Effect of Melt Conditioning on Removal of Fe from Secondary Al-Si Alloys Containing Mg, Mn, and Cr

Abstract

Primary intermetallic sludge formation in hypoeutectic secondary Al7.1Si(1.5-xM)FexMM alloys was investigated in view of potential Fe removal by separation of sludge and Al melt. M is Mg, Mn, and Cr, with Mg as typical alloying element and Mn and Cr as additions to modify the sludge. Differential thermal analysis was used to determine the onset temperatures of sludge formation and solidification of the (Al)-solid solution. These temperatures were compared with results from the semi-empirical sludge factor approach and thermodynamic calculations. The chosen conditioning temperature of 620 °C is slightly above the onset temperature of solidification of the (Al)-solid solution in all alloys. At 620 °C, sludge did not form in alloys with xMg ≥ 0.6 at. pct. In all other alloys, sludge has formed. In the ternary Al7.1Si1.5Fe alloy and the alloy with xMg = 0.3 at. pct, the hexagonal αh phase is responsible for binding Fe, whereas Mg remains in the melt. The cubic αc phase binds Fe from the melt of Mn- and Cr-containing alloys. The (Al,Si)2Cr and Al13Cr4Si4 phases additionally bind Cr. The potential separation of the sludge from the residual melt leads to relative Fe removal up to 67 pct and relative transition-metal removal up to 65 and 83 pct.