Basal plane texture and flow pattern in friction stir weld of a magnesium alloy
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RODUCTION
FRICTION stir welding is a solid-state joining process that was invented at The Welding Institute (TWI) in 1991.[1,2,3] It has enabled the butt welding of aluminum (Al) alloys, some of which are difficult or impossible to fusion weld without voids, cracking, or distortion.[4–22] Furthermore, friction stir welds often exhibit better tensile, bend, and fatigue properties than fusion welds. Taking advantage of these positive factors, this process has already been applied to the construction of large Al structures.[10,11] Friction stir welding, which is characterized by frictional heating and intense material flow arising from the rotation of the welding tool, generally creates an elliptical nuggetshaped stir zone in a plane perpendicular to the welding direction around the weld center.[1,2,4,6,12] Although the stir zone usually has a fine equiaxed recrystallized grain structure, evolution of the elliptical nugget-shaped stir zone has not yet been clarified. A previous study[23] has suggested that the stir zone roughly corresponds to the region experiencing intense material flow during the welding, so that the shape of the stir zone strongly depends on the material flow characteristics. Material flow during welding has been investigated using several methods, and experimental and computational results have been reported.[23–28] Colligan[24] examined the material flow using a tracer material (Fe) embedded at different positions within the butt-joint welds of Al 6061 and Al 7075 plates. The tracer-material distribution, which was obtained by stopping the forward motion of the welding tool while it was still in the seeded material, suggested material movement around the welding tool during the process. He concluded that much of the material was lifted and extruded around the hard pin in the direction of the pin rotaSEUNG HWAN C. PARK, Ph.D. Student, YUTAKA S. SATO, Research Associate, and HIROYUKI KOKAWA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: [email protected] Manuscript submitted June 27, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
tion. Reynolds and co-workers[25,26] visualized the material flow in friction stir welds using a marker insert technique (MIT). The MIT showed that the friction-stir-welding process could be roughly described as an in-situ extrusion process wherein the tool shoulder, the weld backing plate, and the cold material outside the weld zone formed an “extrusion chamber,” which moved relative to the plate of the base material. During the extrusion, the material experienced the upward and downward movement in the “extrusion chamber.” An increase in heat input promoted this movement of the material. Bendzsak et al.[27] examined the material flow in the vicinity of the tool surface by a computational method. The material flow occurred in the direction of tool rotation just below the shoulder. In a region apart from the shoulder, on the other hand, the direction of t
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