内高压成形机理与关键技术

在中国国家杰出青年科学基金资助项目“镁合金热态液力成形技术”、中国国家自然科学基金资助项目“轻体件高内压液力成形机理的研究”、“管材热态内压成形新方法及其机理研究”和“激光拼焊管内高压成形机理”、以及中国教育部高等学校博士学科点专项科研基金资助项目“镁合金热态内高压成形机理研究”共同资助下,开展内高压成形机理及关键技术研究,在内高压成形塑性变形规律、起皱和破裂等失稳行为、提高成形极限和降低成形压力方法,以及液力胀接、热态内压成形和拼焊管内高压成形等方面取得重要进展,并在汽车和航天等领域实现内高压成形技术产业化应用,报告上述研究的理论和工程体系。 根据塑性变形特点,将内高压成形分为变径管内高压成形(IHPF of TPVD)、弯曲轴线管内高压成形(IHPF of TPCA)和多通管内高压成形(IHPF of TPB/BT)等3类,提出IHPF of TPVD由充填、成形、整形等步骤组成,IHPF of TPCA由弯曲、预成形、内高压成形等步骤组成,IHPF of TPB/BT由胀形、补料、整形等步骤组成。以此为出发点,通过实验和理论分析,研究IHPF塑性变形规律与失稳行为。     

On the tube hydroforming process using rectangular,trapezoidal, and trapezoid-sectional dies: modeling and experiments

It is generally known that the contact between tube and die, in the case of tube hydroforming process, leads to the appearance of friction effects. In this context, there are many different models for representing friction and many different tests to evaluate it. In the present paper, the pin-on-disk test has been used and the theoretical model of Orban-2007 has been chosen and developed to evaluate friction coefficient. The main goal is to prove the capacity of theoretical model to present the friction conditions in comparison with the pin-on-disk test. From the Orban model, values of 0.05 and 0.25 of friction coefficient have been found under lubricated and dry tests, respectively. On the other hand, by the classical pin-on-disk test, other values were experimentally obtained as friction coefficient at the copper/steel interface. In the case of pure expansion hydroforming, based on an internal pressure loading only, a “corner filling” test has been run for tube hydroforming. Both dry and lubricated contacts have been considered. Various configurations and shapes have been studied such as the rectangular, trapezoidal, and trapezoid-sectional dies. Finite element simulations with 3D shell and 3D solid models have been performed with different values of friction coefficients. From the main results, it was found that the critical thinning occurs in the transition zone for the square and rectangular section die and in the sharp angle for the trapezoidal and trapezoid-sectional die. The comparison between numerical data and experimental results shows a good agreement. Moreover, the thickness distribution along the cross section is relatively consistent with those measured for the 3D shell model; however, the 3D solid models do not provide a realistic representation of the thickness distribution in the shaped tube. Finally, the results obtained from the theoretical model were more efficient than the results obtained from the pin-on-disk test.     

一种估计管材硬化模型参数的方法

管材力学性能参数的准确性是影响管材塑性成形有限元数值模拟质量的关键因素之一。单向拉伸试验的试件取自滚弯和焊接等制管工序之前的平板坯料 ,所测应力—应变关系无法真实描述管材的塑性变形行为。单向拉伸试验也不能精确反映管材在实际塑性成形中所处的复杂应力状态。基于各向同性硬化假设 ,本文提出了一种轴压胀形、单向压缩试验和数据拟合技术相结合的估计管材硬化模型参数的方法。有限元数值模拟结果显示 ,由这种方法所估计出的管材硬化模型参数是相当准确的。     

Determination of flow stress of thin-walled tube based on digital speckle correlation method for hydroforming applications

The flow stress, used to describe the plastic deformation behavior of thin-walled tube, is one of the most important parameters to ensure reliable finite element simulation in the tube hydroforming process. In this study, a novel approach of on-line measurement based on digital speckle correlation method is put forward to determine flow stress of thin-walled tube. A simple experimental tooling is developed and free-bulged tests are performed for 304 stainless steel and H62 brass alloy tubes. An analytical approach is proposed according to the membrane theory and the force equilibrium equation. The developed method is validated by means of FE simulations. The results indicate that the present method is acceptable to define the flow stress in the tube hydroforming process.     

