The Effects Of Laser Shock Peening (LSP) On Residual Stress Distribution And Fatigue Life Of AA2024-T351 Aluminum Alloy

It is a well-known fact that several fatigue failures initiate from the crack region of stress concentration sites located on the top most surface of most components subjected to cyclic fatigue loading.The inducing of residual stress into the top surface of the material through techniques such as shot peening and laser shock peening can retard both fatigue crack initiation and propagation.Thus,result in fatigue life increase of the component,resist wear and stress corrosion cracking?SCC?.Plastic deformation generated by plasma shock waves at the top most surface of a material is the main induction of residual stresses.The need to study the effects of residual stress on fatigue performance of AA2024-T351 aluminum alloy to retard fatigue failure and improve fatigue life is motivated by the critical role the metallic material plays in the aerospace industry.Thus,the topic the effects of laser shock peening?LSP?on residual stress distribution and fatigue life of AA2024-T351 aluminum alloy.A novel strategy,scanning path gradient in the advancing direction was developed to optimize the laser peening pulse sequence.3-D finite element analysis?FEA?was used to simulate the residual stress distribution,and establish a relation between the simulation results and the experimental work of the metallic material.The developed FEA model was used to predict the effect of the scanning path gradient and the normal scanning path in the advancing direction on residual stress distribution during LSP and was verified experimentally.The two laser pulse sequence scanning path strategies were conducted on a‘dog-bone'specimen.Both strategy of fatigue lives were compared to the untreated specimen,but the LSP specimen revealed an increase in fatigue life.The fatigue life of strategy 1 was increased by 69.3%thus 35,292 cycles,while that of strategy 2 was also increased by 93.5%thus 40,321 cycles and this was achieved by optimization of the laser pulse sequence.The final outcome between the simulated model and the experimental results showed a good correlation.The results further revealed that tensile residual stresses were sandwich between the apex and bottom surfaces of the compressive residual stresses treated by double-sided peening.Hence,LSP can also generate insignificant tensile residual stresses in the mid-thickness of double-sided treated specimens.The effect of laser pulse energy on fatigue life was also investigated.The results indicated a significant increase in the fatigue life extension as the laser pulse energy increased.In addition,as the laser pulse energy increased,the residual stresses increased at the specimen top surface.It was also revealed that when the laser pulse energy is higher than the critical value,internal cracking could occur within the metallic material,which reduces the fatigue life of the material.The crack propagation rate of the laser treated specimen was significantly reduced,while the fatigue crack propagation of the untreated specimen was fast.The laser pulse energy of 3 J,4 J,and 5.7 J,corresponding to the fatigue life gains were 41%,73%and 165%,respectively.Post-failure morphology analysis was carried out on the fractured surfaces via scanning electron microscope.The fracture morphology showed crack nucleation sites,dimples and fatigue striations features.Finally,the effects of multiple impacts on fatigue life and surface integrity was also investigated and analyzed sequentially.There was significant improvement in the residual stress distribution and microhardness after multiple impacts.The increase in the number of impacts,increased the residual stress and microhardness.Additionally,the surface roughness of the multiple impact specimen increased as the number of impacts increased,this was observed when compared with the untreated specimen.After multiple impacts,the specimen experienced grain refinement and microstructure changes due to the ultra-high rates of 10⁶ s^-1.The fatigue lives of the treated specimen increased significantly as the number of impacts increased.The percentage increase in fatigue life of the multiple impacts compared to the untreated specimen were 22.4%,59%and 101%for 1 impact,2 impacts and 4 impacts,respectively.