Search

In this work, the micro-scale damage development and crack propagation features of TA15 titanium alloy with tri-modal microstructure consisting of equiaxed α (α p), lamellar α (α l) and β transformed matrix α (β t) 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 α p /α p , α p /β t and single-α l /β t 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 α p content, the proportion of low-energy-consumed crack paths (local shearing across α p and colony-α l) 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 α l content increases, the disorderly distributed single-α l significantly increases at lower α l content, which improves the crack path tortuosity and energy consumption. However, at a higher level of α l content, single-α l decreases dramatically and the content of colony-α l 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 α l content. [ABSTRACT FROM AUTHOR]

The crystallographic orientation evolution and its dependence on processing parameters during the development of tri-modal microstructure of titanium alloy were studied by the thermal-mechanical processing tests and electron backscatter diffraction (EBSD) examination. It is found that the development of tri-modal microstructure undergoes two stages: firstly, bimodal microstructure consisting of equiaxed α (α p ) and transformed β matrix (β t : a mix of secondary α phase (α s ) and β phase) is formed after first-stage near-β forging; then, the tri-modal microstructure consisting of α p , lamellar α phase (α l ) and β t are obtained after the following heat treatment. The equiaxed α in final tri-modal microstructure does not follow the Burgers orientation relationship (OR) with β phase. Its crystallographic orientation is hardly influenced by the hot processing parameters. The lamellar α in tri-modal microstructure is right the undissolved secondary α of bimodal microstructure (obtained after first step) during the heating process of the second step. Both of them keep the Burgers OR with β phase, however, the dissolution of secondary α present selectivity to some extent making the variant selection degree of lamellar α greater than that of secondary α. The variant selection degree of lamellar α in tri-modal microstructure decreases with increasing the cooling rate, deformation degree and strain rate of near-β forging. The secondary α in tri-modal microstructure is precipitated from β phase and obeys the Burgers OR with β phase during the cooling process of the second step. The existing lamellar α plays a strengthening role in the variant selection during its precipitation. While the cooling rate, deformation degree and strain rate of near-β forging show limited effect on the probabilities of each type of misorientation and variant selection degree of secondary α. [ABSTRACT FROM AUTHOR]

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 (α p ) 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 (α l ). In the third processing step, compared with low-temperature aging, normal annealing provides sufficient driving force for α l and secondary lamellar α (α s ) growing, which leads to thicker α l and α s . As for the nano & micro hardness, in one sample undergoing different process, transformed β matrix (β t ) is always harder than α l and α s due to the interfacial-strengthening effect. In addition, with increasing strain rate of the first step, α p becomes harder due to the constantly enhanced work hardening effect while the hardness of β t varies little because of the competition between interfacial strengthening and distribution disorder degree. However, the nano hardness of α l decreases firstly and then increases with strain rate, which presents the same trend with the micro hardness of integrated hardness at different processing conditions. [ABSTRACT FROM AUTHOR]

It may be a potential way to realize high-efficiency and high-quality local loading forming on the double-action press. However, the deformation characteristics and forming quality of transitional region are the weak link during forming process. In this work, the material flow, deformation inhomogeneity, forming defect of transitional region, and their dependences on the local loading parameters were quantitatively investigated by FE simulation. It is found that due to the constraint clearance, the web material would undergo twice transverse flow with opposite directions successively in two loading steps. This phenomen is basically the same as that in the local loading forming on single-action press, while the extent of transverse material flow on double-action press is greatly smaller. Finally, the transitional region presents different strain distributions in various sub-regions. Region A and region C present a symmetric strain concentration area at rib root, while region B presents a slant strain concentration area at the middle rib. The forming quality on double-action press is better than that on the single-action press. Only a cavum defect at left rib is produced because of the transverse material flow. Constraint clearance plays the most important role in the deformation characteristics and forming quality during the local loading forming on double-action press. The transverse material flow and cavum defect can be suppressed by decreasing the constraint clearance, but the deformation inhomogeneity would increase to some extent at the same time. In addition, the friction factor and radius of left rib are the second significant factors. Increasing the friction factor and radius of left rib is helpful to suppress the transverse material flow and cavum defect, respectively. As far as the forming quality of transitional region is concerned, it is a better way to conduct the local loading forming on double-action press than on single-action press. The results will provide basis for the design of processing parameters in the local loading forming of Ti-alloy rib-web component on double-action press. [ABSTRACT FROM AUTHOR]

