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Friction stir welding - invention, innovations and applications (March 2001)

   

Stephan W. Kallee, E. Dave Nicholas and Wayne M. Thomas
TWI Ltd

INALCO 2001
8th International Conference on Joints in Aluminium
Munich, Germany, 28-30 March 2001

Abstract

Friction stir welding (FSW) is a patented joining process, which was invented at TWI ten years ago [1,2] . A wear resistant rotating tool is used to join sheet and plate materials such as aluminium, copper and lead. In laboratory experiments, aluminium based metal matrix composites, magnesium alloys, zinc, titanium alloys and steels have also been friction stir welded. 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. As it is a solid phase process, FSW permits joining of materials that cannot be welded with conventional fusion welding processes.

In the aerospace industry, large tanks for launch vehicles are being produced by FSW from high-strength aluminium alloys, and a Delta II rocket with a friction stir welded Interstage Module has successfully been launched in August 1999. In the shipbuilding and rolling stock industry several companies now exploit the process, e.g. for the production of large prefabricated aluminium panels, which are made from aluminium extrusions. The automotive industry uses FSW now in the high-volume production of components (e.g. light alloy wheels and hollow aluminium extrusions) and considers the use for making tailored blanks, suspension parts and components of aluminium space frames. Research and production FSW machines are commercially available and include installations for welding up to 16m lengths.

Introduction

Friction stir welding (FSW) was invented and patented [1,2] in 1991 at TWI in Cambridge (UK) and has been developed to a stage where it is applied in series production. Currently 51 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and several of them exploit the process in commercial production, e.g. in Scandinavia, USA, Japan and Australia [3] . They have filed more than 285 patent applications related to FSW.

The workpieces have to be clamped onto a backing bar and secured against the vertical, longitudinal and lateral forces, which will try to lift them and push them apart. Development trials have established that a gap of up to 10% of the sheet thickness can be tolerated before weld quality is impaired.

In macrosections of good quality welds in aluminium alloys a well-developed nugget is visible at the centre of the weld, as schematically shown in Figure 1. Outside the nugget there is a thermomechanically affected zone, which has been severely plastically deformed and shows some areas of partial grain refinement [4] . The overall shape of the nugget is very variable, depending on the alloy used and the actual process conditions.

State of the art friction stir welding tools

Friction stir welding uses a non-consumable rotating tool, which moves along the joint between two components to produce high-quality butt or lap welds. The FSW tool is generally made with a profiled pin, which is contained in a shoulder with a larger diameter than that of the pin ( Figure 1). For butt joining the length of the pin approximates to the thickness of the workpiece. The pin is traversed through the joint line while the shoulder is in intimate contact with the top surface of the workpiece to avoid expelling softened material. Tools with specially profiled pins and optimised shoulder designs have been developed at TWI and are now being used in industrial applications, as they provide large tolerance envelopes. The tool related know-how, optimised welding parameters and clamping requirements have been developed during a large number of confidential studies to serve the industrial demands.

The FSW tools are manufactured from a wear resistant material with good static and dynamic properties at elevated temperature. They are made in a manner that permits up to 1000m of weld to be produced in 5mm thick aluminium extrusions without changing the tool.

The stirring effect of the tool is clearly visible in transverse macrosections if different types of materials have been welded such as aluminium extrusions to wrought aluminium sheets, or wrought aluminium sheets to cast aluminium ( Figure 2) [5] . The onion ring like structure of the nugget is typical of high quality stir welds, in which no porosity or internal voids are detectable.

Fig. 1. Friction stir welding principle and microstructure
Fig. 1. Friction stir welding principle and microstructure
Fig. 2. Transverse macrosection of 6mm thick wrought aluminium welded to cast aluminium
Fig. 2. Transverse macrosection of 6mm thick wrought aluminium welded to cast aluminium

 

The design of the friction stir welding tools is the heart of this remarkable and still relatively new welding process. The development of new tool designs needs to proceed with caution and attention to detail. Otherwise, flooding the manufacturing industry with many different design concepts with varying performances will produce conflicting reports and jeopardise the uptake of the process. The following paragraphs describe the tool development approach taken at TWI and outline the tools, which are currently being investigated in a search for advanced FSW tools for the future.

