Microstructure Evolution during Friction Stir Welding of Mill-Annealed Ti-6Al-4V
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FRICTION stir welding (FSW), invented in 1991 at The Welding Institute,[1] is a solid-state joining process that has received significant attention throughout several industries because of its high success for joining aluminum alloys.[2–4] The process features a nonconsumable rotating tool with a probe that is plunged into the seam between two sheets or plates to be joined. Local heating caused by friction between the rotating tool shoulder and the workpiece(s) reduces the flow stress of the material, and upon translation of the tool, the softened material is plastically deformed by the rotating pin.[5] Details of material flow during processing are generally difficult to measure experimentally and must be inferred from marker studies,[6,7] the resultant stir zone microstructure and texture,[8–14] and numerical simulation.[15,16] As depicted schematically in Figure 1, A.L. PILCHAK, formerly Graduate Research Associate, Department of Materials Science and Engineering, The Ohio State University. Columbus, OH 43210, is Visiting Scientist, Air Force Research Laboratory, Materials and Manufacturing Directorate/ RXLM, Wright Patterson Air Force Base, OH 45433, and Research Scientist, Universal Technology Corporation, Dayton, OH 45432. Contact e-mail: [email protected] W. TANG, Associate Research Professor, H. SAHINER, Graduate Research Assistant, and A.P. REYNOLDS, Professor and Frank B. Herty Bicentennial Chair in Engineering, are with the Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208. J.C. WILLIAMS, Professor and Honda Chair Emeritus, is with the Department of Materials Science and Engineering, The Ohio State University. Manuscript submitted March 2, 2010. Article published online November 20, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
it is generally accepted that there is a layer of material, called the ‘‘shear zone’’ or ‘‘shear layer,’’ that reaches the highest temperature and flows at the highest rate around the pin. This region also accumulates the highest amount of strain during welding and ultimately constitutes the stir zone after the tool has passed and deformation is complete. Outside of the shear zone is the transition zone,[8] which serves as a buffer between the nondeformed material in the heat-affected zone (HAZ) and the material moving at the highest rate in the shear zone. This region also experiences deformation during welding, but the total strains are smaller and the strain rates are lower than in the stir zone. Finally, the HAZ experiences a temperature rise during welding, but does not experience any strain due to the rotating tool. Large scale electron backscatter diffraction (EBSD) analysis of friction stir welds has shown that the rotating pin imposes a state of simple shear that changes depending on the location around the tool where the shear direction (SD) is tangent to the tool rotational velocity and the shear plane normal (SPN) is perpendicular to this tangent at any location.[9,11,13,17] This simple shear reference frame is illustrated
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