Vol.2 No.2 2009

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Research paper : Energy savings in transportation systems by weight reduction of their components (M. Sakamoto et al.)−122−Synthesiology - English edition Vol.2 No.2 (2009) establish a highly efficient mass production process that is also safe and environment-friendly, by turning something considered to be a special material into a core material that can be used readily by everyone.The manufacturing technology for noncombustible magnesium is not only a blessing for the manufacturer in terms of safety at the plant and decreased cost of production (special facilities are not needed), but also satisfies the user’s demand for safety. In addition to psychological fear for magnesium ignition, the issue of safety in accidents and fires was an important point that was often ignored.2.1 Discovery of noncombustibility by addition of calcium and the mechanism of noncombustibilityThe discovery of noncombustibility was like finding a bargain. In the development of lightweight metal matrix composites in which various ceramic fine particles are dispersed in aluminum, to directly mix and disperse the ceramic fine particles into the molten aluminum, it is important to improve the wettability of the molten metal and the particle surface and to optimally control the viscosity of molten aluminum. In the process of investigating the effect of adding various elements to the property of molten metal, it was found that calcium was effective in controlling the viscosity of the molten metal, and the low-cost manufacturing process for fine particle dispersed aluminum alloy composite was developed. The study of noncombustible magnesium alloys started from the accidental discovery of noncombustibility in the atmosphere, or the dramatic change in the property after adding calcium to the molten magnesium in the process of applying this technology to molten magnesium for further weight reduction. Figure 1 shows the ignition temperature in the atmosphere of a major noncombustible magnesium alloy AZX912 (A=Al, Z=Zn, X=Ca, number is wt%). As apparent in this figure, addition of calcium increases the ignition temperature by over 200 ºC. Melting in the atmosphere becomes possible when the ignition temperature rises to this level.On the other hand, just as in ignition property, the state of oxides formed on the surface of molten magnesium differs significantly according to whether it contains calcium or not. In case of pure magnesium, the surface oxide of the sample, which was molten in inert atmosphere, immediately exposed to the atmosphere, and cooled rapidly before it ignited, had grown extremely thick even when the time exposed to the atmosphere in the molten condition was only a few seconds. The structure was porous composed of fine particles, and it could be inferred that the oxide did not provide a protective film for the surface. This is shown in the SEM photograph of the oxide surface in Fig. 2. In the figure, (a) is the overall structure at low magnification, and (b) is the enlarged structure at high magnification. The structure of the oxide can be seen from how it is formed. At melting point temperature, with pure magnesium the volume ratio of the produced oxide (Pilling-Bedworth ratio) is much smaller than 1, and the produced oxide will not function as a protective film that completely covers the surface of the molten metal. This is thought to be the reason why the oxidized film formed on the alloy that does not contain calcium has a porous structure.In contrast, the surface of the oxide of calcium-containing alloys is shown in the SEM photograph in Fig. 3. In this figure, (a) is the overall structure at low magnification, and (b) is the enlarged structure at high magnification. In the calcium-containing alloy, the film formed does not grow thick even if the surface is sufficiently oxidized by exposing to the atmosphere for 1 hr, and the surface structure is extremely dense. The oxidized film formed on the magnesium containing calcium presents a completely different state compared to the magnesium not containing calcium. This shows that when the alloy with calcium is in a molten state, a dense oxidized film forms on the surface, and this functions as an extremely effective protective film against oxidation.Fig. 1 Ignition temperature of noncombustible magnesium alloy (AZX912).Amount of calcium added (wt%)Ignition temperature (°C)Melting temperatureAZ91+Ca1000800600400200001.02.03.04.05.0Fig. 2 Surface structure of oxide film in pure magnesium.(b)PL0024 40KU X5,000 15 mm1 µm(a)PL0022 40KU X50 4 mm100 µm

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