FSW: Invention, Innovations and Industrialisation

E-mail: friction@twi.co.uk
www.frictionstirwelding.co.uk/ and www.eurostir.co.uk

Paper presented at Seminar 'Rührreibschweißen (FSW) - ein modernes Fügeverfahren' (Friction welding - a modern joining process) at Schweißtechnische Lehr- und Versuchsanstalt (SLV) Berlin-Brandenburg, 20 March2002

German version

Abstract

Friction stir welding (FSW) has been developed since its invention at TWI in 1991 ^[1,2] to a stage where it is now applied in production. Currently 67 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and they have filed more than 440 patent applications related to FSW. ^[3] Research and production FSW machines are commercially available and include installations for welding up to 16m lengths.

Introduction

Friction stir welding (FSW) is a patented joining process, which was invented at TWI ten years ago ^[1,2] ( Fig.1). The welds are made below the melting point in the solid phase. The excellent mechanical properties and low distortion are attributed to the low heat input and the absence of melting.

 

In the aerospace industry, large tanks for launch vehicles are being produced by FSW from high-strength aluminium alloys. The first Boeing Delta II rocket with a friction stir welded Interstage Module was successfully launched in August 1999 and one with three pressurised FSW tanks was launched in April 2001.

In the shipbuilding and rolling stock industry several companies now exploit the process, e.g. for large prefabricated aluminium panels, which are made from aluminium extrusions.

The automotive Tier 1 suppliers use FSW now in the high-volume production of light alloy wheels and seat frames and are considering its use for tailored blanks, suspension parts and components of aluminium space frames.

Design of Whorl ^TM and MX Triflute ^TM Tools

Instead of using a cylindrical 'pin' for FSW tools, a 'probe' can be used when producing FSW tools (the more generic term 'probe' includes for example, truncated cones, non-round cross sections, conical spirals and whisks). In an investigation of the Whorl ^TM family of tools, ^[4,5,6] trials were carried out with the tool configurations shown in Figures 2 and 3. The frustum shaped tool probes incorporate a helical ridge profile designed to augur the plasticised weld metal in a downward direction. Some probes also have side flats, or re-entrant features, to enhance the weld metal flow path.

Multi-helix tools, which TWI trademarked MX Triflute ^TM , have an odd number of relatively steeply angled flutes and incorporate a coarse helical ridge around the flutes' lands ( Figure 4). ^[7] These probe profiles reduce the tool volume and therefore aid the material flow, as well as the break-up and dispersion of workpiece surface oxides.

 

MultiStage ^TM Tools, e.g. for Lap Welding

TWI has already developed a number of lap welding tools in its Core Research Programme ^[8] and is now systematically working on advanced probe shapes. One possible feature of FSW tools for lap welds is the application of a second 'shoulder' located at the interface region between the two plates. The lower part of this MultiStage ^TM pin is reduced in diameter and has a pentagonal flattened profile to stir up the oxides and to improve the material flow. The benefits of this were demonstrated in an industrialisation study for Fokker Space, ^[9] where TWI conducted trials with 2.5mm thick7075-T7351 sheets ( Figs.5-7).

New Skew-stir ^TM Motion to Improve the Material Flow

The Skew-stir ^TM variant ^[10] of friction stir welding differs from the conventional method in that the axis of the tool is slightly inclined from the axis of the machine spindle. The face of the shoulder is however perpendicular to the axis of the machine spindle ( Fig.8).

The Skew-stir ^TM tool does not rotate on its own axis, and therefore only a specific part of the face of the probe surface is directly involved in working the workpiece material. Consequently, the inner part of the tool can be cut away to improve the flow path of material during welding. This results in an asymmetrically shaped probe ( Fig.9).

 

Friction skew-stir ^TM welding increases the extent of the plasticised material surrounding the probe. The skew-stir ^TM motion, therefore, provides a method of increasing the 'dynamic to static volume ratio' of the probe. Traditionally, when using centric rotation, this ratio is provided by the geometry of the probe because of its re-entrant features.

