PSI - Issue 37

G. Macoretta et al. / Procedia Structural Integrity 37 (2022) 632–643 G. Macoretta, B. D. Monelli / Structural Integrity Procedia 00 (2019) 000 – 000

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2.4. Metallographic and fractographic examinations Microstructural investigations were carried out both in longitudinal and transverse directions. Specimens were prepared following the ASTM E3-11 standard and were etched with Kalling’s II etchant (n.94 , ASTM E407). The specimen porosity was investigated on the basis of sections extracted both in longitudinal and transversal directions, namely sections containing or orthogonal to the specimen axis respectively, and observed by an optical microscope. By using the software ImageJ the material porosity was characterized in terms of average material density and pore size and shape distribution. Per each set of process parameters, a set of 20 metallographic sections extracted from the gauge length of 3 different specimens per each set of parameters were investigated, both in the longitudinal and transversal direction. Scanning Electron Microscopy (SEM) analyses were carried out at the “Centro per l'Integrazione della Strumentazione Scientifica - Università di Pisa” (CISUP) using a FEI Quanta 450 FEG -SEM equipped with an EDX spectrometer Bruker QUANTAX XFlash Detector 6|10. Fractographic analyses were carried out by stereo microscope as well. 2.5. Surface roughness measurement The surface roughness was measured both by a profilometer, Jenoptik Waveline W812R, and by extracting the specimen profile from sections cut in the specimen longitudinal direction, prepared in compliance to the metallographic examination techniques, examined by optical microscope with a 10x magnification factor. The images were thus elaborated by using the software ImageJ and Matlab®, as shown in Fig. 3. The latter technique was introduced with the aim to investigate in detail the surface irregularities produced by the SLM process, in particular the deep and steep valleys that can act as severe notches and thus produce significant stress concentration. As the length of the sampled profiles was equal to 0.8 mm and the images were extracted parallel to the specimen longitudinal direction, it can be assumed that the contribution due to longwave components of the specimen profile is rejected.

Fig. 3. Extraction of the specimen profile from section cut in the longitudinal direction

3. Results and discussion 3.1. Porosity

The baseline set resulted to produce an almost full-dense material, having a density of 99.97% if examined from sections extracted in longitudinal directions and 99.95% in the transversal direction, due to the presence of some