Microstructural evaluation of dross formation on Mg- and non-Mg-containing Al alloys from industrial furnaces

The microstructural aspects of dross formation in industrial Al re-melt furnaces for several different alloy compositions have been evaluated. The Al alloys used in this study included 1350 (no Mg), 3004 (low Mg), and 5182 (high Mg). Dross specimens were collected directly from the different Al alloy melts in industrial furnaces using a consistent sampling protocol in order to compare the microstructure and phase development of the dross as a function of melt temperature and composition. The results showed that the sequence of phases formed during the re-melt process was the same for all the alloys examined: amorphous-Al₂O₃ forms first followed by either α-Al₂O₃+AlN (for non-Mg-containing alloys) or cubic MgO, then MgAl₂O₄, and lastly α-Al₂O ₃ (for low- or high-Mg content). The formation of MgAl ₂O₄ is associated with accelerated oxidation rates (known as breakaway oxidation) and this reaction proceeds until the Mg is depleted at the molten surface. At this point, aluminum oxidation is predominant and occurs at a significantly lower oxidation rate. The results obtained in this study are consistent with models developed for dross grown on similar Al alloys in laboratory environments and show that Mg oxidation (and the accelerated formation of MgAl₂O₄) dominates the oxidation process during Al melting, whether the Al contains low or high Mg contents. The oxide morphology within the dross layer differed according to the particular alloy being melted and thus the amount of Al recovery from dross can vary with composition.