# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
## 3 Results
### Capillary Depth Measurement Using OCT
#### 3.2.2 Capillary Depth Measurement  
For copper micro welding with green lasers, deep penetration welding with much lower aspect rations compared to fiber laser welding processes can be performed, therefore increasing the potential for the OCT to measure the capillary depth. In the following section, the results of the capillary depth measurement will be presented and compared to the corresponding welding depth. As described in Section 3.1, the welding depth was determined by cross sections in the middle of the weld. For an accurate comparison, the capillary depth measurement in the middle of the weld seam was determined. However, in order to compensate for the positioning error caused by grinding the sample when performing the cross section, the mean depth measurement in a range of \(-50\,\mathrm{\SIUnitSymbolMicro m}\) to \(50\,\mathrm{\SIUnitSymbolMicro m}\) around the middle of the weld was used. In Figure 7, the results of the OCT depth measurement and the welding depth are displayed as a function welding speed and the laser power. 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 \(v=750\,\frac{\mathrm{mm}}{\mathrm{s}}\). To differentiate the OCT depth measurement from the welding depth, the OCT depth measurements are marked with a black outline. The formatting of the background follows the formatting in Figure 3 and can be found in Section 3.1. A good correlation can be observed when comparing both measurements. This correlation seems to be independent of the process stability and the regime. In order to determine if a measurement of the welding depth based on the capillary depth is viable, the percentage deviation between the measured values:  
\[\Delta=\frac{s_{weld}-s_{OCT}}{s_{weld}}\cdot 100, \tag{1}\]  
where \(s_{weld}\) as the welding depth, and \(s_{OCT}\) as the measured capillary depth, was determined. In Figure 8a, the percentage difference is displayed as function of the welding depth and the welding speed. 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 \(v=750\,\frac{\mathrm{mm}}{\mathrm{s}}\). Additionally, both process regimes, heat conduction welding  
Figure 7: Mean welding and capillary depth (\(n=6\)); no outline: weld seam depth; with outline: capillary depth measurement, 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. and deep penetration welding, are marked by the background color (heat conduction welding = white; deep penetration welding = grey). When looking at the percentage difference in the heat conduction welding regime, the majority of the measurements are in the range of 75-125%, which indicates that only minor depth could be measured by the OCT System. A determination of the welding depth based on the OCT depth measurement is not viable here. For values above 100%, the OCT System measured a negative depth. A potential explanation for this spread of measurements could be the axial resolution of the OCT System of 13 \(\upmu\)m which has a significant impact with welding depth being in the range of 50 \(\upmu\)m to 80 \(\upmu\)m. However, one exception can be observed in heat conduction welding. For a welding speed of 750 \(\frac{\mathrm{mm}}{\mathrm{s}}\) and a laser power of 1750 W, a percentage difference of 50% can be observed. Since the measurement error decreases with increasing welding depth due to the axial resolution of the OCT system, this implies that a small pit must have formed in the molten material during heat conduction welding.