Study Of The On-line Coaxial Monitoring Of The Fibre Laser Welding Using An Auxiliary Illuminant

With the rapid development of the automobile industry, laser welding technologyplays a more and more important role in the manufacturing. Fiber laser, because of itsmany advantages, such as good beam quality, high power density, small spot size, canget weld of big depth-width ratio, and minimal distortion. However, it is difficult toguarantee the weld quality in a rapid, high-automated laser welding process. Manyscholars have been researching on the monitoring the weld quality by monitoring theweld pool and the keyhole.In this study, an on-line coaxial monitoring system with an auxiliary illuminantwas established for fiber laser welding. The wavelength range of the plasma generatedduring the welding process was detected using the monitoring system. The images ofthe welding area captured by using three different filters were compared. The filterthat can minimize the effect of the plasma signal and get images of the best qualitywas chosen. To clearly capture the images of the welding zone, a green LED lightingsource was mounted on one side of the laser head in this experiment. The differencebetween the capturing system with and without the auxiliary illuminant was studied.The device for adjusting the auxiliary light source angle was designed to obtain thebest shooting effect. According to the requirement of various welding speed and otherwelding parameters, the appropriate shooting device and data acquisition device weredesigned and chosen.The coaxial monitoring algorithm was proposed according to the characteristicsof the coaxially captured images and the efficiency of the collection system. Byprocessing the images captured during the welding process with the grey processingmethod, self-adaption median filtering method, image enhancement method, regiongrowing algorithm and the Canny algorithm, the profiles of the weld pool, the keyholeand the heat-affect zone can be obtained. The gray level distribution along thedirection perpendicular to the welding direction in the weld zone was obtained, theweld width which denoted as L can be obtained through the gray level image. Thus themonitoring of the weld pool width can be realized. The top diameter of the keyholeswhich denoted as D and the bottom diameter of the keyhole as d can be obtained; themonitoring of the penetration status during the welding process can be realized basedon the d/D ratio. Finally, experiments were conducted applying the on-line coaxial monitoringsystem and the quality monitoring mechanism. The monitoring of the weld width canbe realized through the weld pool width L obtained from the captured images, thevariation of the L means the variation of the weld width. When monitoring thepenetration status, the penetration status can be divided into the following threecategories according to the d/D ratio: incompletely penetrated, moderately penetratedand over-penetrated. In the butt welding process, the pixel line position of the buttwelding gap and the center of the molten pool area in the processed images can bemonitored. By calculating the distance between the two lines, which denoted as L,the seam tracking can be achieved. Thus the monitoring of the weld quality and theseam tracking can be realized simultaneously.