Undercut Formation Mechanism in Keyhole Plasma Arc Welding

Undercut formation is a common weld defect in keyhole plasma arc welding. Based on the comprehensive experimental measurement of the weld pool convection with the help of x-ray transmission, high-speed video camera, and thermal camera systems, undercut formation is suggested to be caused by the irregular fluid flow and uneven energy distribution of the weld pool. As the plasma gas rate is increased, the counterclockwise eddy inside the weld pool is increased, and the backward flow at the top surface is decreased, so more energy is transported to the bottom surface, while less energy is transported to the top surface. The rear part of the top weld pool is easily solidified, so a teardrop-shaped profile is easily formed. Due to the strong inward flow at the top surface caused by the teardrop-shaped top weld pool profile and the dominant counterclockwise eddy inside the weld pool, more energy is transported from the lateral sides to the centerline of the top weld pool. A high-temperature zone is formed at the centerline of the top weld pool, while the lateral sides are easily prematurely solidified, causing the undercut formation at the top surface.