Control of the Tool Heel Plunge Depth

One of the most critical settings to achieve successful friction stir welds is the position of the tool shoulder relative to the work piece surface. TWI has introduced a mechanical position control system, using one or two rollers beside or in front of the tool ( Fig.10). These rollers guarantee that the tool does not plunge too deep into the workpiece and that the plasticised material is sufficiently forged underneath the tool shoulder.

The trend is now towards on-line force and torque measurement for data monitoring and closed-loop control. Especially on articulated arm robots and transportable machines, which may not be rigid enough to withstand deflection, load cells or more complex systems can be installed. TWI has experimented with a Kistler rotating dynamometer and a force measurement table, both of which use piezo crystals for measuring the forces and torque ( Fig.11).

On rigid machines, the tool heel plunge depth can be kept constant by either position or force control or by a combination of both. On these machines rollers are not necessary for maintaining the tool heel plunge depth. Hydraulically adjustable rollers can be used for pressing the sheets onto the backing bar, to avoid bulging of the workpieces near the root of the weld ( Fig.12). These rollers are operated with a compliant system to compensate for changes in material thickness.

Workpiece Materials

Friction stir welding is, as rotary friction welding, a solid phase process, which operates below the melting point of the workpiece material. It can weld all aluminium alloys, including those such as aluminium-lithium alloys that cannot normally be joined by conventional fusion welding techniques. Dissimilar aluminium alloys can also be joined (for example 5000 to 6000 series or even 2000 to 7000 series). No shielding gas or filler is required for welding aluminium alloys. TWI has developed FSW for aluminium alloys in the thickness range of 1.2mm to 75mm.

The process can also be applied to copper, magnesium, zinc and lead. Pilot trials on titanium and steel ^[13,14] are showing considerable success. Preliminary trials have also yielded encouraging results when FSW was used to join aluminium based metal matrix composites (MMCs), and when the process was applied to dissimilar materials such as cast magnesium alloy to extruded aluminium alloy.

Weld Quality

The weld nugget strength in the as-welded condition can be in excess of that in the heat affected zone. In the case of annealed materials, tensile tests usually fail in the un-affected material well away from the weld and heat affected zone. The weld properties of fully hardened (cold worked or heat treated) alloys can be improved by controlling the thermal cycle, in particular by reducing the annealing and overageing effects in the thermomechanically affected zone, where the lowest hardness and strength are found after welding. For optimum properties, it would seem that, for the latter, a heat treatment after welding is the best choice, although it is recognised that this will not be a practical solution for many applications.

Typical tensile properties of friction stir welded 5000, 6000 and 7000 series alloys are given in Table 1. The studies have been conducted by TWI, ^[15] Gränges Technology in Finspång, Sweden, ^[16] and Hydro Aluminium in Håvik, Norway. ^[17] They show that for solution treated plus artificially aged 6082-T6aluminium by post weld heat treatment a tensile strength similar to that of the parent material could be achieved, although the elongation was not fully restored. A further improvement was possible when weld specimens were made from solution treated and naturally aged 6082 base metal in the T4 condition and then after welding subjected to normal ageing. Natural ageing at room temperature led, in the recently developed 7108 aluminium alloy, to a similar effect which resulted in a tensile strength of 95% of that of the base material.

Table 1. Typical strengths of FSW aluminium testpieces
Material	0.2% Proof strength MPa	Tensile strength MPa	Elongation %	Welding factor UTS FSW / UTS Parent
5083-O Parent [15]	148	298	23.5	N/A
5083-O FSWed [15]	141	298	23.0	1.00
5083-H321 Parent	249	336	16.5	N/A
5083-H321 FSWed	153	305	22.5	0.91
6082-T6 Parent [16]	286	301	10.4	N/A
6082-T6 FSWed [16]	160	254	4.85	0.83
6082-T6 FSWed and aged [16]	274	300	6.4	1.00
6082-T4 Parent [16]	149	260	22.9	N/A
6082-T4 FSWed [16]	138	244	18.8	0.93
6082-T4 FSWed and aged [16]	285	310	9.9	1.19
7108-T79 Parent [17]	295	370	14	N/A
7108-T79 FSWed [17]	210	320	12	0.86
7108-T79 FSWed naturally aged [17]	245	350	11	0.95

