# High-Speed Micro-Grooving of Metal by Angled Irradiation of Single-Mode CW Fiber Laser
## 2 Experimental Setup  
A single-mode CW fiber laser (IPG Photonics, Germany, YLR-200-SM) was used as the laser source. The maximum average power, \(P\), and wavelength, \(\lambda\), are 200 W and 1.085 \(\upmu\)m, respectively. The \(M^{2}\) value representing beam quality was approximately 1.1. Polarization of the laser beam was random. The groove formation characteristics of the mild steel SS400 and aluminum alloy A1050 4 mm in thickness were examined.  
The experiments were conducted using two different setups. For the experiments at scanning speeds of less than 2 m/s and high-speed observation of the processed area, the workpiece was moved using a mechanical linear slider, as shown in Figure 1a. The workpiece was mounted horizontally on the jig of the slider. The \(Z\) axis is perpendicular to the \(X\)-\(Y\) plane, which is the surface of the workpiece. The laser beam was angled from the \(Z\) axis in the \(X\)-\(Z\) plane, as shown in Figure 1a. The incident angle \(\theta_{\text{i}}\) is defined as the angle between the \(Z\) axis and the laser beam axis in the \(X\)-\(Z\) plane. The laser beam was focused into a spot diameter of 18 \(\upmu\)m by a fused silica lens with a focal length of 100 mm. The dynamic phenomena in the processed area were observed using a high-speed microscope camera (Keyence, Japan, model VW-9000). The maximum resolution of this camera is 640 \(\times\) 480 pixels at the frame rate of 4000 fps. The processed area was illuminated by a white light source, and a bandpass filter with a center wavelength of 440 nm was used to eliminate the bright light noise from the keyhole and molten metal.  
Figure 1: Schematic diagrams of experimental setups.  
In the other experimental setup, the laser beam was moved using a galvano scanner (GSI LUMONICS, Japan, model XY2026, M3 ST) and an _f0_ lens. This setup was used for relatively high-speed processing (up to 6 m/s), as shown in Figure 1b. The workpiece was tilted (\(\theta_{i}\)) with respect to the ground surface, and the laser beam was irradiated downward vertically with respect to that ground surface. The definition of incident angle, \(\theta_{i}\), is the same as that shown in Figure 1a. The focal length of the _f0_ lens was 165 mm, and the spot diameter was 35 \(\upmu\)m. The focus was placed on the surface of the workpiece, and the micro-grooves were formed by single scanning of the laser beam in all the experiments.  
Figure 2 shows a schematic diagram of the laser-irradiated area. The incident and reflection sides in angled laser irradiation are defined as shown in the figure. The shape of the focused laser beam on the surface of the workpiece becomes an ellipse owing to the projection of the angled irradiation, as shown in the figure. The irradiated laser beam diameter on the \(X\) axis, \(d_{x}\), is elongated with increasing incident angle. The power absorption rate of the random polarized laser beam is the average value of the P and S polarization waves. Its value for steel at wavelengths of approximately 1 \(\upmu\)m is constant at approximately 30% for incident angles of 0-60\({}^{\circ}\)[19]. Therefore, the power absorbed by the workpiece can be regarded as constant within this angular range, and the irradiated power density decreases with increasing incident angle. The power density, \(P_{\text{d}}\), was calculated using Equation (1), where \(P\) is the laser power, and \(d_{\text{s}}\) and \(d_{x}\) are the diameters in the processing direction and along the \(X\) axis, respectively. The energy density, \(E_{\text{d}}\), is defined by Equation (2) in terms of the transit time, \(T_{\text{t}}\), which is the time it takes for the beam to pass through the diameter (\(d_{\text{s}}\)) in the processing direction, as expressed in Equation (3).  
\[Pd=P/(\pi/4\text{-}ds\text{-}dx) \tag{1}\]  
\[Ed=Pd\text{-}Tt \tag{2}\]  
\[Tt=ds/Vs \tag{3}\]  
Figure 2: Schematic diagram of laser-irradiated area.