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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 and β phase 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 of β phase 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.

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 (αp) and lamellar α (αl) grains. Under a low level of deformation, almost all the slip deformation is governed by single basal and prismatic slips for both of αp and αl, despite small amount of -pyramidal slip exists in αl grains. As deformation proceeds, -pyramidal and -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 αl grains whose major axes tilting about 40 °–70 ° from the tensile axis. Slip transfer occurs at the boundaries of αp/αp and αl/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 αl/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 αl/β and αp/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.

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.

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 primary α grain 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.

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.

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.

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 α (αp), 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 αp increases most in the normal-rolling plane, then in the normal-circumferential plane and least in the circumferential-rolling plane, whereas αp content decreases in an opposite trend. Along the ND, the aspect ratio and content of αp is 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.

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.

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.

Ti-55531 (Ti-5Al-5Mo-5V-3Cr-1Zr) is near β titanium alloy, which plays a significant role in manufacturing landing gears and flap tracks in the aerospace industry. This study aims to find out an optimal heat treatment to obtain an excellent balance of strength, ductility and fracture toughness. For this purpose, the Ti-55531 alloy was subjected to solution plus aging treatments and mechanical properties tests. Optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were adopted to observe the microstructural evolution including morphology, distribution and size. Standard tensile and compact tension (CT) tests were carried out to obtain the yielding/tensile strength (YS, TS), elongation (EL) and fracture toughness (KIC) of Ti-55531 alloy. The effects of solution temperature, aging temperature and time on the microstructural evolution and fractography were analyzed. The relationships between the mechanical properties and microstructures were qualitatively described. The influencing mechanism of plastic deformation and crack propagation path on the KIC were discussed. Simultaneously, the values of YS and KIC were predicted based on mechanical property models, the predicted values were compared with tested data. Finally, an optimal heat treatment was proposed for the Ti-55531 alloy, to obtain a good balance of strength, ductility and fracture toughness.

The effect of microstructure heterogeneity on flow instability is investigated during isothermal compression. The mechanism of plastic flow instability is studied from the perspective of the strain mode and geometrical orientation dependence of microstructure. It is found that the instability criteria underestimate the range of unsafe processing region of lamellar microstructure due to its stronger trend to flow instability. The key factors are the geometrical orientation and colony size of lamellar microstructure. In the colonies with hard geometrical orientation, deformation resistance is big since only the pyramidal slip system can be operated. The difference of the deformation resistance among colonies can reach 49% in a primary β grain. The incompatibility of flow among various colonies promotes the generation of micro shear bands in colonies with hard geometrical orientation, which facilitates the slip transmission across α/β interfaces and weakens the Hall-Petch strengthening effect. The large colony size facilitates the rapid expansion of micro shear bands leading to the continuous softening of colonies, which strengthens the strain localization and accelerates the occurrence of flow instability. The critical value of Semiatin's model is modified to 4 for lamellar microstructure. Based on this research, a method to confirm more precise unsafe processing regions of titanium alloys of different microstructures is proposed.

Folding defect is one of the most important forging defects, which can deteriorate the surface quality, mechanical properties, and material utilization of forged parts. In this work, the common folding defects in die forging were classified into three typical types: local-loading-type folding, confluence-type folding, and bending-type folding. Three simple Eigen experiments were employed to analyze the formation mechanisms and rules of three typical types of folding accordingly. By FE simulation, the formation mechanisms of three typical types of folding defects were analyzed based on the evolution of the velocity field. In addition, the effects of geometric and forging parameters on each type of folding defect were investigated by uniform experiment design. Especially, a quantitative folding index was applied to uniformly evaluate the formation possibility and severity of folding defect for any samples whether producing folding or not. It is found that the local-loading-type folding is produced by the formation and collapse of a step, which is sensitive to the reduction amount but insensitive to the feed amount. The confluence-type folding is formed due to the local lack of material, which is sensitive to the reduction degree and insensitive to the rib width and billet height in this work. The bending-type folding is mainly formed by the geometric instability of workpiece during the forming process, which is sensitive to the reduction degree and insensitive to the geometric parameters of billet.

