Study On Formation Mechanism Of White Layer On Workpiece Surface In Cutting Process

In the process of cutting,a layer of micro-structure called "white layer" is produced on the surface of the workpiece.Its hardness is very high,and sometimes it can cause large pieces of material to peel off or become a source of fatigue.It has a significant effect on the surface quality and fatigue life of the workpiece.Therefore,this paper attempts to use positron annihilation technology to study the white layer of the workpiece surface in the cutting process mainly at the atomic scale.In this paper,dry orthogonal turning experiments are carried out for titanium,titanium alloys and alloy structural steels.By using single factor analysis and positron annihilation technology,the relationship between white layer of machined surface and cutting parameters was studied.The main contents and conclusions of this paper are as follows:(1)The formation mechanism of white layer on TA2 and TC4 is mainly plastic deformation.(2)The microstructural defects of surface white layers of TA2,TC4 and 40CrNiMoA were studied.The results show that under different cutting parameters,dislocation defects are dominant in the surface white layer cutting of TA2 and TC4.Defects of 40CrNiMoA surface caused by white layer cutting include dislocation,vacancy and vacancy group.(3)The influence of cutting parameters on the surface white layer defect of TA2 and TC4 was studied.The results show that in order to obtain a lower defect concentration in the surface layer of the workpiece during cutting,lower cutting speed should be selected when processing two kinds of materials,and the situation of 0.2mm/r feed should be avoided as far as possible when processing two kinds of materials.The back feed is 0.2 mm and 0.25 mm when TC4 is processed.Titanium alloy TC4 is superior to industrial pure titanium TA2 in the same cutting parameters.(4)The white layers on the surface of processed industrial pure titanium TA2 and titanium alloy TC4 were detected by slow positron beam Doppler broadening experiment.The experimental results show that the better surface quality can be obtained by turning TA2 with 76.53m/min cutting speed,and by turning TC4 with 30.07m/min cutting speed and 76.53m/min cutting speed.Among the samples with different feed rate,the influence of different feed rate on the surface quality of TA2 is not obvious,and the better surface quality can be obtained by turning TC4 with 1.0mm/r feed rate.The surface quality of TC4 is better than that of TA2 at the same cutting speed or feed rate.(5)The influence of cutting parameters on the white layer of 40CrNiMoA surface was studied.The results show that the main factor affecting the thickness of the white layer is cutting speed.With the increase of cutting speed,the thickness of the white layer of 40CrNiMoA surface increases,and the influence of feed and back feed is not obvious.With the increase of cutting speed,the defect concentration of the white layer on the surface of the sample also increases,that is to say,the surface quality has decreased.;the effect of feed rate on the defect concentration in the white layer of the machined surface is not obvious;with the increase of knife feed rate,the defect concentration decreases as a whole,and it can improve the surface quality.