New peening technology is proposed to improve the fatigue strength of welded joints. By using this technology, compressive residual stress is introduced at weld toe by the developed peening procedure which plastic deformation is only applied to the base material near the weld toe. In this study, improvement mechanism of fatigue strength of weld joints by hammer peening on base metal was clarified by FEA. It was clarified that increasing of stress at weld toe is controlled by depression formed near the weld toe. Not only compressive residual stress at weld toe but also decreasing stress concentration at weld toe by plastic deformation on base metal was indicated as the factor of improving fatigue strength of weld joints.
For gas metal arc welding, the effect of CO2 mixture in a shielding gas on a metal transfer process was investigated through the observation of the plasma characteristics and dynamic behavior at the droplet's growth-separation-transfer by the temperature measurement methods which were suitable respectively to the argon plasma region and the metal plasma region. At the present experimental conditions, the metal transfer process was a spray transfer type with 100%Ar shielding gas. On the other hand, with 85%Ar+15%CO2 shielding gas, the metal transfer process was a globular transfer type in which the arc length was shorter, the width was narrower, and the time interval of the droplet separation was longer. For both shielding gases, the metal plasma region near the arc central axis exhibited 6500-7500 K which was lower than the argon plasma region. With 85%Ar+15%CO2 shielding gas, when the metal droplet grew below the electrode wire, the region below the droplet has a high plasma temperature and a high concentration of iron vapor which surrounded the droplet. The region also exhibited a remarkably high electron number density. At the spray transfer process, the argon plasma region had the electron number density twice higher than the metal plasma region. Meanwhile, at the globular transfer process, the metal plasma region had a higher electron number density than the argon plasma region, which corresponded to a higher electrical conductivity near the arc axis. This means that the electric current goes through the arc axis easier than the spray transfer process. This condition increases the temperature below the droplet. The thermal expansion increases the force preventing the droplet from falling down. In consequence, the metal transfer takes the globular transfer type.