Study On Preparation And Properties Of Aluminum Based Porous Materials By Counter-gravity Infiltration Casting

Due to its unique characteristics of metal and pore structure, aluminum based porous materials combine the characteristics of functional materials and structural materials, and has a broad prospect in application. At present, the infiltration casting is the main method of aluminum based porous materials preparation. However the traditional process has many unavoidable defects, such as "excessive infiltration " and "insufficient infiltration". In order to solve these problems, we innovatively designed a counter-gravity infiltration casting system based on the PASCAL principle and the pressure infiltration theory, then studied the counter-gravity infiltration casting process and found the principles to determinate the process parameters. In this paper, porous aluminum naked materials, Al2O3k/Al materials, porous aluminum core sandwich materials and other aluminum based porous materials were successfully prepared by this system. The related materials mechanics, heat and acoustic properties were studied and the explanations for the different properties of aluminum based porous materials prepared by counter-gravity infiltration casting system and the traditional process were analyzed. The main research results are as follows:(1) The counter-gravity infiltration casting system has an up and down structure. The above section is an infiltration room while the lower section is a heating and melting room, between which is a diversion part for connection. Graphite paper is used to ensure the seal of system between the parts. There are more than one infiltration rooms with different sizes in the system, which can achieve different size of foam metal intermittent and semi-continuous infiltration productions. The effective infiltration space can be determined by the size of the infiltration products.(2) The counter-gravity infiltration casting system mainly consists of the melting of metal matrix, the installing and preheating of porous materials in infiltration room, the process by bottom-up infiltration pressure to make melt into the hole of the porous materials, the holding crystallization steps and so on. Because of bottom-up movement patterns of metal and mould filling and crystallizing of aluminum melt, there is no obvious macro casting defect in the final product.(3) A novel material—MOD was exploited as the pore-forming material, which can meet the requirements of strength and stiffness for infiltration casting. This material is not only soluble in water, but also has a higher melting point and boiling point than the sodium chloride and potassium chloride. Furthermore, it is non-corrosive to the based metal, low cost and environmentally friendly.(4) Contrastive research was carried out to study the acoustics and thermal properties of the porous aluminum naked materials prepared by counter-gravity infiltration casting system and the traditional process respectively. The acoustics performance study suggested the product prepared by the former method is better than the product prepared by the latter. It was the decrease of connected space between adjacent holes in counter-gravity infiltration casting system that caused this difference. The thermal properties research showed the product prepared by counter-gravity infiltration casting had an obviously higher thermal conductivity, it was largely because the product had a low pore degrees and more dense crystallization of the based metal after some processes, for example the constant pressure process.(5) Al2O3k/Al materials with no obvious macro casting defects were produced using alumina bubble brick as pore-forming materials through the counter-gravity infiltration casting system. It was found that along with the increase of the metal temperature and hollow balls preheating temperature and the particle size of hollow balls, the length of infiltration was getting longer. Besides, along with the increase of the particle size of the hollow ball, the pore degrees decreased. Improving the filling and holding pressure, lengthening the holding time and adding metal Mg were the effective measures to improve infiltration process.(6) When compared the quasi-static compression performance of the Al2O3k/Al materials and the porous aluminum naked materials, it could be found that their stress-strain curves also had the "three stage" feature, which were linear elastic stage, yield platform stage and close-grained change stage. The particle size of porous materials, the relative density and the strain rate were the key factors for their compression performance, the smaller particle size of porous materials, the bigger of the relative density and the lower of the pore degrees, the greater of their yield stress and platform stress would be; However, compared with the porous aluminum naked materials, the yield stress and platform stress of the Al2O3k/Al materials were much greater, and its close-grained strain stress was only slightly reduced.(7) A kind of equipment, which could be used to prepare aluminum based porous materials of large size with no obvious macro casting defects, was designed and manufactured. It was creatively to put forward the method that using the small size porous precast block to prepared large size aluminum based porous materials. This method could effectively avoid of the easily broken defects of large size and precast block during the handling process.(8) The three point bending test was taken on the large size Al2O3k/Al core sandwich material, and its bend destructive process was also studied. The results were as follows:(1)The materials showed the classic bend characteristics of porous aluminum sandwich board, which consisted of linear elastic stage, elastic-plastic stage and Instability stage.(5)The materials had a long period of instability, and the bending load was twice as the Al2O3k/Al materials’, nearly18KN; The stiffness of the sandwich material was more than30KN/mm;(3)Bending failure of the materials was mainly caused by the shear cracking destruction of the core materials and bending failure of surface panel, and the cracking phenomena did not appear between the interface of panel and core material during the process of destruction. The metallurgical combination between panel and core material and using alumina bubble brick as porous materials were the critical reasons why Al2O3k/Al core sandwich material had good flexural performance, large stiffness and why the cracking phenomena did not appear during the process of destruction.