Effects of alkaline elements on the metal transfer behavior in metal cored arc welding

Abstract

The aim of this study was to clarify the effects of adding a small amount of an alkaline element to the wire in metal cored arc welding (MCAW) on the metal transfer process. Sodium was selected as the alkaline element, and a standard wire without sodium (wire 1) and three wires with 0.028, 0.056, and 0.084 mass% sodium (wire 2, 3, and 4) were prototyped. They were observed using a high-speed video camera equipped with laser illumination, which clarified that the droplet transfer frequency tended to increase proportionally with the amount of sodium added. Subsequently, the line spectra of the iron atom (particularly the Fe I 537.1 nm line) and sodium atom (particularly the Na I 589.0 nm line) were primarily observed using a bandpass filter. Iron vapor evaporated from the droplet bottom, while sodium vapor mainly evaporated from the molten wire tip or the neck between the wire and the droplet because of its low boiling point. Because the sodium atom has a low ionization energy, we considered that a new current path of sodium plasma was directly formed from the tip or neck of the molten wire through the arc plasma to the molten pool, bypassing the inside of the droplet. The formation of this new current path was considered to reduce the current of the iron plasma flowing from the droplet bottom reducing the arc pressure and further enhance the electromagnetic force acting on the neck to promote droplet detachment, thereby increasing the metal transfer frequency.