This work focuses on the plastic deformation behavior and microstructure evolution in high temperature sheet bulk forming of 2219 aluminum alloy. Uniaxial compression tests were carried out along the normal direction (ND), transverse direction (TD) and rolling direction (RD) of an annealed rolled sheet which has brick-like grains but weak texture. Electron backscattered diffraction (EBSD) was employed to observe the microstructure after compression deformation. Experimental results show strong anisotropy in plastic flow in spite of non-prominent anisotropy in strength. Meanwhile, grain refinement mechanisms transform from geometric dynamic recrystallization (GDRX) in ND compression to discontinuous dynamic recrystallization (DDRX) with enhanced particle stimulated nucleation (PSN) effect in TD and RD compressions. Grain morphology has little effect on the plastic anisotropy. However, the heterogeneous deformation associated with the distortion of brick-like grains in compression accounts for the diverse microstructural developments to a large extent.

For a two-phase titanium alloy, the tensile and damage tolerance properties exhibited in tri-modal microstructure were investigated and compared to the same properties exhibited in widmanstatten and bimodal microstructures. These investigations and comparisons were conducted by analyzing the microstructure characteristics and their influences on deformation behavior and fracture features. The results show that the tri-modal microstructure exhibits superior strength and plasticity that are close to bimodal microstructure and remarkably higher than the widmanstatten microstructure. The underlying reason is that the multiple types of interfaces and refined grains of tri-modal and bimodal microstructures produce greater interface strengthening and better overall deformation homogeneity than the widmanstatten microstructure. Furthermore, the better deformation homogeneity also contributes to suppressing the low-cycle fatigue crack initiation of tri-modal microstructure. Besides, the unique lamellar α in tri-modal microstructure with disordered distributions on both geometric and crystallographic orientations effectively hinders the fatigue crack propagation. Thus, the low-cycle fatigue life (the number of reversals to failure) of tri-modal microstructure is the longest, which is 3.93 times as long as that of widmanstatten microstructure and 1.12 times as long as that of bimodal microstructure. For the fracture toughness, the fracture crack paths of bimodal and tri-modal microstructures with the small sized grains present far lower fluctuation and tortuosity compared to those of widmanstatten microstructure, which means lower energy consumption. However, the crack path of tri-modal microstructure is still more deflected than that of bimodal microstructure due to the contribution of the disorderly distributed lamellar α, which increases the energy consumption to some extent. Thus, the fracture toughness of tri-modal microstructure is lower than the widmanstatten microstructure but higher than the bimodal microstructure. These results indicate that tri-modal microstructure exhibits more excellently combined tensile and damage tolerance properties compared to the other two microstructures.

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 inter-pass 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 inter-pass 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. Keywords: Cooling rate, Hot compression test, Kinetic equation, Ni-based superalloy, Recrystallization

In this work, the micro-scale damage development and crack propagation features of TA15 titanium alloy with tri-modal microstructure consisting of equiaxed α (αp), lamellar α (αl) and β transformed matrix α (βt) were firstly investigated. On that basis, the dependences of fracture toughness of tri-modal microstructure on microstructural parameters were discussed. The results indicate that in the damage and fracture of tri-modal microstructure, voids mainly nucleate and grow up at the triple junctions and the interfaces of αp/αp, αp/βt and single-αl/βt due to the micro-scale strain incompatibility at these locations. By internal necking or local shearing connection with these voids, the main crack achieves an increment. It leads to two main categories of crack paths, i.e., the connection-type path caused by the connection behavior and the natural-extension-type path progressing through voids after the connection with voids. The connection-type path exerts greater influence on the fracture toughness than the natural-extension-type path. Moreover, as for the connection-type path, it can also be classified into two sub-categories, i.e., the internal necking connection-type path and the local shearing connection-type path. As far as the energy consumption is considered, more internal necking connection-type path is favorable for improving the fracture toughness, while more local shearing connection-type path is detrimental. Besides, the above void evolution and crack propagation behavior will affect the tortuosity of crack path to some extent, which will also change the overall energy consumption and final fracture toughness. As for the dependence of fracture toughness on microstructural parameters, it is found that with the increase of αp content, the proportion of low-energy-consumed crack paths (local shearing across αp and colony-αl) increase and the crack path tortuosity decreases, both of which reduce the energy consumption and result in the continuous reduction of fracture toughness. As the αl content increases, the disorderly distributed single-αl significantly increases at lower αl content, which improves the crack path tortuosity and energy consumption. However, at a higher level of αl content, single-αl decreases dramatically and the content of colony-αl becomes high, which reduces the crack path tortuosity and energy consumption remarkably. So, the fracture toughness first increases and then decreases with the increase of αl content.

