Isothermal Local Loading Forming Research Articles

The effect of loading mode on isothermal local loading forming of magnesium alloy rib-web component

The lightweight of aerospace equipment has urgent demand for the large-scale reinforced wall structure with higher strength and toughness. In this research, the isothermal local loading forming (ILLF) process was proposed to decrease the extremely larger forming load and promote the material filling at rib locations. A typical rib-web component with two cross ribs was investigated and the experiment setup was designed to fabricate it under three loading modes (two new local loading modes and one conventional integral loading mode). The parallel finite element (FE) models were established and verified by comparing the experimental points and simulation curve. The material filling effect and forming load were compared under three loading mode, and the Lode parameter together with hydrostatic pressure was applied to analyze these differences during the deformation processes. Based on the principle of more uniform equivalent strain and larger filling rate, the best loading mode was determined and its corresponding process parameters were optimized.

Exploration of the material transfer effect in local loading forming of ultra-large-size integrated component with multi-rib

Isothermal local loading forming is a less-loading and flexible forging technology, which is promising to form the ultra-large-size integrated component with multi-rib by adopting small tonnage equipment. Due to the local loading characteristic, the material in the loading region can transfer into the unloading region in the transitional region. Identifying the affect region of the material transfer is important to control the material flow and obtain high-quality components under local loading way. In this work, the phenomena of the material transfer are explored and the boundaries of the transitional region are clarified. Firstly, the material transfer after each loading step is analyzed by displacement field and strain field based on finite element simulation of the ultra-large component. Meanwhile, the velocity vectors during each loading step are observed. Secondly, the variations of the material volume in different loading zones during each loading step are revealed. Subsequently, the change of material volume located far from the die partition line is analyzed. Furthermore, the induced forming problems by the material transfer effect, i.e., folding and additional strain, are elaborated. Finally, the boundaries between the transitional region and first/second loading zone are determined. The phenomena of the material transfer and the induced folding in the transitional region are verified using an eigenstructure with multi-rib by the FE simulation and physical simulation experiment.

Uncertainty analysis and multi-objective billet robust optimization for transitional region of multi-rib component under isothermal local loading forming

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.

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 rib-web 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 transitional 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.

Prediction of tri-modal microstructure under complex thermomechanical processing history in isothermal local loading forming of titanium alloy

Abstract To control the tri-modal microstructure and performance, a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed. The staged isothermal local loading experiment on TA15 alloy indicates that there exist four important microstructure evolution phenomena in the development of tri-modal microstructure, i.e., the generation of lamellar α, content variation of equiaxed α, spatial orientation change of lamellar α and globularization of lamellar α. Considering the laws of these microstructure phenomena, the microstructure model was established to correlate the parameters of tri-modal microstructure and processing conditions. Then, the developed microstructure model was integrated with finite element (FE) model to predict the tri-modal microstructure in the isothermal local loading forming. Its reliability and accuracy were verified by the microstructure observation at different locations of sample. Good agreements between the predicted and experimental results suggest that the developed microstructure model and its combination with FE model are effective in the prediction of tri-modal microstructure in the isothermal local loading forming of TA15 alloy.

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 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.

Improving the deformation homogeneity of the transitional region in local loading forming of Ti-alloy rib-web component by optimizing unequal-thickness billet

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.

Formation mechanism, prediction and control of the forming defects in transitional region during local loading forming of Ti-alloy rib-web component

Abstract Predicting and controlling the forming defects in transitional region are two key problems needed to be solved in the isothermal local loading forming of Ti-alloy rib-web component. To this end, the authors have conducted systematic works, which are reviewed here. First, according to the structural features of large-scale rib-web component, the finite element (FE) model of transitional region was established and validated by physical experiment. Then, the material flow and deformation inhomogeneity of transitional region are quantitatively studied by the FE model. In addition, the formation mechanisms of forming defects were revealed. Subsequently, an adaptive folding index was proposed to measure the severity of folding defect, based on which a quick prediction model of the folding defect was developed. Finally, the forming limit considering the forming defects in transitional region was determined by the stepwise searching method combined with the defect prediction model.

Influence of die parameters on the deformation inhomogeneity of transitional region during local loading forming of Ti-alloy rib-web component

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.

