Industrialisation of friction stir welding for aerospace structures

TWI Ltd, Cambridge, United Kingdom
E-mail: friction@twi.co.uk

Paper presented at Structures and Technologies - Challenges for Future Launchers
Third European Conference, 11-14 December 2001, Strasbourg France

Abstract

In the aerospace industry, large tanks for launch vehicles are being produced byFSW 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 friction stir welded tanks was launched in April 2001.All major manufacturers of military and civil aircraft are currently investigating the potential for replacing riveting by FSW, and seven large international collaborative projects have been launched to assess the benefits.

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 now applied in production. Currently 65 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. They have filed more than 400 patent applications related to FSW. ^[3]

In macrosections of good quality welds in aluminium alloys a well-developed nugget is visible at the centre of the weld, as shown in Figures 1 & 2. Outside the nugget there is a thermo mechanically affected zone, which has been 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.

Design of Whorl ^TM and MX Triflute ^TM 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 generally has a profiled pin and 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 along the joint line while the shoulder is in intimate contact with the top surface of the workpiece to avoid expelling softened material and provide consolidation.

FSW tools are manufactured from a wear resistant material with good static and dynamic properties at elevated temperature. Current state-of-the-art tools permit up to 1000m of weld to be produced in 5mm thick aluminium extrusions without changing the tool. The design of the friction stir welding tools is the heart of this remarkable and still relatively new welding process.

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). In an investigation of the Whorl TM and MX Triflute ^TM families of tools, ^[6,7,8] trials were carried out with the 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. 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.

 

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 the material flow, the break-up and the dispersion of surface oxides. The tool shoulder profiles under investigation are designed to provide better coupling between the tool shoulder and the workpiece.

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

TWI has already developed a number of lap welding tools in its Core Research Programme ^[10] 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 tostir up the oxides and to improve the material flow.

In an industrialisation study for Fokker Space, ^[11] TWI conducted pilot trials with 250mm square, 2.5mm thick 7075-T73 clad sheet. The cladding was found to contaminate the weld nugget and there was lifting of the free edge of the top sheet. Trials continued with the cladding removed from the mating surfaces. The metallurgical sections looked better but significant upper sheet thinning occurred, as can be seen in the cross-section below ( Fig.6).

The tool design was modified so that less metal moved in the direction of the top-sheet ( Fig.7). Trials then continued with flat 250mm square sheets of 2.4mm thick aluminium alloy 7075-T7351. The results were highly satisfactory. Having demonstrated the feasibility of a high-strength lap weld, it was necessary to move to a more representative configuration. A series of trials was conducted with 1m long flat sheets and a very encouraging microstructure has been achieved ( Fig.8).

 

New Skew-stir ^TM motion to improve the material flow

The skew-stir ^TM variant ^[12] 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.9). The intersection of the tool axis and the spindle axis can be set above the plate, through the plate thickness, or at a position below the plate being welded. By inserting packing pieces, this intersection, or focal point, can be varied to suit the material, the process parameters, and the tool geometry. When the focal point is positioned slightly above or below the surface of the plate, the shoulder contact face makes a nominally orbital movement.

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 substrate component material. Consequently, theinner part of the tool can be cut away to improve the flow path of material during welding. This results in an asymmetric shaped probe ( Fig.10). The skew-stir ^TM technique provides an increased flow path, and the width of the weld nugget region is greater than the diameter of the probe. This feature is ideally suited for lap and T-joints and similar welds, where the interface is 90° to the machine axis, i.e. parallel with the work piece surface.

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, the 'dynamic to static volume ratio' is provided by the geometry of the probe because of its re-entrant features. Skew-stir ^TM can be used to advantage, where tools of complex shape cannot be used. Moreover, the greater volume of plasticised material results in better disruption and dispersion of surface oxide layers. This should minimise the risk of undesirable joint features.

