Investigation on texture evolution during friction stir welding of stainless steel
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I. INTRODUCTION
DURING friction stir welding (FSW), the material experiences a severe combination of deformation and heating and, as a consequence, undergoes a substantial alteration of its microstructure. The alterations can include modification of its crystallographic texture and various features of its subgrain structure, as documented in a number of recent articles. These changes are known to affect the postweld properties such as the strength, the fatigue life, and the toughness. In AA 7075 alloys, for example, microstructures after FSW show fully recrystallized grains in the weld zone and an overaged condition in the heat-affected zone (HAZ). The welded material exhibits low strength and is sensitive to intergranular fracture.[1] The stability of grain structures within the weld zone has also been investigated during solution treatment after FSW.[2] For 304 stainless steel, the welding temperature, grain size, and strength increased with higher tool rotation rates.[3] Deformation textures in materials joined by FSW possess components that arise from shearing.[4,5] Such shear textures are evidence of the intense shear deformations that occur during FSW. Dynamic recrystallization is also a factor in FSW, based on evidence from several experimental observations: equiaxed grains in optical micrographs, low dislocation density in transmission electron microscopy micrographs, and reduced hardness from microindentations. In a 6063 Al alloy, for example, material in the base zone exhibited a Goss texture, while in the weld zone created with FSW, shear textures of dynamically recrystallized grains were observed.[4] Strong textures persist even if recrystallization occurs. Further, texture gradients exist in the through-thickness and transverse directions to the weld direction. Electron-backscatter diffraction (EBSD) measurements have confirmed that such gradients in crystallographic texture are present in joints made by FSW.[5] JAE-HYUNG CHO, Postdoctoral Associate, and PAUL R. DAWSON, Professor, are with the Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853. Contact e-mail: prd5@ cornell.edu Manuscript submitted May 5, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
Strong textures typically imply high strength of anisotropy.[6] Texture gradients imply that the anisotropic properties vary spatially. The severity of the deformation and the strength of its spatial gradients complicate the modeling texture evolution during FSW. Nevertheless, it is possible to employ modeling of texture evolution to explain trends that are observed. In particular, we can examine the textures in light of the dynamics of lattice reorientation during crystallographic slip together with the character of the velocity gradients that arise during FSW and drive the lattice reorientations. To investigate texture evolution, it is necessary to evaluate the histories of deformation and heating that the material experiences in relation to its proximity to the tool and the welding parameters. Welding and to
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