# 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 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. To get an idea of the absolute difference between the measured variables, the absolute difference between the capillary depth and the welding depth is shown as a function of the welding speed and the welding depth in Figure 8b. 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, the laser power when changing from heat conduction welding to deep penetration welding is marked for each welding speed. For a welding speed of 250 \(\frac{\mathrm{mm}}{\mathrm{s}}\) (red triangle), a decrease in measured difference from 80 \(\upmu\)m to 60 \(\upmu\)m can be observed when changing from heat conduction welding to deep penetration welding at a laser power of 1250 W. In deep penetration welding, the measured difference stays constant at a value of \(\sim\)60 \(\upmu\)m with an increase in welding depth, an increase in absolute difference can be observed at a welding depth of 850 \(\upmu\)m. For a welding speed of 500 \(\frac{\mathrm{mm}}{\mathrm{s}}\) (green diamond), the drop in absolute difference (65 \(\upmu\)m to 26 \(\upmu\)m) can also be observed when changing from heat conduction welding to deep penetration welding at a laser power of 1500 W. With increasing welding depth in deep penetration welding, a steady increase in measured difference can be observed from 26 \(\upmu\)m to 62 \(\upmu\)m. When increasing the welding speed to 750 \(\frac{\mathrm{mm}}{\mathrm{s}}\) (blue circle), no significant change in absolute difference by changing from heat conduction to deep penetration welding at a laser power of 1750 W can be seen. Additionally, contrary to the welding speeds of 250 \(\frac{\mathrm{mm}}{\mathrm{s}}\) and 500 \(\frac{\mathrm{mm}}{\mathrm{s}}\), with increasing welding depth, a decrease in absolute difference can be observed. A potential explanation this decrease in difference could be the change in melt flow conditions (melt-jet effect; humping) for high welding speeds, as presented in Section 3.1. In conclusion, by using an OCT System, the capillary depth in welding copper with green lasers can be determined. A good correlation between the capillary depth and the welding depth was shown for deep penetration welding, enabling a welding depth control using an OCT System in copper micro welding with green lasers. By evaluating the absolute difference between the welding depth and the capillary depth measurement, an increase in difference could be observed for welding speeds of 250 \(\frac{\mathrm{mm}}{\mathrm{s}}\) and 500 \(\frac{\mathrm{mm}}{\mathrm{s}}\). For high welding speeds of 750 \(\frac{\mathrm{mm}}{\mathrm{s}}\), the absolute difference decreases, which is potentially caused by the change in melt flow conditions.