A constitutive model for the tensile deformation of a binary aluminum alloy at high fractions of solid
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I. INTRODUCTION
SHAPE casting is a metallurgical process in which liquid metal is poured in a mold to obtain a solid product meeting a set of geometrical requirements. Operations such as continuous casting, direct chill casting (DC casting), and molding belong to that category of processes in which the quality of products is largely dependant upon the behavior of the metal during solidification. The main aspect to consider during solidification of most alloys is certainly the shrinkage associated with the liquid-solid transition. Indeed, solidification shrinkage can induce tensile stresses in certain zones of the castings, especially where metal contraction is partially impeded by the mold or the casting itself. These stresses may in turn initiate cracks, especially where the microstructure is weakened by the presence of a liquid phase. These types of defects have been studied by many investigators and the phenomenon is most often referred to as “hot tearing.” Since Pellini[1] introduced the filmstage concept in its strain theory of hot tearing, a consensus has been achieved in the literature for considering hot tear formation as a three-stage phenomenon taking place as follows.[2] (1) The first solidified crystals are small and isolated. The melt behaves as a viscous fluid. (2) The metal is composed of a three-dimensional (3-D) array of crystals surrounded by a continuous liquid film. At this stage, strain accommodation is possible by a reordering of crystals and liquid flow.
DANIEL LAROUCHE, Professor Assistant, is with the Department of Mining, Metallurgy and Materials Engineering, Laval University, Québec, PQ, Canada, G1K 7P4. Contact e-mail: [email protected] JOSEPH LANGLAIS, Program Manager, is with the Arvida Research and Development Center, Alcan International Ltd., Saguenay, PQ, Canada, G7S 4K8. WEILI WU, formerly Graduate Student, Department of Applied Sciences, University of Québec at Chicoutimi, Chicoutimi, PQ, Canada G7H 2B1, is not currently employed. MICHEL BOUCHARD, formerly Professor, Department of Applied Sciences, University of Québec at Chicoutimi, is retired. Manuscript submitted March 11, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
(3) The liquid phase is no longer continuous and cannot freely flow toward zones where depressions will eventually promote the creation of cavities and cracks. Many authors studied the conditions prevailing in the formation of microporosity and hot tearing in the third stage. Criteria based on stress, strain, strain rate, or other principles were proposed for hot tearing predictions and have been reviewed recently by Eskin et al.[3] These contributions will not be discussed here because the present work is more dedicated to the mechanical behavior of alloys at high fractions solid. The mechanical behavior of semisolid alloys received a lot of attention since the pioneering works of Flemmings and his group.[4] Depending of the fraction liquid, the behavior of semisolid materials under stress has been described with rheological or creep-type laws. Th
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