Influence of alloying elements on the thermal analysis results of Al–Si cast alloys

Abstract

Knowledge about the influence of alloying elements on the eutectic temperature and the eutectic depression is essential for assessing the quality of a modification treatment by means of thermal analysis. In this work, the influence of magnesium on sodium and strontium modification was studied while the effects of Si, Cu and Fe were investigated for sodium modification. In addition, the evaluation methods of different commercial thermal analysis systems were studied critically with respect to the determination of the eutectic reference temperature of the non-modified melt. This is usually done by means of equations from literature. These equations consider the effect of alloying elements on the eutectic temperature. Magnesium is shown to affect the depression values in a different way from that expected up to now. The eutectic depression is found to fall substantially with an increasing Si content and to increase with a growing Cu content while the effect of iron was not very pronounced. Thermal analysis systems which calculate the eutectic reference temperature by means of equations from literature may show systematic errors due to different cooling conditions in the thermal analysis experiment. Setting up these empirical equations with experiments at the right cooling rates leads to a very good agreement with the estimated and measured reference temperatures. Systems where ternary reactions in the cooling curves are used to estimate the concentrations of the corresponding elements proved to be unsuccessful because the effects of the alloying element on the cooling curve were not detected correctly.