The Preliminary Analysis For Droplet Transfer And Weld-bead Undercutting In GMAW

GMAW (Gas Metal Arc Welding) is the most widely used welding process in manufacturing, and droplet transfer is one of the key factors which affect the welding quality and the stability of welding processes. Spray transfer is the most expected mode of droplet transfer in several kinds of transfer modes in GMAW. On the other hand, welding velocity must be increased in order to improve the production efficiency. The increasing of welding velocity will lead to some defects of weld-bead formation, then the undercut appears. In this thesis, these two questions mentioned above are studied. It is of great significance to ensure the weld quality and improve production efficiency in GMAW.Based on the analysis of forces exerted on the droplet, an equation is developed to calculate the critical transition current from droplet transfer to spray transfer. In the determinate condition, the calculated results match to the experimental ones.For correlating the welding process condition to the model for predicting the formation of undercutting, the analytical model is used to calculate the shape of weld pool. On some determinate hypothesis, the arc pressure is considered to analyze the shape of weld bead. The arc pressure distribution is simply in Gauss function. A hydrostatic model to explain the undercut mechanisms is proposed, then it is used to calculate the depth of undercut. The result indicates that higher welding velocity and more welding power will lead to higher undercut depth, which means undercut trend is more obvious. When the welding velocity and welding power are limited below a certain value, the undercut will not appear.