Thermochimica Acta 651 (2017) 22–33 Contents lists available at ScienceDirect Thermochimica Acta j ourna l h omepage: www.elsevier.com/locate/tca An alternative method to identify critical temperatures for semisolid materials process applications using differentiation Gabriela Lujan Brollo, Cecília Tereza Weishaupt Proni, Leandro Cássio de Paula, Eugênio José Zoqui ∗ Materials and Manufacturing Department, Faculty of Mechanical Engineering, University of Campinas – UNICAMP, Campinas, SP 13083-860, Brazil a r t i c l e i n f o Article history: Received 8 November 2016 Received in revised form 6 February 2017 Accepted 9 February 2017 Available online 16 February 2017 Keywords: Semisolid processing Thixoformability Thermodynamic characterization a b s t r a c t A novel method is proposed for identifying critical temperatures in thixoformability analysis. The pro- posed 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 tem- peratures 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 solidifica- tion/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. © 2017 Elsevier B.V. All rights reserved. 1. Introduction The term semisolid materials (SSM) processing refers to the forming of metallic alloys in the semisolid temperature range [1,2]. SSM processing is divided into two main categories: rheocasting, in which controlled cooling is applied to a liquid alloy so it can reach the semisolid state with a solid fraction of approximately 0.2–0.5 [3], and thixoforming (or thixocasting), in which a solid billet is heated in a controlled manner up to a liquid fraction of approxi- mately 0.3–0.6 in the semisolid range [4]. The key to SSM processing ∗ Corresponding author. E-mail addresses: gbrollo@fem.unicamp.br (G.L. Brollo), wyliah@gmail.com (C.T.W. Proni), leandrocp@fem.unicamp.br (L.C. de Paula), zoqui@fem.unicamp.br (E.J. Zoqui). is to ensure that the solid/liquid mixture can be easily controlled and that the semisolid alloy microstructure has an spheroidal rather than a dendritic morphology. This can be achieved using several techniques, such as stirring during casting or reheating the alloy after it has been subjected to forming or grain-refined [4–9]. A spheroidal microstructure, which is extremely temperature- and time-dependent [10,11], ensures that the alloy exhibits semisolid thixotropic behavior when sheared [12,13]. The practical result of this special rheology is smooth, laminar die filling, avoiding defects usually found in conventional die casting, such as porosities, and requiring the use of only a fraction of the force needed in con- ventional metal forming, with consequent energy savings. Other advantages include lower processing temperatures, reduced ther- mal shock to the die, less solidification shrinkage, reduced material loss and lower process costs [1,2]. http://dx.doi.org/10.1016/j.tca.2017.02.010 0040-6031/© 2017 Elsevier B.V. All rights reserved.