Microstructure and property development in spray formed and extruded Al‐Mg‐Li‐Zr alloys for aerospace and autosport applications

Abstract

AbstractLow density Al‐4Mg‐1.3Li‐0.4Zr and Al‐6Mg‐1.6Li‐0.4Zr alloys were spray formed and the billets extruded under a range of conditions. The alloy compositions were selected to provide an attractive balance of tensile strength and elongation, without relying on complex post‐processing and heat treatments – and are therefore suitable for complex cross section extrusions and free from quench distortion. The as‐spray formed microstructures showed fine homogeneous, equiaxed grains with an average size of 10–15 μm. Firstly, a laboratory scale extrusion was carried out to extrude 30 mm diameter billets into 7.5 mm diameter cross‐section and to investigate the effects of temperature and ram speeds on microstructural development. The microstructure after extrusion at 400°C was partially dynamically recrystallised with a ∼︁0.5 area fraction of ∼︁1 μm grains and the remainder were unrecrystallised elongated grains. Electron backscatter diffraction showed a mixed <111> + <100> double fibre texture. After peak age hardening at 150°C for 96 h, the Al‐6Mg‐1.6Li‐0.4Zr alloy showed a 0.2% proof strength of 495 MPa, a tensile strength of 553 MPa and elongation to failure of 8.5%; Al‐4Mg‐1.3Li‐0.4Zr showed a 0.2% proof strength of 392 MPa, a tensile strength of 476 MPa and an elongation of 10%. Subsequently, large‐scale complex cross‐section were extruded at ∼︁400°C. Due to the complexity of the cross‐sections and the non‐axisymetric deformation, the fibre texture was distorted to FCC rolled Copper and Brass orientations. After a dual‐step age hardening treatment of 130°C for 8h followed by 160°C for 8h, the proof strength, ultimate strength and elongations were 340 MPa, 470 MPa and 16% respectively. The combination of relatively simple process steps, high strength and ductility/formability, and low distortion in complex sections demonstrated the strong potential for these low density alloys in aerospace and autosport applications.