| Owing to the advantages of high specific strength,low density and coefficient of thermal expansion and etc.,TC4 titanium alloy has a great application potential in microwave device electronic packaging.However,deformation usually takes place during machining the thin-wall,complex-shape electronic packaging cases,resulting in a low production yield.For this reason,it is of high value to carry out research on the pre-machining process optimization,and to improve the microstructures of the TC4 titanium alloy and further the machinability and production yield.In this dissertation,the microstructure and mechanical properties of the TC4 titanium alloy blanks of large machining deformation were firstly investigated to figure out the reasons for machining disqualification.Secondly,after the TC4 titanium alloy blanks heat-treated under different conditions,the microstructure,properties and milling performance of the TC4 titanium alloy blanks were studied,combining with finite element simulation,to determine the best heat treatment procedure of the TC4 titanium alloy.The main reason for milling deformation of the TC4 titanium alloy cases is that the stripe structures results in the structural anisotropy,inducing high internal stress in the TC4 titanium alloy.Meanwhile,the stiffness of the thin-wall TC4 titanium alloy cases is rather low.So,the machining deformation of the TC4 cases occurs under the combination effect of high milling force and internal stress.As the annealing temperature increased,the stripe structure in the TC4 titanium alloy gradually decreases in amount,which is basically eliminated at 900 ?.The?-phase equiaxial grains grow significantly,and the ?-phase grains are uniformly distributed along the ? grain boundaries.As a result,the strength,hardness and plasticity of the TC4 titanium alloy is moderate.The 900 ? normalized TC4 titanium alloy is of bimodal microstructure composed of the primary ?-phase equiaxial grains and the ?-phase transition structure,which has higher strength and hardness,and lower plasticity.The microstructure of the 930 ? quenched(solid soluted)TC4titanium alloy consists of the basket-like ?' martensite matrix with fine primary ?equiaxial grains.The strength and hardness of the TC4 titanium alloy increase further,while the plasticity and toughness decrease.After the quenched TC4 titanium alloy tempering at 530 ?,the ?' and residual ?' phases both decompose,but the basket-like structure is still retained.The primary ?-phase grains are still in an equiaxial state in which a little bit ?-phase grains disperse.Therefore,the TC4 titanium alloy has the highest strength and hardness,but the lowest plasticity and toughness.As increasing the annealing temperature,the equiaxial grains of the TC4 titanium alloy grow,the degree of chip serration(Gs),the degree of processing deformation and surface roughness all decrease at first and then increase.As for the 900 ?annealed TC4 titanium alloy,the Gs,the degree of processing deformation and the surface roughness are all the lowest,and the processing quality of the workpieces is the best.The solid soluted and the solid soluted plus tempered TC4 titanium alloy has high residual stress and milling force.The milling deformation is so serious that the workpieces do not meet the relevant quality requirement.Linear fitting relationship between the Johnson-Cook intrinsic parameters A,B,m and n and the maximum Mises stress in the side wall of TC4 titanium alloy cases after milling was established using the ABAQUS software.The intrinsic parameters of the TC4 titanium alloy in different heat treatment states were inversely obtained,and the milling simulation was implemented.The results show that the 900 ? annealed TC4 titanium alloy has the lowest milling deformation,corresponding to the experimental result in this work.Meanwhile,the reasonability foe selection of the above intrinsic parameters is proven,which provide a data support for machining simulation of the TC4 titanium alloy. |
PDF Full Text Request