Mechanical Behavior of AA6061 Aluminum in the Semisolid State Obtained by Partial Melting and Partial Solidification
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INTRODUCTION
THE aluminum alloy 6061 is used in many applications such as aeronautics, automotive industries, and architectural structures because of its versatility.[1] This Al-Mg-Si alloy exhibits good corrosion resistance and high mechanical properties with appropriate thermal treatment. The strengthening (caused by aging treatments) results from precipitation of metastable phases from the supersatured Al-rich solid solution. Consequently, the treated alloy contains numerous submicronic particles, which represent obstacles to dislocation motion and, thus, cause hardening.[1] However, these interesting properties can be modified during high-temperature forming processes and more particularly during welding and casting. This alloy, indeed, exhibits a great sensitivity to solidification defects such as porosity and hot cracking, which are very harmful for the subsequent mechanical properties of the alloy, thus limiting its use. Hot cracking is a defect that initiates above the solidus temperature, i.e., in the semisolid state where liquid and solid phases coexist. It occurs in the last stages of solidification when the solid volume fraction is above 85 pct.[2,3] For these solid fractions, the solid phase forms a continuous and dense network of grains with the liquid phase remaining in the form of thin films and pockets. In the course of solidification, deformations (due to solidification shrinkage and thermal contraction) develop and exert tensile strains on remaining liquid films. Therefore, hot cracks initiate into the liquid phase and propagate through this phase in an intergranular manner, which can possibly lead to E. GIRAUD, Ph.D. Student, and M. SUERY, Research Director, are with SIMaP, Universite´s de Grenoble, CNRS UMR5266, 38402 Saint Martin d’He`res Cedex, France. Contact e-mail: eliane.giraud@ simap.grenoble-inp.fr M. CORET, Associate Professor, is with LaMCoS, INSA-Lyon, CNRS UMR5259, 69621 Villeurbanne, France. Manuscript submitted October 20, 2009. Article published online June 15, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
fracture.[2,4] It is now clear that the main reason for the occurrence of hot cracking is the inability of the material to accommodate strains at the very end of solidification. Thus, this phenomenon depends on the mechanical response of the mushy state together with the possibility for the liquid to flow to accommodate deformation of the solid network. It is then obvious that a better understanding of the occurrence of hot cracking requires a better knowledge of the rheology of the mushy alloy. Essentially, three types of mechanical tests have been developed to study the rheological behavior of alloys in the semisolid state: tension,[2,4–10] compression,[2,11] and shear.[2,10] Although the tensile test is the most difficult to perform owing to the very low strength and ductility of a semisolid alloy, it is the most interesting in the context of hot tearing since it allows testing the alloy in conditions close to those prevailing during the formation of this defect. However, to be rea
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