GB2607671A

GB2607671A - Friction stir welding tool insert

Info

Publication number: GB2607671A
Application number: GB2204232.9A
Authority: GB
Inventors: Ghosh Santonu; Rodriguez Suarez Teresa; Åke Andersin Stig
Original assignee: Element Six UK Ltd
Current assignee: Element Six UK Ltd
Priority date: 2021-03-26
Filing date: 2022-03-25
Publication date: 2022-12-14
Also published as: GB202204232D0; GB202104259D0; WO2022200585A1

Abstract

A friction stir welding (FSW) tool insert comprising PCBN, has a longitudinal axis of rotation 12, with coaxial shoulder 16 and stirring pin 14. The shoulder 16 diameter D, and pin 14 height H, satisfy a relative inequality, whereby D < 4.5H, i.e., the ratio between i) the largest linear dimension D (36, fig 1) of the shoulder 16 measured perpendicularly to the longitudinal axis of rotation 12; and ii), the pin height H (34, fig 1) measured parallel to the longitudinal axis of rotation 12 between a base of the stirring pin 14 and a maximum point of extension of the stirring pin 14; is less than 4.5. The integral polycrystalline cubic boron nitride tool may weld steel plates at least 12 mm thick, e.g. 20 mm. The shoulder 16 may have a spiral scroll 30. The pin height H may be 98% of the thickness of the plate.

Description

FRICTION STIR WELDING TOOL INSERT

FIELD OF THE INVENTION

This disclosure relates to a friction stir welding (FSW) tool insert. In particular, it relates to a FSW tool insert for friction stir welding high temperature ferrous alloys and other high temperature alloys. More particularly, it relates to a FSW tool assembly in which the tool insert comprises polycrystalline cubic boron nitride (PCBN).

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 case of FSW in low temperature metals, the whole tool/tool holder can be a single piece of shaped tool steel, in which case it is often referred to as a 'probe'. In the case here where the tool is for welding higher temperature alloys such as steel, the tool is often in two or more parts, with an end element that is in direct contact with the material being welded, often referred to as a 'puck' or tool insert', and the remainder of the tool being the 'tool holder' which holds the puck securely and which fits into the FSW machine, so that the tool puck and tool holder together make up the 'tool' or 'tool assembly'. The tool puck is typically shaped to form a shoulder and a stirring pin, often with a reverse spiral cut into the surface so that during rotation it pulls metal towards the pin and pushes this down into the hole being formed by the pin.

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 tool traverse, when the tool moves laterally along the line in between the workp eces to be joined, and c) an extraction step, when the tool is lifted or traversed out of the workpieces.

The tool traverse, which is the stage primarily forming the weld, is usually performed under constant conditions, typically these conditions are rotational speed, conditions of the plunge, speed of traverse etc. FSW tool inserts must be capable of withstanding high axial forces required as part of the FSW process. There are many tool insert geometries available. However, they do not automatically transfer to materials such as PCBN without issue. It has been demonstrated that PCBN based tool inserts are capable of withstanding the harsh FSW operating environment, where temperatures reach in excess of 1000°C. Also, tool pucks made from PCBN are relatively cost effective and highly durable. Unfortunately, the inventors have found that PCBN tool inserts shaped using known geometries are prone to abrupt failure during use brought about by high stress concentrations experienced at key locations. This reduces tool life and introduces inconsistency, thereby rendering such tools obsolete for welding steel or superalloys.

It is therefore an aim of the invention to provide a PCBN based tool insert that addresses the 20 above-mentioned problem.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a friction stir welding (F SW) tool insert comprising polycrystalline cubic boron nitride (PCBN) and having a longitudinal axis of rotation about which it rotates during use, the tool insert further comprising a stirring pin and a coaxial shoulder region, wherein the stirring pin has a pin height measured parallel to the longitudinal axis of rotation between a base of the stirring pin and a maximum point of extension of the stirring pin, and wherein the shoulder region has a largest linear dimension measured perpendicularly to the longitudinal axis of rotation, a ratio between the largest linear dimension of the shoulder region and the pin height being less than 4.5.

