Numerical Simulation Of Titanium Alloy Ti6Al4V Hot Isostatic Pressing(HIP) Parts Forming Process

A slight deviation of the original shape and size of the part from the Hot Isostatic Pressing?HIP?treated in the HIP process appeares due to the complicated thermal-mechanical coupling problems and non-densification of the alloy powder,which makes how to accurately control the shape of the HIP process is one of the crucial and difficult points during HIP processes.Due to the large shrinkage ratio of alloy powders and the irregular deformation,the numerical simulation method is used to predict the deformation of the steel capsule and the densification process of the compacts,which can provide the basis for the optimization of the capsule structure.However,it is necessary to systematically investigate the constitutive model of porous materials,the thermal properties of materials and the solution of finite element equations in order to perform the numerical simulation analysis of complex structures.In this paper,single-core parts and parts with complex structure with Ti6Al4V alloy powder as a matrix can be formed by HIP technology.The deformation of the compacts was analyzed by numerical simulation and verified through experiments from both mechanical properties and workpiece size.The Shima-Oyane model was used to describe the flow behavior of alloy powders during HIP processes based on the continuum plastic theory of porous materials.The hydrostatic pressure correction factor?and stress partial tensor correction factor?were introduced into the model which two parameters related to relative density describe the influence of density and porosity on the constitutive equations.The modified Maxwell power-law creep model was chosen to describe the viscoplastic flow behavior of the alloy powders based on the creep phenomenon in the HIP process.Through the HIP interrupt experiment,the specimens with relative various densities were obtained and tested by uniaxial compression conditions at high temperature.The least square method was used to fit the parameters?and?in the constitutive equation,and their coefficients b₁,b₂,b₃,b₄ and q₁,q₂,q₃,q₄ were calculated.Based on above results,the equations of equivalent stress and equivalent strain rate of Ti6Al4V powder alloy consdering creep factors were established.In this thesis,advanced nonlinear finite element software MSC.MARC numerical simulation analysis was selected to simuluate deformation of the steel capsule and the densification process of the compacts with a core t and multicomponent cores.The simulation results of the single-core model showed that the simulated size of the compact in the radial direction is larger than that of the testing samples,while the size along the axial direction is the opposite.The maximum relative error along the radial direction is 3.4%and the maximum relative error along the axial direction is 1.8%.The average relative density of the calculated by HIP test was 98.8%,the average relative density of the numerical simulated compacts was95.57%and the error was 3.23%.The numerical simulated results are in good agreement with the experimental results.The simulated results of the multi-core model showed that the error between the simulated result and the tested result is small.The simulated size is larger than the tested size and the maximum deformation error is 4.7%.The relative density of the compacts is located between 93.6%to 97.8%,did not reach full dense state.There are large differences in their internal density.It can be seen from the above results that the numerical simulated analysis of HIP process can accurately predict the deformation of the steel capsule and the densification of alloy powder,and provide guidance for Hot Isostatic Pressing-Near Net Shape?NNS-HIP?components.The mechanical properties of HIP-formed Ti6Al4V single-core parts at room temperature were measured,and the results indicated that the microstructures of HIP parts were uniform,and the?+?phase were distributed with a typical stripe shape.No impurities,holes and other defects were found in the microstructure.Tensile tests at room temperature showed that the tensile elongation of HIP-1 and HIP-2 specimens were 16.45%and 17.20%respectively,the reduction of crosssection area was 39.62%and 35.72%respectively.Both tensile elongation and reduction of crosssection area exceed ASTM specifications for castings.In addition,the mechanical properties of the compacts are higher than that of the Ti6Al4V castings(tensile strength?_b is 890Mpa,yield point?_0.2 is 825Mpa),which is comparable with the mechanical properties(tensile strength?_b is 930Mpa,yield point?_0.2 is860Mpa)of the same size Ti6Al4V forgings.