To predict hot tearing in direct chill (DC) casting and shape casting of aluminum alloys using thermal stress analysis, cooling rate dependence of viscoplastic properties in a partially solidi ed state is indispensable. Based on viscoplastic properties determined from experiments, this study develops a method to predict the temperature dependence of viscoplastic properties at an arbitrary cooling rate through the Clyne-Kurz microsegregation model. For validation of the developed method, tensile tests were performed on Al-5 mass%Mg alloy in a partially solidi ed state at three cooling rates. Results show good agreement between the predicted values and experimentally obtained values, which demonstrates that the developed method is effective for predicting the cooling rate dependence of viscoplastic properties.

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To predict hot tearing of direct chill casting ingot, both the tensile constitutive behavior and elongation of alloy are inevitable during partial solidification. For predicting both the maximum true stress σ ss and the elongation ε elong regardless of alloy systems, their dominant factor was examined in terms of the solidification microstructure. For an Al-Mg and an Al-Cu alloys, (i) temperature T dependences of the maximum true stress and elongation (σ ss =f(T) and ε elong =f(T)) and (ii) dihedral angle θ of liquid phase formed at grain boundary were measured experimentally. Then, fraction of solid cohesion C was determined by the Campbell s model using the angle. Firstly, the solid fraction dependence of the tensile properties (σ ss =f(f s) and ε elong =f(f s)) were compared between the two alloys. The two dependences differ with each other. Secondly, the fraction of solid cohesion dependences of the tensile properties (σ ss =f(C) and ε elong =f(C)) were compared and the result shows that the two dependences were consistent with each other. The fraction of solid cohesion enables to explain the difference in solid fraction dependence of the tensile properties for the two alloys. The result demonstrates that the dihedral angle should be essential to predict the two tensile properties of alloy during partial solidification.

It is known that hot tearing tendency increases with the increase in the cooling rate of casting during the solidification. As the direct control factors of the hot tearing, the cooling rate dependences of the temperature field of the casting and the mechanical property of alloys in the semisolid state have been implied. However, quantitative evaluation is not clarified yet to show which parameters is more important to predict hot tearing tendency. In this study, through the thermal stress analysis using cooling rate-dependent temperature fields of casting and cooling rate-dependent creep parameters in the semi-solid state, hot tearing tendency was predicted for an AlMg alloy during the solidification. For the prediction, the maximum principal creep strain accumulated during the solidification was used as the indicator of hot tearing tendencies. Then, the hot tearing tendencies were compared with experimental results. As a result, in the cooling rate range of this study which was corresponding to gravity die casting, it was found that the temperature fields are relatively more critical to predict hot tearing tendency than the creep parameters.