Formative mechanism of the undercut defect in horizontal variable polarity plasma arc welding

Horizontal welding is important for heavy or huge welding structures.Keyhole mode variable polarity plasma arc welding of aluminum alloy plates with medium thickness was carried out in horizontal position.The characteristic of welding defects was introduced.Preliminary experiments indicated that the undercut defect could not be eliminated easily.The relationship between welding parameters and the undercut defect showed that this defect could be lessened by using higher heat input.The fluid flow of weld pool was observed by a high speed camera.The fluid flow in weld pool was not symmetric and much of molten metal gathered in the lower part.The fluid flow velocity in the lower part was bigger than that in the upper part.To this end,the formative mechanism of the undercut defect was proposed.The flowability of the molten metal was an influential factor for the undercut defect.A preheating method was designed to verify the formative mechanism.