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A Study of Preparation of Pure Al Matrix Foam by Direct Foaming in the Melt

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Tutor: YaoGuangChun
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: pure aluminum matrix foam,direct foaming in melt,preparation techniques,solidifi
CLC: TB383.4
Type: PhD thesis
Year:  2008
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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¡Á1800¡ÁXmm3) 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.
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