Microstructure response and bending fatigue characteristics of typical positions for the integral stiffened structure of 6005A-T6 aluminum alloy after laser peen forming

The comprehensive process of laser peening (LP) for forming and fatigue strengthening of 6005A-T6 integral stiffened structure (ISS) was investigated. The formation characteristics of ISS under the accumulation of multiple pulse lasers were emphasized. The stress distribution, microstructure response, and phase evolution of the LPed region and deformation region were analyzed. The fatigue crack propagation mode and the mechanism for extending fatigue life were revealed. The results demonstrate that a scanning method perpendicular to the target forming direction achieves better forming effects. For specimens scanned in unfold direction scanning, chord direction scanning, and unfold-chord directions, compressive stresses at the bottom of stiffeners are −87 MPa, −134 MPa, and −169 MPa, respectively. The overall bending elongation forming combined with local impact plastic deformation induced by LP refines surface grains, while slightly elongating those in the deformation region through compression. LP enhances the solid solubility of alloy elements, while residual compressive stress (RCS) does not significantly influence the dissolution of the second phase. The fatigue extension percentages for three specimens are 33.9%, 17.3%, and 43.5%, respectively. The increase in grain boundary density of refined grains impedes crack propagation, while RCS that brings materials on both sides of the crack tip closer together induces deviation from the original crack path. Under RCS, micropores and microcracks are closed to prevent further propagation. Fatigue striations propagate simultaneously parallel and perpendicular to the surface, and the multi-level dimple structure consisting of “large dimples containing small dimples” suppresses crack propagation.