 7 mm to approximately 9 mm. The annular shelf may have a width D4 of at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 11 mm, or at least 12 mm, or at least 13 mm, or at least 14 mm. The annular shelf may have a width D4 of at most 2 mm, or at most 3 mm, or at most 4 mm, or at most 5 mm, or at most 6 mm, or at most 7 mm, or at most 8 mm, or at most 9 mm, or at most 10 mm, or at most 11 mm, or at most 12 mm, or at most 13 mm, or at most 14 mm, or at most 15 mm.

Middle section 3b may have a chamfered inner surface which extends from the upper surface to the lower surface and is adjacent to the throughhole. The chamfered inner surface defines a tapering throughhole. The height H4 of middle section 3b measured parallel to the longitudinal axis may be from approximately 1 mm to approximately 10 mm, for example from approximately 2 mm to approximately 9 mm, for example from approximately 3 mm to approximately 8 mm, for example from approximately 4 mm to approximately 7 mm, for example from approximately 5 mm to approximately 6 mm, for example from approximately 3 mm to approximately 5 mm. The height H4 of middle section 3b measured parallel to the longitudinal axis may be at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm. The height H4 of middle section 3b measured parallel to the longitudinal axis may be at most 2 mm, or at most 3 mm, or at most 4 mm, or at most 5 mm, or at most 6 mm, or at most 7 mm, or at most 8 mm, or at most 9 mm, or at most 10 mm.

Lower section 3c may also be present. This serves to connect protrusion 23 to middle section 3b. Lower section 3c comprises a throughhole. As noted above, protrusion 23 is annular and surrounds the throughhole.

Upper section 3a, middle section 3b, lower section 3c and protrusion 23 may be integrally or monolithically formed.

First body 3 and protrusion 23 may have a combined height H1 of from approximately 10 mm to approximately 100 mm, for example from approximately 15 mm to approximately 95 mm, for example from approximately 20 mm to approximately 90 mm, for example from approximately 25 mm to approximately 85 mm, for example from approximately 30 mm to approximately 80 mm, for example from approximately 35 mm to approximately 75 mm, for example from approximately 40 mm to approximately 70 mm, for example from approximately 45 mm to approximately 65 mm, for example from approximately 50 mm to approximately 60 mm, for example from approximately 55 mm to approximately 65 mm, for example from approximately 25 mm to approximately 35 mm, for example from approximately 25 mm to approximately 30 mm. First body 3 and protrusion 23 may have a combined height H1 of at least 10 mm, or at least 15 mm, or at least 20 mm, or at least 25 mm, or at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 45 mm, or at least 50 mm, or at least 55 mm, or at least 60 mm, or at least 65 mm, or at least 70 mm, or at least 75 mm, or at least 80 mm, or at least 85 mm, or at least 90 mm, or at least 95 mm. First body 3 and protrusion 23 may have a combined height H1 of at most 15 mm, or at most 20 mm, or at most 25 mm, or at most 30 mm, or at most 35 mm, or at most 40 mm, or at most 45 mm, or at most 50 mm, or at most 55 mm, or at most 60 mm, or at most 65 mm, or at most 70 mm, or at most 75 mm, or at most 80 mm, or at most 85 mm, or at most 90 mm, or at most 95 mm, or at most 100 mm.

The diameter D2 of the wall of the protrusion 23 measured perpendicular to the longitudinal axis L may be from approximately 1 mm to approximately 15 mm, for example from approximately 2 mm to approximately 10 mm, for example from approximately 3 mm to approximately 9 mm, for example from approximately 4 mm to approximately 8 mm, for example from approximately 5 mm to approximately 7 mm, for example from approximately 5 mm to 6 mm. The diameter D2 of the wall of the protrusion 23 measured perpendicular to the longitudinal axis L may be at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 11 mm, or at least 12 mm, or at least 13 mm, or at least 14 mm. The diameter D2 of the wall of the protrusion 23 measured perpendicular to the longitudinal axis L may be at most 2 mm, or at most 3 mm, or at most 4 mm, or at most 5 mm, or at most 6 mm, or at most 7 mm, or at most 8 mm, or at most 9 mm, or at most 10 mm, or at most 11 mm, or at most 12 mm, or at most 13 mm, or at most 14 mm, or at most 15 mm.

Protrusion 23 may have a height H2 measured along the longitudinal axis of approximately 5 mm to approximately 50 mm, for example from approximately 10 mm to approximately 45 mm, for example from approximately 15 mm to approximately 40 mm, for example from approximately 20 mm to approximately 35 mm, for example from approximately 25 mm to approximately 40 mm, for example from approximately 30 mm to approximately 35 mm, for example from approximately 5 mm to approximately 15 mm, for example from approximately 5 mm to approximately 10 mm. Protrusion 23 may have a height H2 measured along the longitudinal axis of at least 5 mm, or 10 mm, or 15 mm, or 20 mm, or 25 mm, or 30 mm, or 35 mm, or 40 mm, or 45 mm. Protrusion 23 may have a height H2 measured along the longitudinal axis of at most 10 mm, or at most 15 mm, or at most 20 mm, or at most 25 mm, or at most 30 mm, or at most 35 mm, or at most 40 mm, or at most 45 mm, or at most 50 mm.

