【作者】 李兵；

【导师】 姚广春；

【作者基本信息】 东北大学 ， 有色金属冶金， 2008， 博士

【摘要】 泡沫铝是由金属相和气孔组成的复合材料,它把连续相铝的金属特点(如强度大,耐高温等)和分散相气孔的特性(如阻尼,隔热,隔声,消音,减震,屏蔽等)有机结合在一起,可广泛应用于交通运输、建筑机械、冶金化工、电子通讯和航天航空等多个领域,其研究成为当今世界材料科学高技术领域的重要研究、开发内容之一。目前在工业应用中,人们对泡沫铝的力学性能越来越关注,尤其是其能量吸收性能。由于基体金属纯铝是一种很好的韧性材料,纯铝基泡沫铝作为一种韧性泡沫金属材料越来越受到人们的关注。但由于基体材料纯铝的熔点较高,与发泡剂氢化钛的分解温度不相匹配,以及凝固时收缩较大,用熔体直接发泡法制备纯铝基泡沫铝材料较为困难。本文系统地进行了熔体直接发泡法制备纯铝基泡沫铝材料的实验室试验和工程化试验研究。研究了纯铝基泡沫铝材料的凝固过程。研究表明,纯铝基泡沫铝材料具有独特的凝固性质,在自然冷却条件下冷却时,外层泡沫以层状凝固方式进行凝固,内层泡沫以体积凝固方式进行凝固,内层泡沫泡孔泡壁上有许多分散性的微小缩孔；当在强制水冷条件下进行冷却时,凝固得到的泡沫铝泡壁表面粗糙,凝固收缩在垂直于泡壁方向进行,均匀分布于整个泡壁。垂直于泡壁方向的凝固收缩消除了泡壁局部由于凝固而产生的裂纹,收缩的结果使泡壁变薄。由对纯铝基泡沫铝材料凝固特性的分析建立了泡孔凝固模型。通过对泡沫铝凝固过程中基体材料以及气孔内气体的体积变化的计算可知,纯铝基泡沫铝材料在凝固过程中孔隙率有所增大。讨论了熔体直接发泡法制备泡沫铝过程中工艺条件的控制。纯铝基泡沫铝制备过程中,在金属钙的添加量、搅拌温度、发泡时间和冷却条件一定的条件下,通过改变氢化钛的加入量和搅拌时间可以控制泡沫铝的孔隙率。制备的纯铝基泡沫铝材料的最大孔隙率可达92%以上,最小密度可达0.25g/cm3以下。对无泡层的形成与控制进行了研究,在泡沫铝制备过程中无泡层的形成分为三个阶段：第一阶段为发泡初期短时间内形成,在这个过程中,气泡向上运动,部分液体未被气泡运动所携带成为泡沫中的液膜,最后残留在底部形成无泡层；第二阶段和第三阶段为气泡长大过程中,由于Plateau边界处液体与液膜处液体存在压力差,促使液膜处的液体流向Plateau边界处,最后通过Plateau通道流向底部形成无泡层。在泡沫铝制备过程中,当熔体的粘度越大,表面张力越小时,无泡层的厚度越小。向含有3wt.%Ca的铝熔体中再加入0.5wt.%Mg后,熔体的表面张力显著降低,制得的泡沫铝中的无泡层厚度得到了很好的控制。通过工程化生产纯铝基泡沫铝材料的试验,确立了纯铝基泡沫铝材料产业化完整的工艺条件,成功地制造出了可商业化销售的纯铝基泡沫铝板材(1000×1800×Xmm3)。研究了熔体直接发泡法制备纯铝基泡沫铝材料时铝熔体泡沫的稳定性。在熔体直接发泡法制备纯铝基泡沫铝过程中,金属钙加入到熔体中以后产生了金属间化合物Al20CaTi2、Al4Ca和氧化物Al2O3。金属间化合物Al20CaTi2、Al4Ca弥散在铝熔体中使熔体的粘度增大,抑制了熔体的排液；而A12O3存在于界面处,这些氧化物在界面处的存在改变了曲面的毛细曲率,从而减缓了液膜和Plateau边界处间的毛细流动,而且氧化物对氢的吸附作用降低了界面能。上述三种作用提高了铝熔体泡沫的稳定性。对纯铝基闭孔泡沫铝材料和Al-6Si基闭孔泡沫铝材料进行了静态和动态压缩,试验中发现,纯铝基闭孔泡沫铝和铝硅基闭孔泡沫铝材料在静态和动态压缩过程中,均表现出不同的特点：纯铝基闭孔泡沫铝材料的压缩曲线较为平缓,显示出比较明显的塑性泡沫材料的特征；而铝硅基闭孔泡沫铝材料的压缩曲线则波动比较大,显示出比较明显的脆性泡沫材料的特征；通过分析两种基体材料的微观形貌和成分,发现Al基闭孔泡沫铝微观形貌主要是小块状和球状(Al20CaTi2),无大块或长条状金相；而Al-6Si基闭孔泡沫铝微观形貌主要是大片状和长条状相(Al3.21Si0.47、Al2O3、CaAl2Si2和Al3Ti)。对闭孔泡沫铝的能量吸收性能进行了分析,从闭孔泡沫铝的能量吸收能力、能量吸收效率和能量吸收图进行了详细的研究,结果表明：压缩速率相同,密度相等时,纯铝基闭孔泡沫铝的能量吸收能力要大于Al-6Si基闭孔泡沫铝的能量吸收能力；纯铝基泡沫铝材料能量吸收效率最高可达80%；对纯铝基泡沫铝材料能量吸收图的研究发现,纯铝基闭孔泡沫铝材料作为缓冲吸能材料使用时,具有相当好的能量吸收能力,且最大容许应力也较好。更多还原

