Mechanisms Of High Efficiency And High Performance Ball-end Milling Of Hardened Die Steel Cr12MoV

The surface performances of parts such as wear resistance,corrosion resistance,and fatigue resistance,are closely correlated to the surface integrity.Taking automobile drawing dies for example,frictional performance,wear resistance and fatigue resistance of working surfaces directly determine the forming quality of panels,the maintenance costs of dies and the efficiency of production.With the rapid development of high-speed machine tools and the cutting tool technology,the feasibility of high-speed-machining of hardened die steel becomes more and more obvious.High-speed hard milling technology has a wide application prospects in die/mold manufacturing industries because of its ability to simplify the traditional die/mold manufacturing process and shorten the production cycle.However,high-speed hard machining process will cause a series of changes of surface geometry as well as physical and mechanical properties of the machined surface,which may affect the surface performances greatly.Then this dissertation aims at exploring the high efficiency and high performance ball-end-milling mechanism of hardened cold work steel Cr12MoV commonly used in drawing die based on the deep analysis of the relationship among cutting parameters/surface integrity/typical surface performance,and then proposing some guidelines for the selection of cutting parameters.Specifically,the surface performances investigated in this dissertation are determined according to the common failure modes and service environment of drawing dies.Firstly,the generation mechanisms of surface topography together with the main influential factor for ball-end-milled surfaces are investigated by combining theoretical analysis,topographical simulation and experimental verification.The influences of milling parameters on typical surface integrity parameters including surface roughness,microhardness and residual stress are analyzed through response surface tests and single factor tests.Results show that the residual materials left by ball-end-milling process can form micro-pits or micro-grooves features with controllable sizes.The interaction between spindle speed and radial depth of cut significantly impacts the typical parameters of surface integrity.In addition,the corresponding optimum plan is put forward based on response surface method for the purpose of a minimum surface roughness,a maximum surface microhardness or a maximum surface residual compressive stress.Secondly,the directional effect of surface integrity for ball-end-milled surface is discussed by comparing and analyzing the difference in surface roughness,microhardness,yield strength and microstructure in different directions.Moreover,the generation mechanisms of surface defects and its relationship with cutting parameters for high-speed ball-end-milled surface of hardened die steel are revealed.Results show that a high spindle speed or a large feed per tooth can reduce or even avoid the micro-pits formed on the hard milled surface.Thirdly,the antifriction mechanisms of surface topography for ball-end-milled surface are revealed based on the hydrodynamic lubrication theory and the ANSYS Fluent simulation analysis.Then the antifriction effectiveness is analyzed through sliding friction tests.The principles for a milled surface with high antifriction performance are proposed,which can be used to guide the selection of cutting parameters during the ball-end-milling process.Results show that the friction coefficient for high-speed hard milled surfaces decreases about 52.8%～76.2%compared with that of polished ones.The friction coefficient for ball-end-milled surface can be controlled by adjusting feed per tooth and radial depth of cut during ball-end-milling process.Furthermore,the influence of tool path strategies on wear resistance of ball-end-milled surface is analyzed based on tribology theory and dislocation theory.The wear morphology and wear mechanism are compared for surfaces milled with different tool path strategies and the influence essence is illustrated from typical surface integrity parameters perspectives.The optimum selection scheme of process parameters for the acquisition of a highly wear-resistant surface during high-speed ball-end-milling of hardened die steel is put forward taking material remove rate,surface work hardening and surface defects into consideration.Then,the mechanisms of sliding fatigue wear occurred on the high-speed ball-end-milled surface of hardened die steel are analyzed and two kinds of mechanisms are proposed based on the field survey and the analysis of service environment,loading type and surface condition of drawing dies.The pits generation process on the working surface of drawing dies is illustrated according to the proposed hypothesis about sliding fatigue wear mechanism.The hypothesis is verified by conducting sliding friction and wear tests.The correlation between the surface topography strike and the sliding fatigue wear is studied.Additionally,the process technique is also put forward to improve the sliding fatigue wear resistance of high-speed ball-end-milled surface of hardened die steel.Finally,the fatigue resistance of high-speed ball-end-milled surface of hardened die steel is investigated.The influence mechanisms of typical surface integrity parameters such as surface topography,microhardness and residual stress on fatigue resistance are analyzed.The expression of microscopic stress concentration coefficient aiming at the micro-groove features on ball-end-milled surface is proposed based on Neuber model.Fatigue samples are classified according to surface roughness,microhardness and effective residual stress and then the fatigue life is evaluated through approximate single factor three point bending fatigue tests.Then the change trend of fatigue life along with typical surface integrity parameters is obtained.The influence essence of tool path strategies on fatigue resistance is illustrated and two kinds of tool path strategy optimization methods are proposed to improve the fatigue resistance according to the topography features of ball-end-milled surface.These two methods are based on the effective residual stress and microscopic stress concentration phenomenon,respectively.Results show that the microscopic stress concentration coefficient increases with the increase of the ratio of radial depth of cut to the diameter of cutting tool.High surface residual compressive stress and appropriate degree of working hardening contribute to the improvement of fatigue resistance,whereas surface roughness shows no significant influence on fatigue resistance.With the optimization goal of maximizing residual compressive stress and obtaining a microhardness about HV0.05 900,the optimum selection scheme of process parameters for a well fatigue-resistant surface during high-speed ball-end-milling of hardened die steel is put forward taking material remove rate,surface machining defects and microscopic stress concentration coefficient into consideration.