Texture Evolution within the Thermomechanically Affected Zone of an Al-Li Alloy 2195 Friction Stir Weld
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INTRODUCTION
A. Processing Background
NASA is currently exploring spin-forming to reduce the manufacturing costs of 5.5-m-diameter domes for Al-Li 2195 cryogenic fuel tanks.[1,2] The domes investigated in the current study were produced by MT Aerospace in Augsburg, Germany, using their proprietary concave spin-forming process. Since a commercial plate of Al-Li 2195 alloy is not large enough to manufacture a single-piece 5.5-m-diameter dome, two plates were friction stir welded together to create a sufficiently large spin-forming blank. Friction stir welding (FSW) is a solid-state joining process currently being used in automotive, shipbuilding, and aerospace structural applications.[3] In comparison with conventional fusion welding, FSW parameters are controlled to be maintained at less than the melting temperature of the material, leading to improved property retention. In addition, FSW produces a fine, dynamically recrystallized grain structure within the weld zone, which results in better formability of the weld region compared with fusion welding.
WESLEY A. TAYON, MARCIA S. DOMACK, ERIC K. HOFFMAN, and STEPHEN J. HALES, Research Materials Engineers, are with the Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, VA 23681. Contact e-mail: [email protected] Manuscript submitted January 11, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
B. Development of Texture in FSW Materials In the study of FSW texture development, several ideal texture fibers have been widely documented,[4–6] such as the A and B fibers. The dominance of shear deformation has enabled many authors to reference textures developed through torsional deformation to describe the resultant texture characteristics within a FSW weld.[7–9] Specific {hkl}huvwi indices aligned with a common shear plane normal (SPN) and shear direction, respectively, have been identified.[4–9] The A-fiber, defined by the {111}huvwi family of Miller indices, consists of {111} fcc slip planes aligned with the shear plane (i.e., parallel to the tool axis). The B-fiber, defined by the {hkl}h110i family of Miller indices, results in the alignment of the h110i slip direction with the shear direction. The shear direction is mutually orthogonal to the weld direction and SPN. Al alloys develop a strong B-fiber during the FSW process.[10–13] While significant research has examined texture within the weld nugget, less effort has been given to characterize texture within the thermomechanically affected zone (TMAZ). Deviations from ideal texture component locations in both standard pole figure (PF) and orientation distribution function data indicate the occurrence of rigid body rotations as texture evolves during the FSW process.[5,6,10–15] Rigid body rotations, associated with the flow of material and shearing deformation due to the FSW process, alter the texture and grain morphology. Schneider and Nunes[16] proposed a kinematic model of material flow based on various FSW marker/tracer and flow visualization studies. Nunes deconvoluted the flow path within a weld n
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