Microstructure and mechanical properties of ultrafine-grained AA2024 sheets joined by underwater friction stir welding.

High-speed underwater friction stir welding (UFSW) of cryo-and warm-rolled ultrafine-grained AA2024 aluminum alloy sheets composed of high-density nano metastable phase S′, and conventional coarse-grained AA2024-T4 ultra-thin sheets composed of high-density GPB zones has been carried out. The microstructure of the welded joints was characterized by optical and scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, and transmission electron microscopy. The results indicate that by substituting the advancing side (AS) with ultrafine-grained material, the strength and plasticity of the joint can be considerably improved (the yield stress increases by 50 MPa, and the strength coefficient reaches 98% of that of the base metal), providing a promising method for the low-cost and high strength connection of aluminum sheets. [Display omitted] • CWR UFG AA2024 sheets were joined by underwater friction stir welding. • For the AS with UFG material, the strength and plasticity of the joint improved. • CWR/T4 joint showed the best performance, with a YS of 98% of the T4 BM. • Less coarse S phases are generated during UFST in the CWR-AA2024 sheets. [ABSTRACT FROM AUTHOR]