Study On The Formation Mechanism Of Coarse Grain In TB6 Aviation Die Forging

In recent years,with the rapid development of China's aviation industry,near-?titanium alloy has become one of the key materials for long-life and lightweight load-bearing components in aviation manufacturing field because of its high strength and toughness,excellent hardenability and high damage tolerance.In the process of manufacturing large aviation die forgings with complex structure,due to the uneven deformation and temperature of each part,it is very easy to cause the local macro and micro structure of forgings to meet the high requirements of aviation forgings,so some materials need to be removed,which reduces the utilization rate of materials.This paper focuses on the phenomenon of "low times coarse grain" in the low-dimensional structure of TB6 Ti alloy I-shaped die forging formed by one die forging and the problems that lead to the uneven structure and performance.The microstructure and mechanical properties of TB6 titanium alloy are studied;The deformation behavior of TB6 titanium alloy at high temperature was studied.The formation mechanism of macrostructure and the relationship between macrostructure and deformation process were discussed.The quantitative critical conditions for the formation of macrostructure of TB6 titanium alloy were determined.The numerical simulation prediction of macrostructure in die forging was realized by secondary development of finite element method(1)The dark spot macrostructure is produced on the surface of TB6 titanium alloy die forging at a certain thickness,which is called Macrograin.This structure makes the forging structure uneven and the ? grain in local area coarse.The dark spot macrostructure is produced on the surface of TB6 titanium alloy die forging at a certain thickness,which is called Macrograin.This structure makes the forging structure uneven and the ? grain in local area coarse.The microstructure of macrostructure is characterized by large ? grains with small equiaxed ? grains uniformly distributed in the grains,and the normal macrostructure(non macrostructure)of forging is distributed between the large ? grains,which belongs to typical bimodal structure with small equiaxed ? grains and small equiaxed ? grains uniformly distributed in the small equiaxed ? grains.(2)Compared with the normal structure,the tensile strength of TB6 titanium alloy die forging with macrostructure is lower.However,due to the transformation of ? phase to ?''phase of orthorhombic martensite in the micro tensile specimen with macrostructure during tensile deformation,the secondary increase of stress occurs,which improves the elongation of the material.It is necessary to explore its mechanism and find out the way to avoid and eliminate it.(3)The formation of macrostructure of TB6 titanium alloy needs a certain deformation temperature(after cooling)and deformation amount at the same time.In addition,the influencing factors of macrostructure are initial holding temperature,holding time after cooling,strain rate,cooling mode after compression and initial macrostructure of billet.Among them,reducing the strain rate,using air cooling cooling(after hot compression),reducing the initial holding temperature and increasing the holding time after cooling are beneficial to reduce or eliminate the influence of coarse grain.(4)Based on the critical criterion of macrostructure formation,the finite element subroutine is established by secondary development,which can accurately predict the distribution of macrostructure in die forgings.