Probe and shoulder design of Whorl TM and MX Triflute TM tools

Instead of using a cylindrical 'pin', a 'probe' can be used (the more generic term 'probe' includes for example truncated cones, non-round cross sections, conical spirals and whisks). A series of specially profiled FSW tools has been designed and tested. An investigation [6,7,8] of the Whorl TM and MX Triflute TM families of tools is aimed at joining 25-40mm thick plates from aluminium alloy 6082-T6 by single sided welding, and in up to 75mm thick plates by double sided welding. Trials have been carried out with the WhorlTM tool configurations shown in Figure 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.
Fig. 3. Basic variants of TWI's new generation of Whorl TM type FSW tools for welding thick workpieces
Fig. 3. Basic variants of TWI's new generation of Whorl TM type FSW tools for welding thick workpieces

 

The Whorl TM concept provides for non-circular probe cross-sections. In this way, the probe displacement volume is less than its volume of rotation, to enable the easier flow of plasticised material. A prototype Whorl TM tool is shown in Figure 4 superimposed on a transverse section taken from a 75mm (=3inches) thick weld in aluminium alloy 6082-T6. The extent of the overall thermomechanically affected zone compared to the tool profile is evident. Acceptable mechanical and metallurgical properties were achieved using the tool shown. A more effective flow of the plasticised material around the tool is further achieved when the distance between each helical ridge is greater than the thickness of the ridge itself. The core of the probe doesn't need to run parallel with the helical ridge, nor does the ridge have to be of uniform pitch. Thus, the surface area of the helical ridge can be changed to suit different materials.

Fig. 4. Prototype Whorl TM tool, and a section of a weld in 75mm thick AA 6082
Fig. 4. Prototype Whorl TM tool, and a section of a weld in 75mm thick AA 6082

 

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 5) [9] . These reduce the tool volume further and therefore aid material flow and to break-up and disperse surface oxides.

Fig. 5. Prototype Triflute TM tool with three flutes and a helical ridge around the flutes' lands
Fig. 5. Prototype Triflute TM tool with three flutes and a helical ridge around the flutes' lands

 

The tool shoulders of the new generation of Whorl TM and MX Triflute TM tools have a sophisticated profile. The shoulder profiles under investigation are designed to provide better coupling between the tool shoulder and the workpiece. This is to provide like to like frictional contact by helping prevent plasticised material from being expelled. Improved coupling is achieved by entrapping plasticised material within special profiles, such as scoops, spirals and concentric grooves. The shoulder profiles with concentric grooves provide improved movement of the top surface layers of the plasticised workpiece material. Combined rotation and travel mean that these concentric grooves provide a series of continuous cycloidal paths on the weld track.

Workpiece materials

FSW 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 that cannot normally be joined by conventional fusion welding techniques such as aluminium-lithium alloys. Dissimilar aluminium alloys can also be joined (e.g. 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 1.2 to 75mm.

Up to 50mm thick pure copper was friction stir welded at TWI using a purpose built FSW machine that can weld prototype canisters for the encapsulation of spent nuclear fuel [10] . Circumferential welds were made at welding speeds of up to 100mm/min, accompanied by excellent weld quality, which exceeded all original expectations.

The FSW process can be applied to magnesium alloys as well as to zinc and lead [11] . A preliminary study on FSW of 9.5mm thick Elektron Magnesium alloy AZ61A has been successfully carried out at TWI. It was also possible to join magnesium to aluminium by friction stir welding, as demonstrated in an empiric feasibility study at TWI ( Figure 6). Further laboratory experiments were conducted at the Shibaura Institute of Technology in Japan [12] on hot rolled plates of 6mm thick magnesium alloy AZ31. The investigators of the latter study concluded that the strength of the friction stir welded specimens was comparable to that of the parent material. The mechanical performance of the welds was reported to be fairly insensitive to the morphology of the parent metal sheet.

Before steels and stainless steels can be friction stir welded in a production environment, further experiments have to be conducted to establish the best material for the FSW tools [13] . Once a suitable material has been identified, work on developing tool designs and optimised procedures for a number of steels can commence. This work should also examine the mechanical and metallurgical properties of the welds and provide data from which potential users can realistically estimate the costs of using the process in production. The benefits of the process for industrial exploitation will be investigated at TWI in the near future by making a number of prototype components by FSW. When butt joining aluminium to steel, a mechanical interlock can be provided by machining a dove tail into the steel plate and then traversing the tool off centre through the softer aluminium alloy ( Figure 7).

