PSI - Issue 42

Jochen Tenkamp et al. / Procedia Structural Integrity 42 (2022) 328–335 Jochen Tenkamp / Structural Integrity Procedia 00 (2019) 000 – 000

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Poisson’s ratio

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2. Material and experimental setups 2.1. Material and manufacturing

The additively manufactured AlSi10Mg material (short: AM-10) was manufactured vertically by laser-based powder bed fusion (PBF-LB) on an EOS system M290 at Fraunhofer Research Institution for Additive Manufacturing Technologies (IAPT, Germany, Hamburg). The AM cylinders of  20 mm and 120 mm length were drilled in “as built” condition to final geometry, as shown in Tenkamp et al. (2022b) and finally polished to a roughness of R z  0.8 µm to minimize surface effects on mechanical properties. The cast AlSi10Mg and AlSi7Mg materials (short: SC-10, SC-7) were processed by sand casting (SC) at the Institute of Materials Design and Structural Integrity of the University of Applied Sciences Osnabrueck (Germany). The melt got a grain refinement and Si eutectic modification by adding AlTi5B1 and Strontium and was cast off into a step wedge mold of oil sand. The SC step wedge was cut in four parts according to different cooling rates (in total four). The selected step three for AlSi10Mg and AlSi7Mg was further cut to bars of 20 × 20 mm cross section and 80 mm length. To adjust comparable defect sizes to AM material, both SC materials were hot isostatically pressed (HIP) by Densal TM process (Bodycote, Germany), before receiving the following T6 heat treatment: (i) solution annealing at 545 °C for 1 hour, (ii) water quenching at ambient temperature and (iii) artificial age-hardening at 160 °C for 5 hours. The bars were further drilled to final geometry, as shown in Tenkamp et al. (2022b), and finally polished to a roughness of R z  0.8 µm. The high cooling rates of PBF-LB process compared to SC process lead to a significant refinement of AM microstructure, as shown in Fig. 1 based on micrographs of optical (OM) and scanning electron microscopy (SEM). The DAS was quantified for SC alloys and compared to the dendritic width in AM alloy in Tab. 1. Moreover, the macro hardness according to Vickers (HV10) was determined for each alloy.

Fig. 1. Representative micrographs of sand cast (SC) alloys SC-7 and SC-10 compared to additively manufactured (AM) alloy AM-10.

2.2. Experimental setups The quasi-static deformation behavior was characterized by strain-controlled tensile tests at universal testing system (Instron 3369 with 50 kN load cell) at a strain rate of 2.5 ⋅ 10 − 4 s − 1 . The cyclic deformation behavior was determined by strain-controlled low cycle fatigue (LCF) tests at a servo-hydraulic fatigue testing system (Schenck PC63M with 63 kN load cell incl. Instron 8800 controller). The LCF tests were carried out at a strain ratio of R = -1 and a frequency of f = 0.05 Hz. Based on quasi-static (QSS) and cyclic stress-strain (CSS) curves, the Young’s modulus (′) as well as hardening coefficient (′) and exponent (′) were determined according to Morrow equation. The properties are plotted in Tab. 1. The high cycle fatigue (HCF) tests were performed stress-controlled on a resonant fatigue testing system (Rumul Testronic with 20 kN load cell) at a frequency of around 70 Hz and a stress ratio of R  = -1. For each alloy, the fracture surface of selected HCF specimens were further investigated by SEM to determine the failure-initiating or initial defect and its size related to the  area concept of Murakami (2012). Representative fractographs of each alloy are