Characterization of as-cast microstructure of aluminum foams by melt foaming method

The effect of the microstructure of cell-wall matrix on mechanical properties of aluminum foams becomes dominant supposing that the pore structure is well controlled. However, the as-cast microstructure of aluminum foams, including the morphology of primary alpha-Al grains, the constitution and distribution of secondary phases induced by thickening and foaming agents, are not well characterized at present. In this paper, we revealed the morphology of primary alpha-Al grains by anodization treatment, and phase constitution and distribution by SEM equipped with an EDS in cell-wall matrix of aluminum foams by melt foaming route. Results show that the as-cast microstructure is composed of dendritic alpha-Al grains, inter-grain CaAl4 phase, a small amount of Al20CaTi2 and bifilm. alpha-Al grains are of equiaxed type and their morphology is irregular due to the restriction effect of cell walls. alpha-Al grains in cell-walls are much smaller than those in the pore-free layer. The cooling curve of aluminum foams indicates that primary alpha-Al formed first, followed by CaAl4 during solidification. CaAl4 will not be present in the melt and thus not be responsible for thickening.