IMI834钛合金等温和非等温热处理过程中片层α相析出行为的定量表征

为了揭示冷却速度对片层α相析出的影响机制,采用试验分析方法对IMI834钛合金在等温和非等温热处理过程中片层α相的析出行为进行定量表征。通过热膨胀试验获得不同冷却速度下片层α相的临界析出温度。基于金相显微镜和电子探针分析以及数据拟合方法,建立片层α相的平均宽度、体积分数以及α和β相内的溶质浓度随温度和冷却速度的定量演化模型。两种条件下的片层α相析出行为对比发现:非等温冷却过程中片层α相的平均宽度和体积分数均小于同温度下等温相变过程中片层α相的平均宽度和体积分数。随着冷却速度的增加,片层α相的宽度和体积分数显著减小,临界析出温度降低。发生上述现象的原因是溶质元素Al、Mo和Nb的扩散速度随冷却速度增加而减小。

A Review on Ultrasonic-Assisted Forming:Mechanism,Model,and Process

Compared with conventional forming processes,ultrasonic-assisted forming technology with a high frequency and small amplitude can significantly improve the forming quality of materials.Owing to the advantages of reduced forming force,improved surface quality,avoidance of forming defects,and strengthened surface structure,ultrasonic-assisted forming technology has been applied to increasingly advanced forming processes,such as incremental forming,spinning,and micro-forming.However,in the ultrasonic-assisted forming process,there are multiple ultrasonic mechanisms,such as the volume effect and surface effect.The explanation of the effect of ultrasonic vibration(UV)on plastic deformation remains controversial,hindering the development of related technologies.Recently,many researchers have proposed many new theories and technologies for ultrasonic-assisted forming.To summarize these developments,systematic discussions on mechanisms,theoretical models,and forming performances are provided in this review.On this basis,the limitations of the current study are discussed.In addition,an outlook for ultrasonic-assisted forming is proposed:efficient and stable UV systems,difficulty forming components with complex geometry,explanation of the in-depth mechanism,a systematic theoretical prediction model,and multi-field-coupling energy-assisted forming are considered to be hot spots in future studies.The present review enhances existing knowledge of ultrasonic-assisted forming,and facilitates a fast reference for related researchers.

Effect of blank quenching on shear spinning forming precision of 2219 aluminum alloy complex thin-walled components

The quenching-spinning(Q-S)process,i.e.,shear spinning after blank quenching,has been increasingly utilized to form 2219 aluminum alloy complex thin-walled components.However,the changes in material property,shape and stress of the blanks after quenching will affect the spin-ning forming precision.In this study,the rules and mechanisms of these effects are investigated based on a combined finite element(FE)model including blank quenching and component spinning process.The results indicate that the increase of material strength and the existence of distortion of the quenched blank lead to a notable increase in the non-uniformity of the circumferential compres-sive stress in the spinning area and the increase of the flange swing height during spinning.These changes result in an increase in the wall thickness and component-mandrel gap of the components.The quenching residual stress has little effect on wall thickness and roundness but can noticeably reduce the component-mandrel gap.This is because that the existence of quenching residual stress of the blank can lead to the decrease of the maximum circumferential compressive stress of the workpiece in spinning and an obvious drop in the maximum compressive stress after reaching the stress peak.Quenching distortion is the main factor affecting the roundness.Moreover,the opti-mized installation way of the blank for spinning is obtained.

Deformation behavior and microstructure evolution of titanium alloys with lamellar microstructure in hot working process: A review

Titanium alloys have been widely used in many industrial clusters such as automotive, aerospace and biomedical industries due to their excellent comprehensive properties. In order to obtain fine microstructures and favorable properties, a well-designed multi-step thermomechanical processing(TMP) is critically needed in manufacturing of titanium components. In making of titanium components,subtransus processing is a critical step to breakdown lamellar microstructure to fine-structure in hot working process and thus plays a key role in tailoring the final microstructure and properties. To realize this goal, huge efforts have been made to investigate the mechanisms of microstructure evolution and flow behavior during the subtransus processing. This paper reviews the recent experimental and modelling progresses, which aim to provide some guidelines for the process design and microstructure tailoring for titanium alloy research community. The characteristics of the initial lamellar microstructure are presented, followed by the discussion on microstructure evolution during subtransus processing. The globularization of lamellar α is analyzed in detail from three aspects, i.e., globularization mechanism, heterogeneity and kinetics. The typical features of flow behaviors and the explanations of significant flow softening are then summarized. The recent advances in modelling of microstructure evolution and flow behaviors in the subtransus processing are also articulated. The current tantalized issues and challenges in understanding of the microstructure evolution and flow behaviors of the titanium alloys with lamellar microstructure are presented and specified in future exploration of them.

