Effects of welding velocity on metal transfer mode and weld morphology in underwater flux-cored wire welding

Metal transfer process in underwater flux-cored welding are investigated by X-ray transmission method for welding velocity from 0.5 mm/s to 9.5 mm/s. The metal transfer mode of underwater flux-cored wire welding is the mixed transfer mode mainly composed of four fundamental transfer modes i.e. wide-angle globular repelled transfer mode, small-angle globular repelled transfer mode, surface tension transfer mode and short-circuit transfer mode. The category and proportion of fundamental modes constituting the mixed mode depend strongly on the welding velocity owing to the variation of force condition on droplet. With increasing welding velocity, the proportion of the wide-angle globular repelled transfer mode increases continually while that of the surface tension transfer mode and short-circuit transfer mode decreases, and the proportion of the small-angle globular repelled transfer increases firstly and then decreases gradually. The welding appearance and cross-sectional macrostructure of weld specimens at different welding velocities are also investigated. When the welding velocity increases from 0.5 mm/s to 3.5 mm/s, the weld width decreases from 20 mm to 12 mm and the weld penetration increases rapidly from 0.8 mm to 3.7 mm, completely different from in-air welding.