Hidetoshi Ueno

Effects of calcium addition on the foamability of molten aluminum.

A new process for producing porous aluminum by the foaming technique was developed. In the process, molten aluminum into which Ca has been added before the stage of viscosity rise is stirred to disperse oxide films into the melt. Adding Ca results in considerable shortening of the time for viscosity rise because of promoting oxidation of the melt. Another advantage of Ca addition includes that the foaming agent is mixed easily and quickly in the melt by the vortex method. This also allows to diminish a required amount of expensive foaming agents such as TiH2 and ZrH2 and to utilize economical foaming agents such as volcano ash (Shirasu). Only about 0.4%Ca is required both for viscosity rise of the melt and mixing of foaming agents. The optimum viscosity of the melt for foaming is about 8.6cP.

Reaction between vitreous SiO2 and Al

Rate of reaction between Al and SiO2 in vitreous silica or in silicate was measured between 500°C and 600°C in air or vacuum. The reaction couples used in the study were aluminum-silica glass and aluminum-Shirasu Balloon, which was a hollow microsphere of aluminosilicate glass. In the present study, it was found that the rate determining stage of the reaction between SiO2 and Al was associated with breaking of a Si-O bond. Apparen+ activation energies of the reaction are 56.5cal/mol for the silica glass-aluminum system and 78.7cal/mol for the Shirasu Balloon-aluminum system, respectively.

Effects of heat treatment on the structure and the mechanical properties of the Shirasu Balloon-aluminum composites

Effects of heat treatment on mechanical. properties of composites produced from aluminum and Shirasu balloons, hollow microsphere of aluminosilicate glass, were studied. The reaction of Shirasu balloon with aluminum was also examined by an X-ray diffraction method, an electron microprobe analysis and a scanning electron microscopic technique. The results obtained were as follows:(1) The main chemical reaction occurs between Shirasu balloons and Al and is describcd as 3SiO2+4Al→3Si+2Al2O3. Some other oxides such as Fe2O3, Na2O and K2O in the balloons are also reduced by Al.(2) Al oxide, thus formed, replaces Silica or other oxides to form. the balloon walls. The Al oxide has initially a structure of η-alumina and then, changes to θ- and α-alumina.(3) Silicon and iron diffuse into the matrix through the Al oxide layer to form a solid solution.(4) Changes of mechanical properties can mainly be attributed to Si produced. The tensile and the bending strength are improved by conducting heat treatment at temperatures below 577°C, the eutectic temperature of the Al-Si alloy. This is caused by the enrichment of Si in the matrix and the precipitation of supersaturated Si at the balloon walls or the grain boundaries of the matrix. However, heating above the eutectic temperature reduces to strength remarkably because the shrinkage porosities are developed.(5) Iron forms the Al-Fe-Si compound and hardly affects the mechanical properties because of its small content in the balloon.