Effect of base material temper condition on precipitate evolution and mechanical properties of 2195 Al[sbnd]Li alloy friction stir welding joints

Abstract

In this study, 2195 AlLi alloy plates with different temper conditions (O, T4 and T8) were subjected to friction stir welding (FSW). Precipitate evolution of the three FSW joints (2195-O/T4/T8 joint) was investigated by transmission electron microscopy (TEM). Mechanical properties of these joints were analyzed using micro-hardness and tensile test. Efforts were made to explore the correlation between precipitates, mechanical properties and fracture behavior of the joints. It can be found that the precipitates from the base material are unevenly dissolved in the nugget zone (NZ) of the 2195-O joint, while the original precipitates mostly disappear in NZ of the 2195-T8 joint, and change a little in the NZ of the 2195-T4 joint. In the thermal-mechanically affected zone (TMAZ) of the 2195-O and 2195-T4 joint, precipitates are almost unchanged, while in the TMAZ of the 2195-T8 joint, most of the precipitates are dissolved and the residual is significantly coarsened. Little change of precipitates occurs in the heat affected zone (HAZ) of the 2195-O joint. T1 newly forms in the HAZ of the 2195-T4 joint (T4-HAZ), while this precipitate is mainly dissolved and coarsened in the HAZ of the 2195-T8 joint (T8-HAZ). Moreover, both grain boundary phases (GBPs) and precipitate-free zone (PFZ) are generated in the T4-HAZ and T8-HAZ. Hardness profile is represented as a “n” shape in the 2195-O joint due to significant harden of the NZ, a “wave line” shape in the 2195-T4 joint attributed to the slight change of precipitation degree, and a “w” shape in the 2195-T8 joint mainly owing to the dissolution and coarsening of T1. The three joints are all broken in their weak regions during tensile test and have a similar fracture behavior, indicating that the crack initiation and development are related to the GBPs and PFZ.