Strengthening Mechanism Of 2024 Aluminum Alloy By Electric Pulsed Assisted Laser Shock Peening

For the workpiece with high strength and thickness,Laser shock peening（LSP）process has thickness effect which can not achieve the expected strengthening effect,and it is easy to lead to negative gain of fatigue life because of unreasonable distribution of introduced residual stress.The thickness effect can be reduced by increasing laser energy,but the high energy laser has high cost and harsh application environment.In order to solve these problems,electric pulsed（EP）assisted laser shock peening process（EP-LSP）was proposed,in which pulse current was applied synchronously during laser shock processing to reduce deformation resistance and improve plasticity and formability of materials through electroplastic effect.In this paper,the effect of EP-LSP on microstructure and mechanical properties of 2024 aerospace aluminum alloy was investigated by combining theoretical analysis,numerical simulation and experimental characterization methods,and the mechanism of pulsed current assisted laser shock strengthening was proposed.The main conclusions and innovative achievements are as follows:（1）According to the sample material,size and use environment,determine the pulse power generation device and insulation fixture;Based on the laser impact platform,a pulsed current assisted laser impact composite strengthening scheme was designed to ensure the strengthening effect and achieve the insulation safety of the system.A synchronous loading platform for pulsed current assisted laser impact strengthening was established.（2）Based on laser shock strengthening theory and electroplastic effect theory of pulse current,the Johnson-Cook constitutive model of 2024 aluminum alloy and material parameters in EP-LSP process were modified,and the finite element model of electric pulsed assisted laser shock peening was established.Based on the above model analysis,EP-LSP induces higher residual compressive stress and deeper residual compressive stress layer than LSP under the same number of laser shock layers,and EP can reduce the number of LSP strengthening to saturation.The co-induced theory of"laser shock"and"electroplasticity"was analyzed,and the plastic deformation behavior and residual stress formation mechanism of 2024 aluminum alloy were obtained.（3）The microstructure and mechanical properties of 2024 aluminum alloy under pulsed current assisted laser shock strengthening and pulsed current assisted laser shock combined strengthening were investigated.The results show that EP-LSP has more severe plastic deformation and higher grain refinement than LSP induced 2024 aluminum alloy.The XRD pattern of EP-LSP is wider than that of LSP,and the diffraction peak shifts to the right.EP-LSP induces higher grain refinement and introduces larger residual stress.EP-LSP introduced a higher density of dislocation structure than LSP.Compared with the dislocation network and dislocation wall introduced by LSP,THE EP-LSP sample also found dislocation entanglement and cellular dislocation structure.The dynamic precipitation of 2024 aluminum alloy was induced by EP-LSP,and the precipitated phase was Nanoscale S phase（Al₂Cu Mg）.EP-LSP induces higher microhardness,elastic modulus,residual compressive stress and deeper strengthening layer than LSP.（4）The strengthening mechanism of 2024 aluminum alloy by pulsed current assisted laser shock was proposed.It is found that EP-LSP induced residual compressive stress layer depth increase is due to joule effect of electroplastic and pure electroplastic effect of thermal softening and pure electroplastic effect of initial yield stress reduction.EP-LSP increases the number of potential nucleation sites as well as movable dislocations.The microhardness and elastic modulus of the material are improved by dynamic precipitation effect,and the interaction between the dislocations and the dynamic precipitated phase is further enhanced.After repeated EP-LSP,high density dislocation,dislocation cell and subgrain boundary structure appear in2024 aluminum alloy,and the grain refinement degree is higher than that of LSP.The dislocation network and dislocation wall structure induced by these microstructures have stronger resistance to dislocation,and the reinforcement to saturation of EP-LSP is less than that of LSP.