A TOOL ASSEMBLY FOR FRICTION STIR WELDING

Abstract

This disclosure relates to a tool assembly for friction stir welding. The tool assembly comprises a tool holder and a puck each having an axis of rotation. The tool holder comprises a tool post and the puck comprises a pin. The puck is coupled to the tool post. The tool assembly is adapted such that during friction stir welding, run-out of the tool holder, measured as the run-out between the axis of rotation of the tool holder and the axis of rotation of the pin, does not exceed 10 μm.

Claims

1. A tool assembly for friction stir welding, the tool assembly comprising a tool holder and a puck each having an axis of rotation, the tool holder comprising a tool post and the puck comprising a pin, the puck being coupled to the tool post, wherein the tool assembly is adapted such that during friction stir welding, run-out of the tool holder, measured as the run-out between the axis of rotation of the tool holder and the axis of rotation of the pin, does not exceed 10 μm.

2. A tool assembly as claimed in claim 1, wherein the puck is coupled with the tool post by a diffusion bond.

3. A tool assembly as claimed in claim 1, wherein the puck is coupled with the tool post by a friction weld.

4. A tool assembly as claimed in claim 1, wherein the tool holder comprises an annular joining collar mountable about the tool post and about the puck to couple the tool post and the puck in axial alignment.

5. A tool assembly as claimed in claim 4, wherein the puck and the joining collar taper correspondingly inwardly towards the tool post.

6. A tool assembly as claimed in claim 5, wherein the puck tapers at an angle θ.sub.1, angle θ.sub.1 being in the range of 2° to 15°.

7. A tool assembly as claimed in claim 4, wherein the tool post and the joining collar taper correspondingly inwardly towards the puck.

8. A tool assembly as claimed in claim 7, wherein the tool post tapers at an angle θ.sub.4, angle θ.sub.4 being in the range of 2° to 15°.

9. A tool assembly as claimed in claim 4, wherein any one or more of the tool post, puck and joining collar is circular in axial cross-section.

10. A tool assembly as claimed in claim 4, wherein any one or more of the tool post, puck and joining collar is a polygon in axial cross-section.

11. A tool assembly as claimed in claim 10, wherein the puck comprises a set of radially outwardly facing facets and the joining collar comprises a set of radially inwardly facing facets, each set of facets extending radially inwardly towards the tool post.

12. A tool assembly as claimed in claim 10, wherein the tool post comprises a set of radially outwardly facing facets and the joining collar comprises a set of radially inwardly facing facets, each set of facets extending radially inwardly towards the puck.

13. A tool assembly as claimed in claim 11, comprising six, seven or eight facets in each set.

14. A tool assembly as claimed in claim 11, wherein each facet has four sides, two of said four sides being parallel to each other, the remaining two sides converging towards each other.

15. A tool assembly as claimed in claim 11, wherein each set of facets is arranged in series about the central axis.

16. (canceled)

17. A tool assembly as claimed in claim 15, wherein sequential facets about the central axis lay side-by-side connected by a rounded intersection.

18. A tool assembly as claimed in claim 4, wherein the joining collar comprises a material with a coefficient of thermal expansion (CTE) of less than 11 ppm/° C. for temperatures up to 600° C.

19-27. (canceled)

28. The tool assembly as claimed in claim 6, wherein angle θ.sub.1 is in the range of 6° to 8°.

29. The tool assembly as claimed in claim 8, wherein angle θ.sub.4 is in the range of 4° to 7°.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0048] FIG. 1 shows a schematic side view of an assembled prior art tool assembly comprising a tool post, a puck and joining collar;

[0049] FIG. 2 shows a schematic side view of the tool post of FIG. 1;

[0050] FIG. 3 shows a schematic end view of the tool post of FIG. 2;

[0051] FIG. 4 shows a schematic side view of the joining collar of FIG. 1;

[0052] FIG. 5 shows a schematic end view of the joining collar of FIG. 4;

[0053] FIG. 6 shows a schematic side view of the puck of FIG. 1;

[0054] FIG. 7 shows a schematic end view of the puck of FIG. 6;

[0055] FIG. 8 shows a schematic side view of an assembled tool assembly in an embodiment of the invention;

