# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
## 4 Conclusions  
In this article, the applicability of using an Optical Coherence Tomography System for measuring the capillary depth in micro welding of copper using a green laser was presented. Using a focal diameter of \(300\,\mathrm{\SIUnitSymbolMicro m}\) and varying the welding speed from \(250\,\frac{\mathrm{mm}}{\mathrm{s}}\) to \(750\,\frac{\mathrm{mm}}{\mathrm{s}}\), as well as the laser power from \(1000\,\mathrm{W}\) to \(3000\,\mathrm{W}\), it was possible to achieve welding depth from \(55\,\mathrm{\SIUnitSymbolMicro m}\) up to \(1070\,\mathrm{\SIUnitSymbolMicro m}\). Figure 11: (**a**) Resolution in welding direction as a function of the modulation frequency; (**b**) resolution perpendicular to the welding direction as a function of the distance from the middle axis. Figure 12: (**a**) OCT movement path and Intensity in an x-y-plane during a welding process; (**b**) weld seam image overlaid with corresponding OCT movement path and thresholded Intensity. To determine the optimal position of the OCT measurement beam in relation to the laser beam origin, a method was presented in which the OCT beam position is altered during the welding process to measure the depth in multiple positions along the weld direction. With this method, it could be shown that the measuring position with the greatest depth and the highest signal intensity is trailing the laser beam. In addition, a dependency of the optimal measuring position on the process parameters could be shown. Whereas in heat conduction welding, the trailing distance is almost negligible, the trailing distance to be used increases with increasing laser power and welding speed. By comparing the measured capillary depth and the welding depth determined in cross sections, a good correlation between both measured variables could be shown independently on the previously determined process regime and stability. By evaluating the percentage difference, it could be shown that no weld seam depth measurement is possible with heat conduction welding. For deep penetration welding, the percentage difference decreases with an increase in welding depth, making it feasible to determine the welding depth based on the capillary depth measurement. The absolute difference between the capillary and welding depth ranges from \(22\,\mathrm{\SIUnitSymbolMicro m}\) to \(62\,\mathrm{\SIUnitSymbolMicro m}\) for welding depth of \(200\,\mathrm{\SIUnitSymbolMicro m}\) to \(600\,\mathrm{\SIUnitSymbolMicro m}\), as used in typical copper micro welding application. Measuring the capillary depth during the welding process allows for a direct determination of the aspect ratio of the welding process. It could been shown that, for deep penetration, welding aspect ratios as low as 0.35 can be achieved, while, typically, aspect ratios substantially larger than one are the case using state-of-the-art fiber laser welding processes for copper micro welding. Welding with a shallow and wide capillary not only benefits the depth measurement with an OCT System but has also been shown stable regarding spatter occurrence. For heat conduction welding, a maximum aspect ratio of 0.16 was determined, resulting in the conclusion that a small melt pit must have been formed out prior to the change to deep penetration welding. Using the the additional scanning system of the OCT System, multiple information about the welding process can be determined simultaneously. By superimposing the welding motion with a spatial movement perpendicular to the welding direction, additional information, like the capillary width, melt pool width, and approximations, about the capillary form might be determined. By thresholding the signal intensity, a good correlation between the OCT Signal and the weld seam width could be shown. Conceptualization, T.B. ; methodology, T.B. ; investigation, T.B. ; writing--original draft preparation, T.B. ; writing--review and editing, T.B., C.B., M.B., and J.P.B. ; visualization, T.B. All authors have read and agreed to the published version of the manuscript.