Role of Oxide Particles in Aluminum Melt toward Aluminum Foam Fabrication by the Melt Route

Abstract

The aim of this work is to elucidate the role and contribution of oxide particles to aluminum foam fabrication. The melts were internally oxidized by a thickening process in which pure aluminum melt was stirred with or without the addition of 1.5 wt.% calcium for maximum 25 min. After this, each thickened samples were melted again and mixed for 100 s by introducing 1.5 wt.% TiH2 as a blowing agent. In order to investigate the foam evolution, the foam samples were hold in the furnace for 50 to 500 s. The stirring torque (viscosity) of the calcium containing melt increases with thickening time and achieves the stationary value after 17 min. However, the torque of pure aluminum melt does not change during stirring. Oxides have been found on the microstructures of both stirred samples, although the content of oxides of calcium added sample is significantly more than that of pure aluminum. SEM observation results of samples thickened by calcium addition show that the melt contains calcium oxide and Al4Ca in addition to equiaxed aluminum, and the morphology of formed oxide is not granulous but wrinkled bifilm containing calcium and aluminum oxides. The oxides formed in the pure Al melt has less effect on the viscosity thus the foamability of the aluminum melt. It is found that the calcium oxides formed by stirring are responsible for the effective increase of melt viscosity. The foams using oxidized pure Al melt have dense layer at the bottom caused by drainage and coarse foam structure due to strong coalescence. In case of the Al-Ca alloy, uniform pore distribution, lack of the dense layer and homogeneous time dependent increase of the cell size were observed. Besides, the sample held for longer time has thicker cell wall at the bottom compared with that at the top. We confirmed that the oxide bifilms of Al and Ca contributes to decreased drainage rate and coalescence, namely stabilization. The insufficient amount of oxide particles in pure aluminum is the reason for the lack of stabile foam (significant drainage) in that case.