通过对Gasar凝固过程中的传质、气泡形核、气孔生长、中断及脱离等的理论分析, 建立了一个描述单气孔演变过程的非稳态三维模型, 并采用有限差分的方法模拟了不同凝固速率下定向凝固多孔Al的气孔形貌. 基于Al-H₂系的研究结果表明: 当凝固速率在0.15~0.005 mm/s范围内时, 固/气两相能够维持协同生长, 气孔的平均孔径分布在100~1100 mm之间, 但随凝固速率的降低会逐渐增加, 气孔长度亦随凝固速率的降低逐渐增加, 长径比则基本保持在40左右; 当凝固速率为0.015 mm/s时, 气孔孔径的模拟值与实验值吻合较好, 之后随凝固速率的降低, 模拟孔径略低于实验值, 分析认为, 实际凝固过程中熔体上方的H₂向熔体内的不断扩散是导致该差异的主要原因; 随着熔体过热度和H₂分压的逐渐增大, 对应藕状多孔Al固/气两相协同生长凝固速率范围的最大值由不足0.01 mm/s先逐渐增加之后稳定在0.15 mm/s, 最小值则由0.0001 mm/s左右逐渐增加至约0.01 mm/s; 对Al-H₂系和Cu-H₂系相关参数的比较分析表明, H₂在金属熔体中的溶解度是决定Al-H₂系Gasar结构中固/气两相协同生长凝固速率范围的主要参数.

The solid/gas eutectic unidirectional solidification process is a new kind of technology fabricating the lotus-type porous structure. Besides having the properties of large specific surface area, excellent sound absorption, penetrating performance etc. of traditional porous materials, the particularity of the lotus-type porous structure makes it has extraordinary mechanical and thermal properties. There is a great potential for lotus-type porous aluminum in the field of lightweight engineering and heat dissipation of chip owing to its low density, outstanding corrosion resistance and high thermal conductivity. However, the fabrication of lotus-type porous aluminum has always been more difficulty than other metals. Until porous aluminum with excellent pore structure was fabricated under very small solidification rates (0.008~0.015 mm/s) and high superheat degrees of melt (240~340 K), the proper processing parameters were recognized to be essential for the coupled growth of solid/gas phases for Al-H₂ system, especially the solidification rate. In this work, a three-dimensional time-dependent model describing the evolution of single pore was established based on the theoretical analysis of mass transfer, gas bubble nucleation, pore growth, interruption and detachment. The morphology of single pore under different solidification rates were simulated during the Gasar process by using the finite difference method for Al-H₂ system. The research reveals: coupled growth of solid/gas phases can be achieved under the solidification rates from 0.15 mm/s to 0.005 mm/s. The average pore diameter which ranges from 100 mm to 1100 mm increases with decreasing the solidification rate. The pore length also increases while the pore aspect ratio is nearly a constant about 40 with decreasing the solidification rate. The simulated average pore diameter is in good agreement with the experimental values when solidification rate equals to 0.015 mm/s, and then being slightly lower than the experimental ones with decreasing the solidification rate. The diffusion of hydrogen into the melt during the solidification process is regarded as the main reason of the discrepancy between simulated and experimental average pore diameters. The maximum value of the simulated solidification rates for coupled growth of solid/gas phases in Al-H₂ system first increases from less than 0.01 mm/s to 0.15 mm/s and then being a constant, while the minimum ones increase from about 0.0001 mm/s to 0.01 mm/s with improving the overheat degree of melt and hydrogen partial pressure. By comparing the relative parameters of Al-H₂ and Cu-H₂ systems, the solubility of hydrogen is regarded to be the main parameter which determines solidification rates of coupled growth of solid/gas phases for Al-H₂ system.