Ultrafine grain structure in the friction-stir welding of aluminum alloy 2024 at low temperatures

Friction-stir welding involves a solid-state intercalation process facilitated by dynamic recrystallization which accommodates superplastic flow. Adiabatic and frictional heating, a result of high strain and high strain-rate deformation, drive the recrystallization process. In this study we examined the residual grain structures for aluminum alloy 2024 friction-stir welded at starting temperatures which ranged from roughy 30 degrees C to -100 degrees C. The ability to weld any metals or alloys at -100 degrees C is in itself rather extraordinary, especially since the steady state (recrystallized) grain size appears to be somewhere between 0.5 mu m and 0.8 mu m. At higher welding temperatures the residual grain size in the weld zone increased to near 10 mu m as a result of grain growth. At -100 degrees C the welding produced a maximum temperature of around 319 degrees C, but. the residual grain sizes remained in the sub-micron range. In this case the welding was performed with the workpiece submerged in liquid nitrogen. Microstructures have been examined in detail using transmission electron microscopy. The weld zone properties were monitored by measuring and comparing residual microhardness profiles through the stir-welded zone and extending into the base metal (workpieces).