PSI - Issue 28

Mohamed Ali Bouaziz et al. / Procedia Structural Integrity 28 (2020) 1039–1046 M.A. BOUAZIZ et al / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction As ecological considerations and environmental laws become more and more demanding, sustainable development is gradually proposing manufacturing routes that are more environmental friendly. Green manufacturing is a pillar of sustainable development, because reducing the enormous amounts of energy and material consumption helps global warming to be limited. Additive manufacturing (AM) has an essential advantage for future developments. This technology produces very little material waste, does not require any tools, and allows for the production of several parts at the same time. AM design enjoys a unique creative freedom, thereby offering functional integration. It is possible to reduce the environmental cost with regard to the consumption of resources, energy (or pollutant emissions) upstream and during fabrication, assembly and at the end of the life cycle, during the disassembly of the product. For all these reasons and thanks to the improvements in quality of AM parts, a growing interest is observed in industry for metallic additive manufacturing processes. These processes make it possible to achieve complex geometries (e.g., internal, interlocking or lattice structures), to improve the functionality of the components (i.e., weight, ergonomics) or to consolidate larger systems by limiting their number of components. The technology has made it possible to achieve great advances in materials and metal additive manufacturing processes, and 3D printed parts now have mechanical properties that are similar or even superior to their traditional equivalents. The most used AM techniques for the production of metallic parts are powder bed fusion (PBF) and material extrusion (MEAM). These techniques were originally developed for polymeric part manufacturing, then they were adapted for direct and indirect production of metallic parts (Gonzalez-Gutierrez et al. 2018). In PBF, a laser or electron beam selectively fuses metal powders by adding cross-sectional layers on the surface of a powder bed. The powder bed is then lowered by one layer thickness, a new layer of material is deposited onto its top, and the process is repeated until the part is finished (Van Noort 2012). Minimum support, wide material choice and powder recycling are advantages of PBF, but this technique also has drawbacks. The biggest disadvantages are high power usage (since it uses a lot of energy to create parts) and the requirements in terms of process handling, which makes access to this technique reserved for qualified operators (Bibb 2006). Therefore, MEAM also known as fused filament fabrication (FFF) shows great promise as a cost-effective alternative and easy to handle process. The building material is supplied in spooled filaments. The filament is fed into a heating unit with a nozzle. The material is extruded through the nozzle on a platform that moves up and down in the vertical (Z-) direction, and the extrusion head moves in the platform (XY) plane. The idea of using highly filled polymers for additive manufacturing of metallic parts was first introduced in the 1990s; it was named fused deposition of metals or FDMet (Wu et al. 1999; 2002). Recently, Markforged developed and marketed its process called Atomic Diffusion Additive Manufacturing (ADAM) (Campbell and Wohlers 2017). It is based on an FFF principle in which highly-filled polymers with metallic particles are initially extruded as filaments, and then these filaments are selectively extruded at a temperature higher than the melting point of the polymer binder. The shaping step is followed by the removal of the polymer from samples using solvents and by thermal decomposition. Last, fully densified metallic components are obtained after sintering (Agarwala et al. 1996; Wu et al. 2002). In contrast to beam-based additive processes, the microstructure of the part is not generated layer-wise by melting and solidifying small areas, but in a steady manner during sintering from the outside of the part to the inside in order to obtain dense metallic parts. This peculiarity generates a different mechanical behaviour and characteristics affected by the parameters chosen during the numerous process steps. Printing parameters effects on the mechanical properties of material obtained by FFF were studied for polymeric materials (Cuan-Urquizo et al. 2019; Gardan 2015; Mohamed et al. 2015). The behaviour of metallic parts obtained by PBF processes were also investigated (Bajaj et al. 2020; Portella et al. 2020; Yadollahi et al. 2020). However, the mechanical behaviour of metallic materials obtained via FFF have not been investigated a lot since these processes are very recent. In the present work, an experimental investigation was performed to study the behaviour of 17-4PH stainless steel obtained by ADAM. So-called micro single edge notch tension (µ-SENT) specimens were fabricated with the same dimensions but with two different layer thicknesses. The microscale experiments coupled with DIC were then carried out to measure surface displacement and strain fields (Marae-Djouda, et al. 2020a; 2020b) . Based on the measured kinematic fields, effects of layer thickness were investigated by analysing the characteristic sizes of strain fluctuations.