PSI - Issue 23

Sascha Gerbe et al. / Procedia Structural Integrity 23 (2019) 511–516 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Cast aluminum alloys are due to their good strength to weight ratio excellent materials if light weight solutions are required, especially in the case of large and complex geometries, e.g., in automotive drivetrains. A major issue in this context are defects and heterogeneities resulting from casting process. Different microstructural characteristics, like secondary dendrite arm spacing (SDAS) (cf. Houria et al. (2015), Gerbe et al. (2019)), porosity (cf. Ben Ahmed et al. (2017), Tenkamp et al. (2018-1), Gerbe et al. (2018)) and the morphology of eutectic silicon (cf. Siegfanz et al. (2013)) are depending on the solidification rate and influencing mechanical properties. Furthermore, Jang et al. (2009), Campbell (2010) and Siegfanz et al. (2013) discussed the appearance of facet areas in fatigue fracture surfaces. While Siegfanz et al. (2013) correlated the facets to operated {111} slip bands of the fcc aluminum, Campbell (2010) links their occurrence to oxide films and mentions the negative effect on fatigue properties. Affected by the heterogenic appearance of such characteristics, the mechanical properties of cast aluminum alloys are often accompanied by strong scattering. To avoid failures from fatigue, components usually are designed in a conservative manner, which lowers the light weight efficiency. To remove this issue and to increase the light weight potential of cast aluminum alloys comprehensive mechanical investigations, especially in the field of fatigue are necessary. Hence, aim of the present study is to develop systems and strategies for a better description of the microstructural characteristics influences on the fatigue initiation and crack propagation properties. The experimental work in this study was performed on the two hypo-eutectic cast aluminum alloys AlSi8Cu3 and AlSi7Mg0.3. The first material is a secondary metallurgic alloy used for castings of big dimensions, like engine blocks, often produced in sand cast technique. Specimens of this grade were taken from industrial in-series castings at two positions representing the highest difference in solidification rate and respective microstructural appearance (cf. Fig. 1a). AlSi7Mg0.3 is a primary metallurgic alloy (without recycling material). In this work, it was produced in a laboratory gravity die cast process (cf. Fig. 1b). Both alloys were tested in heat treated T6 condition (highest strength). 2. Experimental

Fig. 1. Investigated castings; (a) AlSi8Cu3 in-series sand cast engine block; (b) AlSi7Mg0.3 gravity die cast (cf. Gerbe et al. (2018)).

Figure 1a shows the engine block from industrial in-series sand cast and the extraction regions (grey: bearing seat, low SDAS, fine microstructure; black: stud bold, high SDAS, coarse microstructure). In Fig. 1b, the AlSi7Mg0.3 casting produced by gravity die casting is shown. The red surrounding marks the part, where specimens were extracted. The fatigue properties were determined by means of uniaxial cyclic loading by use of the two resonance testing machines, Rumul Testronic ( f = 70  5 Hz) for the high-cycle-fatigue regime (HCF; ≤ 10 7 cycles) and an ultrasonic testing machine from Boku Vienna ( f up to 20 kHz) for the very-high-cycle-fatigue regime (VHCF; > 10 7 cycles), respectively. The set ups and specimen geometries are shown in Fig. 2. Specimens in HCF were fatigued up to N max = 10 7 cycles (HCF) and N max = 10 9 cycles (VHCF). In latter experiments up heating was prevented by forced air and use of pulse-pause mode. Further information on the experimental setup is provided in Gerbe et al. (2018). Additionally, the evolution of cyclic deformation and damage was monitored for selected AlSi7Mg0.3 specimens in the HCF regime by online stress-strain hysteresis analysis using tactile extensometer with a gage length of 10 mm and alternating-current-potential-drop method (ACPD) using Matelect CGM-5 (cf. Fig. 2b). The ACPD technique was adapted to detect the fatigue evolution with respect to fatigue crack initiation and propagation. As the skin depth δ is higher than the gage radius of the circular cross section, the whole volume of specimen is monitored during fatigue testing, cf. Tenkamp et al. (2019). Further, the fatigue tests were interrupted to quantify and visualize the current 3D