Effect of Lithium Addition on Microstructure, Texture and Mechanical Properties of Al-Mg-Si Alloys

Abstract

A number of Li containing 6××× alloys were prepared and characterized for potential aerospace applications. The alloys composition in weight percent were, Al-0.5Mg-0.2Si-xLi and Al-0.9Mg-0.5Si-xLi, with x = 0,1,2,3 respectively. Zirconium additions were also made in the latter alloy system. In the first phase the effect of Li on low alloy group (LAG alloys) was studied. The microstructure and mechanical properties changed substantially with Li. SEM/EDX and XRD analysis revealed that Li addition promoted the formation of Al3Li phase which strongly affected the mechanical properties. The alloy containing 3% Li showed maximum hardness after aging. In the second step the aging response of Al-0.5Mg-0.2Si-3Li alloy in W, T6 and T8 conditions was evaluated. Cold reductions, through rolling were made up to 10 - 60 % followed by aging at 175 °C for 2 - 12 hrs. Maximum hardness was achieved after 60 % cold work and 10 hours of aging time, attributed to the fine Al3Li precipitates. In the third part, the LAG alloys were investigated for micro-texture evolution. The intensity of the deformation texture increased with Li content, while Li helped in reducing texture strength after aging. Significantly low fraction of 3 type coincident site lattice (CSL) boundaries was observed for all Li containing alloys, as compared to the Li-free compositions in the aged condition. In the fourth phase, the effect of Li/Si and Li/Mg ratio on LAG and High alloy group (HAG alloys) was investigated. Inverse pole figure (IPF) maps, TEM/EDX and XRD indicated that Li addition above 1 wt.% promoted the formation of Al3Li precipitates. It was concluded that Li/Mg ratio played a more effective role in comparison with Li/Si ratio for enhanced aging response. In the final phase, HAG alloys microalloyed with Zr were extruded, quenched and aged. The alloy density was reduced up to 7.8% in comparison to the Li-free alloy while UTS increased by 62% with 3% Li addition. EBSD data revealed that Li effectively reduced the grain size of the modified alloys. The hardness, YS and UTS first decreased for 1 wt.% Li addition and then increased with increasing Li content from 2 to 3 wt.% at the expense of ductility. After T6 condition, the texture intensity of the extruded alloys increased with Li content, contrary to the observation made for the rolled alloys that did not contain Zr. This was attributed to the presence of Al3Zr dispersoids in extruded HAG alloys. Finally, a synergistic effect of Li and other alloying elements in 6××× alloys on mechanical properties and texture is obtained.