Study On The Formation Mechanism Of Cold Die Microstreure During Hot Die Forging Of TC18 Titanium Alloy

With the development of the national aviation industry,the near-βtitanium alloy has become one of the key materials for manufacturing the lightweight load bearing components with long-life in the aviation industry due to theirs high strength and toughness,good hardenability as well as high damage tolerance.However,the macro-and microstructure of the large-scale aerospace forgings with complicated structures are usually difficult to meet the high requirements of aviation forgings since both strain field and temperature field of forgings are heterogeneous.Generally,such macro-and micro-structural flaws in the aerospace forgings need to be removed,which cause a low material utilization rate.This dissertation focuses on the problem of non-uniform microstructure and properties caused by a classical macro structural flaw that is named as the"cold-mold structure"in the TC18 titanium alloy die forgings.Meanwhile,its resultant mechanical properties have been analyzed;the high-temperature deformation behavior of TC18 titanium alloy has been studied,a constitutive model has been established.In particular,the formation mechanism of the cold-mold structure and its dependence of the deformation process have been discussed.A quantification criterion has been proposed whereby a modeling of prediction for the cold die structure in classical die forgings has been obtained by numerical simulation which is on basis of the secondary development of finite element.The main results are as follows:（1）A certain thickness of the macrostructure which is under the surface of TC18titanium alloy die forgings（sticking）is cold-mold structure,its appearance makes the forgings macrostructure ownning stratified phenomenon.The microstructure characteristics of the cold-mold structure are as follows:the originalβgrains are equiaxed and their grain boundaries have a continuous distribution of the grain boundaryα-phase.The equiaxed originalβgrains have a short rod-shapedα-phase distributed along the different orientations to formα-clusters interspersed（Willisian microstructure）,as well as uniformly distributed granularα-phases.The microstructure of the normal macrostructure（non-cold-mold structure）of the forging shows a typical basket structure.Compared with the normal basket structure,the cold-mold structure reduces the strength and plasticity of the alloy.（2）The flow stress of TC18 titanium alloy during hot deformation is sensitive to the deformation temperature and deformation rate,which increases with the decrease of temperature and the increase of strain rate.The Constitutive equation based on Arrhenius equation establishes the constitutive equation considering strain compensation,and can accurately predict the deformation resistance of TC18 titanium alloy under different thermal deformation conditions.It also provides data support for finite element numerical simulation.（3）The formation of the cold-mold structure of the TC18 titanium alloy die forging is mainly related to the formation of the last fire（the deformation process of theα+βdouble phase zone heating in theβsingle-phase zone）.In the deformation temperature range of790-890°C,the critical strain quantification model of the cold-mold structure of TC18titanium alloy be expressed.Whenε≤ε_f,the degree of fragmentation of Willisian microstructure in the alloy is low and it is difficult to form a basket structure,so that the macrostructure appears as a cold-mold structure.（4）Based on the critical criteria for the formation of cold-mold structure,the finite element subroutines were established through secondary development,and the distribution of cold-mold structure for die forgings can be accurately predicted.