PSI - Issue 41

Danilo D’Andrea et al. / Procedia Structural Integrity 41 (2022) 199–207 D’Andrea et al./ Structural Integrity Procedia 00 (2019) 000–000

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4.2. Microstructure The XRD spectra obtained on the traditional alloy sample and on the AM sample are shown in Fig. 4. The models clearly show that both the traditional samples and the AM sample are characterized by the sole presence of the FCC austenite phase. However, as can be seen by comparing XRD spectra, the difference in intensity of the peaks is due to the analysed area of the sample, which in the case of the AM sample is smaller, and therefore the effect of the fluorescence caused by the iron is lower.

Fig. 4. XRD comparison between AM and traditional sample

The tensile fracture surfaces of specimens obtained by traditional processing and AM show a markedly different morphology. By comparing the microscopies obtained with the scanning electron microscope (SEM), it is possible to highlight for the traditional specimen the typical cup-cone fracture surface characterized by the existence of the dimple-like parabolic structures typical of ductile fracture mode. (Fig. 5a), While for the AM specimen there are significant defects, and in particular circular voids and numerous un-melted powders, ranging from about 5-50 μm, as shown in Fig. 5b, And which affect the mechanical properties of SS316L, such as ductility, toughness, and corrosion resistance (Kurgan and Varol (2010)). Furthermore, these defects can represent trigger points for fractures when subjected to static and even more dynamic stresses. As can be seen from Fig. 5, the crack is generated starting from the discontinuities due to the incorrect melting of the powders, inducing stress concentration and consequent failure. In fact, at higher magnifications (Fig. 6), very fine dimples were observed, typical of ductile failure. Spherical metal particles that were not melted during manufacture were also observed within the pores. 5. Conclusions In this work static tensile tests have been performed on traditional and AM specimens of AISI 316L. Infrared thermography has been adopted to evaluate the temperature trend during static tensile tests, applying the STM, and to retrieve the limit stress of the material. It can be though as the macroscopic stress level that introduce in the material the first irreversible damage. Such damage initiation leads to a deviation from the linear trend of the thermoelastic law during static tensile test. The AM specimens show lower mechanical properties compared to the traditional material, due to their microstructure, characterized by several defects (unmelted powder, stress