Simulative And Experimental Study Of 40Cr Steel Subjected To Laser Shock Peening With Different Parameters On The Stress And Strain

As a new type of surface strengthening technology,laser shock peening has the characteristics of high energy,high pressure,ultra fast and high strain rate.It has been widely applied in the surface strengthening of metal parts.A lot of researches have been carried out at home and abroad on laser shock peening(LSP),but they are mainly concentrated on the field of aviation aluminum alloy.40 Cr steel is often used in the aviation industry for the manufacture of high speed and impacted load of small parts.The traditional methods of surface strengthening often have the disadvantages such as Low efficiency,high temperature and poor working conditions.In order to solve the above problems,based on 40 Cr steel as the research object,ABAQUS finite element software system was used to study the effects of different process parameters(impact times,spot diameter,pulse loading time,laser energy and pressure rise time)on strengthening induced by LSP.The different overlap scheme of LSP was analyzed.The difference of double-sided simultaneous impact and double-sided interval impact on residual stress and surface deformation was investigated.The influence of the forward and reverse surface spacing on the residual stress and the residual stress hole was discussed.The LSP treatment of 40 Cr steel block samples was carried out by pulsed laser and the result was compared with the simulation results.The principle of "residual stress hole" was analyzed,and the influence of different process parameters on it was put forward.Two methods of suppressing "residual stress hole" were investigated.(1)The three-dimensional analysis model was constructed by ABAQUS finite element software.The influence of different process parameters on residual stress,strain and surface deformation was investigated and the effect of "residual stress hole" was also analyzed.The effect of different LSP process parameters on the strengthening effect was investigated by comparatively systematic numerical simulation on 40 Cr steel of LSP.Taking the number of impacts as an example,the maximum value of residual compressive stress is 468.36 MPa,540.87 MPa and 556.31 MPa respectively when the number of impacts is 1,2 and 3 respectively,and the depth of residual compressive stress is also increasing along with the number of impacts increasing.As the number of impacts increases,the surface deformation and the equivalent plastic strain also increase.The influence of different parameters on the problem of "residual stress hole" was analyzed.It was not the more the number of impacts,the longer the pulse load time and the greater the energy to enhance the effect of the better.(2)The effect of LSP with different scheme of spot overlapping was studied,which lays the foundation for the subsequent massive LSP.The differences of residual stress,surface deformation and equivalent plastic strain induced by double-sided simultaneous impact and double-sided interval impact were studied.The analysis shows that the deformation of the specimen surface and the distribution of the residual stress became relatively uniform with the spot overlap rate increasing.When the spot overlap rate is relatively small and relatively large,the surface deformation value would be relatively large.The stress distribution of double-sided interval impact is better consistency than double-sided interval impact.(3)The merits of twice double-sided alternate impacts were analyzed.The advantages and disadvantages of three impact sequences were investigated.There were three sequences of twice double-sided alternate impacts.The residual compressive stress of the top surface induced by LSP impact of the order 1 and the order 2 was larger than that of the reverse surface.while,the result of the order 3 were just opposite to the conclusion of the order 1 and the order 2.Compared with the order 1 and the order 3,the "residual stress hole" produced by the order 2 is the smallest.(4)40Cr steel was treated by LSP with different laser energies.The experimental results and the simulation results of residual stress distribution induced by LSP with different laser energies are compared to verify the accuracy of numerical simulation results.The method of suppressing the problem of "residual stress hole" was also studied.The residual stress distributions induced by LSP with different laser energies of the surface and depth direction of the specimen were measured by X-ray stress tester and compared with the simulated numerical value.The maximum compressive stress on the surface of the test is 461 MPa,which is close to the maximum compressive stress of 447.5 MPa by numerical simulation.It is possible to effectively suppress the "residual stress hole" problem by using the two element optical diffraction method to change the circular spot into a square spot and using the laser overlap rate for massive LSP.The conclusion that the optimal spot overlap rate of LSP should be between 50% and 75% was obtained.