This is material remote from the weld, which has not been deformed, and which although it may have experienced a thermal cycle from the weld is not affected by the heat in terms of microstructure or mechanical properties. This has been developed at TWI, but has been discussed with a number of appropriate people in industry and academia, and has also been provisionally accepted by the Friction Stir Welding Licensees Association Unaffected material or parent metal: It is therefore proposed that the following revised scheme is used. However, it has become evident from work on other materials that the behavior of aluminium alloys is not typical of most metallic materials, and therefore the scheme cannot be broadened to encompass all materials. This work was based solely on information available from aluminium alloys. The first attempt at classifying microstructures was made by P L Threadgill (Bulletin, March 1997). 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 DESCRIPTION OF THE ROTATING TOOL PIN It leaves a solid phase bond between the two pieces. The plasticized material is transferred from the leading edge of the tool to the trailing edge of the tool probe and is forged by the intimate contact of the tool shoulder and the pin profile. The maximum temperature reached is of the order of 0.8 of the melting temperature of the material. This heat causes the latter to soften without reaching the melting point and allows traversing of the tool along the weld line. Frictional heat is generated between the wear resistant welding tool and the material of the work pieces. The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. In friction stir welding (FSW) a cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces butted together. It is also a cleaner and more efficient process compared to conventional techniques. Also, FSW is a process that can be automated. Since its invention, the process has received world-wide attention and today two Scandinavian companies are using the technology in production, particularly for joining aluminium alloys. Compared to conventional friction welding, FSW uses a rotating tool to generate the necessary heat for the process. Consistent with the more conventional methods of friction welding, which have been practiced since the early 1950s, the weld is made in the solid phase, that is, no melting is involved.
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