# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
## 3 Results
### Capillary Depth Measurement Using OCT
#### 3.2.1 OCT Positioning  
To determine the capillary depth during the welding process, the positioning of the OCT relative to the laser beam is of particular importance. Due to the movement of the laser beam during laser welding, the front of the capillary is inclined. The inclination  
Figure 3: Mean welding depth (\(n=6\)) of welding copper with a green laser and a focal diameter of \(300\,\mathrm{\SIUnitSymbolMicro m}\); white: heat conduction welding, grey: deep penetration welding, cross pattern: increased spatter formation with spatter sizes \(>100\,\mathrm{\SIUnitSymbolMicro m}\), dotted area: jet effect of the molten material, vertical lines: humping. increases with the welding speed [8] and leads to a shift of the deepest point of the capillary. In order to be able to measure the capillary depth, the position of the OCT beam must be adjusted. In the following section, a method, as well as the results, to determine the position of the deepest point in relation to the center of the laser beam will be presented. In Figure 4, the addressed positional discrepancy is illustrated schematically. The Figure shows the laser beam (green) in a longitudinal cross section of the welding process. Additionally, the OCT beam (red) is displayed for different positions in relation to the origin of the laser beam. In Figure 4a, the position of the OCT beam is aligned with the position of the laser beam. In this configuration, the measured depth is less than the true depth of the capillary. By shifting the OCT beam behind the capillary, as illustrated in Figure 4b, the true depth of the capillary can be measured. Here, the molten surface is presumed to be orthogonal to the OCT beam, resulting in a measured depth with high signal intensity. Lastly, when the position of the OCT beam is shifted out of the deepest point, the OCT beam can be reflected off the capillary surface and lead to a too deep measurement Figure 4c. In order to determine the optimal measuring position of the OCT beam, the position was altered during the welding process relative to the origin of the laser beam. By evaluating the measured depth, as well as the signal intensity, the optimal measurement position could be determined. Figure 5 illustrates the presented method schematically. In Figure 5a, a longitudinal cross section of the welding process is displayed. The OCT beam (red line) is moved along the welding direction and passes the capillary and the origin of the laser beam (green) once. The result of such measurement is displayed in Figure 5b exemplary. The graph shows the measured depth (red) as function of the position of the OCT beam relative to the laser beam origin (dash-dotted line). In the areas where there is no visible measured depth, the signal intensity was low and, therefore, not relevant for the determination. In the displayed example, one can see a depth measurement with an offset of \(\sim\)\(-180\,\mathrm{\SIUnitSymbolMicro m}\) relative to the laser beam origin. This indicates that the lowest point of the capillary is trailing the laser beam. In order to determine the optimal lag distance as a function of the process parameters, the presented method was applied to the investigated process parameters. In Figure 6, the optimal offset values are displayed as function of the welding speed and the laser power. Additionally, both process regimes, heat conduction welding and deep penetration welding, are marked by the background color (heat conduction welding = white; deep penetration welding = grey). The welding speed can be distinguished by color and shape, where red triangles represent a welding speed of \(v=250\,\frac{\mathrm{mm}}{\mathrm{s}}\), green diamonds \(v=500\,\frac{\mathrm{mm}}{\mathrm{s}}\), and blue circles \(v=750\,\frac{\mathrm{mm}}{\mathrm{s}}\).