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.)−124−Synthesiology - English edition Vol.2 No.2 (2009) the molten noncombustible alloy, the high vapor pressure of the molten magnesium appeared to be held extremely low. This, in other words, was the development of a refining technology by a reduced pressure method, and it was a breakthrough for converting the noncombustible magnesium alloy into a practical material. This method is characterized by being extremely simple so it can be executed easily, and therefore can be implemented in large-scale production facilities.The reduced pressure method is a simple method for removing the inclusions by floating and separating them on the surface of the molten alloy, by maintaining the molten state under decompression. Since various gases are dissolved in the molten metal, the inclusion floats to the surface of the molten metal by attaching to the gas bubbles generated by reduced pressure. In ordinary magnesium alloys, since the vapor pressure is high, the pressure cannot be reduced. In noncombustible magnesium alloys, the metal vapor pressure becomes extremely low in appearance due to the oxide film formed over the molten metal surface, and therefore, reduced pressure refining can be applied as in aluminum, iron and steel. The target pressure can be reached sufficiently by the evacuation capacity of an ordinary mechanical pump, and the retention time is several seconds to several minutes depending on the quantity of the molten metal. Therefore, it can be applied readily to a large melting furnace. Figure 4 shows the melting furnace with 100 kg capacity for noncombustible magnesium alloys at AIST. It is equipped with the reduced pressure refining mechanism. In the conventional flux method, the working environment deteriorates due to steam originating from flux, but in this method, the working environment remains safe and clean, and the integrity of the material will not be lost since no flux will remain in the molten metal. This technology, at present, has been transferred to four companies and the development of a mass production process is in progress. This series of smelting process is mainly atmospheric melting. It is basically an atmospheric process where in the final step, refining is done by placing a lid and reducing pressure to remove the inclusions. It leads directly to cost reduction of the casting process, and has been put to actual practice. However in reality, many know-hows are required to cast noncombustible alloys, as will be described later.3.2 Plastic forming processThe second issue pertains to the plastic forming process. Compared to aluminum, iron and steel, the formability of magnesium in the cold processTerm 2 is poor, and there is a great risk when a user introduces this process for the first time. This raises the manufacturing cost of magnesium and becomes a realistic barrier in industry. While alloys of iron, steel, and aluminum have cubic crystal structure with small anisotropy, magnesium is a hexagonal crystal structure with large anisotropy, and therefore does not show rich formability in the cold process, and also has a basic problem in the plastic forming process. The problem is more serious in noncombustible alloys. Since magnesium alloys must contain aluminum to increase its strength and corrosion resistance, high melting point aluminum-calcium intermetallic compound is formed due to the calcium added for noncombustibility, and this becomes a primary crystal that crystallizes as a network into the grain boundary, decreases the flow of the molten metal, and negatively affects the mechanical properties, particularly the ductibility. This is shown in Fig. 5a. Since the aluminum-calcium intermetallic compound has extremely small solid solubility into the metal matrix, it is not easy to control such solidification structure by heat treatment.On the other hand, the situation is different when plastic forming is considered. Even with a noncombustible alloy of poor plastic formability, good formability can be achieved by applying the extrusion process that is a hydrostatic Fig. 4 Smelting furnace for noncombustible magnesium alloy equipped with reduced pressure refinement mechanism (capacity100 kg).Fig. 5 Structure of noncombustible magnesium alloy (AMX602).a: Solidified structure, b: Structure after extrusion.VacuumareaKeroseneburnerFurnace(a)(b)10 μm

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