Investigation of superplasticity in friction stir processed 2219Al alloy

Commercial 2219Al-T6 alloy plates were friction stir processed (FSPed) at a rotation rate of 400 rpm and a traverse speed of 100 mm/min in water and air, producing two fine-grained 2219Al samples with average grain sizes of 1.0 and 2.1 μm, respectively. The 1.0 and 2.1 μm-2219Al retained fine-grained microstructure during annealing treatment at temperatures up to 400 and 425 °C, respectively, above which abnormal grain growth occurred. Superplasticity was observed in 1.0 μm-2219Al within the medium temperature range of 350-425 °C and a maximum ductility of 450% was obtained at 400 °C and 3 × 10−4 s−1. Increasing the grain size from 1.0 to 2.1 μm resulted in a slight increase in the thermal stability, but did not enhance the superplasticity of the FSP 2219Al. The relatively low superplasticity in the FSP fine-grained 2219Al was attributed to unstable grain structure because the pining Al2Cu particles were easily coarsened at high temperatures.