Isothermal local loading forming (ILLF) provides a new way to form large-scale Ti-alloy rib-web components (LTRC). However, the material undergoes complex inhomogeneous deformation in transitional region, which influences the forming quality of the component. The purpose of this paper is to improve the deformation homogeneity of the transitional region in ILLF by optimizing unequal thickness billet (UTB), which can adjust the initial volume distribution and control material flow with low cost and high efficiency. Based on the finite element (FE) simulation, the strain distribution of the transitional region in ILLF was investigated. It is found that the strain concentration at the root of formed rib in the first-loading region is more intense than that in the whole loading forming. Besides, the most strain concentration occurs at the root of partitioning rib on the side of first-loading region, which was not observed in the whole loading forming. The material transferred into the first-loading region during the second-loading step, and subsequent rib shift is the fundamental reasons for the strain concentration. The initial volume distribution of UTB has a significant effect on the strain concentration. In order to optimize the UTB, the average strain of strain concentration zone was correlated with geometric parameters of UTB by using response surface method (RSM). Based on the RSM model, the optimized UTB was achieved. The FE simulation of optimized UTB shows that the transferred material, rib shift, average strain, and maximum effective strain were all effectively decreased compared with equal thickness billet outcomes. The present RSM-based optimization method of UTB is proven to be a promising strategy to improve the deformation homogeneity of the transitional region in ILLF. [ABSTRACT FROM AUTHOR]

The deformation inhomogeneity of transitional region plays a great role in both the macro and micro forming qualities in the local loading forming. In this work, the dependence of deformation inhomogeneity of transitional region on the die parameters in the local loading forming of Ti-alloy rib-web component was studied based on finite element (FE) simulation. To evaluate the deformation inhomogeneity, an area-weighted strain inhomogeneity index was employed, which is calculated through the user subroutine of FE software (DEFORM-2D). It is found that there exist two kinds of strain concentration areas contributing to the deformation inhomogeneity of transitional region. One kind is the almost symmetric strain concentration area at the non-partitioned ribs, which is essentially related to the filling of rib. Another kind is the slant strain concentration area at the partitioned rib, which is caused by the multi-step loading during local loading forming. Besides, the effects of die parameters on the deformation inhomogeneity of transitional region were studied by the combination of orthogonal experiment design and FE simulation. The results suggest that the draft angle of the right rib is the most significant factor for both the deformation inhomogeneities of the partitioned rib region and whole transitional region. And, these two deformation inhomogeneities both decrease with the draft angle of the right rib decreasing. Furthermore, the relationship between die parameters and deformation inhomogeneity of transitional region is developed using the response surface methodology (RSM). Based on the RSM model, the die parameters were optimized to improve the deformation homogeneity of transitional region. The results will provide basis for the design of die parameters to improving the deformation and microstructure homogeneities in the isothermal local loading forming of Ti-alloy rib-web component. [ABSTRACT FROM AUTHOR]

The flow behavior and microstructure evolution of a near α titanium alloy with nonuniform microstructure during hot deformation were studied by isothermal compression test and electron backscatter diffraction technique. It is found that the nonuniform microstructure prior to deformation consists of equiaxed α, lamellar α in the colony form and β phase, and the α colony keeps the Burgers orientation relationship with β phase. The flow stress of nonuniform microstructure exhibits significant flow softening after reaching the peak stress at a low strain, which is similar to the lamellar microstructure. Nevertheless, the existence of equiaxed α in nonuniform microstructure makes its flow stress and softening rate be lower than the lamellar microstructure. During deformation, the lamellar α undertakes most of the deformation and turns to be rotated, bended and globularized. Moreover, these phenomena exhibit significant heterogeneity due to the orientation dependence of the deformation of lamellar α. The continuous dynamic recrystallization and bending of lamellar α lead to the “fragmentation” during globularization of lamellar α. The bending of lamellar α is speculated as a form of plastic buckling, because the bending of lamellar α almost proceed in the manner of “rigid rotation” and presents opposite bending directions for the adjacent colonies. [ABSTRACT FROM AUTHOR]

The under-filling defect is prone to occur in the forging of large-scale titanium alloy rib-web components (LTRC). A rigorous preform design with accurate volume distribution is required for a desirable LTRC. The unequal thickness billet (UTB) design divides the preform into multiple regions of varying thickness and can not only adjust the volume distribution effectively but also control forming defects with low cost and high efficiency. The purpose of this paper is to attain LTRC without any under-filling defects using the UTB methodology. Firstly, cross sections are extracted from a desirable LTRC so as to create the initial UTB according to the calculated neutral layer of material flow between ribs. Then a finite element (FE) model is established, using the Deform-3D software for the isothermal closed-die forging to study the material flow and die cavity filling. Finally, three schemes to modify the initial UTB are proposed for those areas where there is under-filling: (I) increase the number of regions in affected areas, (II) adjust the size parameters of the regions around affected areas, and (III) increase the thickness of the regions in affected areas. In conclusion, it is found that scheme I and scheme II are based on the constancy of volume principle and can be adopted if the distribution of the ribs on those affected areas is quite simple. However, scheme I increases the complexity of the UTB, so scheme II is preferred. Alternatively, if the distribution of ribs on those regions is more complicated, scheme III can be adopted. Although the volume of the UTB is increased slightly, all ribs can be completely filled, allowing the most accurate LTRC to be produced. [ABSTRACT FROM AUTHOR]

