Reduction of Blowholes in High Speed Arc Welding of Hot-Dipping Galvanized Steel Sheet

Abstract

As for the blowholes to be generated in high speed fillet welding of hot-dipping galvanized and annealed steel sheet lap joints, the types of materials to cause them as well as their formation mechanism were examined. It seems to us that the formation of blowholes is not caused by the organic substances usually stuck to the surface of base materials but by the vaporization of zinc. As the result of chemical analysis of the gases sampled from the blowholes by breaking them, it was found that the main contents were hydrocarbon including methane gas. Hydrogen gas was also detected at the same time but it was only about a tenth of methane gas so that the possibility of hydrogen gas causing blowholes is low. The results of chemical analysis of zinc at both blowhole wall faces and root parts seem to indicate that zinc is supplied from heat affected zones to the origin of blowholes wherefrom it pushes up the top side of blowholes for their growth. From the above, the blowholes can be presumed to be caused by the vaporization of zinc. Intending to oxidize zinc as a means to control its vaporization, the effect of adding oxygen gas to the mixed gas of Ar-CO₂ was examined. As the result, it was found that the effective addition rate of oxygen gas for the reduction of blowholes was about 5% and especially effective in the case of joint gaps of 0.2 to 0.4 mm. However, too much addition of oxygen gas spoils this effect. Therefore, using the consumable electrodes of different melting points, the vaporization characteristics of zinc oxide were examined. By selecting the type of filler metal having the melting point of about 1, 400°C and using the mixed gas including oxygen gas, the effectiveness of reducing blowholes was clarified.