Papers by Author: Toshiya Shibayanagi

Abstract: Superplasticity in an AZ80 magnesium alloy subjected to friction stir processing (FSP) has been investigated. FSP was carried out at two traveling speeds of 150mm/min and 300mm/min for grain refinement. Optical microscopy on cross section to processing direction revealed obvious differences in size and feature between the stir zones at the two traveling speeds. The hardness of FSPed sample at the room temperature was about 30HV higher than that of as-received one. The maximum stress of the FSPed sample was reduced remarkably at lower strain rates compared with those of the as-received one at 573K and 673K. On the other hand, the elongation to failure of the FSPed sample showed ten to thirteen times larger than that of the as-received one at 573K and low strain rates. Further surface morphology near the fracture tip was observed by scanning electron microscopy to discuss deformation mechanism at high temperatures.

Abstract: Spot heating system utilizing a fiber-laser has been developed in order to control recrystallization, grain growth process and texture development. The newly developed system enables the laser beam to be focused onto the surface of specimen with a minimum diameter of around 10 m. The maximum power of laser in the present system is 100W. Specimens of cold rolled pure aluminum plates of 1mm thick are used for local recrystallization treatment by the spot heating. After laser-spot heating, preferentially recrystallized grains appeared in the irradiated and heat affected region with a diameter depending on the laser power density. The present system enables spot heating on any critical points of the specimen surface. Some laser-heated specimen show locally recrystallized regions occupied by almost single grain having similar crystallographic orientation as that of cold rolled matrix. But the orientation of recrystallized grain is not always the same as that of the matrix. Selective spot heating on grains having peculiar orientation would bring about a possibility of texture control that has not been achieved by means of conventional materials processing.

Abstract: The present study tried to control grain structure in polycrystalline aluminum or titanium by means of a laser spot heating method as well as by a computer simulation technique. Monte Carlo simulation of spot heating was performed utilizing two-dimensional grain structure model composed of 200 x 200 sites with hexagonal cells. Grain growth proceeded preferentially in the higher temperature region and resulted in a large grain surrounded by small grains. This large grain tended to keep on growing during homogeneous heating after the spot heating, suggesting that the spot heating technique can fabricate a peculiar grain structure such as different grain size distribution and texture that are not realized by utilizing conventional uniform heating process. Laser spot heating utilizing a fiber-laser system realized heating of small area such as 30micorn diameter to bring about peculiar grain size distribution as calculated by the computer simulation.

Abstract: Grain refinement and high temperature deformation in two kinds of magnesium alloys subjected to friction stir processing (FSP) have been investigated. One was a rolled sheet of LA141Mg and another was a cast plate of AZ91Mg. FSP was developed by adapting the concepts of friction stir welding to obtain a fine grain size in a stirred zone. Grain refinement was achieved by FSP to give fine grain sizes of 11.4μm and 8.4μm for LA141 and AZ91 alloys, respectively. For LA141 alloy, the maximum stress of the FSPed sample was higher than that of the as-received one in the range of 300K to 523K while the elongation to failure of the former was considerably smaller than that of the latter. On the other hand, the elongation for the FSPed sample of AZ91Mg showed three times larger elongation with a lower maximum stress than the as-received cast one at 523K and 2.8×10-3s-1. Further difference in high temperature deformation for both magnesium alloys was discussed based on microstructural change and stress-strain curves.

Abstract: The present paper deals with the control of microstructure of friction stir processed aluminum alloys focusing on grain refinement, thermal stability at elevated temperature and texture development in some aluminum alloys such as 5083, 6061 and 7075 commercial aluminum alloys. 3mm thickness plates of 5083, 6061 and 7075 Al alloys were friction stir processed/welded with several rotation speeds and travelling speeds. Optical microscopy revealed the grain refinement in the stirred zone of each alloy and the average grain size decreased with decreasing rotation speed under various travelling speeds. Annealing of the joints brought about abnormal grain growth at temperatures higher than 773K for 5083 alloy. Critical temperature of the abnormal grain growth tended to decrease as the rotation speed decreased for the fixed travelling speed. Dissimilar joining of 5083 Al alloy to 6061 Al alloy also showed abnormal grain growth when annealed at 773K. A peculiar texture development of 7075 Al joint showing (111)//ND-oriented grains existing throughout the nugget was revealed by EBSP analysis.