Fig. 6. Friction stir weld between an aluminium extrusion (AA2219) and a 3mm thick die cast magnesium alloy AZ91 [14]
Fig. 6. Friction stir weld between an aluminium extrusion (AA2219) and a 3mm thick die cast magnesium alloy AZ91 [14]
Fig. 7. Pilot trial on friction stir welding aluminium to a mild steel plate comprising a mechanical interlock
Fig. 7. Pilot trial on friction stir welding aluminium to a mild steel plate comprising a mechanical interlock

 

Preliminary investigations have established that the FSW technique can be applied to titanium and its alloys, and this offers the potential for a rapid, cost-effective and technically simple method of making high quality welds in titanium alloys. A current TWI project aims to develop the process to a point where it can be considered for industrial use. Initial studies have been on Ti-6Al-4V, but it is intended to investigate other alloys in due course. The main interest in these alloys stems from the aerospace industry, but producers of oil pipelines and offshore platforms would like to use friction stir welded titanium alloys when extreme corrosion resistance is required.

International collaborative projects

Five large collaborative projects have been launched in Europe to assess the advantages of FSW. The acronyms, titles and internet links of these projects are shown in Table 1:

 

Table 1. Collaborative Projects on FSW

 

AcronymTitle and WWW addresses of the proposalsValue [Euro]
EuroStir TM 'European Industrialisation of Friction Stir Welding'
http://www3.eureka.be/Home/projectdb/PrjFormFrame.asp?pr_id=2430
6.8M
WAFS 'Welding of airframes by friction stir'
Enter search term 'WAFS' at http://dbs.cordis.lu/EN_PROJl_search.html
5.1M
JOIN-DMC 'Joining dissimilar materials and composites by friction stir welding'
Enter search term 'JOIN-DMC' at http://dbs.cordis.lu/EN_PROJl_search.html
2.0M
TANGO 'Technology application for the near term business goals of the aerospace industry' 88.0M
MAGJOIN 'New joining techniques for light magnesium components'
Enter search term 'MAGJOIN' at http://dbs.cordis.lu/EN_PROJl_search.html
3.0M

 

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 unaffected material well away from the weld and heat affected zone. The weld properties of fully hardened (cold worked or heat treated) aluminium 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 aluminium alloys are given in Table 2. The studies have been conducted by TWI [15] , Sapa in Finspång, Sweden [16] , and Hydro Aluminium in Håvik, Norway [17] and results of the latter two studies have been reported in detail at the Inalco 98 conference, which was held at TWI. They show that for solution treated plus artificially aged 6082-T6 aluminium a tensile strength similar to that of the parent material could be achieved by post weld heat treatment, 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 artificial 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 2. Mechanical properties of friction stir welded Al specimens

Material0.2% 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 have been conducted on friction stir welding specimens made from 6mm thick aluminium alloys 5083-O and 2014-T6 [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 substantially 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 in most 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 necessary to have extendable pins, which can be adjusted dependent on the actual sheet thickness. A suggestion has been made to machine chamfers on the bottom edge of the workpieces or to grind a groove into the backing bar in order to avoid root defects. For filling gaps between the workpieces a slight thickness increase in the joint area seems advantageous or a lateral reciprocating motion perpendicular to the weld direction could be investigated.

Friction stir welded aluminium panels in the shipbuilding industry

The first commercial application of FSW concerned the manufacture of hollow aluminium panels for deep freezing of fish on fishing boats ( Figure 8). These panels are made from friction stir welded aluminium extrusions at Sapa [19] . The minimal distortion and high reproducibility make FSW both technically and economically a very attractive method to produce these stiff panels.

New FSW applications have been reported from Japan, where the process is used to produce aluminium honeycomb panels and seawater resistant panels ( Figure 9). The latter are made from five 250mm wide 5083 aluminium alloy extrusions to make a panel with the size of 1250 x 5000mm. They are used for ship cabin walls because of the good flatness of the weld root [20] .

