The objective of this study was to develop the technology of butt friction stir welding (FSW) for Al–Cu–Li alloy sheets of 1.4 mm thick after hot rolling and subsequent quenching. This initial state of the alloy was chosen in order to provide heat treatment of the received joints after FSW. The TEM observation was performed for the base material and different zones of the joints in order to determine the precipitates and dislocations density and distributions. The grain size in the stir zone (SZ) was increased; the width of the softened region was narrowed down with the decreasing of linear energy. The differences in mechanical properties of the FSWed joints were explained by analysis of microstructural changes and tensile fracture surfaces, supported by the results of the numerical simulation of the temperature distribution and material flow. The fracture of the high linear energy FSW joints occurred in the SZ along the remnant oxide line (ROL), while the low linear energy FSW joint raptured in the transition between HAZ and TMAZ on the AS, where lower hardness value was detected.