Much of the casting research at the University of Tokyo deals with fundamental issues related to continuous casting of steel and aluminum, thin-wall castings, and directional solidification. Extensive studies of the deformation behavior of steel (Nakagawa et al. 1995), stainless steel (Mizukami et al. 1995), and aluminum (Suvanchai, Ikane, and Umeda 1995; Nakagawa et al. 1995b) during solidification have been carried out to define the causes of internal cracking during solidification and the possible means of avoiding such problems in production operations. Although the major emphasis appeared to be process technology, several new cast products were also discussed.
A low density aluminum curtain wall composite material was produced through suction casting a mixture of molten aluminum with hollow SiO2 spheres. The spheres were produced from volcanic sands, which are otherwise a waste material.
As in the United States, Japan also has growing quantities of radiologically contaminated scrap metal (RSM). In an attempt to develop controlled reuse opportunities for such scrap, a process is being developed for the production of nodular iron containers from RSM. The process leads to production of a cylinder without the use of a core through vertical centrifugal casting (Kim et al. 1994). If rotation is stopped at a critical time, the remaining unsolidified metal will drop to form a cast bottom for the cylinder. Fig. 3.1 shows a schematic of the process. Cylinders having a height of 300 mm and outer diameters ranging from 176 mm to 190 mm have been produced from a 196 mm diameter steel mold. Variations in diameter result from different thicknesses of mold coating, which is one of the critical parameters for the process. Similar results have also been demonstrated for producing cylinders from aluminum alloy AC4C (6.7% Si, 0.4% Fe, 0.15% Cu, 0.06% Mn, 0.35% Mg, 0.06% Zn, bal. Al) (Murata et al. n.d.).
Work on directional solidification processes includes studies of unidirectionally solidified W-Cr cast irons to apply phase selection criteria to elucidate the sequence of solidification of the various carbide forms (M3C versus M7C3) (Liu et al. 1995). Theoretical studies of directional solidification of peritectic alloys indicate that phase selection criteria can be applied to project competitive growth processes of stable and metastable phases during solidification. Potential peritectic systems range from carbon and Cr-Ni steels, copper alloys, rare earth permanent magnet materials, and high TC superconductors (Umeda, Okane, and Kurz n.d.).
The bulk of the research effort at the Tohoku University Division of Foundry Engineering relates more to casting process technology than to the development of new cast materials. Several ongoing efforts are directed toward the goal of improved thin-wall castings. These include studies of the fluidity of eutectic and hypereutectic Al-Si alloys (Funakuba, Anzai, and Niyama n.d.), and studies of the deformation of the shell formed at initial melt/chill contact for Fe-C alloys (Dong, Niyama, and Anzai 1995) and low-melting materials (Bi, Sn, Pb, Sn-Pb) (Dong, Niyama, and Anzai 1994).
A new process for casting titanium alloys called ADCAST has resulted from studies of the fluidity, shrinkage, and mold reaction of titanium alloys in precision casting. The graduate student carrying out this research is the son of the owner of the company funding the research.
Much of the research effort discussed at Nagoya University is related to the long-term research interests of Prof. Asai in the application of electromagnetic fields to the processing of materials. Some of the new areas of application included the following applications.
The steel industry program at Nagoya University is funded by Nippon Steel, Kobe Steel, Sumitomo, NKK, Kawasaki, Daido, Nissin Steel Co., Mitsubishi Steel Co., and Mitsubishi Heavy Industries in conjunction with JRDCM. The goal is to develop a process in which steel plate can be rolled immediately after continuous casting without the need for intermediate cooling, scarfing, and reheating. This requires the production of a high quality surface in the as-cast slab so that scarfing prior to rolling can be eliminated. The improvement of the cast surface is achieved by soft contact solidification whereby the slab barely touches the surface of the casting mold. Consequently, oscillation marks are eliminated. The key to the process is synchronous imposition of a magnetic field during mold oscillation. The process may parallel methods used in the aluminum industry for electromagnetic casting of direct chill ingots for sheet production. The use of magnetic fields as a "bottle" for constraining the molten metal permits solidification to occur with no physical mold/metal contact. This, in turn, leads to the formation of very smooth surfaces capable of being hot worked without surface conditioning. Significant work will still be required before the soft contact process can be commercialized. If fully implemented, application of the process by the Japanese steel industry could result in a 0.2% reduction of Japan's total energy consumption. Such potential provides more than adequate incentive for MITI and the companies involved in the program.
Levitation melting is being developed as a method to eliminate contact between the material being melted and the crucible. Major emphasis is on reactive materials such as TiAl in which contact and chemical reaction with ceramic crucible materials pose serious problems. The process can be carried out in a vacuum or in special atmospheres, which also contribute to the ability to produce castings of high purity. By controlling the electromagnetic forces, it is possible to control levitation and the physical shape of the melt without producing turbulence within the hot liquid. Although emphasis is currently on high temperature melting materials where suitably nonreactive crucibles are not available, it may also have application in high volume production for single-shot melting in die casting.
Separation of inclusions is difficult to achieve with flotation or gravity when the density of the inclusion is close to that of the metal. Theoretical analysis indicates that a direct magnetic field magnifies the difference between metallic and nonmetallic particles and aids separation. Lab experiments confirm the ability to reduce the silicon and iron content of molten aluminum. Prof. Asai holds a patent on this process, which could be of great commercial interest for the processing of contaminated aluminum scrap metal.
Prof. Asai described work at Nippon Steel on continuous simultaneous casting of a stainless steel cladding over a carbon steel core. The process produces a fine definition and bonding of the two alloys and thereby allows rolling to thin gauges. The fine definition is produced through the use of a magnetic field that keeps the two liquids separated in the caster's nozzle through suppression of convection. More expensive conventional methods for producing clad steel products involve either hot roll bonding of sheets or plasma spraying of the cladding.