A study on the variation of solidification contraction of A356 aluminum alloy with Sr addition

Abstract

A356 aluminum alloys are very useful engineering materials that have been widely applied in the automotive, machinery, aerospace and defense industries. However, modification treatment of the alloys is needed to change the alloy structure, particularly to change the form of the eutectic silicon from flake like in order to improve mechanical properties [1, 2]. The modifying agents usually used in A356 alloys are sodium and strontium. Sodium is subject to loss from the melt, as it disappears by evaporation or oxidation. In addition, its use presents undesirable environmental problems. Strontium, on the other hand, offers a semi permanent modifying effect and is often used to modify hypoeutectic Al-Si casting alloy [3–5]. From previous research, thermal analysis results indicate that modification depresses the eutectic transformation temperature [6–8]. As a result, the freezing range of the alloy is increased and so is the mushy zone [1]. The mushy zone may be divided into two parts, the semi-liquid zone and semi-solid zone, which are distinguished by physical characteristics. The alloy melt has good fluidity and has no strength in semi-liquid zone, while it has poor fluidity and some strength in semi-solid zone [9]. When the semi-liquid temperature range is increased, the fluidity of melts is improved. On the other hand, as the semi-solid temperature range is decreased, contraction stress during solidification is decreased so that solidification cracking can be restrained. Therefore, the range of the semi-liquid zone or semi-solid zone is one of the most important factors in determining the castability of the alloy [9]. The present study attempts to investigate the effects of Sr addition on the variation of the range of the semiliquid zone and to compare the results with microstructures of each specimen. All melts produced in this work were made with an A356.2 alloy with the composition given in Table I. Melts were prepared in 2 kg quantities using SiC crucibles in an electric furnace and degassed with pure argon which was bubbled into the melt at a rate of 0.2 ml/min for 20 min. Strontium was added to the melt in the form of A1-10wt%Sr mother alloy and the melt was held at temperatures in the range of 740–750 ◦C for 30 min.