Coupled Modeling And Simulation Of Microstructure And Deformation For Ti-22Al-24.5Nb-0.5Mo Sheet Gas Bulging Forming

Ti2AlNb-based alloys have attracted considerable attentions as a potential material to long-time service at 650~750°C,resulting from its advantages of good creep resistance.To predict the effects of forming process on microstructure evolution and control the dimensional accuracy and mechanical properties of the complex shape Ti2 AlNb part,the coupled modeling,hot deformation behaviors,bulging abilities and hot forming limit curve Ti-22Al-24.5Nb-0.5Mo(at.%)were investigated.This study provides the reference for the forming process design for Ti2 AlNb complex-shape thin-walled sheet parts based on the quantitative relationship between micortucture and flow stress.To achieve the effect of heating on the microstrure evolution of Ti 2AlNb alloy,the static phase transformations under different heat treatments at 910~1040°C were studied.The volume fractions of ?2,B2/? and O were analyzed and the microstructure evolution and phase transformations were discussed.To achieve the hot deformation behaviors,the constant strain rate tensile of Ti2 AlNb sheet at 910~1040°C with strain rates 0.1~0.0004s-1 were tested.The Arrhenius equations and microstructure evolution mechanisms were studied.At B2/?+O phase zone,the volume fractions of ?2 and O phases were large.The lamella O-phase grains joined the plastic deformation and the dynamic recrystallization of O grains were observed during the plastic deformation.The globularization of lamella O-phase grains were effected by the deformation degree.It can be found that some cavitations were around the ?2 grains when plastic deformation the was large.At ?2+B2/?+O phase zone,B2/? grains joined the main plastic deformation because of the small volume fractions of ?2 and O phases.The flow stress kept stability under the dynamic equilibrium of dynamic recrystallization and grains growth for B2/? grains.At ?2+B2/? phase zone,the B2/? grains coarsening and the formability reduce because of the reducing for the small ?2 and O grains pinning effect.To quantitatively describe the effects of grain size and phase volume fraction on the flow stress,the different heat treated specimens were tested by constrain uniaxial tension at 930°C and 970°C.With the increase of heating temperature and time,the flow stress increase because of the grains carsening and ?2/O phase volume fraction increasing.Based on the creep equation,the grain factor m and the relationship between B2/? and O strength were calculated,as following: at ?2+B2/?+O phase zone,m=1.1 and 2/=1.14 O BK Kb,while at B2/?+O phase zone,m=1.7 and 2/=1.23 O BK Kb.Meanwhile,the data proved that the lamella O-grains globularization was the main reason of the flow stress softening phenomenon at B2/?+O phase zone.The coupled microstructure and deformation models of uniform viscous plastic constitutive equations were set up,where the relationship between the flow stress and the physical variables,such as rlative islocation density,relative grain size,phase volume fraction,material damage,plastic deformation heat ing and lamella Ograins globularization,were discussed.The model constants have been determined by using a Genetic Algorithm(GA)-based optimization method,which combined the weight objective function.To verify the accuracy of the constitutive equations under the complex deformation conditions,the hot tension tests with the abrupt changing strain rate were tested.Contrasting with the experimental flow stress curves,it is found that the uniform viscous plastic constitutive equations are more accurate than the phenomenological constitutive equations because the effects of deformation history on microstructure evolution were considered.To study the hot bulging ability and forming limit curve,a high temperature bulging testing device was developed,where the top testing temperature is 1000°C and the top gas pressure is 70 MPa.Using the die inserts with 1/4,2/4,3/4 and 1 major-minor axis ratios and the un-bonded bi-layer bulging test method,the forming limit curve of Ti2 AlNb rolled sheet at 970°C with strain rate 0.001s-1 were measured.Meanwhile,the forming limit curve also was predicted by M-K theory.The main reason of the difference between experimental and predicted FLCs at the tension-tension side was analyzed,which was the material damage evolution and grain size evolution were affected by the strain rate.The VUMAT subroutine of uniform viscous plastic constitutive constitutive model was developed.The effects of forming temperature and gas pressure loading path on dislocation density,material damage,grain size and phase volume fraction during the forming process for Ti2 AlNb cup-shape part were simulated by Abaqus.The results show that the predicted strain distribution and microstructure evolutions agreed well with the experimental results.The effects of heat treatments on microstructure evolutions and hot mechanical properities were studied.Based on the experimental and FEM results,the best forming process parameter for complex Ti2 AlNb sheet part was determined: finishing the hot forming at 970~985°C with strain rate 0.0004~0.001s-1,then rapid cooling to 820°C and keeping thermal insulation a few hours.