 

Fatigue tests on friction stir welds made from 6mm thick 5083-O and 2014-T6 have been conducted. ^[15] . The fatigue performance of friction stir butt welds in alloy 5083-O was comparable to that of the parent material when tested using a stress ratio of R=0.1. Despite the fact that the fatigue tested friction stir welds were produced by a single pass from one side, the results have substan-tially exceeded design recommendations for fusion welded joints. ^[18] Analysis of the available fatigue data has shown that the performance of friction stir welds is comparable with that of fusion processes, and inmost cases substantially better results, with low scatter, can be obtained.

The outstanding fatigue results can only be achieved if the root of butt welds is fully bonded. As known from other welding processes, it is also essential in FSW to avoid root flaws. If the pin is too short for the actual material thickness, the workpieces are only forged together without stirring up the oxide layers. These flaws are difficult to detect by non-destructive testing. In cases of large variations in sheet thickness, it could be even necessary to have extendable pins, which can be adjusted dependent on the actual sheet thickness. ^[19,20,21] A suggestion has been made to machine chamfers on the bottom edge of the workpieces ^[22] or to grind a groove into the backing bar in order to avoid root defects. ^[23] Engraving of the backing bar enables the user to imprint information about the manufacturer and the FSW machine at the bottom of the weld. ^[24] For filling gaps between the workpieces a slight thickness increase in the joint area seems advantageous ^[25] or a lateral reciprocating motion perpendicular to the weld direction could be investigated. ^[26]

Commercial FSW Machines

Up to 16m long SuperStir ^TM machines have been designed, built, and commissioned by Esab in Laxå, Sweden. The first Esab SuperStir ^TM machine has been installed at Hydro Marine Aluminium. Five of them have been installed at The Boeing Company for welding fuel tanks of spacecraft. ^[27,28,29] These include one large horizontal machine for welding Delta II fuel tanks from inside and two vertical machines for welding Delta IV tanks from outside ( Fig.13).

Two Esab SuperStir ^TM machines have been installed at Sapa, and one of them is used for the production of large panels and heavy profiles with a welding length of up to 14.5m and a maximum width of 3m. This machine has three welding heads, which means that it is possible to weld from two sides of the panel at the same time, or to use two welding heads (positioned on the same side of the panel) starting at the centre of the workpiece and welding in opposite directions. Using this method, the productivity of the FSW installation is substantially increased. The double-sided concept is also available at the Esab SuperStir ^TM machine that was installed at Tower Automotive in Milwaukee, Wisconsin.

A Powerstir ^TM machine was tailor made by Crawford Swift in Halifax (UK) and was delivered in autumn 1999 to Airbus UK in Filton, for fabricating prototype aluminium wings and fuselage skins for large aircraft,among them the future Airbus A380. The FSW machine was named '360' which refers to its 3-axis CNC capability and 60kW spindle power. The mechanics withstand up to 100kN (10t) downward force with minimum deflection, giving the machine good thick-section welding capability ( Fig.14).

The first examples of Esab's newest series of large gantry machines have now been commissioned at TWI (8 x 5 x 1m, Fig.25) ^[30] and at DanStir in Co-penhagen, Denmark (15 x 3 x 1m,Figs.15 & 16). ^[12]

MTS Systems in Eden Prairie, Minnesota, have developed and built two hydraulically operated FSW machines, ^[31] one of which has been installed at the University of South Carolina ( Fig.17). The system includes a proprietary head assembly with an adjustable, self-load-reacting pin tool (licensed from NASA), and a multi-axis FSW welding head manipulation system. The machine enables FSW development for non-planar and variable thickness structures. The head can be automatically tilted by ±15° and the adjustable pin can supply forces of up to 90kN (9t) at a stroke of more than 30mm. When using conventional pin tools, adownward force of up to 130kN (13t) can be applied. The rotation speed can be varied to up to 2000rev/min at a maximum torque of 340Nm. The machine is used in the NASA EPSCoR programme at the University of South Carolina, which began in April 1997.