Role of processing parameters in the development of tri-modal microstructure during isothermal local loading forming of TA15 titanium alloy

It is critical to precisely control the tri-modal microstructure during the isothermal local loading forming of titanium alloy to obtain high-performance components. To this end, the effect of local loading processing parameters on the development of tri-modal microstructure were experimentally investigated. The key influence factors and laws are revealed as follows. In the first loading step, the deformation temperature plays a decisive role in the volume fraction of equiaxed α, which decreases with the increase of temperature. While, the deformation amount and cooling mode present little effects on the microstructure evolution. In the second loading step, the deformation temperature and amount mainly influence the volume fraction, spatial orientation distribution and globularization of lamellar α. The volume fraction of lamellar α increases with the temperature decreasing. The spatial orientation distribution of lamellar α gradually changes from homogeneous distribution to concentrated distribution with the increase of deformation amount. Besides, the dynamic globularization fraction of lamellar α producing in the second loading step increases with the deformation amount, and their relationship can be well fitted by Avrami type equation. Moreover, higher temperature in the second loading step is beneficial to decrease the critical strain for the initiation of dynamic globularization and promote the kinetic rate of dynamic globularization. On the other hand, the deformation amount of the second loading step has after-effects on the static globularization of lamellar α in the annealing treatment. If the deformation amount exceeded the critical strain for the initiation of dynamic globularization, the static globularization of lamellar α would produce in the annealing, and the static globularization fraction increases with the deformation amount of the second loading step.

Quick prediction of the folding defect in transitional region during isothermal local loading forming of titanium alloy large-scale rib-web component based on folding index

Forming quality of transitional region determines the performance of titanium alloy large-scale rib-web component under isothermal local loading forming. As folding defect is the most sensitive defect in transitional region, its prediction is very critical to the precision shape forming in isothermal local loading forming. In this paper, a quick prediction model for the folding defect in transitional region was established to replace the traditional defect prediction method by time-consuming FE simulation. First of all, the FE model of local eigen model of transitional region during local loading forming of large-scale rib-web component was established and validated by physical experiment. Then, an adaptive folding index was proposed to measure the possibility and severity of folding defect based on the mechanism that there exists a local increase of strain rate when a folding tends to appear. Availability of the adaptive folding index was checked by large numbers of uniform experiments designed by the space-filling Maximin Latin hypercube designs (maximin LHDs). Furthermore, a judging criterion for folding defect based on the folding index was developed and validated by random samples. Subsequently, the folding index was correlated with the geometrical characteristic parameters of transitional region model by RSM model, through which the folding index can be predicted quickly without the time-consuming FE simulation. Finally, a quick prediction model was established for folding defect judgment under various geometrical parameters by combination of the RSM model for folding index and proposed judging criterion. Prediction results are quite consistent with the results of FE simulation and experiment. The folding prediction model can greatly facilitate the design space exploration and engineering design thereby providing guideline to the precision shape forming. Besides, the adaptive folding index and corresponding judging criteria for folding defect put forward in this work have reference significance to the study on folding defect in other forging processes.

Forming defects control in transitional region during isothermal local loading of Ti-alloy rib-web component

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.

Quantitative analysis of the material flow in transitional region during isothermal local loading forming of Ti-alloy rib-web component

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.

Some Advances in Plastic Forming Technologies of Titanium Alloys

To satisfy the ever increasing demands of high performance and light weight in high-end equipments, the titanium components are designed to have complex shape and specific performance. Research and development of advanced plastic forming technologies are of great importance to manufacturing these titanium products with low cost and short cycle. The local loading forming technology with advantages in reducing forming load, enlarging forming size and enhancing forming limit and precision through control of unequal deformation provides a feasible way to manufacture the high performance and light weight titanium components (large-scale, integral, complex, thin-walled) widely used in aircrafts and shows a good developing prospect. This paper presents the state of the art of local loading forming technology and its applications in manufacturing titanium components in the authors’ laboratory, including the isothermal local loading forming of large-scale complex TA15 bulkhead, and heat rotary draw bending of large-diameter thin-walled titanium tube.

Microstructure evolution in the local loading forming of TA15 titanium alloy under non-isothermal condition

In this paper, the microstructure evolution and processing–microstructure relationship in the non-isothermal local loading forming of TA15 titanium alloy were studied through an analog experiment. Some new microstructural mechanisms are found, which are different from those under isothermal local loading forming. In the non-isothermal local loading forming, the tri-modal microstructure consisted of equiaxed primary α, lamellar α and β transformed matrix is achieved. The lamellar α, not produced under isothermal condition, is generated by β→α transformation due to the decrease of component temperature. With the same processing parameters, the volume fraction and grain size of primary α are both greater than those processed isothermally. The content of lamellar α decreases with heating temperature decreasing and little lamellar α can be found when the heating temperature drops to 930°C. Under small deformation degree, the lamellar α distributes randomly in each feature region. As deformation increases, the lamellar α in transitional region and second-loading region present a preferred orientation perpendicular to the compression direction. The primary α content almost decreases linearly with heating temperature, which is different from the regular that under isothermal condition. Non-isothermal local loading forming with a higher heating temperature (near-β region) offers a cost-efficient way for the manufacture of TA15 titanium alloy large-scale integral components.