The ratio between the largest linear dimension of the shoulder region and the pin height is important for PCBN tool inserts used in friction stir welding steel and other high temperature alloys. In prior art tool inserts used for friction stir welding aluminium workpieces, the aluminium flows significantly more easily than steel. Thus, those tool inserts require larger shoulder regions in order to retain the softened aluminium in the welding zone. Larger shoulders require higher applied loads, and these cause higher stresses within the tool insert, leading to more tool failures. When friction stir welding steel, it is possible to reduce the size of the shoulder region. In this way, the applied loads are reduced, and the stresses within the tool insert are consequently much lower, thereby reducing the risk of failure in the PCBN.

Optional and/or preferable features of the first aspect of the invention are provided in claims 2 to 21.

In a second aspect of the invention, there is provided a friction stir welding (FSW) tool insert for welding plate with a thickness of at least 12 mm, the tool insert comprising polycrystalline cubic boron nitride (PCBN) and having a longitudinal axis of rotation about which it rotates during use, the tool insert further comprising a stirring pin and a coaxial shoulder region, wherein the stirring pin has a pin height measured parallel to the longitudinal axis of rotation between a base of the stirring pin and a maximum point of extension of the stirring pin, and wherein the shoulder region has a largest linear dimension measured perpendicularly to the longitudinal axis of rotation, wherein the pin height is within 95% of the thickness of the plate to be welded, and wherein a ratio between the largest linear dimension of the shoulder region and the pin height is less than 4.5. Preferably, the pin height is at least 11.8 mm Optional and/or preferable features of the second aspect of the invention are also provided in claims 2 to 21.

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 side view of a tool insert in a first embodiment of theinvention; Figure 2 is a side view of the tool insert in a second embodiment of the invention; Figure 3 is a side view of the tool insert in a third embodiment of the invention; Figure 4 is an image taken from Finite Element Analysis software, indicating the maximum principle stresses incurred within a prior art tool insert; and Figure 5 is a cross-sectional view through a fourth embodiment of the tool insert; Figure 6 is a perspective view of the tool insert in a fifth embodiment of the invention; Figure 7 is a side view of the tool insert of Figure 6; Figure 8 is a perspective view of the tool insert in a sixth embodiment of the invention; Figure 9 is a side view of the tool insert of Figure 8 The Figures are not drawn to scale.

Throughout the description, similar parts have been assigned the same reference numerals.

DETAILED DESCTIPION

In Figure 1, a first embodiment of a tool insert is indicated generally at 10. The tool insert 10 has a longitudinal axis of rotation 12 about which it rotates during use in the friction stir welding process. Note that this axis of rotation is not an axis of rotational symmetry due to an asymmetric thread pattern machined into the tool insert 10.

The tool insert 10 comprises a stirring pin 14, a shoulder region 16 and a body portion 18, all in axial alignment with each other, with the shoulder region 16 being axially intermediate the stifling pin 14 and the body portion 18. The stirring pin 14, shoulder region 16 and body portion 18 are all integrally formed with each other such that the tool insert 10 is one-piece. The tool insert 10 is machined out of a single PCBN block after the block has been sintered in a HPHT press.

The stirring pin 14 has a conical profile, tapering outwardly from rounded apex 20 towards circular base 22 The stirring pin 14 comprises an inscribed spiral 24 running from the apex 20 downwards towards the shoulder region 16. The spiral 24 has an arcuate working surface 26. Notably, there is no (vertical) overhang on the spiral 24, when viewed from the side, to facilitate laser shaping from the direction of the apex 20.

The shoulder region 16 comprises a shoulder surface 28 that extends from the stirring pin base 22. In this embodiment, the shoulder surface 28 extends generally perpendicularly to the longitudinal axis of rotation 12. The shoulder surface 28 comprises optional spiral 30 (also 10 known as 'scroll') or concentric annular grooves for engaging with the workpiece during FSW.

The shoulder region 16 extends axially downwardly and merges into the body portion 18. The upper body portion 18a is initially cylindrical proximate the shoulder surface 22. The body portion 18 then steps radially outwardly into a cylindrical lower body portion region 18b, which subsequently tapers radially inwardly towards a circular base 32, remote from the shoulder surface 28. The diameter of the body portion base 32 is 18.4 mm. The body portion 18 is adapted to couple with a tool holder, which fits into a F SW machine.

The stirring pin 14 has a pin height 34, which is measured parallel to the longitudinal axis of rotation 12 between the stirring pin base 22 and a maximum point of extension of the stirring pin 14. The shoulder region 16 has a largest linear dimension 36 measured perpendicular to the longitudinal axis of rotation. The ratio between the largest linear dimension 36 of the shoulder region 16 and the pin height is less than 4.5.

