【作者】 杨天武；

【导师】 周荣； 金青林；

【作者基本信息】 昆明理工大学 ， 材料加工工程， 2013， 博士

【摘要】 金属-气体共晶定向凝固是一种利用气体(氢气)在金属固、液两相中存在溶解度差制备规则多孔金属的革命性新工艺,也称为‘’Gasar"或“藕状多孔材料”。采用定向凝固制备的规则多孔铜,除具有传统的烧结型或发泡型多孔金属的性能特点(低密度、高比模量和比强度以及冲击能量吸收能力)外,还具有自身特殊的性能特点,比如小的应力集中、高的机械性能、良好的导热能力等,因此具有广阔的应用前景和重要的潜在应用价值。制备装置是研究规则多孔金属的前提和必要条件。本论文根据Gasar工艺原理,结合设备特点(在真空负压下熔炼,在高压下吸收氢气达到饱和,凝固析出氢气成孔),设计了规则多孔金属制备装置。装置采用倾包法,使熔炼与凝固分开,便于控制。装置中,为保证水冷铜盘的冷却能力,冷却水流速控制在6m/s左右,冷却水进出水温差控制在10℃左右；铸型、熔炼坩埚和加热电阻材质选用石墨；铸型下移的抽拉速度在0.1-10mm/min范围内变动；增加隔热挡板用以提高固-液界面温度梯度。分析了实际试样中无孔层、边沿大孔和顶部大孔、以及弧形区域,总结了规则多孔铜的实际结构特点,得到了实际规则多孔结构的模型。通过控制相关工艺参数得到不同结构的规则多孔铜,对规则多孔铜的气孔结构进行了研究。研究表明工艺参数对规则多孔铜气孔结构有影响,具体表现为：(1)气体压力对气孔结构的影响：当使用纯氢时,气孔率随着氢气分压的增大而减小；当氩气分压一定时,气孔率会随着氢气分压的增大而减小；气体总压一定时,气孔率会随着氢气分压的增大而增大。气体总压力是决定气孔平均直径的最主要因素,随着总压力的增大,气孔平均直径减小。气孔尺寸分布均匀性随氢气压力的增加而增加；在氢气和氩气混合气氛下,随着氩气分压增大气孔尺寸分布均匀性先增加后减小。(2)铸型温度对气孔结构的影响：气孔率随铸型温度的增加而增加；由于不可避免产生侧向散热,气孔生长方向与试样中心轴存在一夹角,随着铸型温度升高,夹角减小,气孔间的平行度增加。(3)抽拉速率对气孔结构的影响：气孔率随下移速率升高略有减小,这种变化过小,可以认为抽拉速率对气孔率无影响,气孔率主要由气体压力决定。在气体压力一定时,气孔平均孔径随下移速率的增加而减小。规则多孔铜的理想结构是进行相关推演和计算的依据。本论文分别对正五边形、正六边形、正七边形、正八边形的气孔率、气孔生成吉布斯自由能和气孔结构的演化进行了分析、计算和推演,结果表明正六边形结构是符合规则多孔材料的最理想结构。在处于稳态生长的试样中,能够找到正六边形排列的气孔结构,进一步证实了正六边形是Garsar材料的理想气孔结构。由于采取措施保证了沿轴向自上而下的单向传热,此时从试样中心散出的热量大于从边壁散出的热量,固-液界面形态要保持凹界面或平界面是不可能的,只能是凸界面。定向凝固时晶体沿热流方向凸向上生长,凸向液相。由于气泡生长不会产生分叉,气泡凸向生长时,其生长只能以三种方式进行,即收缩生长气泡变小、平行生长气泡不变、膨胀生长气泡变大。比较三种生长方式的界面能,发现相同接触角时界面能值由大到小依次为膨胀生长>收缩生长>平行生长,同时平行生长也能满足气泡在液-固界面前的力学平衡,说明中心上凸两侧平行向上的平行生长是符合实际的生长方式。试样中也进一步证实了此生长方式的存在。建立了规则多孔铜的凝固温度场模型,用解析的方法计算了规则多孔铜凝固温度场。认为Gasar工艺凝固时,气孔对传热不起作用,凝固过程不受气孔的影响。获得了凝固层厚度与时间的理论关系,其与经典凝固中平方根定律相似,并进行了验证。结果表明对于凝固速度范围在1.5mm/s～2.5mm/s,高度为240mm的试样,所需凝固时间为93～107s。分析了固-液界面前沿液相温度场,认为加热区的总热阻主要受辐射换热热阻Rr的影响,其它热阻可忽略。固-液界面前沿液相温度梯度GL主要受加热温度Th和试样半径rl的影响。加热温度增加、试样半径减少,温度梯度GL均明显增加。熔炼温度Tp和坩埚壁厚对温度梯度GL的影响不明显,辐射距离δ对温度梯度GL基本没有影响。更多还原

【Abstract】 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.更多还原