Characteristics of dynamically formed oxide films in aluminum-calcium foamable alloys

Characteristics of dynamically formed oxide layers in Al-Ca alloys containing up to 2.0 wt.% calcium, which are commonly used in manufacturing Alporas closed-cell aluminum foams, were investigated using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). A recent modern technique known as oxide-metal-oxide (OMO) sandwich was applied to prepare the samples containing 0, 0.5, 1, 1.5, and 2 wt.% calcium. The results of the intensive research studies clarify that the presence of the calcium in the aluminum intensifies the oxidation of the alloys and subsequently the thickness of the oxide layers. In addition, increasing the calcium content led to an obvious change in the oxidation behavior of the alloys as well as the morphology of oxide layers. The alloys containing less than 1 wt.% calcium, formed nano-sized calcium aluminate particles. These particles were generally formed in regions located between the grain boundaries of solidified alloy and the covering alumina layer where caused the destruction sites in the deformed alumina layer. The increase of the calcium contents of the alloy more than 1 wt.%, not only intensified the formation of under-layer nano-sized calcium aluminate particles but also changed the morphology and thickness of the covering oxide layers. In alloys containing 1 wt.% calcium or higher, the oxide layers were consisted of tile-like, thick, highly brittle and scrappy calcium aluminate layer. These oxide layers can interfere with the molten alloys as a result of the stirring process and enhance the apparent viscosity of the alloys.