**3. Results**

## *3.1. Microstructural Analysis*

In the untreated condition, see Figure 4a, one can identify pores and grain boundaries, also detected in [56]. The post treated conditions differ from the as-built condition, as significant changes in the microstructure are detected. Grain boundaries are no longer clearly visible, and precipitates are formed within the microstructure. This is observed for both post treatments; see Figures 4b and 5. By virtue of the heat influence, the post treatment causes melt pool boundary softening, implying microstructural evolution and precipitation [57]. Additionally, the porosity and the maximum extension of pores are significantly decreased for the HIP condition, also detected in [58] and published within previous work on this topic in [35].

**Figure 4.** Microstructural analysis. (**a**) Microstructure of the AB condition [35]. (**b**) Microstructure of the SA condition.

The changes to the microstructure found in the conditions with a heat-treatment above 500 °C are investigated in detail. Iron-rich precipitates and silicon agglomerations are detected; compare [59]. These microstructural features are also found in [27] for both the HIP and SA conditions. A performed EDX-analysis on a Fe-rich precipitate, the spot marked as 'a' in Figure 5, shows a chemical composition (Al70.24Si15.24Fe14.32) that calculates to Al5Si1.1Fe1.02 and is similar to the *β*-phase Al5SiFe, reported and found in [60–62]. Due to the elevated temperature above the solubility temperature, silicon crystals are precipitated at the grain boundaries which grow to their respective size throughout the subsequent annealing [37,38,63]. An analysis at spot 'c' confirms the labelled agglomerations as Si-particles that are well reported in [64,65]. The detected microstructural features decelerate the long crack growth. The crack front interferes with these microstructural features, and the propagation is obstructed and forced to change its direction, whereby the overall resistance against fatigue crack growth is enhanced. The improved resistance against crack propagation is attributed to deflection and energy dissipation at the crack tip [25,66]. Within this study, this microstructural behavior is observed for the HIP and the SA condition; compare [35]. After the post treatment, the base material in area 'b' shows a chemical composition of Al94.27Si5.73, which differentiates to the as-built matrix due to precipitation.

**Figure 5.** Microstructure in post treated condition including EDX analysis.

## *3.2. Residual Stress Measurement*