The diameter D3 of the protrusion 23 measured perpendicular to the longitudinal axis L may be from approximately 25 mm to approximately 75 mm, for example from approximately 30 mm to approximately 65 mm, for example from approximately 35 mm to approximately 60 mm, for example from approximately 40 mm to approximately 55 mm, for example from approximately 45 mm to approximately 50 mm, for example from approximately 40 mm to 50 mm. The diameter D3 of the protrusion 23 measured perpendicular to the longitudinal axis L may be at least 25 mm, or at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 45 mm, or at least 50 mm, or at least 55 mm, or at least 60 mm, or at least 65 mm, or at least 70 mm. The diameter D3 of the protrusion 23 measured perpendicular to the longitudinal axis L may be at most 30 mm, or 35 mm, or 40 mm, or 45 mm, or 50 mm, or 55 mm, or 60 mm, or 65 mm, or 70 mm, or 75 mm.

Recess 24 is configured to snugly receive protrusion 23, and thus the height and diameter of recess 24 are accordingly determined by the height and diameter of protrusion 23.

Figure 6 shows an embodiment of the invention in which, rather than being directly mounted in the roller-element bearing 4, the friction stir welding tool assembly is instead mounted in sleeve 17, which is itself mounted in the roller-element bearing 4. Sleeve 17 is depicted in Figure 7 and comprises a connection member 25, a lipped member 26, a trunk member 27 for engaging the roller-element bearing 4, a screw thread 28 and one or more holes 24 which are configured to receive axial pins (not shown). The screw thread may receive a retaining nut, for example retaining nut 5 as shown in Figure 1 . The lipped member 26 works in combination with the screw thread 28 and retaining nut to secure the trunk member 27 in contact with the roller-element bearing 4. As shown in Figure 6, the lower surface of the lipped member 26 sits on the upper surface of the inner race of the roller-element bearing 4. The axial pins are used to adjust the displacement of the friction stir welding tool assembly along the longitudinal axis. The nature of the adjustment means is not particularly limited, so long as the displacement of the friction stir welding tool assembly along the longitudinal axis can be adjusted. The thickness of workpiece that can be welded is partly dictated by the length of the pin or probe which is exposed when the apparatus is in a working position, i.e. during welding. This means that typically different lengths of pin, and different tool assemblies, are required to weld different thicknesses of workpiece. However, in this embodiment, because the friction stir tool welding assembly can be displaced along the longitudinal axis, friction stir welding of a range of workpiece thicknesses can be accomplished using the same tool assembly. This is advantageous as it provides a more flexible welding apparatus and it also means that it is not necessary for a user to stock multiple friction stir welding pins of differing heights for welding different workpiece thicknesses. Furthermore, due to the linked rotation of the tool holder of the friction stir welding assembly and the sleeve 17, which occurs as power is delivered from the FSW machine spindle, the resultant force and torque are evenly distributed along the tool holder, sleeve and bearing. This distribution of forces and torques contributes to enhanced welding performance.

A further advantage is obtained when the sleeve is used in combination with the cap, bearing, protrusion and recess and compressible fasteners. In such an arrangement, the sleeve protects the cap from being loaded and generating movement of the tool that would result in more friction between the recess and displacement of the tool which would load the bearing and result in the protrusion rubbing on the part near the workpiece.

The apparatus further comprises a protective bushing. The protective bushing has a first surface, a second surface and a throughhole extending therebetween. Embodiments of protective bushing according to the invention are depicted in Figures 8-11 .

The protective bushing comprises a connection member configured to connect the protective bushing to the stationary shoulder and a contact member configured to contact a workpiece to be welded. The shape of the contact member depends on the shape of the workpiece to be welded. For example, if a fillet weld is being formed, then the contact member would have a substantially V-shaped cross-section, as depicted in Figures 8 and 9. However, if the welding is to form a butt or lap joint, then the contact member would have a substantially square or rectangular cross-section so as to conform to the surface of the workpiece(s) to be welded. A rectangular cross-section is depicted in Figures 10 and 11 .

The protective bushing is formed of a material which does not deform under the high temperatures of the friction stir welding process. Any suitable material may be used. For example, the connection member and/or the contact member may comprise, consist of, or consist essentially of a superhard material, such as polycrystalline boron nitride (PCBN) or polycrystalline diamond (PCD), or a carbide, for example tungsten carbide, for example cemented tungsten carbide. The superhard material may also be any of those listed below in the context of the stirring pin. Alternatively, the connection member and/or the contact member may comprise, consist of, or consist essentially of a refractory metal or a refractory metal alloy. The refractory metal is one or more selected from the group consisting of: Ti, V, Cr, Mn, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os and Ir, for example one or more selected from the group consisting of: Nb, Mo, Ta, W and Re. In some embodiments, the connection member and/or the contact member comprises W or a W alloy. In some embodiments, the connection member and/or the contact member comprises an alloy of W and one or more of the following: Ni, Fe, Re, Cu, Co, Nb, Ti and Mo. In some embodiments, the connection member and/or the contact member comprises an alloy of W and one or more of the following: Ni, Fe, Re, Cu, Co and Mo. In some embodiments, the W alloy comprises at least 90 wt.% W, for example at least 92.5 wt.% W, for example at least 95 wt.% W, for example at least 97 wt.% W.

In some embodiments, the W alloy comprises at most 92.5 wt.% W, for example at most 95 wt.% W, for example at most 97 wt.% W, for example at most 98.5 wt.% W.

In some embodiments, the W alloy consists of from 10 wt.% to 40 wt.% Cu, for example from 10 wt.% to 30 wt.% Cu, for example from 10 wt.% to 20 wt.% Cu, for example from 10 wt.% Cu to 15 wt.% Cu, and balance W and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Cu, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0 wt.% to 5 wt.% Fe, from 0.5 wt.% to 5 wt.% Co, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Fe, from 0.5 wt.% to 5 wt.% Co, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, from 0 wt.% to 5 wt.% Mo, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Fe, from 0.5 wt.% to 5 wt.% Co, from 0 wt.% to 5 wt.% Mo, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, from 0.5 wt.% to 5 wt.% Mo, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Fe, and unavoidable impurities.

