Study On Forming Characteristics Of Hydrogenated Ti-55 Titanium Alloy

Ti-55 titanium alloy is a structural material with excellent comprehensive properties,which not only has good heat resistance and high specific strength,but also has good thermal strength,thermal stability and creep resistance.In industrial applications,the superplastic forming conditions of titanium alloys are required for high forming temperatures and low strain rates.They result in high production costs and long cycle times.The internal microstructure of titanium alloy can be improved by thermohydrogen processing,thereby optimizing the deformation condition of titanium alloy and enhancing the forming performance.In this paper,the effects of thermohydrogen processing on the refinement mechanism and forming properties of Ti-55 titanium alloy were systematically investigated by means of microstructural analysis,gas bulging forming,high-temperature tensile tests and electroplastic tensile tests.By using XRD,OM,SEM,EBSD and TEM microscopic analysis methods,the refinement mechanism of hydrogenation-dehydrogenation treatment to Ti-55 titanium alloy is achieved by recrystallization and phase transformation.The recrystallization refinement is due to the formation of micro-strain field in the matrix by the hydride formed during the hydrogenation process.It results in distortion and dislocation,which induce recrystallization nucleation and growth during the dehydrogenation process.The phase transformation refinement is mainly achieved by the eutectoid transformation ofβ_H→δ+αduring hydrogenation process.Also,in the process of dehydrogenation,the hydride decomposition ofδ→α+H₂↑and the release of hydrogen in the form of solid solution fromα_H→α+H₂↑,β_H→α+β+H₂↑break the matrix structure into smaller microstructure.The forming pressure of Ti-55 titanium alloy for constant pressure gas bulging was determined by MSC.MARC finite element software.On the basis of simulation results,the forming properties of Ti-55 titanium alloys with different hydrogen content were investigated by constant pressure gas bulging experiment.The results show that low-temperature superplasticity can be achieved by adding reasonable hydrogen content to Ti-55 titanium alloys.The ratio of deformation height to radius of the hydrogenated alloy with 0.3 wt%H reaches 1.58 at 825°C,which is close to that of the as-received alloy formed at 925°C.Therefore,the optimum superplastic gas bulging forming temperature is reduced by about 100°C.The volume fraction ofβphase in the hydrogenated alloy with 0.3 wt%H increased.There is a suitableα/βratio,as well as the grain refinement.Consequently,the superplastic formability of hydrogenated alloy is improved.Furthermore,hydrogenation lowers stacking fault energy of Ti-55 titanium alloy and promotes the occurrence of dynamic recrystallization.Finally,the bulging height results of simulation and experiment were compared.It has found that the height error was within 9%,which verifies the reliability of the gas bulging simulation results.Combined with the thermohydrogen processing（hydrogenation-superplastic deformation-dehydrogenation）,the mechanical properties of dehydrogenated Ti-55 titanium alloy after superplastic deformation at its highest service temperature of 550°C were studied.The tensile results at550°C show that with the increase of hydrogen content,the yield strength and tensile strength of Ti-55 titanium alloy after thermohydrogen treatment obviously increase.But the elongation first increases and then decreases.Compared with the as-received alloy,the yield strength and tensile strength of the 0.5wt%H-thermohydrogen processed alloy at 550°C increased by35.6%and 33.9%,respectively.Microscopic analysis shows that the improvement of mechanical properties of Ti-55 titanium alloy after superplastic deformation at 550°C is due to the combination of the microstructural refinement,dislocation strengthening and hardness increase ofαandβphases by thermohydrogen processing.The deformation behavior of hydrogenated Ti-55 titanium alloy was studied by electroplastic tensile test.Under the condition of 30V-200Hz,the peak stress first decreases and then increases with the increase of hydrogen content.And the elongation first increases and then decreases.The peak stress and elongation of the hydrogenated alloy with 0.4wt%H reach605.8MPa and 28.2%,respectively.Compared with the result of as-received alloy under pulse current,its peak stress is reduced by 27.3%and its elongation is increased by 105.8%.The volume fraction ofβphase increases significantly and the dislocation is obviously reduced in hydrogenated alloy with 0.4wt%H.It is considered that the pulse current increases the temperature of the specimen and hydrogenation reduces the transformation temperature ofα→β.The combination of hydrogen and pulse current promotes the transformation ofα→βand increases the content ofβphase.Also,the diffusion ability of hydrogen is strong,hydrogenation can reduce the driving force of dislocation movement and enhance the mobility of dislocations,which benefits to the coordination of plastic deformation.