Local microstructure and strengthening mechanisms of double-sided friction stir welded Al-Mg-Mn-Er alloy joint

Local microstructure and strengthening mechanisms of double-sided friction stir welded Al-Mg-Mn-Er alloy joint were investigated to reveal its softening mechanism. The results showed that a fine equiaxed grain structure (5.61 μm) was formed in the nugget zone (NZ), while the thermo-mechanically affected zone (TMAZ, 45.32 μm) and heat-affected zone (HAZ, 100.73 μm) maintained a fibrous grain structure. The fraction of low angle grain boundaries decreased from 75.6% of the base metal (BM) to 15.6% of the NZ. The annealing effect resulted in obvious reduction in dislocation density from 1.8×1014 m−2 of the BM to 4.5×1012 m−2 of the NZ. The average diameter size and volume fraction of Al3(Er, Zr) precipitates of the NZ, TMAZ and HAZ were close to those of the BM (13.7 nm and 0.13%). The NZ and TMAZ exhibited the lowest yield strength of about 201 MPa while the BM had the highest yield strength of about 295 MPa. The loss of the dislocation strengthening and substructure strengthening was the main reason for the decrease of yield strength from the BM to the NZ.