Semi-solid route is one of the fabrication methods for aluminum alloy foams where the melt is thickened by primary crystals while the conventional foaming methods use thickener particles.[1] The primary crystals can inhibit the drainage in a cell wall of the foam by the clogging effect which is based on the percolation theory.[2] Clogged cell walls where the drainage is inhibited by this effect can hold other cell walls; thus a whole foam can keep stable. On the other hand, it was revealed that the cell walls are stabilized by the oxide film generated on the inner surface of the pore (the solid-gas interface) in the foam fabricated through the conventional methods.[3] In this situation, the thickening particles are fixed on the oxide film and prevent the drainage. It can be considered that the semi-solid route also generates the oxide film on the surface of pores. However, it is doubtful whether the primary crystals are fixed on the oxide film since the primary crystals are around ten times larger than the thickening particles. Therefore, the objective of this research is revealing whether the oxygen in atmosphere increases the stability of the pores in the aluminum alloy foam fabricated through the semi-solid route.

First, 100 g of Al-6.4mass%Si alloy was completely melted in the electronic furnace. Second, the gas in the furnace was replaced with high purity air (20% oxygen) or Ar gas three times to change the concentration of oxygen in the furnace. The oxygen meter measured the concentration of oxygen in the furnace during the entire fabrication. After that, the molten metal was slowly cooled until the temperature of the melt reached and became constant at 613 °C. 2 g of TiH₂ wrapped in an aluminum foil as a blowing agent was added into the semi-solid slurry. The impeller stirred the slurry at a rotational speed of 15 s^-1 for 100 s. The slurry was held at 613 °C for 200 s for the foaming. After the foaming, the foamed slurry was solidified. The porosity p was measured. The inner surface of the pore of the foam were observed by using the optical microscope and EPMA.

Figures 1(a) and (b) show the cross sections of aluminum alloy foams fabricated under 18% and 10 ppm of concentration of oxygen, respectively. The porosity p of 18%-oxygen was higher than 10 ppm-oxygen. Figure 1(c) shows the optical microscopic image of the inner surface of the pore from the cross-sectional direction. Figure 1(d) shows the EPMA mapping images of the same view as Figure 1(c). Some Al₂O₃ particles existed near the inner surface as indicated by the white arrows. Therefore, these results lead to the suggestion that the oxygen in the furnace will improve the stability of the pores by making Al₂O₃ near the inner surface of the pore.