Effect Of Gasar Processing Parameters On Pore Structure Of Ordered Porous Copper

Unidirectional solidification of metal-gas eutectic, also called "Gasar" process, is considered as a revolutionary method for producing porous metals. The produced structure is called as "Gasarite" in which the cylindrical pores are aligned in the solidification direction. Compared with conventional porous metal, the ordered porous metal has many advantages such as lower stress concentration, higher mechanical property and superior capacity of heat conduction. Therefore the ordered porous metal has important potential applications in many industrial fields. In this study, ordered porous copper with various pore structures was successfully fabricated in a Gasar apparatus developed by our laboratory. The influences of preheating temperature of the mold, gas pressure and solidification velocity etc., on the porosity, pore size and distribution were investigated from the theoretical and experimental point of view.Based on the analytical calculation of the temperature field during Gasar solidification, it was found that the temperature gradient of liquid phase is directly propotional with that of solid phase. Through increasing the temperature gradient of solid, the temperature gradient of liquid phase could be increased. Furthermore, the temperature gradient of liquid phase is improved with increasing mold preheating temperature and decreasing diameter of ingot.In Gasar solidifacation, there exists a lateral non-porous skin in the ingot owing to lateral heat transferring at the primary stage of solidifacation. In addition, the direction of porous is non-uniform becauce of lateral heat transferring. The pore arrest is an unavoidale phenomenon due to the formation of the lateral non-porous skin and the diffusional growth of pores, which causing the non-uniform distribution of pore size. Preheating the mold up to a temperature near to the melting temperature of copper can completely eliminate the lateral non-porous skin in the ingot and produce excellent ordered porous copper in which the pore size presents a uniform distribution.By analyzing the experimental results under different processing parameters, the influence of the gas pressure on the porosity, the average pore diameter and the pore size distribution was studied. The results indicated that the porosity was significantly affected by the partial pressures of hydrogen and argon:the porosity decreases with increasing partial pressure of hydrogen when only single hydrogen is used; the porosity decreases with increasing partial pressure of argon when the partial pressure of hydrogen keeps constant; the porosity increases with increasing partial pressure of hydrogen when the total gas pressure keeps constant. The total gas pressure was the key factor influencing the average pore diameter, namely the average pore diameter decreased with increasing total gas pressure. The distribution uniformity of pore size was influenced by the total gas pressure and ratio of partial pressure of hydrogen to partial pressure of argon. In general, the distribution uniformity of pore size improved as the total gas pressure increased and worsened as the ratio of partial pressure of hydrogen to partial pressure of argon increased.In Gasar solidification, the average pore diameter, the pore number density and the porosity was significantly affected by the transference velocity of the mold. The average pore diameter decreased with increasing transference velocity at a given gas pressure, while the pore number density increased with increasing transference velocity. The porosity was independent of the transference velocity but dependent on the hydrogen gas pressure.