The Effects Of Laser Shock Peening On Residual Stress And High Temperature Fatigue Life Of Aluminized Steel

The heat exchange tubes in the solar thermal power generation system usually operate in the corrosive environment created by high temperature heat storage medium.In addition,they also suffer various dynamic loads,resulting in fatigue failure.321 austenitic stainless steel has high strength and low high temperature sensitivity,so it is widely used in heat exchanger tubes.Aluminizing treatment can improve the corrosion resistance of 321 stainless steel,but the aluminizing process cannot meet the requirements of fatigue resistance of heat exchange tubes.Laser shock peening can effectively improve the fatigue performance of metal materials by forming high amplitude residual compressive stress and high density dislocation on the surface of metal materials through laser pulse.Therefore,in this paper,aluminized 321 stainless steel is selected as the research object.Through the combination of numerical simulation and experimental research,in order to improve the fatigue performance of aluminized 321 stainless steel,the distribution law of residual stress and the strengthening effect of high temperature fatigue performance of aluminized steel after laser shock strengthening are analyzed,and the strengthening mechanism of laser shock on high temperature fatigue performance of aluminized steel is explored.The main research contents are as follows:(1)The Johnson-cook equation was used to fit the tensile curve of aluminized steel at different strain rates,and the constitutive of aluminized steel was obtained,and the applicability of the constitutive on aluminized steel was verified.The residual stress formation process of laser shock peening aluminized steel was analyzed by finite element software ABAQUS.The results show that the dynamic stress wave induced by laser shot peening overlaps,reflects and attenuates many times during propagation,and gradually transforms into stable residual stress after 10000 ns.From the residual stress distribution in the aluminized steel,it can be seen that the residual compressive stress in the aluminized layer is significantly higher than that in the matrix.With the increase of thickness direction,the residual compressive stress gradually decreases;When the thickness is greater than 1mm,the area of residual tensile stress is defined as the middle area.(2)The effects of laser shock power density,shock times and coverage area on residual stress distribution of aluminized steel samples were studied.The results show that with the increase of laser power density,the residual compressive stress on the surface of aluminized steel increases gradually and tends to be saturated when the power density is 6.59GW/cm2.When the laser power density is 6.59GW/cm2,with the increase of laser shock times,the surface residual stress changes little,but the thickness of residual compressive stress layer increases gradually.Increasing the laser shock coverage area will increase the range of residual stress field in aluminized steel synchronously,but it has little effect on the distribution characteristics of residual stress in the thickness direction.The simulated residual stress of laser shock peening agrees well with the experimental results.(3)The effects of laser shot peening multiple and stress level on high temperature fatigue life of aluminized steel were studied by experiments and simulations.The results show that compared with the specimens without laser shock strengthening,the fatigue life of aluminized steel increases first and then decreases with the increase of the number of shocks,and reaches the maximum at three shocks.Three times laser shot peening can significantly improve the high temperature fatigue life of aluminized steel under different stress levels.From the perspective of fracture characteristics,laser shot peening does not change the fracture characteristics of aluminized steel.The aluminized layer is cleavage fracture,and the matrix part is ductile fracture.However,after laser shock treatment,the proportion of fatigue crack propagation area of aluminized steel increases,and the proportion of transient fracture area decreases,indicating that laser shot peening increases the fracture toughness of aluminized steel.Due to the large surface roughness of the four times laser shot peening,the fatigue life of the sample is lower than that of the aluminized steel.The numerical simulation results of fatigue life show that three times of laser shock strengthening can significantly improve the fatigue life of nodes in the residual compressive stress layer of aluminized steel.(4)The microstructure of the aluminized steel before and after laser shot peening fatigue was analyzed by TEM.The results show that the ability of the aluminized steel substrate to accommodate dislocations is stronger than that of the aluminized layer.The initial deformation of the austenite substrate occurs until the coating reaches the limit of dislocation proliferation,leading to the formation of microcracks on the surface of the aluminized layer.Therefore,the fatigue failure of aluminized steel is mainly caused by dislocation movement.On the contrary,laser shock induces high residual compressive stress and increases the volume fraction of deformation twins in aluminized steel.In the fatigue process of laser shock strengthened aluminized steel,more obvious dynamic recovery and detwinning phenomena were observed,so the high temperature fatigue life of aluminized steel was significantly improved.