Plasma fluctuation and keyhole instability in laser welding

Abstract

In high-power laser welding, a focused laser beam melts and vaporizes the material to form a metal vapor above the surface of a workpiece. As the focused laser beam continuously irradiates on the metal vapor, the laser beam ionizes the metal vapor and a portion of the shielding gas forming a hot and opaque plasma plume. Previous studies showed that the plasma plume was unstable during laser welding and it influenced laser energy utilization, process stability, and weld quality. In this study, dynamic behaviors of the plasma plume during laser welding were investigated using a high-speed camera at a speed of 9000 frames/second. It was found that the plasma grew in size continuously to a maximum height and then became small with respect to the elapsed time. The plasma fluctuated at an average frequency of 1.2 kHz in laser welding of steel. The plasma plume varied in dimensions dramatically in laser welding of aluminum and the average fluctuation frequency was about 1.3 kHz. The plasma fluctuation was related to the keyhole instability and this fluctuation was likely caused by the variation of the keyhole opening. Laser weld quality would be improved significantly if the keyhole instability or plasma fluctuation was suppressed during laser welding. One of the techniques of minimizing keyhole instability was the use of dual-beam laser welding technology. Experimental results indicated that the plasma plumes in dual-beam laser welding were stable and good-quality welds were produced for both steel and aluminum.In high-power laser welding, a focused laser beam melts and vaporizes the material to form a metal vapor above the surface of a workpiece. As the focused laser beam continuously irradiates on the metal vapor, the laser beam ionizes the metal vapor and a portion of the shielding gas forming a hot and opaque plasma plume. Previous studies showed that the plasma plume was unstable during laser welding and it influenced laser energy utilization, process stability, and weld quality. In this study, dynamic behaviors of the plasma plume during laser welding were investigated using a high-speed camera at a speed of 9000 frames/second. It was found that the plasma grew in size continuously to a maximum height and then became small with respect to the elapsed time. The plasma fluctuated at an average frequency of 1.2 kHz in laser welding of steel. The plasma plume varied in dimensions dramatically in laser welding of aluminum and the average fluctuation frequency was about 1.3 kHz. The plasma fluctuation was relate...