Quality control of laser welding by secondary plasma radiation measurement

Laser beam welding is known by the high quality of the weld. if however, faults are generated (for instance due to mismatch of the seam) it is not possible to correct these during welding in process. An on-line quality control system is required which must be capable of measuring the process status and adapting the process parameters to prevent faults and to ensure a constant weld quality. Therefore, a signal is needed that provides the necessary information about the quality of the welding process. First, the spectral distribution of the radiation generated by the welding process was studied. Based on these results, 2 spectral band widths in the ultraviolet and the infrared, suitable for quality control were selected. Using a measurement configuration with photo diodes, the optical signals in two bandwidths (200-400 nm and 850-1100 nm) were recorded during laser welding. These signals can be correlated to weld quality parameters and can be applied in a control loop. Considering changes in the process parameters of laser power and welding speed, a band width can be indicated in which must signal strength be to ensure a good quality weld. Different defects have been studied, some already present in the work piece prior to welding others caused by positioning errors of the parts. They proved to be detectable and could be identified from the secondary radiation.