Diffusion bonding of B-doped Ni₃Al polycrystals with various grain sizes was performed under various conditions with bonding pressures of 29-147MPa, bonding temperatures of 1173-1323K and bonding times of 0.12-3.6ks. The influences of the bonding conditions on the bond strength and the configurations of the fracture surfaces were investigated focusing on the microstructure near the bonding interface. The bond strength increased with increasing bonding pressure and increasing bonding temperature. On the other hand, the bond strength was almost independent of the bonding time. Moreover, the bond strength was higher as the grain size was smaller. The fracture surfaces of the specimen broken at the bonding interface were composed of two kinds of areas with different configurations. One had rugged configuration with a height of tens micrometers, and another had flat and smooth configuration. The area fraction of the rugged area tended to be larger in the fracture surfaces of the specimens exhibiting higher bond strength. The formation of the rugged fracture surface was attributed to the migration of the bonding interface accompanying the dynamic recrystallization in the bonding process and the fracture along the migrated interface by the observation of microstructure near the interface. The relationship between the bond strength and the distribution of the rugged fracture surface indicated that the increase in the bond strength was mainly due to the relief of the stress concentration at the triple junctions on the bonding interface by the change in the configuration of bonding interface.

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Liquid fuels usually evaporate before they burn in surrounding oxidizer. It is important to understand mixture formation processes between vaporized fuel and oxidizer. Laser induced exciplex fluorescence (Exciplex) technique has been used to observe mixture formation processes in the inter-droplet space of fuel spray flow in spite of various inconvenient defects. In the present study, we have succeeded in developing a similar but new method, the laser induced Pyrene-excimer fluorescence (Excimer) technique. It is based on spectrally separated two-color fluorescent emissions, Monomer fluorescence and Excimer fluorescence, by using only one molecule added into the fuel. Characteristics of Excimer technique are discussed by comparing with those of Exciplex technique.

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A magnesium alloy is said to be an ecological material with high ability of recycling and lightweight property. Especially, magnesium alloys are in great demand on account of outstanding material property as a structural material. Under these circumstances, research and development of welding process to join magnesium alloy plates are of great significance for wide industrial application of magnesium. In order to use it as a structure material, the welding technology is very important. TIG arc welding process is the most ordinary process to weld magnesium alloy plates. However, since the heat source by the arc welding process affects the magnesium alloy plates, HAZ of welded joint becomes wide and large distortion often occurs. On the other hand, a laser welding process that has small diameter of heat source seems to be one of the possible means to weld magnesium alloy in view of the qualitative improvement. However, the low boiling point of magnesium generates some weld defects, including porosity and solidification cracking. Furthermore, precise edge preparation is very important in butt-welding by the laser welding process, due to the small laser beam diameter. Laser/arc hybrid welding process that combines the laser beam and the arc is an effective welding process in which these two heat sources influence and assist each other. Using the hybrid welding, a synegistic effect is achievable and the disadvantages of the respective processes can be compensated. In this study, YAG laser/TIG arc hybrid welding of thin magnesium alloy (AZ31B) sheets was investigated. First of all, the effect of the irradiation point and the focal position of laser beam on the quality of a weld were discussed in hybrid welding. Then, it was confirmed that a sound weld bead with sufficient penetration is obtained using appropriate welding conditions. Furthermore, it was made clear that the heat absorption efficiency is improved with the hybrid welding process. Finally, the tensile tests of welded joints were performed, and it was confirmed that they have sufficient mechanical properties. As a result of this study, it is confirmed that, if the appropriate welding conditions are selected, sound welded joints of AZ31B magnesium alloy are obtainable by the YAG laser/TIG arc hybrid welding process.

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