Papers by Author: Hiroyuki Kokawa

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Authors: Sen Yang, Hiroyuki Kokawa
Abstract: To improve intergranular corrosion resistance of 304 stainless steel, a novel method, laser surface remelting combined with annealing treatment was adopted, which resulted in a high population of low  CSL boundaries, especially, twin boundaries (3) on the surface of the processed specimens. The grain boundary character distribution and effect of laser processing parameters on it were investigated. The experimental results showed that the maximum frequency of the low  CSL boundaries could attain 88.6% under the optimal processing conditions. The high fraction of the low  CSL boundaries led to a high corrosion resistance to intergranular corrosion.
Authors: Yutaka S. Sato, M. Urata, Hiroyuki Kokawa, Kenichi Ikeda
Authors: Ryo Ishibashi, Toshiaki Horiuchi, J. Kuniya, M. Yamamoto, Sadahiro Tsurekawa, Hiroyuki Kokawa, T. Watanabe, Tetsuo Shoji
Abstract: The effect of grain boundary character distribution (GBCD) on intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels in high temperature water was verified experimentally. GBCD control using the strain annealing method increased the fraction of low- S coincidence site lattice (CSL) boundaries and the segmentalized network of random grain boundaries in austenitic stainless steels. The fractions of low- S CSL boundaries of GBCD controlled steels were 75–85%, while those of uncontrolled steels were 60–70%. Creviced bent beam tests were conducted at 561 K in pure water containing 8 ppm dissolved oxygen for stress corrosion cracking (SCC) evaluation. The tests revealed that GBCD control suppressed IGSCC initiation or propagation and that cracks were predominantly propagated along random grain boundaries. It is considered that induced lower- S CSL boundaries result in high resistance to IGSCC.
Authors: Sen Yang, Hiroyuki Kokawa, Zhan Jie Wang
Abstract: In order to modify grain boundary character distribution (GBCD) and to improve intergranualr corrosion (IGC) resistance of 304 stainless steel, laser surface remelting experiments were conducted on 304 stainless steel using a 2kW CW Nd: YAG laser, and the effects of laser processing parameters on GBCD and corrosion resistance were investigated in detail under the optimal annealing condition (1220K 28h). The experimental results showed that combination of laser surface remelting and the following annealing treatment could change the GBCD remarkably and improve the IGC resistance of 304 stainless steel. However, there are no obvious effects of laser processing parameters on the final depth of the processed zone, although the depth of the molten pool increases with the increase of the laser output power or the decrease of the scanning velocity, and the subsequent GBCD and corrosion resistance.
Authors: Yutaka S. Sato, A. Sasaki, A. Sugimoto, A. Honda, Hiroyuki Kokawa
Abstract: Mg alloy has a poor formability at room temperature because of lack of the active slip systems, but the grain refinement improves its ductility. Friction stir processing (FSP) can create homogeneous microstructure consisting of fine grains in Mg alloys, thus it would be expected that FSP enhances the formability of Mg alloys. In this study, multi-pass FSP was applied to Mg alloy AZ31B, and then formability of FSPed alloy was evaluated. Multi-pass FSP produced the fine recrystallized grains in Mg alloy. The stir zone exhibited larger fracture limit major strains than the base material under uniaxial tension and plane strain deformation, and these values increased with decreasing grain size. The stir zone having grain size of 2.9 μm showed the fracture limit major strains which are roughly as same as those of an annealed pure Al. The present study suggests that FSP is an effective method to enhance the formability of Mg alloys.
Authors: Sergey Mironov, Yutaka S. Sato, Hiroyuki Kokawa
Abstract: The Texture Developed in High-Temperature β Phase during Friction Stir Welding of Ti-6Al-4V Was Studied. It Was Demonstrated that 0002 and 1120 Pole Figures Calculated from α Phase May Be Employed for Interpretation of Material Flow in the β Phase. Together with Orientation Measurements in Retained β Phase, this Approach Was Shown to Be a Very Simple and Effective for the Texture Analysis. The β Texture Was Found to Be a Mixture of {hkl}<111>-Fiber Texture and D2(112)[111](Simple-Shear Texture Component.
Authors: Kunitaka Masaki, Yutaka S. Sato, Masakatsu Maeda, Hiroyuki Kokawa
Abstract: Friction stir welding (FSW) makes the stir zone with fine recrystallized grain structure. The recrystallized grains would be formed through dynamic recrystallization at high temperatures and high strain-rate. The present study experimentally simulated the dynamically recrystallized microstructure of a friction stir welded Al alloy 1050 produced at 600 rpm rotation and 100 mm/min travel speed, using combination of the plane-strain compression at various strain rates and the subsequent cooling along the cooling cycle of FSW. The equiaxed grain structures similar to the microstructure of the stir zone were produced at strain rates between 0.1 and 32 s-1; the grain size decreased with increasing strain rate. Strain rate during the FSW could be estimated to be about 1.8 s-1. The present study suggests that plane-strain compression test can simulate the recrystallized grain structure of the friction stir welds.
Authors: Yutaka S. Sato, Mitsuo Fujimoto, Natsumi Abe, Hiroyuki Kokawa
Abstract: Friction stir spot welding (FSSW), developed based on principle of friction stir welding, has been paid attention as a new solid-state spot welding process. Since FSSW can produce high-quality weld in Al alloys more easily than resistance spot welding, this process has been already used for construction of Al components in the automotive industries. Despite the large industrial interests in FSSW, fundamental knowledge on welding phenomena of this process has not been fully understood. In this study, FSSW phenomena, such as the consolidation mechanism, the microstructural evolution and the material flow, were examined in Al alloy 6061. This study clarified that the elliptical zone found in the vicinity of the pin hole on the cross section was characterized by the initially lapped surface of two sheets. Moreover, the following material flow was proposed; capture of the upper material with the threads on the pin surface, spiral flow along the tool rotation, and then release at the tip of the pin.
Authors: Seung Hwan C. Park, Satoshi Hirano, Shinya Imano, Yutaka S. Sato, Hiroyuki Kokawa, Toshihiro Omori, Kiyohito Ishida
Abstract: The authors have developed a new friction-stir welding (FSW) tool that enables to weld high-softening-temperature materials (HSTMs), such as steels, titanium and zirconium alloys. The new tool is made of a Co-based heat-resistant alloy strengthened by precipitating intermetallics, Co3(Al,W), with a L12 structure at high temperatures. The Co-based alloy tool exhibits yield strengths higher than 500 MPa at 1000 deg C, so it might have a great potential as a tool material for FSW of HSTMs. In this study, the feasibility of using the Co-based alloy tool with various HSTMs was examined. Changes in the tool shape during FSW and the weld appearances produced with the Co-based alloy tool will be briefly shown.
Authors: Hiroyuki Kokawa, Masahiko Shimada, Zhan Jie Wang, Yutaka S. Sato, M. Michiuchi
Abstract: Optimum parameters in the thermomechanical treatment during grain boundary engineering (GBE) were investigated for improvement of intergranular corrosion resistance of type 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice (CSL) boundaries indicated a maximum at the small pre-strain. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface.
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