Abstract
Intermetallic beta-Al5FeSi phase and coarse Mg2Si particles have negative effects on extrudability and workability of 6xxx Al alloy billets. To achieve extruded products with a high surface quality as-cast billets are therefore heat-treated before extrusion.
During heat treatment the undesired intermetallic particles, i.e., beta-AlFeSi platelets, are transformed to a rounded alpha-Al(FeMn) Si intermetallic phase. This transformation was studied in-situ by TEM for 6005 and 6082 Al alloys.
It was observed that the Mg2Si particles precipitate in the Al matrix at about 250 degrees C; this precipitation also occurred at the edge and faces of beta intermetallic particles, and the Mg2Si particles were the preferred sites for alpha-Al(FeMn) Si particle nucleation. The transformation proceeded faster and at lower temperatures, 350-450 degrees C, than what has been reported earlier for homogenization studies of bulk samples and industrial billets.
This could be associated with the thin characteristic of used samples in TEM giving contribution from fast surface diffusion, but it was also concluded that the phase boundary layer diffusion was important for the understanding of how the transformations proceed.