Eclipse Aviation Corporation of Albuquerque, New Mexico, has awarded MTS Systems a contract for another unique friction stir welding system for the fabrication of aircraft structures ( Fig.18). This award completes a three-year joint development activity, in which MTS and Eclipse have researched and proved the efficiency and reliability of friction stir welding in the fabrication of structural wing and fuselage members for the revolutionary Eclipse500 jet. ^[32,33]

MCE Technologies Inc in Seattle, Washington, offer production FSW equipment ( Figs.19-21). ^[35] So far two of their machines have been installed at the Marshall Space Flight Center in Huntsville, Alabama. These advanced-technology systems are being used to weld the next generation fuel tanks for the manned Space Shuttle. Initial proof-of-concept tests with actual Space Shuttle fuel tank segments are now being performed. An estimated 3000kg increased payload return will be realised through the use of the patented FSW technology.

The General Tool Company in Cincinnati, Ohio, has produced the first FSW machine with a vacuum clamping table and demonstrated its advantages in the commercial production of aluminium panels made from extrusions joined to wrought sheet ( Fig.22). They are currently building 3 large tank welding machines for space launch vehicles for a prestigious customer ( Fig.23).

International Collaborative Projects

Seven large collaborative projects have been launched in Europe to assess the advantages and limitations of FSW. The acronyms and titles of these projects are shown in Table 2, and Internet links to their proposals are also given:

 

Table 2: Research group projects on FSW
Acronym	Title and url of research proposals Forschungsvorschläge	Value [€]
EuroStir ®	'European Industrialisation of Friction Stir Welding' www.eurostir.co.uk und www3.eureka.be/Home/projectdb/PrjFormFrame.asp?pr_id=2430 (braucht 30sec)	6,8Mio
QualiStir TM	'Development of Novel Non Destructive Testing Techniques and Integrated In-line Process Monitoring for Robotic and Flexible Friction Stir Welding Systems'	2,0Mio
AMTT User 27	'Characterisation of Friction Stir Welded and Laser Welded Aluminium Joints' www.arcs.ac.at/0xc1aa8791_0x000bf90b
WAFS	'Welding of Airframes by Friction Stir' Sucbegriff 'WAFS' auf dbs.cordis.lu/EN_PROJl_search.html eingeben	5,1Mio
JOIN-DMC	'Joining Dissimilar Materials and Composites by Friction Stir Welding' Suchbegriff 'JOIN-DMC' auf dbs.cordis.lu/EN_PROJl_search.htmleingeben	2,0Mio
TANGO	'Technology Application for the near term Business Goals of the Aerospace Industry'	88,0Mio
MAGJOIN	'New Joining Techniques for Light Magnesium Components' Suchbegriff 'MAGJOIN' auf dbs.cordis.lu/EN_PROJl_search.htmleingeben	3,0Mio

 

The QualiStir ^TM Project on Development of Non-Destructive Testing Techniques and In-line Process Monitoring

For automated FSW manufacturing cells a novel FSW system will be developed in a Collaborative Project called QualiStir ^TM . This project is managed by TWI and is jointly funded by an industrial consortium and the European Commission under the CRAFT Initiative (Co-operative Research Action for Technology). The QualiStir ^TM system will be able to control the FSW process by monitoring key weld parameters and will be designed to be easily interfaced with either robots or FSW machines. The system will provide automated in-process monitoring and non-destructive testing (NDT) suitable for welding complex three-dimensional geometries. The NDT techniques applied are based on novel phased array designs and will be able to detect all defects associated with friction stir welding.

