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Friction stir welders provide prefabricated components and panels (July 2004)

   
Stephan W Kallee, TWI Ltd

Paper published in Aluminium International Today, July/August 2004.



Since the invention of friction stir welding at TWI in 1991, companies from all parts of the world have implemented the process in the fabrication of aluminium components and panels.

Trendsetters were the Scandinavian aluminium extruders, which were in 1995 the first to apply the process commercially for the manufacture of hollow aluminium deep-freeze panels and for ship decks and bulkheads. Friction stir welded structures are now revolutionising the way in which high-speed ferry boats, hovercraft and cruise ships are built from prefabricated lightweight modules ( Fig.1&2).

Fig.1. Hydro Marine Aluminium's prefabricated FSW deck panels for 'The World' cruise ship
Fig.1. Hydro Marine Aluminium's prefabricated FSW deck panels for 'The World' cruise ship
Fig.2. Fosen Mek's cruise ship 'The World' contains friction stir welded decks
Fig.2. Fosen Mek's cruise ship 'The World' contains friction stir welded decks

 

In the US aerospace industry, large tanks for launch vehicles are being fabricated by FSW from high-strength aluminium alloys. The first rocket with a friction stir welded interstage module was successfully launched in August 1999.Recently, the first approval has been obtained for the use of the FSW process in the manufacture of American business jets ( Fig.3&4).

Fig.3. FSW gantry at Eclipse Aviation for welding stringers and spars to aluminium cabin panels
Fig.3. FSW gantry at Eclipse Aviation for welding stringers and spars to aluminium cabin panels
Fig.4. Take-off during the first test flight of an Eclipse 500 friction stir welded business jet
Fig.4. Take-off during the first test flight of an Eclipse 500 friction stir welded business jet

 

The railway rolling stock industry initially exploited the process for the production of heat sinks for high-power electronics. Now several companies supply large pre-fabricated aluminium panels, which are made from extrusions. In Japan, complete trains are being assembled from double-skin aluminium extrusions using the FSW process.

The automotive industry uses FSW in the series production of aluminium components. Friction stir welded light alloy wheels are available in the Australian after market. In Sweden, hollow aluminium panels for the rear seats of station wagons are friction stir welded with less than 60 sec cycle time using a fully automated FSW machine with carousel type loading system. Suspension arms are mass-produced in Japan and USA from aluminium extrusions. Articulated arm robots can be used with friction stir spot welding (FSSW) guns in the body-in white or closure production ( Fig.5-7). A big break-through for the FSSW process was the implementation of for the volume production of rear doors and bonnets of Japanese cars that are friction stir spot welded from deep drawn aluminium sheets.

Fig.5. Friction Stir Link's computer controlled friction stir spot welding gun
Fig.5. Friction Stir Link's computer controlled friction stir spot welding gun
Fig.6. Friction Stir Link provides pre-production prototypes, systems integration and job shop services for automotive aluminium space frames
Fig.6. Friction Stir Link provides pre-production prototypes, systems integration and job shop services for automotive aluminium space frames
Fig.7. Friction Stir Link fabricates deep drawn FSSW body-in-white parts
Fig.7. Friction Stir Link fabricates deep drawn FSSW body-in-white parts

 

The newest application is used in the production of the Ford GT, a US sports car, for which the multi-piece centre tunnel is friction stir welded. This is a structural part that increases the rigidity of the chassis and is also used as a fuel tank ( Fig.8&9). Crash modelling verified that the centre tunnel is the preferred location for the fuel tank, because it helps reducing risks in collisions. As an added benefit, the location keeps the overall weight distribution and the centre of gravity relatively consistent at differing fuel levels. The 'ship-in-a-bottle' design of the fuel tank is an industry first. The mechanical components, including the fuel pumps, level sensors and vapour control valves are first mounted on a steel rail. Then, the single-piece tank is blow-molded around the rail. This method maximizes fuel volume and reduces the number of connections to the fuel system.

