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Simulation of surface solidification in direct-chill 6xxx aluminum billets

Using the Bridgman technique, the solidification of aluminum alloys of types 6005, 6063, and 6082, was studied. The solidification process differs both between the alloys and between billet surface and bulk, due to higher concentrations of Fe, Si, Mn and Mg near the surface. Previously determined surface concentrations were used to calculate the Fe, Si, Mn and Mg additions needed for Bridgman experiments that simulate surface region solidification. Microstructures were studied and grain size, secondary dendrite arm spacing, intermetallic particles, segregation, and coarsening were evaluated. The increased alloy element concentrations at the surface were found to influence both the structure coarseness and the kind of intermetallic precipitation. In addition, clear differences could be determined between the alloy types, depending on their pull rate in the furnace.