Intermetallic Sludge Formation in Fe Containing Secondary Al–Si Alloys Influenced by Cr and Mn as Preparative Tool for Metal Melt Filtration

In view of filtration of Fe-enriched intermetallics to decrease Fe content in secondary aluminum alloys, the formation of so-called sludge particles has been investigated depending on dwell time and chemical composition using an AlSi9Cu3 secondary alloy with high Fe content. To evaluate the dwell time dependency, samples are cast into ceramic crucibles and held at 620 °C for varying times. Furthermore, Mn and Cr have been added to the melt in different amounts and the alloys are treated for 6 h at 620 °C. The samples are analyzed using light optical micrographs and image analysis to reveal particle sizes and distribution as well as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) for phase identification. It is found that the growth of Fe containing sludge particles stagnates after reaching a specific value and is not significantly affected by ultra-long dwell times. Mn addition has a minor effect on the the particle size distribution preserving large particle sizes, whereas Cr addition leads to a smaller sludge particles. Generally, the sludge consists of the cubic α-phase. Additionally in the presence of Cr, Fe, and Cr concentration gradients occur in the cubic α–Al–(Fe,Mn,Cr)–Si phase and the Al13Cr4Si4-phase is contained in the centers of the sludge particles. It is concluded that Mn addition is favored due to larger particle size and higher efficiency to bind Fe in the α-phase than by Cr or Cr and Mn addition.