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 42CrMo billet at 1123–1423 K and 0.01–1 s−1. 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. Keywords: Cast preformed forming process, Flow stress, Grain size, Non-uniform as-cast 42CrMo billet, Ring rolling, Unified model

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 rib-filling 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.

Revealing the microstructural parameters-mechanical properties relationship is very critical for the microstructure tailoring and properties optimization of titanium alloy. In this work, the dependence of mechanical properties on microstructural parameters of TA15 titanium alloy with tri-modal microstructure was investigated. The results indicate that at a certain lamellar α (αl) content (28%), both of yield strength (σ0.2) and ultimate tensile strength (σb) decrease continuously with the increase of equiaxed α (αp) content, while the decrease speed slows down at higher αp content. At a certain content of αp of 20%, σ0.2 and σb both increase first and then decrease as the content of αl increases. The above effect laws result from the competition between the softening effect caused by increasing the softer αp and αl contents and the strengthening effect relating to αl/βt interfaces and βt hardening. As for the elongation (δ) and reduction of area (Φ), they show an increase trend with αp content increasing at a certain αl content of 28%. However, they decrease first and then increase with αl content increasing at a certain αp content of 20%. The early plasticity decrease at lower αl content is due to the popular rapid void nucleation and cleavage fracture along αl/βt interfaces. When αp or αl content increases to higher level, the deformation compatibility and homogeneity improves, which suppress the rapid void nucleation and cleavage fracture along αl/βt interfaces. Meanwhile, the fracture mechanism changes from the coexistence of void coalescence and cleavage fracture to the void coalescence dominated fracture, thus the plasticity increases. Moreover, a back-propagation neural network model was developed to correlate the mechanical properties with microstructural parameters of tri-modal microstructure. The prediction results suggest that better combination of strength and plasticity can be achieved by controlling the contents of αp and αl in the range of 10–15% and 22–27%, respectively.

A combination of scanning electron microscopy and electron backscatter diffraction is used to investigate the deformation in the fatigue crack tip plastic zone and its role in the crack propagation of Ti-6Al-2Zr-1Mo-1V alloy with a tri-modal microstructure. The results show that heterogenous slip and secondary micro-cracks are the main features of the fatigue crack tip plastic zone. These features play important roles in fatigue crack propagation. In particular, the unique lamellar α in the tri-modal microstructure can deflect and delay the crack propagation effectively, thus, improving the fatigue life.

In this paper, the microstructure development and flow behavior in the near isothermal forming of two-phase titanium alloys were studied through designing an off-equilibrium analog experiment. The results show that deformation decreases the volume fraction of primary alpha ( α p ) phase significantly at high temperature. This can be ascribed to the occurrence of dynamic transformation of α p →β by calculating the Gibbs energy barrier based on solution thermodynamics and the drive force from stress difference between α p and β phases. With the decrease of temperature, α p phase fraction varies little with deformation as a result of the counteraction between dynamic transformation of α p →β and strain-induced phase transformation of β→ α p . However, the deformation under the off-equilibrium condition can accelerate the precipitation kinetics of secondary alpha ( α s ) phase evidently. Moreover, a large amount of fine equiaxed α s phase can be formed by intragranular nucleation. Finally, it is found that, with the deformation at high temperature, the loss of Hall–Petch strengthening is principal source of flow softening. Meanwhile, the quantity of flow softening is less than that of the loss of Hall–Petch strengthening, which is associated with pronounced precipitation of α s phase. At the low temperature, flow softening by the loss of Hall–Petch strengthening is minor compared to that by α s laths rotation.

Magnesium-lithium (Mg-Li) alloys, due to their high specific strength, have great potential in the modern aviation industry. The superplastic characteristics, microstructure evolution, mechanical properties, and forming process of LZ91 Mg-Li alloys at 200°C–300°C were studied in this paper. Tensile tests show that the optimal elongation of tensile specimens is 812.6% at 300°C under a strain rate of 5 × 10−4 s−1. The flow stress of the material increases with strain during the high-temperature tensile testing due to the grain coarsening . Meanwhile, low dislocation densities were observed during tension, which is not sufficient for dynamic recrystallisation to occur. The finite element method is used to investigate the forming schemes of a narrow-mouth box component. Finally, this component was manufactured by the powder pressure bulging process at 300°C experimentally.

