Study On The Volatilization Of TC4 Alloy Elements In Electron Beam Cold-bed Melting And The Microstructure And Properties Of Direct-rolled Plate

Titanium alloy is an important supporting material for modern national defense construction and an indispensable important material for national economic development,but the high cost limits its wider application.Electron beam cold hearth melting technology has the advantages of strong decontamination ability,diverse ingot types,strong adaptation to raw materials,etc.The application of electron beam cold hearth furnace（EB furnace）to cast titanium alloy master ingots,and then directly rolled into titanium alloy materials such as plates,rods,wires and tubes,is one of the main paths for the development of low-cost titanium alloy materials.Previous research in China has mainly focused on titanium alloys for vacuum arc melting（VAR）of vacuum self-consuming electrode,while the research on titanium alloy smelting in EB furnaces and its plastic processing is still in its infancy.In this paper,the research work is focused on smelting TC4（Ti-6Al-4V）titanium alloy in EB furnace and rolling it.In the aspect of alloy melting,the volatilization loss behavior of elements during EB furnace melting was studied emphatically,and the calculation model of element volatilization flux was established and verified.TC4titanium alloy flat ingot with stable composition was melted and cast.The hot deformation behavior of as-cast TC4 titanium alloy was studied,the constitutive equations of high temperature deformation inβsingle-phase region andα+βtwo-phase region were established,and the thermal processing map of the alloy was established.In the aspect of slab rolling,the evolution law of microstructure and mechanical properties of alloy plate under different processes was studied.TC4titanium alloy plates prepared by different rolling processes were annealed,and the effects of annealing temperature on the microstructure and mechanical properties of alloy plates were studied.The main conclusions are as follows:（1）When EB furnace melts TC4 titanium alloy,it is mainly the volatilization loss of Ti and Al,and the volatilization of V can be ignored.The volatilization of Ti in the whole smelting process and Al in the melting stage of raw materials are not controlled by liquid mass transfer,and the volatilization flux obeys Langmuir equation.The volatilization of Al in cold-hearth melting and crystallizer melting stage is obviously affected by melt flow.By introducing the factors of melting speed,cold-hearth size and crystallizer cross-section size,the calculation models of liquid phase mass transfer coefficient of Al volatilization in cold-bed and crystallizer were established,respectively.It was verified that the measured composition of Al content in the experiment was basically consistent with the calculated results,which indicated that the established volatilization model was reliable.Furthermore,the corresponding relationship between melting speed,initial Al contentof raw materials and Al content of ingot was established.Based on this,EB furnace melting and casting experiment with the raw material ratio of Ti-7.2Al-4V was carried out,and a large TC4 flat ingot with a size of 125cm×21cm×780cm was obtained.Compared with the double VAR process,the amplitude of Al element was higher and the V element was equivalent.（2）Based on the Arrhenius model and Z parameters,the thermal deformation activation energy Q_（α+β）=746.33k J/mol,Q_β=177.84k J/mol of the cast TC4 titanium alloy is obtained.The constitutive equation of the alloy is established,and the average relative error between the predicted value and the experimental value is5.04%,and the model established can well trace the thermal deformation behavior of the alloy.Based on the dynamic material model and Prasad instability criterion,the thermal working map of the cast TC4 titanium alloy was established,and the plastic hazard zone of the alloy is 850～960°C,the strain rate is 0.01s^-1～0.4s^-1,850～1000℃,the strain rate is 0.26s^-1～10s^-1,1050～1100℃,the strain rate was 0.03s^-1～1.0s^-1,the suitable processing area is 1050～1100°C,and the strain rate is less than 0.03s^-1.（3）The as-cast TC4 titanium alloy was directly rolled by conventional process and improved process,and the alloy plate with thickness of 8mm was prepared by three rolling times.Due to the increase of single heat rolling temperature（1065℃）,the plate surface cracking of the improved process is significantly improved compared with the conventional process（single heat heating temperature is 965℃）.The microstructure of semi-finished plates rolled in single heat rolling by the two processes is quite different.The most obvious change in the conventional process is the disintegration of lamellarα,andβtransformation structure with thin lamellar is obtained after the improved process is rolled in single heat rolling.Primaryαequiaxed is a significant change feature of microstructure after two-fire rolling,and the equiaxed degree is close,which indicates that lamellar microstructure is easier to break and spheroidize under T_β.The microstructure uniformity is further improved after three-fire rolling by the two processes,and the mechanical properties meet the requirements of TC4 titanium alloy in GB/T 3621—2007 Titanium and Titanium Alloy Plate.（4）The effect of annealing temperature on the microstructure and properties of alloy sheet is studied.The results show that the primaryαgrain grows gradually at780℃,the secondaryαbecomes thicker and wider with the increase of annealing temperature,and the strength and plasticity index decrease after annealing at 810℃or above.The suitable annealing temperature is 750℃.The morphology ofαphase of the alloy plate rolled by the improved three-fire process has no obvious change after annealing at 750℃,theαgrains tend to merge after annealing at 800℃,theαgrains merge and grow obviously after annealing at 850℃,and the rolled fibrous structure characteristics are completely destroyed after annealing at 900℃.With the increase of annealing temperature,the grain orientation concentration and texture strength of the alloy plate show some ups and downs.For plates of triple reheat rolling by improved process,the best matching of room temperature strength and plasticity is after annealing at 750℃.