The effect of europium on the oxidation of A356 alloy and pure aluminum

Abstract

The modification processes made during rapid solidification and heat treatment are called thermal, and the modification process made by preventing the growth of Si by adding elements from the outside is called the chemical method. The chemical method is frequently used in the industry in terms of ease of applicability. It has been observed that modifications have been made with elements such as Na, and Sr from past to present. Recently, it has been discovered that modifications occur with rare earth elements such as Eu, Er, and Yb. It is known that Sr, which is the most used element in the industry, modifies the structure by using it at the addition level of ppm. However, the Sr element has two noted disadvantages. Because of its high oxygen affinity, strontium tends to oxidize from within the liquid metal and mix with the dross. In this case, since the amount of Sr is lost over time, the modification effect decreases. In addition, it has been observed that making intermetallic with element B, which is used as a grain refiner, reduces both the grain refinement effect and the modification effect . For this reason, a search for new modifier elements has been started. Recent studies have shown that a small amount of Eu addition can modify the structure. However, the relationship of the Eu element with other elements and its oxidation behavior in the alloy has not been investigated yet. When the Richardson & Ellingham diagram is examined, it is seen that the oxygen affinity of Eu+2 is higher than that of Al. Therefore, in this study, it is aimed to investigate the effect of the Eu element on the oxidation behavior of A356 alloy in both solid and liquid states. In order to understand the Al-Eu interaction and to examine the change in oxidation behavior, the oxidation behavior of 10% Eu added Al was also investigated. Two different europium ratios (0.1 and 0.4wt.%) were selected for addition to the alloy. Characterization of these alloys was carried out. Afterwards, they were subjected to oxidation tests for 2, 4, 6, 12 and 24 hours at 500 and 750 °C. The oxidation kinetics were obtained over time by calculating the weight gain/surface area. No significant change was observed at 500 °C. The oxide layer which is formed at 750 °C and internal structure were analyzed using an optical microscope, SEM and XRD. It was observed that a MgO layer was formed on the surface after oxidation, 0.1 Eu amount had no effect on oxidation, and 0.4 Eu increased oxidation kinetics at 750 °C. In addition, it was determined that Al2Si2Eu intermetallics precipitated on the oxide surfaces. Oxidation tests were carried out similarly to the Al master alloy containing 10% Eu. In addition, the 48-hour examination was carried out to find the moment when the oxidation slowed down. As a result of the oxidation tests, it was observed that the addition of Eu greatly increased the oxidation kinetics of pure aluminum at 750 °C. When the oxide structure was examined, it was observed that an oxide layer containing Eu-Al-O was formed according to SEM analysis. According to XRD results, it is thought that Eu3O4 and Al2O3 oxides are intermixed. In addition, the hardness values were measured and it was observed that the hardness values decreased as the oxidation time increased. The oxidation behavior of the Al10Eu master alloy in the semi-solid state was investigated by exposing it to oxidation for 24, 48 and 72 hours at 650 °C. According to the results of microstructure analysis, it was observed that EuAl4 intermetallics were selectively oxidized.