Characteristics of Dynamically Formed Oxide Films in Al–Zn Melt

Abstract

The entrainment of surface oxide films in the melt leads to the formation of double oxide defects in the casting parts. Oxide/metal/oxide (OMO) sandwich technique is a method for physical simulation of the formation of surface oxide film. In this method, air bubbles are artificially introduced into the melt in order to trap two adjacent bubbles and extract the interface between them. In this study, to prepare OMO samples, air bubbles were blown with a pressure of 0.8 atm. into the mold filled with Al melt containing 3, 5 and 7 wt% of Zn. Using scanning electron microscopy and energy-dispersive X-ray spectroscopy, characteristics of dynamically formed oxide films such as morphology and film thickness were investigated. Results showed that in higher zinc content, more cracks appear on the oxide films. Thickness of the oxide films in Al–Zn alloys was estimated to be 95–1070 nm. According to the measurement performed on the folds, adding more than 3% zinc caused a reduction in the oxide film thickness. Thermodynamics suggests the presence of spinel phase along with aluminum oxide at 700 °C which is in agreement with energy-dispersive X-ray spectroscopy’s results. The presence of the spinel phase at the interface is the reason behind the reduction in the thickness of the oxide film.