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Friction stir welding - process variants

 
W M Thomas, G Verhaeghe J Martin, D G Staines and C Stanhope

TWI Ltd

Paper presented at INALCO 2007, October 24-26, Tokyo, Japan.

Abstract

Friction stir welding (FSW) is now extensively used in aluminium industries for joining and material processing applications. The (FSW) technology has gained increasing interest and importance since its invention at TWI almost 16years ago. Recent applications are reviewed and some of the variants of FSW are introduced. Furthermore, this paper describes some of the feasibility work that has been carried out to develop self-reacting, (bobbin) stir welding forwelding 25 mm thick aluminium alloy material.

Introduction

The systematic development of Friction stir welding (FSW) has led to a number of variants of the technology. The following describes preliminary studies being carried out on Bobbin stir and Friction stir-lock. [1-7]

Currently, FSW is used particularly for joining aluminium alloys in shipbuilding, marine industries, aerospace, automotive and the rail industry. Furthermore, the technology provides significant advantage to the aluminium extrusionindustry. Automotive suppliers are already using the technique for wheel rims and suspension arms. Fuel tanks joined by FSW have already been launched in spacecraft, and many other space advances are under development; commercial jetswelded by FSW have successfully completed flying trials, with high volume commercial production forthcoming. Aluminium panels for high speed ferries and panels for rail vehicles are also produced. Furthermore, friction stir welding ismaking an impact as a material processing technique.

Bobbin stir welding

Self-reacting FSW has been shown to be effective for joining hollow extrusions and lap joints. Essentially there are two types of self-reacting techniques one known as the 'bobbin tool' [1] and one known as the 'adaptive technique' (AdAPT). [5 and 7] The bobbin technique provides a fixed gap between two shoulders, while the adaptive technique enables adjustment of the gap between the shoulders during the welding operation. Figure 1 shows a fixed bobbin tool with three-sided tapered probe.

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Fig.1. Basic principle of the self-reacting 'bobbin tool'

The self-reacting principle of the bobbin technique means that the normal down force required by conventional FSW is reduced. The reactive forces within the weld are contained between the bobbin shoulders ( Figure 2).

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Fig.2. Bobbin tool showing self-contained reactive forces

Trials in 25mm thick 6082-T6 aluminium using the above arrangement produced good quality welds. Metallurgical sections showing the width of the larger diameter (drive side) shoulder and the smaller opposed shoulder are shown in Figure 3a. Higher magnification sections are shown in figure 3b and c. Unlike single sided stir welds the weld profile revealed is narrower in the mid-thickness than at the shoulder regions. Several flow features within the thermo-mechanically affected zone (TMAZ) are shown in Figure 3a.

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Fig.3. Bobbin weld in 25 mm thick 6082-T6 aluminium

a) Macrosection

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b) Detail of retreating side at mid thickness (with hardness indents)

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c) Detail advancing side at mid thickness (with hardness indents)

The hardness distribution across the transverse direction in the 25mm thick 6082-T6 aluminium weld is shown in Figure 4. The minimum hardness is located in the HAZ near the interface between the TMAZ and the HAZ.

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Fig.4. Hardness survey mid-thickness in 25mm thick 6082-T6 aluminium weld

Three point bend testing confirmed that the weld provided good mechanical integrity ( Fig.5).

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Fig.5. Three point bend test on 25 mm thick 6082 T6 aluminium alloy bobbin weld

Bobbin type tools, are similar to other standard FSW tools that are driven from one side, in that the tool behaves as a rotating cantilever. The use of a tapered probe provides for a more uniformly stressed tool which displacessubstantially less material during welding than a cylindrical pin type probe. The use of a tapered probe for the bobbin tool enables a proportional reduction in the diameter of the lower shoulder of the bobbin tool. A reduction in thelower shoulder diameter results in lower frictional contact and resistance, therefore less torque and bending moment on the tool. The additional frictional contact provided by the lower shoulder and the absence of a backing anvil,which acts as a heat sink, means that the operating temperature will be higher than that of similar conventional welds. [6] Tool design and process conditions will need to be adjusted to allow for the welding travel speed to be increased benefiting from this additional heat generation.

Bobbin welds essentially eliminate partial penetration, lack of penetration or root defects. Preliminary trials have shown that lap welds produced by the bobbin technique have less problems with the adverse orientation of the notchat the edge of the weld. This work is continuing at TWI and will be reported later.

Certain bobbin welds can reveal a mid-thickness 'blip'. [3] Non-optimised welds can also be characterised by imperfections that appear in the mid-thickness of the weld on the advancing side see Figure 6. The latter is usually caused by insufficient static and dynamic volume ratio of the probe to provide an adequate flow path.

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Fig.6. Non-optimised bobbin welds showing a mid-thickness 'blip' and imperfections. 25mm thick 6082-T6 aluminium alloy

Double driven bobbin techniques (DDB)

For certain applications bobbin tools that are driven from both ends are envisaged ( Figure 7a and b)

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Fig.7. Bobbin tool

a) Driven from both ends

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b) Driven from both ends and reactive force applied from both ends

With both sides of a fixed gap bobbin tool driven, the probe part of the tool no longer behaves as a rotating cantilever. A bobbin tool that is driven from both ends and designed for uniform stress, means that the aspect ratio ofthe probe can be altered (decrease in cross-section area and/or increase in length) to provide an improved flow path. However, while the torque and bending forces can be shared between both ends, the cross-section of the probe must beable to accommodate the reactive forces that tend to push the shoulders apart.

