Effect Of Cryogenic Pretreatment On Surface And Fatigue Properties Of TC4 Titanium Alloy After Barrel Finishing

TC4 titanium alloy is based on titanium and other elements.It has low density,high strength,high temperature resistance and corrosion resistance,so it is widely used in aerospace and other fields.In practical application,the failure of TC4 titanium alloy and its surface and fatigue properties have great influence on the damage of the material.Considering that the cryogenic treatment and the barrel finishing,as a heat treatment process and a machining process respectively,have a significant impact on the properties of TC4 titanium alloy.In order to accurately grasp and clarify the influence and mechanism of surface quality and fatigue life of TC4 titanium alloy after cryogenic pretreatment in the barrel finishing,the cryogenic treatment of TC4 titanium alloy at different times is carried out to study the effect of cryogenic pretreatment on the properties of TC4 titanium alloy.The surface quality and fatigue life of TC4 titanium alloy before and after cryogenic treatment are analyzed in the process of barrel finishing.The difference of machining mechanism and machining effect is analyzed by discrete element simulation.The main research results and conclusions are as follows:(1)The change rule of surface properties of TC4 titanium alloy before and after cryogenic treatment is as follows:theβphase structure of TC4 titanium alloy transforms into secondaryα′phase after cryogenic treatment,and the transformation fromβphase toα′phase has a certain accelerating effect.The hardness of TC4 titanium alloy after cryogenic treatment for 11 hours is the highest,and after cryogenic treatment for more than 50 hours,the hardness has no significant change and tends to be stable.The grains of rolled TC4 titanium alloy have certain directionality.Under the effect of cold shrinkage and internal stress release,the cryogenic treatment makes the structure of TC4 titanium alloy more uniform,improves the homogeneity and compactness of the material,and causes the strengthening and toughening of TC4 titanium alloy.(2)The effect of cryogenic pretreatment on the surface quality of TC4titanium alloy by barrel finishing is studied.The results show that the cryogenic treatment can significantly improve the barrel finishing effect of TC4 titanium alloy,and the reduction of surface roughness of TC4 titanium alloy after 11hours of cryogenic treatment is significantly better than that non cryogenic treatment,the decrease of Ra is 162.1%of that of non cryogenic treatment,and the decrease of R_Z is 208.6%of that of non cryogenic treatment;The MRR is higher than that non cryogenic treatment,but the overall material removal rate is lower,the difference is not obvious;The improvement of surface texture,surface smoothness and surface integrity is more obvious than that of non cryogenic treatment,the surface anisotropy is improved,and the trend of isotropy is gradually obvious.(3)The fatigue life simulation software FE-SAFE is used to investigate the influence of the barrel finishing of cryogenic treatment on the fatigue life of TC4 titanium alloy.The results show that the cryogenic pretreatment has a good effect on the fatigue properties of TC4 titanium alloy.The fatigue life of TC4titanium alloy increases with the decrease of surface roughness.However,when the surface roughness is small enough,the influence of surface roughness on the fatigue life of TC4 titanium alloy is small.The residual compressive stress is introduced into the surface of the workpiece after barrel finishing.The fatigue life of TC4 titanium alloy under the residual compressive stress is larger,which is 1.782 times of that under the same residual tensile stress.The fatigue life of TC4 titanium alloy increases with the increase of residual compressive stress.There is an interaction between surface roughness and residual stress on the fatigue life of TC4 titanium alloy.Residual stress is the main effect on improving the fatigue life.(4)Discrete element method is used to study the flow pattern of the machining medium and the changes of the stress and wear of the workpiece during the barrel finishing of TC4 titanium alloy before and after cryogenic treatment.When the transmission ratio is i=-1,there is a slip layer in the processing medium,which forms a forced flow.In the process of movement,the workpiece moves between the friction zone and the extrusion zone of the processing flow state in a nearly elliptical track on the basic horizontal plane15mm to 20mm from the bottom of the roller.The energy and speed of the workpiece show periodic changes,there is no significant change in speed before and after cryogenic treatment,but the contact energy after the cryogenic treatment was lower than that of non cryogenic treatment.The wear amount after cryogenic treatment is 1.44 to 1.77 times of that without cryogenic treatment,and the average wear depth of the surface changes from 0.0089mm to0.0138mm.Compare with the effect of sliding and scratching,the workpiece is mainly impacted and rolled by the rolling polishing block.