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Aluminum foams from A356 base alloy were produced by both the melt foaming process and gas injection foaming process. A comparison of microstructures between the two kinds of aluminum foams was carried out. The related causes were analyzed to form the difference in microstructure. Results indicate that aluminum foams produced by different processes are distinct in metallurgical structure. The average thickness of cell wall, the species and area fraction of secondary phases or particles and other metallurgical features have been all comparatively studied. The difference in microstructure features of the cell walls will also make the aluminum foams different in mechanical properties. Therefore, we need to select proper foaming process for aluminum foams according to the property requirements in practical application.

Particle coverage ratio on bubble interface was investigated by metallographic analysis when the foam is critically stable in the gas injection foaming process. It is found that the critical coverage ratio was almost the same under different conditions, such as different particle amount, particle size, or particle wettability. The particle coverage ratio is proposed as the criterion to judge the foam stability, and a criterion equation was deduced. Adsorption coefficient of particles, effective coverage ratio of a particle and critical coverage ratio on the bubble interface are stability influencing parameters of the criterion equation. Their determination methods were presented and examined by air injection foaming of A356/Al2O3 melt. The adsorption coefficient calculated by measuring cross-section of foam cell wall gave values agreeing with the experimental results. The effective coverage ratio, which is about 0.9 in this article, can be calculated by combining measurements on surface and on the cross-section of the foam cell wall. The critical coverage ratio, that is about 14% in this article, can be determined by measuring the cross-section as well as the surface of the foam cell wall.