# Complexity Modeling of Steel-Laser-Hardened Surface Microstructures
## 2 Materials and Methods
### Experimental Setup  
As far as the procedure followed in the research work is concerned, a laser cell, equipped with an RV60-40 robot (produced by Reis Robotics Company, KUKA AG, Zuggwitzstrasse 140, 86165 Augsburg, Germany) (Figure 1a) was used to harden steel standard tools labeled EN 100083-1. The maximum power of the laser cell was 3000 W. Several dozens of specimens were hardened with an output constant power of 1500 W, with spot size 8 \(\times\) 13 mm. This controls the position of the laser head and, consequently, the orientation of the laser beam of the diode laser source. The advantage of the source used was that the manufacturing cell was characterized by a thermal scanner by means of which the surface temperature produced in the heat treatment process was detected and accurately measured. In this way, all the specific process parameters studied were controlled during tests. The parameters analyzed in the study and their variations were: (1) laser beam velocity (v), variable between 2 and 5 mm/s; (2) surface temperature (T), variable between 1373 and 1673 K; (3) impact angle (\(\varphi\)), variable between 30\({}^{\circ}\) and 150\({}^{\circ}\). Furthermore, since there is no contact between the beam diode and the hardened specimens' surface, the authors considered that any robot vibrations can be valued as negligible with respect to the process parameters studied. Each laser-hardened specimen was etched and polished at the Ljubljana Institute of Metals and Technology (IMT) and then studied with a JSM-7600F SEM microscope produced by JEOL Company at the Ljubljana Jozef Stefan Institute (IJS). The impact angle parameter is expressed with two corresponding notations: the first notation (angles indicated as \(\varphi\), as shown in Figure 1b) fixes the origin on the side of the hardened surface and considers the angles as positive because they are generated by a counterclockwise rotation of the laser beam. The second notation (angles indicated as \(X_{3}\)) reports positive values when the horizontal component of the laser beam agrees with the forward velocity of the laser beam (Figure 1c) and negative values when the above-mentioned component is in opposition to the forward velocity (Figure 1d). Therefore, the angular value of \(\varphi=150^{\circ}\) according to the first notation is equivalent to the angular value of \(X_{3}=-30^{\circ}\) according to the other notation and \(X_{3}=90^{\circ}\) coincides with \(X_{3}=-90^{\circ}\). The second notation is useful to highlight whether the laser beam agrees with or is in opposition to the forward velocity and will be used below. The changes in the impact angle were forward \(\varphi\in[90^{\circ}\), \(150^{\circ}]\) and backward \(\varphi\in[30^{\circ}\), \(90^{\circ}]\). In a preliminary sensitivity analysis, it was found that the above-mentioned parameters had the strongest impact on the final surface geometrical microstructures. Using JSM-7600F, the field emission scanning electron microscopy (SEM) identified the images which were employed to characterize the material microstructure both before and after the implementation of the surface treatment. The microstructure of laser heat-treatedsteel was clearly identified through the initial solution with which the specimen surface was arranged and etched. The magnification used was 5000 (Figure 2). The analysis of the surface microstructure was carried out in the proximity of the laser heat-treated zone classified as parent material (PM), positioned outside the laser affected zone (LAZ) (Figure 1b). Within the LAZ microstructure, partially transformed martensite was traced at the boundary with the martensite transformation being complete at the  
Figure 1: (**a**) RV60-40 robot and laser-hardened specimen.