Deformation, Cracking and Fracture Behavior of Dynamically-Formed Oxide Layers on Molten Metals

Abstract

This study investigates the cracking behavior of oxide layers formed on molten metals and alloys including pure zinc, Zn–4 wt%Al (ZAMAK3) and Al–(0.5–2) wt%Ca in dynamic oxidation condition by injecting gas bubbles into the molten metal during the pouring process. The crack characteristics of the oxide layers were studied using a field emission scanning electron microscope. The results show that various stresses initiated from turbulence flow in the molten metal promote the deformation of the oxide layer, particularly at the initial stages of oxidation. Different coefficients of thermal expansion of the oxide layers and the metals can also result in deformation/cracking the oxide layers. Simultaneous aspiration of the molten metal and solidification phenomenon within the casting process may lead to various morphological changes, e.g. folded-, wrinkled- and cracked-oxide layers. In addition, a splitting and reforming phenomenon of multiple oxide layers is observed, called as strips of ‘ruffled tape’. An illustrative mechanism is suggested and discussed quantitatively for the formation of such phenomena. It is assumed that the unique appearance of these strips depends on the formation time and complex stress gradients on the oxide layers.