Revealing the influence of Fe on Fe-rich phases formation and mechanical properties of cast Al-Mg-Mn-Fe alloys

In this study, the recycled Al-Mg-Mn-Fe alloys having different Fe levels (0.1%, 0.5%, and 0.8%) were successfully developed by applying squeeze casting without heat treatment. Optical and scanning electron microscopy, synchrotron X-ray tomography and radiography, X-ray diffraction, and tensile tests combined with thermodynamic calculations were used to study the correlation between the microstructural evolution and mechanical properties. The results showed that for the alloys with an applied pressure of 75 MPa, as the Fe increase from 0.1% to 0.8%, the yield strength (YS), and ultimate tensile strength (UTS) increased from 122 MPa and 244–146 MPa and 289 MPa, and elongation decreased from 34% to 12%. Even though Fe additions increased the volume fraction of Fe-rich intermetallic phases, it significantly increased the UTS and YS. The synchrotron X-ray tomography and deep-etched results both show that the 3D morphology changed from individual Chinese-script to interconnected plate-like. The 3D morphology of 0.8Fe alloy clearly demonstrate that the hole partially or whole penetrated the entire rectangle-shaped Al6(FeMn) phases, which is due to the close-packed plane growth. In-situ synchrotron X-ray radiography results showed the facet growth behaviour of Al6(FeMn) in 0.8Fe alloys during solidification with a long needle-like shape. The size of primary Al6(FeMn) phases decreases, whereas their number increases with increasing cooling rate. Moreover, the applied pressure was beneficial in refining the size of α-Al grains and Fe-rich phases and reducing the volume fraction of pores, thus contributing to the improvement of strength and elongation. The in-situ tensile test results indicated that the crack initiated in the Fe-rich phases and pores, and the slip lines were blocked by the Fe-rich phases resulting in the strengthening of the secondary phases.