Numerical Analysis of Metal Transfer Process in Plasma MIG Welding

Abstract

In plasma MIG welding, inert gas introduced from the torch nozzle is ionized in the up-stream region of the MIG arc, which is termed “plasma”. This study aims to clarify the effect of the plasma on the metal-transfer process in the plasma MIG welding through numerical analysis. As a result, the plasma with a temperature of approximately 10,000 K was found to be formed around the wire tip. The MIG arc temperature around the wire tip was 11,000 K at the maximum, which was lower than that of the conventional MIG welding by approximately 1000 K. This difference was caused by the decreased current density around the wire tip due to the influence of the plasma. The droplet temperature was also decreased by 400 K due to this lower current density. The amount of the metal vapor evaporated from the droplet was decreased compared to that of the conventional MIG welding due to the lower droplet temperature. This might lead to a decrease in fume formation generally known in the plasma MIG welding. In the conventional MIG welding, the arc attachment was concentrated around the wire tip, leading to a higher current density. However, in the plasma MIG welding, the plasma transported to the surrounding of the wire tip increases the electric conductivity in that region, due to the influence of the metal vapor mixture. This leads to the dispersion of the arc attachment toward the wire root. Consequently, the current density in the plasma MIG welding was found to decrease compared with that of the conventional MIG welding. The lower current density in plasma MIG welding decreases the Lorenz force acting on the wire neck, thus delaying droplet detachment to make the droplet diameter larger and the metal transfer frequency smaller. The latter was about 20 % of that in the conventional MIG welding.