For example, the pin height 34 is measured parallel to the longitudinal axis of rotation between the shoulder region 16 and the rounded apex 20. In this embodiment, the pin height 34 is 5.3 mm. The shoulder region 16 has a largest linear dimension 36 (in this case, a diameter) of 20.0 mm. Therefore, the ratio between the largest linear dimension 36 of the shoulder region 16 and the pin height 34 is 3.8.

In Figure 2, a second embodiment of the tool insert is indicated generally at 50. Tool insert 50 is similar to tool insert 10, and so only the differences are described. Unlike the first tool insert 10, here the body portion 18 is not stepped. The body portion 18 has the same diameter 36 as the shoulder region 16, and this diameter 36 is uniform throughout the height of the body portion 16. Also, the proportion of tapered body portion 16 to cylindrical body portion 16 is greater than in the first tool insert 10. Furthermore, the diameter of base 32 is 17.0 mm. Finally, in this embodiment, the pin height 34 is 5.4 mm. The shoulder region 16 has a diameter 36 of 22.8 mm. Therefore, the ratio between the largest linear dimension 36 of the shoulder region 14 and the pin height 34 is 4.2.

In Figure 3, a third embodiment of the tool insert is indicated generally at 100. In this embodiment, the shoulder region 16 slopes downwards, away from stirring pin base 22. More specifically, the shoulder region 16 is arcuate and comprises a convex shoulder surface 102.

The shoulder surface 102 has a radius R in the range of 10 to 40 mm. Preferably, the shoulder surface 102 has a radius R in the range of 15 to 35 mm.

A circumferentially extending locking groove 104 is provided in an upper region of the body portion 18, proximate the shoulder region 14, to mechanically engage with a locking collar (not shown), as part of a tool holder retention mechanism. The locking groove 104 extends around the entire circumference of the body portion 16. It is important to note however that the locking groove 104 is not essential to the invention and may be omitted.

Two segment shaped slots 106 cut into a lower end of the body portion 16. The segment shaped slots 104 are diametrically opposed to each other. Forming part of an anti-rotation mechanism, the segment shaped slots 106 cooperate with two segment shaped steps within a specially adapted FSW tool holder (not shown) when the tool insert 100 is in position and supported by the tool holder. The anti-rotation mechanism prevents relative rotational movement between the tool insert 100 and the tool holder about the axis of rotation. Again, it is important to note that the anti-rotation mechanism is not essential to the invention and may be omitted.

In this embodiment, the pin height 34 is 11.8 mm. The shoulder region 16 has a diameter 36 of 36.0 mm. Therefore, the ratio between the largest linear dimension 36 of the shoulder region 16 and the pin height 34 is 3.1. This tool is ideally suited for welding plate with a thickness of 12 mm.

Turning now to Figure 4, a tool insert with a known geometry is indicated generally at 150. During FSW, maximum principal stresses are incurred in a region 152 of the shoulder region 14. When the same geometry is applied to a PCBN based tool insert, such stresses can be catastrophic.

To mitigate this issue, in addition to some of the features described in each of the embodiments, the tool geometry may be further adapted. These adaptations are specifically intended for PCBN based tool inserts.

The external transition between the shoulder region 16 and the body portion 18 preferably extends in a curved manner. Optionally, a fillet (i.e. an arcuate surface) 200 is disposed between the shoulder region 16 and the body portion 18, for example see Figure 2. The fillet 200 has a radius of at least 1.0 mm. This fillet 200 reduces stress concentration along the peripheral edge of the shoulder region 14.

If present, spirals 30 on the shoulder region 16 preferably have a pitch height 202 (see Figure 1) measured parallel to the longitudinal axis of rotation. The pitch height 202 must be at least 0.2 mm. This helps to extract maximum life from the tool insert since the shoulder region 16 wears much faster than the stirring pin 14 in use. Preferably, the pitch height 202 is in the range of 0.2 and 0.5 mm.

As best seen in Figure 5, when the stirring pin 12 is generally conical, the stirring pin 12 has a pin angle 204 equivalent to half the internal cone angle. The pin angle must be in the range of 20 to 50 degrees Preferably, the pin angle 204 is in the range of 25 to 40 degrees Turning now to Figures 6 and 7, a further embodiment of the invention is indicated at 300. In this embodiment, the pin height 302 is 14.78 mm. The shoulder region has a diameter 304 of 35.0 mm. Therefore, the ratio between the largest linear dimension of the shoulder region and the pin height is 2.36. This tool is suitable for welding plate with a thickness of 12 mm or more since the pin height is greater than the thickness of 12 mm plate. This tool is ideally suited for welding plate with a thickness of 15 mm.

