## 3 Results and Discussions
### HCF and VHCF Behavior  
The damage behavior that was investigated by load increase tests with the measurement of plastic strain amplitude (Figure 8a) is shown in Figure 8b. As-built hybrid specimens without post-build stress-relief were not tested under load increase tests due to the fracture mechanism activated at the joint interface. In the response of cyclic loading in terms of plastic strain amplitude, the sensitivity of the damage development under different conditions can be evaluated. Generally, all the batches show two different regions of response starting at the quasi-constant magnitude of plastic strain amplitude which increases slightly as the microcracks start initiating in the structure. After a critical value of stress amplitude, in this case around 110 MPa, the response parameter increases exponentially showing the growth of the initiated crack until failure. Therefore, the constant amplitude tests performed after this critical stress amplitude will allow for mechanism-based understanding of the fatigue phenomenon under different processing conditions. \begin{table}
\begin{tabular}{c c c} \hline
**Test Mode** & \(\boldsymbol{\sigma_{\text{UTS}}}\) **[MPa]** & \(\boldsymbol{\varepsilon_{\text{t,max}}}\) **[10\({}^{-2}\)]** \\ \hline Interrupted & 324.10 \(\pm\) 23.10 & 6.36 \(\pm\) 1.29 \\ Continuous & 361.10 \(\pm\) 4.50 & 4.05 \(\pm\) 0.15 \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of tensile properties in interrupted and continuous tests. Figure 7: Damage state in no-load condition (stage 0), at two stages during the tensile test (stage 1, stage 2), and after fracture (stage 3) represented by interrupted CT scans. Further constant amplitude tests were performed at a stress amplitude of 120 MPa for the same processing conditions, and Figure 9 elaborates their cyclic deformation behavior. From the pure Al4047 conditions, the as-built batch A endured the longest fatigue life conforming to the life obtained in load increase test in Figure 8b. After stress-relief in batch B, the fatigue life was reduced, and the reduction in fatigue life continued further for the batch C built with base plate heating. All of these three conditions of pure Al4047 (batch A-C) experience an initial rise in plastic strain amplitude followed by a drop up to around 60 load cycles. The following behavior can be explained by the process-related structure. Very small cyclic softening is experienced in the as-built condition (batch A), and the proceeding changes in the damage behavior are only minor until it increases in the second stage of cyclic softening towards the final stage of fatigue life. As the microstructural features in this condition are very thin, as shown by the dendritic width of 0.29 \(\upmu\)m in Figure 9b, which offer high resistance to dislocation movement in the structure resulting in very small deformation, i.e., at plastic strain amplitude of 0.06 \(\times\) 10\({}^{-3}\) at fracture. After post-process stress-relief in batch B, having a dendritic width of 0.35 \(\upmu\)m, the strain at fracture increased to 0.09 \(\times\) 10\({}^{-3}\), and an observable cyclic softening can be seen. In the specimen processed with base plate heating (batch C), the plastic strain amplitude at fracture increased to 0.14 \(\times\) 10\({}^{-3}\) where a dendritic width of 0.56 \(\upmu\)m allowed for plastic deformation in the material and the resulting fatigue life decreased. For the hybrid configuration (batch E), the  
Figure 8: (**a**) Schematic representation of used load increase test with accompanied measurement of plastic strain amplitude (\(\epsilon_{\text{a,p}}\)); and, (**b**) the resulting damage behavior of the investigated batches. Figure 9: (**a**) Cyclic deformation behavior of the investigated batches at 120 MPa stress amplitude; and, (**b**) cyclic fracture strain (\(\epsilon_{\text{a,p}}\)) as a function of dendritic width. plastic strain amplitude remains below in all phases of fatigue life as compared to the pure Al4047 configurations.