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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As shown in Fig. 1, these process parameters exploit the region of the upper part of the feasible region and are characterized by a higher meltpool aspect ratio, still preserving a similar safety margin on the occurrence of lack fusion phenomena. The gain in the process productivity rate introduced by the A1 and A2 parameters can be evaluated by considering the job build time necessary to print a bath of 20 prismatic bars having a cross-section of 10x10 mm and a height of 100 mm, built in a vertical direction, keeping constant the recoater time. As reported in Table 2, the job build time is almost halved in both cases.

Table 2 Job build time in function of the selected process parameters set

Set

Build time, hr

Variation, %

Baseline

13.3

Literature highest values

9.0

-32%

A1

7.3

-45%

A2

7.0

-47%

2.3. Mechanical tests Tensile tests were carried out at room temperature on an MTS servo-hydraulic machine having a load capacity of 50 kN (MTS Systems Corporation, Eden Prairie, MN USA), equipped with an extensometer featuring a gauge length of 10 mm (MTS 634.21-F25), and a data acquisition system and control unit (RT3, Trio Sistemi e Misure S.r.l., Dalmine, Italy). It was employed a circular specimen, designed in compliance with the ASTM E8-16 standard and having a diameter of 5 mm. For each set of process parameters, five specimens were printed in different locations on the build plate and tested to investigate the process repeatability. A Wöhler curve was obtained for each set of process parameters, investigating the HCF behavior in the range comprised between 10 5 and 10 6 cycles to failure. It was employed a cylindrical specimen designed in compliance with the ASTM E466 standard and having a uniform diameter along the gauge length because it allows maximizing the effects of the material inner defects or the surface roughness on the fatigue response, shown in Fig. 2. Specimens were tested in an axial loading configuration on a Rumul Mikrotron (Russenberger Prüfmaschinen AG) resonant testing machine with a loading ratio R = 0.05 and a frequency of about 150 Hz. The tests were carried out at room temperature. In all the cases, the specimens were tested in the as-built condition, preserving the surface roughness produced by the different sets of adopted SLM process parameters.

Fig. 2. Specimen employed in the HCF tests.