Determination of Optimal Modifier Level for Eutectic Silicon Phase in Hypoeutectic AlSi7Mg0.3Cu0.5 Cast Alloy

Abstract

Silicon particles in as-cast aluminum alloys can be modified using various modifiers, with Strontium (Sr) and Sodium (Na) being the most employed in the aluminum casting sector. However, their concentration diminishes (fades) during the melting furnace process. Modifier fading within the melt can result from three principal chemical reactions: vaporization of the modifier under high vapor pressure conditions at melting temperatures, oxidation driven by a significant affinity for oxygen, or interaction with other elements in the melt leading to the formation of intermetallic compounds. Given the very low vapor pressure of both Na and Sr, their vaporization from the aluminum melt was excluded as a reason for the modifier's fading. Oxidation seems to be like the primary chemical reaction leading to the fading of Na and Sr from the aluminum melt. This study examined the effect of Sr and Na fading on the modification level of silicon particles in the AlSi7Mg0.3Cu0.5 (356) aluminum alloy. The cooling curve analysis has been utilized to quantify modifier fading. Approximately 105 ppm of Sr in the form of master alloy (AlSr10) and 67 ppm of Na in the tablet form were separately added to the melt. Both modifiers were kept in the melt for approximately 200 minutes to monitor their fading. The present paper aimed to quantify the fading of Na and Sr in an AlSi7Mg0.3Cu0.5 alloy. The depletion of modifiers (Na and Sr) while the melt is retained in a furnace can be quantitatively determined through equations derived from existing literature. The surface reaction rate constant (ks), once calculated, serves as a tool for predicting the extent of modifier loss during the melt-holding process in both industrial and laboratory furnaces.