In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0.5 wt.% to 5 wt.% Fe, from 0.5 wt.% to 5 wt.% Mo, and unavoidable impurities. In some embodiments, the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, from 0 wt.% to 5 wt.% Mo, from 0 wt.% to 5 wt.% Nb, from 0 wt.% to 5 wt.% Cu, from 0 wt.% Ti, and unavoidable impurities.

In some embodiments, the W alloy consists of from 1 wt.% to 30 wt.% Ni, for example from 1 wt.% to 20 wt.% Ni, for example from 1 wt.% to 10 wt.% Ni, for example from 2 wt.% to 7 wt.% Ni, for example from 1 wt.% to 5 wt.% Ni, from 0 wt.% to 15 wt.% Fe, for example from 0 wt.% to 10 wt.% Fe, for example from 0 wt.% to 5 wt.% Fe, for example from 0 wt.% to 3 wt.% Fe, for example from 0 wt.% to 2 wt.% Fe, for example from 0 wt.% to 1 wt.% Fe, for example from 0.5 wt.% to 5 wt.% Fe, for example from 1 wt.% to 5 wt.% Fe, from 0 wt.% to 10 wt.% Cu, for example from 0 wt.% to 5 wt.% Cu, for example from 1 wt.% to 5 wt.% Cu, from 0 wt.% to 5 wt.% Mo, for example from 0 wt.% to 3 wt.% Mo, for example from 1 wt.% to 5 wt.% Mo, balance W and unavoidable impurities.

In some embodiments, the W alloy consists of from 1 wt.% to 30 wt.% Ni, for example from 1 wt.% to 20 wt.% Ni, for example from 1 wt.% to 10 wt.% Ni, for example from 2 wt.% to 7 wt.% Ni, for example from 1 wt.% to 5 wt.% Ni, from 0 wt.% to 15 wt.% Fe, for example from 0 wt.% to 10 wt.% Fe, for example from 0 wt.% to 5 wt.% Fe, for example from 0 wt.% to 3 wt.% Fe, for example from 0 wt.% to 2 wt.% Fe, for example from 0 wt.% to 1 wt.% Fe, for example from 0.5 wt.% to 5 wt.% Fe, for example from 1 wt.% to 5 wt.% Fe, from 0 wt.% to 10 wt.% Cu, for example from 0 wt.% to 5 wt.% Cu, for example from 1 wt.% to 5 wt.% Cu, from 0 wt.% to 5 wt.% Mo, for example from 0 wt.% to 3 wt.% Mo, for example from 1 wt.% to 5 wt.% Mo, for example from 0 wt.% to 5 wt.% Nb, for example from 0 wt.% to 3 wt.% Nb, for example from 1 wt.% to 5 wt.% Nb, for example from 0 wt.% to 5 wt.% Cu, for example from 0 wt.% to 3 wt.% Cu, for example from 1 wt.% to 5 wt.% Cu, for example from 0 wt.% to 5 wt.% Ti, for example from 0 wt.% to 3 wt.% Ti, for example from 1 wt.% to 5 wt.% Ti, balance W and unavoidable impurities.

In some embodiments, the W alloy consists of 95 wt.% W, 3.5 wt.% Ni and 1 .5 wt.% Cu.

In some embodiments, the W alloy consists of 92.5 wt.% W, 5.0 wt.% Ni and 2.5 wt.% Fe.

In some embodiments, the W alloy is a W base, high-density metal as defined in ASTM B777- 07.

As depicted in Figures 8 and 9, the protective bushing 8 comprises a connection member 29 configured to connect the protective bushing to the stationary shoulder and a contact member 30 configured to contact a workpiece to be welded. In this embodiment, the connection member 29 comprises an annulus which is configured to connect the protective bushing 8 to the stationary shoulder. In this and all other embodiments, the connection member and the contact member may be integrally formed or monolithic.

Connection member 29 may have a height L1 measured parallel to the longitudinal axis L of from approximately 1 mm to approximately 20 mm, for example from approximately 2 mm to approximately 15 mm, for example from approximately 3 mm to approximately 10 mm, for example from approximately 4 mm to approximately 9 mm, for example from approximately 5 mm to approximately 8 mm, for example from approximately 5 mm to approximately 7 mm. Connection member 29 may have a height L1 measured parallel to the longitudinal axis L of at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 11 mm, or at least 12 mm, or at least 13 mm, or at least 14 mm, or at least 15 mm, or at least 16 mm, or at least 17 mm, or at least 18 mm, or at least 19 mm. Connection member 29 may have a height L1 measured parallel to the longitudinal axis L of at most 2 mm, or at most 3 mm, or at most 4 mm, or at most 5 mm, or at most 6 mm, or at most 7 mm, or at most 8 mm, or at most 9 mm, or at most 10 mm, or at most 11 mm, or at most 12 mm, or at most 13 mm, or at most 14 mm, or at most 15 mm, or at most 16 mm, or at most 17 mm, or at most 18 mm, or at most 19 mm, or at most 20 mm.

