Papers by Author: Atsushi Yamamoto

Abstract: A pure iron tape with cube orientation was fabricated by cold rolling and annealing. The orientation characteristics of the pure iron tape were evaluated using electron back-scattering diffraction (EBSD) analysis. The secondary recrystallized grains with cube orientation was formed on the tape surface for the pure iron tape. The coarse grains with a grain size of ca. 1mm were observed on the tape surface. The areal fraction of cube orientations with an angular deviation ≤ 20 ̊ amounts to ca. 81%.

Abstract: The fracture behavior and adhesion strength of CrN/Cr and CrN films on SKD61 substrates were investigated by a three-point bending test. The SKD61 substrates were coated with a film with multilayer of CrN/Cr and a single film of CrN by a physical vapor deposition (PVD). The fracture behaviors of CrN/Cr and CrN films at three-point bending test were observed by scanning electron microscopy (SEM). Adhesions of the CrN/Cr/SKD61 and the CrN/SKD61 were also evaluated using a scratch tester. The scratch test resulted in no great difference in the adhesion strength between the CrN/Cr/SKD61 and the CrN/SKD61. On the other hand, the three-point bending test showed a high adhesion of the CrN/Cr with the intermediate layer of Cr film compared to the CrN on the SKD61.

Abstract: The formability of the magnesium-lithium alloy was examined by multi-stage deep drawing. Long cups of magnesium alloy were formed at ambient temperatures by multi-stage deep drawing processes. In multi-stage deep drawing, the magnesium-lithium alloy sheets were employed and a flat sheet blank is formed into a cylindrical by a punch. Various cups were drawn by exchanging the punch and ringed die. The die was flat in the first stage, and was taper without a blankholder in the subsequent stages. The effects of the ratio of blank diameter to punch diameter and blank thickness on the deep drawability were examined. It was confirmed that the Mg-Li alloy long-cups were successfully formed by a multi-stage deep drawing operation in cold.

Abstract: Resistance microwelding of fine crossed wires is of increasing industrial importance for medical devices, but the understanding of the process is very limited. A study has been carried on the feasibility of small-scale resistance welding of crossed TiNi fine wire. Welding mechanism is similar to that of small-scale resistance welding of crossed Ni wire, which is mainly four stages (wire cold deformation, surface melting, molten phase squeeze-out and, solid-state bonding). The optimum microstructure and welding parameters were proposed.

Abstract: Precipitation phenomena in an austenitic stainless steel, SUS316L cold-rolled with various reduction rates were studied by transmission electron microscopy and synchrotron radiation diffractmetry. After the aging at 573 K for 15000 h, two of precipitates were observed, which were identified as M7C3 and M23C6 by SR diffraction and electro diffraction measurements. The precipitates M7C3 were formed at both innergranular and grain boundary, while the precipitate M23C6 was formed at innergranular. The precipitation was promoted with increasing cold rolling reduction. Also segregation of phosphorous was detected along grain boundaries. Besides, the residual stresses were measured with side inclination method using a synchrotron radiation facility, SPring-8. The residual stresses were increased with increasing the cold rolling reduction rate.

Abstract: A low impurity magnesium alloy has bean prepared. Deformation behavior in cold-rolling and corrosion behavior of the alloy were compared with those of a commercial alloy. The specimens were cold-rolled at room temperature with reduction rates of 0~80 %. Transmission electron microscopic observations on the cold-rolled specimens were carried out. In the case of the low impurity magnesium alloy, recrystallization easily occurred. Analyses of microstructures in the deformed specimens were carried out by means of EBSP, and the recrystallization phenomena have been discussed. Another effect of lowering the impurities is to improve a corrosion resistance with changing the corrosion morphology.

Abstract: Resistance microwelding is an important joining process used in the fabrication of miniature instruments. The small-scale resistance spot weldability of Zr based glassy thin sheet alloys was investigated. Metallic glass sheets were successfully welded and amorphous structure was maintained in both weld nugget and heat affected zone. Joint strength that is evaluated by shear breaking force increased as the welding current increased, resulting from nugget growth. Fractured surface showed typical vein pattern.

Abstract: High strengths and impact values are required for carburizing steels used for automotive drive train parts. It is well known that the finer-grain microstructures are favorable for these properties. In this study, with a view to effective refining of grain size with controlling the alloying elements, influence of Mn and Cr content on prior austenite grain size after carburizing and repeated quenching was investigated on Ti-modified carburizing steels with ferrite and pearlite microstructure. It was found that the increase in Mn and Cr contents in steels were effective for the grain refining. These effects were attributed to fine ferrite and pearlite microstructure caused by the lowered Ar3 point due to higher Mn content, and retarding the grain growth of austenite by the fine dispersion of carbides in the carburized layer of specimen due to higher Cr content.

Abstract: A new technique has been developed, which enables one to randomize the basal plane texture formed in wrought magnesium alloys, AZ31B. Rolls with about 60 mm in diameter having wavy surface of 8 mm in wave length and 3 mm in peak to valley depth were used. The peak lines were perpendicular to the roll axis. The alloy sheet specimen of 50 x 50 mm with 2.5 mm in thickness was heated at temperatures ranging from 623 to 723 K, and then immediately wavy-roll formed. The specimen was re-heated at the same temperature, and then wavy-roll formed with rotating at 90 degree. Such a process was repeated at eight times, then finally flat-rolled with a slight reduction. Electron backscattered diffraction analyses showed that the basal plane texture was predominantly formed in the start material, while after the cross wavy roll-forming, accumulation of basal plane tended to be diffusely distributed.

Abstract: Crack formation by a rolling contact fatigue in a high carbon chromium bearing steel has been discussed. Newly developed method for preparing specimens including pre-existing voids enabled one to observe the early stage of fatigue crack formation. Many fatigue cracks were formed around the voids. The positions of crack formation and the direction of the cracks were consistent with those forecasted by finite element analysis. Fatigue crack formation was followed by formation of the WEAs.