Camera based closed-loop control of laser micro-welding processes by observation of the full penetration hole

In macro welding-processes, monitoring and closed-loop control by using the so-called “full penetration hole” is well known. This paper reports first results for the transfer of this technology to laser micro-welding processes for lap joints as they are used for housings or fuel cells. The laser source was a 400 W cw fiber laser with a Gaussian beam profile and a spot size of 28 μm. A cellular neural network (CNN) camera was used to measure the image feature of the full penetration hole within the thermal image of the welding process. For full penetration weldings on stainless steel samples, the laser power was controlled by the rate of full penetration hole detection. The effect of the feedback system is that the laser power is automatically adapted to changes in sheet thickness or feeding rate. The sheet thickness was varied between 220 and 350 μm and the feeding rate between 10 and 40 m/min without significant change in the weld seam quality. The closed-loop system increases the robustness of the process against perturbations and the process is always guided at the minimum laser power necessary for full penetration thus reducing spatter and smoke residues.