Influence of copper content on the structure and properties of aluminium alloys

Designing new aluminium alloys always requires an assessment of the influence of modified contents of the main alloying additions on the required properties for a specific application. The aim of this study was to develop a new alloy with a higher tensile strength than the tensile strength obtained in typical 6xxx series alloys, with a satisfactory hot extrusion ductility. The article presents the results of studies on the influence of copper additions, in three variants, on the structure and mechanical properties of extruded aluminium profiles in various heat treatment states. The base reference for the presented studies was the following aluminium alloys: AlMgSi (EN AW-6063) and AlCuMgMn (EN AW-2017). On this basis, three alloy variants with Cu contents of 2.5, 3.5 and 4.5 weight % were selected. These alloys were cast by vertical semi-continuous casting in the form of ingots with a diameter of 100 mm. Hot deformation studies were carried out in the process of co-extrusion on a semi-industrial line consisting of a horizontal 5 MN press together with auxiliary devices. The extruded profiles were free from defects that could have resulted from the process, such as overheating, cracks and blisters. The materials in the form of extruded profiles, in the heat treatment states T1 and T4, were characterised in terms of structure and mechanical properties. The variants with Cu contents of 2.5 wt% and 3.5 wt% had comparable structure and grain size, and the variant with 4.5 wt% Cu had a finer microstructure. For the studied alloys after heat treatment, higher the strength, higher the Cu content in the alloy, ranging from 445 to 543 MPa. It has been found that it is possible to develop modified aluminium alloys based on the 6xxx series with an increased copper addition (but still below the content typical for Al alloys of the 2xxx series) that will have a strength above 400 MPa. The first variant of the new alloy with a Cu content of 2.5 wt% meets the set goal and meets the requirements set at the beginning of the studies.