Microstructure And Mechanical Property Evolution Of Titanium Alloy In Flow Forming Process

Thin-walled titanium alloy cylindrical parts are crucial for high-end equipment utilized in aviation and aerospace fields.Hot flow forming(or tube spinning)is an effective method to produce these parts due to combing the advantages of enhancing material plasticity,reducing forming force,expanding the limit size,refining microstructure and improving mechanical performance of the components.Therefore,hot flow forming process becomes an advanced technology needed to develop urgently.However,the process couples multi-fields,multi-steps and multi-parameters,and the material undergoes a complicated stress state,which would intensify the complexity of microstructure,texture and properties evolution.Therefore,it is a great challenge to tailor the microstructure and mechanical performance of the thin-walled titanium alloy cylindrical parts in hot flow forming process.For these purposes,in-depth and systematic investigations on the microstructure,texture and properties evolution of TA15 titanium alloy in flow forming process were carried out by combining experiment,finite element(FE)simulation and theoretical analysis.The main contents and results obtained are as follows:Based on the FE simulation and microstructure observation,the microstructure evolution and its mechanism of TA15 titanium alloy in flow forming process were revealed.It is found that the morphology evolution and grain refinement of primaryαgrain(α_p)are the dominant phenomena in this process.Theα_pgrains were compressed in the normal direction(ND)and elongated in the rolling and circumferential directions(RD/CD).The difference in strain components finally results in the inhomogeneous morphology ofα_p in different planes of the cylindrical part.Meanwhile,with the forming pass increasing,the variation of grain morphology is more obvious.The grains are refined via three mechanisms,viz.,discontinuous dynamic recrystallization(DDRX),continuous dynamic recrystallization(CDRX)by progressive lattice rotation and deformation induced intense shearing.The grain refinement degree and microstrcutre homogeneity gradually improves with the forming pass increasing.The significant compressive and shear deformation in flow forming strongly affects the mechanism and kinetics of dynamic recrystallization (DRX).In both deformation modes,DDRX and CDRX coexist,while the dominant mechanisms are quite different.The dominant mechanism in compression deformation changes from CDRX to DDRX with strain.However,shear deformation makes CDRX always prevail and present faster DRX kinetics,thus improving the grain refinement degree and grain size homogeneity.Through the crystal plasticity theory,the FE model coupled with texture evolution of titanium alloy in flow forming process was established,and the texture evolution and its mechanism during this process were revealed.The results indicated that non-standard{0002}basal plane texture is gradually formed with the increase of forming pass.The formation of the{0002}basal texture mainly results from the basal slip system activated under the predominant compression in the ND.However,the deviation of the texture is caused by the significant shear deformation in the CD and the RD as well as the possibility of prism<a>and pyramidal<a+c>slipping in the flow forming process.Meanwhile,the texture distributes inhomogenously along the ND of the cylindrical part.The{0002}poles in the outer surface are more close to the ND and present the stronger instensity than that in the middle and inner surface area.In addition,if the c-axis of grain is closer to the ND of the cylindrical part,the deformation will get harder and the texture would be weakened.Conversely,if the c-axis of grain deviates from the ND,the deformation will be easier and the texture would be strengthened.The mechanical performance evolution and anisotropic mechanism of the titanium alloy cylindrical part after flow forming process were investigated.The results indicated that the yield strength in the RD is lower than that in the CD,while the RD presents higher plasticity than the CD after multi-pass flow forming.The value of yield strength is mainly determined by the difficulty of prism slip activation inα_p grains when loading along the different directions of the spun cylindrical part.As loads along the CD,the prism slip have higher Schmid factor due to the formation of non-standard{0002}basal plane texture after spun,thus raising the yield strength in the CD than in the RD.On the other hand,lower Schmid factor of prism slip in the CD makes basal slip,pyram slip and cross slip occur easily,thus inducing the microcrack and cleavage plane formed.Finally,the plasticity of the spun cylindrical part in this direction reduces.Meanwhile,the microcrack will be propagated easily along theα_p morphology orientation when loading along the CD.This is another reason for the lower plasticity in the CD.Based on the above results,the effects of processing parameters on the microstructure,texture and mechanical property evolutions of the spun cylindrical part were explored.It is found that with the deformation temperature increasing,the grain refinement mechanism gradually changes form CDRX to DDRX.The grain refinement degree increases first and then decreases.The texture intensity and the yield strength in RD gradually decrease,while the plasticity in RD increases first and then decreases.Decreasing the roller feed rate or increasing the forming pass can improve the grain refinement degree,thus enhancing the strength and plasticity of the spun cylindrical part.