Electron Beam Precise Preparation And Organization Control Of Ti-48Al-2Cr-2Nb Alloy

Ti-Al intermetallic compound base alloy is a new kind of structure materials.It is widely used in aerospace engine and military field because of its low density,high specific strength and excellent performance at high temperature.Research has shown that the purity especially O content of TiAl alloy has a great influence on the use of performance.Now casting Ti-48Al-2Cr-2Nb alloy is one of the most widely used ?-TiAl alloy,and this alloy remains a question of brittleness at room temperature.Therefore,in order to improve the performance of the alloy deeply,expand its service temperature and application field,we need develop a new type of melting process from the point of the purity,grain refining and etc.Electron beam melting technology with high energy density,high vacuum degree,and the advantages of good controllability,can greatly improve the purity of TiAl alloy.And this technology uses a water-cooled copper crucible,which not only can`t introduce impurities,also can effectively speed up the cooling of the ingot.All these have effect of refining grain,reducing composition segregation.However,due to the high vacuum degree of the technology,the component of the alloy with different vapor pressure has different volatilization capacity in the smelting process,which leads to the deviation of the composition of the alloy ingots obtained after smelting,further affect the performance of the ingot.Therefore,in this paper,the volatilization behavior of different components in the process of preparing TiAl alloy ingot by EBM is studied,and the elemental volatilization law of the multi-component alloy is analyzed,which provides a theoretical basis for the accurate preparation of the multi-component alloy ingot by EBM.The ingot with the nominal composition of Ti-48Al-2Cr-2Nb was obtained by re-proportioning the raw materials of the alloy.In order to obtain the optimal microstructure,the heat treatment of the ingot was studied.In this study,the initial alloy was firstly prepared under different electron beam smelting conditions.The average composition of ingot was analyzed by X-ray fluorescence spectrometer,and the volatilization law of each component in the process of electron beam smelting was obtained.Then,according to the law,the alloy raw materials were re-proportioned and the melting was carried out.The ingot with the nominal composition of Ti-48Al-2Cr-2Nb was obtained.After heat treatment,the microstructure,phase composition,lamellar spacing and mechanical properties were analyzed by metallographic microscope,X-ray diffractometer,transmission electron microscope and vickers hardness tester,and the following conclusions were obtained:1.From the perspective of thermodynamics and kinetics alloy computing research,through the Miedema and Kolher model to calculate the Ti-Al-Cr-Nb quaternary alloy system,saturated vapour pressure value of the activity.Combined the Langmuir equation with the experimental results,the four elements` theoretical volatilization rates and the experimental evaporation rates were obtained,thus obtained the volatile law of each element during the melting of EBM;2.According to the law of volatilization,the alloy raw materials were re-proported.After electron beam melting,the ingot with the nominal composition of Ti-48Al-2Cr-2Nb was obtained;3.The ingot was heat treated at different temperatures and times.It was found that with the increase of temperature,the alloy microstructure became coarser and the interval between layers became larger.With the increase of time,the amount of microstructure precipitation in the middle layer of the alloy decreases.In summary,when the heat treatment parameter is 1320? for 30 min,the obtained alloy structure is optimal;4.The microhardness of the as-cast and heat-treated ingot were tested,and it was found that the microstructure regulation had a great influence on the mechanical properties of the alloy.Combined with the obtained microstructure and microhardness,it can be seen that 1320? for 30 min is still the optimal heat treatment process parameter,and the microhardness can reach 400 HV.According to the experimental researches,the volatilization law of each element in the preparation of TiAl alloy by EBM was obtained,and the ingot with the nominal component Ti-48Al-2Cr-2Nb was prepared based on this model.In other words,this model can be used to predict the element volatilization behavior of alloy melt in the EBM process to some extent.The microstructure control mechanism of Ti-48Al-2Cr-2Nb alloy was obtained,and the process parameters with better microstructure and mechanical properties were obtained.