Electrically-assisted manufacturing (EAM) has been proved to be very efficient in homogenizing the alloy with heterogeneous grain distribution due to the local Joule heating effect, which completely differs from the common heat treatment. However, the interaction between pulsed current and texture evolution during deformation have been rarely investigated even though texture is critical for the plastic behaviour of anisotropic material. Here, the effect of pulsed current on texture evolution behaviour has been studied by comparing the evolution behaviours of a Ni-based superalloy under electrically-assisted and quasi-static tension. The result showed that the evolution of easy-to-deform texture into hard one was promoted by the pulsed current, but the further enhancement of the hard-to-deform texture’s intensity was inhibited. This difference could be attributed to the more pronounced local Joule heating effect in the hard-to-deform grain due to larger lattice distance here, and then the softened hard-to-deform grain could rotate itself towards other orientations more easily. This is called “target effect” of pulsed current on the hard-to-deform texture in present work, which further indicates that EAM is effective in the control of homogeneous behaviour of anisotropic materials not only from the microstructure distribution but also from the mechanical property.

Ductile fracture is the most common defect in plastic forming of tailor-welded blank (TWB). Revealing the damage and fracture characteristics of welded joint is very critical for accurately predicting the fracture defect. In this study, the fracture strain and microscopic damage development of a 2219 aluminum alloy welded joint were systemically investigated by in-situ-SEM testing. The interactive effects of microstructure and stress state on void growth and coalescence were quantitatively explored. Further, a micromechanical model based on actual microstructure was established to reveal the underlying mechanisms. The results show that the prominent void growth rates are affected by the particle size, particle volume fraction and grain size, and the effects vary with the stress triaxiality. The growing voids coalesce through one or both of ligament necking and shearing, which compete with each other dependent on the microstructure and stress state, making the variations of critical void spacing ratio. Attributing to these effects, the fracture strains of different microstructures in welded blank under various stress states are different, and the relationship between the fracture strain and microstructure relies on the stress state. These damage and fracture rules are explained by the microscopic heterogeneous deformation characteristics under different microstructures and stress states.

Understanding the heterogeneous globularization phenomena and the underlying mechanisms is important to process design for primary hot working of titanium alloys. To this end, hot compression of a TA15 titanium alloy with initially colony structure was carried out at 900 °C and strain rate of 0.1 s−1 on a Gleeble thermal simulator. Comprehensive electron back scatter diffraction (EBSD) examination was conducted to check how the heterogeneous microstructural developments were affected by the geometrical and crystal orientations of the colony. A crystal plasticity model was employed to quantify the heterogeneous deformation and orientation evolution during compression. The heterogeneous globularization behavior is interpreted from the orientation perspective by relating the orientation spread within alpha lamellae to the slip activation, strain partitioning in the constituent phases as well as microstructural phenomena such as deformation banding and dynamic recrystallization. The activation of the prism and basal slips in alpha phase and strain partitioning between the alpha and beta phases would enhance the orientation spread, which accounts for the relatively high globularization efficiency in inclined colonies. Keywords: Titanium alloy, Heterogeneous globularization, Crystal orientation, Geometrical orientation, Globularization efficiency, Crystal plasticity

Dwell fatigue failure of titanium alloys has threatened flight safety for over five decades. To quantitatively evaluate the component life, experimental dwell fatigue tests are generally conducted in the lab. However, the loading profile in the lab is generally shorter than those of the realistic in-service conditions by several orders of magnitude, not only for the stress hold but also for the stress rise and fall periods. Although the dependence of fatigue life on the dwell period has been extensively studied, the effect of the rise and fall time has been ignored. Investigating such a topic is extremely time-consuming, especially when the applied stress is lower than the yield strength under the in-service loading. The fatigue tests could require years of loading when the rise and fall periods reach the order of magnitude of 10 3 s , for example, which is experimentally infeasible. Modeling techniques provide a solution to systematically study the effect of the rise and fall on dwell fatigue responses benefit from the high computing power availability. In this study, crystal plasticity models representing equiaxed α and dual-phase α-β lamellar microstructures have been developed and used to study the effects of the rise and fall time over a wide range ( 10 − 1 - 10 3 s ) on the load shedding behaviors of IMI834 titanium alloy under dwell fatigue loadings. The time for the peak stress at the soft-hard grain boundary reaching equilibrium under different load profiles is quantitively investigated. The rise and fall time have been demonstrated to influence the load shedding at different degrees. The microstructural features in the soft grains of dual-phase titanium alloys play a significant role in affecting the local stress evolutions. The correlations between the load shedding and the rise and fall time are also influenced by the microstructure. The strain partitioning between the α and β phases in the soft grains under cyclic dwell fatigue of titanium alloys has been examined to elucidate the underlying mechanisms of the rise/fall time dependence.

