No abstract

Lotus-type porous aluminum with cylindrical pores was fabricated by unidirectional solidification through thermal decomposition of calcium hydroxide, sodium bicarbonate, or titanium hydride. The pore-forming gas decomposed from calcium hydroxide, sodium bicarbonate, and titanium hydride is identified as hydrogen. The elongated pores are evolved due to the solubility gap between liquid and solid when the melt dissolving hydrogen is solidified unidirectionally. The porosity of lotus aluminum is as high as 20 pct despite the type of the compounds. The pore size decreases and the pore density increases with increasing amount of calcium hydroxide, which is explained by an increase in the number of pore nucleation sites. The porosity and pore size in lotus aluminum fabricated using calcium hydroxide decrease with increasing argon pressure, which is explained by Boyle's law. It is suggested that this fabrication method is simple and safe, which makes it superior to the conventional technique using high-pressure hydrogen gas.

Lotus-type porous 8,12,14, and 18 wt pct) alloys were fabricated using the continuous casting technique under a hydrogen gas pressure of 0.1 MPa at various transference velocities, and the effects of the silicon content level and transference velocity on the pore morphology and porosity were investigated. Both the porosity and the average pore diameter increase as the silicon content level increases and decrease as the transference velocity increases. In particular, the velocity dependence is obviously exhibited at a silicon content level higher than 12 wt pct. The pore shape is changed from irregular in the higher-dendrite fraction to nearly circular in the lower-dendrite fraction. The porosity and the pore morphology are influenced by the silicon content level and transference velocity. In the model, these results can be understood with the explanation that the pores, which contribute to the increase in porosity, are generated at the eutectic fronts. This indicated that the porosity and the pore size in lotus-type porous Al-Si alloys can be well controlled by varying the silicon content level and the transference velocity.