Modelling, dynamical simulation and online monitoring in laser beam welding

Efficient and high quality processing requires identification of the underlying phenomena and on-line monitoring of the achieved performance. The reliability for prediction of the weld quality can be improved by using the Coaxial Process Control (CPC) system. The camera images taken from the process are processed on-line with a time of 80 ns to readout one single array element having a spatial extent of 16 µm. The CPC signal depends on the length of the geometrical path of observation within the capillary plasma and the plasma density, resulting in a linear dependence of the signal on penetration depth and the sensitivity on the movement of the capillary. Both, the simulation of the two moving boundaries involved - capillary surface and melting front - as well as the CPC monitoring signals allow to reconstruct capillary movement and supports the identification of the different dynamical process domains. Within the two transition regions between the three main process domains - heat conduction welding, partial and full penetration welding - the capillary shape and its dynamics changes qualitatively. The shape and its dynamics are closely related to the formation of caustic surfaces due to laser light reflected at the curved capillary surface.