Simulation And Experiment Research Of Fatigue Life Improvement Of Titanium Alloy By Laser Shock Processing

Laser shock processing is a new surface modification technology, which makes use of the mechanical effect of high pressure shockwave generating in the process of interaction between metal materials and high power density,short pulse.It causes metal materials to yield and plastically deform,thereby the surface layer develops dislocation of high density,twins and high level compressive residual stress.Those prolong the fatigue life of metal materials greatly.LSP has extensive applied foreground in some modern manufacturing such as aerospace industry,automotive engineering and so on.According to the theory of residual stress field formation by laser shock wave,Laser shock processing of TC4 Ti alloy was researched by the methods of experiments and simulations.The effection of residual stress field to fatigue life of center-hole specimen as well as the prediction methods of which were researched.The achievements were:According to the theory of plastic-elasticity dynamics,the basic theory of the shock wave generation and propagates in material as well as the formation mechanism and evaluation method of residual stress field was described.Based on ANSYS/LS-DYNA software,the 3D FEA and semi-2D FEA analysis models were founded,considering the finite element theory. Some key problems were discussed and the distribution of the residual stress field generated by LSP was obtained.The effects of laser shock power density was simulated which indicated that the peak surface residual compressive stress increased as the power density increased when the laser power density was below a fixed value,and then decreased,even to tension residual stress.At the same time,the maximum compressive residual stress appeared below the surface was increasing all the time.The local stress-strain method was described by focusing the stress concentration and the fatigue notch factor k_f The laser shock processing experiments and fatigue experiments of 2024 Al alloy and 7050 Al alloy were collected and discussed.The results indicated that the qualification of the shocked region make a big effect to fatigue life improving, especially the roughness.Fatigue life can be increased by 6～9 times under good-improved roughness and 2～3 times under the normal roughness.The amplification of fatigue life will be increase when the fatigue loads level down.The improvements of yield stress and elastic modual induced by laser shock processing make good effords to prevent the crack propagation.Fatigue load simulation and life prediction was done for center-hole specimen of Ti alloy with and without LSP.The fatigue lives gained by the stress results at the middle position of the notch root in the case of no plane strain are shorter than which gained by the stress results in the case of plane strain.The fatigue lives are increased about 1.9～3.3 times.