## 4 Discussion  
L-PBF is a highly advanced metal additive manufacturing technology used to produce complex metallic components layer by layer that have a fine microstructure due to the rapid cooling rates involved in the process. Despite its advantages, including the ability to create intricate geometries and reduced material waste, L-PBF is susceptible to certain defects, such as porosity and surface roughness, which can significantly affect the fatigue life of components [26]. Drawing a parallel with this study, the L-PBF process produces a fine cellular-dendritic microstructure, resulting in increased strength compared to cast material, as confirmed by [PERSON] et al. [3]. However, the application of the heat treatment led to the coarsening of the Si-network and partial Si precipitation within the \(\alpha\)-Al cells, which reduced the mechanical properties of the alloy. The coarser Si network and partial Si precipitation weakened the grain boundary, solid solution, and dislocation strengthening mechanisms. Although some improvement from precipitation strengthening was expected, its effect was marginal compared to the overall reduction in other strengthening mechanisms, as observed by [PERSON] et al. [21]. The process-induced defects introduce stress concentrators, which lead to premature crack initiation and growth, ultimately reducing the overall fatigue resistance of the material. Porosity refers to the presence of small voids or cavities within the material. In L-PBF, these voids typically form due to incomplete melting, trapped gas, or unstable melt pools. Porosity in L-PBFed parts can be categorized into two types: gas-induced porosity and lack-of-fusion porosity [27]. Gas-induced porosity occurs when inert gases such as argon or nitrogen, used in the L-PBF process or hydrogen of atmospheric air, become trapped within the melt pool and form gas pockets upon solidification. These usually present a spherical shape. Lack-of-fusion porosity, on the other hand, results from insufficient energy input, causing poor fusion between successive layers or adjacent powder particles. These pores are irregular and often more detrimental due to their sharp crack-like shapes. In terms of fatigue performance,  
Figure 10: Crack initiation position in the as-built and heat-treated crank arms. porosity acts as a stress concentrator, where local stresses significantly exceed the average applied stress [28]. Under cyclic loading, these stress concentrations become sites for fatigue crack initiation. Once a crack initiates at a pore, it can propagate rapidly under repeated loading, reducing the component's fatigue life. The larger the pore size and the closer the pores are to the surface, the more severe their impact on fatigue life. Pores near the surface are especially critical due to the free surface effect, which causes higher stress concentrations [29]. This issue is particularly problematic in high-cycle fatigue (HCF) regimes, where the component is subjected to a large number of stress cycles at relatively low-stress amplitudes. In such cases, fatigue life is heavily dependent on the initiation phase, and rough surfaces lead to rapid crack initiation due to the localized stress concentration. As shown in Figure 9, the presence of pores, mainly gas-induced porosity, and the surface roughness have contributed to an early failure of the crank arm. The combined presence of porosity and surface roughness due to unfused particles in L-PBF components creates a compounding effect that further shortens fatigue life. Porosity located near the surface exacerbates the impact of surface roughness by introducing larger and deeper stress concentrators. On the other hand, the heat treatment allowed the crank arm to reach 100,000 cycles, which can be attributed to the reduced sensitivity of the heat-treated condition to micro-notches (defects), as already confirmed by [PERSON] et al. [12].