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INTRODUCTION

FRICTION stir processing (FSP) and friction stir welding (FSW) are allied technologies involving localized severe plastic deformation induced by the action of a nonconsumable tool on a deformable material. In both FSP and FSW, the tool generally consists of a cylindrical shoulder portion with a projecting, concentric, and smaller-diameter pin. The tool is rotated while the pin is forced into the surface of the workpiece material. A combination of frictional and adiabatic heating leads to softening and allows the tool to penetrate until the shoulder comes into contact with the surface of the workpiece. Welding may be accomplished by traversing the tool along abutting edges of the restrained materials that are to be joined, so that metal flow around the pin leads to coalescence and the formation of a solid-state bond between similar or SRINIVASAN SWAMINATHAN, Research Scientist, is with General Electric–Global Research, Bangalore 560066, India. KEIICHIRO OH-ISHI, Postdoctoral Scientist, is with the National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan. ALEXANDER P. ZHILYAEV, RyC Fellow, is with the Centro Nacional de Investigaciones Metalu´rgicas (CENIM), CSIC, 28040 Madrid, Spain, and Leading Research Scientist, with the Institute for Metals Superplasticity Problems, RAS, 450001 Ufa, Russia. CHRISTIAN B. FULLER, Research Engineer, is with General Electric Co., Healthcare Division, Milwaukee, WI, 53219. BLAIR LONDON, Professor, is with the Department of Materials Engineering, California Polytechnic State University, San Luis Obispo, CA 93407. MURRAY W. MAHONEY is Independent Consultant, Midway, UT 84049. TERRY R. MCNELLEY, Distinguished Professor, is with the Center for Materials Science and Engineering, Department of Mechanical and Astronautical Engineering, Naval Post Graduate School, Monterey, CA 93943-5146. Contact e-mail: [email protected] Manuscript submitted June 4, 2009. Article published online December 22, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

dissimilar materials.[1] In FSP, the tool may be traversed in a predetermined pattern to process a volume in the workpiece defined by the pin tool profile and the traversing pattern. When applied to a cast metal, FSP may convert the as-cast stir zone (SZ) microstructure to a wrought condition, thereby improving physical and mechanical properties in the absence of a macroscopic shape change.[2,3] Heating during FSP/FSW is due to a combination of adiabatic deformation in a volume of material surrounding the tool pin and friction at the workpiece-tool interface. The equilibrium of forces and moments, traversing of the tool, and localized plastic deformation induced by tool rotation result in rapid transients and steep gradients in the SZ strain, strain rate, and temperature. The extent of the SZ and surrounding thermomechanically affected zone (TMAZ) depends on the constitutive behavior of the material as well as the tool design and processing conditions. Tool features such as threads, flutes, or stepped spirals result in complex