An alternative method to identify critical temperatures for semisolid materials process applications using differentiation

Abstract

A novel method is proposed for identifying critical temperatures in thixoformability analysis. The proposed method applies the principles of differential calculus to semisolid transformation curves. The method is compared with the conventional tangent method for determining the solidus and liquidus temperatures using DSC curves and with the visual recognition method for identifying the “knee”, or unstable main eutectic transformation temperature. Numerical simulation is performed with Thermo-Calc® (under the Scheil condition) for A356 alloy, and the same alloy is used to generate experimental DSC data for cooling and heating cycles under different kinetic conditions (5, 10, 15, 20 and 25°C/min) in order to compare the conventional methods for identifying critical temperatures and the novel method described here. The findings indicate that the proposed method, referred to here as the differentiation method, is an efficient tool for identifying critical points such as the solidus, liquidus and knee temperatures and that the results obtained using the method are in good agreement with results obtained with traditional methods. The method also proved to be operator-independent, as it uses well-defined mathematical/graphical criteria to identify critical points. Furthermore, the new method is easy to use, as after differentiation the critical points become easily recognizable visual and numerical features such as valleys and peaks (for the knee temperature) and zeros (for the solidus and liquidus) on the curve being analyzed. The following formal definition for the eutectic knee, the first of its kind in the literature, is proposed: “the peak/valley in the expected eutectic range of the differentiated DSC curve of the solidification/melting transformation”. Application of the proposed definition yields results that agree well with results obtained by the traditional visual recognition method, indicating its suitability for identification of the eutectic knee, especially when a comparative study of different shaped curves is being performed. A definition that allows suitable lower and upper limits for the SSM processing window to be identified is also proposed.