Friction Stir Welding is a cleaner and better joining technique.
Invented in 1991 at The Welding Institute (TWI) in the UK, Friction Stir Welding (FSW) is perhaps the most remarkable and potentially useful new welding technique to be introduced in the recent past. In the second half of the 1990s, the research laboratories at Suresnes and Ottobrunn began evaluating this new process and acquired licenses from TWI to work on the process. With the foundation of EADS, the two labs have been merged to form EADS Innovation Works, where FSW development continues in joint and coordinated research programmes.
FSW is a purely mechanical process. It is clean, ecological and produces weld seams of the highest quality, while it offers the further advantage of being suitable for joining dissimilar metallic materials and others that are difficult to weld by conventional fusion methods. FSW is not restricted to sheets and plates: it can also weld tubes, tanks, T-joints and corner joints. The FSW process can utilize existing and readily available machine tool technology, it is suitable for automation and most recently has been made adaptable for use with robots.
The basic process is remarkably simple: frictional heat is generated by a fast-rotating, cylindrical and shouldered welding head with a profiled probe. The heat causes the material of the work pieces to soften into a plastic state without reaching the melting point. The tool is plunged into the two parts to be joined and the material is then stirred, or better, forged by the intimate contact of the probe and the tool shoulder. This produces a welded joint with a finer micro-structure than that of the parent material. The relatively low residual stresses and the fine-grained, stirred zone result in very good strength properties and good ductility, characteristics that are of key importance in aerospace applications.
As a rule, all extrudable aluminium materials can be welded, including wrought and cast aluminium materials, and even metal-matrix alloys. The process is also well suited for joining alloys of magnesium, copper and zink. The welding of steel and titanium has also been explored in the laboratory.
FSW applications in the future will include the capability to join other ‘unweldable’ aluminium alloys or promising aluminium/lithium compounds, as well as armor plate aluminium materials. The pieces to be welded need only rudimentary surface preparation and the process can within limits tolerate gaps and misalignment. This represents a major advantage when compared to fusion welding, which calls for special surface preparation. Welding filler metals or inert gas are also unnecessary.
The FSW process generates very little noise and does not emit smoke, dust, dangerous plasma or x-rays.Welding speeds vary between 100 to 2,000 mm/min, which is comparable to many fusion welding processes. Workpieces must be rigidly clamped and a strong backing bar is required. A keyhole remains at the end of each weld line.
Limitations of the process are being overcome by intensive research and development. In their research, EADS Innovation Works at Suresnes, together with the French Welding Institute, have been using a purpose-built FSW machine since early 2002. The machine is designed to weld aluminium alloys up to a thickness of 15 mm. It can produce longitudinal weld seams of up to 2 meters and is able to accommodate circumferential and curvilinear welding with different radii. This machine was the first in Europe to be equipped with advanced process options, such as the bobbin and retractable pin tools. Both eliminate some of the limitations of the original process: the bobbin shape does not require a backing bar, since all vertical force is exerted within the welding head itself, while the retractable pin tool enables the processing of materials of varying thickness and does not leave a keyhole at the end of the seam. Three-dimensional profiles are possible with both techniques.
Researchers at Ottobrunn began to validate the process in 1997 with various aerospace aluminium alloys. Their work is currently focussed on reducing welding forces by improving tool geometry and process parameters, including innovative hybrid processes, such as the application of external heat. Their colleagues at Suresnes are concentrating on developing bobbin tool technology and on improving the weldability of both thin and thick components. They are also conducting analyses of thermal behaviour during welding.
Rapid progress continues to be made in Friction Stir Welding and its associated technologies. New applications continue to emerge and the variety of materials that can be welded is increasing, as is the understanding of metallurgical and engineering aspects of the process. Joint activities with several business units are currently underway with the aim of introducing Friction Stir Welding into series production.
FSW is a purely mechanical process. It is clean, ecological and produces weld seams of the highest quality, while it offers the further advantage of being suitable for joining dissimilar metallic materials and others that are difficult to weld by conventional fusion methods.
