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I. INTRODUCTION

FRICTION stir welding (FSW) was developed and patented in the early 1990s[1,2,3] and has rapidly become an important industrial joining process. The advantages of FSW over conventional fusion welding have been recognized by many researchers, particularly for industries that rely heavily on joining aluminum alloys.[1–19] A few significant advantages in the manufacturing of aluminum structures include the elimination of cracking in the weld fusion and heataffected zones (HAZ), weld porosity, filler metals, shielding gases, and costly weld preparation.[1,4] Despite these potential benefits, there remains a vast amount of research and development needed to better understand the microstructure and long-term service integrity associated with friction stir welds. An understanding of microstructural evolution during FSW and the mechanisms by which the associated microstructures affect such mechanical and physical properties as strength, fatigue, creep, and corrosion is critical. Such an understanding will bring broader acceptance and, inevitably, new applications for this innovative joining technology. Various authors have reported some of the fundamental materials aspects associated with FSW.[5–17] Of primary importance in aluminum metallurgy for strength, formability, and corrosion resistance considerations is the postweld precipitate morphology. Although limited, several investigators have reported on various precipitate morphologies and their effects on fracture and corrosion in the weld and HAZ associated with FSW.[5,9] The heterogeneity of fracture in the weld metal and HAZ is a function of precipitate volume fractions and distributions that are directly dependent upon grain boundary diffusivity. Grain boundary diffusivity is, in turn, proportional to the grain boundary energy and dependent upon the crystallographic structure of the boundary. Boundaries with low DAVID P. FIELD, Assistant Professor, is with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920. TRACY W. NELSON, Assistant Professor, and YURI HOVANSKI, Graduate Student, are with the Department of Mechanical Engineering, Brigham Young University, Provo, UT 84604. KUMAR V. JATA, Senior Researcher Materials Scientist, is with the Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted January 26, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

energy configurations will typically have low diffusivity, which retards the formation and growth of detrimental precipitates on crystallite interfaces. While not directly related, except through the assumption of a random distribution of crystallite lattice orientations,[20,21] possible grain boundary geometries are a function of crystallographic texture. Spatial gradients of the texture likely delineate regions of varying grain boundary structure. In addition, deformation response, corrosion susceptibility, surface appearance, and various other material prop