Suppression of weld slag formation has been studied to improve corrosion resistance of ultra-high strength steel sheet for automotive chassis parts. Although Metal Inert Gas welding can prevent slag formation, its unstable arc phenomena on carbon steel induces weld bead shape defects. This study aims to develop a highly stable MIG welding process for hot-rolled UHSS which forms sound bead shape with decreasing slag. Detailed observation of metal transfer behavior revealed that the combination of higher pulse peak current and lower pulse base current than conventional MIG welding realize cyclic short-circuit transfer and it can lead to proper bead shape. Low base current leads to a stable growth of metal droplet and the metal droplet are pushed down by accelerated plasma flow in peak period. The MIG welding utilizing high current pulse offered sound weld bead shape and slag-less simultaneously. Through these findings, the combination of intensive pulse current and extremely low base current is proposed as a new pulsed current pattern for hot-rolled UHSS.

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A simple technique of restraining fatigue crack growth utilizing wedge effects of electrodeposition products was newly proposed and experimentally tested. Crack growth restraining sheets, which consist of a high water content gel and an anode layer, were manufactured by way of trial and applied to notched plate specimens of a steel and an aluminium alloy, and the efficacy of the sheets was evaluated by fatigue tests. In the fatigue tests, the crack growth restraining sheet was renewed at proper timing to maximize its performance. In order to find a most suitable electrochemical environment for the crack growth restraint by wedge effects of electrodeposition products on the crack surfaces, water components absorbed in the gel and effects of an anode layer as promoting factors of the electrodepositon were comparatively examined. For the steel specimens, it was found that all the proposed crack growth restraining sheets were effective and extend the failure life in the range of 1.6～6.0 times as compared with the base metal specimen, and the efficacy of the gel sheet impregnated with synthetic sea water covered by an anode layer of zinc-leaf tape was maximum. Also for the aluminium alloy specimens, it was found that almost all the proposed crack growth restraining sheets were effective and extend the failure life in the range of 3.6～6.8 times as compared with the base metal specimen, and the efficacy of the gel sheet impregnated with synthetic sea water covered by an anode layer of aluminium-leaf tape connected to an external DC power supply was maximum. From fracture mechanical analyses with FEA, it was found that the crack restraining effect of the gel sheet greatly depends on the water gel content. In situ observations around the cracks and microscopic observations of the fracture surfaces were also performed for a comparative study.

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Toward a sustainable society, there is a demand for multi-material structures that utilize aluminum, which has high electrical conductivity, is lightweight, and is easy to recycle. To realize the manufacturing of multi-material structures, the challenges are to develop a low-cost and versatile aluminum dissimilar material joining technology and a structure that facilitates a circular economy. With the aim of reducing costs and improving versatility, we have established a high-speed eutectic bonding method that generates and expels eutectic between aluminum and copper in a short time of less than one second by induced current flow in air, and are developing a joint bonding technology with the bonding strength to fracture the base material. In this study, in developing a joining judgment system for dissimilar metal bonding method using high-speed eutectic reaction, we developed a thermal analysis method that reduces the discrepancy between the temperature distribution obtained by energized experiment and that obtained by thermal analysis. This thermal analysis method consists of a process of deriving interface heat with pressure as a variable including energizing environmental factors, using energization heat balance experiment model, and a process of incorporating the obtained interface heat and uniform heating zone into general-purpose heat conduction analysis software to perform a thermal analysis of dissimilar metal bonding of any shape. The accuracy of thermal analysis was compared based on temperature distribution of experimental values and analyzed values, using bonding conditions of multi-material structure that obtains base metal fracture strength through high-speed eutectic reaction. Error rate was about 5% within 4 mm from the interface, which is a reasonable accuracy for determining conditions for bonding by eutectic reaction, and prospects for developing bonding judgment system were obtained.

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