PSI - Issue 34

Raffaele Sepe et al. / Procedia Structural Integrity 34 (2021) 172–177 R. Sepe / Structural Integrity Procedia 00 (2021) 000–000

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standard). This material was chosen for its numerous applications thanks to a favorable combination of mechanical performances, wear and corrosion resistance, see Davis (2014). Plates were manufactured in nitrogen atmosphere, with residual oxygen content below 1%, by means of a commercial EOS M270 laser sintering machine operating in full-melting mode. A solid-state disc laser, mounted onto a 6-axis robot for the laser head positioning, was used for the LBW process. The welding line was equipped with a patented device (see Caiazzo et al. (2015)), supplying Ar shielding at 25 L/min to prevent oxidation at the top- and back-side and He at 30 L/min to remove the plasma plume. Weldment was performed along the side orthogonal to the growth direction defined by the AM process. The welding was executed as autogenous (no filler metal was needed) by setting up the optimal welding process parameters (Table 1) already found by some of the Authors in previous researches, see Caiazzo et al. (2021). Laser power and travelling speed were set up, as well as the position of the beam focus underneath the top surface of the plates (negative defocusing). The geometry of the welding bead was observed in the cross-section via light microscopy upon cutting, grinding, mirror polishing, and chemical etching. Welding imperfections were checked and compared to the corresponding allowed threshold as defined by the referred standard, see ISO 13919-1 (2019). The so obtained joints were heat treated at 650 °C for 1 h, in vacuum, to enhance their main mechanical properties, as strongly suggested by literature, see Rafi et al. (2014). Finally, seven slices of welded plates were cut perpendicular to the welding direction and consequently milled to achieve the final shapes as requested by the standards, see Figures 1-2. A total of nine specimens were tested by fatigue and two were tested statically. Static tests were carried out according to ASTM E8 standard by electromechanical testing machine Zwick Roell Z250 having a load cell of 250 kN. An extensometer with gage length of 50 mm was mounted along the specimen working section so as to measure the axial strain during the test. The specimens were tested under displacement control with a crosshead speed v = 0.7 mm/min, up to failure. The fatigue tests were carried out according to ASTM E466 under load control, with a frequency of 10 Hz, at room temperature. The specimens were tested with a servo-hydraulic testing machine INSTRON 8502, equipped with a load cell of 50 kN. A sine wave load (thus R = -1) was defined as fatigue load with an amplitude ranging from 8 to 12 kN. The failure criterion was the total failure/rupture of the specimens. Table1: Welding process parameters. Laser power [kW] Travelling speed [mm/s] Focus position [mm] 2.1 45 -1.5

Fig. 1. (a) Laser welded joints made of two AM plates; main specimen dimensions for (b) static and (c) fatigue tests.