Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451
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I. INTRODUCTION
COMPARED to many of the fusion-welding processes that are routinely used for joining structural alloys, frictionstir (FS) welding is an emerging solid-state joining process[1–12] in which the material that is being welded does not melt and recast. Therefore, when alloys are friction-stir welded (FSW), phase transformations that occur during the cool down of the weld are of a solid-state type. Due to the absence of parent-metal melting, the new FS welding process is observed to offer several advantages over fusion welding. The benefits that stand out most are welding of difficult-toweld aluminum alloys such as the 7xxx series, better retention of baseline material properties, fewer weld defects, low residual stresses, and better dimensional stability of the welded structure. Also, FS welding is an environmentally cleaner process, due to the absence of a need for the various gases that normally accompany fusion welding. A schematic of the FS welding assembly is shown in Figure 1. The FS welding process uses a nonconsumable pin made from a high-strength material that extends from a cylindrical shoulder. The shoulder and the pin rotate at several hundred revolutions per minute. The work pieces that are to be joined are firmly clamped to the work table, and the pin is plunged into the work pieces where the weld bond K.V. JATA, Senior Research Materials Scientist, is with the Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/ MLLM, Wright-Patterson Air Force Base, OH 45433. K.K. SANKARAN, Technical Fellow, is with the Boeing Company, St. Louis, MO 63166. J.J. RUSCHAU, Research Engineer, is with the Materials Engineering Division, University of Dayton Research Institute, Dayton, OH 45469-0136. Manuscript submitted September 22, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
line is desired. The height of the pin is slightly smaller than the thickness of the alloy plates that are being joined, so the penetration of the pin into the work pieces stops as soon as the shoulder of the cylinder makes contact with the surface of the work piece. The rotating pin (extending from the cylindrical shoulder) produces the stirring action in the material along the bond line and produces the required thermomechanical deformation. Frictional heating is produced from the interaction of the cylinder shoulder with the work piece and the downward applied forging pressure. To produce a longitudinal weld, the work piece assembly is translated relative to the shoulder and pin assembly. To produce an ideal defect-free weld, the revolutions per minute of the cylinder shoulder-pin assembly, travel speed, downward forging force, and pin tool design have to be optimized. Although the development of FS welding technology to make complex welds is proceeding at an extremely rapid pace, primarily due to the efforts of the industry, understanding of the microstructural transformations that occur during the welding process and of the postweld mechanical properties[5–13] has been slow. There has also been signi
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