Oxide Scale Behavior and Surface Protection of Al–Mg Alloys Containing a Trace of Ca

Abstract

In this study, the solid and liquid oxidation behaviors of Al–Mg alloys that had a trace of Ca were investigated. With increasing Ca content, the grains of α-Al dendrites in Al–7.5mass%Mg alloys were refined by the formation of two Ca-containing particles at grain boundaries. Using thermal gravimetric analysis result at 515 °C for 24 h under O₂ atmosphere, Al–7.5mass%Mg alloy exhibited a parabolic behavior in its weight gain during oxidation. However, there was nearly no difference in the weight change during oxidation of the Al–7.5mass%Mg alloys that contained a trace of Ca. From the auger electron spectroscopy and transmission electron microscopy/energy-dispersive X-ray spectroscopy results for the oxidized surface, it is believed that the improvement in the oxidation resistance in Al–7.5mass%Mg alloys that contain Ca may be attributed to the formation of a mixed oxide layer that includes CaO, MgO, and Al₂O₃ on the surface. After a melt holding test for 3 h, there was no notable difference in Mg loss during oxidation between Al–7.5mass%Mg and Al–7.5mass%Mg–0.1mass%Ca alloys. However, for a melt oxidation time for 24 h, the Mg reduction of Al–7.5mass%Mg alloy reached approximately 15%, whereas in Ca added alloy, the reduction was controlled under 8%. The cross section of Al–10mass%Mg alloy that solidified after the melt holding test showed a significant contamination due to oxide inclusions. However, Ca added alloys had good internal soundness throughout all the conditions. It was confirmed that the protective layer formed by Ca-containing can suppress the formation of Mg-based oxide inclusions and result in an increase in the alloy internal soundness during the melting process.