PSI - Issue 19

Wolfgang Schneller et al. / Procedia Structural Integrity 19 (2019) 556–565 Author name / Structural Integrity Procedia 00 (2019) 000–000

558

3

The specimens, which are intended to be machined after manufacturing, are built with a certain machining allowance in order to subsequently remove the as-built surface and eliminate surface related effects. The final geometry of the specimens is shown in Figure 1. Partially, some of these results are already published in [16,17].

Fig. 1. Specimen geometry for HCF testing

The fatigue testing is performed at a load stress ratio of R=-1 on a resonant testing rig with a frequency of about 106 Hz. The abort criterion is defined either as total fracture or as run-out at 1e7 load cycles. Specimens are gripped using collets. Run-outs are reinserted at higher stress levels to obtain more data in the finite life regime, conservatively not ruling out pre-damaging at stress levels lower than the endurance limit, as suggested by the authors in [18]. 3. Results and discussion 3.1. Microstructural analysis The impact of the heat influence on the microstructural evolution regarding the post treated test series is characterized by performing a microstructural analysis. A HIP treatment, as well as SA is performed above the solubility temperature of AlSi10Mg [19]. SEM images of microsections of the AB, HIP and SA conditions are taken with a Carl Zeiss EVO MA 15 microscope. Figure 2.(a) shows the microstructure of the AB condition in which pores and grain boundaries can be detected. In comparison, Figure 2.(b) represents the microstructure of the HIP condition, in which such boundaries are no longer visible because of the heat input, this is also observed for the SA condition. Fe-rich precipitates and Si-particles, which are also detected in [20,21], are formed during the post treatment for the HIP as well as the SA condition. Microstructural features like Silicon agglomerations and needle-shaped, Fe-rich precipitates obstruct a propagating fatigue crack. For that reason, the resistance against fatigue crack growth is generally improved. Such microstructures favor crack deflection and energy dissipation at the crack tip, whereby the long crack growth is decelerated and consequently the fatigue strength enhanced [22,23]. Utilizing image-processing tools, the microsections are converted to binary pictures in which pores remain as black spots within a white matrix. This enables the assessment of the porosity and maximum pore extension. A comparison of the investigated conditions reveals that the porosity as well as the maximum pore extension is decreased for the HIP and SA condition, referring to the AB condition.