# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
## 3 Results  
### Welding Depth and Process Stability  
In order to be able to evaluate the measurement results of the OCT system, the laser welding process must first be investigated with respect to the welding regime and process stability. Only then can the applicability of using an OCT system to determine the welding depth in laser microwelding of copper with green lasers be assessed. To avoid misinterpretations of the measurement results, as well as measurement errors due to instabilities, the performed laser welding processes were classified with regard to the process regime and stability and will be presented in the following section, along with the achieved weld seam depth. The occurring process regimes were evaluated by analyzing the weld seam surface. The evaluation criteria are displayed exemplary in Figure 2(a1,b1). For heat conduction welding, a bright and smooth weld seam surface, as shown in Figure 2(a1), can be observed. With increasing laser power, material evaporation sets in, changing the process regime from heat conduction to deep penetration welding. For this regime, an irregular surface of the weld seam can be detected, as displayed in Figure 2(b1). The process stability was determined by the visual appearance of the weld seam surface, as well as the evaluation of the recorded high-speed-videos. The process stability was assessed on two criteria, melt flow effects (Figure 2(a1-d1)) and spatter occurrence (Figure 2(a2-d2)). Figure 1: Schematic illustration of the experimental setup; green beam: laser beam (515 nm), grey beam: imaging path (VIS \(\
ot\)515 nm), light red beam: OCT beam (840 nm). While, for heat conduction welding, shown in Figure 2(a1), no instabilities regarding the melt flow were observed, the deep penetration regime could be divided into three categories, stable,'melt-jet effect', and humping [7]. For stable melt flow conditions in deep penetration welding, the weld seam surface has an uneven surface, as shown in Figure 2(b1). For high welding speeds, a melt jet is forming behind the capillary which can also be seen on the weld seam surface, as displayed in Figure 2(c1). Lastly, for high welding speeds and high laser power, humping can be observed. This regime is characterized by regularly spaced humps on the weld seam surface, as displayed in Figure 2(d1). By superimposing the high-speed-video images, the spatter occurrence of a full process can be evaluated within a single image. The results of superimposing the high-speed images are displayed in Figure 2(a2-d2) for the corresponding weld seam images displayed in Figure 2(a1-d1). An increased amount of spatter formation and the occurrence of spatters with a diameter \(>100\) um, was used as criteria for unstable processes. The formation of start spatters was not taken into account. Using the presented criteria, the process displayed in Figure 2(b2) with a laser power of \(1500\) W and a welding speed of \(250\)\(\frac{\text{mm}}{\text{s}}\) is considered unstable due to the occurring spatter amount and size, while the displayed processes in Figure 2(a2,c2-d2) are considered stable. Furthermore, no blowouts could be observed in the conducted experiments. The welding depth was determined by metallographic investigations. For each weld, a cross section in the middle of the weld was performed. In Figure 3, the mean welding depth for \(n=6\) repetitions is displayed as function of the welding speed and the laser power. The welding speed can be distinguished by color and shape, where red triangles represent a welding speed of \(v=250\)\(\frac{\text{mm}}{\text{s}}\), green diamonds \(v=500\)\(\frac{\text{mm}}{\text{s}}\), and blue circles \(v=750\)\(\frac{\text{mm}}{\text{s}}\). Based on the previously introduced criteria, the present welding regime is  
Figure 2: Exemplary weld seams to illustrate the criteria used for the assessment, as well as superimposed images of the high-speed-imaging, for determining the spatter occurrence in the laser process.