Friction stir welding (FSW). Friction stir welding (FSW) is used extensively in the primary structure of the Eclipse 500™. Eclipse Aviation pioneered the use of friction stir welding in aircraft structures. (Oh, sure, FSW has been used by rocket scientists for the space shuttle’s fuel tank, but Eclipse has figured out how to use it on thin-gauge aircraft aluminum.) Approved for use on the Eclipse 500 by the FAA in May 2002, friction stir welding replaces the riveting in the majority of the aircraft’s primary structure. At 20 inches per minute, friction stir welding is approximately 10 times faster than manual riveting and provides a continuous join for improved structural rigidity. The friction stir welding patterns are computer-controlled to ensure accuracy, virtually eliminating time-consuming rework. About friction stir welding. Friction stir welding (FSW) is an important innovation in manufacturing technology. By using FSW to reduce or replace traditional riveting and bonding processes, Eclipse can build high-performance aircraft with fewer labor hours. The resulting airframe structures are also stronger and lighter than those built using traditional processes. Invented and patented in 1991 by The Welding Institute (TWI) in England, FSW has already gained acceptance in land and sea transportation systems. Aerospace applications are rapidly expanding as exemplified in the Delta rocket program, military aircraft applications, and most recently, by the commitment from NASA to incorporate friction stir welding technology into the Space Shuttle External fuel tank. How friction stir welding works. In friction stir welding, a cylindrical, shouldered tool with a profiled pin is rotated and slowly plunged into the joint line between two pieces of sheet or plate material. The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. Frictional heat is generated between the wear-resistant welding tool and the material of the work pieces. This heat causes the latter to soften without reaching the melting point. The plasticized material is transferred from the leading edge to the trailing edge of the tool pin as the tool traverses along the joint. The resulting extrusion/forging process leaves a solid phase bond between the two pieces. The process can be regarded as a solid phase keyhole welding technique since a hole to accommodate the probe is generated, then filled during the welding sequence. Traditional mechanical fastening is slow and labor intensive, and even auto-riveting is comparatively slow. Aerospace composite technologies are not compatible with high-rate production. The joint is left with a fine-grained join with no entrapped oxides or gas porosity. Higher properties & no heat distortion. Since friction stir welding is not a fusion process requiring significant heat input, the properties of the metal in the joined area are higher than those from any other known welding process and distortion is minimized or eliminated. Works with multiple aluminum alloys. Unlike fusion welding techniques (laser, etc.), friction stir welding can be used to join dissimilar aluminum alloys (wrought or cast products). Superior weld quality. FSW is unlike traditional welding because the bonding occurs below the melting point of the aircraft material. The process provides clean and consistent high-strength bonds with minimal distortion. The repeatable quality of the solid-phase welds can improve existing products and lead to a number of new product designs previously not possible. Joints with the highest quality can be achieved through friction stir welding. The crushing, stirring and forging action of the FSW tool produces a joint with a finer microstructure than the parent material. As demonstrated by the tensile tests performed on riveted and lap welded 2024 aluminum, friction stir welded joints are stronger than traditional riveted joints. The friction stir welded samples have twice the ultimate strength. Joint geometries. The process has been used for the manufacture of butt and lap joints. For each of these joint geometries, specific tool designs are required which are being further developed and optimized. Longitudinal and circumferential butt joints in Al alloy fuel tanks for space flights have been friction stir welded and successfully tested. The FSW process can cope with circumferential, annular, non-linear, and three-dimensional welds. Since gravity has no influence on the solid-phase process, it can be used in all positions:
- Horizontal
- Vertical
- Overhead
- Orbital
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