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Friction stir welding - how to weld aluminium without melting it (May 2001)

   

Stephan W Kallee
TWI Ltd, Granta Park, Great Abington, Cambridge CB1 6AL, UK,

friction@twi.co.uk

Innovations for New Rail Business, IMechE, London, 24 May 2001

Abstract

Recent train accidents demonstrated the problems inherent in the use of existing technology for welding aluminium railway vehicles. This paper describes a new industrial process for welding that is being used in the construction of Japanese railway trains and that has the potential to significantly improve the crashworthiness of vehicles.

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.

In the 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. Research and production FSW machines are commercially available from several manufacturers and include installations for welding up to 16m lengths.

Introduction

Modern passenger rail cars are increasingly produced from longitudinal aluminium extrusions with integrated stiffeners. Using this concept the whole body shell can be made from either single wall or hollow double skin extrusions. The crashworthiness of these structures is reported to be much higher than that of conventional vehicles, caused by the absence of transversal welds and the high buckling strength of the panels under longitudinal compression [3] .

Large aluminium extrusions with complicated shapes are available and are being used in the manufacture of single and double deck trains. For some of the Japanese Shinkansen trains (e.g. Type 500) vacuum brazed honeycomb panels are used for the side walls below and above the windows and these are very effective for reducing the weight of the structure. However, at present, their relatively high production cost does not allow a more extensive use [4] .

Aluminium extrusions have to be welded with the lowest possible heat input, because the most commonly used alloys of the 6000 series gain their strength from a heat treatment [5] . If the heat input is too large, the strength of the heat treated alloy is reduced in the heat affected zone and problems with distortion can occur. Pulsed MIG welding is commonly used by train manufacturers. To avoid porosity in MIG welds it is important to clean the joints immediately before welding and in some cases to use a double shaved MIG wire, from which the surface layers have been mechanically removed twice during its manufacture in the drawing line [5] .

In Japan a cumulative total of 3km of friction stir welds for rolling stock of subways has been produced by 1998 at Nippon Light Metal Company in Shizuokaken and satisfied all acceptance criteria [6] . In series production 20m long extrusions for commuter train car bodies (known as A trains) are now being joined by Hitachi in Kudamatsu-city (Japan) using friction stir welding [7] and special joint designs have been developed [8,9,10] . Sapa in Finspång (Sweden) supply also European rolling stock manufacturers with friction stir welded aluminium panels.

The friction stir welding process

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 53 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 [11] . 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 [12] . 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.
Fig. 1. Friction stir welding principle and microstructure
Fig. 1. Friction stir welding principle and microstructure

 

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) [13] . 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. 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 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.

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). An investigation [14,15,16] of the Whorl TM and MX Triflute TM families of tools trials has been carried out to improve the material flow around the probe. The frustum shaped tool probes incorporate a helical ridge profile designed to augur the plasticised weld metal in a downward direction ( Figure 3). 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

 

A prototype Whorl TM tool is shown in Figure 4 superimposed on a transverse section taken from a 75mm thick weld in aluminium alloy 6082-T6. Acceptable mechanical and metallurgical properties were achieved using the tool shown. 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) [17] .

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

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 [18] . 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 [19] . A preliminary study on FSW of 9.5mm thick 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 empirical feasibility study at TWI ( Figure 6).

Fig. 6. Friction stir weld between an aluminium extrusion (AA2219) and a 3mm thick die cast magnesium alloy AZ91 [20]
Fig. 6. Friction stir weld between an aluminium extrusion (AA2219) and a 3mm thick die cast magnesium alloy AZ91 [20]

 

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 [21] . 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. 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.

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 [22] , Sapa in Finspång, Sweden [23] , and Hydro Aluminium in Håvik, Norway [24] . 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 [22] 148 298 23.5 N/A
5083-O FSWed [22] 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 [23] 286 301 10.4 N/A
6082-T6 FSWed  [23] 160 254 4.85 0.83
6082-T6 FSWed and aged [23] 274 300 6.4 1.00
6082-T4 Parent [23] 149 260 22.9 N/A
6082-T4 FSWed [23] 138 244 18.8 0.93
6082-T4 FSWed and aged [23] 285 310 9.9 1.19
7108-T79 Parent [24] 295 370 14 N/A
7108-T79 FSWed [24] 210 320 12 0.86
7108-T79 FSWed naturally aged [24] 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 [22] . 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 [25] . 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.

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 [26] . The minimal distortion and high reproducibility make FSW both technically and economically a very attractive method to produce these stiff panels.
Fig. 8. Sapa FSW aluminium panel for pressing of fish blocks before quick freezing. The panel is welded from both sides [23]
Fig. 8. Sapa FSW aluminium panel for pressing of fish blocks before quick freezing. The panel is welded from both sides [23]

 

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 [27] .

Fig. 9. Large aluminium ship panel made from 5083-H112 aluminium alloy extrusions, made by Sumitomo Light Metal [27]
Fig. 9. Large aluminium ship panel made from 5083-H112 aluminium alloy extrusions, made by Sumitomo Light Metal [27]

 

Pre-fabricated wide aluminium panels for high-speed ferryboats can be produced by FSW and are commercially available [28] . 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) [29,30] . 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 [30]
Fig. 10. Esab SuperStir TM machine at Hydro Marine Aluminium to weld aluminium extrusions for shipbuilding panels [30]
Fig. 11. Prefabricated FSW panel for catamaran side-wall, rolled for road transport (at Hydro Marine Aluminium) [30]
Fig. 11. Prefabricated FSW panel for catamaran side-wall, rolled for road transport (at Hydro Marine Aluminium) [30]

 

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 [31,32] .

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 ® Project [33] which is part funded by the Eureka programme and focuses on 'European Industrialisation of Friction Stir Welding'. The first machine was delivered to TWI and is currently being commissioned. 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. Initially the machine will be used 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 more than 100mm 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 12). 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.

Fig. 12. Crawford Swift's Powerstir TM machine with 3 CNC axes and 60kW spindle power. It can react up to 10t force
Fig. 12. Crawford Swift's Powerstir TM machine with 3 CNC axes and 60kW spindle power. It can react up to 10t force

 

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.

TWI owns and operates several FSW machines to weld a wide range of workpieces. Its modular laboratory machine was built to accommodate large sheets and structures ( Figure 13). 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. 13. TWI's modular laboratory FSW machine to weld large workpieces
Fig. 13. TWI's modular laboratory FSW machine to weld large workpieces
 

Conclusions

  • Friction stir welding is being exploited in the rolling stock industry to produce aluminium panels from 6000 and 5000 series extrusions.
  • Friction stir welding can compete with MIG welding in terms of productivity, cost and weld performance.

References

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  19. Thomas W M, Temple-Smith P (TWI, rights transferred to Lead Sheet Association): 'Friction welding sheet material'. UK Patent Application GB 2 319 977 A.
  20. Lockyer S A and Russell M J: 'Mechanical properties of friction stir welds in Magnesium alloys'. International Conference on Magnesium Alloys and their Applications, 26-28 September 2000, Munich.
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  33. 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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