Insights into the influences of nanoparticles on microstructure evolution mechanism and mechanical properties of friction-stir-welded Al 6061 alloys.

Forging and welding are common necessary procedures for AA6××× alloys, and recrystallization is an inevitable process. However, it is still difficult to realize the control of the recrystallization behavior, while realizing the strengthening of mechanical properties of the base metal and welded joint. In this study, we overcome this problem through 0.5 wt% TiC–TiB 2 nanoparticles, and fully reveal the influence of the nanoparticles on the recrystallization behavior of the base metal and nugget zone of a friction-stir-welded (FSW)-ed joint from the perspectives of dislocation rearrangement and grain boundary motion. The strengthening mechanisms of the base metal and FSW-ed joint are clarified. The recrystallization driving force of the base metal and nugget zone was increased. Dislocations with a higher density rearranged and formed more grain boundaries in the nugget zone. Besides, nanoparticles distributed on the grain boundaries restrained the vanishing of low-angle grain boundaries. The nugget zone microstructures were refined from 3.1 to 2.3 μm, and the recrystallization ratio was increased from 7.2% to 10.4% at 800 rpm. The grains in the nugget zone were refined from 2.2 to 1.9 μm, and the recrystallization ratio was increased from 16.1% to 18.4% at 1200 rpm. The promoted recrystallization in the nugget zone accelerated the release of stress. Nanoparticles weakened the precipitate coarsening in the nugget zone. After strengthening, the ultimate tensile strength and plastic strain of the FSW-ed joint at 800 rpm were increased by 4.7% and 18.8%, respectively. This study provides new approaches for a systematic microstructure evolution control in FSW-ed 6061 Al alloys. • Trace nanoparticles double the recrystallization driving force of base metal. • More dislocation tangles promoted recrystallization of nugget zone. • Nanoparticle also inhibited grain coarsening of nugget zone. • Precipitate coarsening in the nugget zone was weakened. • FSW-ed joint softening was alleviated and the mechanical properties were enhanced. [ABSTRACT FROM AUTHOR]