###### Abstract  
This study proposes a genetic algorithm-optimized model for the control of the fatigue life of AISI 1040 steel components after a high-power diode laser hardening process. First, the effect of the process parameters, i.e., laser power and scan speed, on the fatigue life of the components after the laser treatment was evaluated by using a rotating bending machine. Then, in light of the experimental findings, the optimization model was developed and tested in order to find the best regression model able to fit the experimental data in terms of the number of cycles until failure. The laser treatment was found to significantly increase the fatigue life of the irradiated samples, thus revealing its suitability for industrial applications. Finally, the application of the proposed genetic algorithm-based method led to the definition of an optimal regression model which was able to replicate the experimental trend very accurately, with a mean error of about 6%, which is comparable to the standard deviation associated with the process variability. laser hardening; diode laser; genetic algorithm; fatigue life 2020 10 1401 10.3390/app10041401 10.3390/app10041401 10.  
which facilitates their manipulation [5; 6; 7]. In fact, lasers can provide localized surface heating. Part of this heat is transferred to the workpiece, leading to an increase in its surface temperature and resulting, therefore, in its austenitization. In fact, alternating heating and cooling phases very quickly on the surface can transform it into martensite, which is harder than the bulk material [8; 9]. Moreover, with a laser, the heat input can be controlled very precisely and selectively on the surface of the components, due to the ability to accurately define the illuminated area, together with the short timescale of energy transfer into the material [10]. This ensures a reduced and localized deformation of the part, therefore limiting, and eliminating in the best case, any further post-processing operation [10; 11]. However, it must be noted that there are two main sources of residual stresses during the heat treatments of steels: (i) stresses caused by the transformation of the austenite; (ii) thermal stresses caused by the different cooling rates and strain incompatibilities [12]. In particular, the resulting stresses are of the compressive type, which lead to the improved hardness of the surface [13]. In fact, the internal stresses are limited only to the depth at which the martensitic transition occurs, thus resulting in the limited distortion of the component. Among the different laser systems available today, high-power diode lasers (HPDL) are particularly suited for the hardening treatments due to the rectangular or elliptical shape of the laser beam profile, which is able to ensure a uniform heating on large areas of the treated surface [14; 15]. This is an essential requirement for the laser treatment to be effective [16]. In fact, typical industrial CO\({}_{2}\) lasers are not able to produce an energy flux into the stainless steel, and metals in general, high enough to promote the hardening process [17]. This is usually due to the reflection of the metal materials at the CO\({}_{2}\) wavelength. However, the application of a coating on the component to be heat treated can increase the absorptivity at the CO\({}_{2}\) wavelength [18]. But this introduces additional costs to the process both for its application and the subsequent removal. However, the use of high-power diode lasers avoids coating the surface of the workpiece because they produce wavelengths in which metals show greater absorptivity [17; 19; 20]. In thermal-based surface treatments, as it is for laser hardening, the main target is to achieve the requested mechanical performance, e.g., strength, hardness, wear resistance, etc., while minimizing thermal deformation. However, very often, these requirements are mutually exclusive and, therefore, the optimization of the process becomes a challenging task.