Microstructure and mechanical properties of underwater friction stir welding of CNT/Al-Cu-Mg composites

The friction stir welding (FSW) joints of CNT/Al-Cu-Mg composites have severe flash defects and softening phenomena due to the excessive heat input. In this paper, underwater friction stir welding (UFSW) was used to transfer the heat input and increase the cooling rate of the joint, which was beneficial to suppress the coarsening of the strengthening precipitates in the thermal mechanically affected zone (TMAZ) and narrow the range of TMAZ. The DIC results demonstrate that the joints obtained by UFSW alleviate the strain concentration in the TMAZ. The nugget zone (NZ) with outstanding performance could synergistically engage in plastic deformation. Thus, the mechanical properties of the UFSW joint increase to 523 MPa, reaching 94.7% of the CNT/Al-Cu-Mg composites, providing further improvement of the FSW joints. Additionally, the flash defects have been improved due to the descent of the thermal cycle, which alleviates the difficulty of post-weld processing and promotes welding efficiency.