Double driven and double adaptive bobbin techniques (DDDAB)

The concept of a double driven bobbin also includes for the use of a double adaptive technique whereby both shoulders can be adjusted and a load applied from both ends see Figure 7b . The latter arrangement will reduce the reactive forces transmitted through the probe and enable FSW to tackle thicker plate material than currently possible. The (DDDAB) concept is expected to be able to increasethe welding speed significantly above that which is possible using conventional bobbin techniques and may even provide welding speeds faster than conventional FSW for thick plate welding.

The use of bobbin type techniques require run on and off regions for the tool to be exited from the workpiece. Bobbin techniques are best suited to flat two-dimensional applications but could be developed for more complexshapes.

Stir-lock TM

Stir-lock TM is an 'in-process' forge/forming seam joining technique. One side of the Stir-lock TM joint can be compared with riveting, whereby a rivet head is formed into a countersunk hole, for example, to provide a mechanical interlock between two or more plates. The countersunk holes are made in thecomparatively harder sheet or plate material. However, the material that forms the interlock or 'rivet head' remains integrally part of the comparatively softer, more easily formable sheet or plate material. The Stir-lock TM technique can also be applied to any perforated material. Figure 8 shows a possible application for steel-to-aluminium joining in a T-joint configuration.

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Fig.8. Stir-lock TM technique for joining dissimilar metals

Demonstration examples of steel to aluminium transition joints are shown in Figures 9 and 10.

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Fig.9. Double transition joint showing hole cross-section

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Fig.10. Single sided, Stir-lock TM aluminium-to-steel transition joint

a) Friction treated near-side, continuous weld track;

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b) Far-side showing aluminium extruded into re-entrant holes

A simple tensile test on initial samples showed promising results and failed in the steel along the line of holes. In this respect, the joint can be designed to fail in the steel or in the aluminium material, depending on the holepattern.

Composite transition joints using Stir-lock TM

Transition joints between metals and composite materials are also becoming increasingly important in the aerospace, marine and automotive industries. Using the Stir-lock TM technique, reinforcement transition joints can also be produced for composite/metal applications. Figure 11 shows a stainless steel mesh joined to aluminium sheets by friction. The mesh provides a skeleton reinforcement for the application of resin based, polymer or rubber materials. This technique differs from othertransition jointing techniques in that the reinforcement itself can provide a degree of flexibility, which can be important for certain applications e.g. for polyurethane or rubber-to-metal composite applications, where appropriatecompliance and flexibility is required.

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Fig.11. Stainless steel mesh reinforcement joined to aluminium sheets by the Stir-lock TM technique

Peel tests were carried out on initial welded samples, which showed that the mesh was substantially joined to the aluminium sheet material. Figures 12a and b show the mode of failure of the peel tested sample in which both the aluminium sheet material and stainless steel mesh have undergone significant deformation prior to joint failure. The results of test showthat the weld region remained attached to one side of the sheet, and pulled material out of the other sheet.

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Fig.12. Transition joint between stainless steel mesh and aluminium sheets

a) Weld region pull-out with embedded and part ruptured mesh

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b) Weld region attached with some embedded mesh

 

Different forms of material, such as perforated metal or other non-solid forms could be welded as an alternative to mesh. Furthermore, different steels, uncoated and coated could be welded, depending on the application and otherweldable materials could also be considered.

Discussion and concluding remarks

The basic principles and the continuing development of the FSW technology such as Bobbin stir, and friction stir-lock have been described in the paper and the following concluding remarks are made:

The bobbin technique shows promise for welding 25 mm thick aluminium plate material, using extremely low axial load.

The bobbin technique provides full penetration welds free from lack of penetration and associated root defects.

The investigation of transition joints has demonstrated the potential for using FSW for producing mechanical joints between dissimilar metals and for providing skeletal reinforcement for composite materials.

Development work will continue at TWI to ensure these techniques can be used commercially.

Acknowledgements

The Authors wish to thank C S Wiesner, I M Norris, S Smith, P J Oakley, P Evans, M J Russell, A Duncan and D Saul for their support and contributions.

References

  1. Thomas W M, Nicholas E D, Needham J C, Murch M G, Temple-Smith P and Dawes C J: 'Improvements relating to friction welding'. European Patent Specification 0 615 480 B1.
  2. Colligan, K.J. and Pickens, J.R., 'Friction Stir Welding of Aluminium Using a Tapered Shoulder Tool,' Friction Stir Welding and Processing III, eds. K.V. Jata, M.W. Mahoney, R.S. Mishra, and T.J. Lienert, TMS Annual Meeting, San Francisco, 2005, pp. 161-170.
  3. Graham L D: 'Low Cost Portable Fixed-Gap Bobbin Tool FSW Machine', poster presentation at the 86th Annual AWS Convention/2005 Welding Show.
  4. Stol I and Cobes W.J: 'Multi-shoulder fixed Bobbin tools for simultaneous friction stir welding of multiple parallel walls between parts'. United States Patent US 7,198189B2, April 3rd 2007.
  5. Thomas W M, Sylva G: 'Developments in Friction Stir Welding', ASM Materials Solutions 2003, Conference & Exposition, 13-15 October 2003 Pittsburgh, Pennsylvania, USA.
  6. Marie F, Allehaux D, and Esmiller B, 'Development of the Bobbin Tool technique on various aluminium alloys' TWI's Fifth International Symposium on Friction Stir Welding, Metz, France, 14-16 September 2004.
  7. Sylva G, and Edwards R, 'A Feasibility study for self Reacting Pin Tool Welding of Thin section Aluminium', TWI's Fifth International Symposium on Friction Stir Welding, Metz, France, 14-16 September 2004.

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