Development of advanced metal transfer model for the enhancement of gas metal arc welding process stability

Abstract

Welding is a dominant technology for determining the quality and productivity of manufacturing industries. Gas metal arc welding (GMAW) is one of the most widely applied welding process in which a continuous and consumable electrode are delivered to the workpiece. Even though it is a complex phenomenon which is influenced by welding current, arc voltage, shielding gas composition, electrode polarity and contact tip to workpiece distance, the better understanding of metal transfer process is still needed because it is closely related to the quality and stability of the welding process. Therefore, the qualitative growth of the domestic manufacturing industries requires the growth of the welding industry, and the better understanding of metal transfer is required to acquire the fundamental technology of the welding technique.

Representative analytical models, such as static force balance theory (SFBT) and the pinch instability theory (PIT), provided a physical understanding of the specific metal transfer mode and improved its performance by considering additional forces. However, existing models couldn’t consider both globular and spray transfer at once. In addition, the effects of $CO_2$ when Ar-$CO_2$ mixture is used as the shielding gas could not be considered in existing metal transfer models.

This thesis proposes the advanced metal transfer model. It is based on the attaching and detaching forces acting on the molten metal at the electrode tip, and both growth and detachment processes of the molten drop are modelled. In order to consider different behaviors of the metal transfer according to the magnitude of welding current, the equation for calculating the magnitude of transition current is derived and applied as the bifurcation point between globular and spray transfer. The tapering effect at the side of electrode and the corresponding forces are considered to model the metal transfer when the welding current is above the transition current. In addition, an equation, which depends on the $CO_2$ proportion, to estimate the arc-covered region in the molten drop is proposed. The dynamic detachment process of a molten droplet during the metal transfer process is described as the formation and breakup of the necking region between the electrode tip and the molten droplet, and the behavior of a molten droplet is modelled as second order differential system. The proposed model is applied for both direct current and pulsed current GMAW, and the simulation results are compared with experimental results