The forming quality in the transitional region largely determines the formability of local loading forming of the large-scale rib-web component. Thus, it is very critical to determine the forming limit considering defects in the transitional region during local loading forming. In this paper, the stepwise searching method based on Kriging metamodels of defect indexes is adopted to determine the above forming limit. First, the finite element (FE) simulation of the transitional region during local loading forming and the evaluation indexes for folding and cavum defects are introduced. Then, the Kriging metamodels of defect indexes are developed based on quantities of FE simulation results under various geometry parameters. During Kriging metamodeling, in order to guarantee the accuracy of the metamodel in the global design space and interesting region simultaneously, the space-filling maximin Latin hypercube designs (maximin LHDs) and sequential sampling approach are employed to design sample points. After metamodeling, extra random samples designed within the whole space are simulated to validate the developed Kriging metamodels. At last, the method of determining the forming limit considering defects in the transitional region is presented and applied to two specific transitional region models. It is realized through stepwise searching based on Kriging metamodels of defect indexes. The application results show that the proposed method is an effective and reliable method to determine the forming limit considering defects in the transitional region. It would provide an important guideline in the processing design of local loading forming of titanium alloy large-scale rib-web components. [ABSTRACT FROM AUTHOR]

The forming quality of transitional region plays a key role in the performance of titanium alloy large-scale rib-web component under isothermal local loading forming. In this work, a local eigen model of transitional region in the local loading forming of large-scale rib-web component is extracted so as to catch the detail-forming characteristics of transitional region with low computation time in simulation and experiment cost. Then, based on the local eigen model, the mechanism of forming defects in transitional region and their dependence on processing parameters are studied by the combination of physical experiment and verified finite element (FE) simulation. It is found that the second loading step can be divided into two forming stages according to the material flow pattern: (1) the transverse flow stage and (2) the stable forming stage. In the transverse flow stage, there exists long-range transverse flow of web material and excessive transverse flow would lead to the formation of folding and cavum defects. Moreover, the severities of defects are well correlated with the quantity of material transferred into the first-loading region in this stage. For a certain reduction amount, decreasing spacer block thickness and increasing friction both can decrease the quantity of transferred material and then suppress the development of folding and cavum defects, while the deformation temperature and loading speed have little effect on the quantity of transferred material and defects. In the end, an effective way is put forward to suppress and prevent the defects in transitional region during the local loading forming of titanium alloy large-scale rib-web component. [ABSTRACT FROM AUTHOR]

The material flow in transitional region plays an important role in the forming quality of transitional region in the isothermal local loading forming of titanium alloy large-scale rib-web component. To study the material flow in transitional region, the finite element (FE) model of transitional region was established based on DEFORM-2D software and validated by physical experiment. Then, a quick and easy method, which can measure the area of different local regions of forged part in DEFORM-2D via user subroutine, was proposed to achieve the quantitative analysis of material flow mechanism. This technique can also be used in the analysis of other forming process, such as the calculation of fill ratio in forging process. The material flow pattern of transitional region during local loading forming was analyzed step by step and compared with integral forming. The results show that the material flow of transitional region during local loading process can be divided into six stages according to the material flow pattern and load-time curve. Twice transverse material flow with opposite directions occurred in the first and second loading steps sequentially, which does not exist in the integral forming. Four characteristic values evaluating the transverse flow of material, which are associated with the formation of defects and their severities, are defined and quantitatively measured at various processing conditions. It is found that decreasing the spacer block thickness and increasing friction both can decrease the four characteristic values, thus weaken the transverse material flow, which are helpful to improve the forming quality in transitional region. However, the transverse flow of material is little affected by the loading speed. [ABSTRACT FROM AUTHOR]

To control the microstructure and performance of large scale complex titanium alloy component by local loading forming, the effect of processing on microstructure in local loading forming was investigated by a through-process finite element model. It is found that the volume fraction of primary equiaxed α decreases with temperature, deformation speed and loading pass. The distribution of α fraction becomes more uniform with decreasing temperature and increasing deformation speed and loading pass. The α grain size increases with temperature and loading pass but decreases with deformation speed. The homogeneity of the grain size can be improved by increasing deformation temperature and loading pass, or decreasing loading speed. [ABSTRACT FROM AUTHOR]

