Delamination of near-surface layer on cold rolled AlFeSi alloy during sheet forming

A near-surface layer existed on a cold rolled and annealed foil stock AlFeSi alloy due to severe shear strain during twin-roll casting and subsequent cold rolling. The near-surface layer, with a thickness up to ~ 700 nm and consisting of ultrafine grains with diameters up to 100 nm and high population of voids/microcracks, was present as randomly distributed patches on the alloy surface. The grain boundaries within the near-surface layer were decorated by oxide particles and inclusions of lubricant residue, which reduce the ductility of the near-surface layer. During sheet forming, further shear deformation occurred in the near-surface region due to the friction caused by the sliding contact between the alloy sheet and the forming tool. The differences in the microstructure and the ductility between the near-surface layer and the underlying bulk alloy resulted in cracking within the near-surface layer and/or at the near-surface layer/bulk alloy interface, finally leading to the delamination of patches of the near-surface layer. The delamination generated flake-like alloy debris may significantly modify the property of forming lubricant and influence the tendency to galling during sheet forming.