Effect of powder oxide content on the expansion and stability of PM-route Al foams

The oxide content in Al powders has been found to have a significant effect on the expansion and stability of foams made via a PM route. With low oxide contents (O < 0.3 wt%) expansion is moderate and the foam structure is unstable. Larger expansions, improved foam stability and more homogeneous foam structures are achieved if the amount of oxide in the powder is moderate (O = 0.3-0.6 wt%). Foaming precursors with excessive oxide contents (O > 0.6 wt%) results in small expansions but very stable foam structures. Oxides were observed to form clusters of crumpled films, which at higher levels form a network which restricts the drainage of liquid from the cell walls and Plateau borders, but which also inhibit foam expansion. Oxide clusters in the cell walls lead to a decrease in the minimum cell wall thickness, resulting in an increase in foam expansion.