The arrangement shown in Fig.9 results in the shoulder face remaining in the same plane as the plate top surface. Tilting the work piece or the machine spindle produces an angle between the workpiece and the tool face that can be varied to suit conditions. It is also possible to assemble tools with the tool shoulder not perpendicular to the spindle axis to increase the forging action.

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 successfully introduced a mechanical position control system, using one or two rollers beside or in front of the tool ( Fig.11). 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 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.12).

On rigid machines, the tool heel plunge depth can be kept constant by either position or force control ( Fig.13). 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 near root of the weld. These are operated with a compliant system for compensating changes in material thickness.

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) aluminium alloys can be improved by controlling the thermal cycle, in particular by reducing the annealing and over ageing effects in the heat affected zone, where the lowest hardness and strength are found after welding. For optimum properties, it would seem that for some alloys 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 1. The studies have been conducted at TWI, ^[15] Sapa inFinspång, Sweden, ^[16] and Hydro Aluminium in Håvik, Norway. ^[17] They show that for solution treated plus artificially aged 6082-T6aluminium a tensile strength similar to that of the parent material can be achieved by post weld heat treatment, although the elongation is 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 subjected to artificial ageing after welding. 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. The strength can also be improved by using advanced tool designs at high welding speeds, as demonstrated in recent TWI studies.

Table 1. Mechanical properties of friction stir welded aluminium alloy specimens
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 artificially 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 artificially 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]	295	370	14	N/A

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 found to be comparable to that of the parent material when tested using a stress ratio of R=0.1. Analysis of the available fatigue data has shown that the performance of friction stir welds is comparable with that of welds made by fusion welding processes, and in most cases substantially better results, with low scatter, can be obtained.

 

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. Boeing has applied FSW to the Interstage Modules of a Delta II rockets, and the first of these was launched successfully in August 1999. The Mars Odyssey launch in April 2001 utilized the first pressurised structures. The Mars Odyssey spacecraft lifted off on a Delta II rocket, which demonstrated the strength and quality of longitudinal friction stirwelded joints on all three cylindrical tank components. ^[18] Friction stir welding technology for the Delta IV common booster core tanks increases the weld strength by 30 to 50% and lowers cycle time by nearly 80%.

The key milestones in Boeing's FSW activities were the successful production and testing of a subscale prototype FSW tank at TWI and then the delivery of the first Esab production machines ( Figs.14 & 15). 2100m of defect free friction stir welds have been produced for Delta II rockets, and 1200m for the larger Delta IV rocket. The FSW specific design of Delta IV and Delta II achieved 60% cost saving, and reduced the manufacturing time from 23 to6 days. The temperature range to which the friction stir welds are submitted during service is -195°C to +183°C.

 

Friction stir welding is also being considered for producing Ariane 5 motor thrust frames ( Figs.16 & 17). A study by Fokker Space ^[11] has shown that FSW can readily be applied to lap joints in aluminium alloy 7075-T7351. Although the tensile strengths measured in this investigation are 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.

 

Potential for using friction stir welded aluminium panels in aircraft production

The FSW process offers tremendous potential for low-cost joining of lightweight aluminium airframe structures for large civil aircraft such as the Airbus A380. Researchers at Airbus Deutschland see a high potential for joining aluminium alloys by FSW for skin-to-skin fuselage connections. They presented data that demonstrate that the mechanical and technological properties of these welds approach the properties of the parent material. ^[20] This could lead to the reduction of cost and weight through improved joint quality and the possibility of new design.

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. ^[21] The production start of friction stir welded non-structural parts for the Boeing commercial aircraft was scheduled for October 2001. ^[22]

It has been reported that Eclipse Aviation Corporation of Albuquerque, New Mexico, has decided to use FSW to replace traditional riveting and bonding processes. ^[23] 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. They are pioneering a new-economy model of air travel which couples the fractional jet business with the new 'dispersed operation' air travel concept using the more than 5,000 small under-utilised airports in the United States to provide individual air service.

