In-situ study of phase transformations during homogenization of 6060 and 6063 Al alloys

In as-cast 6xxx aluminium billets beta-Al5FeSi intermetallic phase and coarse Mg2Si particles have negative effects on subsequent processes such as extrusion. To achieve extruded products with a high surface quality the billets are heat-treated by a so-called homogenization before extrusion. During the heat treatment undesired intermetallic particles, i.e. beta-AlFeSi platelets, are transformed to rounded alpha-Al(FeMn)Si intermetallic particles. The transformations occurring during homogenization have up until now only been studied by interrupted experiments giving some insight into the process, but a complete picture of how these phase transformations are coupled to the geometrical changes has not evolved. In the present study transformations in 6060 and 6063 alloys have been observed in-situ by TEM, and several nucleation and growth modes have been identified. Boundary layer diffusion was found to be important for the understanding of how the transformations proceed, and also diffusion on the free surface contributed to higher transformation rates compared to industrial processes due to thin characteristics of TEM samples. Together with an earlier study of 6005 and 6082 alloys the homogenization process for all important industrial 6xxx alloys is now characterised in detail by in-situ monitoring.