Research On Laser Shock Processing And Composite Strengthening Of Thick Plate With Small Holes

Thousands of connection holes and fastening holes exist in the aircraft.The small hole structure is a typical stress concentration detail and is prone to failure under fatigue loads.This not only causes economic losses,but also presents a security threat.Therefore,it is of great significance to improve the fatigue life of small hole component.The laser shock processing can make the small hole component plastic deformation,introduce the residual compressive stress layer,thereby the small hole component fatigue life is improved.However,due to the limited depth of residual compressive stress formed on the surface,the laser shock effect for the thick plate is limited.Ultrasonic vibration assisted hole extrusion can reduce the mandrel pull force and improve the service life of the mandrel,and that the small hole extrusion can be achieved,but the corner after the extrusion stress is larger.In order to make up for the deficiencies of the above two strengthening methods and improve the fatigue life of the small hole components largely,this paper proposes a composite strengthening method of laser shock processing(LSP)and ultrasonic vibration extrusion(UVE).In this paper,a combination of theoretical analysis,numerical simulation and experimental verification are used to compare the effect of LSP,UVE,composite strengthening on the7050-T7451 aluminum alloy small hole components,explore formation and mechanism of residual stress and fatigue fracture change law.The main work and conclusions of the study are as follows:(1)Through the LSP theoretical analysis,residual stress simulation analysis and fatigue contrast test of holes with different thicknesses,it was found that there was thickness effect in the LSP of small hole components.The effect of LSP to different thicknees small hole components was mainly related to the stress distribution of the hole wall.Under the same laser power density,the depth of the residual compressive stress formed on the surface of the small hole components with different thicknesses was certain,but with the increase of the thickness,the range of residual tensile stress on the hole wall increased.The source of fatigue tended to initiate in this range,so the LSP effect of thick plate small hole components was lower than that of thin plate small hole components.When the laser power density was increased,the residual tensile stress range in the middle of the thick plate hole wall decreased,but the stress peak increased,and the surface was easily ablative,the shock pits became larger,and the plate deformation increased.Therefore,the use of LSP for thick plate small hole components had certain limitations.(2)Based on the ABAQUS simulation software,the influence of UVE characteristics on the mandrel extrusion process was explored.The best mode shape and frequency of the mandrel were determined by modal analysis.The friction force with and without ultrasonic and different ultrasonic amplitude,the variation and the distribution of residual stress in the hole wall were analyzed.The results showed that the ultrasonic vibration played a role in reducing the friction force during the extrusion of the mandrel bar,and the use of amplitude in the UVE was determined.Finally,an ultrasonic vibration extrusion test platform was set up.(3)Through the above analysis and research,it was proposed to combine the two methods of strengthening and form a new composite strengthening method.ABAQUS was used to investigate the influence of the critical parameters such as the laser power density and squeeze volume on the distribution of residual stress in the small hole.The results showed that at a power density of 2.83 GW/cm~2 and a squeeze rate of about 3%,the relatively stable and large residual compressive stress appeared.The composite strengthening method had passed the test verification.It was found that the composite strengthening effect was indeed more significant than other methods.(4)By scanning electron microscopy,three fatigue fractures that under three methods of strengthening were analyzed.It was found that most of the fatigue sources occurred in areas with greater stress after strengthening,and the crack expansion had a tendency to bypass the residual compressive stress areas.The composite strengthening fatigue source appeared on the surface of the sample and outside the laser shock zone,and the fatigue crack grate rate was lower than the other two methods.It was indicated that the composite strengthening formed a good residual compressive stress field on the wall and the surface of the small hole,which effectively inhibited the fatigue crack initiation and expansion in the stress concentration area of the small hole.In this paper,through the contrast study of LSP,UVE and composite strengthening,a new method for improving fatigue life of small hole components had been proposed.