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

• The Al 7075-T6 alloy was friction stir processed over two kinds of backing anvils.
• The FSPed grain sizes depend on the refrigeration temperature for each backing anvil.
• Large refinement down to 0.5 (in air) and 0.3 (N2 refrigerated) μm allows GBS.
• Low temperature high strain rate superplastic window at 250–350 °C was obtained.
• A eF = 270% record at 250 °C and 10−2 s−1is obtained for the finest FSPed material.

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.