Research On The Machining Mechanism And Technical Fundamental In Micro-milling Single Crystal Nickel-based Superalloy

Facing the increasingly serious problem of energy and living space,the miniaturization of products has become an urgent trend.With the development of micro manufacturing technology,especially in the fields of aerospace,defense industry,microelectronics and biomedicine,the urgent need for high strength micro structures and parts in high temperature environment is needed.The microstructures/parts made of single crystal superalloy materials are widely used in high temperature resistant working environment with the combination of micro features and high temperature resistance properties of high temperature alloy materials.In recent years,micro cutting technology,especially micro milling,has made great progress.It has promoted the progress of micro cutting technology in cutting mechanism,process equipment,parameter optimization and workpiece materials.Continuously improving the surface quality and accuracy of micro parts becomes an important issue in the field of micro cutting.The minimized component made of a single crystal has only one grain,which eliminates the transverse grain boundary perpendicular to the high temperature stress axis,and greatly improves the high temperature performance and service life of the single crystal parts.The single crystal superalloy material has high cutting stress,high cutting temperature,and serious work hardening and low tool life.Single crystal materials have obvious mechanical anisotropy properties.The mechanism of material removal is different from the shear slip along grain boundaries of polycrystalline materials and the processing characteristics of each crystal face are significantly different.In the past,the micro cutting technology is more focused on the research of the processing of polycrystalline materials.At present,there is a lack of micro cutting mechanism and technology research for single crystal materials,especially single crystal superalloys.Therefore,in order to guide the micro milling process of single crystal nickel-based superalloy,the key technical problems of the crystal nickel-based superalloy are studied in this paper based on the theoretical study,the finite element simulation and the experiment.The main research is reflected in the following aspects:(1)By analyzing the properties and mechanical anisotropy of single crystal nickel-based superalloy materials,the material removal mechanism of single crystal is proposed based on the P-N model and dislocation slip theory of single crystal,and the dense surface and easy slip crystal direction are indicated.Based on the molecular dynamics simulation method,the molecular dynamic cutting model of single crystal superalloy is established.Scanning electron microscope is used to observe the shape and depth of subsurface slip zone of single crystal material,and the results verify the deformation mechanism of single crystal material.The difference between the removal mechanism of single crystal materials and polycrystalline materials is compared and verified based on experimental methods.Based on the chip morphology of single crystal nickel-based superalloy,the plastic instability condition of the single crystal material and the saw-tooth chip caused by the larger strain rate are proposed.(2)A theoretical model for surface roughness of single crystal nickel-based superalloy is established.Based on minimum quantity lubrication(MQL)technology and dry cutting conditions,the influence of process parameters on surface roughness is compared under singlefactor test conditions.Based on response surface methodology,a roughness prediction model is established to analyze the influence of interaction factors on surface topography.The cutting parameters are optimized by orthogonal test.The factors that influence the surface quality of workpiece are analyzed,such as dissimilar materials,different crystal faces,and cutting tools with different diameter and number of blades.(3)The cutting area is divided into shear zone,plow zone and recovery zone,and the axial micro-milling force is introduced.The mathematical model of micro milling force is established.In this model,the shear zone is mainly sheared and the plow zone is dominated by plow,and the blade surface friction is mainly after the recovery zone.The 3D simulation of micro milling force for single crystal material is realized.The regression prediction model of micro milling force is established by orthogonal experiment,and the influence rule of cutting parameters on micro milling force is analyzed.The paper verifies the consistency and accuracy of theoretical milling force,simulated milling force and experimental milling force.(4)Based on the material characteristics of single crystal nickel-based superalloy DD98,the HILL constitutive model parameters are derived and calculated.The 3D simulation of temperature field of single crystal material is realized.The temperature field and chip generation stage of micro cutting are defined,and the distribution of temperature field is obtained.The artificial thermocouple method of temperature field measurement is proposed,the influence rule is obtained that micro milling process parameters on the micro milling temperature in(100)typical crystal surface,and the distribution of temperature field of different crystal surfaces.(5)A mathematical model for the wear of micro-milling cutter is established.In view of the characteristics of high stress and high hardness in high temperature alloy cutting process,scanning electron microscopy(SEM)is used to detect and analyze the wear mechanism of single crystal nickel-based superalloy by micro-milling cutter.The form of cutter wear is presented,and the influence of element changes in cutter and workpiece materials on the cutting process and workpiece performance is analyzed by energy spectrum.(6)In order to study the deformation mechanism of surface and subsurface of single crystal nickel-based superalloy,the mechanism and reason of white layer phenomenon are observed and analyzed by optical microscope.By scanning electron microscopy(SEM),metallographic examination and energy spectrum analysis are carried out to study the influence mechanism of white layer elements,and the microhardness of subsurface is measured.The high-temperature working environment of single crystal superalloy is simulated,and different heat treatment processes are carried out for the workpiece after micro-milling.The mechanism of recrystallization of single crystal superalloy after cold plastic deformation under high temperature is explored and an effective method of inhibiting recrystallization is proposed.In this paper,the mechanism and key technical problems of micro milling of single crystal nickel-based superalloy are explored by means of theoretical research,finite element simulation and experiment.It is beneficial to expand the range of micro cutting technology in difficult-to-machine materials,and perfect the theoretical system of micro cutting.The paper has guiding and practical significance for the application of single crystal materials and the improvement of machining quality and accuracy.