Multiscale Analysis of the Strength and Ductility of AA 6056 Aluminum Friction Stir Welds
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ENTIONAL welding of aluminum alloys is a limiting process for structural applications, which leads to the introduction of large ‘‘knock down factors’’ on the properties of the base alloys. These limitations stem both from the microstructure evolutions associated with the thermal history of welding and from the intrinsic defects (hot tearing and inclusions) associated with Dr. C. GALLAIS, formerly Ph.D. Student, ONERA, 92322, Chaˆtillon Cedex, France, is Research Technology Engineer, Hispano-Suiza. Dr. A. SIMAR is with the De´partement de Me´canique, Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium. Dr. D. FABREGUE, Associate Professor, is with the Institut National des Sciences Applique´es de Lyon, GEMPPM/MATEIS, 69621 Villeurbanne Cedex, France. Dr. A. DENQUIN and Dr. G. LAPASSET are with ONERA. Dr. B. de MEESTER, Professor, is with the De´partement de Me´canique, Universite´ catholique de Louvain. Dr. Y. BRECHET, Professor, is with the Institut National Polytechnique de Grenoble, LTPCM-ENSEEG, Domaine Universitaire de Grenoble BP75, F-38402 Saint Martin d’Heres, France. Dr. T. PARDOEN, Professor, is with the De´partement des Sciences Mate´riaux et des Proce´de´s, Universite´ catholique de Louvain. Contact e-mail: [email protected] Manuscript submitted July 10, 2006. Article published online May 22, 2007. 964—VOLUME 38A, MAY 2007
fusion and solidification.[1] Friction stir welding (FSW) allows for bypassing most of these limitations by joining aluminum alloys without a melting step. As shown in Figure 1, the FSW process is relatively simple in its principle.[2,3] A spinning tool progresses between the two plates to be joined, inducing both the stirring and the heating of the matter. By convention, the advancing side will always be located on the left-hand side of all micrographs and figures in the following. Both large deformations and nonisothermal treatments contribute to the microstructure evolution and to the subsequent modification of the mechanical properties. As a result, the mechanical properties of the joint are highly heterogeneous. One can distinguish various regions in a FSW joint (Figure 1(c)). (1) The central region, called the welding nugget (WN), in which the deformation and the maximum temperature were the largest, is fully recrystallized with very small grain sizes. (2) The zone next to the WN, exhibiting elongated and deformed grain shapes, has undergone both nonisothermal heat treatment and plastic deformation, but deformation was not large enough to lead to recrystallization. In this thermomechanically affected zone (TMAZ), the microstructure evolves METALLURGICAL AND MATERIALS TRANSACTIONS A
Figure. 1—Basics of the FSW process: (a) schematic description of the welding process, (b) maximum temperature and strains associated to the welding process, (c) metallography of a FSW joint between two AA 6056 plates and identification of the different zones, and (d) typical hardness evolution.
mainly through the dissolution or coarsening of the hardening phase. (3) Further away
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