PSI - Issue 2_A

W. Reschetnik et al. / Procedia Structural Integrity 2 (2016) 3040–3048 W. Reschetnik et al. / Structural Integrity Procedia 00 (2016) 000–000

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Fig. 7 shows optical micrographs of the aluminum alloy EN AW-7075 after SLM processing. Arch-shaped lines resulting from an equally shaped melt pool during processing are visible, see Fig. 7 a. Solidification cracks were observed due to the high crack sensitivity of the aluminium alloy, Figures 7 b and 7 c. Alloying elements such as magnesium and copper can lead to a relatively large range of solidification temperature compared with binary alloys and increase potential cracks which leads to an early crack initiation under loading.

Fig. 7. Optical micrographs of the aluminium alloy EN AW-7075 after SLM processing: (a) cube region (polished and etched) with arch-shaped lines; (b) cube region (polished and etched) with cracks; (c) cube region (polished) with cracks.

These cracks seem to be a reason for the weak mechanical properties in the presented work, cf. Table 3. Moreover, the process parameters i.e. laser power, hatch distance and scan speed play a major role in affecting the mechanical properties. Consequently, different process parameters on the EN AW-7075 powder and possible post treatments for the improvement of the mechanical properties have to be investigated in future work. Figures 8 a and 8 b show scanning electron microscope (SEM) images of the crack surfaces after tensile tests for heat treated specimens, which were built horizontally (Fig. 8 a) and vertically (Fig. 8 b) regarding to the building platform.

Fig. 8. Scanning electron microscope images of the fracture surfaces after tensile tests for heat treated specimens, which were manufactured with load direction (a) perpendicular and (b) parallel to the building direction.

In Fig 8 a, a crack propagates nearly perpendicular to the direction of the applied load. This perpendicular fracture leads a relatively flat surface at the break. The crack grows through the grain of the material and chooses the path of the least resistance. Therefore, it seems to be transgranular brittle fracture. In addition, the vertically built specimen (Fig. 8 b) shows dimples area after tensile test in comparison to the horizontally built specimens, which