Shear Behavior of AA6061 Aluminum in the Semisolid State Under Isothermal and Nonisothermal Conditions
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INTRODUCTION
SOLIDIFICATION of metallic alloys involves the transformation of the liquid phase into one or more solid phases over a given temperature interval. During this transformation, several transitions are usually defined separating various types of behavior.[1–3] The first transition corresponds to the coherency solid fraction, which separates the domain where the solid grains are free to move in the liquid from the domain where they begin to mechanically interact. Below the coherency solid fraction, the viscosity of the material is close to that of the liquid. Just above this solid fraction, the material is able to transmit shear strains, but the bonds between the solid grains are too weak so that the material cannot transmit tensile strains. In the case of Al alloys, this transition occurs for solid fractions lower than 0.6.[1,2,4] The second transition separates the domain where the liquid is able to flow between the solid grains from the domain where intergranular flow is no longer possible. At this point, the liquid is present only as films between the solid grains. The material can then transmit shear and tensile strains. In general, this second transition starts at a solid fraction of about 0.9.[1,2] Finally, the coalescence solid fraction corresponds to the point where the solid starts to coalesce and E. GIRAUD, formerly with the Universite´ de Grenoble, is now Postdoctoral Researcher with the MMS, University of Lie`ge, 4000 Lie`ge, Belgium. Contact e-mail: [email protected] M. SUERY, Research Director, is with the Universite´ de Grenoble, SIMaP, UMR CNRS 5266, 38402 Saint Martin d’He`res Cedex, France. M. CORET, Associate Professor, is with the Universite´ de Lyon, LaMCoS, INSALyon, UMR CNRS 5514, 69621 Villeurbanne, France. Manuscript submitted July 30, 2010. Article published online June 1, 2011 3370—VOLUME 42A, NOVEMBER 2011
form a continuous skeleton with a mechanical strength close to that of the solid. Several investigations[1,2,4–7] showed that this transition occurs for solid fractions close to 0.97 in the case of Al alloys. The solid fraction for which liquid flow can no longer be possible is very critical during casting and fusion welding processes, since it corresponds to the possible occurrence of hot tearing.[1,6,8–10] Indeed, at this stage, strains generated in the material by solidification shrinkage, thermal contraction, and external loading cannot be accommodated by liquid flow, thus leading to crack formation.[11] The study of the hot tearing phenomenon, therefore, requires investigation of the behavior of the alloy in the range 0.90 to 0.97, where the alloy is relatively brittle.[1,5,6,8,9,12] Modeling of this phenomenon requires, on the one hand, the knowledge of the rheological behavior of the alloy in this solid fraction range and, on the other hand, a criterion for the formation of cracks. Since cracks are usually the result of tensile strains, the tensile behavior of the alloy close to the end of solidification is of prime importance. This behavior was investigated recently in
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