Fourier Thermal Analysis of Eutectic Al-Si Alloy with Different Sr Content
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Fourier Thermal Analysis of Eutectic Al-Si Alloy with Different Sr Content R. Aparicio 1, G. Barrera 2, G. Trapaga 1 and C. Gonzalez 3 1 CINVESTAV Queretaro, Libramiento Norponiente 2000, Real de Juriquilla, 76230 Queretaro Qro. Mexico, [email protected] 2 Instituto de Investigaciones Metalúrgicas, UMSNH. Apdo. Postal 888 Centro, 58000. Morelia Mich. México. [email protected] 3 Department of Metallurgical Engineering, Facultad de Química, UNAM, Edificio “D” Circuito de los Institutos s/n, Cd. Universitaria, 04510 México D. F., México. [email protected] ABSTRACT The purpose of this work is to explore the capability of Fourier Thermal Analysis (FTA) to detect differences in solidification kinetics between unmodified and Sr modified eutectic Al-Si alloy obtained from the same base alloy. Experimental melts are produced in silicon carbide crucibles using an electrical resistance furnace and burdens of A356 alloy and commercial purity Si. The addition of strontium to the melts is accomplished using Al10 pct Sr master alloy rod. Chemical composition is controlled using spark emission spectrometry. The changes in microstructure are characterized using optical microscopy. Thermal analysis are performed in cylindrical stainless steel cups coated with a thin layer of boron nitride, using two type-K thermocouples connected to a data acquisition system. Experimental cooling curves are numerically processed using FTA. Results show changes in solidification kinetics of eutectic Al-Si alloy with different Sr content. These changes, measured at the beginning and during solidification of the probes, can be related to the changes in nucleation and growth causing the differences detected during microstructural characterization of the probes. INTRODUCTION For commercial Al-Si cast alloys it is a common practice during melt treatment to include the addition of little quantities of strontium in order to refine the coarse flake-like structure of the silicon phase of the eutectic constituent to a fine fibrous structure [1], improving properties of cast components. There are two main theories trying to explain eutectic modification. The first theory is focused on changes of the growth of eutectic silicon by a mechanism named impurity induced twinning [2]. The second theory is based in an inhibition of nucleation of eutectic silicon that results in more undercooling and the refinement of microstructure [3, 4].The real mechanism of modification is probably related both to the reduction in nucleation and the changing growth of eutectic silicon by impurity induced twinning. However it is still not yet understood [5]. Taking into account that modification mechanisms have been explained in terms of changes in nucleation and growth of eutectic silicon it is interesting to explore the changes in solidification kinetics of Al-Si alloys as a result of addition of eutectic modifiers. Computer aided cooling curve analysis (CA-CCA) methods have been used to study solidification kinetics of various alloy systems of metallurgical interest
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