PSI - Issue 7

Theo Persenot et al. / Procedia Structural Integrity 7 (2017) 158–165 Persenot et al. / Structural Integrity Procedia 00 (2017) 000–000

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investigated : 30 and 45 min. The specimens were rinsed under water immediately after being removed from the etchant in order to eliminate any trace of acid.

2.3. Material characterization

Laboratory X-ray tomography was used in order to obtain a 3D characterization of the samples. The scans were performed with a phoenix | x-ray v | tome | x laboratory tomograph using a 2.5 µ m voxel size, i.e. a spatial resolution of ∼ 4 µ m . In order to visualize the entire height of the cylindrical area, 5 scans were performed along the sample gauge length. The scanning time per sample was 1h (720 projections per scan). A standard filtered back projection method was used to reconstruct the 3D images (phoenix datos x software). Porosity level was obtained through thresholding of the grey level images ; the pores with a volume smaller than 8 voxels (2*2*2 cube) were discarded from the analysis as such features might correspond to noise in the reconstructed images. Optical microscopy was used to characterize the microstructure. Slices from the as-built EBM samples were me chanically polished using SiC paper down to 2500 grit and a 90% colloidal silica suspension and 10% H 2 O 2 (30 %) solution. Before observation, the polished sections were etched with Kroll’s reagent (2% HF, 4% HNO 3 ) in order to reveal the microstructure. Fatigue tests were carried out on a servo-hydraulic Instron 8516 machine. Constant stress amplitude tests (stress ra tio R = 0.1) were carried out at room temperature with a 10Hz frequency (sinusoidal wave form). The cyclic maximum and mean stress levels were calculated based on the average section measured from the tomographic scans. Fatigue tests were performed on as-built and etched samples until failure or stopped after 10 7 cycles (except for two etched samples which were stopped after 2.10 6 and 3.10 6 cycles). The tensile yield strength of as-built samples was found equal to 780MPa.

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Fig. 2. Pore size distribution in as-built samples

3. Results and analysis

3.1. Material characterization

The pore volume fraction in as-built samples obtained from the 3D images is of the order of 0.024% (Standard Deviation (SD) = 0.004%). This low value confirms the possibility of making very dense bulk samples using EBM. Figure 2 shows that more than 80% of the pores have an average equivalent diameter smaller than 30 µ m . According