Effect of the eddies formed inside a weld pool on welding defects during plasma keyhole arc welding

摘要

In this study, the formation process of welding defects in plasma keyhole arc welding was investigated based on the formation of eddies inside the weld pool. An experimental approach using an advanced X-ray visualization system was employed to clarify this relationship. The results indicated that two large eddies are produced in the weld pool. One exhibits an upward tendency (upper eddy) and the other a downward tendency (lower eddy). The relative strength of each eddy significantly changed depending on the welding current through the change in the keyhole profile (shape and size). For a suitable welding current, the lower eddy was larger and much longer than the upper eddy. Virtually no welding defects were found out in the cross-section and longitudinal sections of the weld pool and keyhole. In the case of low current, the upper eddy was larger and longer than the lower eddy. Severe undercut and humping phenomena were observed on the top side. Contrarily, for a high welding current, the upper eddy disappeared to only form a significantly large lower eddy that covered the entire weld pool. Here, a severe concave defect on the top surface and a severe burn-through defect on the bottom side of the weld bead were seen. These results indicated that when the penetration ability of an arc on the base metal is sufficient, the relative strength of each eddy should be adjusted to a suitable scale with a larger downward eddy and smaller upward eddy. This can ensure a stable and high welding quality without welding defects.