Fig. 8. Sapa FSW aluminium panel for pressing of fish blocks before quick freezing. The panel is welded from both sides [19]
Fig. 8. Sapa FSW aluminium panel for pressing of fish blocks before quick freezing. The panel is welded from both sides [19]
Fig. 9. Large aluminium ship panel made from 5083-H112 aluminium alloy extrusions, made by Sumitomo Light Metal [20]
Fig. 9. Large aluminium ship panel made from 5083-H112 aluminium alloy extrusions, made by Sumitomo Light Metal [20]

 

Pre-fabricated wide aluminium panels for high-speed ferryboats can be produced by FSW and are commercially available [21] . The panels are made by joining extrusions, which can be produced in standard size extrusion presses. Compared to fusion welding, the heat input is very low and this results in low distortion and reduced thermal stresses. Annually more than 70km of defect free friction stir welds have been produced over the last three years at Hydro Marine Aluminium in Haugesund (Norway) mainly for shipbuilding panels ( Figure 10) [22,23] . After welding, the panels can be rolled for road transport, as they are stiff only in the longitudinal direction ( Figure 11).

Fig. 10. Esab SuperStir TM machine at Hydro Marine Aluminium to weld aluminium extrusions for shipbuilding panels [23]
Fig. 10. Esab SuperStir TM machine at Hydro Marine Aluminium to weld aluminium extrusions for shipbuilding panels [23]
Fig. 11. Prefabricated FSW panel for catamaran side-wall, rolled for road transport (at Hydro Marine Aluminium) [23]
Fig. 11. Prefabricated FSW panel for catamaran side-wall, rolled for road transport (at Hydro Marine Aluminium) [23]

 

Friction stir welded aluminium tanks and boosters for spacecraft

An increasing number of fuel tanks for spacecraft are now being produced from difficult-to-weld aluminium alloys. Friction stir welding procedures for welding end domes into cylindrical high-quality tanks have been developed for cryogenic oxygen storage vessels. It has been proposed to shrink fit an aluminium backing ring into the tank before the last circumferential weld is made. The joint is then simultaneously made between three members: the end dome, the cylindrical shell and the backing ring. The latter may remain in
the tank after welding. Boeing has applied FSW to the Interstage Module of a Delta II rocket, and this has successfully been launched in August 1999.

Friction stir welding is also being considered for producing Ariane 5 motor thrust frames. A study by Fokker Space [24] has shown that FSW can readily be applied to lap joints in Aluminium 7075-T7351. Although the tensile strengths measured in this investigation were lower than those that can be obtained with friction stir welded butt joints, they are at an acceptable level to replace bolted lap joints. For unpressurised structures, lap joints offer the significant advantages of generous tolerances at interfaces between components and ease of assembly. The fact that FSW can now be applied to such connections adds the advantages of a simple; low energy; automatic; high strength; high stiffness; minimum mass and low cost welding process.

Potential for using friction stir welded aluminium panels in the aircraft production

The FSW process offers tremendous potential for low-cost fastenerless joining of lightweight aluminium airframe structures. EADS sees a high potential for joining aluminium alloys by FSW for skin-to-skin fuselage connections of their future Airbus 380 production. They presented data recently that demonstrates that the mechanical and technological properties of these welds seem to approach almost the properties of the parent material [25] .

The Phantom Works of The Boeing Company are pursuing FSW of thin butt, lap and T-joints and thick butt joints for various aircraft missile and space applications. There is a strong desire for welding these joint configurations on curvilinear paths thus enabling welding of complex aircraft parts. Boeing has demonstrated curvilinear FSW of a complex aircraft landing gear door by using a patented force actuator. Boeing has also successfully demonstrated FSW of sandwich assemblies by welding thin T-joints for a fighter aircraft fairing, which has been flight tested [26] .

It has been reported that Eclipse Aviation Corporation has decided to use FSW to replace traditional riveting and bonding processes for a new generation of business jets [27] . This could be the first application of this welding process in high-volume aviation applications with the potential to dramatically lower assembly time and cost. To meet the current development schedule, which calls for first flights in 2002, Eclipse must start building structures at the end of 2001. The company will decide in the first half of 2001 whether FSW is ready for full implementation.

Friction stir welded hollow aluminium panels in the rolling stock industry

Hollow profiles and T-stiffener extrusions are used and are currently being friction stir welded in the railway industry for the manufacture of commuter and high-speed trains. Innovative joint designs have been proposed to compensate for lateral and angular offset by using the FSW process, and a Group Sponsored Project is currently being conducted to evaluate them experimentally [28,29] .

