The friction-stir welding (FSW) of 0.6 cm plates of 2024 Al (140 HV) to 6061 Al (100 HV) is characterized by residual, equiaxed
grains within the weld zone having average sizes ranging from 1 to 15 mm, exhibiting grain growth from dynamically recrystallized
grains which provide a mechanism for superplastic flow; producing intercalated, lamellar-like flow patterns. These flow patterns
are visualized by differential etching of the 2024 Al producing contrast relative to 6061 Al. The flow patterns are observed to be
complex spirals and vortex-like, among others, and to change somewhat systematically with tool rotation (stirring) speed between
400 and 1200 rpm; depending on tool orientation. The equiaxed grain and sub-grain microstructures are observed to vary
according to estimated temperature profiles (varying from 0.6 to 0.8 TM, where TM is the absolute melting temperature) referenced
to the rotating tool axis. Dislocation spirals and loops are also observed in the 2024 Al intercalation regions within the weld zones
at higher speeds (800 rpm) corresponding to slightly elevated temperatures introducing dislocation climb, and residual
microhardness profiles follow microstructural variations which result in a 40% reduction in the 6061 Al workpiece microhardness
and a 50% reduction in the 2024 Al workpiece microhardness just outside the FSW zone. © 1999 Elsevier Science S.A. All rights
reserved.