Restoration of exceptional mechanical properties of Al-Cu-Li (AA 2198) alloy post-friction stir processing by recovery of T1 precipitates through controlled temper conditions

Friction stir processing (FSP) is considered to be an effective technique to improve the mechanical properties of metallic materials through grain refinement. However, the fine-grained Al-Cu-Li (AA2198) alloy exhibits a drastic reduction in strength post-FSP as compared to its peak aged temper condition due to the dissolution of its primary strengthening phase, T1 (Al2CuLi) during processing. The present study is aimed at recovering T1 precipitates, thereby restoring the alloy strength. The evolution of microstructure in both the base alloy and FSP alloy subjected to different temper conditions was examined by detailed electron microscopy and was analyzed through a differential scanning calorimetry study. The mechanical properties of the alloy were evaluated by Vickers hardness measurements and tensile testing at room temperature. Experimental findings reveal that the direct aging treatment is ineffective in recovering T1 in FSP alloy due to the delayed precipitation kinetics. However, prior cold working led to the recovery of substantial amounts of T1 during further aging of the FSP alloy. The variation in hardness and tensile properties were in alignment with the precipitation states, and the exceptional mechanical properties were restored in the FSP alloy with the recovery of T1 by cold work prior to aging. The strength modeling indicated that the experimental yield strength after T1 recovery is lower than the predicted value, and these are attributed to the incomplete recovery of the T1 due to trapping of the solute in Curich Al based intermetallics. To dissolve the intermetallics solution treatment was performed after FSP, but it led to significant abnormal grain growth (AGG) and is analyzed using Humphrey's framework.