Fundamental Laser Welding Process Investigations

In a number of systematic laboratory investigations the fundamental behavior of the laser welding process was analyzed by the use of normal video (30 Hz), high speed video (100 and 400 Hz) and photo diodes. Sensors were positioned to monitor the welding process from both the top side and the rear side of the specimen.Special attention has been given to the dynamic nature of the laser welding process, especially during unstable welding conditions.

In one series of experiments, the stability of the process has been varied by changing the gap distance in lap welding. In another series of experiments, the stability has been changed by varying the focus point position.In the investigations a clear correlation between the signal emitted from the front side of a seam and the signal emitted from the rear side has been observed by photo diodes.It turns out that simple intensity measurements of normal video and high speed video (100 Hz) can not reveal any instability in the process when humping occurs.

Contrary to this, photo diode signals (sampled at 3 kHz) clearly indicate a characteristic signal when humps occur.When the seam area and seam width have manually been measured on high speed video pictures (400 Hz), a clear impact on the seam characteristics has been identified when a hump occurs.Finally, a clear correlation between the position of the focus point, the resultant process type and the corresponding signal intensity and signal variation has been found for sheets welded in the bead-on-plate configuration.