A Study On Fe-rich Intermetallics Removal And Modification Methods For Recycled Al-Si Alloy

With the market holding Al approching its saturation, scrap aluminum recyclinghas become sole choice for the sustainable development of the aluminum industryworldwide. However, scrap aluminum is inevitably contaminated by Fe in its recyclingprocess, which will significantly reduce the mechanical performance and corrosionresistance of the recyclced Al alloys if Fe concentration is too high. Thus, Fe reductionhas become the key to the success of the scrap aluminum’s recycling.In the present study, the effects of S, Ce-Rich mischmetal and borides additions onthe modification of Fe-rich intermetallics in secondary Al-Si alloy are studied bymelting experiments followed by optical microscope, X-ray diffraction(XRD), energydispersive X-ray (EDS) and differential scanning calorimeter(DSC) characterization.The main conclusions are as follows:①S prompts the transformation of skeleton-like α-Fe phase from needle-like β-Fephase and the size of α-Fe phase increases with the addition of S. When S and Ce-Richmischmetal are added together, the enrichment of Re on the solid-liquid interface refinesthe skeleton-like α-Fe phase by influencing its nucleation and growth rates. Furthermore,the aluminum melt is puried and a new phase-rare earth phase becomes observable inthe microstructure.②Borides (KBF4) addition prompt transformation from needle-like β-Fe phase toprimary α-Fe phase. The size of primary α-Fe phase increases with the addition ofborides. When the boride addition reaches1.0%, Chinese script-type α-Fe phase appears.This α-Fe phase is found nucleating and growing based on the primary α-Fe phase.③When borides (KBF4) and Ce-Rich mischmetal were added togwether, rareearth elements, espceially Ce, appear in the primary α-Fe phase, which,in turn, changethe interface of primary α-Fe phase and melt. As a consequence, the microstructure ofprimaryα-Fe phase is changed from the polygon to the star. Furthermore, added Ce-richmischmetal inhibit the growth of the primary α-Fe phase and the formation of Chinesescript-type α-Fe phase.④When borides (KBF4) and Al-30Mn were added togwether, the formation ofprimary α-Fe phase is promoted by borides, the size of primary α-Fe phase increases bythe content of Mn. Because the density of primary α-Fe phase is higher than that ofaluminum melt, the primary α-Fe phase settles on the crucile bottom. ⑤If the melt subjetced to borides and Al-30Mn combined addition treatment wascold in the furnace to form ingot. It beocmes observable that the Fe concentrationincreases with the distance from the top of the ingot, and the iron content of the upperpart is much lower than the original amount. When the original melt Fe concentration is0.85%, the Fe content on the top of ingot is0.5744%,0.7287%on the middle,indicating an iron removal efficiency of31.83%and13.53%, respectively.