Microstructural features of TA15 titanium alloy under different temperature routes in isothermal local loading forming

Abstract In this paper, an analogue isothermal local loading experiment was carried out to study the microstructure of TA15 titanium alloy in different temperature routes. The experimental results show that four different types of microstructure are obtained in various temperature routes: Widmanstatten microstructure consisting of α plates in coarsen β grains and continuous grain boundary α layer is produced by β forging in each loading step; bi-modal microstructure composed of equiaxed primary α and β transformed matrix with α plates and residual β phase is obtained by deformation at the same temperature in two-phase region in each loading step; basket-weave microstructure consisting of lamellar α and β transformed matrix can be achieved by β forging in the first loading step followed by near-β forging in the second loading step; tri-modal microstructure with equiaxed primary α, lamellar α and β transformed matrix can be got through near-β forging in the first loading step combined with conventional forging in the second loading step. For a certain microstructure above, the contents of each constituent phase (equiaxed primary α, lamellar α or β transformed matrix) are all close for different loading regions. The lamellar α in basket-weave and tri-modal microstructure present varied morphology and distribution in different loading regions due to various deformation paths.

Effects of Parameters on Inhomogeneous Deformation and Damage in Isothermal Local Loading Forming of Ti-Alloy Component

Isothermal local loading (ILL) forming technology provides a new way to form largescale rib-web (LSRW) components of Ti-alloy, widely used in the aero-space fields as key load-bearing structures. However, the metal undergoes complex plastic inhomogeneous deformation and microstructural evolution, this will lead to macroscopical forming defects and further damage due to multi-process parameters and local loading method, making the process and forming quality hard to control. Using numerical simulation, combined with experiment, influences of various process parameters on forming process, inhomogeneous deformation and damage have been explored for ILL of LSRW components, such as the types of die-forging mode, frictional conditions, and local loading parameters (partitioning of the loading zone, constraint conditions, and loading pass). Then the reasonable forming conditions for LSRW components of Ti-alloy to be studied are proposed. The practical forming experiment of TA15 LSRW component was achieved successfully and the forging with good forming quality, excellent microstructure, and comprehensive mechanical properties was obtained, which indicates the reliability and practical value of results obtained in this article.

Some advances in local loading precision forming of large scale integral complex components of titanium alloys

The large scale integral complex components of titanium alloys have been attracting increasing applications in the aerospace industry because they can meet the requirements of high performance, lightweight and high reliability. The local loading method which can control unequal deformation is an essential way to form such components. There are key problems to be solved urgently in the research and development of local loading method, and finite element numerical simulation in combination with theoretical analysis and experimental investigations has been playing a more and more important role in this field. This paper presents the state of the art of some local loading forming technologies and their applications in manufacturing titanium components, including the isothermal local loading forming of large scale integral rib web components, hot rolling of large seamless rings and hot spinning of thin walled shells.

Microstructure evolution of the transitional region in isothermal local loading of TA15 titanium alloy

The microstructure of the transitional region determines the performance of TA15 component under isothermal local loading forming. To understand the characteristic of microstructure evolution in transitional region, a set of analogue experiments for isothermal local loading were designed and carried out to investigate the microstructure development under different temperatures and complex strain path. It is found that the isothermal local loading does not change the microstructural composition of TA15 alloy with initially equiaxed microstructure in the transitional region, though a small fraction of lamellar α phases appear at temperatures above 950°C due to reheating and small deformation in the second loading step. Recrystallization takes place in the β matrix but not in the primary equiaxed α phases. The β grains are fine and equiaxed irrespective of strain path due to recrystallization. The existence of lamellar α refines the β grains. Kinked and disordered primary equiaxed α phases are produced under complex strain path. The grain size of primary equiaxed α phases increases slightly due to static coarsening and strain induced grain growth in multi-heat processing. The volume fraction of primary equiaxed α phases is not influenced by strain path but determined by working temperature.

Quantitative analysis of the microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy

In this paper, an analogue experiment was carried out to study the effect of processing parameters including deformation temperature, deformation degree, cooling mode and loading pass on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. The volume fraction, grain size and aspect ratio of primary α phase of transitional region were quantitatively characterized. It is found that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At a certain deformation degree, primary α grain size increases first and then decreases with increasing temperature. However, primary α grain size varies little with deformation degree at higher temperature (in upper two phase region) but increases firstly and then decreases with deformation degree at lower temperature (in lower two phase region). Primary α aspect ratio increases with deformation degree at lower temperature but varies little at higher temperature. The morphology of transformed structure in β matrix is greatly influenced by deformation temperature and less influenced by deformation degree under air-cooling. The precipitated Widmanstatten α phase in β matrix is in lamellar form and arranges in colonies under air-cooling, but it is in thinner acicular form and distributes disorderly under water quenching. Loading pass has little influence on the morphology of microstructure.