| The high-temperature alloy GH4169 is a key material for manufacturing of aero-engine hot-end components,especially for turbine disks.As the aero-engine suffered from the high temperature,high pressure,high speed and high alternating load during long-term service,the surface integrity of its turbine disk affects fatigue life and aerodynamic performance.As an important indicator of surface integrity,the fatigue performance of components depends on the residual stress of machined surface.Therefore,according to the requirement of long-term service,the research on residual stress and fatigue life of GH4169 superalloy finished by hybrid turning and burnishing is carried out to obtain the fatigue properties,which provides a new theoretical basis and technical support for further improving the fatigue life of GH4169.Based on the study of the residual stress on the machined surface of Superalloy GH4169,the prediction model of the stress intensity factor range is established,and the influence of the residual stress on the stress intensity factor at the crack tip is revealed.By revealing the influence law of the surface integrity characteristics induced by hybrid turning and burnishing on the low cycle fatigue life of Superalloy GH4169,the evolution model of the surface "forming process" that induced by hybrid turning and burnishing is proposed,the effective control and optimization of the residual stress on the machined surface has been realized,and provide theoretical basis and technical support for the fatigue life prediction of superalloy GH4169.The geometric model of the tool nose radius and the undeformed chip is established to reveal the effect of the tool radius on the residual stress of the machined surface of superalloy GH4169.Based on the principle of X-ray diffraction,the prediction model of stress intensity factor range of crack tip in the crack source area is established by using cosa method,which aims to reveal the influence of residual shear stress and the residual normal stress on stress intensity factor range.The most significant factor affecting the fatigue life of GH4169 is revealed by investigating the effect of hybrid turning and burnishing processes on fatigue life of GH4169.The evolution model of surface material modification of GH4169 is proposed to achieve optimal control of surface residual stress.First of all,based on the changes in cutting force,cutting temperature and phase of the machined surface of Superalloy GH4169 are analyzed,the main factor influencing the generation of residual stress of the machined surface of GH4169 is explored.The geometric model of the tool nose radius and the undeformed chip is established to reveal the effect of the tool radius on the residual stress of the machined surface of GH4169.The results show that there is no phase transformation during machining GH4169,which indicates that no residual stress is generated as a result of phase transformation.The cutting temperature is increased with the increase of cutting speed,the cutting force is not increased,but the residual tensile stress on the machined surface of the sample is not increased,which indicates that the cutting force produced in the turning process is the main factor affecting the residual stress on the machined surface of the sample.The cutting edge angle and maximum chip thickness decrease with the increase of tool nose radius.The reduction of chip thickness leads to the increase of compression plastic deformation on the machined surface,so the residual tensile stress on the specimen surface increases.Secondly,based on the principle of X-ray diffraction,the model of residual shear stress is established by using cos a method.The prediction model of stress intensity factor range of crack tip in the crack source area is established by considering the influence of residual shear stress and normal stress.The comparison analysis is made with the AK value calculated by Moussaoui model,and the prediction model is verified by the fatigue crack growth test data of GH4169 in the literature.The results show that the surface residual stress along a single direction is not a suitable indicator to evaluate the fatigue life of GH4169 superalloy.With the equivalent stress decreases by 5.2%,the fatigue life of the specimens increases by 39.4%.The values of AK calculated by considering the residual shear stress is in good agreement with the threshold value of fatigue crack growth ?kth of GH4169 superalloy,while the values of AK calculated by Moussaoui model without considering the residual shear stress is about 30%-37% higher than the threshold value Akth.Thirdly,the low cycle fatigue performance of GH4169 superalloy is studied.The biggest factor affecting fatigue life of GH4169 finished by hybrid turning and burnishing is revealed by analyzing the changes of the surface integrity characteristics of GH4169 under different hybrid turning and burnishing conditions Based on the macroscopic and microscopic features of the fatigue fracture,the formation and propagation of the fatigue crack in low cycle fatigue are analyzed,and the mechanism of the fatigue crack initiation by hybrid turning and burnishing Superalloy GH4169 is revealed.The results show that the surface roughness is decreased by 64.3%-70.6%after burnishing as compared to turning;Meanwhile,the minimum principal residual stress is increased by a factor of 2.5-4.2,and the peak value of the minimum principal residual stress of machined surface layer is increases by a factor of 3.6-4.3 after burnishing;After burnishing,the microhardness is increased by 10.5%compared with that of turning,with the increase of burnishing pressure,the microhardness is increased by 1.4%-3.3%;With the increase of burnishing pressure,the X-ray diffraction peak of the sample surface gradually shifts to a higher diffraction angle,indicating that the residual compressive stress increases;After burnishing,the fatigue life of the is increased by 37.1%-82.4%compared with turning,and the longest fatigue life of the sample is 1.55×104 cycles when the burnishing pressure is 18Mpa;the minimum main residual stress is the biggest factor affecting the fatigue life,followed by the roughness of the machined surface;After burnishing,the fatigue crack initiated at the surface,and the crack growth rate is decreased by 23.0%-38.7%compared with turning,which shows that the compressive residual stress produced by burnishing delays the crack fatigue crack growth rate and prolongs the fatigue life of sampleFinally,based on Hertz point contact theory,the calculation model of stress induced by burnishing is established;the initial residual stress caused by turning is considered,and the equivalent stress model is proposed according to the principle of superposition;the independent "load-unload" closed-loop effect of each machining process is used,and the evolution model of stress is proposed by considering isotropic hardening and kinematic hardening of Superalloy GH4169 during hybrid turning and burnishing,the influence of initial residual stress and burnishing parameters on the residual stress is analyzed.The results show that when the influence of initial residual stress caused by turning is not considered,the predicted value of residual stress is much lower than the measured value.When the influence of initial residual stress is considered,the predicted value of residual stress is consistent with the measured value,which shows that the initial residual stress caused by the previous process directly affects the residual stress which was produced by the next process;the surface residual stress value and the depth of residual stress layer increase with the increase of burnishing pressure.Compared with the burnishing pressure,the ball diameter has more influence on the surface compressive residual stress and the depth of compressive residual stress layer,while the burnishing pressure and the ball diameter have less influence on the peak value of compressive residual stress. |
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