Effect of Weld-Pass Morphology and Molten-Pool Flow on Lack of Fusion Formed in the Fully Automatic Welded Joint at 6 o’Clock Direction

Compared with traditional shielded metal arc welding and semi-automatic welding, fully automatic welding has the advantages of fast welding speed, high welding quality as well as a high qualification rate on the premise of ensuring the comprehensive performance of the girth weld joint. Meanwhile, the proportion of welding defects also changed from porosity and slag inclusion to lack of fusion. Therefore, from the view of the macroscopic and dynamic evolution behavior, the paper aims to reveal the influence of the coupling of weld-pass morphology and molten-pool flow on the formation of lack of fusion using macroscopic metallographic observation and high-speed photography. The results indicate that the weld morphology is prone to convex reinforcement and larger penetration depth in the 6 o’clock direction of the pipeline girth weld under the reference parameters. The excessive reinforcement of the pass to be welded is one of the sufficient conditions for the formation of a lack of side fusion in the 6 o’clock direction. Excessive reinforcement could lead to welding arc heat mainly used to melt the raised weld metal to be welded, which results in insufficient heat flow to both sides of the molten pool metal. Furthermore, due to the larger curvature radius at the groove and the increase of the surface tension of the molten pool with lower relative temperature, the flow resistance of the molten pool increases and the fluidity decreases.