Optimizing the Zn and Mg contents of Al–Zn–Mg wrought alloys for high strength and industrial-scale extrudability

Abstract

High-strength Al alloys of the 7xxx series are promising candidates for further light-weighting of car bodies. However, they are difficult to process by extrusion: complex geometries in particular pose a major challenge due to high extrusion pressures and tool wear. In addition to process optimization, alloy chemistries can be tailored for properties such as good extrudability or high strength. In this work, several experimental alloys based on EN AW 7108A were investigated with respect to the contents of the main alloying elements Zn and Mg, as well as heat treatment (e.g., homogenization), to achieve good extrudability and favourable mechanical properties. The experimental alloys were initially extruded on a small-scale extrusion press and evaluated with regards to their warm formability, microstructure, extrudability, and mechanical properties in T5 temper. Alloy compositions with Mg contents <1.2 wt.-% are optimal and lead to good extrudability as well as high yield strengths well above 350 MPa. The Zn content, on the other hand, has minor influence on the extrudability, but significantly increases strength at higher contents (∼60 MPa). However, the combined Zn + Mg content should not exceed 7 wt.-%. Based on these findings, two optimized alloys were designed, direct chill cast on industrial scale, and extruded on an industrial extrusion press. The alloys showed good extrudability and high strength in the T5 condition. The results also reveal that the limits of the EN AW 7108A alloy are exhausted and higher strengths could only be achieved with the optimized EN AW 7003 alloy.