Machined Surface Integrity Of High-strength Steel In High-speed Milling Based On Fractal Theory

AISI 4340 steel is a typical high-strength steel with high lateral ductility,high toughness,high hardness,and excellent fatigue resistance.It is widely used in aerospace and automobile manufacturing.However,it is a typical difficult-to-machine material,which has the problems of unstable processing quality,slow processing efficiency,and high processing cost."Made in China 2025"put forward"Focusing on Improving Quality and Efficiency",while referred to the basic policies of"Driven by Innovation,Quality First,Green Development,Structural Optimization,and Talent-oriented".Therefore,AISI 4340is tested by high-speed dry milling,which has the advantages of ensuring the processing quality,improving the processing efficiency,low energy consumption and pollution-free,in this thesis,to achieve the processing of AISI 4340 with high quality,high efficiency and low cost.In order to achieve the research goals,high-performance coated tool is identified at first by the control variable analysis of chip morphology,cutting force,cutting temperature and tool wear mechanism.Subsequently,fractal and multi-fractal theories are introduced,while the fractal features and multi-fractal features of the machined surface are analyzed.Moreover,it is introduced into the optimization of cutting parameters,and the optimal cutting parameters are obtained by response surface method to achieve stable machining surface quality.Then,Gilbert's method is used to complete the processing cost analysis of high-speed milling 4340,while the processing efficiency is compared.Finally,machining of AISI 4340 is achieved with guaranteed processing quality,efficient and low-cost.First,a three-dimensional finite element simulation of the high-speed milling process is performed with ABAQUS,and the effect of milling speed on cutting force,stress field,and temperature field is studied.The real-time changes of cutting force and cutting temperature during high-speed milling are revealed.The results show that the coated carbide tool with Ti Al N composition can be used for high speed milling of AISI 4340 steel,and the best cutting speed range is v_c=280?440 m/min.The chips are obviously serrated,while the cutting force and cutting temperature increase with the increase of milling speed.The relationship between the stress and the deformation degree of workpiece grid and the distribution of temperature field of coated tool are also summarized.Secondly,the high-speed dry milling test of AISI 4340 steel is carried out.The cutting performance of the coated tool is evaluated from four aspects:chip morphology,cutting force,cutting temperature and tool wear.It is determined that the Ti N/Ti CN/Ti Al N multi-coated tool is the most suitable for high-speed milling of AISI 4340.Then,the fractal features and multi-fractal features of the machined surface are studied by introducing the fractal and multi-fractal theories.And,fractal and multi-fractal representations of the two-and three-dimensional topography of the machined surface are constructed.It is found that the surface defects would increase the 2D fractal dimension of the machined surfaces,and the surface pits would increase the 2D multi-fractal spectral width difference and spectral height difference of the machined surfaces.By analyzing the multi-fractal features of the machined surfaces'three-dimensional topography,it is known that the troughs are the dominated characteristic of the machined surface.In addition,the generalized Taylor formula between the residual stress and the cutting parameters can be used to predict the residual stress,while the 2D fractal dimension of the machined surfaces can be used to improve the prediction accuracy.Finally,the surface roughness is taken as the optimization target to optimize the cutting parameters with the fractal and multi-fractal features of the machining surface are used as auxiliary responses.This process is achieved by the response surface method.At the same time,machining cost analysis is calculated by Gilbert's method,and machining efficiency is compared.As a result,the optimized cutting parameters with high efficiency and low cost are obtained:v_c=408.35 m/min,f_z=0.02 mm/z,a_p=0.2 mm and a_e=2 mm.