| Foreign object damage(FOD)would occur when aeroengine blades’ leading edges are impacted by sands or gravels during service,which will affect their fatigue properties.As an advanced surface treatment technology,laser shock peening(LSP)can effectively improve the fatigue performance of blades by introducing compressive residual stress and refining surface grains.However,the traditional one-sided LSP can cause uneven stress distribution and reverse bending on the surface of the specimen,which is not suitable for thin-walled specimens such as aeroengine blades.In this research,two-sided laser shock peening technology(TSLSP)is used to strengthen Ti-6Al-4V titanium alloy specimens,which are commonly used in engine blades.The mechanism of improving fatigue performance by TSLSP is studied by combining experiment with simulation.In view of the limitation of direct TSLSP(D-TSLSP)in reducing the FOD resistance of specimens,a "soft-hard" alternating energy absorbing structure was obtained by optimizing the laser shock process,and the mechanism of improving the fatigue and FOD resistance properties of the structure was studied.The main research contents and results are as follows:(1)The strengthening mechanism of TSLSP on Ti-6Al-4V titanium alloy was analyzed theoretically.Besides,the optimum process parameters of TSLSP used for Ti-6Al-4V titanium alloy were obtained by analysis.Based on the stress concentration theory,the influence of FOD notch on the fatigue performance of Ti-6Al-4V specimens was studied.The damage notch and fatigue fracture morphology of untreated and TSLSP treated specimens were observed and analyzed.The results suggested that the damage notch was the key factor for the decreasing fatigue performance of the specimen.(2)The evolution of laser induced residual stress in FOD/fatigue fracture process and the influence mechanism of residual stress on fatigue performance of the specimen were investigated by means of experimental and simulation comparison methods.The surface stress distribution of the sample after TSLSP,TSLSP + FOD,TSLSP + FOD + fatigue were measured,and the surface stress evolution process of the samples were simulated.The results indicated that the damage caused by FOD changed the stress distribution on the surface of the specimen,but the compressive residual stress field induced by TSLSP was relatively stable,and no significant relaxation was observed during the fatigue process.(3)In this thesis,TSLSP induced high magnitude compressive residual stress andincreased grain boundary area composite strengthening mechanism was investigated to explain the improvement of fatigue properties.TSLSP could not only increase the fatigue crack growth threshold by introducing high magnitude compressive residual stress,but also decrease the fatigue crack growth rate(FCG rate)by increasing the grain boundary area caused by grain refinement,which indicated that the synergistic effect of laser shock induced compressive residual stress and grain refinement could improve the fatigue performance.(4)Through the observation of fatigue fracture morphology and quantitative analysis and calculation,combined with the residual stress simulation results,the mechanism of improving the material’s resistance to FOD and fatigue properties by interval TSLSP(I-TSLSP)was summarized,and the fatigue crack growth law of the specimens after I-TSLSP was studied.The results suggested that the "soft-hard" alternating energy-absorbing structure induced by I-TSLSP absorbed impact energy through plastic deformation in the "soft" zone,reduced the fatigue crack growth rate by using the "hard" zone,thus improved the FOD resistance and fatigue properties of the specimens,while the FCG rate showed a high trend in first and low trend in late. |
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