PSI - Issue 7

A. Brueckner-Foit et al. / Procedia Structural Integrity 7 (2017) 36–43 A. Brückner-Foit/ Structural Integrity Procedia 00 (2017) 000–000

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damage process [Dezecot et al. (2017) and Limodin et al. (2017)]. Fatigue failure of cast components is generally caused by internal defects, since the casting skin tends to be free of defects and insulates the internal flaws from the environment [Brueckner-Foit et al. (2017)]. In this study, fatigue specimens were die-cast and fatigue tested without additional machining. As expected, the size and morphology of the internal defects strongly depend on the cooling conditions which can be varied systematically in the casting process [Samuel et al. (2016)]. Two types of defects were analysed in more detail: pores (both gas and shrinkage pores) and plate-like iron inclusions. The resulting structure were transferred to a finite element mesh. Finally the resulting stress concentrations were calculated and the various pores were classified according to their propensity to fatigue crack initiation. These findings are confirmed by CT analysis of fatigued specimens which clearly give evidence for secondary internal cracks. The second type of internal defects, platelet-like iron inclusions, play an important role in recycled aluminium alloys. Commercial Al-Si alloys always contain certain amounts of Fe as natural impurity, which cannot be removed from the primary aluminium metal in a cost efficient way. Fe tends to precipitate in combination with other elements during solidification [Wanderka et al. (2017), Bjurenstedt et al. (2017)], forming intermetallic phases that adversely affect mechanical properties and hence the formation of different casting defects. This effect is studied by systematically varying the iron content of the alloys and relating to the fatigue properties to the three-dimensional defect structure found in the CT scans. 2. Material and experiments In this study, a near-eutectic Al-Si-Cu alloy with the chemical composition given in Table 1 was used. Various amounts of Fe were added in order to cast specimens with different Fe-contents. The specimens were cast by gravity die casting at 760°C in a steel mold pre-heated at 350°C achieving a fixed cooling rate. The specimens were die cast with finished shape (see Fig. 1) in order to consider the effect of the casting skin, since it is well known that the casting skin can insulate the casting defects from the air environment and promote sub-surface crack initiation. Table 1 Chemical compositions of the Al-Si-Cu alloy with different Fe-additions (wt. %). Alloy Si Cu Mg Fe Zn Mn Al A 12.96 1.52 0.68 0.6 0.48 0.17 rest The main microstructural features are the eutectic silicon particles, and the Fe-rich and the Cu-rich inclusions. Among the Fe-rich inclusions, the Chinese-script- like α -phase and the platelet-like ß-phase can be distinguished. The ß-Al 5 FeSi plates are brittle and tend to form large clusters of particles that reduce the ductility and the ultimate strength of the alloy [Bacaicoa et al. (2017)]. Some specimens were heat-treated in order to analyze the effect of heat treatments on the size and morphology of these inclusions, as well as on the mechanical properties. As it can be observed in Fig. 2, a high concentration of shrinkage pores together with gas pores were formed as a result of the cooling rate associated with the casting process. The variability of the casting parameters led to a dispersion of the cooling rates, and therefore, to specimens with different porosity levels. Specimens with different Fe-content were tensile tested in a MTS E45 testing machine at a strain rate of 1.5 10 -4 s 1 in order to analyze the influence of the Fe-content on the tensile properties of the alloy. Fatigue tests were performed with the cast specimens at a load ratio of R = -1 and test frequency of 50 Hz in a servo-hydraulic testing machine. After a certain amount of cycles, the fatigue specimens were scanned by micro-computed tomography at intervals of B 12.87 1.64 0.58 1.37 0.47 0.23 rest C 12.99 1.8 0.43 1.85 0.51 0.3 rest D 12.62 1.45 0.51 2.28 0.45 0.26 rest