Connection member 29 may have a diameter G1 measured perpendicular to the longitudinal axis L of from approximately 5 mm to approximately 50 mm, for example from approximately 10 mm to approximately 45 mm, for example from approximately 15 mm to approximately 40 mm, for example from approximately 20 mm to approximately 35 mm, for example from approximately 25 mm to approximately 30 mm, for example from approximately 10 mm to approximately 20 mm. Connection member 29 may have a diameter G1 measured perpendicular to the longitudinal axis L of at least 5 mm, or at least 10 mm, or at least 15 mm, or at least 20 mm, or at least 25 mm, or at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 45 mm. Connection member 29 may have a diameter G1 measured perpendicular to the longitudinal axis L of at most 10 mm, or at most 15 mm, or at most 20 mm, or at most 25 mm, or at most 30 mm, or at most 35 mm, or at most 40 mm, or at most 45 mm, or at most 50 mm.

The slope angle 9_b is the included angle between the longitudinal axis L and a line traced and intersected along the outer surface of contact member 30, as depicted in Section B-B of Figure 9. In this embodiment, slope angle 9_b is 45 degrees, i.e. suitable for contacting two workpieces which are to be fillet welded.

Contact member 30 may have a height L2 measured parallel to the longitudinal axis L of from approximately 1 mm to approximately 20 mm, for example from approximately 2 mm to approximately 15 mm, for example from approximately 3 mm to approximately 10 mm, for example from approximately 4 mm to approximately 9 mm, for example from approximately 5 mm to approximately 8 mm, for example from approximately 5 mm to approximately 15 mm. Contact member 30 may have a height L2 measured parallel to the longitudinal axis L of at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 11 mm, or at least 12 mm, or at least 13 mm, or at least 14 mm, or at least 15 mm, or at least 16 mm, or at least 17 mm, or at least 18 mm, or at least 19 mm. Contact member 30 may have a height L2 measured parallel to the longitudinal axis L of at most 2 mm, or at most 3 mm, or at most 4 mm, or at most 5 mm, or at most 6 mm, or at most 7 mm, or at most 8 mm, or at most 9 mm, or at most 10 mm, or at most 11 mm, or at most 12 mm, or at most 13 mm, or at most 14 mm, or at most 15 mm, or at most 16 mm, or at most 17 mm, or at most 18 mm, or at most 19 mm, or at most 20 mm.

Contact member 30 may have a diameter G2 measured perpendicular to the longitudinal axis L of from approximately 5 mm to approximately 50 mm, for example from approximately 10 mm to approximately 45 mm, for example from approximately 15 mm to approximately 40 mm, for example from approximately 20 mm to approximately 35 mm, for example from approximately 25 mm to approximately 30 mm, for example from approximately 25 mm to approximately 35 mm. Contact member 30 may have a diameter G2 measured perpendicular to the longitudinal axis L of at least 5 mm, or at least 10 mm, or at least 15 mm, or at least 20 mm, or at least 25 mm, or at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 45 mm. Contact member 30 may have a diameter G2 measured perpendicularto the longitudinal axis L of at most 10 mm, or at most 15 mm, or at most 20 mm, or at most 25 mm, or at most 30 mm, or at most 35 mm, or at most 40 mm, or at most 45 mm, or at most 50 mm.

As depicted in Figures 10 and 11 , the protective bushing 18 comprises a connection member 129 configured to connect the protective bushing 18 to the stationary shoulder and a contact member 130 configured to contact a workpiece to be welded. In this embodiment, the connection member 129 comprises an annulus which is configured to connect the protective bushing 18 to the stationary shoulder. The connection member and the contact member may be integrally formed or monolithic.

Connection member 129 may have a height L1 and a diameter G1 as described above in the context of connection member 29.

Contact member 130 may have a height L2 and a diameter G2 as described above in the context of contact member 30. In both of the embodiments shown in Figures 8-11 , the diameter G3 of the throughhole measured parallel to the longitudinal axis L may be from approximately 1 mm to approximately 20 mm, for example from approximately 2 mm to approximately 15 mm, for example from approximately 3 mm to approximately 10 mm, for example from approximately 4 mm to approximately 9 mm, for example from approximately 5 mm to approximately 8 mm, for example from approximately 5 mm to approximately 15 mm. The diameter G3 of the throughhole measured parallel to the longitudinal axis L may be at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 11 mm, or at least 12 mm, or at least 13 mm, or at least 14 mm, or at least 15 mm, or at least 16 mm, or at least 17 mm, or at least 18 mm, or at least 19 mm. The diameter G3 of the throughhole measured parallel to the longitudinal axis L may be at most 2 mm, or at most 3 mm, or at most 4 mm, or at most 5 mm, or at most 6 mm, or at most 7 mm, or at most 8 mm, or at most 9 mm, or at most 10 mm, or at most 11 mm, or at most 12 mm, or at most 13 mm, or at most 14 mm, or at most 15 mm, or at most 16 mm, or at most 17 mm, or at most 18 mm, or at most 19 mm, or at most 20 mm.

