| Titanium alloys have been the main structural material in aerospace andaeronautical fields due to their lower density, high specific strength, excellentcorrosion as well as high heat-durability. However, titanium alloys are sensitive todeformation temperature and strain rate. It is difficult to form large complex structuraltitanium alloy by the conventional forging. Superplasticity is characterized by lowflow stress,high uniformity of plastic flow and large elongation. Therefore,superplastic forming is an effective approach to solving the problem about forminglarge complex titanium alloy components. The major works and the research results inthe thesis are as follows.TC4-DT is a kind of damage-tolerance titanium alloy. The average grain size ofcommercial TC4-DT titanium alloy is70μm，Relatively coarse grains, the superplasticformability of the alloy is lower. To improve the superplasticity, the average grain sizein15μm of isometric TC4-DT alloy was prepared by three-dimensional upsetting andcogging ausform process firstly inwhich the alloy wasβforged firstly at1000℃andsubsequently cooled to room temperature rapidly by water, then subjected to final α+βforging at950℃and also subsequently cooled to room by Air. The mechanicalproperties of the Refinement TC4-DT alloy at room temperature and high temperaturewere enhanced greatly. The SEM micrographs of room temperature tensile fractureabout forged TC4-DT alloy showed that: After thinning processing specimens fracturedeformation is ductile fracture, samples of coarse grain supply state fracture is prevailcleavage fracture. The results of TC4-DT alloy microstructural evolution in thethermo-mechanical process show that: the main mechanism of TC4-DT alloythermomechanical processing grain refinement is deformation induced grainrefinement, makes the grain refinement and crusher by severe plastic deformation,Obtained small martensite after β forging by water-cooled conducive to further refineand isometric the microstructure during in the α+β region deformation.Superplastic tensile tests of the fine-grained TC4-DT alloy were carried out bythe constant strain-rate, the maximum m-value and the strain-induced superplasticitymethods, respectively. When the constant strain-rate method was employed, theelongations of TC4-DT alloy after forging between185%and631%were obtained at temperatures ranging from870℃to900℃and strain rates ranging from3.3×10-4s-1to1.1×10-2s-1.When the maximum m-value method and the strain-induced superplasticitymethod were applied, the elongations of1233%and1033%were obtainedrespectively at870℃Additionally, compared with the maximum m-value method, thedeformation efficiency is improved86%by the strain-induced superplasticity method.At the same time, the superplastic mechanics behavior and microstructure evolution ofthe fine-grained TC4-DT alloy were studied. During the constant strain-ratesuperplastic deformation (SPD), the flow stress curves exhibited the obvious strainhardening effect at higher temperatures and lower strain rates, which induced by thehigh dense dislocations of the fined-grain TC4-DT alloy and dynamic grain growth.The microstructure was studied based on quantitative metallography and SEM Theobtained results show that：(1) There exist coarsened grains with different levels (2)Dynamic recrystallization occurs during superplastic deformation. During the constantstrain-rate superplastic deformation (SPD),The apparent deformation activationenergy Q value is similar from the grain boundary diffusion activation energy athigher temperatures and lower strain rates, Indicates that the rate of grain boundarydiffusion is the primary control mechanism at this point. The alloy during plasticdeformation at high temperature, the flow stress with strain rate’s increasing, butdecrease with the rise of temperature When Obtained thermal deformation activationenergy Q was153.016KJ/mol.Meanwhile the flow stress, strain rate, deformationtemperature satisfy the following relationship:=3.74×108[sinh(0.02385527)]2.70981333exp[﹣153.016225×103/RT]The mechanism of TC4-DT alloy by the maximum m-value SPD is GBSaccommodated by diffusion creep coordinated grain boundary sliding mechanism;Obtaining the maximum elongation of1233%in superplastic deformation underoptimum temperature of870℃.Strain-induced superplastic deformation in the early constant speed stages, Dueto a short time deformation, strain increased caused hardening by the lack of timelyslack deformation is mainly within the crystal slip. In the late maximum msuperplastic deformation stages. Strain hardening phenomenon exhibited in thedeformation initial, The flow stress rapidly increased to peak, Then the combinedeffect of strain softening produced by the recrystallization and strain hardening caused by deformation increased, caused by the stress jagged, the stress overall trend declineuntil specimen fracture. Summary,The main deformation mechanism of strain-inducedsuperplasticity method is grain boundary sliding, and the auxiliary mechanism ofdeformation is dynamic recovery and recrystallization Changed after forging TC4-DTorganizational superplastic significantly higher change before forging, Elongationincreased by an average203%.With increasing of the pre-strain at the sametemperature, elongation increases firstly and then decreases for TC4-DT alloy，withincreasing of the temperature at the same pre-strain, elongation increases firstly andthen decreases for TC4-DT alloy. The optimum parameters of the strain-inducedsuperplastic deformation process is870℃, the pre-strain is1.5, in this condition thealloy elongation up to1033%.The deformation structure is observed in three different programs doubleannealing treatment under a constant strain rate and temperature at870℃, The resultsshow that:the superplastic forming of TC4-DT alloy sample adopted at870℃and3.3×10-4s-1, and then the basketweave structure is observed in the deformed structureby solution treatment at950℃for1h and aging at720℃for4h. |
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