Study on Fabrication Technology and Pore Structure of Ordered Porous Copper with Directional Solidification
|School||Kunming University of Science and Technology|
|Course||Materials Processing Engineering|
|Keywords||Ordered porous copper Unidirectional solidification Pore structure Process parameter Heat transfer|
Directional solidification of metal-gas eutectic, which is based on the gap of gas (hydrogen) solubility between liquid and solid metals, is a novel revolutionary process for fabricating porous material with pores aligned in solidification direction. The process is also called "Gasar" or "lotus type porous material". Ordered porosity copper with elongated pores can be fabricated by a unidirectional solidification method under a pressurized gas mixture of hydrogen and argon. Besides possessing the property of traditional porous materials (like lower density, higher specific modulus, higher specific strength and energy absorbing ability), the lotus-structured metals perform particular properties such as lower stress concentration, higher mechanical property and superior capacity of heat transmission, therefore have a wide application prospect and important potential applications.The manufacturing equipment is premise and requirement to study lotus-structure metals. According to the technology principle, metals are smelted in vacuum and absorb hydrogen at high pressure. When hydrogen saturation has been reached, hydrogen would separate out and formes pores. The equipment used leaning ladle.lt can separate smelting and solidification.The flow velocity of cooling water is about6m/s.The water temperature difference of get in and out cooling water is about10℃.These can ensure cooling capacity of the equipment.The material of mould, smelting crucible and heating resistor is graphite.The withdrawal rate is within the scope of0.1-10mm/min.Useing thermal baffle is effective to increase temperature gradient of solid-liquid interface.The non-porous layer, the big pore at edge and top, the arc shaped pore area in the actual sample are analysed and summarized. The structure characteristics of ordered porous copper are summarized.Then the practical porous structure model is obtained.The influence of process parameters on the the structure was investiaged and the results show that:(1) 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.(2) The porosity increased with increasing mold temperature. Because the radial heat dissipation is inevitable, there is an angle between pore growth direction and the sample center shaft. The angle decreased with increasing the mold temperature,(3) The porosity was independent of the transference velocity but dependent on the hydrogen gas pressure.The average pore diameter decreased with increasing transference velocity at a given gas pressure.In this paper, the ideal structure of ordered porous copper has been studied. Based on the ideal structure symmetry hypothesis, the porosity, pore formation gibbs free energy and the evolution of pore is analyzed and calculated. It is showed that hexagonal pore structure is in accord with the actual pore structure.As the heat flows from top to bottom of the sample, the solid-liquid interface morphology can be divided into concave, flat and convex interface. The bubble can not bifurcate. Its growth can only be in three ways:contraction growth, parallel growth, expansion growth. It is demonstrated that the relationship of interfacial energy of them is that expansion growth> contraction growth> parallel growth. The parallel growth meets mechanical equilibrium condition.The temperature field is calculated by establishing solidification model of garsar process. The heat transfer in Gasar process has been analysed. It is considered the gas has no influence on heat transfer. The relationship between solidified shell thickness and solidified time has been deduced, which is quiet simlar to the traditional square root law. It is shown that the solidification velocity should be in the range of1.5mm/s～2.5mm/s. The solidificattion time should be93-107s for sample with a height of240mm. The relationship between temperature gradient and process factors has been studied through calculating the temperature gradient in liquid. The results show that the radiant heat resistance is the most important factor in all thermal resistance. The heating temperature and the radius of sample are the main factors for the temperature gradient. The temperature gradient increases with increasing heating temperature and decreasing radius, The effect of melting temperature and the wall thickness of crucible are not obvious. The radiation distance has no effect on temperature gradient.