Comparison of different process monitoring methods for laser beam micro welding

Although laser beam welding is an established joining technology in the field of precision engineering, welding of copper materials is still a delicate issue. The fluctuating energy deposition resulting from the unsteady thermodynamic and optical properties of copper causes intermittent welding failures. To make welding processes more reliable, failure detection by online process monitoring is a common technology for large-scale welding applications. For micro welding, where material thickness far below 1 mm and focus diameters of 100 µm and less are used, the existing methods must be adapted to meet requirements such as extreme accuracy and processing times of a few milliseconds. Innovative monitoring systems based on ultra-fast cameras and optical sensors with signal processors capable of sampling rates up to 20 kHz provide new opportunities. In this paper monitoring of micro welding processes with pulsed Nd:YAG lasers and fiber lasers is examined. Different methods for process monitoring are compared, regarding what kind of information about the welding process can be gained and how failures can be identified. A special topic is the use of galvanometric scanners and how an integrated system consisting of a processing head and a monitoring system can be put into practice