[0056] FIG. 9 shows a schematic front view of the tool post of FIG. 8;

[0057] FIG. 10 shows a schematic end view of the tool post of FIG. 9;

[0058] FIG. 11 shows a schematic side view of the joining collar of FIG. 8;

[0059] FIG. 12 shows a schematic end view of the joining collar of FIG. 11;

[0060] FIG. 13 shows a schematic side view of the puck of FIG. 8;

[0061] FIG. 14 shows a schematic end view of the puck of FIG. 13;

[0062] FIG. 15 shows how angle θ.sub.1 is measured relative to the puck of FIG. 8;

[0063] FIG. 16 shows how angles θ.sub.2 and θ.sub.3 are measured relative to the joining collar of FIG. 8;

[0064] FIG. 17 shows how angle θ.sub.4 is measured relative to the tool post of FIG. 8;

[0065] FIG. 18 shows schematic end views of two alternative embodiments of the joining collar;

[0066] FIG. 19 indicates an enlarged portion of the puck of FIG. 8 and various significant external angles α.sub.1 and α.sub.2 thereof;

[0067] FIG. 20 indicates an enlarged portion of the joining collar of FIG. 8 and various significant internal angles β.sub.1 and β.sub.2 thereof;

[0068] FIG. 21 is a graph indicating the average CTE of various alloys;

[0069] FIG. 22 is a graph indicating the tensile strength of various alloys; and

[0070] FIG. 23 is a graph indicating the creep rupture properties of various alloys. In the drawings, similar parts have been assigned similar reference numerals.

DETAILED DESCRIPTION

[0071] Referring firstly to FIGS. 1 to 7, a prior art tool assembly is indicated generally at 10. The tool assembly has a central longitudinal axis 11. The tool assembly comprises an elongate tool post 12, a puck 14 and a joining collar 16 mounted about the tool post 12 and the puck 14 to secure the tool post 12 and the puck 14 in axial alignment.

[0072] Under perfect FSW conditions, the tool assembly 10 is rotational about the same central longitudinal axis 11. However, when run-out occurs, the rotational axis of the puck 14 becomes displaced, and out of alignment with the rotational axis of the tool post 12. Such misalignment is commonly understood to be measured linearly, for example, the amplitude of an oscillation about the central longitudinal axis 11.

[0073] The tool post 12 comprises conjoined first and second body portions 12a, 12b, the first body portion 12a being nearest the puck 14. The first body portion 12a is octagonal in axial (i.e. lateral) cross-section. The second body portion 12b is circular in axial cross-section. The tool post 12 s radially stepped part-way along its length.

[0074] The metal joining collar 16 is externally cylindrical and has a central bore 18 extending axially along its length, as best seen in FIGS. 4 and 5. The bore 18 is octagonal in lateral cross-section, to enable coupling with the first body portion 12a of the tool post 12.

[0075] The puck 14 is octagonal in lateral cross-section. The size of the puck 14 matches that of the first body portion 12a of the tool post 12, as shown in FIG. 1. At one opposing end of the puck 14, distant from the tool post 12, the puck 14 is shaped into a stirring pin 20. The puck tapers radially inwardly (indicated in FIG. 7 by concentric circles) to a tip, which comes into contact with the components being welded in use.

[0076] The puck 14 and the tool post 12 are separated axially by a gap 22 and secured in position relative to each other by virtue of the joining collar 16 shrink fitted onto the puck 14 and tool post 12. Conventionally, the puck 14 and tool post 12 abut one another though and are mechanically locked in place, as mentioned earlier.

[0077] Turning now to FIGS. 8 to 14, a first embodiment of the tool assembly according to the invention is indicated generally at 100. The tool assembly comprises a tool post 102, a super-abrasive puck 104 and a joining collar 106. The joining collar 106 is shrink fitted onto the tool post 102 and the super-abrasive puck 104.

[0078] The tool post 102 comprises conjoined first and second body portions 102a, 102b, best seen in FIG. 9, and the first body portion 102a is nearest the puck 104. The first body portion 102a is octagonal in axial (i.e. lateral) cross-section. The first body portion 102a is tapered radially inwardly towards the puck 104. In other words, it is a truncated pyramid with an octagonal base and flat pyramidal sides. The second body portion 102b is circular in axial cross-section and its diameter is constant along its length. At the intersection of the first and second body portions 102a, 102b, the tool post 102 is radially stepped inwardly.

