## 1 Introduction
Many researchers have investigated the quasistatic mechanical behavior of SLM-processed alloys, and it is found that their quasistatic tensile strength is comparable to their cast or wrought counterparts, while surpassing in some instances, e.g., [10; 13; 14]. The maintenance or increase in tensile strength is attributed to several factors, with the fine microstructure being the most dominant one for all of the materials. This feature has an accompanying aspect of a decrease in ductility due to a decrease in microstructural size. The fatigue behavior of SLM parts has been intensively investigated in the last few years. It has been found that the fatigue mechanisms in these parts are significantly influenced by the process-incited aspects, like remnant porosity, microstructure, roughness, and residual stresses. The few existing studies on fatigue behavior of SLM materials--Ti-alloys [15; 16], Al alloys [1; 17; 18], and steels [19; 20] have shown that the fatigue strength of SLM-processed parts in their as-built condition is far inferior to their surface-finished parts, as the surface roughness of these parts usually lie in the range of Rz = 30 to 50 \(\upmu\)m. Process-induced differences in microstructure and properties have been investigated, as, for example, the influence of application of laser beam or electron beam melting on properties of Ti-6 Al-4V [21; 22]. Such high roughness initiates the surface crack at an early stage of fatigue life. However, after post-process surface finishing of these parts, their fatigue strength is, in general, comparable to that of their wrought counterparts. But, the reliability of fatigue strength is found to be low due to multiple fatigue mechanisms in these materials. The most influencing factor related to fatigue mechanisms is remnant porosity. Though the SLM parts have generally higher than 99.5% relative density, a small fraction of the remnant porosity drives the mechanism of fatigue crack initiation, and, therefore, the fatigue failure. In the absence of significant porosity near the part contour, surface weakness plays the dominant role in causing a fatigue crack initiation from the surface [20; 23]. In the presence of critical defects in the core, these defects are generally responsible for fatigue crack initiation, and the intensity of their deteriorating effect on fatigue depends on their location relative to the component boundary. Near the contour, pores are more influential in initiating a fatigue crack, and their propagation, until they reach the component boundary, is usually very abrupt, resulting in much-reduced fatigue life. Pores of similar size, found far from the part contour, can still survive until the fatigue crack is initiated and the part will endure longer life [15; 23; 24]. The microstructure is usually the most important factor for determination of the fatigue behavior of parts; however, its role is diminished in SLM parts under the presence of porosity. Once remnant porosity is controlled, it is the next process-specific feature influencing the fatigue mechanisms. The ultra-fine microstructure that was obtained in the SLM process poses impediment to the movement of dislocations which creates resistance to fatigue crack initiation and make these parts endure longer under fatigue loading; however, this behavior can depend on the region of fatigue life under consideration, as the grain size generally have a contrasting effect on crack initiation and crack growth. The process-specific thermal evolution influences the microstructural features, which, in turn, have their effect on crack initiation and crack propagation phases [25; 26]. For laboratory testing of specimens, the crack initiation phase is dominant in high cycle fatigue (HCF) at lower applied stresses and very high cycle fatigue (VHCF) regions at very low stresses, even below the conventional fatigue limit, and the role of porosity and microstructure and their interaction effects should be deeply investigated.