The throughhole of the protective bushing is sized such that the distance (measured perpendicular to the longitudinal axis L) between at least a portion of the inner surface of the throughhole of the protective bushing and at least a portion of the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole is less than or equal to 1 % (e.g. 1 .0%), optionally less than or equal to 0.9%, optionally less than or equal to 0.8%, optionally less than or equal to 0.7%, optionally less than or equal to 0.6%, optionally less than or equal to 0.5%, optionally less than or equal to 0.4%, optionally less than or equal to 0.3%, optionally less than or equal to 0.2%, optionally less than or equal to 0.1%, of the diameter of the throughhole of the protective bushing. The distance (measured perpendicular to the longitudinal axis L) between at least a portion of the inner surface of the throughhole of the protective bushing and at least a portion of the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole may be 0.001% or more, for example 0.01% or more, for example 0.1% or more, for example 0.3% or more of the diameter of the throughhole of the protective bushing. The distance (measured perpendicular to the longitudinal axis L) between at least a portion of the inner surface of the throughhole of the protective bushing and at least a portion of the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole may be from approximately 0.001% to approximately 1%, for example from approximately 0.01% to approximately 1%, for example from approximately 0.1% to approximately 1%, for example from approximately 0.1% to approximately 0.5%, for example from approximately 0.3% to approximately 1.0% of the diameter of the throughhole of the protective bushing. In an embodiment, the distance (measured perpendicular to the longitudinal axis L) between the inner surface of the throughhole of the protective bushing and the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole is less than or equal to 1 % (e.g. 1.0%), optionally less than or equal to 0.9%, optionally less than or equal to 0.8%, optionally less than or equal to 0.7%, optionally less than or equal to 0.6%, optionally less than or equal to 0.5%, optionally less than or equal to 0.4%, optionally less than or equal to 0.3%, optionally less than or equal to 0.2%, optionally less than or equal to 0.1%, of the diameter of the throughhole of the protective bushing. The distance (measured perpendicular to the longitudinal axis L) between the inner surface of the throughhole of the protective bushing and the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole may be 0.001% or more, for example 0.01% or more, for example 0.1% or more of the diameter of the throughhole of the protective bushing. For example, the distance (measured perpendicular to the longitudinal axis L) between the inner surface of the throughhole of the protective bushing and the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole may be from approximately 0.001% to approximately 1%, for example from approximately 0.01% to approximately 1%, for example from approximately 0.1% to approximately 1%, for example from approximately 0.1% to approximately 0.5%, for example from approximately 0.3% to approximately 1.0% of the diameter of the throughhole of the protective bushing. This limited tolerance between the outer surface of the friction stir welding tool assembly and the inner surface of the throughhole of the protective bushing ensures that the friction stir welding tool assembly is free to rotate unencumbered by the protective bushing, but that softened metal cannot flow up into the throughhole of the stationary shoulder and onto the upper part of the friction stir welding tool assembly. The cooled softened metal can upon hardening create a layer of metal on the tool assembly and/or the inner surface of the throughhole to impede the rotation of the tool assembly, thereby interfering with the welding process. While low temperature metals such as aluminium can be relatively easily removed from the friction stir welding tool assembly, high temperature metals such as steel cannot be, which means that without the protection the tool assembly would need to be replaced much more often. Thus, to friction stir weld steel using a stationary shoulder apparatus in a commercially viable manner, the protective bushing as described herein is particularly important.

As well as the above-mentioned advantage of the protective bushing, the protective bushing also acts as a shoulder and, in combination with the bearing, protrusion and recess and compressible fastener, works to ensure alignment of the tool and to reduce transverse movement. When the protective bushing is further combined with the cap, it helps to ensure that there is only axial movement of the tool assembly with respect to the stationary shoulder. Where the protective bushing is not present, loading on the tool assembly can be transferred to the cap and cause misalignment of the compressible fasteners and the protrusion and recess. This effect is further enhanced by the provision of the guide pins.

The apparatus further comprises a friction stir welding tool assembly comprising a tool insert 7 and a tool holder 2. An example tool insert 7 is depicted in Figure 12. The tool insert 7 comprises a first body portion 33, a second body portion 34, a connection member 32, and a stirring pin 35. The second body portion 34 is entirely optional. As shown in Fig. 12, the first body portion 33 has a first boundary 33a and a second boundary 33b which are spaced apart along a longitudinal axis L. The longest linear dimension of the first boundary 33a is less than the longest linear dimension of the second boundary 33b such that the first body portion 33 tapers along the longitudinal axis L between the first boundary 33a and the second boundary 33b. In this context, the term “taper” is understood to describe an object which diminishes or reduces in thickness towards one end. In this embodiment, the first and second body portions 33, 34, connection member 32 and stirring pin 35 are integrally formed. It is also envisaged that the first and second body portions 33, 34 and connection member 32 may be integrally formed and then joined to the stirring pin 35, for example by brazing. Typically, at least the first and second body portions 33, 34 and the connection member 32 are integrally formed.

The embodiment of the tool insert shown in Figure 12 has a first body portion 33 with a linear taper. However, non-linear tapers are also envisaged as part of the invention. A non-linear taper may comprise multiple steps. Another example of a non-linear taper is a curved taper.

The shape of the stirring pin 35 is not particularly limited, so long as it is suitable for FSW. The stirring pin 35 may have a conical or cylindrical profile. The stirring pin 35 may have a cone angle of from about 15 degrees to about 75 degrees, for example from about 30 degrees to about 45 degrees.

In some embodiments, the tool insert 7 consists of a stirring pin 35, a first body portion 33, a second body portion 34 and a connection member 32. In some embodiments, the tool insert 7 consists of a stirring pin 35, a first body portion 33 (termed a body portion) and a connection member 32.

In some embodiments, the tool insert 7 does not comprise a stirring pin 35. Instead, the tapered first body portion 33 or, if present, the second body portion 34 also fulfils the function of the stirring pin 35.

The stirring pin 35, where present, may consist of or comprise a superhard material. The superhard material may comprise or consist of a sintered polycrystalline super-hard material, such as polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) (as used herein, PCBN comprises grains of cubic boron nitride (cBN) dispersed within a matrix comprising metal or ceramic material), or silicon carbide-bonded diamond (SCD) (as used herein, unless otherwise specified, the term “diamond” will include both natural and fabricated diamond). The stirring pin 16 may comprise or consist of diamond enhanced carbide (DEC), 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).

