Elucidation of alkali element's role in optimizing metal transfer behavior in rutile-type flux-cored arc welding

This study investigates the effects of alkali elements on metal transfer behavior in rutile flux-cored arc welding. Four types of prototype flux-cored wires with different sodium contents in the flux were fabricated. By using these wires, the influence mechanism of sodium on the metal transfer behavior was elucidated through shadowgraph measurements of the metal transfer behavior as well as spectroscopic and color image observations of the arc characteristics. It was found that the metal transfer between 190 A and 310 A was in the projected transfer mode and could be further classified into two sub-modes (type A and type B) based on the droplet formation process. A larger droplet was formed on the side of flux column in type A, while a smaller one was formed in the center covering the flux in type B. The metal transfer frequency became larger in the latter case for the same wire feeding speed. Type A tended to dominate in the lower current and lower sodium content conditions, while type B dominated in the opposite conditions. The dominant sub-mode was determined to depend on the Lorentz force acting on the droplet. At medium currents (250 A and 280 A), both sub-modes appeared in similar proportions. The maximum metal transfer frequency occurred at a particular sodium content. When the sodium content was smaller or larger, type A or type B became dominant, respectively. The sodium content at which the maximum frequency occurred decreased when the current increased. In type A, the iron plasma was widely distributed on the droplet side of the flux, while the sodium plasma was concentrated near the flux on the opposite side, so both were separated. In contrast, in type B, the sodium plasma was concentrated around the flux at the center and the iron plasma was widely distributed in the arc column, so both overlapped around the center. Sodium has a low boiling point and low ionization potential. In type A, the sodium vapor greatly increased the electrical conductivity of plasma around the flux column, so part of the current flowed from the wire through the sodium plasma to the weld pool. Accordingly, the current flowing through the bottom of the droplet to the arc decreased, leading to a lower arc pressure and recoil pressure under the droplet, and causing the metal transfer frequency to increase with sodium content. On the other hand, in type B, the sodium vaporization increased around the center, increasing the recoil pressure. In addition, the current density at the bottom of the droplet increased due to the current concentration in the arc, causing the arc pressure to rise. Therefore, the metal transfer frequency tended to decrease with sodium content. Due to the balance of these factors, the metal transfer frequency has a maximum at a particular sodium content.