Process control and quality assurance in remote laser beam welding by optical coherence tomography

Abstract

Remote laser beam welding significantly outperforms conventional joining techniques in terms of flexibility and productivity. This process benefits in particular from a highly focused laser radiation and thus from a well-defined heat input. The small spot sizes of high brilliance laser beam sources, however, require a highly dynamic and precise positioning of the beam. Also, the laser intensities typically applied in this context result in high process dynamics and in demand for a method to ensure a sufficient weld quality. A novel sensor concept for remote laser processing based on optical coherence tomography (OCT) was used for both quality assurance and edge tracking. The OCT sensor was integrated into a 3D scanner head equipped with an additional internal scanner to deflect the measuring beam independently of the processing beam. With this system, the surface topography of the process zone as well as the surrounding area can be recorded. Fundamental investigations on aluminum, copper, and galvanized steel were carried out. Initially, the influence of the material, the angle of incidence, the welding position within the scanning field, and the temperature on the OCT measuring signal were evaluated. Based on this, measuring strategies for edge tracking were developed and validated. It was shown that orthogonal measuring lines in the advance of the process zone can reliably track the edge of a fillet weld. By recording the topography in the trailing area of the process zone, it was possible to assess the weld seam quality. Comparing the results to microscopic measurements, it was shown that the system is capable of clearly identifying characteristic features of the weld seam. Also, it was possible to observe an influence of the welding process on the surface properties in the heat-affected zone, based on the quality of the measuring signal.

A Novel Approach to the Holistic 3D Characterization of Weld Seams – Paving the Way for Deep Learning-Based Process Monitoring

“…In general, the high intensities in the area of the process zone during laser beam welding make the use of optical systems challenging. Low-coherence interferometry, as the first method to allow dimensional measurements directly in the process zone, allows high-precision in-situ measurements of the weld seam topology [20].…”

Welding depth measurement for different mode lasers using optical coherence tomography

“…A fast Fourier transform (FFT) is performed on the interference spectrogram to resolve the keyhole depth. By deflecting the measuring beam independently of the processing beam using a scanner, OCT can also be used to preprocess or postprocess seam tracking [8,9] .…”

Prediction of electrical resistance of laser-welded copper pin-pairs with surface topographical information from inline post-process observation by optical coherence tomography

“…One potential inline monitoring technology that overcomes this limitation is optical coherence tomography (OCT). OCT is an interferometric measurement technology that enables the assessment of the welding bead by non-destructive distance measurement [3][4][5][6][7]. OCT can be applied to identify misalignments in pre-process Faculty 1 (Physics/Electrical Engineering), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany monitoring, the weld depth in in-process monitoring, and the shape of the resulting weld in the post-process monitoring stage [8,9].…”