Metallographic Methods for Oxide Skin Segregation Studies

This work is a study of the segregation of alloying elements and salts to the surface of Direct Chilled-cast (DC-cast) ingots of alloy: (i) AA5083; and (ii) AA5182. The ingots only have slightly different chemical compositions. Alloy AA5182 contains Cu, while alloy AA5083 is grain refined, contains Cr, and is lower in Si and Fe compared to AA5182. The ingot microstructures were investigated using Optical Microscopy, Scanning Electron Microscope (SEM) and Secondary Ion Mass Spectrometry (SIMS). The results showed that the surfaces contained aluminum dendrites, large intermetallic crystals (Al6(MnFe) and Al3(Fe,Mn)) and impurities, and small crystals between dendrite arms. Between the dendrites small salt crystals containing Na, K, Ca, Mg, F and/or Cl were found. The conclusion is that salt and minority impurities are mainly located to grain boundaries in the segregation zone.