Vol.2 No.2 2009

43/98

Research paper : Energy savings in transportation systems by weight reduction of their components (M. Sakamoto et al.)−123−Synthesiology - English edition Vol.2 No.2 (2009) The significant difference between the surface structures of pure magnesium and magnesium-calcium alloys shows that there are differences in the constituent phase and formation mechanism of the oxidized film. Studying the oxidized film carefully, it was clarified that the outermost layer of the surface oxides of calcium-containing alloys was mainly composed of oxidized calcium[1]. Since this oxide has a dense structure, it acts as an effective barrier against oxygen supply from the atmosphere and against evaporation of magnesium from the molten surface, and the ignition temperature is thought to increase due to this protective action.Although why such a dense oxide film forms only on calcium-containing alloys is a very important and interesting issue, not much is known about the formation mechanism. Calcium is more active than magnesium, and a dense surface oxide film is not formed in calcium alone, as in magnesium alone (or a magnesium alloy without calcium). What is clear is that the coexistence of magnesium and calcium is significant. When oxidation occurs when calcium and magnesium are coexisting, various interactions occur. Since calcium is one of the few elements that can be expected to reduce magnesium oxides at temperature range near the melting point, the formation of the outermost oxidized film layer composed of calcium oxide is probably a complex process that involves the reduction of magnesium oxide by calcium and production of calcium oxide, in addition to the preferential oxidation of calcium over magnesium.After the discovery of the effect of calcium, the search for a similar effect among various elements was futile, and no additive element that showed dramatic ignition suppression like calcium was discovered. To the present, we found no element that enhanced noncombustibility as much as calcium. It is thought that this is an appropriate conclusion, since, considering the above mechanism, the only element with lower free energy to form oxide than magnesium is calcium.3 Solution of individual elemental technology issuesFor practical application, it is necessary to appropriately set the individual elemental technology issues and solve them one at a time. However, this is easier said than done. If issues are dealt with as they arise, a long time will be required to reach the goal. A clear scenario is necessary to send out a new material to the world. In the development of noncombustible magnesium, (1) following the discovery of a new raw material described above, (2) the development of refining technology to raise the raw material to material, (3) the technology for fabricating members (forming and processing technology), (4) the reliability assessment, and (5) a technological breakthrough in the product realization phase were necessary.Through the mechanism described in the previous section, calcium-containing alloys can be melted and cast in the atmosphere safely, and the manufacturing process can be done safely and simply using the magnesium-based alloy. To be simple is important from the industrial perspective. The most important point here is to be conscious that the original motivation for employing magnesium as a core material is to realize a low environment load society that may become possible if magnesium is used. Therefore, this development will be meaningless unless all elemental technologies follow the guiding principle. The outline is described as follows.3.1 Clean molten metal refining technologyIn noncombustible alloys, active metal calcium is added to active magnesium, and, inclusions such as oxides that are produced in the smelting processTerm 1 mix into the molten metal in larger amounts compared to other alloys, and this may affect strength and corrosion resistance. Since the specific gravity of the inclusions is close to that of the molten metal, it is difficult to separate them by precipitation or floating, and therefore, complete separation and removal are difficult. Normally, they are removed using flux such as chlorides or fluorides of magnesium or calcium. However, the corrosive resistance is compromised if the flux remains even in small quantity, and the material yield may be decreased if attempt is made to avoid this problem. Also, there is the problem of industrial waste composed of chlorides and fluorides. Even if an atmospheric melting process that does not use fireproof gas or flux is developed, the value of this technology is greatly diminished if flux or other materials are used in the refining process. Our condition for survival was to develop an environmentally clean, simple, and yet effective method.We focused on the fact that in addition to the prevention of oxidation by the dense oxide film formed on the surface of Fig. 3 Surface structure of oxide film in Mg-5Ca binary alloy.(b)(a)

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