Study On Preparation Of Aluminum Foam Using Novel Foaming Agent Two Steps Foaming Process

Closed-cell aluminum foams can be widely used in many fields like transportation, architecture and machine, electrommunication, aerospace industry and environmental protection due to its characteristics of light-weight structure, high strength, energy absorption, sound and heat insulation, and electromagnetic shielding, etc. It is the most commercial valuable approach to fabricate closed-cell aluminum foam in the melt foaming method, which allows the large scale production of aluminum foam with regular pore structure. However, traditional melt foaming process adopt metal calcium as thickening agent, and TiH2 as foaming agent with rapid decomposing rate, which make the operation rigorously and inefficient. The use of expensive TiH₂ and calcium increase the production cost, and discontinuous operation reduce the produce efficiency greatly, so it is important to develop novel substitute material for TiH₂ and calcium.The thickening mechanism and foaming behavior of a new substitute material--NFA which can take the place of calcium thicken and TiH₂ foam, were studied. A new production technology called NFA two steps process was developed, the interior defects of aluminum foam and the recycle of aluminum foam scrap were also investigated in this paper. The main conclusions are as follows:(1) The thickening process of NFA was studied. The results show that the thickening mechanism of NFA is that NFA can release CO₂ gas which can react with liquid aluminum melt and form a great amount of tiny oxide particles, these oxide particles as well as the NFA decomposed leftover solid particles will thicken the melt; 1～2 wt% addition of metal Mg is necessary in order to improve the wettability of NFA with molten aluminum.(2) The NFA thermal decomposing behavior was studied, and the results show that NFA decomposing reaction is controlled by chemical reaction mechanism. NFA decomposed gently during 600～700℃and rapidly during 700～800℃.The thermal decomposing behavior of NFA indicated that NFA can completely take the place of TiH₂ to produce aluminum foam or aluminum alloy foam ; in addition, due to long decomposing time of NFA the operation conditions such as temperature, time and stirring intensity are controlled less rigorous than that of TiH₂ .(3) The foaming process of NFA was studied. The results show that the melt foaming process is actually a homeostasis process of bubble nucleation, growth, rheology and collapse. The bubble grows mainly in the prophase of holding time, the pressure of carbon dioxide generated by decomposing of NFA is the driving force of bubble's growth. In the anaphase of holding time the collapse of the bubble become the dominant factor, while the collapse of the bubble mainly attributed to two factors: drainage and coalescence, both are spontaneous process. The drainage phenomenon leads to formation of free-bubble layer and the rheology of bubbles leads to formation of gradient pore structure. The CO₂ gas released by NFA during the foaming process can react with the liquid aluminum melt, and form a continuous oxide film on the surface of the cell, which is important for foam stabilization, and can slow down cell coalescence and growth.(4) Based on the mechanism of thickening and foaming of NFA and its thermal decomposition characteristics, a NFA two steps foaming process was developed to produce aluminum foam continuously, the technology parameters were determined by experimentation in lab, and the quadrate aluminum foams with symmetrical pore structure were obtained successfully.(5) There are three main interior structure defects in the preparation of aluminum foam with melt foaming process such as free-bubble-layer at the bottom, partial free-bubble-zone and heat crack. Increasing aluminum melt viscosity can reduce the thickness of free-bubble layer effectively. Adoption of double-oblique-propeller mixed with high stirring speed can eliminate partial free-bubble-zone. To prevent from heat crack, the choose of aluminum alloy with small crystal temperature range and the use of metal cast model, as well as adoption of gradual solidification style will be effective.(6) All kinds of aluminum foam production wastes can be reclaimed by remelted; It is not necessary to thicken while to produce aluminum foam with secondary aluminum whose foaming performance is similar to that of original aluminum; the final porosity and uniformity of secondary aluminum foam will improve obviously if the secondary aluminum and the original aluminum is mixed at the ratio of 1:1, with 1～2% addition of Mg during the foaming process.