Fourier thermal analysis applied to sodium eutectic modification of an Al-Si7 alloy

The so-called “Fourier Thermal Analysis” (or FTA) is an evolution of the “integral thermal analysis”, which is actually used as a process control in Aluminium and cast iron foundries. It has been developed since late 80’s in order to investigate nucleation and growing kinetics of the various phases in multi-component alloys. FTA is based on the evaluation of the thermal gradient in one-dimensional thermal field that arises in a cylindrical solidifying specimen. During the last twenty years, both the traditional thermal analysis and FTA have been applied to the experimental determination of the solid fraction during solidification, in order to assess results from numerical simulation. Nevertheless, FTA has not been applied to foundry process control or optimisation. Eutectic modification is extensively used in low-pressure permanent mould processes, in order to improve tensile properties and toughness of Al-Si alloys. The effectiveness of the treatment is subjected to the presence a minimum amount of modifying elements, such as Sr, Na or other elements. Traditional thermal analysis is useful in determining modification level of the alloy, then to control the modification treatment. Aim of this work is to verify the potentiality of gradient-based thermal analysis method, such as FTA, in eutectic modification investigation. An A356-type hypoeutectic Al-Si alloy has been modified with metallic sodium at four different modification levels. Two-thermocouple thermal analysis curves have been recorded, in order to perform FTA analysis. Fraction solid versus time (fs(t)) and temperature (fs(T)) have been determined at different modification levels. Microstructural characterization has been made using automatic image analysis. Average values of dimension and roundness of eutectic Si have been compared to thermal analysis results. A significant correlation between the so-called “eutectic depression” and silicon morphology has been observed. More relevant is the behaviour of the fraction solid curves, from which it is possible to note a significant delay in the start of eutectic reaction. This delay has been observed not only in time but also in temperature and fraction solid domain.