INFLUENCE OF BISMUTH IMPURITY ON THE SOLIDIFICATION KINETIC AND MICROSTRUCTURE OF Gd-CONTAINING Al-7Si-0.3Mg CAST ALLOYS

Abstract

Recycled Al-Si alloys are an important raw material for sustainable and green production due to their low energy consumption. However, various impurity elements, such as Bi, coming from the scraps or recycling process can deteriorate the mechanical properties of the final product. After the limitation of Pb-containing products due to their toxicity by the European Union, Bi content in secondary Al-Si alloys has continuously increased. Bi can interact with other alloying elements affecting the final microstructure and consequently the tensile properties. On the other hand, several investigations about the effect of lanthanide addition into the Al-Si alloys have been performed in recent years. Some lanthanide elements can refine the primary α-Al and modify/refine the eutectic Si crystals. The aim of this work is to study the influence of Bi as a trace element over Gd-containing Al-7Si-0.3Mg alloys. Alloys containing 200 ppm Bi as impurity have been treated by Gd. Two-thermocouple thermal analysis and microstructural investigations were performed to investigate the changes in the solidification sequence and microstructure of the alloys. The results demonstrated that Gd is ineffective on the refinement of α-Al dendrites while it decreases the growth temperature of eutectic reaction and refines the eutectic Si crystals of the Al-7Si-0.3Mg alloy. Unlike traditional eutectic Si modifiers, such as Na and Sr, the efficiency of Gd addition on the eutectic was not influenced by Bi impurity. No Bi/Gd interaction was detected. Additionally, SEM investigations showed that Bi-rich phases are located in the growth front of the GdAl2Si2 phases.