Effect of arc distance on back appearance of root welding without backing plate by PMAG-TIG twin-arc welding

A novel pulsed metal active gas-tungsten inert gas (PMAG-TIG) twin-arc tandem welding process is applied to make stable back beads in the first layer weld during one-side multilayer welding without backing plate. The effect of different arc distances on the arc plasma behavior, molten pool flow pattern, and back appearance of root welding during the twin-arc tandem welding was studied. Results indicated that when arc distance is about 22 mm during the welding process, about 30% heat energy of the arc heat is utilized to melt the root face of base metal, and the rest heat energy acts on the molten pool. In this condition, the temperature distribution of the molten pool is appropriate, the flow of the front edge of the molten pool is suitable, the back appearance is continuous, stable, and uniform, and the back reinforcement and weld width is moderate. When arc distance is about 17 mm, more arc heat energy is utilized to melt the root face, the fluidity of the front edge of the molten pool is stronger, which leading to the back reinforcement is relatively larger. While arc distance is about 27 mm, less arc heat acts on the root face, the fluidity of the front edge of the weld pool is weak, resulting that in the back reinforcement is relatively small. This twin-arc tandem welding process is a new welding process giving high quality and efficiency in root welding for medium or thick plates.