# High-Speed Micro-Grooving of Metal by Angled Irradiation of Single-Mode CW Fiber Laser
## 3 Experimental Results and Discussion
### Micro-Groove Formation Mechanism  
The mechanism of micro-groove formation is discussed by evaluating the formed micro-groove shapes and results of high-speed observation. Figure 7 schematically illustrates temperature distribution and scattering of spatter during angled laser irradiation.  
Temperature distribution is symmetrical with respect to the laser beam axis for perpendicular irradiation of the surface of the workpiece. By contrast, temperature distribution is asymmetrical with respect to the laser beam axis for angled irradiation owing to the lack of geometrical balance. Therefore, a higher temperature region is formed on the surface of the reflection side because of the lower thermal conductivity of air, as shown in Figure 7. In addition, the keyhole edge is close to the free surface exposed to the air, and many spatters are scattered toward the reflection side. On the incident side, the molten metal is thought to remain. Therefore, the molten metal distribution is  
Figure 6: High-speed observation of spatter.  
Figure 7: Schematic illustrations of temperature distribution and spatter during angled laser irradiation.  
asymmetric around the keyhole, and this is considered to be an important factor in groove formation by angled laser irradiation.  
The behavior of the molten metal and the groove formation mechanism are illustrated in Figures 8 and 9, respectively. Figure 8 shows a schematic diagram of the behavior of molten mild steel at a lower scanning speed. In this case, the metal is melted more deeply owing to the higher energy density, \(E_{\text{d}}\), which results in a larger volume of molten metal, as shown in Figure 8a. The molten metal begins to flow into the groove formed on the reflection side after the laser beam has passed (Figure 8b). If the scanning speed of the laser beam is equal to or slower than the flow speed of the molten metal, the molten metal fills the groove before solidification. Consequently, no noteworthy groove is formed, as shown in the experimental result at \(V_{s}=1\) m/s in Figure 4. The small upheaval is considered to be the result of surface tension and contraction during solidification (Figure 8c).  
Figure 8: Schematic illustrations of molten metal behavior during angled laser irradiation with low-speed scanning (mild steel).  
Figure 9: Schematic illustration of molten metal behavior under angled laser irradiation during high-speed scanning (mild steel).  
The groove formation process at a higher scanning speed is schematically illustrated in Figure 9. The groove is formed on the reflection side, and the molten metal remains on the incident side, as in the case of a low scanning speed (Figure 9a). However, the pressure that forces the molten metal out of the keyhole is thought to be strong, especially ahead of the leading edge of the keyhole. The extruded molten metal from the keyhole has a large flow vector and changes direction because it is constrained by the surrounding solid metal. The flow vector opposite to the scanning direction and the strength can be maintained for a certain distance, \(L\), behind the keyhole, and it is larger than the flow vector into the groove on the reflection side (Figure 9d). As a result, the molten metal is thought to be cooled and solidified while staying on the incident side, and the groove that is formed on the reflection side remains after processing is complete (Figure 9c).