Superplasticity of friction-stir welded Al-Mg-Sc sheets with ultrafine-grained microstructure

The commercial Al-5.4Mg-0.2Sc-0.1Zr alloy was subjected to equal-channel angular pressing at 300 °C to a true strain of ∼12 followed by cold rolling to a total thickness reduction of 80%. The produced ultrafine-grained sheets were joined by friction stir welding (FSW). To evaluate the superplastic properties of the obtained joints, the tensile samples including all of the characteristic FSW microstructural zones were machined perpendicular to the welding direction and pulled up to failure in the temperature range of 400 to 475 °C with initial strain rates of 2.8×10−3 s−1 to 5.6×10−1 s−2. The friction-stir welded material exhibited superplasticity over the entire studied range. However, material flow was found to be mainly localized in the stir zone of the joints, whereas it was almost completely retarded in the base material zone. This effect was attributed to the work-hardened nature of the base material and to its propensity for abnormal grain growth.