Influence of small additives of calcium on the structure and properties of ML5 alloy (AZ91)

Abstract

The influence of the calcium additive (from 0.1 to 1.0 wt %) on the phase composition and solidus temperature of ML5 magnesium alloy is investigated. Calcium transfers into the intermetallic compound of the variable composition during alloy crystallization. This compound contains Al (53.4–57.4%), Ca (42.6–42.8%), and Mg (0.002–3.8%) and is transformed with a decrease in temperature into the Al₂Ca compound. The influence of calcium on the amount of phases Mg₁₇Al₁₂ and Al₂Ca and its distribution in the casting and thermally treated ML5 alloy structures are investigated. It is revealed with the help of the electron probe microanalysis that calcium and aluminum are concentrated along the boundaries of the magnesium solid solution. It is shown that, in order to acquire satisfactory mechanical properties and pouring of calcium-containing magnesium alloys should be performed according to the production process preventing the contamination of metal of the coarse inclusions. It is established that small additives of calcium (up to 1 wt %) increase the ignition temperature and lower the alloy oxidability at elevated temperatures (up to 715°C). The influence of the sulfur hexafluoride (SF6) for the calcium loss during flux-free melting was established.