Study On The Formation Mechanism Of Surface Defects In Cutting Process

In the actual cutting process of titanium alloy,due to the plastic deformation of the cutting tool and the sharp friction of the cutting surface,the microstructure of the machined surface of the workpiece is changed,resulting in different types of defects on the surface,which affects the rigidity,strength,wear resistance,fatigue life and other properties of the workpiece.The former researches mainly focus on the analysis of the surface integrity data of the cutting test,but the research on the formation mechanism of the surface defects of the titanium alloy cutting is less,and the relationship between the surface microstructure and its performance is still lack of in-depth study.Therefore,in this paper,TA2 and TC4 are taken as the research objects.The turning test,surface roughness characterization and positron annihilation technology are used to systematically study the surface microstructure.The valence electronic structure of TA2 and TC4 is calculated.The formation mechanism of machined surface defects of titanium alloy is clarified,and the mapping relationship between machined surface integrity and alloy composition is revealed,which improve the performance of titanium alloy.The main contents and conclusions are as follows:(1)By using the single factor method and changing the cutting parameters,turning tests of TA2 and TC4 are carried out.The metallographic samples of the machined workpiece are made,the surface microstructure is observed,the types of surface defects are obtained,and then the influence of different cutting parameters on the surface roughness is studied.The results show that for the same material,the larger the feed rate is,the larger the change of surface microstructure is,and the influence of cutting speed and depth is not obvious.Under different cutting parameters,the type of surface defects of TA2 and TC4 is mainly dislocation.(2)By using positron annihilation technique,the mapping relationship between cutting parameters and machined surface defects of TA2 and TC4 is revealed.Theresults show that a lower surface defect concentration can be obtained by reducing the cutting speed,and when processing two kinds of materials,the feed of rate is 0.2mm/r should be avoided as much as possible.When processing TA2,it is preferred to select0.2mm cutting depth,and when processing TC4,it is preferred to select 0.25 mm cutting depth.(3)By means of slow positron beam technology,the electronic density,microstructure and defect formation process of dislocation defects are deeply characterized.The results show that when the cutting speed is 76m/min,the surface quality of TA2 is high,and when the cutting speed is 30m/min and 76m/min,the surface quality of TC4 is relatively good.When the feed rate is 1.0mm/r,TC4 can get better surface quality.(4)The plane valence electron structure of TA2 and TC4 phases is calculated,and the relationship between the interface valence electron structure and the mechanical properties of TA2 and TC4 is studied,then the influence of adding Al and V elements on the properties of the alloy is analyzed.The results show that covalent bond and metal bond coexist in TA2,and TA2 is prone to fracture and plastic deformation,due to the effect of bond force.However,due to the presence of Al and V elements in TC4 alloy,so the probability of plastic deformation and fracture is low.Combined with the calculation results of electron density,it can be seen that the electron density of TC4 interface is discontinuous and the probability of dislocation type defects is low.This is consistent with the analysis result of cutting test.