Grain boundary sliding during friction stir spot welding of an aluminum alloy

The grain and dislocation structures in an Al–4.3Cu–1.4Mg alloy friction stir spot weld made using tool rotational speeds from 750 to 1500 rpm are investigated. The stir zone region contains grains with random boundaries, where grains with dimensions <250 nm are free of dislocations, and grains having average dimensions >500 nm possessed high dislocation densities. Dislocation annihilation is negligible during the spot welding process. Grain boundary sliding accounts for the low dislocation density in grains having sizes <250 nm.