In Figures 8 and 9, a yet further embodiment of the invention is indicated at 400. In this embodiment, the pin height 402 is 19.71 mm. The shoulder region has a diameter 404 of 40.0 mm. Therefore, the ratio between the largest linear dimension of the shoulder region and the pin height is 2.02. This tool is suitable for welding plate with a thickness of 12 mm or more since the pin height is greater than the thickness of 12 mm plate. This tool is ideally suited for welding plate with a thickness of 20 mm.

In common with each of the embodiments, the pin height is sized to be within 95% of the thickness of the plate to be welded. More preferably, the pin height is sized to be within 96% of the thickness of the plate to be welded. Yet more preferably, the pin height is sized to be within 97% or 98% of the thickness of the plate to be welded.

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.

For example, transitions between the body portion 18 and the shoulder region 16 may extend in a rectilinear manner.

For example, the ratio between the largest linear dimension of the shoulder region and the pin height being less than 4.5 applies whether the shoulder surface is planar or arcuate.

Although many FSW tool inserts have a circular lateral cross-section, other shapes are envisaged as part of this invention. For example, for a tool insert having an octagonal lateral cross-section, the largest linear dimension would be a straight line between two diametrically opposed sides

Claims

CLAIMS1 A friction stir welding (F SW) tool insert comprising polycrystalline cubic boron nitride (PCBN) and having a longitudinal axis of rotation about which it rotates during use, the tool insert further comprising a stirring pin and a coaxial shoulder region, wherein the stirring pin has a pin height measured parallel to the longitudinal axis of rotation between a base of the stirring pin and a maximum point of extension of the stirring pin, and wherein the shoulder region has a largest linear dimension measured perpendicularly to the longitudinal axis of rotation, a ratio between the largest linear dimension of the shoulder region and the pin height being less than 4.5.
2. The tool insert as claimed in claim 1, wherein the ratio is in the range of 2.0 to 4.5.
3. The tool insert as claimed in any preceding claim, wherein the shoulder region comprises a shoulder surface for engaging with the workpiece during use.
4. The tool insert as claimed in claim 3, wherein the shoulder surface is planar.
5. The tool insert as claimed in claim 3, wherein the shoulder surface is arcuate. 20
6. The tool insert as claimed in claim 5, wherein the shoulder surface is concave
7. The tool insert as claimed in claim 5, wherein the shoulder surface is convex.
8. The tool insert as claimed in claim 7, wherein the shoulder surface has a radius n the range of 10 to 40 mm.
9. The tool insert as claimed in claim 8, wherein the shoulder surface has a radius in the range of 15 to 35 mm. 30
10. The tool insert as claimed in any of claims 3 to 9, further comprising a body portion extending from the shoulder surface, in a direction away from the stirring pin.
11. The tool insert as claimed in claim 10, wherein transitions between the shoulder surface and the body portion extend in a rectilinear manner.
12. The tool insert as claimed in claim 10, wherein transitions between the shoulder surface and the body portion extend in a curvilinear manner.
13. The tool insert as claimed in claim 12, comprising a fillet between the shoulder surface and the body portion, wherein the fillet has a radius of at least 1.0 mm.
14. The tool insert as claimed in any of claims 3 to 13, wherein the shoulder surface comprises one or more spirals.
15. The tool insert as claimed in claim 14, wherein the spirals have a pitch height measured parallel to the longitudinal axis of rotation, the pitch height being at least 0.2 mm.
16. The tool insert as claimed in claim 15, wherein the pitch height is in the range of 0.2 and 0.5 mm.
17. The tool insert as claimed in any preceding claim, wherein the stirring pin is generally conical, and has an apex and a base.
18 The tool insert as claimed in claim 17, wherein the stirring pin has a pin angle equivalent to half an internal cone angle, the pin angle being in the range of 20 to 50 degrees.
19. The tool insert as claimed in claim 18, wherein the pin angle is in the range of 25 to 40 degrees.
20 The tool insert as claimed in claims 17, 18 and 19, wherein the stirring pin comprises a spiral extending between the apex and base.
21. The tool insert as claimed in claim 20, wherein the spiral is devoid of any overhang