Deformation Mechanisms of TC17 Alloy with Basketweave Microstructure and Numerical Simulation of Plastic Deformation Bonding

Author:Sun Jin Zuo

Supervisor:li zuo quan

Database:Doctor

Degree Year:2018

Download:9

Pages:138

Size:8949K

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Titanium alloy lightweight components made by plastic deformation bonding process can simultaneously meet the requirements of weight and performance in aerospace equipment.However,the plastic deformation generated from bonding process is not conductive to the precision manufacturing.So as to realize the precise control of deformation during plastic bonding process,this paper studied the deformation mechanisms of TC17 alloy with basketweave microstructure,established the multi-scale constitutive model of plastic deformation and analyzed the forming rule of hollow component during plastic deformation bonding process combining with finite element numerical simulation technique.The main results and conclusions can be drawn as follows.Based on the isothermal compression tests of TC17 alloy,the evolution of microstructure morphology affected by technological parameters was studied by optical microscope.The microstructure observations indicate that the deformation temperature,height reduction and strain rate significantly influence the phase volume fraction and the globularization ofαplatelets.The deformation temperature and height reduction mainly affect the process of globularization inαplatelets,which results in the volume fraction change of globularizaiton.The strain rate obviously affects the way of globularization ofαplatelets,which results in the preferential orientation being vertical to the compression axis at the high strain rate and the terminal coarsening behavior at the low strain rate.The dynamic recovery,dynamic recrystallization and interaction effect between them inαandβphases during isothermal compression of TC17 alloy were studied by transmission electron microscope and electron backscatter diffraction technics.The recrystallization mechanisms ofαplatelets are geometrical dynamic recrystallization(GDRX,dominated by dislocation movement mechanism)and rotational recrystallization(RRX,depending on diffusion mechanism).The main recrystallization mechanism is GDRX at the high strain rate.With a decrease in strain rate,the RRX gradually increases.The main recrystallization mechanism ofβis classical continuous dynamic recrystallization(CDRX,dominated by dislocation movement mechanism).The RRX inβ,as an auxiliary recrystallization mechanism always occurs near the intersection ofαplatelets so as to coordinate the plastic deformation fromαplatelets.In addition,the macrozone withα“soft grain”preferentially deforms,which results in a higher degree of fragmentation ofαplatelets and DRX ofβthan that in the mcrozone withα“hard grain”.Based on the flow stress-strain curves and microstructure examination of isothermally compressed TC17 alloy,the flow behavior affected by microstructure evolution was studied.The process of dynamic recovery withinαandβphases is related to the initial flow softening.The crystallographic orientation ofαgrains spreading from“hard”orientation to“soft”orientation,the globularization ofαphase and the dynamic recrystallization ofβphase are responsible for the following softening behavior.The value of strain rate sensitivity exponent(m)firstly increases and then decreases with an increase in deformation temperature at the strain rates of 0.1 s-1 and 0.0002 s-1,which is respectively related to the grain boundary sliding accommodated with dislocation mechanism and grain boundary sliding accommodated with diffusion mechanism.The value of m presents a relatively stable tendency at the strain rates of 0.01 s-1 and 0.001s-1,which is related to the interaction effect between dislocation and diffusion mechanisms.The flow softening behavior of TC17 alloy results in the decrease of the value of strain hardening exponent(n)with an increase in strain.The value of apparent activation energy for deformation(Q)decreases with an increase in strain during isothermal compression of TC17 alloy.The average value of Q is 374.3?75 kJ·mol-1.The multi-scale plastic deformation constitutive model of titanium alloy is established based on dislocation and diffusion mechanisms.This model quantifies the different micro-mechanisms during hot deformation and is proven to be reliable to descript the deformation behavior of TC17 alloy at elevated temperature.The observations of bonding interface obtained by scanning electron microscopy and relative statistical results of interface voids indicate that theβphase firstly contacts and results in the fragmentation of voids.The fragmentation process stops until the size of voids decreases into the same scale of the thickness ofαplatelets(1μm).The relationship between the strain and bonding ratio suggests that the bonding ration experienced rapid growth,parabolic growth and slow growth stages with an increase in strain during plastic deformation bonding process of TC17 alloy.When the strain reaches to0.05,the bonding ration arrives at95%.The multi-scale plastic deformation constitutive model was imported by VUMAT,and the plastic deformation bonding process of TC17 alloy was simulated by ABAQUS numerical simulation software.The geometrical parameters do not affect the distribution of stress,the distribution of strain and the maximum value of stress,but have a marked impact on the load and the maximum value of strain.When the number of rib is 2,the aspect ratio of rib(G)is 1.5 and the wall thickness is 2.25 mm,the maximum value of strain in whole and hollow cavity is highest,the deformation uniformity is the best and the deformation degree is appropriate.The influence of bonding temperature(T)and height reduction(h)to the average strain value at easy deformation position(?FPI)and the parameter representing the accomplishment of outstanding growth in bonding ratio(fFPII)are studied by the center composite design and response surface analysis.The technology parameters are optimized by modified NSGA-II method.The optimized results show that the bonding temperature(T)is1073K and the height reduction(h)ranges from 0.5 mm to 0.65 mm.