Electron force-induced dislocations annihilation and regeneration of a superalloy through electrical in-situ transmission electron microscopy observations

What effect does electric current do on dislocation evolution of metals keeps being a confusing question to be answered and proved. To this end, the dislocation evolution of a superalloy with electric current was directly observed by electrical in-situ transmission electron microscopy in this work. Dislocations annihilation at first and then regeneration was found for the first time, which directly proves the existence of electron force during the electrically-assisted manufacturing. Dislocations regeneration would be driven by the electron force and the resistance softening by the local Joule heating effect. Resultantly,a base could be provided for future electrically-assisted research.

Unequal-thickness billet optimization in transitional region during isothermal local loading forming of Ti-alloy rib-web component using response surface method

Avoiding the folding defect and improving the die filling capability in the transitional region are desired in isothermal local loading forming of a large-scale Ti-alloy rib-web component（LTRC）. To achieve a high-precision LTRC, the folding evolution and die filling process in the transitional region were investigated by 3 D finite element simulation and experiment using an equal-thickness billet（ETB）. It is found that the initial volume distribution in the second-loading region can greatly affect the amount of material transferred into the first-loading region during the second-loading step, and thus lead to the folding defect. Besides, an improper initial volume distribution results in non-concurrent die filling in the cavities of ribs after the second-loading step, and then causes die underfilling. To this end, an unequal-thickness billet（UTB） was employed with the initial volume distribution optimized by the response surface method（RSM）. For a certain eigenstructure, the critical value of the percentage of transferred material determined by the ETB was taken as a constraint condition for avoiding the folding defect in the UTB optimization process,and the die underfilling rate was considered as the optimization objective. Then, based on the RSM models of the percentage of transferred material and the die underfilling rate, non-folding parameter combinations and optimum die filling were achieved. Lastly, an optimized UTB was obtained and verified by the simulation and experiment.

A novel unified model predicting flow stress and grain size evolutions during hot working of non-uniform as-cast 42CrMo billets

The cast preformed forming process(CPFP) is increasingly considered and applied in the metal forming industries due to its short process, low cost, and environmental friendliness, especially in the aerospace field. However, how to establish a unified model of a non-uniform as-cast billet depicting the flow stress and microstructure evolution behaviors during hot working is the key to microstructure prediction and parameter optimization of the CPFP. In this work, hot compression tests are performed using a non-uniform as-cast 42 CrMo billet at 1123–1423 K and 0.01–1sà1. The effect laws of the non-uniform state of the as-cast billet with different initial grain sizes on the flow stress and microstructure are revealed deeply. Based on experimental results, a unified model of flow stress and grain size evolutions is developed by the internal variable modeling method. Verified results show that the model can well describe the responses of the flow stress and microstructure to deformation conditions and initial grain sizes. To further evaluate its reliability, the unified model is applied to FE simulation of the cast preformed ring rolling process.The predictions of the rolling force and grain size indicate that it could well describe the flow stress and microstructure evolutions during the process.

Forming the transverse inner rib of a curved generatrix part through power spinning

How to form high-quality transverse inner ribs through power spinning is a key issue for complicated integrated curved generatrix parts with transverse inner ribs. In this study, the forming characteristics and laws during the power spinning process were investigated using a finite element simulation based on the orthogonal design method. The results show that the transverse inner rib distributes homogeneously along the circumferential direction but inhomogeneously along the generatrix direction. Depressions occur easily in the middle zone of the rib (MZR). The roller nose radius is the most significant parameter of the MZR underfill degree. A larger roller nose radius is helpful to decrease the MZR underfill degree. Furthermore, the preformed billet thickness also plays a vital role in the underfill degree of the front zone of the rib and the back zone of the rib, as well as the depression degree of the outer surface of the rib. By combining the ribfilling characteristics and laws, the optimized forming process window for obtaining high-quality inner ribs was obtained by regression analysis, thus laying a basis for improving the forming quality of curved generatrix parts with transverse inner ribs in power spinning.

