PSI - Issue 24

Giulia Morettini et al. / Procedia Structural Integrity 24 (2019) 349–359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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while a more uniform fatigue crack initiation from the rough edge (i.e. downward face) was observed for the horizontal specimens, Solberg (2019). The surface contour of the edge from which fatigue is initiating is highlighted in Figures 4 (c) and (d), where the maximum depth of the defect region is indicated. The maximum depths of the surface defects obtained from the fractography (ie, distance between the two dashed lines) are 192 μm and 324 µm for vertical and horizontal specimens, respectively.

Fig. 4. SEM fractography of the tested specimens; (a, c) vertical specimen and (b, d) horizontal specimen.

3.3. Residual stress

During the fabbrication of the specimens, a residual stress due to the high temperature (Yadroitsev (2014) testified a maximum temperature of 2436 °C in the Ti-6Al-4V molten pool produced by a general SLM process) correlates with a fast solidification process (cooling rate is in a range of about 104 – 106 °C/s, Gokuldoss (2017), Qiu (2017) ) during the AM process, was induced in the specimens. Although the track length and the strategy of the layer deposition have had a great influence on the residual stress, however, using an appropriate set of process parameters and supporting structure (especially in the Horizontal specimens) was possible to limit effect of the residual stress. Generally speaking, in order to produce products for load-bearing industrial application, a relaxing process by applying proper post heat treatment is required. It is a difficult task to analyze the effective residual stress, overall the peak value occurs at or near the free surface of the last deposition layer, Edwards (2014), Vastola (2016), Liu (2016). Layer by layer, infact, the residual stress increases until it reaches, in extreme cases, the yield strength of the material, Mercelis (2006). ANSYS Additive Suite is one of the most powerful simulation solution, able to predict the residual stress for metal AM component. Inserting , as a input, the process parameters and the geometric information of the specimen, it was