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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plotted in Fig. 2. The average size and standard deviation of  area as well as the number of analyzed specimens per alloy are listed in Tab. 1.

Fig. 2. Representative fractographs of sand cast (SC) alloys SC-7 and SC-10 compared to additively manufactured (AM) alloy AM-10. The characterized properties of macro hardness, defect size  area and position (surface) as well as Young’s modulus (average between quasi-static and cyclic value) were further used to estimate the fatigue strength at 1E7 cycles according to equation (1) listed in Tab. 1 to evaluate the Murakami-Noguchi concept. Tab. 1. Microstructure, defect and deformation characteristics and properties of AM and SC alloys

E | E’ ∗ , ( ′ ) . K | K’ [GPa] [MPa] [MPa] [MPa]

Property

DAS

HV

 area | n

n | n’

[Value] AM-10

[µm] 0.46

HV10

[µm]

106 ± 3 115 ± 3 101 ±3

256 ± 152 237 ± 67 304 ± 90

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70.8 | 76.6 77.0 | 71.8 75.1 | 73.6

84.5 91.0 79.6

211 | 218 214 | 285 221 | 284

1008 | 818 403 | 406 390 | 413

0.253 | 0.212 0.096 | 0.058 0.096 | 0.062

SC-10

80 ± 8 78 ± 9

4

SC-7

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3. Results and discussion The structural characteristics are listed in Tab. 1. SC materials have a comparable DAS of around 80 µm while AM material is refined to around 0.5 µm (factor of more than 100). The hardness of all Al-Si alloys is in the same range, AM-10 and SC-7 show comparable hardness and SC-10 has an increased hardness. The failure-initiating defect sizes are comparable for all Al-Si materials. The effect of the structural characteristics on deformation and fatigue behavior is discussed in the following subchapters. 3.1. Deformation behavior The quasi-static stress-strain (QSS) curves are plotted in Fig. 3a. AM-10 has the highest ultimate tensile strength (UTS) and fracture strain, while the yield strength (YS) is reduced compared to SC materials. All Al-Si alloys show a significant strain hardening. Hereby, SC-10 and SC-7 have the same strain hardening behavior while the microstructure of AM-10 leads to a significantly increased strain hardening (higher hardening exponent ). In Fig. 3b, the cyclic stress-strain (CSS) curves are shown. While both SC materials SC-10 and SC-7 experienced a significant cyclic hardening (increased cyclic yield strength p ′ 0.2 ), the AM-10 material shows no significant cyclic hardening or softening (slight change of cyclic yield strength p ′ 0.2 ). Due to this, the yield strengths of SC materials are further increased by cyclic loading and hardening effects compared to AM material.