Thermal behavior and fluid flow during humping formation in high-speed full penetration gas tungsten arc welding

An unusual type of humping defect with periodic humped region and perforated region may occur in high-speed full penetration GTAW, which deteriorates the homogeneity of weld property seriously. In this paper, the thermal behavior and fluid flow during full penetration humping (FP-humping) formation is revealed by a numerical investigation to explain the defect's physical mechanism. Self-adaptive double-ellipse distributions of arc heat flux, arc pressure, arc shear stress and electromagnetic force varying with transient free surface evolution are proposed. For the first time, the actual morphology of FP-humping in GTAW can be predicted numerically. The numerical results are well verified by experimental weld geometry and high-speed melt pool images captured by a color camera. The workpiece is melted under arc heat to form the melt pool, and its free surface is gouged significantly under strong arc forces. When the depth of gouging region approaches workpiece thickness, the bottom thin liquid layer in gouging region is easily disrupted to initiate FP-humping formation. The lateral channel becomes the only transfer channel for liquid metal backward flow. The transition region between lateral channel and rear part of melt pool will be necked, and solidified instantly without the arc heat. Both scaling model of thermal conduction and numerical results imply that the premature solidification of necked lateral channel is a predominant factor to influence the FP-humping formation.