## 3 Results  
The microstructure of the as-built crank arms for the tested material conditions is presented in Figure 6a. This figure illustrates the main features of the as-built (AB) condition. Overall, this material condition exhibits a supersaturated Si cellular-dendritic microstructure, where aluminum is surrounded by a fibrous network of silicon. This outcome has already been confirmed in a previous study by the authors [19] and by other research teams [20]. The low-temperature stress-relieving heat treatment strategy (HT) did not introduce macroscopic alterations due to the moderately low thermal exposure, keeping the cellular substructure of the AB condition (Figure 6b). However, the high supersaturation of Si and Mg atoms in the Al lattice led to the development and coarsening of \(\beta\)-Mg\({}_{2}\)Si forerunner phases (red arrows) and Si precipitates inside the \(\alpha\)-Al cells, compared to the microstructure observed in as-build samples [21; 22; 23]. The EDX map of Al, Si, and Mg  
Figure 5: **(a)** Fatigue tests device and **(b)** NP EN ISO 4210-8:2019 standard proposal [15]. elements (Figure 6c) showed the main alloy element distribution that verified a similar distribution in both cases. The previously described morphological modifications can be associated with a decrease in hardness in the HT condition compared to the AB condition. For the AB crank arms, the average hardness was HV\({}_{0.1}\) = 165.6, while for the HT ones, the average hardness value was HV\({}_{0.1}\) = 115.3. These values represent a decrease of approximately 30% compared to the AB condition, similar to the results presented in [4]. As a result of this reduction in hardness, lower monotonic tensile properties were observed, along with increased ductility in the heat-treated condition. Table 2 summarizes the related mechanical properties [9]. Regarding the fatigue results, three crank arms from each condition were tested, and it was verified that three of the as-build crank arms failed in the region where the maximum load was achieved in the FEA (see Figure 3). They failed before reaching the target of 100,000 cycles, being 94,312 the highest number of cycles achieved. In contrast, all the heat  
Figure 6: SEM and EDX images of the cellular structure in transversal cross-section: As-build condition (**a,b**) heat-treated condition, (c) EDX map of Al, Si, and Mg elements. treated crank arms managed to reach 100,000 loading cycles without fracturing. Table 4 can be consulted as a resume of fatigue tests. Concerning the fractography analysis, Figure 7 shows a macrograph highlighting the crack initiation zone in the as-built crank arms. The crack initiation occurred in the same region where the maximum stress was achieved in the FEA (see Figure 3). Figure 8 displays an image from the normal plane of the crack initiation and propagation regions. In this figure, the crack initiation zone and the entire crack growth region can be observed up to the fragile fracture (blue arrows). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Condition**} & \multicolumn{2}{c}{**Standard**} & \multirow{2}{*}{**Number of Cycles**} & \multirow{2}{*}{**Number of Cycles**} & \multirow{2}{*}{**Number of Cycles**} & \multirow{2}{*}{**Load (N)**} & \multirow{2}{*}{**Requirements Achieved**} \\  & \multicolumn{2}{c}{**(Number of Cycles)**} & & & & & \\ \hline As-build & 100,000 & 65,924 & 73,312 & 94,312 & 1300 & No \\ \hline HT & 100,000 & \
\begin{tabular}{c} 100,000 \\ (without fail) \\ \end{tabular} &
\begin{tabular}{c} 100,000 \\ (without fail) \\ \end{tabular} & 125,549 & 1300 & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fatigue tests resume. Figure 7: Crack initiation position in the as-built crank arm. A detailed observation of the SEM images presented in Figure 9a,b clearly shows that the crack initiation was localized in a superficial porosity; these are representative of both cases (as-build and HT). In Figure 9b, unfused particles (blue arrow) and internal porosities (yellow arrows) can also be observed.