| Liquid metal infiltration into porous preforms is an economic and efficient process to fabricate high volume fraction(55~75%) Silicon Carbide particles reinforced Aluminum matrix(SiCp/Al) composites. The characteristics of pores in the preforms have significant effects on the flow behavior of liquid Aluminum alloys during infiltration as well as the microstructure and properties of finally prepared composites. The characteristics of pores are majorly related to the process parameters of preforms preparation. In this paper, the pores in the SiCp preforms were investigated by high resolution(~1.0μm) three-dimensional X-ray micro-computed tomography(3D X-ray μ-CT) inspection and conventional analysis methods.Starch as the pore-forming agent and SiC particles with different sizes were cemented by Aluminum dihydrogen phosphate in preforms. Based on the analysis of raw materials, the mass ratio of each material was determined. The SiCp preforms were prepared by compress forming with a self-designed mold, and hence were sintered with the optimized heating parameters. The fabricated preforms contained adjustable porosity contents, high compressive strength, flat surface and complete appearance.The SiCp preforms with different starch contents were prepared for studying the effect of the starch content on the pore characteristics. The results show that in the sinter process, pores were generated due to the burning of starch and the gaps expansion between the stacked particles caused by the decomposed gas escaping out from preforms. With the increase of the starch content, the total porosity and pore connectivity of preforms as well as small size pores or throats in the pore-network were significantly increased, whereas the spatial homogeneity of pore volume distributions were decreased. When the starch content added to 8wt%, more “poreneck” structures were built up in pore spaces, leading to the increase of small size pores or throats and the decrease of the pore connectivity. But the “pore-neck” structures were broken up by notable gaps expansion when the starch content increased to 15wt%. As a result, small pores were connected into larger pores and hence the pore connectivity was increased.The SiCp preforms with different particle sizes were prepared for studying the effect of the particle size on the pore characteristics. When the particle size was small, the “pore-neck” structures were formed among the small particles, then the pore spaces were segmented and became more complicated. With the increase of the particle size, the effects of pore generation and gaps expansion from the starch were weakened and the interval pore spaces were depended on the gaps among particles. Therefore in the case of larger particles used, the total porosity and the spatial homogeneity of pore volume distributions were decreased, and as for the porenetworks, the average coordination number of pores and the number of both pores and throats were rapidly decreased, while the average effective sizes of pores or throats were increased. When the particle size was increased to 150μm continuously, fine particles were more prone to be filled into the large interval voids among coarser particles, reducing the pore connectivity and the spatial homogeneity of pore volume distributions, and more pores or throats with smaller size were found in the pore-networks increasing the average pore coordination number.The comparisons of pore characteristic data obtained by the different methods has shown that high resolution(~1.0μm) 3D X-ray μ-CT inspection combined with the pore-network models can accurately characterize the pores in quantitative. However the mercury intrusion porosimetry(MIP) method may underestimate the pore fraction and size. This is ascribed to that the porous structures with lower compressive strength would be compacted under the high pressure required for intruding mercury into small pores, and the assumption of irregular pores as cylindrical models was used in the MIP calculation. |
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