PSI - Issue 43

Matthias Oberreiterr et al. / Procedia Structural Integrity 43 (2023) 240–245 Matthias Oberreiter/ Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Cast aluminum alloys possess a good relation between mechanical properties, density as key value for lightweight designs, and excellent castability for manufacturing complex components (Feikus et al. (2020), Yang et al. (2019)). But such complex cast parts inherit casting process dependent imperfections such as gas- or shrinkage porosity, oxide films, persistent slip bands or inclusions, which reduce the local fatigue strength significantly (Zerbst et al. (2019), Zhang et al., Boileau (2000), Leitner et al. (2017)). Inhomogeneities such as pores can be evaluated quite well destructively by fractography (Murakami and Endo (1994)), or non-destructively by computed tomography (Nourian Avval and Fatemi (2020), Nudelis and Mayr (2021)). Fatigue assessment of micropores is often done by the √ approach of Murakami (2012). This approach evaluates the projected area perpendicular to the load direction. The fatigue initiating defect can be described by extreme value statistics, e.g. the generalized extreme value distribution (Romano et al. (2019), Tenkamp et al. (2018), Tiryakioğlu (2008), Beretta (2021)). Murakami (2012) and Beretta et al. (1998) state that the largest initiating defects are Gumbel-distributed. This largest extreme value distribution (LEVD) possesses a location λ and a shape δ parameter (Gumbel (1958)). Tiryakio ğlu (2009) links the failure initiating defect size of the cumulative distribution to the finite fatigue life of AlSi-cast parts by fracture mechanics assuming stable crack growth. Aigner et al. (2018) proposed an extension considering the long-crack threshold Δ ℎ, and the coefficients m and C of the Klesnil and Lukáš (1972) approach, resulting Eq. 1 and Eq. 2. = ( − + ̃ Δ −ℎ , 2− 2 ) − 1 (1) = exp {−exp [ λ − √ 2 ( − + ̃ Δ −ℎ , ⋅ − ) 2− 2 ]} (2) The variable represents the area of the fracture initiating defect and is the number of load cycles until failure. In the study of Aigner et al. (2018), the value was set to zero for the AlSi-cast alloy EN AC 46200. 2. Materials and Methods The investigated materials cover two commonly used aluminum alloys EN AC 46200 and 42100, both in T6 heat treatment, nominal chemical composition according to EN 1706 in Table 1. The investigated specimens are taken out of a complexly shaped, gravity casted bulk production component, featuring strongly varying local casting process conditions, e.g. cooling rate. Table 1 Nominal chemical composition of the investigated materials Material Si [%] Cu [%] Mg [%] Fe [%] Mn [%] Ti [%] Al [-] EN AC 46200 7.5-8.5 2.0-3.5 0.05-0.55 max. 0.8 0.15-0.65 max. 0.25 balance EN AC 42100 6.5-7.5 max. 0.05 0.25-0.45 max. 0.19 max. 0.1 0.001-0.25 balance Two samples are taken from the material EN AC 46200 (AlSi8Cu3) , the first labelled as ‘Position A’ reflects a high, chill-enforced cooling rate and thus quite fine secondary dendrite arm spacing λ 2 and few micropores, see Fig. 1a. The second, labelled as ‘Position B’, possess es a comparably low cooling rate and hence increased  2 -value, exhibiting a quite huge degree of porosity, see Fig. 1b. The third investigated sample series is taken from alloy EN AC 42100 (AlSi7Mg0,3) , labelled as ‘Position C’ and also featuring high local cooling rate, see Fig. 1c. Tensile tests were carried out at a constant strain rate of 3.6 10 −3 1/ at all positions using an Instron ® strain-controlled servo hydraulic test system. The alternating fatigue tests were conducted using a Rumul ® Microtron resonance test rig. The