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Research paper : New material development by the integration of cast technology and powder metallurgy technology (K. Kobayashi et al.)−291−Synthesiology - English edition Vol.3 No.4 (2011) intermetallic compounds using casting technology, synthesizing intermetallic compounds including titanium aluminides (TiAl, Ti3Al, etc.) and iron aluminides (FeAl, Fe3Al, etc.). Because the aluminide intermetallic compounds are composed of elements with large specific gravities and large melting point differences, the degree of segregation proved too large for the traditional melting method or casting technique. Therefore, a new process technology called levitation melting and casting was developed. However, through simply casting the molten metal, the microstructure of the iron aluminide intermetallic compound became coarse, and sufficient strength could not be obtained. At the same time, AIST was developing the semi-solid forming technology as a casting technique for shaping magnesium alloy with fine microstructure. This technique involves the application of high pressure to partially molten alloy, and produces both the near net shaping by thixotropic properties and the high strength given by fine microstructure. Moreover, we considered powder metallurgy technology as a synthetic method for the production of aluminide intermetallic compounds, and investigated the synthesis of aluminide intermetallic compounds with fine microstructure using the mechanical alloying method[2]-[4].With the technologies for the material design and manufacturing processes almost completed for the WC-Co cemented carbides, the research was conducted mainly for the development of finer hard particles. The demand for cemented carbides has increased with the increased speed and precision of machining technology, and as a consequence, there have been further demands for cost reduction. Particularly, an alternative to, or means of reducing the amount of, cobalt (which can be subject to extreme price fluctuation) and tungsten (which suffers from variable resource availability) was sought.Therefore, we started the development of a new process technology for a new hard material using an aluminide intermetallic compound as the binder phase by fusing technologies, i.e. the various known technologies for intermetallic compound synthesis and technologies based on our knowledge of cemented carbides. By using Fe and Al instead of Co as the binder phase of cemented carbides, we developed a process in which only Al was liquefied in the sintering process, followed by the synthesis of a FeAl intermetallic compound. By using this technology, we created a new hard material with heat resistance, as a result of compositing the hard particles of tungsten carbide, titanium carbide, or titanium boride using this iron aluminide intermetallic compound as the binder phase. To enhance the feasibility of this new material replacing conventional WC-Co cemented carbides, we set the goal for the newly developed composite material to exhibit 900 Hv or more in hardness and to surpass 2 GPa in three-point bending strength.3 Research scenarios to realize our goalsIn this technological development, the mechanical strength of the composite material was determined by the close contact force of hard particles and iron-aluminide intermetallic compounds, which filled the gaps between these particles. The method of fabricating a porous preform of hard particles and then pressure-injecting the molten iron aluminide intermetallic compound, and the method of mixing and stirring the hard particle powder into the molten iron aluminide intermetallic compound were investigated, but sufficient strength could not be obtained owing to low adherence between hard particles and intermetallic compound in this method. Therefore, we studied a new process in which hard particles of high melting point are forcibly mixed by mechanical stirring[4] with the iron and aluminum powders that are the components of the intermetallic compound as the binder phase. We believe that the hard particles were coated with metallic powders, since the metallic powder is highly ductile.Following this, we fabricated a WC-FeAl alloy (WC-8.6 mass% Fe-1.4 mass% Al) using a method that was similar to a manufacturing process for conventional cemented carbides. WC powder, Fe powder and Al powder were wet-mixed by attrition ball milling at desired constituent and sintered at 1440 ºC in a vacuum. The conventional cemented carbide (WC-Co) was fabricated by liquid-phase sintering in which the binder phase was melted; high-temperature sintering was also necessary in the case of WC-FeAl to increase the adherence of the binder phase and hard particles. The obtained sintered compact showed excellent resistance to oxidation, even when heated to 800 ºC in air, and when the compact was treated by hot isostatic pressing (HIP), the bending strength was maximum 1.8 GPa. However, strength variations were observed in the obtained compacts, and it proved difficult to manufacture a stable compact; the composition and volume of the binder phase could not be accurately controlled as Al with a low melting point evaporated during vacuum sintering. In addition, the evaporated Al may adhere to the graphite electrode and other devices in the vacuum sintering furnace, and the fabrication of this WC-FeAl hard material using the conventional process was considered impractical.The evaporation of Al during sintering may be caused by the insufficient reaction of Fe and Al during wet mixing. To address this, we then applied the mechanical alloying (MA) method, which is a dry mixing method that uses large mixing forces to synthesize the alloy, to produce WC-FeAl. It has already been established that the MA method for aluminide intermetallic compounds can be achieved using an amorphous alloy powder and that a significant time is needed for the alloying of Fe- and Al to progress. When the long-term MA was conducted at high energy, the adherence of the hard particles and the binder phase

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