One of the most critical aspects in understanding the deformation behavior of two-phase titanium alloys subjected to thermomechanical processes (TMPs) lies in being able to describe the flow stress accurately. To this end, in this study, initially, hot tension tests were conducted on a two-phase Ti-6Al-2Zr-1Mo-1V alloy. It was observed that the flow stress at a given temperature and strain rate exhibits work hardening, followed by flow softening. Flow softening occurs at the peak strain, where the maximum stress is observed; peak strain decreases with increasing temperature and decreasing strain rate. Variations in peak strain are more obvious at low temperatures and high strain rates. Later, the microstructure of the alloy was analyzed and the results show that work hardening and flow softening are caused by dynamic recrystallization (DRX). The DRX volume fraction was found to exhibit an increasing trend and discontinuous dynamic recrystallization (DDRX) was observed at increasing temperature and decreasing strain rate. With respect to microstructure evolution, a unified model consisting of a thermally activated stress component and an athermal stress component was developed. In the athermal stress term, dislocation density and the Hall-Petch effect were used to describe the work-hardening and flow-softening behavior. In the case of the dislocation term, the DRX effects were modeled considering the critical strain for DRX initiation and the DRX rate, which are both temperature-and strain rate-dependent. In the Hall-Petch effect term, the dependence of the Hall-Petch coefficient on the processing conditions was considered and the loss of Hall-Petch strengthening with deformation was modeled. Using the proposed model, the work-hardening and flow-softening behavior and microstructure evolution in Ti-6Al-2Zr-1Mo-1V alloys subjected to TMP were predicted. A good agreement could be observed between the experimental and predicted results. This study provides a solution for modeling work-hardening and flow-softening behavior and helps us understand the deformation behavior of two-phase titanium alloys subjected to TMP.

Low-cycle fatigue (LCF) behavior and property characteristics of titanium alloy with a tri-modal microstructure, consisted of equiaxed α (αp), lamellar α (αl) and β transformed matrix (βt), were explored in this work. The results show that, at different strain amplitude (eta) levels, the cyclic hardening/softening is determined by the competition (eta

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. Keywords: Electromagnetic incremental forming (EMIF), Forming uniformity, Integral panel, Parameters, Shape deviation

Two-phase titanium alloys deform heterogeneously in primary hot working due to the strong anisotropy of plastic deformation of colony structure associated with the transformation crystal structure, morphology and orientation relationship between the constituent phases. To understand the heterogeneous deformation in primary hot working, a homogenized crystal plasticity constitutive model is developed for a single colony which relates anisotropic deformation behavior to microstructural features. Efforts are made to model the morphological effects which cause the abnormally low measured critical resolved shear stress (CRSS) of two basal slip systems and the anisotropic Hall-Petch strengthening associated with the Burgers orientation relationship. The model is able to capture the deformation characteristics and texture evolution in compression of colony structure. It is found that the morphological effects cause the formation of transverse texture and continuous flow softening in hot compression.

Isothermal local loading forming is promising to manufacture the large-scale Ti-alloy multi-rib component, but the folding, die underfilling, and strain concentration (SC) are undesired characteristics existing in transitional region due to the reciprocating material transfer between loading region and unloading region. The current research showed that the optimized billet could eliminate or improve these defects. However, it may also lead to unacceptable results due to the variation of the uncertainty factors, such as the manufacturing tolerance of the billet, the fluctuation of the stroke length, friction factor, and forming temperature. Aiming at this issue, an uncertainty analysis and the multi-objective robust optimization of the billet are performed based on 3D finite element simulation and dual-response surface model. Firstly, a valid FE model of the eigenstructure is established to represent the transitional region. Then, the significance analysis of the uncertainty factors is carried out and the significant uncertainty factors are obtained. Subsequently, a multi-objective robust optimization model concerning the mean and standard deviation of the die underfilling rate and average strain of SC zone under the condition of avoiding folding is established. Finally, the Pareto-optimal solutions are obtained by NSGA-II and the minimum distance selection method is employed to acquire the satisfied solution. The comparison results between the robust optimization and deterministic optimization showed that not only the folding is effectively avoided under the uncertainty factors, but also more robust die filling and deformation homogeneity can be achieved.