[0079] The joining collar 106 is externally cylindrical and has a central bore 108 extending axially along its length, as shown in FIGS. 11 and 12. The bore 108 is octagonal in axial cross-section. However, the size of the bore is not uniform along the length of the tool post 102. The bore 108 tapers radially inwardly from one end 110 of the joining collar 106 before inflecting at or near the midway point 112 to taper radially outwardly to the other end 114 of the joining collar 106, in an hourglass manner. In this way, the bore is divided into two adjoining cavities, a first bore cavity 108a for receiving the puck 104 and a second bore cavity 108b for receiving the tool post 102.

[0080] The puck 104 is octagonal in lateral cross-section. The size of the puck 104 matches that of the first body portion 102a of the tool post 102, as shown in FIG. 1. At one opposing end of the puck 104, distant from the tool post 102, the puck 104 is shaped into a stirring pin 20. The puck 104 tapers radially inwardly (indicated in FIG. 14 by concentric circles) to a tip in a known manner.

[0081] The puck 104 and the tool post 102 are separated axially by gap 22 and secured in position relative to each other by virtue of the joining collar 106.

[0082] One feature of the invention is that the faceted super-abrasive puck 104 has a slight taper (taper angle θ1—see FIG. 15), with the corresponding bore 108 in the joining collar 106 which has facets in a tapered form (taper angle θ2—see FIG. 16), such that as the joining collar 106 expands, the super-abrasive puck 104 is pushed further into the joining collar 106 under the applied axial load, and thus remains a tight fit with the axis of the pin 116 both parallel with the axis of rotation 11 and in line with it.

[0083] The joining collar 106 may have a second slightly tapered set of facets entering from the other end (taper angle θ3—see FIG. 16), which fit to a similar set of tapered facets (taper angle θ4—see FIG. 17) on the W-C shaft 102. The design is such that both tapers allow the components to remain tightly fitted, and to this end when assembled, there remains a gap 22 between the tapered end of the W-C shaft 102 and the (smaller) tapered end of the super-abrasive puck 104 to ensure both are free to move further into the joining collar 106 to tighten in the taper.

[0084] The arrangement of the facets in the tool post 102, the puck 104 and/or the joining collar 106 is preferably rotationally periodic, with the number of facets being any number in the range four to eight inclusive, and being preferably six. For example, the left hand puck 104 in FIG. 18 has six facets X1 and the right hand puck in FIG. 18 has seven facets X1.

[0085] The facets X1 do not necessarily join at their edges, and as shown in FIG. 19; there may be a small segment of a cylindrical or conical surface X2 exposed between facets X1 forming a circular segment on any given cross-section. As a general rule, the angle of this circular segment X2 is much smaller than the angle of the facets X1, and preferably is there to simply break the corners between the facets X1 and improve robustness of individual elements 102, 104. The angle of the round sections X2 must be equal to or greater for external facets X1 on the inserted components (puck 104, tool post 106) than for similar internal facets Y1, Y2 of the joining collar 106 (see FIG. 20), to ensure a good fit between the components.

[0086] The minimum value and maximum value of the taper angle suitable for the application is set by the need to transfer sufficient torque, which provides for a minimum value of 2°, and a maximum value of 15°.

[0087] The precise angle of the tapers is significant in determining the extent to which the tapers are self-locking, and the ease with which they can be released. The two mating taper surfaces typically have the same or similar angles of taper, that is taper angle θ1 is the same or similar to taper angle θ2, and likewise is taper angle θ3 is the same or similar to taper angle θ4, but taper θ1 may differ significantly to taper angle θ3, depending on the details of the design used. The taper angles are generally chosen such that the assembly 100 self-locks under normal FSW operating conditions. That is, when the taper is under sufficient longitudinal compression, and with sufficient clearance to move, then any tendency for the joining collar 106 to expand away is mitigated by further mechanical insertion of the taper. As with most ceramic and brittle materials, super-abrasives and sintered super-abrasives are generally good under compression, so as long as the taper is designed to spread the compression load reasonably uniformly (e.g. taper angle θ1 is the same or similar to taper angle θ2), then the resulting high compression of the puck and W-C post after cool down of the tool is not an issue.

