| Titanium alloys are widely used in aerospace field due to their high specific strength,excellent weldability,good corrosion and heat resistance.Compared with traditional welding process,there is no crack or pore induced by solidification microstructure at diffusion bonding(DB)joint.Furthermore,the joint quality is stable.DB technology for titanium alloy has great application potential in aerospace field.The quality,dynamic mechanical property,failure mode and fatigure crack propagation behavior of joint are highly related to safe service of DB components.It is long period,high cost and low efficiency to choose and optimize DB process parameters by using conventional physical experiment.How to consider the realistic surface asperities in DB model and develop a universal geometry model to accurately predict the void closure require deep-going research.Proper hydrogen addition can induce phase composition and microstructure variations and improve mechanical properties of titanium alloy.But the effect and mechanism of hydrogen addition on joint quality of titanium alloy are lack of systemical study.Additionally,the response and failure mode under dynamic loading for DB joint of titanium alloy remain unclear and need to be studied.It also needs to be solved to improve the fatigure crack propagation resistance of DB joint.This work focuses on prediction and improvement of DB joint quality,and subsequent service characteristics of homogeneous and heterogeneous joints.Under the support of National Natural Science Foundation of China(51875350),International Government Cooperation Project of Shanghai Science and Technology Commission(19110712700)and Aeronautical Science Foundation of China(2016ZE57008),deep studies are performed by the theoretical analysis,finite element simulation and experiments.The main research contents and findings are briefly obtained as follows:A two-wavelength model considering real surface morphology is derived,based on the difference of surface morphology prepared by using different processing method.In two-wavelength model,the void closure process includes two stages.In the first stage,the large flat voids formed by long-wavelength asperities shrink and divide into more microvoids formed by short-wavelength asperities.In the second stage,the microvoids close completely.This model has two advantages: the wavelength of long-wavelength asperities is considered to improve the prediction precision of the contribution of grain boundary diffusion;the void height change rate is calculated based on the actual volume change rate,then the contribution of surface diffusion mechanism is accurately predicted.The model can predict the bonding time needed for sound joint,bonding ratio and void distribution state in DB process at any bonding time.The proposed model is universal.It is applicable to different roughness surfaces prepared by using different processing method.The second stage of the two-wavelength model can predict bonding ratio and time as an independent one for surfaces with single-wavelength feature.Based on verification and analysis,the two-wavelength model shows a higher prediction accuracy.The dynamic mechanical behavior of joint is very important for safe service of DB components.Quasi-static and Hopkinson pressure bar tests are conducted to measure the mechanical behavior of DB interface for Ti6Al4 V alloy at different strain rates.A modified Johnson-Cook(JC)model is proposed to describe the stress-strain behavior of DB interface.The strain rate hardening coefficient has a nonlinear relationship with the strain and the strain rate.The effect of strain rate on fracture mode is studied by the SEM fractography analysis.Furthermore,the parameters of JC failure model are determined.Through a VUHARD subroutine,the modified JC model and JC failure model are integrated into Abaqus.Then the numerical simulation of dynamic mechanical response for DB joint is implemented firstly.By comparing the prediction and experimental results,the prediction error of the maximum load and fracture strain are 2.66% and 3.64%,respectively.The models show high precision and are capable of describing the dynamic mechanical behavior of DB interface for Ti6Al4 V alloy.The influence law and mechanism of heterogeneous interface on fatigure crack propagation behavior are revealed.It is found that the fatigue trace lines show discontinuity at heterogeneous interface.Along the laminate thickness direction,the fatigue crack growth rate(FCGR)changes significantly at heterogeneous interface.For TA2/Ti55/TA2 laminate,FCGR is decreased by 44% when the fatigure crack propagates into Ti55 layer from TA2 one.For Ti55/TA2/Ti55 laminate,FCGR is reduced by 74% when the fatigure crack propagates into Ti55 layer from TA2 one.For both homogeneous and heterogeneous laminates,the cleavage fatigue fracture happens in the stable stage of crack growth,while the fatigue final rupture region is dominated by dimples.By further research,it is found that the variation of FCGR at heterogeneous interface results from the differences of grain size and plastic zone size of crack tip in Ti55 and TA2 layers.The mechanism of hydrogen-induced low temperature diffusion bonding for hydrogenated Ti55 alloy is revealed.When bonded at 700?,bonding ratio and shear strength prominently improve with the increase of hydrogen content,which results from residual hydrogen in bonded sample.Hydrogen addition can increase the fractions of high angle grain boundaries(HAGBs)and ? phase as well as break up original long-strip ? grains.However,when bonded at 800?,hydrogen almost escapes from hydrogenated alloy even though grain refinement and volume fraction increase of ? phase occurred.The bonding ratio and shear strength slightly increase with increasing hydrogen content.It is concluded that residual hydrogen plays a key role in improving diffusion bonding quality during DB process. |
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