A prediction model for the folding defect in transitional region during local loading forming of titanium alloy large-scale rib-web component was developed by the combination of finite element simulation, space-filling Maximin Latin hypercube designs method and back-propagation neural network. The FE model of local eigen model of transitional region during local loading forming of large-scale rib-web component was established based on DEFORM-2D and validated by physical experiment. The Maximin Latin hypercube designs method was applied to design uniform experimental schemes with geometrical conditions and reduction amounts as experimental variables. After simulations of the designed experiments, the back-propagation neural network was used to synthesize the data sets (results of folding defect). Finally, a prediction model was established for folding defect judgment in transitional region under various geometrical conditions and reduction amounts during local loading forming of large-scale rib-web components. The predicted results are quite consistent with the results obtained from FE simulation and experiment. [ABSTRACT FROM AUTHOR]

Understanding the mechanism of high temperature deformation is important for controlling the forming quality of the titanium alloy forgings. In the present work, the flow softening mechanism in subtransus deformation of titanium alloys with equiaxed structure was investigated by interrupted isothermal compression tests. The results show that limited strain hardening followed by continuous flow softening occurs in high temperature deformation of a two-phase TA15 titanium alloy. The flow softening can not be rationalized by dynamic recrystallization. Instead, the increase of mobile dislocations during deformation is an important reason for flow softening. The grain boundaries (including the α-β interfaces) act as an important source for the generation of mobile dislocations. The continuous flow softening results from the significant deformation heterogeneity in subtransus working. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flow stress and globularization of titanium alloy with lamellar structure are molded. [•] Physically based model couples the constitutive behavior and microstructure change. [•] Globularization and Hall–Petch strengthening are considered in microstructure model. [•] The model can realize unified prediction of flow stress and globularization kinetics. [Copyright &y& Elsevier]

Objective: Surgical treatment of a subglottic stenosis can lead to a cricoid cartilage resection, cervical trachea resection, or both that has to be reconstructed with an artificial prosthesis. However, there is at present no standard treatment in the numerous reconstruction procedures, and many prostheses encounter inflammatory rejection and mechanical constraint problems. A porous, metastable β-type titanium alloy prosthesis was developed to improve subglottic rehabilitation. Methods: We have designed a porous type of airway prosthesis made of porous metastable β-type titanium alloy. We used this prosthesis (20 mm in length) to reconstruct the cricoid cartilage and the first tracheal ring in 10 mongrel dogs and evaluated its efficacy. Results: One dog died of an accident with anesthesia at 1 week, 1 dog died of pneumonia about 1 month after the operation, and 6 dogs were killed between the third and eighth months after the operation. At death, all the prostheses had become completely incorporated into the host tissue. Endoscopic examination showed no airway obstruction for a postoperative period of 3 to 8 months in 8 dogs. Granulation was observed in 4 cases, and the porous, metastable β-type titanium alloy plate was exposed in 1 case, but these dogs were asymptomatic. Light microscopic and electron microscopic analysis showed a normal mucous membrane growth on the surface of the prosthesis and no visible granulation tissue in the reconstructed cricoid and tracheal cavity. Conclusions: This airway prosthesis provided good results in canine cricotracheal reconstruction, and this study presents the possibility of successful reconstruction of the cricoid and trachea with epithelial regeneration. [ABSTRACT FROM AUTHOR]

Grain-scale strain heterogeneity characteristics play a critical role in the ductile damage behavior and mechanical properties of two-phase titanium alloys. In this work, the grain-scale strain distribution, strain heterogeneity, and strain localization of titanium alloy with tri-modal microstructure (consisting of equiaxed α (αp), lamellar α (αl), and β transformed matrix (βt)) during tensile deformation were experimentally investigated. The results show that the strain probability distribution of the whole microstructure obeys normal distribution during deformation. Significant strain heterogeneities exist in each constituent (αp, αl, and βt) and the whole microstructure. At lower macro-strain, αp and αl exhibit higher average strain than those of βt and the whole of the microstructure. Meanwhile, strain heterogeneity of each constituent is small and has a negligible change. The strain heterogeneity of the whole microstructure is mainly determined by αp. At larger macro-strain, some highly deformed regions produce and their positions do not change during further deformation. As a result, the strain heterogeneity of each constituent increases fast, and the strain heterogeneity of whole microstructure is mainly related to αl in this deformation stage. On the other hand, two types of strain localization may be generated within αp and αl and at the αp/βt and αl/βt boundaries, respectively. The former type is caused by transgranular intense slip deformation and presents crystal orientation dependence. The latter type is related to the boundary sliding and presents spatial distribution dependence for αl. These strain localizations greatly determine the micro-damages, thus forming the corresponding micro-voids within αp and αl and the micro-cracks at αp/βt and αl/βt boundaries in tri-modal microstructure at larger deformation. [ABSTRACT FROM AUTHOR]