Percolation of primary crystals in cell walls of aluminum alloy foam via semi-solid route

Herein, a uniform aluminum alloy foam was fabricated by the addition of TiH₂ as a blowing agent to Al-6.4 mass % Si in the semi-solid state and subsequent solidification. This was aimed at propounding the stabilization mechanism of the proposed foaming process. The microscopic images, which were the cross section on the center of the foam etched with Weck’s reagent, showed the primary crystals in the semi-solid state and solidifying segregation surrounding the crystals. Thus, it became evident that the area ratio of primary crystals in the semi-solid state approximately equals to the set solid fraction. According to the percolation theory for the cell wall model, the drainage in the cell walls with primary crystals above the percolation threshold was found to be inhibited. By considering that each cell wall is a flow path of the foam, the percentage of the cell walls with inhibited drainage to all the other cell walls was observed to exceed the percolation threshold of the lattice model (0.33) as per the percolation theory. Therefore, it can be concluded that the primary crystals inhibit drainage in some cell walls, ensuring that the stability of the foam is maintained.