Abstract: The reaction between the zinc plate (ZP) and the IF steel with near surface ultra fine grains (NSUFG) structure with grain size of about 89 nm was studied in temperature range of 473K to 623K in order to elucidate the temperature dependence of the reactions and its mechanism, by comparison with the reactions of ZP to coarse grains (CG) sheet, superficial cold rolled CG sheet (CG+R) and superficial cold rolled NSUFG sheet (NSUFG+R). It was found that this NSUFG structure considerably affected reactions between IF steel and ZP. There was almost no effect of superficial cold rolling on their reactions, but the NSUFG structure dramatically enhanced the reactions. The incubation times for appearance of the reaction layer and its layer width of ZP /(NSUFG or NSUFG+R) reactions are shorter and thicker than those of the ZP/(CG or CG+R) ones. The activation energy for reaction was 107kJ/mol, which indicates that the volume diffusion in zinc side and the grain boundary diffusion in the iron side play an important role in the reaction. The layer growth up to the layer thickness less than about 10µm was controlled mainly by the interface reaction and it over about 10µm mainly by the diffusion mass transfer.

Abstract: The microstructure of the solid-state diffusion bonded interfaces of silicon carbide (SiC) and titanium aluminide (TiAl) were investigated. A 100-µm-thick Ti-48at%Al foil was inserted between two SiC specimens and then heat-treated in vacuum. The interfacial microstructure has been analyzed by scanning electron microscopy, electron probe microanalysis and X-ray diffractometry. Four layers of reaction products are formed at the interface by diffusion bonding: a layer of TiC adjacent to SiC followed by a diphase layer of TiC+Ti2AlC, a layer of Ti5Si3CX containing Ti2AlC particles and a layer of TiAl2. However, the TiAl2 layer is formed during cooling. The actual phase sequence at the bonding temperatures of 1573 K and 1673 K are described as SiC/TiC/(TiC+ Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti1-XAl1+X/TiAl and SiC/TiC/(TiC+Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti5Al11 /Ti1-XAl1+X/TiAl, respectively. The phase sequences are successfully expressed on the basis of the Ti-Al-Si-C quaternary chemical potential diagram.

Abstract: The present paper describes microstructure and mechanical properties of a grain refined Al alloy foils fabricated by RF magnetron sputtering method utilizing a commercial 7475Al alloy as a sputtering target. Microstructure of deposited foils was evaluated by means of SEM, TEM, SEM-EPMA and X-ray diffraction methods. Mechanical properties were analyzed by the Vickers hardness test and by tensile test at various temperatures. Columnar grains having a mixed grain size ranging from some hundreds microns to 1 micron on the planar section was obtained, and the foils were super-saturated solid solution with FCC structure. The foils contained less Mg content than the sputtering target. Superplastic deformation was observed for the foils at elevated temperature and the maximum elongation of 370% was obtained when deformed at 863K with the initial strain rate of 1.4 x 10-1s-1.

Abstract: In the present work, the effects of reinforcement type, reinforcement surface treatment, alloying additions, remelting time and remelting number upon the chemical and property degradations of discontinuously reinforced Al matrix composites, are investigated by means of Differential Scanning calorimeter (DSC), Transmission Electron Microscopy (TEM) and thermodynamic calculations. The results show that the SiCp/Pure Al and SiCp/wrought Al alloy composites could not be recycled via remelting, nevertheless SiCp/foundry Al alloy and Al2O3p/Al composites could be recycled via remelting. The remelt-recycling mechanism of discontinuously reinforced Al matrix composites is also discussed in detail in the present work.