The rheological behavior of a solidifying alloy is modeled by considering the deforming material as a viscoplastic porous medium saturated with liquid. Since the solid grains in the mush do not form a fully cohesive skeleton, an internal variable that represents the partial cohesion of this porous material is introduced. The model parameters are identified using shear and compressive stress states under isothermal conditions on an Al-Cu model alloy. The model is partially validated with nonisothermal conditions and we complete this study with tensile conditions. Such conditions, when applied on the mush, may lead to severe defects in many casting processes. The model has been implemented into a commercial finite-element code to simulate a tensile test. Comparison with experimental data shows that the model is able to reproduce the main features of a solidifying alloy under tension, although fracture is not directly addressed here. We show that two critical solid fractions must be introduced in the model to account for the rheology: the coherency solid fraction at which the mush acquires significant strength and the coalescence solid fraction at which solid grains start to form solid bridges.

This article presents the first results of a new experimental technique developed to investigate the evolution of the morphology of the solid and liquid phases during the solidification of a metallic alloy. It consists of ultrafast X-ray microtomography observations of a solidifying aluminum-copper alloy carried out at ESRF. These experiments allow investigating in-situ the formation of the casting microstructure and of the evolution of the morphology of the solid and the liquid phases. It allows also the in-situ determination of the solidification path, of the variation of the copper content in both the liquid and solid phases, and of some other characteristic parameters of the microstructure. Provided that some forthcoming technical improvements on the experimental setup are performed, more quantitative results can be obtained as well as better image quality and resolution.

During casting of aluminum alloys, the partially solidified material is submitted to thermally induced strains that can lead to severe casting defects, such as hot tearing. In this work, carried out in the frame of the European project VIR [CAST], the rheological behavior of a partially solidified AA5182 aluminum alloy has been investigated in order to provide constitutive equations to predict hot tearing in direct chill (DC) casting. Shear and tensile experiments have been performed using specific experimental devices and procedures previously designed for Al-Cu alloys. In the small strain (<0.2) and high solid fraction (>0.8) domain investigated here, the mushy zone is coherent. The stress-strain behavior is therefore dominated by the viscoplasticity of the solid phase, but exhibits a significant strain hardening. The behavior of the mushy zone is modeled by a compressible constitutive equation in which an internal variable, C, representing the state of cohesion of the mush, is introduced. The model accounts for solid fraction, stress state, strain rate and strain effects. The parameters that govern the evolution of C with strain have been determined and appear to be comparable to those for Al-Cu alloys.

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