Dipl.- Ing. Stephan Kallee IWE, Technical Consultant, AluStir
Thirty Years of FSW
Where is friction stir welding being used today?
Where is friction stir welding being used today?
Friction stir welding was invented 30 years ago by Wayne Thomas at TWI in England. Only five years later, it was industrially used by Scandinavian producers of aluminium panels and is now being exploited worldwide in all industrial sectors.
Friction Stir Welding (FSW) operates below the melting point of the work pieces to be joined. The process is used in to make light-weight structures not only from aluminium extrusions, castings or sheets, but also from copper, steel, stainless steel, titanium and plastics or even mixed material combinations.
Some of the metallurgically most difficult FSW applications are related to tubular parts, e.g. for making high-strength steel risers or underwater pipelines in the oil & gas industry or for the encapsulation of nuclear fuel in canisters that are made from copper or metal matrix composites.
Benefits and Limitations
Significant cost benefits can be achieved, when making long welds in aluminium extrusions, e.g. for joining aluminium extrusions side by side to obtain pre-fabricated aluminium panels, that will subsequently be installed in aluminium ships, trains and cars (Figs. 1 and 2). Aluminium bridges, façades and concrete formwork are typical applications in the construction industry.
Very rigid machines and fixtures are typically required, especially for thick workpieces. This limitation can be overcome by using Bobbin tools, which have each a shoulder above and underneath the work piece, by stationary shoulder friction stir welding, by friction stir spot welding or by using special spindles with a closed-loop force control system. The end hole at the end of each weld can be closed using retractable pin tools.
Industrial Applications
Friction stir welding enables the cost-effective production of low-distortion aluminium panels from extrusions, which are most commonly pre-fabricated by aluminium extruders and then installed into trains, ships or offshore platforms by MIG welding. A bespoke FSW gantry machine for welding 15 × 3 m large panels had been installed and successfully used by DanStir around the year 2000 on the site of DSN and the Force Institute in Brøndby, before it was transferred to a Dutch shipyard, to simplify the logistics of importing extrusions to Denmark and road-transporting finished panels to customers abroad.
Manufacturers of space exploration rockets achieve significant weight savings by applying FSW to high-strength 2000 and 7000 series aluminium alloys, which are very difficult to weld by fusion welding processes (Fig. 3). They got a very quick return on investment after installing very large, and thus expensive FSW machines all over the world.
Cumulatively, the highest numbers of meters per day are probably achieved for friction stir welding of IGBT coolers, heat exchangers, electronic cabinets, electric motors and battery trays for electric locomotives, trains and automobiles (Fig. 4). Some factories produce more than 1000 m weld length per day. The welds in these parts are absolutely water tight, and even helium tight, if suitable raw materials and welding procedures are being used.
FSW Machines
Several manufacturers offer FSW machines and FSW robots, or even build hybrid machines that can be used both for FSW with high forces and CNC milling with high rotation speeds. Most FSW machines contain complex control algorithms for parameter monitoring and quality control.
The record for the largest number of FSW machines known to be installed on one site is related to making the very slim aluminium housings of Apple iMac desktop computers, which are manufactured in high-volume production.
FSW Know-How
Friction stir welding looks, on the first sight, as simple as CNC milling, but it requires an additional skill set, to fulfil the demands of ISO 25239. Selecting suitable tool designs and tool materials is similarly critical as paying attendance to detail during clamping. The most successful industrial users of FSW maintain strict confidentiality about their activities and their know-how, so that only very few experts have an insight in what is really going on. Buying a complete package of a high-quality FSW machine, industrially-proven FSW tools and operator training is the fast track to success.
Most experts focus on understanding the material flow during FSW and the effect of tool profiles. They measure, compute and control the forces, torques and other parameters such as rotation speed, welding speed and tool temperature.
Research and Development
Encouraging research and development is being conducted and published all over the world. The use of Artificial Intelligence (AI) in the Computer Aided Manufacturing (CAM) of Friction Stir Welds (FSW) is now being investigated in the collaborative aiCAMstir project. The vision is, to develop a software-package that adjusts the FSW parameters automatically based on computational fluid dynamics, analytical modelling, parameter monitoring, image analysis and non-destructive testing. Participation is free of charge for students or employees of universities and institutes, while industrial companies fund the project by paying an annual participation fee, as shown on www.aicamstir.com.
Non-Destructive Testing
The table in Appendix A of ISO 25239-5 explains typical flaws and the acceptance criteria, such as incomplete penetration (colloquially also known as kissing bonds), cavities (internal voids) and hooks (hooking and thinning).
It lists the methods for inspection and testing: Visual testing and phased array ultrasonic testing are the most effective non-destructive testing methods to be used alongside destructive testing incorporating metallography, bend tests and tensile tests, which are most essential during the welding procedure qualification.
Upcoming Events
The ‘Joint International Symposium on Friction Stir Welding and Processing’ will be held on 28-30 September 2022 at the University of Lüneburg in Germany, just 215 km south of the Danish border.
Link to the Original Publication