Abstract
As a surface treatment technology to improve the surface properties of metallic materials, laser shock peening (LSP) has been widely researched and applied. LSP process produces the high-temperature and high-pressure plasma. The violent expansion of the plasma generates shock waves applied to the surface of the material. Compared to other surface strengthening methods, LSP has a high strain rate. LSP process can increase the surface hardness of the material, introduce compressive residual stress and improves the mechanical properties of the material. However, the conventional LSP may be limited by the energy conversion efficiency and compressive stress depth, which limits the application of LSP to some extent. Further optimization of the LSP process to achieve better reinforcement effect has become one of the important research directions.
In this study, a new LSP method, magnetic field assisted laser shock peening (MA-LSP), was used for AA7075-T6. In the MA-LSP process, the sample was placed in a constant magnetic field environment to conduct LSP. The effect of peening was improved by the synergy of a magnetic field and LSP. The effects of MA-LSP process on the surface morphology, microstructure, surface hardness and residual stress of AA7075-T6 were investigated and compared with the conventional LSP to explore the influence of magnetic field on the peening effect. The results showed that the surface peening marks of MA-LSPed sample were deeper compared to those of conventional LSP, indicating a greater impact energy received. After MA-LSP, the surface grains of sample were more significantly refined, and the surface hardness of the sample was significantly improved with a deeper hardened layer produced. In addition, the results of the in-depth residual stress test indicated that the surface compressive residual stress of the sample after MA-LSP was increased by 19.46% compared with that of the conventional LSP, and the maximum compressive residual stress occurred at a depth of 100 μm from sample surface, which was an increase of 16.85%. Meanwhile, the compressive residual stress layer of MA-LSPed sample was also deeper.
The improved effect of magnetic field on LSP is mainly attributed to the fact that the magnetic field can effectively constrain the plasma, so that the expansion of the plasma is limited, which can promote the shock wave induced by the plasma expansion is more effective transmission to the interior of the material. Besides, due to the magnetoplastic effect, the magnetic field can effectively promote the dislocation motion, thus LSP is more effective in improving the microstructure and properties of materials. These synergistic effects ultimately increase the strengthening effect of the LSP process, which is expected to further improve the mechanical properties of the material and extend its fatigue life.
