Evolution behavior of Fe and Fe-rich phases in high-Fe concentration Al-Si-Mg alloy with Mn, Cr, and Co alloyed

In this work, we report the evolution behavior of Fe and Fe-rich phases in high-Fe concentration recycled Al-Si-Mg aluminum melt with and without alloying elements (Mn, Cr, Co) introduced. Firstly, the thermodynamic calculations reveal that the addition of Mn, Cr, and Co could improve the nucleation temperature of the Fe-rich phase. Moreover, four solidified ingots for Al-10Si-0.5Mg-1Fe alloy(base alloy) and the base alloy with 1.3 wt% Mn, 0.8 wt% Cr, and 0.8 wt% Co added, respectively, were prepared by holding the melt for 30 min at 610 ℃ after all the raw materials were fully melted. Microstructure examination revealed that the addition of Co resulted in the formation of the needle-like Fe-rich phase, which further leads to a poor settling effect and low iron removal efficiency. In contrast, the addition of Mn and Cr favors converting β-Fe into α-Fe, and forming the Chinese script and fish-bone Fe-rich phase. The size of the Fe-rich phase formed after adding Cr is smaller, while the residual Fe content after adding Mn is less. Also, the sludge phase at the bottom of the ingot presents a polygonal shape and long rod shape after adding Mn and Cr, respectively. Further, the Mn alloying increases the particle size of the sludge, while Cr refines the sludge phase and increases the aspect ratio (length/width ratio) of the sludge. Particularly, for Al-Si-Mg alloy with 1.0% Fe content, the superior mechanical properties are obtained by adding 0.8 wt% Cr, and the tensile strength reaches 136 MPa. After adding 1.3 wt% Mn, the Fe content of the upper section is reduced to 0.37 wt%, and the Fe removal rate achieves 63%. The obtained results provide some guidelines for achieving high iron separation efficiency in recycled aluminum-silicon alloy melts.