[0088] Thus, the angle of the taper can be within the range typically considered self-locking in more conventional applications, e.g. <7°, or as a result of the relatively high surface roughness of the super-abrasive composite, self-locking can be supported to slightly larger angles, up to 10°. Thus taper angles θ1, θ2 typically lie in the range 2°-15°, more typically 5°-10°, more typically 6°-8°.

[0089] In contrast, the taper angle for the tool post 102 may be smaller, since there is generally no intention to disassemble this part of the assembly. Thus, taper angles θ3, θ4 typically lie in the range 2°-15°, more typically 3°-8°, more typically 4°-7°.

[0090] Another feature of the invention is to be able to re-use the tool holder (i.e. tool post 102+ joining collar 106) and replace the super-abrasive puck 104, thereby reducing the overall cost of the tool. By re-useable, we mean that the tool holder can be used more than once for different super-abrasive pucks 104, typically 3-5 times or more. This is not possible with prior art designs of tool-holders for two reasons—i) the tool holder is not designed for removal of the puck 14, being parallel sided, and ii) the joining collar 16 invariably suffers damage from movement of the puck 14 if the puck 14 is not tightly clamped at operating temperatures. Puck 104 removal and replacement in the tool-holder does not necessarily have to be an operation suitable for the end user, provided it can be completed somewhere in the tool supply chain.

[0091] To facilitate puck 104 removal, a number of options can be adopted. For example, the joining collar 106 can be provided with two access apertures, typically located symmetrically on opposite sides of the joining collar 106, which allow the use of a wedge insert or similar to push out the puck 104. Alternatively, the tool post 102 can have a central hole running down its length, and an ejector rod can be used down this hole. A third alternative is to destructively remove the puck 104 by drilling into it and inserting an extractor pin which binds to the puck 104 using a screw thread, or expanding barbs, or similar. The precise design selected may depend on other aspects of the tool performance required, and on the type of heating used during the extraction process. The requirement to remove the puck 104 tends to push the wedge angles (θ1, θ2) associated with the puck 104 to higher angles, so that removal is made easier. The process of removing the puck 104 comprises heating the joining collar 106 to facilitate expansion and then driving the wedge in or using one of the other methods described above in order to facilitate release of the puck 104.

[0092] The means by which the tool 104 (i.e. puck) is heatable are various. One arrangement is to rapidly extract the tool 104 during a FSW operation and use the operating conditions for release. A second solution is to provide a heater module which fits around the joining collar 106 and heats it directly, either by flame, radiation, conduction or induction, in part dependent on the material used for the joining collar 106. Where suitable, induction is often the most effective solution, providing heat rapidly and directly to the component most requiring heating.

[0093] Another feature of the invention is in the choice of joining collar 106 materials. Having made the tool holder (tool post 102 and joining collar 106) re-useable, there is a much wider range of materials which can be considered commercially viable, (e.g. meeting a market acceptable price point), since more expensive materials can be considered. Conventional strong metals (e.g. based on iron) have CTE values around 11 ppm/° C., compared with CTE values of 4 ppm/° C. to 5 ppm/° C. of sintered PCBN and W-C. As such, the large difference in CTE is the major cause of the tool 104 becoming a sloppy fit at operating temperatures, with the use of a multi sided shrink fit collar. Strictly speaking, the CTE of a material is itself usually a function of temperature, and the key parameter becomes the total expansion from room temperature to operating conditions, which is equivalent to integrating the CTE as a function of temperature across the temperature change.

[0094] Although generally significantly more expensive than conventional metals, a number of bespoke alloys are known with CTE values substantially below 11 ppm/° C., at least over a portion of the temperature range from room temperature to 600° C., whilst at the same time retaining strength to high temperatures-see FIGS. 21, 22 and 23. In particular, alloys HRA 929, 909 and 903 all to varying degrees have a lower CTE at temperatures up to 600° C. than conventional steels, and 929 has a very similar CTE to W-C up to 400° C. This would minimise the risk of the collar expanding away from the PCBN or W-C elements it surrounds and mechanically clamps during normal operation, whilst still allowing for a higher temperature excursion to be used for assembly and disassembly of the tool.