Electrically-assisted manufacturing (EAM) is used for forming difficult-to-form materials such as the Ni-based superalloy in recent years. Compared to the hot deformation, EAM is a more convenient and effective method. However, the mechanism for the macro and micro behaviors of the materials with EAM is still a controversial issue. Thus, in this work, the electrically-assisted (EA) tension tests of the Ni-based superalloy were carried out with different parameters, and the macro and micro behaviors of the material were studied and discussed. Besides the reduction of yield strength (YS) and flow stress, the Portevin-Le Chatelier (PLC) effect appears during the EA tension and is more significant than that during the hot tension at the same temperature. The critical temperature for the PLC effect is the same with different strain rates under a fixed peak current density. Essentially, the directional distribution of dislocations and the earlier precipitation of the second phase can also be the main causes for the PLC effect. According to the existing theories and the electric treatment experiments that the higher percentage of defects or the second phase in metal result in more significant temperature rise, the local high temperature which induced by local Joule heating effect exists in the critical areas. It may be the main mechanism resulting in the macro and micro behaviors of alloy during the EA tension. Furthermore, recrystallization is experimentally observed when the measured temperature is much lower than the critical temperature for recrystallization due to the local Joule heating effect.

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 3D 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. Keywords: Die filling, Folding defect, Isothermal local loading forming, Transitional region, Unequal-thickness billet optimization

Recrystallization behavior of Ti-55531 alloy in β annealing is explored for grain refinement during primary hot working. Four typical pre-processing conditions were adopted to characterize the diverse processing routes of primary hot working: deformation and annealing in the two-phase region, deformation of globularized structure in two-phase region, deformation of lamellar structure in two-phase region and deformation in single β region. The microstructure evolution in β annealing was quantified by Optical Microscopy (OM), Electron Backscattered Diffraction (EBSD), X-ray diffraction (XRD) and quantitative metallography. The results indicate that the nucleation mode of static recrystallization is determined by the size and distribution of substructures prior to annealing which is related to pre-processing route. The uniform substructures in as-deformed globularized structure produce a higher fraction of low angle grain boundaries after full recrystallization. Fine β recrystallized structure can be obtained when β recrystallization is accompanied by α-to-β phase transformation.

To characterize the plastic deformation behavior and globularization in multistage primary hot working, isothermally interrupted compression was carried out for a TA15 titanium alloy with initial colony structure at temperature of 900 °C and strain rate of 0.1 s−1. Through-process microstructural developments were examined by Electron Backscattered Diffraction (EBSD) and Scanning Electron Microscope (SEM). The globularization efficiency was analyzed via the evolution of intra-α boundaries, α/β interfaces and α grain size. The analyses show that the globularization efficiency is greatly enhanced by short time holding after a true strain of 0.73. The acceleration in globularization is associated with the formation of transverse intra-α boundaries across the α lamellae, the loss of α/β interfacial coherency, the change in deformation mode and the improvement of deformation homogeneity. Thus, the globularization efficiency increases with interrupted strain, holding time and loading pass. The results can be used to optimize the primary hot working of titanium alloys.

The concurrent hot deformation and temperature drop is an important phenomenon in hot forging of two-phase titanium alloys. Understanding the microstructure development in this process is critical to control microstructure and tailor the mechanical properties. For initial equixed structure, primary and secondary alpha evolution, and deformation behavior are revealed by designing concurrent hot compression and controlled cooling experiments. The results show that the growth of primary alpha phase is retarded by deformation at low cooling rate. The morphology of primary alpha cannot be changed by deformation. However, concurrent hot deformation and slow cooling can promote the precipitation of secondary alpha phase. The equixed secondary alpha can be obtained at low strain rates, which can be ascribed to the change in the mechanism of β→α＋β phase transformation by EBSD orientation analysis, the strain weakened anisotropic growth and globularization of alpha laths. Furthermore, without considering the change of alpha phase fraction, the relative difference between calculated and experimental flow stress even can reach 53.4%, which confirms that phase fraction has a significant influence on rheology. Moreover, the strength of transformed beta matrix is improved greatly when the precipitation of secondary alpha is considered. Finally, it can be found that flow stress increases with strain in approximately sigmoidal way. This is due to that temperature drop and increasing phase fraction lead to the increase of flow stress, whereas the rotation and globularization of secondary alpha laths can cause flow softening. The flow stress increases obviously with cooing rate, which can be attributed to fine alpha laths and significant Hall–Petch strengthening effect.

