Evolution of Precipitate Microstructure During Creep of an AA7449 T7651 Aluminum Alloy
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TRODUCTION
ALUMINUM alloys of the Al-Zn-Mg-Cu series are among the major materials for aerospace applications.[1] Their classical process route for modern airplane structures involves a heat treatment carried out on thick plates or forgings, followed by machining to the desired shape. The quench step from the solution treatment temperature, carried out on plates of large dimensions, results in high levels of internal stresses due to through thickness temperature gradients and heterogeneous plastic flow during quenching. These internal stresses are only partially removed by stress-relieving plastic deformation carried out prior to the aging treatment.[2] They can result in part distortion after machining and reduced fatigue resistance. An alternative process route is creep forming, where the material is loaded elastically and subsequently heated to the heat treatment temperature, therefore deforming plastically to the desired shape.[3] Creep forming has the advantage of reducing the level of internal stress due to the reduced flow stress at the aging temperature and allowing for the forming of complex shapes (e.g., with double curvature) with reduced springback effect. Creep forming of precipitation hardening aluminum alloy has received some attention in the literature, mainly from the structural mechanics point of view, where authors aimed at providing appropriate constitutive laws to describe G. FRIBOURG, formerly PhD Student, Grenoble Institute of Technology, Grenoble, France, is now a Research Engineer, SNECMA, Gennevilliers, 92702 Colombes, France. Y. BRE´CHET and A. DESCHAMPS, Professors, and J.L. CHEMIN, Research Engineer, are with the SIMAP Laboratory, INP Grenoble-NRS-UJF, 38402 St Martin d’He`res Cedex, France. Contact e-mail: alexis.deschamps@ grenoble-inp.fr Manuscript submitted December 6, 2010. Article published online July 20, 2011 3934—VOLUME 42A, DECEMBER 2011
the deformation behavior of the material under the combination of temperature and stress (e.g., References 4 and 5). However, one concern arises about the evolution of the precipitate microstructure specifically induced by the plastic deformation, and therefore about the possibility of obtaining inhomogeneous materials properties in the manufactured part due to inhomogeneous levels of experienced plastic strain. Dynamic precipitation under plastic strain in the aluminum alloy is a relatively well-chartered area;[6–8] however, several phenomena can occur depending on the combination of the initial state of the material (supersaturated solid solution or containing precipitates), temperature, and strain rate. Precipitation can be enhanced by plastic strain or, alternatively, precipitates can be dissolved. The microstructure evolution during creep forming has received sparse attention in the literature.[9–13] In 7000 series aluminum alloy, Li et al.[14] very recently showed some accelerated precipitate growth and coarsening that led to a reduction of mechanical properties. However, a quantitative analysis of the evolution of precipitates as a function
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