## 1 Introduction
Residual stresses in the SLM parts are another important factor in shaping the mechanisms of fatigue damage. Their role needs to be investigated together with porosity and microstructure to determine the extent of their impact on fatigue strength as well as fatigue scatter. Another aspect of this study is the investigation of fatigue mechanisms in the VHCF range. An increasing number of components are now subjected to fatigue loading higher than the conventional fatigue-limit or the endurance limit, e.g., for aluminum alloys. The previous design criterion used these limits for structural designs, but the latest research in the VHCF region has questioned the existence of these limits [27]. It has been found that the materials continue to fail when they are subjected to stresses lower than the conventional limits, with increasing number of cycles. The conventional fatigue limit can be envisaged only as an outset to persistent slip bands (PSBs) or termed as plastic strain fatigue limit. With the availability of the high-frequency fatigue machines, e.g., ultrasonic fatigue testing system, the investigation of the fatigue mechanism in the VHCF region is important to investigate the fatigue mechanisms in that region as well as to have more reliability in designing structures. For fcc materials, like Al, where fatigue limit is non-existing, the influence of microstructural features and porosity in VHCF range and their local interactions are important to be understood [28; 29; 30]. Additionally, an important aspect to be investigated when ultrasonic fatigue behavior comes into the discussion is the influence of the test frequency. Traditionally, fatigue testing has been performed under 50 Hz. Therefore the effect of performing fatigue tests at an ultrasonic frequency as compared to conventionally used frequency is an important research aspect. The effect of frequency in the available literature has been found material-dependent. An increase in fatigue strength at ultrasonic frequency has been reported by some researchers [31; 32] for carbon steels. At the same time, no significant effect was also reported by some other researchers, e.g., [33] when tests were performed at frequencies of 150 Hz and 20 kHz. For aluminum alloys, the trend has been found similar with both types of results--significant frequency effect [34], as well as mixed effect [35] of frequency at different test temperatures. Therefore, the effect of frequency needs to be further investigated for each specific case of material, process, and test conditions. The numerical modeling of the damage process relies on the application of the phase-field method [36; 37; 38]. This up-to-date approach is advantageous when compared to the traditional strategies with regard to different aspects. The latter ones assume \(J\)-integral as the driving force of the crack propagation, which does not always match the experimental observations, as _J_-integral vanishes in some cases although the crack propagation is expected. On the contrary, a variational free-discontinuity formulation enables the simulation of crack evolution only by energy minimization with respect to the crack geometry and displacements [39; 40]. In this approach, the phase-field variable taking the values in the range [0,1] is introduced to describe damage state. The simulation of fatigue by using the phase-field strategy has recently started [41]. The preceding synopsis about the aspects related to SLM process parameters influencing their part and mechanical properties unfold several specific guidelines. However, the existing studies focus more on the process development side with respect to maximizing/minimizing some or the other objective criterion like porosity, microstructural development in-process and by post-process heat treatments. This study aims at developing an extended profile of processing and post-processing aspects, and their corresponding part properties, like porosity, microstructure, and residual stresses for the selected Al-Si alloy.