| TiAl alloy has been popular in the aerospace and automotive applications because of its low density,high temperature strength,creep resistance and strong oxidation resistance.However,the low ductility and poor molding performance have hindered its further application in the project.The introduction of powder metallurgy technique is effective in solving the forming problems,especially eliminating macroscopic segregation and playing an effective role in the refinement of the microstructure for the complexforming parts..The titanium alloy mainly uses the elemental powder method and the pre-alloy powder method.Elemental powder method is characterized for its low cost of raw materials(such as HDH powder),but not suitable for hot forming.Pre-alloy powder method can effectively improve the purity of the embryo for preventing alloy oxidation,thus the alloy has a high temperature superplasticity.The powder has a good spherical shape and obvious fast cold tissue characteristics with the development of the technology such as the mature process of innovate argon atomization preparing high-quality titanium alloy pre-alloy powder.During the aerosolization process,the molten liquid is broken into very small spherical droplets by a high velocity air stream and the droplets are rapidly cooled under strong convection.However,the titanium alloys prepared by the powder metallurgy method still have microscopic defects which affecting the mechanical properties of the materials after molding.The previous researches show that the hot working can improve the microstructure of the material and the mechanical properties of the material.Thus the thermal deformation behavior of powder metallurgy titanium aluminum alloy are needed in-depth study.The current researchesare focused on the as-cast titanium-aluminum alloy powder,while the research on the titanium alloy pre-alloy powder are lacking especially on the argon atomization preparation of the pre-alloy powder.As different preparation processesresulting in different organizational properties of embryos,the thermal deformation behaviorr esearch of titanium alloy powder is very important.In this thesis,the cylindrical samples with size of?8mm × 12 mm are fabricated using the hot isostatic pressing billet made by the argon atomization pre-alloy Ti-47.5Al-2Cr-2Nb-0.2W-0.2B powdermade of,and then both surfaces of the specimens are polished until smooth.The thermodynamics of Ti-47.5Al alloy was investigated by thermocompression simulation experiment on the thermal simulation experiment.Then the constitutive equation of this alloy are establishmented and the activation energy Q was calculated to be~365.6 KJ / mol.The fitting correlation coefficient and error of the equation are 0.98922 and 3.30415%,respectively,which is in good agreement with the experimental results.The microstructural evolution of heat-deformed Ti-47.5Al alloy with a strain rate of 0.001-0.5 s-1 powder was observed at temperature range of 1050-1200 oC andthe microstructurewas also analyzed by SEM,EBSD and TEM.It was found that small recrystallized structure appeared and lamellar structure remained in the deformed tissue.Mainwhile,the machining diagram of the alloy deformation process is finally establishedbased on the Prasad instability criterion and the dynamic DMM model.the optimum thermal processing of the alloy is in the temperature range of 1080-1180 °C and the low strain rate of 0.001-0.1 s-1 according to the constructed pattern,.These results obtained in this study provide the theoretical basis for the hot working and production of high temperature TiAl-based alloy parts,also play a guiding role for the hot working process of Ti-47.5Al alloy. |
PDF Full Text Request