Modification of commercial and high purity AlSi5Cu1Mg alloys by Ca and Sr additions

Due to their good castability and excellent mechanical properties at high temperatures, C355 (AlSi5Cu1Mg) alloy is the most important and widely used commercial casting Al alloys in aerospace components. Further increase in mechanical properties can be achieved through modification treatment with Calcium and Strontium, or by using high purity alloys, which generally contain a very low amount of Phosphorus-based compounds. In foundry, the P content of the commercially use alloy is analysed by emission spectroscopy or spectrophotometry, which show a detection limit of 10 ppm [1]. The purpose of this work is to evaluate the use of thermal analysis (TA) to explore the solidification kinetics of commercial and high purity modified C355 alloys. Metallographic and TA techniques, based on two thermocouples’ method [2], were used to quantitatively examine the microstructural changes occurring with modifier additions (Ca and Sr) in commercial and high purity C355 alloys. The cooling curves and the corresponding derivative curves were plotted to determine the nucleation, minimum and growth temperatures during Al-Si eutectic reaction [2]. The results indicate how the presence of P in unmodified commercial purity alloy promotes nucleation of the eutectic Si at lower undercooling, confirming AlP compounds as potent nuclei for the Al-Si eutectic (Fig. 1). In the unmodified commercial purity alloy, the eutectic Si shows a fully unmodified lamellar morphology; on the contrary, a refined plate-like shape is observed in high purity alloy. The addition of Sr to the high purity alloy does not significantly alter the eutectic nucleation. An analysis of the cooling curves reveals a negligible variation of eutectic growth during solidification of the Sr modified high purity alloy, despite a flake-fibrous transition in the eutectic Si morphology. This indicates that eutectic growth undercooling is not a fundamental characteristic of Sr modification but is caused indirectly due to the presence of additional impurities in commercial purity alloys.