| Pure iron thin-walled spherical shell with high dimensional precision,high surface quality and high thickness uniformity is the key part for a test.It is difficult to ensure the product dimensional accuracy and surface quality due to the large chip deformation,built-up edge,large cutting force and serious cutting tool wear during turning pure iron material.It is very important to study the key technology and basic scientific issues that are associated with the machining accuracy,and verify them by experments.This would promote technological progess for pure iron thin-walled spherical shell.Therefore,the main research works are as follows:(1)The machinability of pure iron,10 steel and 45 steel are studied by orthogonal cutting experiments.The chip formation mechanisms and the influence of cutting parameters on the chip formation are analyzed.The results show that the pure iron is a typical difficult-to-machining material with high plasticity and high toughness.The machining of pure iron is characterized by large cutting force,serious chip deformation,large contact length and significant shear slip deformation.The machinability of pure iron could be improved by adopting high cutting speed,larger tool rake angel and water-based coolant.(2)The dynamical and static mechanical behaviors of pure iron are obtained with split Hopkinson pressure bar apparatus and materials testing system.The stress-strain curves of pure iron are measured at high temperatures(20-800℃)and high strain rates(104-5×104-1s).A constitutive equation of pure iron is established based on the Power-Law constitutive model.According to the thermal physical properties of pure iron,as well as the average friction coefficient between the rake face and chip,the stain strengthening coefficient in the constitutive eqation is modified.A cutting process simulation model of pure iron is established.The simulation results of cutting force and chip morphology are compared with those of orthogonal cutting experiments.It indicates that the simulation model can be used to optimize the cutting parameters in rough machining of pure iron.(3)The effects of cooling/lubrication conditions and cutting parameters on uncoated cutting tool life in finish turning pure iron are investigated.The results show that minimum quantity lubrication(MQL)can be conductive to improve tool life.The main tool wear modes are flank wear and severe notches wear distributed on both major cutting edge and minor cutting edge.Diffusion wear,adheasive wear and oxidation wear are the main wear mechanisms.The influences of cooling/lubrication conditions on tool life are analyzed.The tool life during finish turning of pure iron can be imporved considerably by adopting K313+WS2 and KC5010 coated inserts under MQL condition.(4)Based on the thermal-mechanical coupled principles,the influences of cutting parameters on workpiece surface integrity are studied by cutting experiments.The defects on machined workpiece surface consist of tearing,grooves,chip,scales,plasticity flow and side flow.The surface roughness measured is larger than that of the theoretical value,and the surface work hardening is very serious.The surface grains show severe tensile distorted plastic deformation.The surface residual stresses on tangential and axial direction are both residual tensile stresses and the tangential direction stresses are larger than that on axial direction.(5)The key factors related to the machining accuracy of pure iron thin-walled spherical shell are analyzed.The effects of tool setting error and tool radius contour error on spherical contour accuracy,as well as the control methods are investigated.The influence of clamping force,flatness error of end face and cutting force on workpiece deformation are studied using finite element method.The clamping method and cutting parameters are optimized.A control method for flatness error is poposed.The dimensional accuracy and surface quality of the pure iron thin-walled spherical shell can meet the design requirements. |
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