Lowering the temperature for high strain rate superplasticity in an Al–Mg–Zn–Cu alloy via cooled friction stir processing

An Al–Zn–Mg–Cu alloy was friction stir processed over two kinds of backing anvils, at two different cooling rates. A finer grain size, 0.3 vs 0.5 μm, was obtained by processing at the highest cooling rate. Both materials showed superplastic behavior with a maximum elongation to fracture of about 510%. Grain boundary sliding was the operative deformation mechanism. Furthermore, the finer grain size material showed high strain rate superplasticity, at 10−2 s−1, at lower temperatures, as low as 250 °C.