Role of the inter-pass cooling rate in recrystallization behaviors of Ni-based superalloy during interrupted hot compression

The microstructural evolution of a Ni-based superalloy under interrupted hot compressive deformation with different cooling rates in the inter-pass stage is investigated. It is found that metadynamic recrystallization(MDRX) in the inter-pass stage is more sensitive to the accumulated strain than the deformation temperature which is above the recrystallization temperature. The variations of both the grain distribution and the texture intensity caused by MDRX during the interpass stage result in variations of the yield stress(YS) and the work hardening(WH) rate in each stage. Results also show that the MDRX process in the inter-pass stage has a considerable influence on the final microstructure of three-pass compression. The final grain distribution is more uniform,and the compression texture gradually transforms into recrystallization texture with an increasing degree of MDRX. In order to predict the MDRX fraction in the inter-pass cooling stage, a modified kinetic equation is established, which can reasonably predict the MDRX behavior under multi-pass compression with different conditions in the inter-pass stage. Meanwhile, the influence of the interpass cooling stage on the mechanism of dynamic recrystallization(DRX) is studied. It is universally acknowledged that the discontinuous dynamic recrystallization(DDRX) process is the major deformation mechanism for the Ni-based superalloy. However, the continuous dynamic recrystallization(CDRX) process is promoted in the compression stage with a decrease of the cooling rate in each inter-pass stage.

A constitutive model coupling damage and material anisotropy for wide stress triaxiality

A constitutive model that can describe the damage evolution of anisotropic metal sheets during the complex forming processes which experience wide stress triaxiality history is essential to accurately predict the deformation and rupture behaviors of the processes.In this study,a modified Lemaitre damage criterion which couples with the anisotropic Barlat 89 yield function is established.The effects of stress triaxiality,Lode parameter and shear stress on damage accumulation are considered in the constitutive model.The model is numerically implemented and applied to fracture prediction in tensile tests with different stress triaxialities and a complex deformation process with wide stress triaxiality history.The good consistency of predictions and experiments indicates that the modified Lemaitre damage model has excellent fracture prediction ability.Finally,the accuracy of the model is analyzed and discussed.

Mesoscale deformation mechanisms in relation with slip and grain boundary sliding in TA15 titanium alloy during tensile deformation

Revealing the mesoscale deformation mechanisms of titanium alloy with tri-modal microstructure is of great significance to improve its mechanical properties. In this work, the collective behavior and mechanisms of slip activities, slip transfer, and grain boundary sliding of tri-modal microstructure were investigated by the combination of quasi-in-situ tensile test, SEM, EBSD and quantitative slip trace analyses. It is found that the slip behavior presents different characteristics in the equiaxed α(α_(p)) and lamellar α(α_(l))grains. Under a low level of deformation, almost all the slip deformation is governed by single basal and prismatic slips for both of α_(p)and α_(l),despite small amount of < a >-pyramidal slip exists in α_(l)grains. As deformation proceeds, < a >-pyramidal and < c + a >-pyramidal slip systems with high Schmid factors were activated in quantities. Specially, certain coarse prismatic slip bands were produced across both of single and colony α_(l)grains whose major axes tilting about 40 °–70 ° from the tensile axis. Slip transfer occurs at the boundaries of α_(p)/α_(p)and α_(l)/β under the condition that there exists perfect alignment between two slip systems and high Schmid factors of outgoing slip system. The slip transfer across α_(l)/β boundary can be divided into two types: straight slip transfer and deflect slip transfer with a deviation angle of 5 °–12 °, depending on the alignment of slip planes of two slip systems. The grain boundary sliding along boundaries of α_(l)/β and α_(p)/β was captured by covering micro-grid on tensile sample. It is found that the crystallographic orientation and the geometrical orientation related to loading axis play great roles in the occurrence of grain boundary sliding.

Dependence on forming parameters of an integral panel during the electromagnetic incremental forming process

Nowadays, more and more attentions are paid to electromagnetic incremental forming（EMIF）, especially for a part with a large-scale size, e.g., an integral panel with stiffened ribs. In this work, the bending of a panel into a double-curvature profile via EMIF is carried out experimentally and evaluated by comparing the formed profile with the desired profile. During the process,discharges at four positions along different discharge paths are designed. The effects of forming parameters on the die-fittingness of the workpiece are discussed, for which two evaluation indices are used to judge forming results. The results show that a discharge voltage in an incremental mode is helpful to improve the fittingness and avoid the collision rebound against the die at the same time.Discharging at the diagonal positions with the ‘‘X＂ discharge path exhibits the minimal shape deviation and the best forming uniformity. On the contrary, discharging at the parallel positions with the ‘‘Z＂ discharge path obtains the worst forming quality. Overlap of the coil at different positions should be given during EMIF; however, a lower overlap rate of the coil helps improve the forming quality. The results obtained in this work are useful for forming integral panels with stiffened ribs via the EMIF process.

