Influence of lithium additions on precipitation and hardening of 6061

Abstract

The influence of lithium content on the precipitation behaviour and on the hardening response of 6061 Al–Mg–Si has been investigated for a range of lithium additions. In naturally aged samples, the lithium containing alloys show lesser hardening responses than the 6061 alloy, but hardening in 6061 appears to be due to solute clustering rather than to the formation of resolvable Guinier–Preston (GP) zones. The limited hardening in the lithium containing alloys is due to the slow formation of AlLiSi in 0·8 wt-%Li alloy and to δ′ in 2 wt-%Li alloy. In artificially aged samples, it has beenfound that addition of 0·8 wt-%Li leads to precipitation of AlLiSi ternary phase, which inhibits the formation of the GP zonesformed in 6061, and therefore reduces the extent of age hardening. In 2 wt-%Li alloy, the ordered δ′ phase becomes the predominant precipitate and neither GP zones nor AlLiSi precipitates are observed. The 2 wt-%Li addition produces a strong artificial aging response, which is close to that of 6061. The results show that there is a complex interaction between lithium and the other solute elements in 6061 and the observations are briefly discussed in terms of the development of alloys based on this system.

MST/1224