Ductile fracture always occurs in plastic forming processes of thin-walled metal sheets. Damage models, both coupled and uncoupled ones, have been extensively developed to predict fracture of the sheets. Among these, uncoupled damage models, owing to their simple form are widely used. While for an uncoupled damage model, there exist three forms for its fracture prediction: (1) damage threshold (DT), which is related to the damage accumulation; (2) fracture strain (FS), which is always a function of stress triaxiality and Lode parameter; (3) fracture forming limit diagram (FFLD), which is always related to the principal strains. However, for an uncoupled damage model, its three forms (DT, FS and FFLD) for fracture prediction have never been compared before, and thus scholars have always been confused about how to choose the right form of the three in different loading processes. To clarify this confusion, two popular uncoupled damage models of the MMC4 and DF2016 are embedded into an anisotropic yield function first. Then the above-mentioned three forms of the two models are separately applied for fracture prediction in shear test, dog-bone specimen tensile test, hole specimen tensile test, notched specimen tensile test, bulging test in proportional loadings, and a complex non-proportional loading process of spin forming, respectively. Finally, the predicted results with these three forms are systematically compared and a new explanation for their applicability under different stress states is displayed.

In this work, the dependences of tri-modal microstructure parameters and corresponding nano & micro hardness on through-process processing parameters were quantitatively studied during the three-step thermo-mechanical processing of TA15 titanium alloy. It is found that the processing parameters of first step, especially for the deformation temperature and strain rate, mainly affect primary equiaxed α (αp) through the α → β phase transformation and the competition between dynamic recovery and dynamic recrystallization. The second processing step primarily affects the content and thickness of lamellar α (αl). In the third processing step, compared with low-temperature aging, normal annealing provides sufficient driving force for αl and secondary lamellar α (αs) growing, which leads to thicker αl and αs. As for the nano & micro hardness, in one sample undergoing different process, transformed β matrix (βt) is always harder than αl and αs due to the interfacial-strengthening effect. In addition, with increasing strain rate of the first step, αp becomes harder due to the constantly enhanced work hardening effect while the hardness of βt varies little because of the competition between interfacial strengthening and distribution disorder degree. However, the nano hardness of αl decreases firstly and then increases with strain rate, which presents the same trend with the micro hardness of integrated hardness at different processing conditions.

The electromagnetic force is the key factor affecting the electromagnetic forming (EMF) process of integrally stiffened panels. In this work, the influence of rib distribution and height on the distribution and variation rules of electromagnetic force in the EMF of stiffened panels is analyzed via the finite element simulation. The results show that the distributions and peaks of electromagnetic forces on the no-rib panel, panel with X direction ribs, panel with Y direction ribs, and grid-rib panel are very different. The electromagnetic force on the stiffened panels distributes irregularly and concentrates on the ribs because of the skin effect of the eddy. The grid-rib panel presents larger peak force than the other two panels with single direction ribs and the panel without ribs, which may due to the more efficient restraint of the magnetic leakage. However, the maximal forces on the webs of the four panels are very close. With the increase of the rib height, the maximal values of the normal and the horizontal electromagnetic forces both increase rapidly, and the ratio of the horizontal force to the normal force firstly increases and then tends to a certain value. Both the simulated result and verification experiment show that when the rib height increases to 12 mm, the horizontal electromagnetic force would cause obviously transversal bend for the ribs along the X direction; thus, its effect on deformation cannot be ignored.

The significant variation of microstructure features, i.e. volume fraction, size of secondary α phase, due to β→α phase transformation in heat treatment of two-phase titanium alloys can directly affect the mechanical properties. Understanding the nucleation mechanism and modeling the precipitating of secondary alpha phase with existed primary equixed-alpha phase are critical to precisely control the microstructure. In the present work, sympathetic nucleation mode is proposed by crystallographic orientation analysis and validated by calculating the nucleation energy barrier. Then, it is found that the growth of primary α phase competes with precipitation of secondary α phase by consuming the matrix supersaturation related to the driven force for precipitating. By considering the effects of the growth of primary α phase and thermal history on diffusion field around secondary α phase, and introducing a ledge growth mechanism-related factor, a physically founded fast-acting model is developed to predict the precipitation process during cooling. Good agreements between the experimental and computed results are obtained when applied to TA15 alloy with different original microstructures.