Homogenization of Extrusion Billets of a Novel Al-Mg-Si-Cu Alloy with Increased Copper Content.

Within the present work the homogenization of DC-cast (direct chill-cast) extrusion billets of Al-Mg-Si-Cu alloy was investigated. The alloy is characterized by higher Cu content than currently applied in 6xxx series. The aim of the work was analysis of billets homogenization conditions enabling maximum dissolution of soluble phases during heating and soaking as well as their re-precipitation during cooling in form of particles capable for rapid dissolution during subsequent processes. The material was subjected to laboratory homogenization and the microstructural effects were assessed on the basis of DSC (differential scanning calorimetry) tests, SEM/EDS (scanning electron microscopy/energy-dispersive spectroscopy) investigations and XRD (X-ray diffraction) analyses. The proposed homogenization scheme with three soaking stages enabled full dissolution of Q-AlCuMgSi and θ-AlCu phases. The β-MgSi phase was not dissolved completely during soaking, but its amount was significantly reduced. Fast cooling from homogenization was needed to refine β-MgSi phase particles, but despite this in the microstructure coarse Q-AlCuMgSi phase particles were found. Thus, rapid billets heating may lead to incipient melting at the temperature of about 545 °C and the careful selection of billets preheating and extrusion conditions was found necessary.