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) 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 I 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 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 first and second body portions 33, 34 of the tool insert may also consist of or comprise a superhard material as detailed above.

The stirring pin 35, the first body portion 33, the second body portion 34 and/or the connection member 32 of the tool insert may comprise cemented tungsten carbide, for example, cobait- 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 stirring pin 35. In some examples, the super-hard material of the stirring pin 35 may be formed joined to (i.e. integrally formed with) the first body portion 33 or, if present, the second body portion 34, by which is meant that the super-hard material of the stirring pin 35 is produced (for example sintered) in the same general step in which the stirring pin 35 becomes joined to the first or second body portion 33, 34. In some examples, the first body portion 33, the second body portion 34 and/or the connection member 32 may comprise cemented tungsten carbide material including at least about 5 weight per cent and at most about 10 weight per cent or at most about 8 weight per cent binder material, which may comprise cobalt (as measured prior to subjecting the first and/or second body portion 33,

34 and/or the connection member 32 to any high-pressure, high temperature condition at which the super-hard material of the stirring pin 35 may be produced; the actual binder content after such treatment is likely to be somewhat lower). For example, the body portion 35 and/or the connection member 32 may comprise cobalt-cemented tungsten carbide material comprising about 92 weight per cent tungsten carbide (WC) grains and about 8 weight per cent cobalt (Co). The tungsten carbide grains may have a mean size of at most about 6 microns, at most about 5 microns or at most about 3 microns. The mean size of the tungsten carbide grains may be at least about 1 micron or at least about 2 microns. The cemented carbide material may have Rockwell hardness “A” of at least about 88 HRa, for example about 88.7 HRa, or at least about 90 HRa; transverse rupture strength of at least about 2,500 megapascals, for example about 2,800 megapascals (MPa); and/or magnetic saturation of at least about 8 G.cm³/g (Gauss times cubic centimetre per gram) and at most about 16 G.cm³/g (Gauss times cubic centimetre per gram) or at most about 13 G.cm³/g (Gauss times cubic centimetre per gram), for example from about 10.5 to about 12.8 G.cm³/g (Gauss times cubic centimetre per gram) or from about 7 G.cm³/g (Gauss times cubic centimetre per gram) and at most about 11 G.cm³/g (Gauss times cubic centimetre per gram), and magnetic coercivity of at least about 6 kA/m (kiloampere per metre) and at most about 14 kA/m (kiloampere per metre), for example from about 7.2 to about 8.8 kiloamperes per metre (kA/m). The fracture toughness may be about 14.6 megapascals (MPa) and the Young’s modulus may be about 600 megapascals (MPa). Cemented carbide having relatively low binder content is likely to provide enhanced stiffness and support for the stirring pin 35 in use, which may help reduce the risk of fracture, and is likely to exhibit good wear resistance.

The connection member 32 is configured to mate with a tool holder 3 so as to prevent rotation of the tool insert 7 relative to the tool holder 3 when in use. Prevention of rotation of the tool insert 7 relative to the tool holder 3 when in use serves to transfer torque from the shank of the tool holder s to the stirring pin 35 (when present) and the first and/or second body portions 33, 34. The configuration of the present invention also provides a larger contact area between the connection member 32 and the tool holder than in conventional tool designs, and this provides better grip. In the embodiment depicted in Figure 12, the connection member 32 is configured to mate with the tool holder 3 so as to prevent rotation of the tool insert 7 relative to the tool holder 3 when in use by means of two cut-out portions 31a, 31 b in the connection member 32. In this embodiment, the cut-out portions are opposing circular segments when viewed in the axial projection.

The shape of the cut-out portions on the connection member 32 is not particularly limited so long as they are configured to mate with a tool holder 3 so as to prevent rotation of the tool insert 7 relative to the tool holder 3 when in use. As an alternative or in addition to cut-out portions as depicted in Figure 12, the tool insert 7 may contain protrusions. The combination of protrusions and cut-out portions in this manner helps to prevent rotation of the tool insert 7 in the tool holder when in use, and thereby serves to transfer torque from the shank of the tool holder 3 to the stirring pin 35, by providing additional contact surfaces between the tool insert 7 and the tool holder 3. In other embodiments (not shown), the connection member 32 may comprise a central cut-out portion or a central protrusion. In other embodiments (not shown), the central cut-out portion or protrusion may have a polygonal profile when viewed in the axial projection, for example the polygonal profile may be triangular, quadrilateral, pentagonal, hexagonal, heptagonal, octagonal, nonagonal, decagonal, hendecagonal or dodecagonal.

Whatever the nature of the cut-out portions and/or protrusions on the connection member 32, the tool holder 3 must have corresponding cut-out portions and/or protrusions. For example, the tool holder 3 depicted in Figure 13 has protrusions 36a, 36b which correspond to the cutout portions 31 a, 31 b on the tool insert 7.

The tool holder 3 further comprises a shank 38, which may be solid and cylindrical. The purpose of the shank 38 is to facilitate connection of the tool holder 3 to FSW machinery.

The tool holder 3 may comprise or consist of steel, or 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 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.

The tool insert 7 is secured in the tool holder 3 by means of a collar 6. An exemplary collar 6 is shown in Figure 14. The collar 6 has an inner surface and an outer surface. As part of a retention means to attach the collar 6 to the tool holder 3, the collar further comprises a screw thread 39. The tool holder 3 comprises a corresponding screw thread 37 as part of the retention means, as shown in Figure 13. Where it is adjacent to the first body portion 33 of the tool insert 7 the inner surface of the collar s substantially conforms to the taper of the first body portion 33 of the tool insert 7. This ensures that the tool insert 7 is held securely in position and limits movement of the tool insert along both the longitudinal axis and the radial axis, thereby reducing run-out.

