Effect of grain size refinement and precipitation reactions on strengthening in friction stir processed Al–Cu alloys

Strengthening by grain size refinement and precipitation in friction stir processing (FSP) of Al–Cu alloys was investigated. Interestingly, even with significant grain refinement (<1 μm), there was a substantial reduction in microhardness. This is attributed to the loss of precipitation hardening due to the formation of equilibrium θ in lieu of metastable θ′ and θ″ precipitates. Theoretical calculations suggest that the average grain size in the FSP zone must be smaller than 120 nm to compensate for this loss.