Microstructure Characterization And Mechanical Properties Of White Layer On GH4169 Machined Surface

Nickel-based superalloy GH4169 has been widely applied in aeroengine turbine disks due to its excellent high fatigue strength,high thermal stability,and high corrosion resistance properties.The service conditions of high temperature and variable loading put forward strict requirement for the machined surface quality of turbine disks.White layer formed on the machined surface has negative effect on the surface quality and service performance of the machined parts.Therefore,research on the microstructure characterization,the formation mechanism,and mechanical properties of the white layer are required to improve the machined surface quality of superalloy.In this thesis,the microstructure including metallographic phase and grain structure of the white layer on GH4169 machined surface is characterized.The prediction method of white layer thickness is studied.The research results will provide data basis to suppress the white layer formation.Firstly,the etching visualization methods for the white layer on machined surface of GH4169 are proposed and the displaying characteristics are studied.The white layer is generated with orthogonal turning experiments of GH4169.With etching treatments,the microstructural morphologies and surface defects of the white layer are analyzed.The influence of cutting speeds and tool wear on the white layer thickness is discussed.Secondly,the mechanical properties of white layer are analyzed.The hardness,elastic modulus,and surface residual stress of white layer are detected with nano-indentation and Xray diffraction tests.Micro-scratch tests are performed to characterize the strain-induced cracks of white layer.Then,the formation mechanism of white layer is investigated based on the metallographic examination.The element distribution,phase compositions and grain structure are measured by energy dispersive spectroscopy,X-ray diffraction,and electron backscatter diffraction techniques.The deformation and dissolution of precipitated phases and continuous dynamic recrystallization mechanism in the white layer is presented.Finally,the simulating prediction method of white layer thickness is proposed.The critical strain criterion of white layer formation based on Zener-Hollomon dynamic recrystallization model is developed.The GH4169 orthogonal cutting process is simulated by finite element simulation method.Based on the analyses of strain,strain rate and temperature fields within machining affected layer,the prediction model of white layer thickness on machined surface is established,and the prediction results are verified by experiments.