Understanding material behaviour of ultrafine-grained aluminium nano-composite sheets with emphasis on stretch and bending deformation

This study investigates the forming limits of cryo-rolled Ultrafine Grained (UFG) aluminium sheet for loading conditions relevant to sheet metal forming. It is shown that a combination of nano-particle strengthening with an annealing heat treatment enables the production of cryo-rolled UFG aluminium with up to 12 % higher material strength and 20 % higher formability compared with the unreinforced UFG condition. While nano-particle strengthening does not significantly influence the tensile ductility, it reduces the bend fracture limit by over 50 % due to an increased dislocation density and a diffuse sub-grain structure. The level of the forming limit curves (FLD) of all UFG materials was up to 5 times higher than their uniform tensile elongation and hardening index; this is attributed to a high strain-rate sensitivity, which increased with annealing heat treatment. In roll forming, the UFG aluminium showed similar material behaviour and shape defects as observed for conventional sheet metals in previous studies. However, given the reduced bend fracture limits observed with nano-particle addition, this study suggests that roll forming may not be a suitable manufacturing solution for nano-particle strengthened UFG sheet metal.