Analysis of anisotropy mechanism in the mechanical property of titanium alloy tube formed through hot flow forming

Anisotropy of mechanical property is an important feature influencing the service performance of titanium(Ti)alloy tube component.In this work,it is found that the hot flow formed Ti alloy tube exhibits higher yield strength along circumferential direction(CD),and larger elongation along rolling direction(RD),presenting significant anisotropy.Subsequently,the quantitative characteristics and underlying mechanism of the property anisotropy were revealed by analyzing the slip,damage and fracture behavior under the combined effects of the spun{0002}basal texture and fibrous microstructure for different loading directions.The results showed that the prismatic slip in primaryαgrain is the dominant deformation mechanism for both loading directions at the yielding stage.The prismatic slip is harder under CD loading,which makes CD loading present higher yield strength than RD loading.Additionally,the yield anisotropy can be quantified through the inverse ratio of the averaged Schmid Factor of the activated prismatic slip under different loading directions.As for the plasticity anisotropy,the harder and slower slip development under CD loading causes that the CD loading presents larger external force and normal stress on slip plane,thus leading to more significant cleavage fracture than RD loading.Moreover,the micro-crack path under RD loading is more tortuous than CD loading because the fibrous microstructure is elongated along RD,which may suppress the macro fracture under RD loading.These results suggest that weakening the texture and fibrous morphology of microstructure is critical to reduce the differences in slip,damage and fracture behavior along different directions,alleviate the property anisotropy and optimize the service performance of Ti alloy tube formed by hot flow forming.

Development of microstructural inhomogeneity in multi-pass flow forming of TA15 alloy cylindrical parts

Revealing the development of microstructural inhomogeneity in the multi-pass flow forming of titanium alloy components is of great significance to the microstructure control and property tailoring.To this end,the microstructural inhomogeneity of TA15 alloy spun cylindrical parts was analyzed based on the deformation history.The results indicate that the material underwent significant compressive strain in the normal direction(ND),tension strain in the rolling and circumferential directions(RD and CD),while tension strain in the CD is slightly small due to the limited material flow in this direction.These strain characteristics make the microstructure,especially the primary a(ap),present different morphologies in the different planes of the part.Meanwhile,the combined effects of inhomogeneous deformation and temperature distribution in the ND also cause the inhomogeneity of microstructure morphology and parameters in this direction.Quantitative analyses show that with the forming pass increasing,the aspect ratio of apincreases most in the normal-rolling plane,then in the normal-circumferential plane and least in the circumferential-rolling plane,whereas apcontent decreases in an opposite trend.Along the ND,the aspect ratio and content of apis relatively high in the outer and inner surface areas but lowest in the central area,and these inhomogeneous characteristics can be gradually diminished with the forming pass increasing.Furthermore,the variation of hardness inhomogeneity factor indicates that a four-pass forming with the total reduction ratio of 63%could obtain a homogenous microstructure along the ND of the TA15 alloy spun cylindrical part.

Formability enhancement in hot spinning of titanium alloy thin-walled tube via prediction and control of ductile fracture

The damage and fracture in hot spinning of titanium alloy is a very complex process under the combined effects of microstructure evolution and stress state.In this study,their dependences on processing parameters were investigated by an integrated FE model considering microstructure and damage evolution,and revealing the effects of microstructure and stress states on damage evolution.The results show that the inner surface of workpiece with the largest voids volume fraction is the place with the greatest potential of fracture.This is mainly attributed to the superposition effects of positive stress triaxiality and the smallest dynamic recrystallization(DRX)fraction andβphase fraction at the inner surface.The damage degree is decreased gradually with the increase of initial spinning temperature and roller fillet radius.Meanwhile,it is first decreased and then increased with the increases of spinning pass and roller feed rate,which can be explained based on the variations ofβphase fraction,DRX fraction,stress state and tensile plastic strain with processing parameters.In addition,the dominant influencing mechanisms were identified and discussed.Finally,the thickness reduction without defect in the hot spinning of TA15 alloy tube is greatly increased by proposing an optimal processing scheme.

