PSI - Issue 23

I. Bacaicoa et al. / Procedia Structural Integrity 23 (2019) 33–38 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

34 2

1. Introduction

Cast Al-Si-Cu alloys are widely used in the aeronautic and automotive industry due to their excellent mechanical properties, low weight-to-strength ratio, castability and recycling possibilities [Kaufman et al.]. The use of the recycled grade alloy for the production of structural parts has received a considerable interest in recent years since secondary alloys can significantly reduce production costs as well as the CO 2 emissions associated with the production of the aluminum raw material and alloying elements [EAA/OAA]. The manufacturing of recycled aluminum requires 5% of the energy compared with the primary alloy [Das et al.]. However, the recycling process of the aluminum alloy leads to the accumulation of iron, which can only be removed by costly processes [Gaustada et al.]. The Fe-content causes the formation of brittle Fe-rich intermetallic inclusions, such as the so-called Chinese script α -Al 15 (Fe.Mn) 3 Fe 2 and the plate- like ß -Al 5 FeSi inclusions [Stein et al.]. Recent studies have reported the detrimental effect of the complex three-dimensional morphology on the mechanical properties of the alloy using high-resolution X-ray tomography [Bacaicoa et al.]. The results of these studies revealed that the complex morphology of the Fe-rich inclusions and the disposition in complex clusters can considerably decrease the ductility and strength of the alloy [Bacaicoa et al.]. Moreover, a high Fe-content can promote the formation of shrinkage pores during the casting process as result of the reduction of the melt permeability [Puncreobutr et al., Bacaicoa et al.]. A recent study characterized the damage mechanism in a secondary alloy in which the shrinkage pores located in the inner part of the specimens act as the main crack initiation sites [Brueckner-Foit et al.] since the fatigue specimens produced without additional machining after casting did not present significant defects such as Fe-rich inclusions in the cast skin. However, the addition of iron in the alloy composition can lead to the formation of large Fe-rich inclusions in the cast skin as well accelerate the rupture of the Fe-rich particles located between the material bridges of the dendritic arms of the shrinkage pores. The aim of this work is to study the influence of the Fe-content of the damage behavior of an Al-Si-Cu alloy by X ray tomography. Fatigue tests were conducted and the fracture surfaces were analyzed by scanning electron microscopy as well as X-ray tomography in order to analyze the crack initiation and propagation processes. Table 1 shows the chemical composition of the Fe-rich near-to-eutectic Al-Si-Cu alloy used in the present study. A certain amount of iron was added into a normalized secondary alloy in order to reach the considered iron content. The fatigue specimens were cast by gravity die casting at a melt temperature of 760°C in a steel mold pre -heated at 350°C. A certain variability in the cooling rates associated with the different specimens was allows in order to achieve different defect morphologies. The specimens were die cast without additional machining in order to consider the effect of the casting skin, since it has been recently reported that the casting skin can insulate the casting defects from the air environment and promote sub-surface crack initiation [Luetje et al.]. The Mn:Fe relationship is 0.28, which can promote partial substitution from ß -Al 5 FeSi to the less detrimental α -Al 15 (Fe,Mn) 3 Si 2 [Ashtari et al. ], although a significant amount of ß -phase is formed. 2. Materials and experimental methods

Table 1. Chemical composition of the studied alloy. Si Cu Mg

Fe

Zn

Mn

Al

12.62

1.45

0.51

2.28

0.45

0.26

bal.

The main microstructural feature of the studies alloy are the eutectic silicon particles, Fe-rich inclusions and Cu-rich compounds. The Chinese-script-like Fe-rich α -Al 15 (Fe,Mn) 3 Si 2 and the platelet- like ß -Al 5 FeSi are the main iron rich inclusions present in the alloy. Fig. 1 shows an example of the complex clusters formed by the brittle plate-like ß Al 5 FeSi inclusions and Fig. 2 shows a volume element with large shrinkage pores typical in the studied alloy obtained by X-ray tomography.