It is well known that the solidification cracking susceptibility of stainless steel changes depending on the amount of ferrite in the weld metal at room temperature. However, the cause and effect of the amount of ferrite at room temperature and solidification cracking have not always been clarified. When considering the temperature range in which solidification cracks occur, it must be necessary to focus on changes in microstructure and behavior during solidification. In this study, the solidification cracking susceptibility of stainless steel was investigated according to the above reasons, and its susceptibility was summarized using the solidification mode, in other words, an amount of delta ferrite during solidification. A quenched microstructure at the solid-liquid coexistence region was obtained for each stainless steel, and the solidification mode and the amount of the delta ferrite during solidification were investigated. The solidification cracking susceptibility was evaluated using the Trans-Varestraint test, and the effect of solidification mode on brittle temperature range (BTR) was clarified. In addition, a theoretical study on BTR was conducted, the effect of solidification mode on solidification segregation at each stainless steel was considered. It was found that the solidification segregation behavior changes depending on the solidification mode. Especially, the effect of solidification and segregation of impurity elements such as sulfur and phosphorus were large in the austenite, in the case of ferrite, the carbon was affected to BTR more than impurity element compared with the austenite. Thus, it has been suggested that the BTR changes due to the solidification segregation of solute elements according to each solidification mode.

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Friction stir spot welding (FSSW) is one of the solid state welding methods and dissimilar material joining by FSSW has been investigated. “Scrubbing refill FSSW (Sc-RFSSW)” has developed as a new dissimilar material joining method, and it was shown that Sc-RFSSW provides higher joint strength than conventional FSSW methods in joining the aluminum alloy and non-coated steel. In this study, the microscale testing was performed to evaluate the dissimilar material interfacial strength of Sc-RFSSW joints. The relationship between the interfacial strength distribution and the macroscopic strength of the Sc-RFSSW joint was analyzed. In addition, the microstructure of the dissimilar material interface was observed, and the relationship between the microstructure and the interface strength was shown.

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In this study, flow observation in friction stir welding using a transparent fluid followed by PIV analysis were conducted to discuss and verify the relationship between the thread shape of the probe and the layered microstructure. Subsequently, friction stir welding of 6061 aluminum alloy were conducted to confirm the validity of the verification. In the flow observation, the tracer swirled around the probe was observed to be discharged from the probe tip and formed a layered deposit behind the probe. This layered deposition showed different periodicity depending on the tool screw geometry. The PIV analysis showed that the tracer was discharged from the threaded part at the probe tip. Therefore, it is considered that the discharge from the probe tip contributes to the layered deposition. In the joining experiments, the spacing of the layered microstructure appeared on the cross-section altered depending on the thread shape of the tool. Therefore, it is considered that the layered microstructure is strongly related to the geometry of thread of welding tool.

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A simple repair method of fatigue cracks using stop-holes reinforced with slope-type wedge members, which was previously proposed by the author, has been examined for the case of reinitiated cracks. By using this method, the stress intensity factor range around a reinitiated crack tip is expected to be reduced by the wedge load effect of the wedge members. The chief advantages of this method are that the repair work can be easily performed from only one side of a cracked structure, and that the wedge member can be set so adaptive as to maintain the wedge load automatically and effectively as the reinitiated crack grows. In addition to the conventional adaptive wedge member using a pulley and a wire-type displacement meter, a compact adaptive wedge member with a reverse slope and using elastic bands has been newly proposed and comparatively tested with the conventional one. Fatigue tests both under constant and variable amplitude loads were performed on a steel plate specimen with a drill hole and a notch, and validity of the above repair method was experimentally examined using both of conventional and compact adaptive wedge members. As a result of the fatigue tests under constant amplitude loads, it was found that the strain range on the specimen side is reduced to 40.7% and the fatigue life is prolonged by 26.1 times by application of the compact adaptive wedge member as compared with the case of the conventional stop-hole. Under variable amplitude loads, both of the conventional and the compact adaptive wedge members reduced the strain range on the specimen side and prolonged the fatigue life, however, the efficacy of the former was even stronger (30.6～27.8 times in fatigue life as compared with the case of the conventional stop-hole).

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The mechanism of stable weld bead formation in fiber laser-MAG arc hybrid welding at high speed was investigated by observation of welding phenomena. Arc shape and droplet transfer were observed for arc welding and laser arc hybrid welding at welding speeds of 3m/min and 6m/min. The interaction between arc column and laser irradiation point, which some previous reports mentioned, was not observed at both welding speeds of 3m/min and 6m/min. In arc welding at welding speed of 6m/min, droplets transferred to the unmelted base metal in front of the molten pool and scattered as spatter. On the other hand, in laser-arc hybrid welding at welding speed of 6m/min, droplets transferred to the molten pool caused by the laser beam and spatter generation was suppressed. The formation of a molten pool caused by the laser beam in front of the molten pool caused by arc welding is effective in reducing spatter and stabilizing beads.

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