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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1. Introduction Contrarily to subtractive manufacturing methods, Additive Manufacturing (AM) techniques bring the opportunity to fabricate lightweight components with optimized sizes and shapes, complete customization and lightweight properties, as requested in many industrial fields, e.g. see Paoletti (2017), Citarella et al. (2021) and Borrelli et al. (2020). At present, AM has moved from the fabrication of specimens and prototypes to functional metal parts for direct industrial applications. The unprecedented opportunities given by AM technologies in terms of design freedom triggered noteworthy progresses of topology optimization procedures in many application fields with the ending purpose of redesigning the existing mechanical components e.g. see Armentani et al. (2020), Usera et al. (2017). Despite the presence of numerous AM processes, Electron Beam Melting (EBM) and Laser Powder Bed Fusion (LPBF) can be considered as the more mature technologies currently existing, having nowadays numerous applications in industry such as aerospace (Caiazzo et al. (2013)) and automotive (Caiazzo et al. (2017), Sepe et al. (2020a) and De Luca et al. (2021)). One of the disadvantages of these technologies is given by the unfeasibility to fabricate large components due to the limited sizes of the building chamber, or to the large deformations and residual stresses caused by the thermal gradients occurring during the building, see Wang et al. (2009). Increasing the size of LPBF or EBM machines is challenging and expensive, due to the need for accurate calibration of the electron or laser beams over larger areas, and/or by the controlled vacuum or atmosphere required inside the chamber to prevent oxidation, see Prashant et al. (2017) or Sun et al. (2019). Some recent researches were devoted to investigate the potential application of the nowadays existing joining technologies, already mature for traditional materials and processes, to AM parts, see Gill et al. (2020). There is currently a growing interest towards flexible methods, such as Laser Beam Welding (LBW), thanks to its limited energy source supplied during welding, to the reduced Heat-Affected Zone (HAZ) dimensions and reduced distortions of the final components, see Blackburn (2012), Boccarusso et al. (2017) and Sepe et al. (2020b). In addition, the welding efficiency is higher if compared to conventional methods, see Steen et al. (2003). The investigation reported in this document was aimed at evaluating the potential usage of LBW to join AM parts made of 17-4 PH steel and manufactured by LPBF so as to obtain larger products. In particular, Selective Laser Melting (SLM) technology was adopted to manufacture four 3 mm thick plates by using the optimized set up in terms of welding process parameters already found by some of the Authors, see Caiazzo et al. (2021). 14 slices were cut out from the two welded joints and specimens, were machined according to the standards ASTM E8 and ASTM E466 for static and fatigue tests respectively, see ASTM E8-21 (2021) and ASTM E466-21 (2021). Static tests were carried out with an electromechanical testing machine Zwick Roell Z250, whereas fatigue tests were carried out with a servo-hydraulic testing machine Instron 8502 that applied a sine wave load with load ratio R of -1. Finally, stress strain curves were measured whereas stress-life data were arranged in a Wohler curve so as to highlight the endurance limit for the welded specimens.

Nomenclature A

Nominal specimen cross-section;

Probability of survivals;

P s

Quasi-static load;

Load ratio;

F

R

Step slope of S-N curve;

Scatter of the stress amplitudes;

k

T σ

Cycles;

Axial strain; Axial stress;

N

ε σ

Cycles to rupture; Fatigue load amplitude;

N R

Stress amplitude;

P

σ a

2. Materials and Methods Six plates having sizes of 150 x 70 x 3 mm × mm × mm were manufactured, with the building direction shown in Figure 1, by using a commercial pre-alloyed nitrogen-atomized EOS GP1 powder with 36-μm mean grain size and chemical composition matching the 17-4 Precipitation Hardenable stainless steel (according to UNS S17400