Friction stir welded aluminium components in the automotive industry

FSW is now being used in the high-volume production of aluminium automotive components at Sapa in Finspång (Sweden). They installed an Esab SuperStir TM machine, which has two welding heads to weld hollow extrusions from both sides simultaneously ( Figure 12). The machine has a carousel-type loading and unloading station and is automatically loaded by an articulated arm robot. The FSW machine is at the same site as the extrusion press, and therefore the surface quality and tolerances of the extrusions are extremely good prior to welding. The automotive components have very tight tolerances, which would be difficult to achieve with conventional fusion welding processes. Now they are produced by FSW with less than 60sec cycle time, and Sapa reported that their customer was very pleased with the weld quality. Sapa developed a quality management system based on computerised statistical control of all major dimensions of the welded workpieces, and this leads to virtually no parts being rejected or re-machined. The station for measuring the dimensions of the welded component is located directly behind the FSW cell and loaded and supervised by the operator of the FSW cell. The measured actual data is fed back into the welding machine for parameter adjustment.

In Japan Showa Aluminum and Tokai Rubber in Oyama City joined extruded end-pieces to 20-30mm diameter tubes for the manufacture of suspension arms [30] . The rubber of the suspension arm can be vulcanised before welding due to the low heat input of the new assembly method.

In Norway, an innovative technique of joining two parts of a car wheel by FSW has been invented [31] and successfully demonstrated by Hydro Aluminium in Håvik for the manufacture of prototype parts ( Figure 13) [32] . Optional design concepts have been developed to either butt or lap weld cast or forged centre parts to rims that are made from wrought alloys. Branched designs of the centre part are possible by providing two parallel friction stir welds per wheel, which run along a cavity to achieve good load transfer and weight reduction.

Fig. 12. Sapa's Esab-SuperStir TM machine with two welding heads for welding hollow aluminium extrusions simultaneously from both sides
Fig. 12. Sapa's Esab-SuperStir TM machine with two welding heads for welding hollow aluminium extrusions simultaneously from both sides
Fig. 13. Hydro Aluminium's light alloy car wheel, where the rim was friction stir welded to the hub [32] .
Fig. 13. Hydro Aluminium's light alloy car wheel, where the rim was friction stir welded to the hub [32] .

Simmons Wheels in Alexandria (Australia) [33] developed a new method of producing a car wheel rim section from a rolled aluminium 6061-O sheet. They described a method of making initially a pre-formed cylinder; cutting it into several individual rim sections; then post weld forming it into the desired rim profile; and finally subjecting this part to heat treatment to the required T6 temper.

For prefabricating rims for light alloy car wheels, Hayes Lemmerz in Michigan [34] considered chamfering the ends of the originally flat sheets prior to FSW in the weld root area to achieve full penetration. Additionally, flats could be formed on the ends to give better contact between the FSW tool shoulder and the workpiece. After friction stir welding, the rims of these lightweight wheels can be subjected to a series of flow spinning and roll forming operations with the intention to produce a generally uniform thickness wheel rim.

Recent publications released ideas about friction stir welded hem joints and sandwich panels [35] . They proposed that further applications to automotive lightweight structures, such as friction stir welded aluminium tailored blanks, should be experimentally assessed. Designers of aluminium prototype cars are considering the use of tailored aluminium blanks with a variation in thickness, although a number of problems regarding forming of the blanks have to be overcome. A smoother thickness change than that of laser welding could be achieved if a tilted FSW tool was plunged into the stepped side of the joint instead of plunging into the flat side. The advantages of the FSW process are currently being evaluated by several automotive companies and their tiered suppliers, e.g. for the manufacture of components for space frame vehicles.

Commercially available Powerstir TM and SuperStir TM machines

Esab in Laxå (Sweden) started in December 2000 with the manufacture of two new SuperStir TM machines. These machines are representing the state of the art of commercially available FSW equipment and will be used in the EuroStir TM Project [36] which is part funded by the Eureka programme and focuses on 'European Industrialisation of Friction Stir Welding'. The first contract was signed between Esab and TWI. The equipment has a gantry type design with a welding area of 8 x 5m and two heads of different sizes. The first head will be used for welding thin sheets with high rotation speeds. The second head can be used for thick sheets while applying high downward forces. The machine will be capable of welding aluminium alloys with more than 15mm thickness over the full working envelope. Up to 25mm thick aluminium plates can be welded in the centre line of the machine. It is expected that the first machine will be delivered to TWI in April 2001 and will then be used initially for test welding and laboratory operations. At a later stage the machine will be used for welding full size industrial prototypes. The machine is large enough to weld for example a complete side panel of a double-decker bus.

