Preparetion And Hot Deformation Of Ti₂AlNb Alloy Prepared By Powder Metallurgy

Ti2AlNb intermetallic compound alloy is a new generation structural material used in hot end parts of advanced aeroengines.Ti2AlNb intermetallic compound breaks the upper limit of service temperature(～600 ℃)of traditional titanium alloy and can meet the mechanical properties requirements in temperature range of 650 ℃～750 ℃.Besides,Ti2AlNb alloy has higher room temperature ductility and better processability than TiAl and Ti3Al intermetallics.The mass ratio of Nb element in Ti2AlNb alloy exceeds 40%,which increases the control difficulty of the smelting process.The defects such as macrosegregation and porosity of the Ti2AlNb alloy are easily generated in ingot,which adds great difficulty to the development of Ti2AlNb alloy and products.At present,the research on the forming method of Ti2AlNb alloy mainly focuses on the hot deformation of ingot and powder metallurgy(PM).Domestic and foreign scholars have done a lot of research work on the composition design,microstructure evolution and heat treatment control of Ti2AlNb alloy.Ti2AlNb alloy is close to practical engineering application stage.However,in the field of combustor parts preparation that needs to satisfy both large-size and high-performance conditions,there are bottlenecks in the conventional methods of forming Ti2AlNb alloys and products.Based on the application background of Ti2AlNb alloy in advanced aeroengine parts,the feasibility of preparing Ti2AlNb alloy and ring combustor by the combined process of PM+ hot deformation is studied.The hot deformation behavior of PM Ti2AlNb billet and the deformation effect on the properties of PM Ti2AlNb alloy are analyzed.In this paper,the research on hot deformation of PM Ti2AlNb alloy provides a new idea for the preparation of large-size and high-performance Ti2AlNb parts.At the same time,by comparing to other preparing methods,the research of this paper is helpful to understand and solve the key problems affecting the engineering application of Ti2AlNb alloy as a whole.The main research contents and results are as follows:The Ti2AlNb prealloyed powder was prepared by electrode induction melting gas atomization(EIGA)method.The chemical composition,morphology and particle size distribution of Ti2AlNb prealloyed powder were characterized The oxygen content,hollow powder ratio and fluidity in Ti2AlNb powder with different particle size distribution were compared.By analyzing the microstructure of the fully dense PM alloy prepared by hot isostatic pressing(HIP)and the element microsegregation of Ti2AlNb powder,it is found that the grain structure of powder metallurgy Ti2AlNb alloy is affected by the grain size of prealloyed powder.The increase of powder boundaries will hinder the HIP compaction process,which produces large pores(>30μm)inside the PM alloy and will deteriorate the ductility.While,some pores of the hollow powder could be left in the PM alloy to form small pores(<30 gm).The Ti2AlNb prealloyed powder with a medium average grain size(～100 μpm)and a reasonable particle size distribution range is preferred.The PM Ti2AlNb alloy prepared by the preferred powder has the least porosity and the best mechanical property combination.The hot deformation behavior of powder metallurgy Ti2AlNb alloy was studied by thermal mechanical simulation tests in a comparitive way.Compared with the wrought alloy(casting Ti2AlNb ingot + cogging at single phase temperature),the PM Ti2AlNb alloy has the same deformation resistance and similar stress-strain curve.Whether the heat treatment before deformation has no effect on the hot deformation behavior of the PM Ti2AlNb alloy.The deformation behavior of PM Ti2AlNb alloy is sensitive to temperature response,and the stress fluctuation is more obvious near the phase transition point temperature.The deformation behavior at the initial stage of deformation is also significantly affected by the strain rate.Therefore,the thermal deformation process of PM Ti2AlNb alloy needs to be strictly controlled.Temperature around 1030 ℃was selected as the preferred hot deformation temperature for PM Ti2AlNb alloy.The PM alloy can withstand lower temperature and higher strain rate in thermal deformation without cracking due to its uniform composition distribution and microstructure characteristics.Ti2AlNb pre-deformed billets(bar billet,rectangular shaped ring billet and sepecial shaped ring billet)of different sizes were prepared by powder metallurgy near net shape forming method(PM NNS).Free forging and horizontal ring rolling tests were conducted on the PM Ti2AlNb pre-deformed billets.The actual thermal deformation results further verify that the hot deformation behavior of PM Ti2AlNb alloy is sensitive to temperature.Good thermal insulation system and improved rolling parameters can significantly increase the deformation efficiency of PM Ti2AlNb billet and ensure that no crack is found during the rolling processes.Hot deformation promotes the microstructure transformation of PM Ti2AlNb alloy,heals the pore defects,and improves the tensile strength and creep rupture life of the material.The actual thermal deformation results of the PM Ti2AlNb alloy verify the feasibility of the combined process of PM+hot deformation in preparing Ti2AlNb parts.For the engineering application background,the ring rolling test of large-sized PM Ti2AlNb sepecial shaped ring(D>800 mm)was successfully implemented.The maximum diameter of the Ti2AlNb ring after rolling was close to 1000 mm,and the rolling parameters was optimized and solidified.The heat treatment testing results of Ti2AlNb special shaped ring show that increasing the solid solution temperature(913℃<Ta2+B2+o<1000 ℃)and lowering the aging temperature(TB2+o<913 ℃)can increase the strength of material.The size and the distribution of O phase are the key parameters affecting the high temperature performance of Ti2AlNb.The results of rotational bending fatigue(RBF)tests show that the room temperature fatigue limit(520 MPa,107 cycle)of Ti2AlNb ring is close to the 650 ℃ fatigue limit(520 MPa,107 cycle),and the fatigue life at high temperature is better.The heat treatment reduces the strength of the Ti2AlNb alloy,thereby reducing the fatigue limit of the material.Increasing the heat treatment cooling rate and performing the secondary hot isostatic pressing will increase the tensile strength of the large-sized Ti2AlNb special shaped ring.Compared to the ingot billet + cogging + ring rolling process in preparing the Ti2AlNb combustor parts,PM billet+ring rolling process has better feasibility.