| The nickel-based superalloy GH4169 is widely used in the hottest components of aerospace jet engines,various industrial gas turbines,and other applications due to its high temperature strength,excellent oxidation resistance,corrosion resistance,and fatigue resistance.However,GH4169 is a typical difficult-to-machine material due to its low thermal conductivity,multiple hard phases,high hardness,and severe work hardening.During machining,it is prone to surface deterioration,which has a significant negative impact on the material’s service life and fatigue life,with the most significant effect being the occurrence of a white layer in the affected zone.High-speed cutting is currently an effective method for machining superalloys,aimed at improving machining efficiency and surface quality.Therefore,studying the surface integrity of GH4169 during highspeed milling is particularly important.This thesis analyzes the influence of cutting parameters in high-speed milling on the surface integrity of GH4169.Microstructural observation,energy-dispersive X-ray spectroscopy(EDS),X-ray diffraction(XRD),and electron backscatter diffraction(EBSD)were used to observe and analyze the changes in the surface microstructure,metallographic distribution,and grain structure of GH4169 after high-speed milling.Additionally,finite element analysis was used to reveal the formation mechanism of the white layer on the machined surface and investigate the physical and mechanical properties of the surface layer.The main research contents and conclusions are as follows:Firstly,different machining parameters(milling speed,radial milling depth)were selected for the milling of GH4169,and the effects of milling parameters on cutting force,cutting temperature,surface deteriorated layer and white layer thickness,as well as surface hardness,were studied based on single-factor milling experiments.The study found that the metallographic structure of the white layer and the deteriorated layer on the machined surface of GH4169 was significantly different from the base material.The white layer exhibited compactness and no apparent structural features.The transition layer below the white layer showed obvious plastic deformation and grain boundary sliding.With an increase in milling speed at the same radial milling depth,the cutting force showed a decreasing trend,while the cutting temperature increased.The thickness of the deteriorated layer increased by about 40 μm,the white layer thickness increased by about 6 μm,the microhardness increased by about 200 HV,and the hardened layer depth increased by about 100 μm.At the same milling speed,with an increase in milling depth,the changes in the thickness of the deteriorated layer and white layer were more pronounced.For every 0.1 mm increase in milling depth,the thickness of the deteriorated layer increased by about 30 μm,the white layer thickness increased by about 3 μm,the hardness increased by about 20 HV,and the hardened layer depth increased by about 20μm.Secondly,microscopic observation and analysis were conducted on the processed surface of GH4169.It was found that the outermost layer of the processed surface of GH4169 superalloy exhibited a microstructure significantly different from the base material,appearing densified and without apparent structural features.Below the outer layer was the plastic deformation zone,which exhibited distinct features such as grain tilting,elongation and compression of pores,and slip lines.Shear flow was also observed on the processed surface.EDS and XRD analysis indicated that phase transformation occurred in the white layer,resulting in changes in the distribution of elements.This suggests that the white layer is not a result of chemical reactions with the environment.Additionally,the study found that the white layer had lower crystallinity and finer grain size compared to the base material,indicating grain refinement in the white layer.EBSD analysis revealed that the dynamic recovery and recrystallization occurred in the white layer,leading to the absorption of dislocations,reduction of surface defects,and consequently higher density in the white layer.The proportion of low-angle grain boundaries in the white layer was much higher than that in the base material,gradually decreasing from the white layer region towards the plastic deformation zone.In the plastic deformation zone,the dislocation density significantly decreased,and the proportion of high-angle grain boundaries increased,indicating grain slip and deflection resulting in plastic deformation.Finally,based on the orthogonal cutting of metal and the thermal coupling effect,a finite element simulation model of cutting process for GH4169 was established.The effectiveness of the simulation model was verified by comparing the numerical values and trends of cutting forces and temperatures between simulation results and experimental results.Finite element analysis was employed to analyze the stress,strain,and temperature distribution during the cutting process,as well as the changes in residual stresses on the machined surface.The study revealed that increasing cutting speed primarily affected the stress and cutting temperature in the primary deformation zone during cutting.Stress mainly concentrated in the primary deformation zone,resulting in higher cutting forces in that area,while strain mainly concentrated in the secondary deformation zone.The strain distribution map indicated that strain during cutting mainly occurred at the contact between the chip and the tool,similar to the distribution of cutting temperature.Through finite element simulation of residual stresses on the material’s machined surface,the changes in residual stresses were analyzed.It was observed that a significant amount of residual stresses,primarily in the form of residual tensile stresses,were generated on the machined surface.Combining the experimental conclusions,it was inferred that the formation mechanism of the white layer involved dynamic recovery,dynamic recrystallization,grain refinement,and the generation of the white layer due to the high stresses,strains,and temperatures occurring during the cutting process.A theoretical model for determining the formation of the white layer was established through finite element simulation,and a predictive model for white layer formation was developed.This study clarifies the mechanism of white layer formation and reveals the changes in grain structure that occur during the formation process.The critical conditions for white layer formation and the processing conditions to reduce white layer formation were explored,providing theoretical support for improving the surface integrity of superalloy components in practical production processes and having important guiding value for the production and manufacturing of superalloy components. |
PDF Full Text Request