Papers by Author: Yo Kojima

Abstract: ZWK510 (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) magnesium alloy containing Mg3YZn6 quasicrystal phase was prepared by conventional permanent mold casting. Part of the cast ingot was subjected to equal channel angular pressing (ECAP) directly; another part of the cast ingot was extruded initially, then ECAP was applied to the extruded alloy. After 4-pass ECAP, the fraction of coarse grains of the as-cast alloy was decreased to about 30%, and the grain size of fine grain was decreased to about 2 μm. Both strength and ductility of the as-cast ZWK510 alloy were significantly improved with increasing ECAP passes, which was resulted from broken and dispersed I-phase, and fine grains formed due to recrystallization. The as-extruded ZWK510 had an initial grain size of about 2 μm and bands of quasicrystal phase parallel to the extrusion direction. After the extruded alloy was subjected to ECAP, the grain size of the extruded alloy was further refined, the grain size was refined to below 0.5 um after 8-pass ECAP; and the quasicrystal phase was further broken and dispersed in the matrix. After ECAP, the elongation to failure of the extruded alloy was improved. However, both yield strength and ultimate tensile strength were decreased, which is considered to be resulted from the texture modification during ECAP.

Abstract: Mg-0.5%Zn-4%Al-0.5~2.0%Ca-0.5~2.0%La alloys are investigated in order to determine the optimum composition with superior casting properties and excellent creep resistance for automatic transmission case applications. The results show that regardless of the La content, the hot-tearing susceptibility of alloys containing 0.8%Ca or more is low. However, at 0.5%Ca content, hot-tearing occurs in alloys containing more than 1.2%La. The creep resistance of the alloys increases as the Ca content increases and Mg-0.5%Zn-4%Al-0.9%Ca-0.8%La alloy is found to be the optimum alloy exhibiting good casting properties and adequate heat resistance for automatic transmission cases.

Abstract: 4-pass ECAE processing was applied to two kinds of magnesium alloys, AZ31 and ZK31. Subsequently, hot rolling was carried out as a secondary forming process. After ECAE processing, the samples have fine and equiaxed grains. The basal plane of AZ31 alloy is inclined at about 450 to the extrusion direction, while that of ZK31 alloy is parallel to the extrusion direction. As a result, tensile strength and 0.2% proof stress of ZK31 alloy are higher than those of AZ31 alloy. On the other hand, the basal plane orients parallel to the rolled surface in the Subsequently rolled samples and, therefore, tensile strength and 0.2% proof stress are improved significantly. Furthermore, the AZ31 alloy has higher ductility than ZK31 alloy.

Abstract: Equal channel angular extrusion (ECAE) was applied to an extruded ZW1101 (Mg - 11wt%Zn - 0.9wt%Y) Mg alloy containing quasicrystallines. The as-extruded ZW1101 alloy had an initial grain size of about 12 µm and bands of quasicrystalline phases parallel to the extrusion direction. After the extruded alloy was subjected to ECAE processing, the grain size was refined to about 0.5 µm, and the quasicrystalline phases were further broken and dispersed in the matrix. After the ECAE processing, the micro-hardness and yield strength of the alloy were increased, however, the ultimate tensile strength and the ductility of the alloy were slightly decreased.

Abstract: Ingots of AZ31 alloys were prepared for the present study. The ingots were hot rolled to 0.5 mm thickness. In order to develop rolled magnesium alloy sheet having excellent plastic formability, the authors attempted the grain refinement and randomization of the rolling texture of the alloy by means of surface modification utilizing wet-type shot blast (wet blast) and subsequent annealing. Then the microstructural observations including texture analysis and tensile tests were carried out. Furthermore, the bendability and stretch formability were investigated. After wet blast and subsequent annealing at 573K for 1h, the region of 50 µm depth from the sheet surface that experienced deformation by wet blast is recrystallized, and fine and equiaxed grains are generated. Although rolling texture remained in the central part of the sheet, the recrystallized grains near the surface of such specimens have random crystallographic orientation. The tensile strengths of both rolled and wet-blasted specimens are almost the same but the elongation of the latter increases due to grain refinement and randomization of crystallographic orientation. Such a microstructural feature realizes bending with the radius of 1 mm without cracks and wrinkles and leads to an increase in limiting stretching depth by 1.5~2 mm.

Abstract: In the current four-year term project in Japan, new platform science and technology is proposed as a core concept of research and development of advanced magnesium alloys together with understanding of their intrinsic characteristics. The research fields related to advanced super-light magnesium alloys for 21st Century have been focused to the selected three categories; ecomaterial design and processing, high qualification of mechanical performance, and high performance design and processing in functionality. On the basis of the obtained results, platform science and technology for environmentally benign and high performance magnesium alloys is constructed as an industrial base material for the next generation. As a result, numerous large-scale joint research and development projects on magnesium alloys based on partnerships between industries, academia and government has already started towards practical utilization since last year.

Abstract: In this study, it refined by the vacuum distilling refining and the billet was produced for the obtained 99.99% or more of purity magnesium deposit by compression forming, extrusion process was performed and the corrosion resistance of extrusions and the tensile characteristic which were acquired were investigated. The specimen showed good corrosion resistance rather than commercial AZ91D magnesium alloy die-castings. The specimen has high ductility and it is good, and compared with 99.9% of purity magnesium.

Abstract: Equal channel angular extrusion (ECAE) was performed on as-cast ZW1101 (Mg-11wt%Zn- 0.9wt%Y) Mg alloy containing quasicrystallines. The grain size of α-Mg was effectively refined, and coarse eutectic quasicrystalline phases were broken and dispersed in the alloy by ECAE. The alloy processed by ECAE exhibited a good combination of high strength and high ductility, which is due to the grain refinement and fine dispersed quasicrystallines in the alloy.