Synergistic effects of Mn and B on iron-rich intermetallic modification of recycled Al alloy

Individual manganese (Mn) modification cannot prevent the formation of coarse and blocky Iron-rich phases in recycled Al–Si alloys. In the present work, scanning/transmission electron microscopy (SEM/TEM), thermodynamic calculations, and synchrotron X-ray imaging were comprehensively applied to reveal the synergistic effect of Mn and B (boron) on Iron-rich phase formation in Al–Si–Fe alloys during solidification. The results showed that with addition of Mn and B, the morphology of Iron-rich phase translated from the coarse primary into a uniform and hollow-shaped multibranched Chinese-script morphology. An appropriate boron amount significantly reduced the initial nucleation temperature of the primary Iron-rich phase and increased the number of nucleation cites. The TEM results show that there was no obvious orientations relationship between the AlB2 and α-Al15(MnFe)3Si2 phases. The main modification mechanism is that B was adsorbed on the oxide film surface, which hindered the formation of the Iron-rich phase at high temperature. This Mn + B compound modification method provides an alternative way to enhance the mechanical properties of recycled Al–Si alloys in industry.