Superplastic Formability And Microstructure Evolution Of Ti750 High Temperature Titanium Alloy

With the development of aircraft, it demand that the titanium alloy has highterservice temperature and better high temperature properties. at present, there are Ti-1100,BT18,Ti60,IMI 829 etc high temperature titanium alloy have been developed indifferent country, their operating temperature is all between 550℃and 650℃. Ti750titanium alloy is developmented by Chinese academy of sciences, it is a kind of near-αphase titanium alloy whose short-time service temperature can reach 750℃, and can beused in the supersonic aircrafts. Although the cold processing of titanium alloy is verypoor, it is a kind of natural superplastic material, some titanium alloy's elongation ratecan reach more than 1000%. superplastic forming has the advantages of good formabilityand small flow stress, which can process parts of complex shape, so the using ofsuperplastic forming in forming titanium alloy aerospace structures has widely extended.In this paper, the superplastic ability,forming technology and evolution ofmicrostructure was studied through the superplastic bulging process of a Ti750 alloysheet into a deep cup. Confirm the best superplastic forming temperature and strain ratesensitivity index of Ti750 alloy by high temperature tensile tests. The superplasticbulging method was adopted by the result of finite element analysis, and with thisunderstanding,we got the Ti750 alloy deep cup part. finally we discussed the evolutionof microstructure through heat treatment.First, the heat expansion effect in the Ti750 alloy superplastic forming was studied.Ti750 alloy plate does not have significant directional character, the elongation rate ishigher than 200% between 930℃and 960℃and strain rate sensitivity index is 0.3 at940℃, so the superplastic ability of Ti750 alloy is poor. Under the situation ofsuperplastic deformation, the yield strength of material was lower than 50MPa.FEA simulation of Ti750 alloy superplastic bulging was carried out onMSC.MARC2005 before superplastic forming, At last, we could got the dangerous zonein the forming process, thickness distribution of the component. We defined the strainrate to 5×10-4s-1 and it was shown that lower friction coefficient could improve thesituation of non-uniform thickness distributing of the bulged part. After studing the heatexpansion effect in Ti750 alloy superplastic forming, a formula was developed to compute the quantity of dimensional modification of the die. At last, we formed theTi750 alloy cup at 940℃and obtained the part that met the requirement. The materialbecame brittler after forming, because the original basketweave microstructure wasdestroyed.At last, this paper studied on the microstructure evolution of Ti750 alloy. thoughquenching under different temperature we could know how the microstructure changedwhen the material was heated, the temperature ofαphase changing toβphase is 1000℃.When the material was quenched above transformation temperature,we could getmartensitic structure,When it was cooled in the air above transformation temperature,we got basketweave microstructure. the heat treatment temperature should lower than1000℃otherwise it may have negative effect on the quality of material.