The evolution of precipitation and microstructure in friction stir welded 2195-T8 Al–Li alloy

Precipitate and microstructure evolution in friction stir welding of 2195-T8 aluminum alloy was characterized by transmission electron microscopy. The results show that precipitations in the base metal primarily consist of T1 (Al2CuLi) platelets and small amounts of θ′ (Al2Cu) and τ2 (Al7Cu2Fe) phase. In the heat affected zone (HAZ), these precipitations dissolve during welding, allowing the re-precipitation of δ′ (Al3Li) and β′ (Al3Zr) during cooling. δ′ and β′ phase are the major strengthening phase in the weld nugget zone (WNZ), which results in the observed lower microhardness of the nugget region. Differential scanning calorimetry (DSC) curve is used to confirm and interpret the results provided by the microscopy.