# High-Speed Micro-Grooving of Metal by Angled Irradiation of Single-Mode CW Fiber Laser
## 1 Introduction  
The laser source, processing method, and applications of laser surface micro-grooving have been widely investigated. Short-pulse lasers (less than a nanosecond) have generally been used as the laser source in these investigations, and the desired surface texture can be obtained by varying the laser parameters such as wavelength, pulse width, and energy fluence. The ablative processing of metals using pulsed fiber lasers at a high repetition rate (several MHz) was reported [1], and the effects of various laser parameters on the ablation efficiency and quality were studied. High-speed laser beam scanning is needed for industrial applications. For this purpose, a polygon scanning system for a high-repetition-rate pulsed laser was developed [2], and high productivity and large-area processing were demonstrated. There are many attractive applications of laser micro-grooving, which affects material surface properties such as adhesiveness, friction coefficient, wettability, and appearance. The surface wettability of stainless steel [3] and a polymer [4] were investigated using short-pulse lasers. Periodic line-like or pillar-like structures were created by changing the laser fluence and spatial overlap of the laser pulse, and the obtained micro-structures exhibited increased water repellency. Accurate friction control of stainless steel was obtained using a nanosecond pulsed laser [5]. Increases in not only the friction coefficient but also the hardness were reported. Periodic micro-craters werecreated on a cold-rolled surface using a pulsed laser. The periodic structure could be transferred to a steel sheet of the product to improve the appearance of paint [6]. Micro-structuring can be applied to biocompatible materials such as Ti alloys to improve their integration with surrounding tissue. The micro-grooving of Ti-6 Al-4V by a UV pulsed laser was studied, and the groove structure was controlled by varying the pulse repetition rate, scanning speed, and focal spot size [7]. Direct joining of a metal and a plastic by laser heating is one of the most promising techniques for automotive and aircraft applications because the components of these hybrid materials exhibit both strength and lightness [8]. Micro-grooves formed on the metal surface by laser processing could improve the bonding strength owing to their anchoring effect [9]. The relationship between the bonding strength and surface structures was investigated experimentally and theoretically [10; 11]. Short-pulse lasers are suitable for controlling surface structures; however, their average power is currently less than 1 kW owing to their small pulse energy or low pulse repetition rate. An ultrashort-pulse laser system with a maximum average power of 1.4 kW at a pulse width of 8 ps was developed [12]; however, cost reduction and commercialization of the system are expected to require a long time. Therefore, productivity of short-pulse laser processing is limited. By contrast, a continuous-wave (CW) laser has the advantage of high average power at an affordable cost. Single-mode CW fiber lasers with an average power of more than 5 kW are commercially available. In addition, a single-mode laser can be focused down to several tens of micro-meters in diameter, resulting in a high power density of more than 10 MW/cm\({}^{2}\), even at an average power on the order of 100 W. This power density is sufficient to generate a keyhole, and it is expected that micro-grooves can be formed by controlling phenomena such as melting, evaporation, and resolidification on the metal surface. High-speed micro-grooving using a combination of a 2-kW single-mode CW fiber laser and a polygon scanner was demonstrated, and micro-grooves more than 100 \(\upmu\)m in depth were formed on steel by multiple scanning of the laser beam at speeds greater than 10 m/s [13]. However, a relatively high-power laser source and multipass scanning are required to generate deeper grooves. Characteristics of groove formation by nanosecond pulses and CW lasers were examined and compared [14].