【Abstract】 Aluminum foam is a composite material consisting of metallic matrix and gas bubbles. It has an extensive application in different areas, including automotive, transportation, architecture, machinery, metallurgy, chemical engineering, communication and aerospace, due to its advantages such as, high strength, fire-resistance, sound and energy absorption, sound isolation, impact absorption and interception of electric wave. At present, tremendous research activities concerning aluminum foam have been implemented.In industry applications, mechanical behavior, especially, energy-absorbing performance has attracted significant attention. Because of excellent toughness of the matrix metal, pure aluminum matrix foam can be utilized as toughness reinforcing metal foam material. Current knowledge reveals that the preparation of pure aluminum matrix foam by direct foaming in melt is difficult because of the higher melting point temperature, which does not match the decomposition temperature of foaming agent, and the bigger shrinkage during solidification. In this paper, both laboratory scale and pilot plant trials of the direct foaming in melt method for fabricating pure aluminum matrix foam are investigated and discussed in details.Solidification process of pure aluminum matrix foam material was studied. Results showed that pure aluminum matrix foam material had special solidification character. When pure Aluminum matrix foam was cooled by free cooling, outer foam solidified in the layer solidification way, inner foam solidified via bulk solidification way, and some minute shrinkage voids formed dispersedly at the cells of inner foam; when pure Aluminum matrix foam was cooled by water, the surfaces of cells were ragged, solidification shrinkage occurred uniformity at the cell walls along the vertical direction. The vertical direction shrinkage eliminated the cracks, and made the cells wall thinner. Solidification model of cells were established. Volume change of bulk material and gas during solidification were explained, showing that the porosity of pure aluminum matrix foam become bigger during solidification.In addition, the control method of preparation techniques conditions of aluminum foam by direct foaming in melt was discussed. When Ca addition、stirring temperature、foaming time and cooling condition were constant, the porosity of aluminum foam could be controlled by changing the TiH2 addition and the stirring time. The biggest porosity could reach above 92%, and the lowest density could reach below 0.25g/cm3.The formation and control of bubble-free layer were researched. A triple-step process was established for the formation of bubble-free layer:for the initial stage, bubbles moved upwards, and a portion of liquid was remained at the bottom and a bubble-free layer was formed; the second and the third processes were during the process of growing of bubbles, the bigger pressure of liquid stored in films made liquid in films flow to plateau borders during these stages, and then the liquid in plateau borders flowed to the bottom through plateau channels, and formed bubble-free layer. When 0.5wt.%Mg was added into aluminum melt which contained 3wt.%Ca, the surface tension of melt decreased significantly, and aluminum foam block with thin bubble-free layer could be gained.Industrialization technique and formation conditions of pure Aluminum matrix foam were established through the experimentation in semi-industry, and the commercial pure aluminum matrix foam slabs (1000×1800×Xmm3) were obtained.The stability of aluminum melt foam during the preparation of pure aluminum matrix foam material by direct foaming in melt was investigated. Intermetallics, such asAl2oCaTi2、Al4Ca and Al2O3 were confirmed in the melt after adding Ca. Those intermetallic compounds Al4Ca and Al20CaTi2 will enhance the viscosity, and restrained the drainage of melts; Al2O3 at the surface of cell walls changed the curvature of curved face, which slowed up the capillary flow between liquid film and the plateau borders, and also had an adsorption force-field for H2, moreover, the adsorption action could reduce the interface energy. The three above processes stabilized the pure aluminum matrix foam.Researches on static and dynamic compressions of pure aluminum matrix foam and Al-6Si matrix foam have been conducted. Results showed that there were different compression processes between aluminum matrix foam and Al-6Si matrix foam during static compression and dynamic compression. The compression curves of pure aluminum matrix foam were flat, indicating that more obvious characteristics of the plastic foam; while the compression curves of Al-6Si matrix foam were more undulated, apparently showing that more obvious characteristics of the brittleness foam. The microscopic patterns and phase compositions of matrix were studied, the results showed the microstructures of pure aluminum matrix foam existed basically in small pieces (Al20CaTi2); whereas the microcosmic pattern of Al-6Si matrix foam mainly existed as big pieces、long needles (Al3.21Si0.47、Al2O3、CaAl2Si2 and Al3Ti).Energy absorption characteristic of closed-cell aluminum foam was systematically analyzed, with energy absorption capabilities, efficiencies and figures of aluminum foams were studied. The results showed that the energy absorption capability of pure aluminum matrix foam were more bigger than that of Al-6Si matrix foam under the same compressing speed and the density; The energy absorption efficiency of pure aluminum matrix foam could be as high as 80%; it is also found, from the energy absorption figure of pure aluminum matrix foam, that pure aluminum matrix foam had goodish energy absorption capability and the maximal allowable stress as energy absorption material.更多还原