Stephan W. Kallee, TWI Ltd
Paper published in 'Automobil Produktion', December 2004.
Friction Stir Welding (FSW) was invented in 1991 at TWI (The Welding Institute, www.twi.co.uk) and has since then been developed to a stage where it is being applied in production. Currently 114 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and they have filed more than 1125 patent applications related to FSW.
Friction Stir Welding (FSW) is conducted below the melting point by pressing a rotating tool into the joint line. The wear-resistant FSW tool has a profiled pin and a shoulder with a larger diameter than that of the pin ( Fig.1). The pin length is similar to the required weld depth. The tool is traversed along the joint line, while the shoulder is pressed onto the surface of the workpiece, to provide consolidation of the plasticised workpiece material.
Fig.1. Principle and microstructure of the friction stir welding process invented at TWI
In 1998, TWI started an investigation on aluminium tailored blanks for door inner panels and demonstrated new concepts on friction stir welded drive shafts and space frames in a confidential group sponsored project involving BMW,DaimlerChrysler, EWI, Ford, General Motors, Rover, Tower Automotive and Volvo.
As a consequence of the encouraging results of this project, Friction Stir Welding and its variant Friction Stir Spot Welding (FSSW) are now being used in the series production of aluminium automotive components at several locations world-wide:
Ford in Detroit (USA) uses a friction stir welded centre tunnel in the production of the sports car Ford GT. The centre tunnel is a structural part that increases the rigidity of the chassis and is also used as a vapour tight fueltank ( Fig.2 & 3). The location of the tank provides good weight distribution and crashworthiness. 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, a single-piece tank is blow-moulded around the rail. This concept maximizes fuel volume and reduces the number of connections to the fuel system.
Fig.2. Friction stir welding of the aluminium centre tunnel of the Ford GT
Fig.3. The centre tunnel of the Ford GT housing the fuel tank is friction stir welded
Tower Automotive in Grand Rapids (Michigan, USA) produces suspension links for Lincoln Town Cars designated as stretched limousines. These have heavy-duty rear axles installed, while the rest of the rear suspension remains unchanged. Tower Automotive manufactures an aluminium link for these cars. This is made from two identical extrusions, friction stir welded simultaneously with two spindles from both sides. This provides excellent fatigue properties.
Sapa in Finspång (Sweden) use an FSW machine with two welding heads for welding hollow aluminium extrusions from both sides simultaneously, to produce foldable rear seats of the Volvo V70 station wagon. The machine has acarousel-type loading and unloading station and is automatically loaded by an articulated arm robot.
Mazda in Hiroshima (Japan) uses friction stir spot welding for the rear doors and bonnet of the Mazda RX-8. The hood of this sports car has an impact-absorbing structure aimed at enhancing pedestrian protection. They use this process, to avoid spatter and to reduce the energy consumption significantly in comparison to resistance spot welding.
Showa Denko in Oyama City (Japan) joins extruded end-pieces to 20-30 mm diameter tubes for the manufacture of suspension arms. The rubber of the end-pieces of the suspension arms can be vulcanised prior to welding due to the low heat input of the new assembly method.
Simmons Wheels in Alexandria (Australia) developed a new method of producing a wheel rims from rolled aluminium 6061-O sheet. From this they form a cylinder with a longitudinal friction stir weld. After cutting this into rim sections they spin form it into the desired rim profile and finally subject this part to heat treatment to the required T6 temper. The company is now supporting UT Alloy Works in Guandong (China) during FSW production ramp-up of light alloy wheels ( Fig.4-6).
Fig.4. UT Alloy Works in China use friction stir welding for prefabricating a cylinder from aluminium sheets...
Fig.5. ... which will be subsequently shaped by spin forming (FSW seam in six o' clock position) ...
Fig.6. ... and assembled to an aftermarket three-piece wheel for tuning sport cars
Hydro Aluminium in Håvik (Norway) invented an innovative technique of joining two parts of a car wheel by FSW. Optional design concepts have been developed, to either butt or lap weld cast or forged centre parts to rims that are made from wrought alloys.
DanStir in Copenhagen (Denmark) collaborates with a leading Norwegian manufacturer of light metal wheels. DanStir applies the friction stir welding process to the assembly of aluminium wheels from spin formed rims and cast structures. This reduces the wheel weight by 20-25% providing the supplier with a business advantage over its competitors. Following the completion of the development phase, DanStir will assist in industrialising the technology, taking part in production machine design, commissioning and future operation.
Riftec in Geesthacht (Germany) provides subcontract production and engineering consultancy, e.g. during the installation of FSW robots in automotive manufacturing lines. One of their automotive related projects concerned the production of welded test specimens for study of the Berlin-based company Inpro.
Friction Stir Link in Waukesha (Wisconsin, USA) is a service supplier focussing on the automotive industry. It provides FSW process development, technology transfer, moderate-volume production and friction stir welding system integration services ( Fig.7-9).
Fig.7. CNC controlled FSSW gun on an articulated arm robot at the company Friction Stir Link in Detroit
Fig.8. The company Friction Stir Link provides pre-production prototypes, systems integration and job shop services for automotive aluminium space frames
Fig.9. The company Friction Stir Link fabricates FSSW body-in-white parts from deep drawn aluminium sheets
TWI (The Welding Institute, www.twi.co.uk) has developed an amazing array of joining technologies applicable to the transport industry. It is headquartered near Cambridge, UK, and has a long association with the automotive and motor-sport industry sectors. TWI gives access to world-class expertise and knowledge exclusively to its industrial members. It supports international vehicle manufacturers and suppliers during the development, implementation and industrial application of welding procedures.
With increasing international competition and the drive to reduce the weight of cars, automotive suppliers in particular wish to obtain licences for patented but cost-effective manufacturing processes such as FSW. Therefore, TWI's strategy is to support its industrial members during the application of innovative manufacturing processes that reduce costs or add functionality.
The Core Research Programme (CRP) of TWI consists of a series of applied research projects, which underpin TWI's highly confidential contract research services. Industrial members of TWI have access to 20 CRP reports on friction stir welding so far. Having a multi-million pound annual budget, this research programme aims to develop production relevant knowledge and skills for transfer into industry.
Two projects in TWI's current core research programme focus on friction stir welding, as follows: In the first project, friction based processes for repair are being developed including the development of portable friction stir welding machines for thin sheets. In the second CRP project, the relationship between spindle rotation speed, applied force, and material softening response by friction heating is being investigated. This project will investigate in a later phase particularly 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 superplastic forming than those of the parent material.
Now, a new group sponsored project is being launched at TWI, to establish tool designs, process parameters and tolerances for Friction Stir Spot Welding (FSSW) of a range of light alloys. The FSSW process ability to weld through coatings, adhesives and sealants will be explored, and data for the specification of welding machines and robots will be gathered.
Stephan Kallee is Sector Manager in TWI's Industrial Membership Services Group and focuses on the transport industry (email@example.com). He believes that friction stir welding will significantly change the way of assembling cars. It started with the low-volume production in the supply chain and is now being industrialised by several large companies for the body-in-white production.