The EuroStir ^® Project on European Industrialisation of Friction Stir Welding

The overall objective of the EuroStir ^® project is to accelerate the use of friction stir welding in Europe. ^[37] FSW will be applied to a range of materials and will be researched to achieve high welding speeds in increased joint thickness. The process will be industrialised for real components and applied in commercial production.

EuroStir ^® was launched in December 2000 and will last for 5 years. It is part-funded by EUREKA, which is a pan-European initiative for promoting collaborative research in advanced technology. EUREKA aims to improve Europe's competitiveness in global markets for civil applications of new technology. The Research and Development Phase of the €6.8M project will take 2½years, during which six Tasks will be addressed ( Table 3). The Task objectives are to demonstrate weldability by feasibility studies with both robots and gantries, and to develop methods and procedures for weld assessment and quality assurance ( Table 4). The project currently has 33 collaborators and is open for further participants from EUREKA countries.

Table 3: Tasks in the EuroStir project
(1) Tool development
(2) Workpiece materials range
(3) FSW productivity
(4) FSW flexibility
(5) Weld properties
(6) Dissemination

Table 4: Objectives of the EuroStir project, financed by UK (51%), France (21%), Germany (10%), Sweden (9%), Denmark (7%) and Poland (2%) (a) High speed welding (>2 m/min) of aluminium sheet
(b) Welding of aluminium alloys of over 20 mm in thickness
(c) Friction stir welding of dissimilar materials e.g. cast to rolled sheet, aluminium to magnesium or steel
(d) Robot welding in three dimensions and power compensating friction stir welding with a shaped 'bobbin' tool
(e) Improvement of mechanical properties e.g. in the HAZ
(f) Development of a procedure for Ti, Ni, stainless steels and ferritic steels
(g) Industrialisation of (a)-(f) for real components and application in series production
(h) Industrial usage by 50% of the participants within 5 years

 

Deliverables will be proven welding procedures for test pieces and prototypes in comprehensive detail. Equally important will be the comparison between types of equipment ( Figs 24 & 25), which will enable potential users to make investment choices. The role of the job shops in providing a service to potential users has been proven to be catalytic to investment. A vital project achievement will be the establishment of at least 25 user organisations in Europe within 5 years.

The Dissemination Phase of the EuroStir ^® project will be mainly funded by industry and is planned to take also 2½ years. It will involve seminars, workshops, provision of job shop services and low cost feasibility studies for potential users who choose not to proceed via the job shop route. Manufacturing economics will feature strongly in this Phase.

Conclusions

• Friction stir welding is being exploited in the shipbuilding, automotive and rolling stock sectors to produce aluminium panels from 6000 and 5000 series extrusions.
• The aerospace industry is applying friction stir welding successfully for the serial manufacture of spacecraft made from high-strength aluminium alloys and is investigating its application for civil and military aircraft.
• FSW can also be used for copper, lead, magnesium, zinc and titanium alloys, as well as for steel and stainless steel.
• The experiments with Whorl ^TM , Triflute ^TM , MultiStage ^TM , and Skew-stir ^TM tools have produced promising results and proved that the FSW process can be applied for joining 1-50mm thick aluminium plates in one pass.
• A total of 33 companies has teamed up in the EuroStir ^® project, to get friction stir welding out of the laboratories and into the industrial manufacturing workshops.