Study on experimental approaches of forming limit curve for tube hydroforming

This paper proposes a set of experimental approaches to establish the forming limit curve (FLC) in different forming modes for tube hydroforming. In tension–compression strain state, analytical models are constructed to determine the linear strain paths at the pole of the hydroformed tube, and a self-designed free hydroforming apparatus with axial feeding and internal pressure are used to carry out the bulge tests. In plane strain state, the difference is that both ends of the tube are fixed with different punches. In tension–tension strain state, a novel hydroforming apparatus are designed. The novel device requires the simultaneous application of lateral compression force and internal pressure to control the material flow under tension–tension strain states. The linear strain paths for the right hand side of FLC by finite element method simulation are calculated. The linear strain paths in different strain states are verified and the FLC of roll-formed QSTE340 seamed tube is constructed through the proposed experimental approaches. Comparison between simulation and experimental results for hydroforming process of front crossmember shows that the experimental FLC is accurate and valid for tube hydroforming.     

Plastic instability in dual-pressure tube-hydroforming process

The tube-hydroforming process has become an indispensable manufacturing technique in recent years. Successful tube hydroforming requires bulging to take place without causing any type of instability such as bursting, wrinkling or buckling. The dual-pressure tube-hydroforming process was introduced to achieve a favorable tri-axial stress state in the deformation process. In this paper, the effect of applying counter pressure on plastic instability of thin-walled tubes is analyzed. It is concluded that in dual-pressure tube hydroforming, the onset of plastic instability is delayed and the ductility of the metal is increased.     

摩擦对管件液压成形结果影响的数值模拟

在简要介绍液压成形技术在国内外的现状以及常用分析方法之后,对管坯与模具之间的摩擦机理进行了分析并建立了有限元模型.针对成形与回弹的不同特点,分别采用动态显示算法与静态隐式算法进行了仿真分析,讨论了摩擦对汽车仪表板梁成形过程中的补料量、管坯壁厚以及回弹量等参数的影响,为实际生产中补料量的确定、润滑方案的选择、模具的设计等提供理论参考.     

Analysis and finite element simulation of the tube bulge hydroforming process

To investigate the effect of the loading path on the forming result and get the reasonable range of the loading path in tube bulge hydroforming process, a mathematical model considering the forming tube as an ellipsoidal surface is proposed to examine the plastic deformation behavior of a thin-walled tube during the tube bulge hydroforming process in an open die, and thus different loading paths are gained based on this model. The finite element code Ls-Dyna is also used for simulating the tube bulge hydroforming process. The effect of the loading paths on the bulged shape and the wall thickness distribution of the tube are discussed, and then the reasonable range of the loading path for the tube bulge hydroforming process is determined.     

Analysis and finite element simulation of the tube bulge hydroforming process

To investigate the effect of the loading path on the forming result and get the reasonable range of the loading path in tube bulge hydroforming process, a mathematical model considering the forming tube as an ellipsoidal surface is proposed to examine the plastic deformation behavior of a thin-walled tube during the tube bulge hydroforming process in an open die, and thus different loading paths are gained based on this model. The finite element code Ls-Dyna is also used for simulating the tube bulge hydroforming process. The effect of the loading paths on the bulged shape and the wall thickness distribution of the tube are discussed, and then the reasonable range of the loading path for the tube bulge hydroforming process is determined.     

基于径压胀形确定管材的摩擦因数

摩擦对管材液压成形有极大的影响,管材摩擦因数的确定是一项极其重要的工作。在分析比较现有测试方法的基础上,基于径压胀形原理及其变形规律提出确定管材液压成形胀形区摩擦因数的新模型。该模型以恒定内压力下圆形管材径压胀形成方形断面后,以断面对角线长度差作为确定摩擦因数的测量指标。对比对角线长度差的有限元数值模拟结果及实测结果,以此确定管材液压成形时胀形区的摩擦因数。对低碳钢及不锈钢管的有限元数值模拟分析表明：对角线长度差与摩擦因数及内压力均成指数关系,该长度差对摩擦力很敏感且可方便测量,也可作为针对管材液压成形胀形区润滑剂特性的评定指标。所提出的新模型具有简单、实用等优点。     

