## 3 Results and Discussions
### HCF and VHCF Behavior
The fusion of the two materials at the joint of conventionally manufactured and additively melt interface has resulted in decreased plastic strain amplitude and a correspondingly increased fatigue life. The relationship of the processing conditions with the corresponding dendritic width and the related softening and hardening mechanisms explain the fatigue life conditions, showing that the processing conditions can be controlled for the required microstructural features. Complete Woehler curves were generated for the configurations with post-process stress-relief, i.e., batches B and C for the pure Al4047 and batch E for the hybrid condition. These tests were performed at 20 Hz on the servohydraulic system and at 20 kHz on the ultrasonic system, as elaborated in the legend in Figure 10. For the pure alloy condition Al4047 built by SLM process, the influence of base plate heating has been found critical under the influence of applied stress amplitude. At high-stress amplitude, finer microstructure (batch B) resulted in higher fatigue life as compared to relatively coarse microstructure (batch C) obtained when base plate heating is employed; however, as the applied stress amplitude is lowered, the trend is inversed where base plate heated specimens in batch C endured longer. Here the special feature of the SLM process, i.e., the role of remnant porosity comes into play. In general, HCF resistance is dominated by resistance to crack initiation where fine grain size offers more fatigue life; however, the difference in the remnant porosity of the two batches explains the inversed behavior at lower stress amplitudes. At a lower value of stress amplitude, the very small-sized defects remain inactive, and the mechanism of fatigue failure has more than one dimension as compared to higher stress amplitude where a single mechanism dominates the fatigue behavior. The ultrafine grains, even after base plate heating, has resulted in higher fatigue lives. But, when the existing pore size is relatively large, they become active and cause early failure, as in batch B. For the hybrid configuration of batch E, they have outperformed the pure alloy condition, as could also be seen in the result of load increase test that they offer less cyclic softening as compared to the pure alloy condition, due to better joining in the interface region of the two materials, which shifted the mechanism of fatigue failure out of the interface region. The fractures in hybrid specimens were from the SLM side. Influence of test frequency on the resulting fatigue life is an important aspect for the utilization of the ultrasonic fatigue tests. The Woehler curves in Figure 10 show that the tests performed at 20 Hz and 20 kHz show similar trend lines. Investigation of the effect of frequency was performed by using the homogeneity of variance tests using a significance level of 0.05. [PERSON]'s test with squared deviations as well as Brown-Forsythe test were conducted to calculate the statistical significance. [PERSON]'s test with squared deviations resulted in a \(p\)-value of 0.116 (F:3.995) and the Brown-Forsythe test resulted  
Figure 10: Woehler curve until very high cycle fatigue range for pure Al4047 by SLM process (with stress-relief batch B, and with base plate heating and stress-relief batch C), and for hybrid batch AI7020+Al4047 with stress-relief batch E.  
in a \(p\)-value of 0.338 (F:1.179), which shows that the \(p\)-values lie far below the significance level of 0.05. Therefore it can be concluded that the underlying null hypothesis of different fatigue lives is not acceptable and it can be concluded with statistical significance that there is no difference in the fatigue lives resulting from the two test frequencies. A square plate with two holes, shifted relative to each other, is chosen as a numerical example. It consists of AlSi12 with the parameters listed in Table 5. The plate dimensions are 100 \(\times\) 100 \(\upmu\)m and it is discretized by a mesh of approximately 7000 elements.