Esab received also another order from the first European FSW job shop, DanStir ApS in Copenhagen (Denmark). This machine is of the same gantry type design as that for TWI but is designed for a welding area of 5 x 3m and has one welding head. It has a vertical clearance under the gantry of 0.85m. It is equipped with a high-power main drive and a hydraulic actuator for vertical spindle movements. The versatile control system allows for numerous operations modes including position or force control and/or various combinations hereof. The power-full design facilitates up to 100kN (10t) down force on the parts to be welded, and up to 40kN (4t) transverse trust on the welding head. This allows the welding of all aluminium alloys in thickness up to 15mm and beyond.

A Powerstir TM machine has been tailor made by Crawford Swift in Halifax (UK) and delivered in autumn 1999 to BAE Systems in Filton (UK) where it is being used 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, and thus enable the machine for FSW of workpieces with a virtually unlimited thickness. The machine is 11.5m long x 5.7m wide x 4.7m high and takes the basic form of a moving table machine ( Figure 14). The table, onto which the workpieces are clamped, moves underneath the gantry and is accelerated by the latest servomotor and ball-screw technology to speeds of up to 8m/min.

TWI owns and operates several FSW machines to weld a wide range of workpieces. Its currently biggest laboratory machine was built to accommodate large sheets and structures ( Figure 15). It can run linear and circumferential welds on specimens with 3.0m length x 4.0m width and 1.15m height or diameter with welding speeds of up to 1.7m/min. The modular construction enables it to be enlarged for specimens with even greater dimensions.

Fig. 14. Crawford Swift's Powerstir TM machine with 3 CNC axes and 60kW spindle power. It can react up to 10t force.
Fig. 14. Crawford Swift's Powerstir TM machine with 3 CNC axes and 60kW spindle power. It can react up to 10t force.
Fig. 15. TWI's modular FSW machine to weld large workpieces.
Fig. 15. TWI's modular FSW machine to weld large workpieces.

Up to 16m long SuperStir TM machines have been designed, built, and commissioned by Esab since autumn 1996. One of them has been installed at Hydro Marine Aluminium ( Figure 10). Surveying bodies such as Germanischer Lloyd, Det Norske Veritas and Registro Italiano Navale have given approval to the welding procedure for specific applications, after successful testing of the machine in Haugesund.

Another SuperStir TM machine has been installed at Sapa and 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.

Conclusions

  • Friction stir welding is being exploited in the shipbuilding, automotive and rolling stock industry sectors to produce aluminium panels from 6000 and 5000 series extrusions.
  • The aerospace industry applies the process successfully for the manufacture of spacecraft made from high-strength aluminium alloys.
  • The experiments with Whorl TM and Triflute TM tools have to date produced promising results and proved that the FSW process can be applied for joining up to 75mm thick aluminium plates.
  • FSW can also be used for copper, lead, magnesium, zinc and titanium alloys, as well as for steel and stainless steel.

References

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  18. Dawes C J: 'Friction stir welding aluminium ships'. 3 rd Int Forum on Aluminium Ships, Haugesund, Norway, 27-28 May 1998.
  19. Sato S, Enomoto M, Kato R and Uchino K: 'Application of aluminum extrusions to suspension arms'. IBEC'98, Detroit, 29 Sept - 1 Oct 1998.
  20. Aasen E J, Morley E J, Midling O T, Sandvik A: 'Two-piece wheel'. Int Patent Appl No PCT/NO96/00254.
  21. Midling O T: 'Friction stir welding - a valuable processing route'. Aluminium '97 Conf, Sept 1997, Essen, Germany.
  22. Simmons A G (Simmons): 'Vehicle wheel rim section'. International Patent Application WO 00/03818.
  23. Lawrence G (Hayes Lemmerz): 'Wheel rim and method for producing same'. Int Patent Appl WO 99/33594.
  24. Kallee S W, Nicholas E D: 'Application of friction stir welding to automotive lightweight structures'. IBEC'98, Detroit, Michigan, 29 Sept - 1 Oct 1998, SAE P-334, Vol 5, pp191-198.
  25. Eureka Project Σ!2430: 'EuroStir® - European industrialisation of friction stir welding'. http://www3.eureka.be/Home/projectdb/PrjFormFrame.asp?pr_id=2430 (needs 90-120sec for opening).

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