| The fatigue life of high temperature alloy used in aero-engine turbine disk is low due to the poor surface integrity induced by cutting process.Therefore,this paper takes the surface integrity of turned GH4169 as research objects,to reveal the distribution characteristics and formation mechanism including the microstructure and physical mechanical properties of turned surface materials.Through analysis of influence law between turning process parameters and fatigue life,the optimization of turning parameters will be realized.It will provide theoretical foundation for improving safe service life of high temperature alloy GH4169 parts.In this paper,based on the quantitative evaluation of the time domain and spatial domain distribution characteristics of surface mechanica/thermal loadings imposed on turning workpiece,the formation mechanism of microstructure,working hardening and residual stress in cutting metamorphic layer for GH4169 were expounded.The prediction model of turned surface integrity was established.By revealing the influence law of machined surface integrity of GH4169 on its fatigue performance and the sensitivity analysis of fatigue life to cutting process parameters,the optimization of turning process parameters were proposed to improve the fatigue life.Research results are helpful to the establishment of integrated technology system of material-manufacture-service for high temperature alloy components.The main contents of the thesis include:the quantitative evaluation of the time domain and spatial domain distribution characteristics of surface mechanica/thermal loadings imposed on turning workpiece;microstructure characterization and formation mechanism of cutting degenerating layer for high temperature alloy GH4169;Study of physical and mechanical properties of cutting degenerating layer for high temperature alloy GH4169;and the optimization of anti-fatigue processing parameters for turning GH4169.First,for typical turning superalloy GH4169,based on the analysis of mechanical/thermal loadings in tool-workpiece contact zone,the finite simulation model of turning process is established.By analyzing the model results,the time domain and spatial domain distribution characteristics of surface mechanica/thermal loadings imposed on turning workpiece are obtained.The results show that the time of temperature rise and drop on the workpiece surface is in millisecond range during cutting process.The strain,strain rate and temperature of the machined metamorphic layer are gradient distributed along the depth direction.Secondly,the microstructure in machined metamorphic layer of superalloy GH4169 are characterized using OM/SEM/EBSD/TEM in micro-meso-nano scale.Based on this,the grain gradient distribution characteristics(amorphous-nanocrystalline-micron),dislocation and twin morphology of the machined metamorphic layers were revealed.The recrystallization driving conditions of machined metamorphic layer for the high temperature alloy GH4169 were expounded,the recrystallization mechanism under the interaction of twin and dislocation of machined metamorphic layer was revealed.At the same time,the influence of turning process parameters such as cutting speed and feed rate on the grain gradient distribution characteristics of the machined metamorphic layer by turning GH4169 superalloy is analyzed.The results show that,at the beginning of cutting,the formation of lamellar twins is caused by the increase of strain and strain rate of the surface metamorphic layer.As the cutting process continues,the cutting deformation increases,and the sub-crystal structure in the metamorphic layer rotates under the action of the mechanical force of the cutting tool,resulting in the increase in the orientation difference of the sub-crystal boundary,and finally the formation of the recrystallized grains in the form of an equal axis.Thirdly,based on the grain gradient distribution characteristics of the machined metamorphic layer,the prediction model of microhardness in machined metamorphic layer applicabling to multi-scale grain is established.The influence of cutting parameters on the depth and degree of work hardening were revealed.The FEM simulation prediction model of residual stress in machined metamorphic layer is established.Through FEM results,the influence of thermo-mechanical loadings on residual stress in machined metamorphic layer by turning GH4169 superalloy is analyzed.The influence of grain layer thickness and hardening layer thickness on the residual stress distribution in the machined metamorphic layer is discussed.The results show that the microhardness of the machined metamorphic layer induced by turning GH4169 shows a gradient distribution along the cutting depth.For the residual stress distribution in machined metamorphic layers,the thermal loading mainly affects the maximum tensile residual stress(σsur),and the mechanical load mainly affects the depth of the tensile stress layer(dsub)and the maximum compressive residual stress(σcom).Finally,for turning nickel-based superalloy GH4169,the influences of residual stress,work hardening and microstructure changes in machined metamorphic layer on fatigue life at different temperatures are researched.Based on the macroscopic and microscopic morphological features of the fracture,the origin,propagation and transient fracture characteristics of the fatigue crack in the low-cycle fatigue test of GH4169 were analyzed.The response relationship between low cycle fatigue life and the fatigue stress concentration factor Kf,degree of work hardening RH1’ residual stress S22 and S33,the degree of grain refinement RD of turning GH4169 is investigated.Research results show that the sensitivity order of high temperature(650℃)low cycle fatigue life on surface integrity characterization parameters are strain hardening RHV,residual stress in cutting speed direction S22,surface fatigue stress concentration factor Kf,degree of grain refinement RD and residual stress in feed direction S33.For room temperature(25℃)low cycle fatigue life,the sensitivity order of surface integrity characterization parameters is sensitivity of strain hardening RHV,fatigue stress concentration factor Kf,degree of grain refinement RD and surface residual stress(S22,S33).Within the selection range of cutting parameters in this paper,the cutting speed should be 80-110m/min,and the feed should be 0.10-0.12mm/rev to obtain a higher low cycle fatigue life. |
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