# High-Speed Micro-Grooving of Metal by Angled Irradiation of Single-Mode CW Fiber Laser
## 1 Introduction
Groove formation by a pulsed laser was an ablative process, whereas melting was the primary mechanism in CW laser processing. Consequently, it was shown that the groove depth and width were unstable during CW laser processing. These earlier studies indicate the possibility of micro-grooving by a CW laser; however, further reduction of the required laser power and stabilization of the groove shape are desirable for industrial applications. It was shown that, in high-speed micro-welding by a CW laser, undercuts were generated at both edges of the molten bead and remained as shallow grooves [15]. If the shallow grooves could be appropriately controlled, effective micro-grooving by a CW laser might be possible, even at low average power. Formation of these molten beads and shallow grooves are attributed to molten metal flow and solidification, which are symmetric with respect to the laser irradiation axis. When the balance of the molten metal behavior can be controlled by varying the incident angle of laser irradiation, it can be expected that deeper micro-grooves can be obtained by using a CW laser at lower power. Therefore, micro-grooving by angled laser irradiation of a low-average-power CW laser can be proposed as a new approach to deepening the shallow grooves that are formed at both edges of the molten bead. The effect of the laser incident angle on the grooving quality of a ceramic was examined using a pulsed laser [16]. The laser axis was inclined in the beam scanning direction in this study. Therefore, the processing behavior was symmetric about the laser axis, and it was confirmed that the groove depth was decreased by increasing the irradiation angle. However, the processing mechanism is thought to be different from that of metal, and ablation is a major removal process for ceramic materials. Groove formation on steel by angled irradiation by a nanosecond pulsed laser was also reported [17]. In this study, an angled groove in the top surface of the workpiece was formed along the laser irradiation axis, and it was shallower than that obtained by vertical irradiation. It was concluded that reducing the power density resulted in a shallow groove owing to the enlarged spot size caused by its projection in angled laser irradiation. In addition, the main mechanism of groove formation was ablation in short-pulse laser processing. Therefore, molten metal behavior is not thought to contribute to groove formation in this method. In our previous study [18], micro-grooving by angled laser irradiation was investigated using a 200-W single-mode CW fiber laser. Unique micro-groove profiles were formed on the side opposite to the incident laser beam, and formation of micro-grooves effectively by a relatively low-average-power CW laser was demonstrated. However, the detailed mechanism of micro-groove formation has not been clarified, and further investigation is needed to optimize this process. Therefore, in this study, the characteristics of micro-grooves formed by angled laser irradiation were experimentally investigated by controlling process parameters such as scanning speed and incident angle. The formation mechanism of the micro-grooves was discussed by making high-speed observations, evaluating the shapes of the formed micro-grooves, and considering the thermophysical properties of the metal.