Phase fraction evolution in hot working of a two-phase titanium alloy： experiment and modeling

In the present work, the coupled effects of initial structure and processing parameters on microstructure of a two-phase titanium alloy were investigated to predict the microstructural evolution in multiple hot working. It is found that microstructure with different constituent phases can be obtained by regulating the initial structure and hot working conditions. The variation of deformation degree and cooling rate can change the morphology of the con- stituent phases, but do not alter the phase fraction. The phase transformation during heating and holding determi- nes the phase fraction for a certain initial structure. ^--at-~3 transformation occurs during heating and holding. [3 to ct transformation leads to a significant increase in content and size of lamellar ~. The ct to [3 transformation occurs simultaneously in equiaxed at and lamellar ct. The thickness of lamellar ~t increases with temperature, which is caused by the vanishing of fine a lamellae due to phase transformation and coarsening by termination migration. By assuming a quasi-equilibrium phase transformation in heating and holding, a modeling approach is proposed for predicting microstructural evolution. The three stages of phase transformation are modeled separately and combined to predict the variation of phase fraction with temperature. Model predictions agree well with the experimental results.

Robust optimization of the billet for isothermal local loading transitional region of a Ti-alloy rib-web component based on dual-response surface method

Billet optimization can greatly improve the forming quality of the transitional region in the isothermal local loading forming （ILLF） of large-scale Ti-alloy ribweb components. However, the final quality of the transitional region may be deteriorated by uncontrollable factors, such as the manufacturing tolerance of the preforming billet, fluctuation of the stroke length, and friction factor. Thus, a dual-response surface method （RSM）-based robust optimization of the billet was proposed to address the uncontrollable factors in transi- tional region of the ILLF. Given that the die underfilling and folding defect are two key factors that influence the forming quality of the transitional region, minimizing the mean and standard deviation of the die underfilling rate and avoiding folding defect were defined as the objective function and constraint condition in robust optimization. Then, the cross array design was constructed, a dual-RSM model was established for the mean and standard deviation of the die underfilling rate by considering the size parameters of the billet and uncontrollable factors. Subsequently, an optimum solution was derived to achieve the robust optimization of the billet. A case study on robust optimization was conducted. Good results were attained for improving the die filling and avoiding folding defect, suggesting that the robust optimization of the billet in the transitional region of the ILLF was efficient and reliable.

Advances in quantum dot-mediated siRNA delivery

Nano-sized quantum dots（QDs） exhibit uniquely optical properties that are tunable with different sizes and shapes.QDs can emit narrow symmetric bands under a wide excitation range,possess antiphotobleaching stability,and be bio-functionalized on the large surface area.Therefore,QDs are attractive vectors for imaging-guided therapy.Small-interfering RNA（siRNAs）-based therapeutics hold great potential to target a large part of the currently undruggable genes,but overcoming the lipid bilayer to deliver siRNA into cells has remained a major challenge to solve for widespread development of siRNA therapeutics.In this mini-review,we focus on theranostic QD/siRNA assembles for enhancing delivery of siRNA and facilitating evaluation of therapeutic efficacy via imaging of QDs,with special attention to carbonaceous QDs for delivery of siRNA.

Whole-process modeling of titanium disc forming for gradient distributions of temperature and deformation

Gradient distributions of temperature and deformation(GDTD)are crucial for achieving dual-performance discs of titanium alloys which is required by the service environment of aeroengine.However,heating,cooling and deforming sequence in the whole process of the titanium disc forming,which leads to difficulties for achieving GDTD due to a lot of parameters.To solve this problem,a whole-process model of the titanium disc forming for GDTD has been established.In the model,heating and cooling via heat radiation,conduction and convection,and deforming by local loading with mold chilling are all considered.Experiments on heating and cooling as well as deforming were carried out by using a furnace and the Gleeble-3500 machine.The experimental data are used to determine thermal parameters and constitutive relations of the IMI834 titanium alloy,and then to verify the reliability of the model.Then the model was used to simulate the evolution rules of temperature and deformation of the titanium disc.The results show that the heating surface,furnace temperature,billet profile and loading rate play the core role for the control of GDTD,and thus a set of parameters were determined.Therefore,this work provides a base for developing a new forming technology of the dual-performance titanium discs with the approach of local heating and local loading.