Commercial FSW machines used for welding aerospace components

MTS Systems in Eden Prairie, Minnesota, has developed and built two hydraulically operated FSW machines, one of which has been installed at the University of South Carolina ( Fig. 18). This machine is described as an Innovative and Flexible Process Development System. ^[24] This high-force, self-contained system is designed, instrumented and controlled to conduct reliable FSW process development. 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 higher strength alloys fornon-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, a downward 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.

MTS Systems announced in June 2001 that Eclipse Aviation has awarded them a contract for a unique friction stir welding system for the fabrication of aircraft structures ( Fig.19). 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 Eclipse 500 jet.

MCE Technologies Inc in Seattle, Washington, offer production FSW equipment ( Figs.20-22). ^[26] These systems are being used now in demanding environments. Currently 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 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 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 wroughtsheet ( Fig.23). They are currently building 3 large tank welding machines for space launch vehicles for a prestigious customer ( Fig.24).

A Powerstir ^TM machine has been tailor made by Crawford Swift in Halifax (UK) and was delivered in autumn 1999 to Airbus UK in Filton, where it is being used for fabricating prototype aluminium wings andfuselage 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. The machine is 11.5m long x 5.7m wide x 4.7m high and takes the basic form of a gantry-type moving table machine ( Fig.23). 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. Their modular laboratory machine was built to accommodate large sheets and structures ( Figure 26). 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.

Up to 16m long SuperStir ^TM machines have been designed, built, and commissioned by Esab in Laxå, Sweden. Five of them have been made for The Boeing Company for welding fuel tanks of spacecraft. These include one large horizontal machine for welding fuel tanks from inside and two vertical machines. One of the Esab SuperStir ^TM machines has been installed at Hydro Marine Aluminium for producing panels from extrusions. Another Esab 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. Esab's newest series of large gantry machines have now been installed at TWI (8 x 5 x 1m, Figs.27 &28) ^[28] and DanStir in Copenhagen, Denmark (15 x 3 x 1m).

International collaborative projects

Seven large collaborative projects have been launched in Europe to assess the advantages 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: Collaborative projects on friction stir welding
Acronym	Title and WWW addresses of the proposals	Value [Euro]
EuroStir ®	'European Industrialisation of Friction Stir Welding' www.eurostir.co.uk and www3.eureka.be/Home/projectdb/PrjFormFrame.asp?pr_id=2430 (needs 30sec)	6.8M
QualiStir TM	'Development of Novel Non Destructive Testing Techniques and Integrated In-line Process Monitoring for Robotic and Flexible Friction Stir Welding Systems'	2M
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' Enter search term 'WAFS' at 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 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 dbs.cordis.lu/EN_PROJl_search.html	3.0M

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. ^[29] FSW will be applied to a range of materials and will be researched to achieve high welding speeds in increased joint thickness. The FSW process will be industrialised for real components and applied in commercial production.

EuroStir ^® was launched in December 2000 and will last for five 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). Matrix management of Tasks and Applications will be conducted by European leaders in their respective fields. The Task objectives are to demonstrate weld ability byfeasibility 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 of EuroStir ® project

(1) Tool development (2) Workpiece materials range (3) FSW productivity (4) FSW flexibility (5) Mech. properties (6) Dissemination

Table 4. Objectives of the EuroStir ® Project, funded by UK (51%); France (21%); Germany (10%); Sweden (9%); Denmark (7%); and Poland (2%)

(a) High-speed (>2m/min) welding of sheet aluminium alloys without loss in quality (b) Welding aluminium alloys with >20 mm thickness (c) FSW of dissimilar materials, i.e. cast-wrought, Mg-Al and Al steel (d) Robotic, all positional and bobbin tool FSW (e) Improving mechanical properties, i.e. raising strength of the weld and HAZ (f) Developing FSW techniques for Ti, Ni, stainless and ferritic steels (g) Industrialise (a)-(f) above for real components and apply in commercial production (h) Achieving industrial implementation by 50% of the participants 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.

Development of novel non-destructive testing techniques and integrated 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 theEuropean 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.

Conclusions

• 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.
• 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.

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