Fig.8. Friction stir welding of the aluminium centre tunnel of the Ford GT
Fig.8. Friction stir welding of the aluminium centre tunnel of the Ford GT
Fig.9. The centre tunnel of the Ford GT housing the fuel tank is friction stir welded
Fig.9. The centre tunnel of the Ford GT housing the fuel tank is friction stir welded

 

Other applications include the high-volume production of housings for electrical motors, loudspeakers and electronics. Research and production FSW machines are commercially available from several machine manufacturers and include capability for welding up to 16m lengths. The latest machines and robots are being used for non-linear and three-dimensional joint lines. Several machines have multiple heads, which can be used simultaneously.

Cost Savings by Implementing Friction Stir Welding

The following comments on cost savings were published by users of the FSW process and speak for themselves:

  • Doug Waldron of The Boeing Company reported that 'the FSW specific design of Delta IV and Delta II achieved 60% cost saving, and reduced the manufacturing time from 23 to 6 days.'
  • For 'Slipper', the US Army's new cargo interface pallet, 'FSW processing reduced the sandwich assembly cost, including raw materials, extruding, and welding, from 61% to only 19% of the total fabrication cost. The Air Force estimates the total cost savings attributed to FSW (for a projected buy of 140,000 Slippers) at $315 million.'
  • Ole Midling of Hydro Marine Aluminium reported that at shipyards using prefabricated FSW panels the 'improvement in the aluminium fabrication has resulted in 15% reduction in the man-hour per ton rate.'
  • Stig Oma of Fjellstrand claimed 'a total fabrication cost saving of approximately 10% based on improved ship design, streamlined fabrication at the shipyard and by supply of prefabricated FSW panels and structures based on extruded profiles.' He said that using prefabricated FSW panels 'has enabled the yard to reduce the production period for a 60m long aluminium catamaran hull from 10 to 6 months, which means a 40% increase in production capacity and turn-over at the yard.'

Current and Future Developments in TWI's Core Research Programme

TWI's Core Research Programme (CRP) consists of a series of applied research projects, which underpin TWI's contract and consultancy services. Having a multi-million pound annual budget, this research programme aims to develop a relevant knowledge and skills base for transfer into industry in order to reduce manufacturing costs, encourage innovation, improve product quality and meet safety and reliability requirements. Industrial members of TWI can so gain and maintain competitive edge in the market place. Two projects in TWI's current core research programme focus on friction stir welding of aluminium alloys, as follows:

Friction based processes for repair are now being developed. Applications where friction repair techniques would be beneficial will then be investigated in a CRP project on 'friction based repair techniques including the development of portable friction stir welding'. In this project the technical requirements of portable equipment for making friction stir welds in thin aluminium sheets will be specified.

The relationship between spindle rotation speed, applied force, and material softening response in the generation of heat by friction is being investigated in a CRP project on 'fundamentals of friction welding and friction processing of materials'. This project will produce guidelines for effective rubbing velocities for FSW in a range of commonly used workpiece materials. The project is to be extended in a later phase particularly to friction stir processing (FSP) of a number of alloys. It has already been demonstrated that the refined FSP microstructures of aluminium alloys provide better formability during super plastic forming than those of the parent material.

Reports on Pre-competitive Research in TWI's Core Research Programme

Industrial members of TWI have access to 20 CRP reports on friction stir welding so far. For instance, the microstructures of friction stir and arc welds have been characterised in 'a study of arc and friction stir welding of two aluminium alloys containing a low level scandium addition'. The researchers compared solidification crack susceptibility of weld metal with and without scandium and determined the mechanical properties of TIG and friction stir welds in similar scandium and non-scandium containing alloys.

Macro and microstructural features of friction stir welds were examined in various materials, and a microstructural classification scheme for friction stir welds has been introduced as part of the CRP. The 'corrosion resistance of friction stir welds in aluminium alloys 2014A-T651 and 7075-T651' and 'fracture toughness of friction stir welds in 2014A, 7075 and 5083 aluminium alloys' has been determined experimentally.