References

1. Thomas W M, Nicholas E D, Needham J C, Murch M G, Temple-Smith P and Dawes C J (TWI): 'Improvements relating to friction welding'. European Patent Specification EP 0 615 480 B1. l2.espacenet.com/dips/viewer?PN=EP0615480
2. Midling O T, Morley E J, Sandvik A (Norsk Hydro, rights transferred to TWI): 'Friction stir welding'. European Patent Specification EP 0 752 926 B1. l2.espacenet.com/dips/viewer?PN=EP0752926
3. 440 FSW patents is available
4. Thomas W M, Nicholas E D, Needham J C, Temple-Smith P, Kallee S W K W, Dawes C J (TWI): 'Friction stir welding', UK Patent Application GB 2 306 366 A. l2.espacenet.com/dips/viewer?PN=GB2306366
5. Connect, July/August 2000.
6. Connect No. 113, July 2001, p.3.
7. TWI Bulletin, Nov/Dec 2000. (for TWI Industrial Members only)
8. www.danstir.dk/superstir.htm and www.eurostir.co.uk/eurostir_danstir.html
9. Connect September/October 1998.
10. Connect Nov 97.
11. Bulletin, November/December 1995. (for TWI Industrial Members only)
12. Backlund J, Norlin A, Andersson A: 'Friction stir welding - weld properties and manufacturing techniques'. 7th Inalco Conference, Cambridge, 15-17 April 1998, ISBN 1 85573 417 6, www.woodhead-publishing.com/engineering/welddesign.html#inalco
13. Midling O T, Oosterkamp L D, Bersaas J: 'Friction stir welding aluminium - Process and applications'. 7th Inalco Conference, Cambridge, 15-17 April 1998, ISBN 1 85573 417 6, www.woodhead-publishing.com/engineering/welddesign.html#inalco
14. 'European recommendations for aluminium alloy structures fatigue design', European Convention for Constructional Steelwork, No 68, 1992.
15. Wykes D H (Boeing): 'Adjustable pin for friction stir welding tool', United States Patent 5,697,544.
16. 'Programmable friction stir welding process', Holt E S, Lang J L (Rockwell), United States Patent 5,713,507.
17. Colligan K J (Boeing): 'Friction stir welding tool for welding variable thickness workpieces', United States Patent No 5,781,366.
18. Rosen C D (Rockwell): 'Friction stir welding total penetration technique', United States Patent No 5,611,479.
19. Colligan K J (Boeing): 'Weld root closure method for friction stir welds', European Patent Application EP 0 810 054 A1.
20. Thomas W M, Fielding P B, Nicholas E D and Temple-Smith P: 'Friction stir - the way ahead. Identify yourself with FSW'. Connect, Jan 99,
21. Enomoto M, Tasaki S, Nishikawa N, Iwai I, Hashimoto T (Showa): 'Friction welding method', European Patent Application EP 0 867 254 A1.
22. Kawasaki T, Sagawa T, Ezumi M (Hitachi): 'Friction stir welding and friction stir welding apparatus'. European Patent Application EP 0 947 280 A1.
23. www.boeing.com/defense-space/space/delta/id/inde0601.pdf (needs 30sec)
24. www.boeing.com/companyoffices/gallery/images/space/delta_iv/delta_iv_mfg.htm
25. Talwar R, Bolser D, Lederich R and Baumann J: 'Friction stir welding of airframe structures'. Second International Symposium on FSW, Gothenburg, 26-28 June 2000.
26. eurostir_nl1e.html
27. Minneman T, Beduhn B, Skinner M: 'MTS Systems receives order for friction stir welding system'. www.mts.com/pr/pr990908.html and www.mts.com/pr/2001/pr20010703_2.htm
28. Velocci A L: 'Eclipse presses ahead amid wide skepticism'. Aviation Week & Space Technology, 16 Oct 2000.
29. 'Cabin Right-Hand Panel Assembly' and 'Friction Stir Welded Lower Panel Assembly at Albuquerque'. www.eclipseaviation.com/progress/index.htm
30. Photograph: www.engr.sc.edu/research/fsw/apparatus/apparatus.html
31. www.mcetechnologies.com/stirnich.htm
32. Thompson J: 'FSW for cost savings in contract manufacturing'. Second International Symposium on Friction Stir Welding, Gothenburg, 26-28 June 2000 and www.gentool.com/pages/fabrication.html
33. The start and navigation page for information on EuroStir ^® is: www.eurostir.co.uk