The collar 6 may comprise or consist of the same materials as those listed above for the tool holder 3. The collar 6 and the tool holder 3 may be made of the same materials or different materials. Where the collar 6 and the tool holder 3 are made of the same material, they will advantageously have the same thermal expansion coefficient. This would mean, for example, that the screw threads 37, 39 would expand at the same rate under the high temperature operating conditions.

In the embodiment shown in Figures 13 and 14, the retention means comprises screw threads 37, 39. However, the nature of the retention means is not particularly limited so long as it securely attaches the collar 6 to the tool holder 3. For example, the retention means may comprise a circumferentially extending groove in the tool holder 3 and a circumferentially extending and corresponding flange in the locking collar 6, and vice versa. Additionally or alternatively, the retention means may comprise a locking pin that couples with a locking aperture in the collar 6 and the tool holder 3. The retention means may comprise two or more locking pins, for example, two locking pins which are diametrically opposed.

A friction stir welding tool assembly is formed from the tool insert 7, the tool holder 3 and the collar 6 as depicted in Figures 1 and 2. In the embodiment shown, the connection member 32 mates with the tool holder 7 by means of two-cut portions on the connection member 32 and two corresponding protrusions on the tool holder 7. In this manner, rotation of the tool insert 7 relative to the tool holder 3 is prevented.

While the protective bushing is shown and described above in the context of the retractable stationary shoulder apparatus of Figures 1 and 2, it is important to note that the protective bushing may be used with any suitable stationary shoulder apparatus. An example of this is depicted in Figure 15, which shows a cutaway perspective view of a stationary shoulder friction stir welding apparatus in combination with a protective bushing according to the invention.

The apparatus comprises a stationary shoulder 40 which comprises a throughhole through which a friction stir welding tool assembly is inserted in the direction indicated by the arrow in Figure 15. The friction stir welding tool assembly shown in Figure 15 comprises a tool insert 7 and a tool holder 2; however, any suitable friction stir welding tool assembly may be used. By suitable it is meant that the tool assembly has a pin or probe at one end which can be used to perform a friction stir welding process and has some means distal to the pin or probe which permits it to be attached to a spindle so the tool assembly can be rotated. The tool holder 2 comprises a shank which may be solid and cylindrical. The purpose of the shank is to facilitate connection of the tool holder 2 to FSW machinery. The protective bushing 8 is as described above.

The stationary shoulder, protective bushing and the friction stir welding apparatus as described herein may be used in a friction stir welding process. In particular, the stationary shoulder, protective bushing and the friction stir welding apparatus as described herein may be used for friction stir welding processes involving joining ferrous metals, such as steel, or non-ferrous metals, such as aluminium, magnesium, titanium and copper or alloys thereof. The stationary shoulder, protective bushing and the friction stir welding apparatus as described herein may also be used for friction stir welding processes involving polymers, for example thermoplastic polymers. The stationary shoulder, protective bushing and the friction stir welding apparatus 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 ferrous metals, 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 ferrous metals, 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 stationary shoulder for friction stir welding, comprising: a first body comprising a first surface, a second surface, and a throughhole extending from the first surface of the first body to the second surface of the first body; and a second body comprising a first surface which is configured to abut the second surface of the first body, a second surface, and a throughhole extending from the first surface of the second body to the second surface of the second body; wherein the throughholes of the first and second bodies are coaxial about a longitudinal axis and are configured to receive a friction stir welding tool assembly; wherein a rolling-element bearing is mounted on the first body in the throughhole; wherein the lower surface of the first body comprises a protrusion and the upper surface of the second body comprises a corresponding recess, or vice versa; and wherein the first body is connected to the second body by means of one or more compressible fasteners.

2. The stationary shoulder of claim 1 , wherein the protrusion and recess are annular.

3. The stationary shoulder of claim 1 or claim 2, wherein the protrusion and recess are adjacent to the throughhole.

4. The stationary shoulder of any one of the preceding claims, wherein the rollingelement bearing is a ball bearing or a roller bearing.

5. The stationary shoulder of claim 4, wherein the roller bearing is a cylindrical roller bearing, a spherical roller bearing, a tapered roller bearing or a needle roller bearing.

6. The stationary shoulder of claim 1 , wherein the rolling-element bearing is a cylindrical roller bearing.

7. The stationary shoulder of any one of claims 1 to 6, wherein the compressible fasteners are spring-loaded pins.

8. The stationary shoulder of any one of the preceding claims, further comprising one or more guide pins inserted through the first body and received by the second body.

9. The stationary shoulder of any one of the preceding claims, further comprising a sleeve adjacent to or abutting the surface of the bearing which faces the throughhole.

10. The stationary shoulder of claim 9, wherein the sleeve comprises adjusting means configured to adjust the displacement of the friction stir welding tool assembly along the longitudinal axis.

11 . The stationary shoulder of claim 10, wherein the adjusting means are one or more axial pins.

12. The stationary shoulder of any one of the preceding claims, further comprising a cap, wherein the cap has a first surface and a second surface, wherein the second surface of the cap abuts or is adjacent to the first surface of the first body, and wherein the cap comprises a throughhole extending from the first surface to the second surface, wherein the throughhole is coaxial about the longitudinal axis and is configured to receive the friction stir welding tool assembly.

13. An apparatus for stationary shoulder friction stir welding, comprising: the stationary shoulder of any one of the preceding claims; and a friction stir welding tool assembly comprising a tool insert and a tool holder.

