# Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing
## 2 Simulation Model and Experimental Methods
### Laser Setup, Materials and Characterization Methods  
In this work, a continuous wave (CW), high-power diode laser LDM 1000 from Laserline GmbH with a 980 nm wavelength was coupled to a fixed focusing head. At the focus point distance of 297 mm, the spot diameter is 0.9 mm with a flat top distribution. The displacement is provided by an \(x\)-\(y\) table that allows movement in both directions mounted below the process chamber. To avoid oxidation during the process, the chamber is filled with argon. A schematic drawing as well as a picture of the setup is shown in Figure 4.  
In situ observation of the melt pool was carried out with a Motionpro Y4 high-speed camera from Videal mounted with a Moritex ML-Z07545 telecentric objective. The camera was positioned as shown in Figure 4 and allows observation of the melt pool with a small angle compared to the laser beam. The objective was fitted with low pass filter (cut-off 800 nm) in order to block the back-reflected laser light from entering the camera. Additional light in the visible range was provided by a light fixed on the right-hand side of the setup. In these conditions and taking into account the size of the melt pool observed, the frame rate was 10 kHz.  
For static experiments, the laser and the camera were triggered together. The laser power was kept constant at the desired level and was switched off after a given time. The camera kept recording 100 ms afterwards to capture the solidification.  
For each moving experiment, the motion of the workpiece was started first in the \(x\)-direction. Once the workpiece reached constant speed, the laser and the camera were triggered together. The laser stayed on for a defined displacement. After the laser was stopped, the table was decelerated to a stop. For the case where several lines were observed, the table was moved by a hatching distance in the \(y\)-direction. Then, the process could start again in the opposite direction along the \(x\)-axis. The process was repeated for the number of lines desired.  
Figure 3: **(a)** Coarse mesh in the \(xy\) plane and the possible position of the fine mesh in gray and (b) Fine mesh in the \(xy\) plane and a representative coarse cell (in gray).  
The workpieces were made of X38 CrMoV5-1 hardened tool steel rectangular plates with dimensions of 70 \(\times\) 70 \(\times\) 7 mm\({}^{3}\). On the top surface, a region of 50 \(\times\) 50 mm\({}^{2}\) was prepared by EDM. The finishing state of the surface was CH30 on the Charmilles scale for EDM which corresponds to a roughness \(Ra\) of 3.15 \(\upmu\)m. This is also the initial state prior to the LP process.  
The total reflection of the steel EDM surface was measured at room temperature with a Lambda 900 spectrometer from Perkin Elmer fitted with an integrating sphere. The absorption is then defined as \(a=1-R\), where \(R\) is the total reflection (both diffuse and specular).  
The lines and points produced were observed under an optical microscope to determine the width of the melt pools. The workpieces were then cut in cross sections to observe the size and shape of the re-melted layer and the heat-affected zone (HAZ). The cross sections were ground and polished up to colloidal suspension to obtain a mirror polish. During the next step, they were etched for 20 s with 5% Nital solution and observed with an optical microscope.