PSI - Issue 34

Radomila Konecna et al. / Procedia Structural Integrity 34 (2021) 135–140 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Metal additive manufacturing technologies especially the powder-bed-fusion (PBF) technology have been available to industry for over twenty years. The level of penetration of L-PBF in industry is challenged by high part cost, low productivity, lack of standardization, insufficient technical knowledge and design skills equivalent to what is available for traditional metals, etc. Further technical knowledge for the design and qualification of critical load-bearing metal PBF parts is still lacking. Among the aluminum alloys suitable for PBF, AlSi10Mg has been significantly investigated to produce customized parts of complex geometry, Brandl et al. (2012). However, unacceptable cost increase or/and geometrical inaccessibility of the part surfaces may restrict post fabrication finishing. Therefore, the fatigue performance of as built AlSi10Mg parts is often insufficient as shown in various studies. For example, Mower and Long (2016) tested vertically built specimens in rotating bending and determined significantly lower fatigue resistance of as-built PBF AlSi10Mg compared to conventional machined Al6061 alloy specimens. Uzan et al. (2017) investigated the fatigue resistance, hardness and tensile stress of L-PBF AlSi10Mg specimens printed in the vertical direction and heat treated under various conditions. They found that the highest fatigue resistance of specimens tested in rotating bending was obtained for the as-built alloy with machined and polished surfaces while the lowest fatigue resistance was displayed by stress-relieved and HIPed at 500 °C specimens due to microstructure coarsening and reduced mechanical properties. Alternatively, to conventional T6 treatments, direct aging treatments have been proposed because capable of precipitate fine reinforcements while maintaining a unique microstructure that exists in the PBF Al – Si alloy, Baek et al. (2021). While the influence of surface quality on fatigue in conventional metal alloys and machined surfaces is accounted for with knock-down factors depending on mean surface roughness R a , an equivalent understanding of the fatigue performance in the presence of the realistic as-built surface quality of L-PBF parts is not yet available, Yadollahi and Shamsaei (2017). The surface quality of additive manufactured parts depends in a complex way on raw material quality, particle powder size and processing parameters of L-PBF system and layer-by-layer printing strategy. Further, fatigue properties of alloys in the as-build state (i.e., no post heat treatment) may depend on residual stresses that are also affected by printing process and part geometry. This contribution investigated the fatigue performance of as-built AlSi10Mg and links it to the near-surface quality affected by L-PBF fabrication. Three sets of specimens oriented in different directions with respect to build direction were fabricated in an industrial-grade system. The specimens were left in the as-built condition, that is as-printed surfaces and process residual stresses and subjected to high cycle fatigue testing. The quality of the as-built surfaces in the different specimens was characterized in terms of surface roughness and near surface microstructure on etched metallographic sections to explain the directional fatigue data of L-PBF AlSi10Mg. 2. Experimental details This study involved a gas atomized AlSi10Mg alloy powder with granules characterized by a mean size of 37 µm, D10 = 21 µm and D90 = 65 µm and a flowability of 80 s/50 g. The L-PBF system SLM 280 HL (SLM Solution Group AG, Germany) operated by an experienced AM service provider (BEAM-IT Fornovo Taro, Italy) was used to process the AlSi10Mg powder. During the manufacturing process, the chamber was flooded with argon to reduce the oxygen content to below 0.2 %. The printing parameters were the following: beam power 350 W, layer thickness 50 µm, hatch distance 0.13 mm and scan speed 1650 mm/s for an energy density of 32.63 J/mm 3 . Specimens were manufactured heating the building platform up to 150 °C. The scan strategy is stripes which rotate each layer of 67° and the scanning order is two contours followed by the hatch scanning. The specimens used in this study were kept in the as-built condition (i.e., no post-fabrication heat treatment and no surface post-processing). The microstructure of the near surface layers for the three specimens was determined by longitudinal specimen sectioning, metallographic polishing and etching. Optical microscopy was used to reveal the printing strategy effect on microstructure. The surface roughness of the specimens was obtained with a profilometer. The roughness parameters R a , R z , etc. were determined (R a mean roughness value; R z average maximum height of the profile etc.). The innovative test method using miniature specimen geometry and a cyclic plane bending loading at load ratio R = 0, was used, Nicoletto (2017). This geometry offers the possibility of obtaining the smooth fatigue response for