Papers by Author: Fujio Tamai

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Authors: Shigeki Morita, Nobuyoshi Ohno, Fujio Tamai, Yuji Kawakami
Abstract: This paper deals with fatigue crack propagation behavior of rolled AZ31B magnesium alloy. Two types of specimens with the loading axis parallel to rolling direction were machined; fatigue crack propagation direction was parallel to transverse direction (L-T specimen), and short transverse direction (L-S specimen). Fatigue crack propagation tests were performed with center cracked plate tension specimen with stress ratio R=0.1 and frequency of 10Hz at room temperature. Crack propagation rate of L-T specimen was approximately 10 times higher than that of L-S specimen. SEM-EBSD observations revealed that the c-axis direction is unfavorable for the fatigue crack propagation in textured polycrystalline magnesium alloy.
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Authors: Shigeki Morita, Nobuyoshi Ohno, Fujio Tamai, Yuji Kawakami
Abstract: This paper describes the fatigue crack propagation behavior of extruded AZ31B magnesium alloys (average grain size: approximately 15 and 119 μm, respectively). Fatigue crack propagation tests were performed on center cracked tension (CCT) specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axis was parallel to the extrusion direction; crack face was perpendicular to basal plane of each grain. The crack growth rate (da/dN) of the coarse-grained specimen was approximately 5 times higher than that of the fine-grained specimen. Fracture surfaces of the fine-grained and coarse-grained specimens showed various directional steps independent of macroscopic crack growth direction.
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Authors: Yuji Kawakami, Fujio Tamai, Takashi Enjoji, Noboru Shikatani, Tatsuya Misawa, Masaaki Otsu, Kazuki Takashima
Abstract: Pulsed Current Sintering (PCS) process possesses some problems that need to be resolved. We, therefore aims at understanding phenomena of PCS process by presenting some basic data on in situ sintering behavior of PCS. In order to observe in situ sintering behavior of PCS, a special graphite mold equipped with thermo couple and electrodes were designed to measure the temperature, electric current and voltage inside the powder during PCS process. We apply three types of raw materials, especially for ZnO (thermoelectric material) as semiconductor, Al2O3 as non-conductor and WC (Tungsten Carbide) as good conductor. The observation succeeded and some valuable data were obtained. The results showed that the temperature in the Al2O3 powder is 100 K higher than the graphite mold at the temperature of 1473K and ZnO powder is 150 K higher than the graphite mold at the temperature of 1373K. The electric current and voltage were measured for each powder during PCS process. In addition, their electric resistance properties were calculated. The electric resistance showed different behavior.
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Authors: Yuji Kawakami, Fujio Tamai, Takashi Enjoji, Kazuki Takashima, Masaaki Otsu
Abstract: In this investigation, functionally graded technique has been applied to prepare tungsten Carbide alloys (WC-Co alloys)-austenitic stainless steel to improve corrosion and wear resistance and reduce their fabrication costs. Two types of specimens were prepared by Pulsed Current Sintering (PCS) method. One is fabricated from WC alloy (WC-20 wt% Co, WC-20 wt% Fe-46Ni) powder and SUS316L stainless steel powder, and is designated “powder / powder process“. The other is fabricated from pre-sintered WC alloy and SUS316L plate, and is designated “bulk / bulk process”. Both types of specimens have four functionally graded layers. Mechanical properties of these specimens were evaluated by three points bending test, and microscopy observations were made to their microstructures. Although the proper bending strength of bulk / bulk process specimen was obtained, the optimum sintering condition is sensitive to the sintering temperature and function graded layers.
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Authors: Fujio Tamai, Yuji Kawakami
Abstract: This work is concerned with the proposition of high contrast multi-layer structure for the fabrication of reflecting surface in the visible range. The multi-layer SiO2/Ta2O5 system has superior heat resistance and reflecting properties. Ta2O5 and SiO2 thin films were prepared by RF plasma sputtering in Ar-O2 mixed gas using Ta2O5 and SiO2 target respectively. Secondary ion mass spectroscopy and SEM observation were performed in order to investigate the multi-layer structure. The XRD and XPS analyses were also performed in order to investigate the crystal structure and the chemical state of the films. The optical constants of Ta2O5 single film were investigated by the ellipsometric analysis. Using the Ta2O5 film doped Bi2O3 for a reflecting multi-layer structure improved the reflecting properties and the heat resistance of the multi-layer system because of the high refractive index of the doped film.
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Authors: Yuji Kawakami, Fujio Tamai, Takashi Enjoji, Kazuki Takashima, Masaaki Otsu
Abstract: Austenitic stainless steel has been used as a corrosion resistance material in tough corrosive circumstance. However, austenitic stainless steel has poor wear resistance property due to its low hardness. Tungsten Carbide alloys (WC) are widely used as tooling materials, because of their high hardness and excellent wear resistance property. In this investigation, we apply powder composite process to obtain hard layer of Stainless steel. The composite material was fabricated from planetary ball milled WC powder and SUS316L stainless steel powder and sintered by Pulsed Current Sintering (PCS) method. We also added TiC powder as a hard particle in WC layer. Evaluations of wear properties were performed by pin-on-disk wear testing machine, and a remarkable improvement in wear resistance property was obtained. The weight loss rate of the composite was 1/10 of SUS316L. In addition, it was found that TiC hard particle addition has a positive effect on the wear resistance property. EPMA investigation showed good dispersion of WC hard phase and TiC hard particle with SUS316L matrix.
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