Hierarchical gradient plasticity and strength-ductility synergy in thin-walled Al-Zn-Mg-Cu Alloy enabled by dual-sided laser shock peening

The service requirements for high strength and ductility present challenges to manufacturing methods specifically designed for thin-walled aluminium alloy plates. This study proposes a low-power density dual-sided laser shock peening (LSP) process to strengthen thin-walled Al-6.22Zn-2.53Mg-1.56Cu alloy component. The simulation and experimentation were combined to quantify the residual stress distribution at both time and spatial scales. Multiscale characterization was conducted to revealed grain refinement and precipitation mechanism along the depth direction. It is found that laser shock wave resulted in the effective grain refinement, severe plastic deformation (SPD) with remarkable residue stress and the recrystallized nano precipitates in the affected zone on the sample surface. The residual stress-affected depth induced by LSP treatment could reach 1.5 mm from the sample surface when applying an LSP power density of 2.7 GW/cm². The SPD generated by LSP induced fragmentation of large original precipitates in the Al-Zn-Mg-Cu alloy, in which the average size of precipitates was 62 nm and 17 nm, respectively, in the original samples and LSP treated samples, respectively. As a result, LSP treatment enables a simultaneous improvement in strength and ductility. The yield strength increased from 376 MPa to 487 MPa, the ultimate tensile strength increased from 418 MPa to 572 MPa and the elongation increased from 9.7 % to 12.5 % in the original samples and the samples treated by LSP at a laser power density of 2.7 GW/cm². The strength improvement in LSP treated samples was mainly attributed to dislocation strengthening and precipitation strengthening. This study provides new insights into the manufacturing of high strength thin-walled aluminium alloy component.