| With increasing complexity of service environment of thin-walled rotating components in the aviation industry,titanium alloys have become excellent structural materials due to their high strength,low density,excellent heat resistance and corrosion resistance.Ti55 high temperature titanium alloy,which is independently developed by China,is a kind of near-αtitanium alloy which can be used at 550°C in long term.Since Ti55 high temperature alloy has low room-temperature ductility,its thin-walled rotating components need to be fabricated by high-temperature forming process,such as spinning or sheet thermoforming,while the influence of high temperature deformation behavior and welding process of the titanium alloy is still not investigated deeply.In addition,the hot spinning and sheet hot forming processes of Ti55 high temperature titanium alloy are seldom studied,which directly limits the application of Ti55 high temperature titanium alloy.Therefore,the research on high temperature deformation behavior and precision forming process of Ti55 high temperature titanium alloy has great significance for improving the service performance of aviation components.The isothermal compression experiment of Ti55 high temperature titanium alloy was carried out,and the plastic deformation behavior and microstructure evolution were analyzed.Besides,the hot processing map of the alloy was established based on dynamic material model（DMM）and the optimized working window of process parameters was obtained.The material deformation softening mechanism is mainly controlled by dynamic recrystallization ofαphase and dynamic recovery ofβphase.The deformation activation energy of the alloy in theα+βdual phase region and theβsingle phase region are 453.00 KJ/mol and 279.88 KJ/mol,respectively.The discontinuous yielding of the alloy mainly occurres in the temperature range of950-1050°C and the strain rate range of 0.01s-1–1s-1.As the strain rate increases,the discontinuous yielding phenomenon is weakened mainly because severe work hardening masked the discontinuous yielding phenomenon.As the content ofβphase increased in theα+βdual phase region,the discontinuous yielding phenomenon is enhanced probably because the stress concentration at the interface of theα/βphase can activate the new movable dislocations.For the Ti55 high temperature alloy,the optimum processing parameter range of the alloy should be850°C-975°C/0.001s-1-1.0s-1.After hot compression in this processing parameter range,the microstructure undergoes sufficient dynamic recrystallization,and transformed into a mixedαandβtransformation microstructure.The effects of electron beam welding（EBW）and heat treatment on the tensile properties of Ti55 high temperature titanium alloy were analyzed through high temperature tensile test.With the increase of temperature and the decrease of strain rate,the initial Ti55 high temperature titanium alloy sheet and electron beam welded sheet exhibited increases strength and elongation.The welded joint of the EBW sheet presents high strength and low plasticity,which were broken in the base metal region in the tensile test.The specimens under the heat treatment condition of 940℃/1h/AC+600℃/2h/AC exhibites high elongation at room temperature.The specimens under the heat treatment condition of 940℃/1h/AC+600℃/2h/FC have a relatively high elongation at 750°C and 15 mm/min.The Ti55 high temperature titanium alloy sheet specimens after EBW show high joint strength and low ductility,which is difficult to be processed.However,the microstructure of the welded joint could be tuned by heat treatment to be close to the base material,which is conducive for subsequent hot processing.The hot power spinning experiment of Ti55 high temperature alloy tubular workpieces was conducted,the evolution of microstructure and mechanical properties was analyzed,and the grain orientation and deformation texture were also investigated,based on which the mechanism of bi-directional strengthening during tube spinning revealed.The results show that the hardness of initial fusion zone was greater than that of the base metal.As the thinning rate increases,the hardness and tensile strength of the fusion zone gradually approach the base metal.The tangential strength of the base metal in single-direction and double-direction spinning increase progressively with the increase of the pass number,and the double-direction spinning is strengthened more evidently than the single-direction spinning.As the thinning rate increases,the grain orientation and morphology changes gradually.The grains in the base material region are significantly elongated along the axial direction,contributing to the formation of the axial fibrous structure.The grains in the base metal area are also elongated along the tangential direction,but the elongation degree is smaller than that along the axial direction,and theβgrain boundaries of the fusion zone are also elongated and broken down.The Schmid factor of the original base material is greater,while the axial and circumferential Schmid factors are significantly reduced with the increase of the thinning rate.Through comparing the Schmid factor of 5 passes in single-direction and double-direction spinning,it can be concluded that the double-direction spinning regulated the grain orientation effectively,and strengthened is better than the single-direction spinning.The sheet hot forming process and hot spin forming process of Ti55 high temperature titanium alloy complex thin-walled components were researched,respectively.In the hot press forming process of the sheet,the forming temperature of the Ti55 high temperature titanium alloy complex thin-walled components was 850°C and the strain rate was 0.001s-1.The Ti55 high temperature titanium alloy complex thin-walled components is prone to wrinkle during one-pass hot press forming process,which made it difficult to manufacture a complex T55 titanium alloy complex curved-generatrix component by one-pass hot press forming process.Through utilizing the pre-forming process and an intermediate mold with a larger transition fillet radius,it is more conducive for uniform deformation of sheet blank and significantly reduced the wrinkling of the final workpeice.The hot-formed Ti55 high temperature titanium alloy complex shape component possesses uniform microstructure,fine grains,and excellent mechanical properties,which is improved obviously compared to the initial sheet blank.In the hot spinning process,a process scheme of shear spinning preforming and multi-pass curve trajectory final forming is adopted.In the multi-pass spinning process,if the spinning temperature was too high,the workpiece is over oxidized,and the ductility of the material is reduced,resulting in cracking.If the interval of the passes is too large,the excessive thinning of the transition zone or excessive tensile stress may occur,leading to cracking.By optimizing the multi-pass spinning process parameters,the cone-cylindrical composite curved-generatrix components could be well formed.After shear spinning and multi-pass power spinning,the microstructure of the component is refined more evidently,and the mechanical properties are significantly enhanced compared with the initial sheet blank,which is also superior to that of the hot formed workpeice. |
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