14. The apparatus of claim 13, further comprising a protective bushing, the protective bushing comprising: a connection member configured to connect the protective bushing to the stationary shoulder; and a contact member configured to contact a workpiece to be welded; wherein the protective bushing has a first surface, a second surface and a throughhole extending therebetween.

15. The apparatus of claim 13 or claim 14, wherein the friction stir welding tool assembly further comprises a collar.

16. The apparatus of claim 15, wherein the friction stir welding tool assembly further comprises a retention means to attach the collar to the tool holder.

17. The apparatus of any one of claims 13 to 16, wherein the tool insert comprises a body portion and a connection member; wherein the tool insert has a longitudinal axis of rotation; wherein the body portion has first and second boundaries spaced apart along the longitudinal axis; wherein the longest linear dimension of the first boundary is less than the longest linear dimension of the second boundary such that the body portion tapers along the longitudinal axis between the first boundary and the second boundary; wherein the second boundary of the body portion abuts or is integral with the connection member; and wherein the connection member is configured to mate with a tool holder so as to prevent rotation of the tool insert relative to the tool holder when in use.

18. The apparatus of any one of claims 13 to 17, further comprising a stirring pin, wherein the first boundary of the body portion abuts or is integral with the stirring pin.

19. Use of the stationary shoulder for friction stir welding of any one of claims 1 to 12 or the apparatus of any one of claims 13 to 18 in a friction stir welding process.

20. A protective bushing for stationary shoulder friction stir welding, comprising: a connection member configured to connect the protective bushing to a stationary shoulder; and a contact member configured to contact a workpiece to be welded; wherein the protective bushing has a first surface, a second surface and a throughhole extending therebetween.

21 . The protective bushing of claim 20, wherein the connection member comprises an annulus which is configured to connect the protective bushing to the stationary shoulder.

22. The protective bushing of claim 20 or claim 21 , wherein the connection member and/or the contact member comprises, consists of, or consists essentially of a refractory metal or a refractory metal alloy.

23. The protective bushing of claim 22, wherein the refractory metal is one or more selected from the group consisting of: Ti, V, Cr, Mn, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os and Ir.

24. The protective bushing of claim 23, wherein the refractory metal is one or more selected from the group consisting of: Nb, Mo, Ta, W and Re.

25. The protective bushing of claim 24, wherein the connection member and/or the contact member comprises W or a W alloy.

26. The protective bushing of claim 25, wherein the connection member and/or the contact member comprises an alloy of W and one or more of the following: Ni, Fe, Re, Cu, Co and Mo.

27. The protective bushing of claim 25 or claim 26, wherein the W alloy comprises at least 90 wt.% W, for example at least 92.5 wt.% W, for example at least 95 wt.% W, for example 98.5 wt.% W.

28. The protective bushing of claim 27, wherein the W alloy consists of from 90 wt.% to 98.5 wt.% W, from 2 wt.% to 7 wt.% Ni, from 0 wt.% to 5 wt.% Fe, from 0 wt.% to 5 wt.% Co, from 0 wt.% to 5 wt.% Mo, and unavoidable impurities.

29. The protective bushing of claim 25, wherein the W alloy is a W base, high-density metal as defined in ASTM B777-07.

30. The protective bushing of claim 20 or claim 21 , wherein the connection member and/or the contact member comprises, consists of, or consists essentially of a superhard material.

31. The protective bushing of claim 30, wherein the superhard material is polycrystalline boron nitride (PCBN) or polycrystalline diamond (PCD).

32. The protective bushing of any one of claims 20 to 31 , wherein the contact member has a substantially square or rectangular cross-section.

33. The protective bushing of any one of claims 20 to 31 , wherein the contact member has a substantially V-shaped cross-section.

34. An apparatus for stationary shoulder friction stir welding, comprising: a stationary shoulder which comprises first and second surfaces and a throughhole therebetween; and the protective bushing of any one of the preceding claims; wherein the protective bushing is connected to the stationary shoulder by means of the connection member.

35. The apparatus of claim 34, further comprising a friction stir welding tool assembly which is inserted through the throughholes of the stationary shoulder and the protective bushing, the friction stir welding tool assembly comprising a stirring pin.

36. The apparatus of claim 34 or claim 35, wherein the distance between at least a portion of the inner surface of the throughhole of the protective bushing and at least a portion of the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole is less than or equal to 1 % of the diameter of the throughhole of the protective bushing.

37. The apparatus of claim 36, wherein the distance between the inner surface of the throughhole of the protective bushing and the outer surface of the friction stir welding tool assembly which is adjacent to the inner surface of the throughhole is less than or equal to 1% of the diameter of the throughhole of the protective bushing.

38. Use of the protective bushing according to any one of claims 30 to 33 or the apparatus of any one of claims 34 to 37 in a method of stationary shoulder friction stir welding.

39. Use of the protective bushing or the apparatus according to claim 38 in a method of stationary shoulder friction stir welding of steel.

40. A stationary shoulder for friction stir welding, comprising: a first body comprising a first surface, a second surface, and a throughhole extending from the first surface of the first body to the second surface of the first body; and a second body comprising a first surface which is configured to abut the second surface of the first body, a second surface, and a throughhole extending from the first surface of the second body to the second surface of the second body; wherein the throughholes of the first and second bodies are coaxial about a longitudinal axis and are configured to receive a friction stir welding tool assembly; wherein a rolling-element bearing is mounted on the first body in the throughhole; and wherein the lower surface of the first body comprises a protrusion and the upper surface of the second body comprises a corresponding recess, or vice versa.