PSI - Issue 24

Jacopo De Nisi et al. / Procedia Structural Integrity 24 (2019) 541–558

542

Paolo Folgarait et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Stainless Steels are nowadays used in almost every application field. Thanks to their peculiar combination of properties - namely, strength and corrosion resistance – that made their fortune since its discovery in the early 19 th century, they are adopted in automotive (Rufini et al. (2018), Podder et al. (2013)), construction and building (Badoo (2008), Corradi et al. (2018)), energy (Di Schino et al. (2006), Di Schino et al. (2019)), medical (Tahla et al. (2013) and food (Boulaneè-Petermann, (1996), Bregliozzi et al. (2004), Di Schino et al. (2002)) applications. Additive Manufacturing (AM), also known as 3D-printing, is a relatively new class of technologies that differ from conventional manufacturing for its underlying concept. AM process accomplishes components’ production by adding material instead of subtracting or forming it (according to ISO/ASTM 52900-15 Standard), without using specifically developed tools (e.g. molds): this approach resulted in a completely new production paradigm, see DebRoy et al. (2018) and Poprawe et al. (2015), that brought AM to success. First-developed AM systems were using polymeric materials and a major boost to the spread of technology came from the possibility to process also metal alloys: this feature was appealing for almost every industry sector. Main benefits coming from AM adoption are the possibility to produce customized components with complex geometrical features if needed, even in single or small batches, with little or no-extra cost, (see Yang et al. (2017)). Among metal AM technologies, Laser-based Powder Bed Fusion (L PBF) reached the widest diffusion and, consequently, attention in development and physics understanding. L-PBF allows to obtain relative densities (with respect to conventionally manufactured alloys) up to 100% (see Zitelli et al. (2019)) with the best geometrical and dimensional tolerances. L-PBF-produced components are already employed in the aerospace industry (see Kumar et al. (2017) and Kellner (2017)), biomedical industry (Zhang and Attar (2015)), automotive and hydraulic components (see Meboldt and Klahn (2018)). At the same time, L-PBF is correlated to some typical defect inside processed alloys which limits its usage (Zhang et al. (2017)), indeed non-proper parameter setting results in insufficient alloy density, deformations and cracks due to arisen residual stresses. The peculiar nature of LPBF process, see King et al. (2015) for deep diving into physics-related aspects, determines different mechanical properties along different testing directions. Moreover, EOS systems install a stationary laser whose spot position is controlled by a galvanometer: the described approach could result in spot defocus, that in turn could cause precision loss while rastering the powder bed. Inside EOS equipment defocus is avoided thanks to the presence of a F-theta lens, guaranteeing constant density and geometrical tolerance along the whole building area. Scope of the performed work is to assess metallurgical and mechanical properties of a Precipitation Hardening (PH) stainless steel alloy processed through L-PBF, aiming at evaluating its suitability for operating conditions comparable to those applied to an analogous alloy from conventional technologies (e.g. rolled, forged). Contemporarily, samples to be tested were located and oriented, inside the building chamber, in order to assess also system capability to guarantee uniform alloy properties all over the building area.

Nomenclature AM

Additive Manufacturing DMLS Direct Metal Laser Sintering L-PBF Laser-based Powder Bed Fusion PH Precipitation Hardening SLM Selective Laser Melting

2. Materials and methods

Experimental work has been performed on 15-5 TM PH alloy (according to UNS S15500 Standard or X5CrNiCuNb15-5) samples obtained from virgin metal powders produced by gas-atomization (using Argon) supplied by EOS Gmbh (with the commercial name of PH1), with chemical composition in the range reported in Table 1 and maximum particle size 63μm. 15-5 TM PH (Fe-Cr-Ni) is a precipitation hardening martensitic stainless steel, delta ferrite-free compositional modification of 17-4 TM PH, containing less Chromium and slightly higher Nickel.