[0095] In a second embodiment of the invention, the tool post 102 is sintered or diffusion bonded to the super-abrasive puck 104, and the joining collar 106 is omitted.

[0096] Since the puck 104 no longer suffers the high forces of excess run-out, or chattering impact within the joining collar 106 when it becomes loose in the joining collar 106, the toughness of the puck 104 can potentially be reduced and traded for increased wear resistance. As such, a range of other materials can be used for the metal binder within the super-abrasive puck 104. The advantage of this is that it then enables a range of other joining and assembly solutions, one option then being sintering or diffusion bonding a metal or W-C post 102 to the super-abrasive puck 104.

[0097] The sintered or diffusion bonded interface lies at some point along the longitudinal axis of the tool holder and generally orthogonal to it and rotationally symmetric about it, although particularly a sintered interface may have additional structures at the interface which break this rotational symmetry. Alternatively, it may take the form of a thin walled cone, filling the gap between two conical shaped and mating components. The interface may comprise of a single layer, or multiple layers. There remains a problem of dealing with the potential CTE mismatch between this interface layer and the rest of the assembly. Since the temperature excursion occurs mainly in connection with the puck 104 getting hot, and the puck 104 has a CTE around 4 ppm/° C. to 5 ppm/° C., then the three options are to: [0098] 1) Position the interface region sufficiently far away from the hot regions of the tool assembly in use, or to provide sufficiently effective cooling to ensure it stays cool and below a particular temperature threshold, [0099] 2) Keep the CTE of the interface region low, and in particular below a defined threshold, such that when the interface region gets hot the CTE mismatch between that and the puck is not excessive and does not cause thermal stresses sufficient to exceed the strength of the join or the adjacent components, or [0100] 3) To keep a smallest dimension of the interface region low, and below a specific threshold, such that the strain is accommodated within the interface region and the stress applied external to it is kept small.

[0101] As an example, the high strength and high entropy alloy TZM (TiZrMo) has a CTE of around 6 ppm/° C., which is fairly closely matched to the super-abrasive puck 104 (typically 4.5 ppm/° C.-5 ppm/° C.) where the CTE is dominated by the super-abrasive component such as PCBN. TZM can be used as the binder for the super-abrasive puck 104, and can also be used as the metal post 102 which is bonded to the back of the super-abrasive puck 104. Bonding may be by diffusion-bonding. Alternatively, the post 102 could be W-C, particularly in circumstances where the cost of a superalloy post would be greater than the cost of a W-C post, which depends on the particular superalloy chosen.

[0102] Diffusion bonding is a reversible process, in that at bonding temperatures it is also possible to disassemble the join if required, typically by sliding the components off sideways.

[0103] Alternatively, the super-abrasive puck 104 could be sintered to a backing layer of W-C during manufacture, and the subsequent bonding then take place to the W-C layer. One option here may be to bond to a post 102 also made of W-C, with the interface between the two W-C elements being a diffusion bond using a thin metal layer. As noted earlier, direct sintering onto a W-C post sufficiently large for mounting the tool directly into a FSW machine is difficult for tools of any significant size, (e.g. >4 mm pin length, as might be used in structural applications) because of the overall length of the shaft needed to both transfer the high torque from the FSW machine and at the same time minimise run-out would be large compared to the dimensions of the sintering capsule. However, it may be a possible solution for smaller pin lengths, such as might be used in automotive and fine metal engineering, when pin lengths of <4 mm and typically 2 mm would be appropriate.

[0104] As an alternative to more conventional metals such as the superalloy TZM, the super-abrasive binder may be a refractory high entropy alloy, comprising five or more metallic elements in a single phase metal, where the alloy remains single phase because of the high entropy (and thus low Gibbs free energy) associated with the entropy of the multiple constituents.

