During the extrusion of aluminum alloys profiles using porthole dies, the temperature of the material in the welding chamber is one of crucial parameters determining the quality of longitudinal welds. In order to extend the permissible temperature range, the billets intended for this process should be characterized by the maximum attainable solidus temperature. Within the present work, the homogenization of AlZnMgCu alloys DC-cast (Direct Chill-cast) billets was investigated, with the aim of solidus temperature maximization. Conditions of soaking and cooling stages were analyzed. The materials were homogenized in laboratory conditions, and the microstructural effects were evaluated on the basis of DSC (Differential Scanning Calorimetry) tests and SEM/EDS (Scanning Electron Microscopy/Energy-Dispersive Spectroscopy) investigations. For all examined alloys, the unequilibrium low-melting microstructure components were dissolved during soaking, which led to the significant solidus temperature increase, in comparison to the as-cast state. The values within the range of 525–548 °C were obtained. In the case of alloy with highest Cu concentration, the application of two-step soaking was necessary. In order to take advantage of the high solidus temperature obtained after soaking, the cooling rate from homogenization must be controlled, and the effective cooling manner is strongly dependent on alloy composition. For high-Cu alloy, the solidus decreased, despite the fast cooling and the careful billets preheating being necessary.

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-Al₅Cu₂Mg₈Si₆ and θ-Al₂Cu phases. The β-Mg₂Si phase was not dissolved completely during soaking, but its amount was significantly reduced. Fast cooling from homogenization was needed to refine β-Mg₂Si phase particles, but despite this in the microstructure coarse Q-Al₅Cu₂Mg₈Si₆ 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.

In the first part of the work, the 3-step homogenization procedure was carried out for AlMgSi(Cu) alloy, aimed at obtaining billets microstructure with numerous fine particles of strengthening phases and increased solidus temperature. In the second part of the work, an original on a global scale prototype device for the dynamic stretching of extruded profiles before artificial ageing, was proposed and tested in semi-industrial conditions. The thermo-mechanical treatment of extruded hollow profiles from AlMgSi(Cu) alloy with 1.22% content of copper was performed with dynamic deformation range: 0.25%, 0.5%, 1%, 1.5% and 2%. The microstructural inspection was performed in the welding area and outside the weld for variant after only extrusion and for variant after extrusion and dynamic stretching before artificial ageing. It was revealed that the designed three-stage homogenization treatment led to dissolution of soluble phases present in the as-cast billets microstructure, which caused significant increase in solidus temperature, from 509 °C in as-cast state to 573 °C after completed soaking. The application of dynamic deformation during the straightening of aluminium profiles after hot extrusion with water cooling on the press run, led to the induced favourable changes in the microstructure of the alloy, which translated into a significant increase in UTS of about 11%.