Elucidation of droplet detachment mechanism in metal-cored arc welding

It has been demonstrated experimentally that a globular transfer mode tends to occur in argon gas metal arc welding using a metal-cored wire (called metal-red arc welding (MCAW)) even under conditions where a spray transfer mode occurs when a solid wire is used. This study aimed to elucidate the droplet detachment mechanism in MCAW using numerical simulations. It was observed that in the initial stage of droplet growth, the argon arc attachment (which carried a large portion of the current) was located on the metal sheath surrounding the unmelted flux. Moreover, an eddy containing an upward flow at the wire axis was formed inside the droplet. When this eddy was formed, the droplet volume increased owing to wire melting. However, neck formation did not occur. This is considered to be because only a small portion of the current is conducted through the droplet, and the Lorentz force acting on the droplet is significantly weak. Thereafter, when the droplet grew to a certain extent and the arc attachment moved below the unmelted flux position, the Lorentz force acted strongly and almost uniformly on the droplet. Accordingly, this eddy disappeared, and the flow abruptly transitioned to a downward unidirectional high-speed flow. Consequently, the droplet began to extend downward to form a neck, which caused it to detach. For these reasons, the time required for droplet detachment increased, and the metal transfer frequency decreased compared with the case of the solid wire. This explains the observation that globular transfer persists in metal-cored wires at currents higher than those for which the transition to spray transfer has occurred for solid wires.