Interactions Between Strontium and Sodium Fluorosilicate in the Combined Treatment of A319 Al Alloy

Abstract

The mechanical properties of cast aluminum–silicon alloys are typically improved by eutectic silicon modification and melt degassing. First, trace strontium additions can modify the silicon phase and relieve stress concentrations at the naturally coarse, acicular particles. Second, chemical degasser additions, such as sodium fluorosilicate, can remove dissolved hydrogen gas from the aluminum melt to prevent the formation of deleterious gas porosity. However, combining both treatments is difficult, due to the tendency of the two chemicals to react and reduce their effectiveness. Nonetheless, there is limited research outlining specific procedures to enable their maximum potency, with both additions. In this work, the effects of strontium and sodium fluorosilicate on the eutectic silicon morphology and casting porosity in A319 aluminum alloy were investigated. This included a comprehensive study of the fading of their individual additions as well as the influence of separating the two chemical additions by various time intervals. The results of the melt treatments were characterized using optical microscopy, density measurements, and analysis of strontium contents using optical emission spectrometry. It was found that the maximum potency of both treatments was achieved by degassing prior to strontium additions, featuring both satisfactory silicon modification and porosity reductions.