Reports on 'flaws in aluminium alloy friction stir welds', and more specifically 'the significance of root flaws in friction stir welds' are available to industrial members of TWI. The former report describes microstructurally the types of flaws in Al-Cu-Mn-Si-Mg aluminium alloy friction stir welds, when the welding conditions diverge from the established operating window. The latter report covers the effect of root flaws on the static and fatigue performance of friction stir welds made from one side.

In a study on 'forces in friction stir welding of aluminium alloys' a commercially available dynamometer measured the horizontal and vertical forces and the torque generated during FSW operations. These data were used to evaluate the effects of friction stir welding parameters and tool geometry on the forces and torques generated during friction stir welding of selected aluminium alloys.

Earlier reports covered the 'tool developments for FSW of 6mm thick aluminium alloys', e.g. describing FSW tools capable of operating with zero tilt or FSW bobbin tools that can contain the weld metal about the tool pin and react the weld metal forging forces necessary for making sound welds. A prototype FSW tool for making lap joints that does not exhibit top sheet thinning or serious oxide related flaws in the weld nugget has also been developed.

New Technology Centres in Wales and Yorkshire

TWI has opened new laboratories in Wales and Yorkshire, where projects aligned to the regional emphasis on advanced manufacturing are carried out complementary to the activities at the headquarters in Cambridgeshire ( Fig.10). In Port Talbot TWI Technology Centre (Wales) focuses on the development and use of non-destructive testing technologies. Phased array ultrasonic systems are being developed there for instance for assessing characteristic flaws in friction stir welds. These offer the possibility of performing inspections with ultrasonic beams of various angles and focal lengths using a single array of transducers. Software control over beam angle and focusing is achieved by application of precisely controlled delays to both the emission pulse and received signal for each element in an array of transducers.

Fig.10. ESAB SuperStir machine with a vacuum clamping table used in the EuroStir ® project at TWI for FSW of 8 x 5m prototypes
Fig.10. ESAB SuperStir machine with a vacuum clamping table used in the EuroStir ® project at TWI for FSW of 8 x 5m prototypes

 

In Sheffield TWI Technology Centre (Yorkshire) focuses on laser and friction stir welding. Two new FSW machines are now being commissioned for this laboratory: Transformation Technologies Inc. builds an FSW machine with a very accurate spindle to weld a range of steels and aluminium-based metal matrix composites at TWI Technology Centre (Yorkshire). The accuracy of the spindle increases the lifetime of the brittle tools that are used for FSW of these and high-melting temperature materials.

Smart Technology Group Ltd has been contracted to supply a high-force machine that can supply 150kN (15t) welding force ( Fig.11). It has a twin head configuration allowing simultaneous welding from both sides. Eleven CNC programmable axes achieve a true three-dimensional welding capability, to weld contoured and complex shapes. The machine also has a sophisticated data acquisition system for data logging, analysis and control.

Fig.11. Smart Technology Group's high-force FSW machine for TWI Technology Centre (Yorkshire) to weld up to 150mm thick aluminium plates and billets
Fig.11. Smart Technology Group's high-force FSW machine for TWI Technology Centre (Yorkshire) to weld up to 150mm thick aluminium plates and billets
 

Fifth International Friction Stir Welding Symposium

As friction stir welding and related technologies continue to grow in industries around the world at a rapid pace, opportunities to catch up with developments in institutes and industry are vital to stay fully informed. Access to such information is essential to any company involved in aluminium fabrication. The Friction Stir Welding Licensees Association will hold their fifth International Symposium in Metz, France, on 14-16 September 2004. This symposium is intended for technical managers, research and development staff, designers, welding engineers and metallurgists who are actively involved in FSW or have a strong interest in the subject.

For more information please email:


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