Papers by Author: Taichi Murakami

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Authors: Takuya Tsumura, Taichi Murakami, Soong Keun Hyun, Hideo Nakajima, Kazuhiro Nakata
Abstract: Effects of pore directions on the profile of fusion zone for lotus-type porous magnesium by laser welding has been investigated by comparing the experimental observations and the results of numerical simulation. The three-dimensional finite element calculations were performed, which takes into account equivalent thermal properties and anisotropy of thermal conductivity. There is the pore anisotropy in the profile of fusion zone by laser beam irradiation and the good weldability was obtained when the growth direction of the original pore equaled to the direction of the laser beam irradiation. A good agreement was obtained between the calculated profile of the weld fusion zone and the experimental results.
Authors: S. Hotta, Taichi Murakami, Takayuki Narushima, Yasutaka Iguchi, Chiaki Ouchi
Abstract: Currently new continuous casting processes such as thin slab caster or strip casting are industrialized or under developing in the world steel makers. In these casting processes, cooling rate after solidification becomes much faster compared with thick slab caster, and hot rolling mill connected directly with casting machine tends to be installed. The present study was conducted to investigate variations of austenitic grain size and micro segregation with cooling rate after solidification and also direct hot deformation conditions in austenite immediately after solidification in HSLA steels. HSLA steels were 0.15%C-0.25%Si-1.50%Mn, 0.028%Nb and 0.028%Nb-0.015%Ti with the same basic compositions. A hot working simulator of THERMECMASTER-Z was used, and the center part of tensile specimen set up in this machine was partially or fully levitation-melted by induction heating under argon gas atmosphere. After melting, specimens were cooled at cooling rate from 0.4K/s to 40K/s, and this range covered cooling rates after solidification in heavy thick slab caster and strip casting. Direct hot tensile straining in austenite after solidification was conducted at strain rates from 1.4×10-3s-1 to 2.6s-1, corresponding to an extracting speed in a respective caster. The increase of cooling rate refined continuously as cast austenitic grain size, and it was enhanced in micro alloyed steels. Micro segregation such as Mn was improved by faster cooling. Direct straining after solidification markedly refined austenitic grain size through dynamic or static recrystallization occurring depending on strain rate.
Authors: Hwa Soon Park, Byung Woo Lee, Taichi Murakami, Kazuhiro Nakata, Masao Ushio
Abstract: The mechanical properties of the friction stir welds of the oxygen free copper (OFC) and 60%Cu-40%Zn copper alloy(60/40 brass) were investigated. The defect-free welds were obtained in a relatively wide range of welding conditions; the tool rotation speed had rpm of 1000 to 2000 in the OFC and 1000 to 1500 in the 60/40 brass, with the welding speed of 500 to 2000 mm/min. The SZ hardness values of the OFC welds were almost the same or slightly lower than those of the base metal. However, the SZ hardness values of the 60/40 brass in all welding conditions were much higher than those of the base metal. The SZ hardness values of both metals increase with a decrease in heat input. The tensile properties of the all-SZ showed relative correspondence to the variation of the SZ hardness values.
Authors: Takuya Tsumura, Fu Xing Ye, Taichi Murakami, Hideo Nakajima, Kazuhiro Nakata
Abstract: Lotus-type porous metals, whose pores are aligned in one direction by unidirectional solidification, have a unique combination of properties. These are expected as revolutionary engineering materials with anisotropy of the properties. For the industrial use of the lotus-type porous metals, a reliable joining technology is required. We already reported the melting property of a few lotus-type porous metals by laser welding. These results indicated that these materials possessed anisotropy of melting property with the pore direction perpendicular and parallel to the specimen surface, especially remarkable anisotropy was obtained for the copper specimen owing to the difference of the laser energy absorption to the specimen surface. In this report, the three-dimensional heat transfer analyses, which take into account the difference of the laser energy absorption comparing with the anisotropy of thermal conductivity inherent to lotus-type porous metals, were performed by commercial code with user-defined subroutine. Predicted profile of weld fusion zone is in good agreement with the cross-sectional view obtained by experiments.
Authors: Behrang Poorganji, S. Hotta, Taichi Murakami, Takayuki Narushima, Yasutaka Iguchi, Chiaki Ouchi
Abstract: New α+β type titanium alloy with Ti-4.5Al-6Nb-2Mo-2Fe was developed on the basis of using biocompatible elements and eliminating the cytotoxic ones such as Vanadium, while achieving the desirable mechanical properties such as appropriate strength, cold workability and low superplastic forming (SPF) temperature. The present study was conducted to investigate the effect of yttrium addition of less than 0.05% into this alloy on static and under superplastic deformation grain growth behavior. The new alloy bar manufactured by α+β processing and annealed at 1073K yielded extremely fine two-phase microstructure with α grain size around 2μm. Specimens were heated at temperatures of 1048, 1073 and 1098K and kept for times between 3.6 to 172.8KS. Yttrium forms in-situ Y2O3 particles, and the presence of these particles yield finer two phase microstructure due to their retardation effect on β phase grain growth. Grain growth behavior during hot deformation was investigated by hot compression test in use of a hot working simulator of THERMEC-Master Z. Strain rate was varied from 2×10-2 to 2×10-4S-1 and strain was 0.69. Grain size of both α and β phases increased with a reduction of strain rate, and Y2O3 particle was also effective to retard grain growth under hot deformation. It was confirmed from comparison of grain growth during isothermal heating with and without hot deformation that grain growth was much accelerated by deformation. All of these results were discussed based on grain growth mechanism or model for two-phase microstructures as well as superplastic deformation mechanism.
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