Influence of Cu/Li Ratio on the Microstructure Evolution of Bobbin-Tool Friction Stir Welded Al–Cu–Li Alloys

Two modern aluminum lithium alloys were welded by semi-stationary bobbin tool friction stir welding. The influence of the Cu/Li ratio on precipitation phenomena under process heat impact was investigated by comparing the response of low Cu/Li alloy 2196-T8 and high Cu/Li alloy 2060-T8. Identical process parameters with a weld pitch of one rotation per mm were used to conduct flawless weldments. The thermal history and microstructural features were studied and correlated to the resulting mechanical properties of the welds. Analysis of microstructure using differential scanning calorimetry and high energy X-ray diffraction technique showed significant differences in the precipitation sequence of the base metal and in the welded samples of the two alloys of interest. A low Cu/Li ratio led to a higher softening resulting in a reduction of 43% of base metal yield strength while the high Cu/Li ratio alloy AA 2060 could demonstrate more thermal stability (38% reduction). Severe dissolution of the T1 precipitate and presence of equilibrium phases were confirmed for the stirred zone of both alloys. The heat affected zone suffered dissolution and overaging reactions leading to a mechanically unfavorable microstructure. The low Cu/Li alloy 2196 developed a higher process temperatures and exhibited a more evolved precipitation sequence.