Friction Stir Welding to Produce Aluminium Components/Panels

Paper published in New Zealand Engineering News, August 2006

Friction Stir Welding (FSW) was invented and patented in 1991 by TWI (The Welding Institute) in Cambridge, UK, and has since then been developed to a stage where it is being applied in production. Currently 148 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and they have filed more than 1500 patent applications related to FSW. Over the last 15 years, companies from all parts of the world have implemented this new solid-phase welding process in the fabrication of aluminium components and panels. Trendsetters were Scandinavian aluminium extruders, which were in 1995 the first to apply the process commercially for the manufacture of aluminium deep-freeze panels, 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).

Friction Stir Welding in New Zealand

The first New Zealand fabricators have recently begun to use this patented welding process. In 2004 it was announced that a number of 55 metre Inshore Patrol Vessels would be procured for use by the Royal Australian Navy and RoyalNZ Navy - and now the opportunity existed for both Australia and New Zealand to be involved in building them. The naval architect of these vessels specified that a significant portion of the structure had to be friction stirwelded.

The NZ industry was initially not in a position, to provide the necessary FSW components and panels, but Auckland University of Technology (AUT), New Zealand Trade and Enterprise (NZTE), Marine Industry Association (MIA), ION Automotive NZ, Circa Marine & Industrial and Ullrich Aluminium teamed-up and made a concerted effort on bringing New Zealand up to speed. In mid 2005, the Donovan Group in Whangarei implemented FSW for the manufacture of these vessels. The Donovan Group has since then modified a large CNC gantry milling machine to be used as a FSW machine for large scale production, which is required for the patrol vessel building.

Another example of FSW in New Zealand has the objective to potentially replace traditional MIG welding for joining aluminium extrusions. Production Solutions Ltd in Auckland, together with AUT and NZTE, has demonstrated the process, by which machining and friction stir welding of the aluminium extrusions can all be done in one machining centre. Being able to carry out the entire production process in the same fixture could offer significant productivity gains. Low distortion, good tensile properties and better surface appearance after anodising of these products have also been demonstrated.

AUT has also developed the application of FSW in the production of alloy wheel rims and structures using thick aluminium alloy plates. All of the FSW carried out by AUT has been achieved on existing manufacturing equipment that the university has modified, as opposed to custom-built FSW machinery.

Contractually, the industrial uptake of FSW has become much easier in New Zealand than in other parts of the world, since the Australian/New Zealand Standard on 'Welding of aluminium structures' was issued (Reference AS/NZS 1665:2004). This is one of the very first standards that covers FSW, although it actually focuses on arc welding. It includes an innovation clause within its first section, which states that 'The Standard can be applied to other welding processes such as friction welding, including friction stir, [...] provided all requirements of the Standard are met, as well as specific constraints of needs, demands and operation of the individual welding processes.'

Overseas use of Friction Stir Welding

Friction stir welding is most commonly used for joining aluminium sheets, extrusions and castings and combinations thereof, and a FSW tool can be used for making thousands of metres of joints in these relatively soft materials without being worn. Typical FSW applications include the production of aluminium freezer and freeze-drying panels for the food industry, aluminium heat sinks for cooling high-power electronics as well as aluminium housings of 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 length. 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. Several industrial companies are investigating the suitability of FSW for making joints in mild steel, stainless steel, NiAl bronze and titanium, but so far FSW has not found an application in volume production of steel, bronze or titanium parts mainly because of tool wear.

FSW is being exploited in various industry sectors for prototypes, on-off components, small batches and high-volume production: A Japanese company uses it for making large copper panels of sputter targets. A Swedish organisation applies it to 50mm thick copper canisters for encapsulating nuclear waste. An Austrian aluminium supplier uses it for making copper to aluminium joints, which are attached to aluminium cathodes for the zinc smelter industry and a German job-shop manufactures large batches of joints between stainless steel and aluminium sheets for the production of vacuum chambers.

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).

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 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.5-6). A big break-through for the FSSW process was its implementation for the volume production of rear doors, bonnets and boot lids of Japanese cars that are friction stir spot welded from deep drawn aluminium sheets.

The newest automotive application is used in the production of the Ford GT 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.7&8). 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-moulded around the rail. This method maximizes fuel volume and reduces the number of connections to the fuel system.

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 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.'

Sixth 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 6th International Friction Stir Welding Symposium in Canada on 10-13 October 2006. This will be the premier event on this topic in 2006. 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. It will enable NZ engineers, to network with leading FSW experts, researchers, practitioners, suppliers and customers from around the world.

Pre-competitive research in TWI's Core Research Programme

The Core Research Programme (CRP) of TWI 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. Industrial members of TWI have access to 28 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 UK based 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, whenthe 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 has been developed for making lap joints that do not exhibit top sheet thinning or serious oxide related flaws in the weld nugget.

New technology centres in Yorkshire and Wales

TWI has opened two new UK based laboratories in Yorkshire and Wales, where projects aligned to the regional emphasis on advanced manufacturing are carried out complementary to the activities at the headquarters in Cambridgeshire ( Fig.9). TWI Technology Centre (Yorkshire) in Rotherham focuses on laser and friction stir welding. Two new FSW machines and an articulated arm robot have recently been commissioned for this laboratory:

Transformation Technologies Inc. built 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 high-melting temperature materials.

Crawford Swift Ltd has supplied a high-force machine that can provide 150kN (15t) welding force ( Fig.10). 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.

Friction Stir Link Inc provided a RoboStir ^TM system, which combines the powerful friction stir welding technology with the flexibility of a heavy-duty articulated arm robot. This latest acquisition by TWI Technology Centre (Yorkshire) in Rotherham adds anew dimension to the company's capabilities. The robotic welding system will provide the opportunity for TWI to develop further 3D welding techniques and applications, and also friction stir spot welding as an alternative to resistance welding. The addition of this machine will create a suite of FSW equipment unrivalled in the world and this, coupled with the wealth of knowledge of FSW at TWI, will create a centre of excellence in this exciting technology.

TWI Technology Centre (Wales) in Port Talbot focuses on the development and use of non-destructive testing technologies. Phased array ultrasonic systems are being developed there for assessing characteristic flaws in friction stirwelds. 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.

Conclusions

TWI supports international fabricators during the development, implementation and industrial application of welding processes and provides licenses for its patented processes such as FSW. TWI is able to transfer know-how on an array of joining technologies applicable to the New Zealand industry. With increasing international competition and the need to reduce the weight of transport structures, an increasing number of companies within the NZ supply chain become Industrial Members of TWI, to benefit from TWI's consultancy and contract R&Dmp;D services.