Processing of Keyhole Depth Measurement Data during Laser Beam Micro Welding

Abstract

Analysing the quality of weld seams is still a challenging task. An optical inspection of the surface is giving limited information about the shape and depth of the weld seam. An application for laser beam welding with high demands regarding the weld depth consistency is the electrical contacting of battery cells. The batteries themselves have a limited terminal or case thickness that must not be penetrated during the welding process to avoid leakage or damage to the cell. That leads to a minimum weld depth to ensure the electrical functionality, and a maximum weld depth indicated by the case thickness. In such applications, a destructive analysis is not suitable which leads to the demand for a non-destructive measurement during the process. Using a coaxial, interferometric measurement setup, the keyhole depth during the deep penetration welding is measureable. For a keyhole with a depth of a couple of millimetres, such a system is commercially available. In micro scale, however, these systems are facing several challenges such as scanning systems, small spot diameters of a few tens of micrometres and narrow keyholes. This study contains an investigation of an interferometric measurement of the keyhole depth and the suitability for laser micro welding. Therefore, the data processing of the achieved measurements is investigated, and the results are compared with the depth measurement of metallographic analysed samples. Stainless steel is used to investigate the behaviour and the stability of developed data processing strategy and the resulting depth values.

OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers

“…Additionally, to achieve a stable welding process in copper, process strategies, like spatial beam modulation, must be applied, providing additional challenges. In recent investigations, the authors of Reference [4] showed the difficulties for measuring the welding depth in micro welding with a spot diameter of 82 µm, while, for many applications, even smaller focal diameters are being used.…”

Development of an Intelligent Quality Management System for Micro Laser Welding: An Innovative Framework and Its Implementation Perspectives

“…In a different study, Mr ňa et al designed a feedback control for laser welding applications based as well on the frequency analysis of the light reflected. Furthermore, Hollatz et al concluded that an Optical Coherence Tomography system, OCT, can be used to measure the keyhole depth of laser micro-welding processing [48]. Optical emissions and welding defect formation exhibit statistical correlation that results in defect identification.…”

Algorithms for Weld Depth Measurement in Laser Welding of Copper with Scanning Optical Coherence Tomography

“…One solution to observe the weld penetration depth inline is optical coherence tomography (OCT). OCT is an interferometric measurement technology that enables the measurement of weld penetration depth coaxially to the processing laser in a fixed optic [5][6][7][8][9][10][11][12] or scanning optic [13][14][15].…”

“…The process noise can be caused by changes in the optical path by the vapor plume in the welding process and thermal noise [16]. To date, this issue was treated with data analysis methods such as histogram analysis [9,17], different filtering (e.g., percentile filter [9,10], Kalman filtering [14]), or kernel density estimation (KDE) [1,7]. The simplest method is the evaluation of histograms at every measurement point [17] and the extraction of the local maximum [9].…”

“…The application of a running percentile filter after removing the noise can be seen as beneficial [10], where a percentile filter is applied to the measurement information along the measurement time for weld depth identification. A further increase in accuracy can be achieved with a combined usage of bandpass and Kalman filtering [14]. However, filtering requires parametrization of the relevant percentile and moving window according to the weld settings, and histograms are not deterministic as maxima could be found at measurement positions with multiple reflections.…”