Uncovering the effects of neutralizing elements (Co, Mn and Cr) on the Fe-rich intermetallic formation in Al–Si–Cu alloys

As Fe impurity in recycled Al–Si–Cu alloys always form Fe-rich intermetallics which is detrimental to the mechanical properties, many studies have been searching for the effective neutralizing elements for transforming the brittle β-AlFeSi intermetallics into less harmful α-AlFeSi. In this study, three transition metals (Co, Mn, and Cr) have been studied systemically by comparing not only their percentage in 3D using X-ray computed tomography but also the elemental mapping using scanning electron microscopy equipped with energy dispersive spectrometer. It was found that the morphology and the amount transformation from β-AlFeSi to other phases largely depends on the neutralizing element additions. The addition of Co forms a high melting point intermetallic compound, Al₇CoCu₂, and adsorbs a large amount of Fe elements to form Al₇(FeCo)Cu₂ increasing plate-like intermetallics, leading to a decrease in mechanical properties from 385.7 MPa to 309.6 MPa. The addition of Mn reduces the size of Fe-rich phase and transform structure of Fe-rich phase from β to α, leading to the improvement of mechanical properties from 385.7 to 427.9 MPa. The addition of Cr forms the Al₁₃Cr₄ compound and absorb Fe atoms to reduce the β-AlFeSi by forming the Al₇(FeCr) compound. However, its effect is much lower than that of the Mn element, improving the mechanical properties by only 3.1%.