Friction stir in aerospace - the industrial way Part II

TWI Bulletin, May/June 2002

Part I was published in TWI Bulletin, March/April 2002

Stephan Kallee gained his Diploma in Mechanical Engineering at the Technical University of Munich, where he specialised in production technology and business administration. After completing a course as a European Welding Engineer, he joined TWI in May 1995, where he works as Collaborative Project Manager for friction and forge welding processes. His recent activities include the application of friction stir welding to automotive lightweight structures, aluminium pipelines and high-speed ferryboats. Now he manages the EUREKA Eurostir ^® project on Industrialisation of Friction Stir Welding.

Dave Nicholas is Business/Technology Development Manager of the Friction and Forge Processes Group in the Electron Beam, Friction and Forge Processes Department at TWI. His interests centre on solid phase welding processes with the focus on friction welding processes. He developed several welding technologies from initial laboratory experiments to their industrial exploitation such as friction surfacing, linear friction welding and, since 1991, friction stir welding. He gained an honours degree in Metallurgy from the University of Wales and has been at TWI since 1967. He is a Chartered Engineer, a Member of the Institute of Metals and a Fellow of The Welding Institute.

Wayne Thomas is a Principal Research Engineer in the Innovation Unit. He joined TWI in 1983. He gained his MPhil from Brunel University, (Materials Technology) and has recently gained Eur Ing, CEng and FWeldI status. He is the author of many technical papers and has been responsible for the conception and/or development of a number of emergent technologies.

In the aerospace industry, large tanks for launch vehicles are being produced by friction stir welding (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 friction stir welded tanks was launched in April 2001.

As Stephan Kallee, Dave Nicholas and Wayne Thomas reported in Part I of this feature, various FSW tool designs have been developed in TWI's Core Research Programme to achieve high-quality welds. Part II now examines weld quality, some potential aerospace applications and non-destructive techniques for testing friction stir welds. 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.

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 HAZ. 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 HAZ, 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 in Finspång, Sweden, ¹⁶ and Hydro Aluminium in Håvik, Norway ¹⁷ . They show that for solution treated plus artificially aged 6082-T6 aluminium 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

Fatigue tests have been conducted on friction stir welding specimens made from 6mm thickness aluminium alloys 5083-O and 2014-T6. 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 Delta II rockets, and the first of these was launched successfully in August 1999. The Mars Odyssey launch in April 2001 used the first pressurised structures. The spacecraft lifted off on a Delta II rocket, which demonstrated the strength and quality of longitudinal friction stir welded joints on all three cylindrical tank components. ¹⁸ Friction stir welding technology for the Delta II common booster core tanks increases the weld strength by 30 to 50%.

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 (Fig.14 and 15). Some 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 to six days. The temperature range to which the friction stir welds are submitted during service is -195° to +183°C.

Friction stir welding is also being considered for producing Ariane 5 motor thrust frames (Fig.16 and 17). A study by Fokker Space 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 lower assembly time and cost dramatically. To meet the current development schedule, which calls for first flights in 2002, Eclipse had to 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 5000 small under-used 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 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, a downward force of up to 130kN (13t) can be applied. The rotation speed can be varied up to 2000rev/min at a maximum torque of 340Nm. The machine is used at the University of South Carolina in the NASA EPSCoR programme, which began in April 1997.

MTS Systems announced in June 2001 that Eclipse Aviation awarded them a contract for a 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 FSW in the fabrication of structural wing and fuselage members for the revolutionary Eclipse 500 jet.

MCE Technologies Inc in Seattle, Washington, offers production FSW equipment (Figs.20-22). ²⁶ These systems are now being used 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 wrought sheet (Fig.23). They are currently building three 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 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. 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.25). The table, on to 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 (Fig.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 (8m x 5m x 1m, Fig.27 and 28) ²⁸ and DanStir in Copenhagen, Denmark (15m x 3m 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

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 two and a half years, during which six tasks will be addressed (Table 3). Matrix manage-ment of tasks and applications will be conducted by European leaders in their respective fields. 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 of EuroStir® Project

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

Deliverables will be proven welding procedures for test pieces and prototypes in comprehensive detail. Equally important will be the detailed comparison between types of equipment (Figs.27 and 28), which will enable potential users to make informed 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 across Europe within five years.

The Dissemination Phase of the EuroStir® project will be mainly funded by industry and is planned to take two and a half 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 the European Commission under the CRAFT Initiative (Co-operative Research Action for Technology). The QualiStirtm 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 monitor-ing 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 FSW out of the laboratories and into the industrial manufacturing workshops.

References

Contacts