Heterogeneous microstructure and mechanical property of thin-walled tubular part with cross inner ribs produced by flow forming

Flow forming was employed to manufacture the 2219 aluminium alloy thin-walled tubular parts with cross inner ribs (longitudinal and transverse inner ribs, LTIRs) integrally, which have been widely used in the aviation and aerospace fields. Electron backscattered diffraction (EBSD) examination along with micro-hardness tests was conducted on the typical zones of the flow formed part, thus to make a detailed study on the deformation modes as well as strain gradient induced heterogeneous microstructure and mechanical property. The wall thickness thinning is dominant at non-rib zone, while it is the die filling that mainly prevails at TIR and LIR zones, resulting in the occurrence of three different deformation modes. The plane strain deformation contributes to the severe fragmentation of ribbon-like grains at non-rib zone, while the axial tensile strain during longitudinal inner rib (LIR) forming and small radial tensile strain during transverse inner rib (TIR) forming cannot make the large ribbon-like grains refined effectively. In order to refine the microstructure and obtain uniform mechanical property, modifying the initial microstructure and changing the strain types at rib zones by full die filling, as well as subsequent annealing treatment should be considered.