Laser Shot Peening Induces Residual Stress Of 6082-T6 Aluminum Alloy And The Strengthening Effect

Laser shot peening?LSP?is becoming an advanced surface strengthening technology due to its ultra-fast?ns level?,high pressure?GPa level?and ultra-high strain rate(10⁷ s^-1 level).Although 6082-T6 aluminum alloy is widely used in industrial fields,its medium strength and corrosion resistance limits the further use.Applying LSP to the surface strengthening of 6082-T6 aluminum alloy can greatly expand the application fields.In this paper,the influences of different LSP parameters on the residual stress fields were analyzed by means of finite element simulation and experiment.The stress strengthening and microscopic strengthening mechanisms of LSP were explored.The main research contents and conclusions of this paper are as follows:?1?The 6082-T6 aluminum alloy LSP finite element model,in which the impact pressure belong to Gaussian distribution,was established based on the Johnson-Cook strain-sensitive plastic model.The effects of different LSP parameters such as shock wave peak pressure,pulse width,spot overlap rate,spot size and times of LSP on residual stress distribution were studied.The changes of the residual stress layer structure of“compressive-tensile-compressive”were studied.Besides,The formation mechanism of“residual stress holes”phenomenon was explained.The results show thar the surface residual compressive stress and the thickness of compressive stress layer increased gradually with the increase of peak impact pressure.The residual stress layer structure gradually changed in the form of enlarging at both ends and shrinking in the middle.If the pulse width was less than 10 ns,the surface residual compressive stress distribution was uneven.The fluctuation of surface residual compressive stress decreased with the increase of spot overlap rate.The small size spot was more likely to cause“residual stress holes”.The first three LSP had a more significant influence on residual stress.?2?The propagation and attenuation of stress waves inside the target were studied for laser-induced high-speed transient impact pressure.The dynamic characteristics of the target were analyzed by the combination of finite element method and plane wave theory.The structure of elastic and plastic coexistence of stress wave during propagation were studied,which could induce ultra-high strain rate effects.The results show that the stress wave velocity was related to the strain of the target.If the strain increased,the wave speed decreased.The attenuation of stress wave during propagation could be characterized by stress amplitude attenuation and particle velocity attenuation,which were both subject to the exponential function.?3?The surface residual compressive stresses of the specimen were measured by X-ray diffraction method to verify the accuracy of the LSP finite element model.The microhardness of the target was measured.It was found that the microhardness of superficial layer increased significantly.Further more,the thickness of microhardness increasing layer and the compressive stress layer were uniform.The cross-section microstructure of the LSP specimen was observed by OM.As a result,the fine equiaxed grains were found in the superficial layer.A large number of dislocation lines were found inside the grains by TEM.High-density dislocation lines were entangled to form dislocation walls,which could refine the original grains into subgrains.Finally,the fine equiaxed crystals were formed.Besides,the fine precipitates were found near the dislocation lines.Distortion near the dislocation lines could provide energy for heterogeneous nucleation.On the contrary,the precipitations acted as a“pinning”effect to the dislocation lines,causing them to aggregate and providing the necessary conditions for grain refinement.