[0105] In a third embodiment of the invention, the tool post 102 is joined to the super-abrasive puck 104 with a friction spin join, and again, the joining collar 106 is omitted. This is where a join described above as a diffusion bonding is instead formed by using a friction spin weld or some other form of friction bonding such as a linear friction welding or ultrasonic friction welding. Such a bond would normally include a metal layer at the interface, in which the metal layer has a lower melting point than the two major elements being joined, and in which the layer has a smallest dimension which does not exceed 3 mm, preferably 2 mm, 1.5 mm, 1 mm, 0.5 mm, in part to minimise the stresses associated with the likely higher CTE of such a metal layer. Said interface layer is contiguous, and may comprise more than one material or sub-element.

[0106] For example, the interface material could be Al or Cu. In principle, the metal layer could even be steel, since friction bonding between W-C and steel has been demonstrated. The advantage of using a sufficiently low melting point metal is that, although the join may initially be formed by friction generated heating, the join may be disassembled by heating the entire unit to soften the join and then mechanically separating them, much as with the diffusion bond. Conversely, the melting point or softening point of the join material needs to be sufficiently high to not fail in tool use, although this can be supported by cooling of the tool holder as described later.

[0107] In each of the embodiments above, once a metallic element is connected to the super-abrasive puck, much more conventional solutions can be used to complete the remainder of the tool holder, for example a converter post which adapts the bespoke tool post of the FSW tool holder to a more standard sized tool holder as used on the FSW machine. A metal tool holder post also allows for a post which is tapered, but has a metal ‘key’ arrangement to transfer the torque. Typically such a metal key arrangement comprises a rectangular metal bar lying in a groove in the post taper, which groove runs in the plane of the longitudinal axis of the post and parallel to the wall of the taper, and with the rectangular metal bar engaging with a suitably matching groove in the taper within the FSW machine.

[0108] A further feature of the invention is to design a tool holder to manage and modify heat flow during operation, to reduce the deleterious effect of differential thermal expansion on reducing the binding between components, and ultimately to reduce the temperature excursion required to disassemble the tool again. This objective can be achieved in a number of ways, the first of which is to insert low thermal conductivity components, typically ceramics into the overall construction of the tool holder. A thermal barrier element, for example thin plate(s), could be inserted into the taper between the super-abrasive puck and the joining collar. This design would keep the ceramics under compression, and provide an additional option for disassembly which would be chemical attack on the ceramic spacers. Alternatively, in the gap 22 between the ends of the tool post 102 and the super-abrasive puck 104, one could place a thermal barrier element, this being a barrier to conduction, convection and/or radiation, in the form of a rock wool which was not compressed to the point of being significantly load bearing.

[0109] In addition to such passive solutions, active solutions for thermal management are also envisioned. A conventional solution would be a water-cooled jacket, either rotating with the tool and with a water feed and return that accommodate this, or static and positioned close to the tool. Alternatively water cooling could be provided down cooling channels in the post, for example by having a hole running down the centre of the post, perhaps with a tube feeding water to the bottom of the hole where the shaft attaches to the super-abrasive puck, and the return being constrained by the hole within the shaft. Methods of providing water-cooling into the centre of such a rotating shaft are known. To provide better control over the cooling effect, the liquid used may be other than water, for example an oil. One limitation of liquid cooling is that the potential phase change of the liquid to gas at the chosen pressure of operation provides a discontinuity in cooling rate and thus usually acts as an upper temperature limit on the allowable temperature at the boundary between cooled solid and cooling liquid. Such a limitation can be avoided by using gas cooling, where there is no further phase change to generate such a discontinuity in cooling effect. One option for gas cooling would be a set of fan blades, each conducting heat from the collar and driving the air motion to cool them. For safety reasons, this fan may need to be in an enclosing cylinder segment (static, or rotating along with it). Airflow would thus approximately parallel to the axis of the tool, typically directed towards the work piece, and may be used to cool the weld area as well. Rapid cooling of the weld (for example when welding under water) can result in a finer and better performing microstructure, and so the air-cooling can also be beneficial. Alternatively, gas cooling could be used down the hollow centre of the shaft replacing the water-cooling described above.

[0110] In brief, a friction stir welding tool assembly has been developed to minimise deleterious run-out during operation. This has been addressed by careful materials selection to reduce CTE mismatch and by astute structural design. The tool holder is reusable and the puck is replaceable.