STATE-OF-THE-ART OF MONITORING AND IMAGING OF LASER WELDING DEFECTS

Several weld defects like lack of fusion, blow-out holes, pores, cracks or undercuts can occur during laser welding. They can be crucial for product failure. Due to its complexity the laser welding process and the origin of its defects are only partially understood. Both experimental observation and numerical simulation is difficult. Beside generally accepted knowledge on welding defects, to a limited extent high speed imaging, particularly by X-ray transmission, and mathematical modelling have generated some understanding. However, experimental observation suffers from the small size of the process zone, from its high dynamics and from the hot environment, while the physical process is to complex for complete simulations. Besides avoiding welding defects through process understanding, detection is of importance in industrial production. It can be distinguished between pre-, in- and post-process inspection, and between on- and off-line. Off-line post inspection is often expensive. Today in-process monitoring is provided by photodiodes or cameras, but owing to the lack of understanding it is limited to empirical correlations between the appearance of a defect and signal changes. The present review provides a survey on laser welding defects, on their experimental observation, on their theoretical treatment by modelling or simulation and on their detection by process monitoring. Despite large research efforts the understanding and detection of laser welding defects is still very limited and unsatisfactory, hindering industrial implementations. Further research will be needed to fully control this critical welding process and in turn to guarantee reliable production and safe product function