Development of automotive engine cradle by hydroforming process

The hydroforming technology may bring many advantages to automotive applications in terms of better structural integrity of the parts, lower cost from fewer part count, material saving, weight reduction, lower springback, improved strength and durability and design flexibility. In this study, the whole process of front sub-frame parts development by tube hydroforming using steel material having tensile strength of 440 MPa grade is presented. At the part design stage, it requires feasibility study and process design aided by CAE (Computer Aided Design) to confirm hydroformability in details. Effects of parameters such as internal pressure, axial feeding and geometry shape in automotive engine cradle by the hydroforming process were carefully investigated. Overall possibility of hydroformable engine cradle parts could be examined by cross sectional analyses. Moreover, it is essential to ensure the formability of tube material on every forming step such as pre-bending, preforming and hydroforming. At the die design stage, all the components of prototyping tool are designed and interference with the press is examined from the point of deformed geometry and local thinning.     

Multi-objective optimization of forming parameters for tube hydroforming process based on the Taguchi method

Tube hydroforming is an attractive manufacturing technology which is now widely used in many industries, especially the automobile industry. The purpose of this study is to develop a method to analyze the effects of the forming parameters on the quality of part formability and determine the optimal combination of the forming parameters for the process. The effects of the forming parameters on the tube hydroforming process are studied by finite element analysis and the Taguchi method. The Taguchi method is applied to design an orthogonal experimental array, and the virtual experiments are analyzed by the use of the finite element method (FEM). The predicted results are then analyzed by the use of the Taguchi method from which the effect of each parameter on the hydroformed tube is given. In this work, a free bulging tube hydroforming process is employed to find the optimal forming parameters combination for the highest bulge ratio and the lowest thinning ratio. A multi-objective optimization approach is proposed by simultaneously maximizing the bulge ratio and minimizing the thinning ratio. The optimization problem is solved by using a goal attainment method. An example is given to illustrate the practicality of this approach and ease of use by the designers and process engineers.     

Analytical and numerical approach to prediction of forming limit in tube hydroforming

Analytical and numerical analyses of forming limit in tube hydroforming under combined internal pressure and independent axial feeding are discussed in this paper. To predict the initiation of necking, Swift's criterion for diffuse plastic instability is adopted based on Hill's general theory for the uniqueness to the boundary value problem. In addition, in order to predict fracture initiation, Oyane's ductile fracture criterion is introduced and evaluated from the histories of stress and strain calculated by means of finite element analysis. From the comparison with a series of tube bulge tests, the prediction of the bursting failure based on the plastic instability and the ductile fracture criterion demonstrates to be reasonable so that these approaches can be extended to a wide range of practical tube hydroforming processes.     

A gradient-based decomposition approach to optimize pressure path and counterpunch action in Y-shaped tube hydroforming operations

In tube hydroforming, the concurrent actions of pressurized fluid and mechanical feeding allows obtaining tube shapes characterized by complex geometries such as different diameters sections and/or bulged zones. Main process parameters are material feeding history (i.e., the punches velocity history), internal pressure path during the process, and (in T- or Y-shaped tube hydroforming) counterpunch action. What is crucial, in such processes, is the proper design of operative parameters aimed to avoid defects (for instance underfilling or ductile fractures). Actually, the design of tube hydroforming operations is mainly aimed to prevent bursting or buckling occurrence and such issues can be pursued only if a proper control of process parameters is performed. In this paper, a design procedure for Y-shaped tube hydroforming operations was developed. The aim of the presented approach is to calibrate both internal pressure history during the process and counterpunch action in order to reach a sound final component. The approach utilized to optimize the aforementioned parameters is founded on gradient-based techniques and the optimization problem here addressed depends on a considerable number of design variables. In order to reduce the total number of numerical simulations/experiments necessary to reach the optimal values of the design variables, the basic idea of this paper is to develop a sort of decomposition approach aimed to take into account subsets of design variables in the most effective way. The proposed decomposition approach allows avoiding about 50% of the numerical simulations necessary to solve the same problem by traditional gradient technique.     

A prediction of bursting failure in tube hydroforming process based on plastic instability

Based on plastic instability, an analytical prediction of bursting failure on tube hydroforming processes under combined internal pressure and independent axial feeding is carried out. Bursting is an irrecoverable phenomenon due to local instability under excessive tensile stresses. In order to predict the bursting failure, three different classical necking criteria – diffuse necking criteria for a sheet, and a tube, and a local necking criterion for a sheet – are introduced. The incremental theory of plasticity for an anisotropic material is adopted and the hydroforming limit, as well as a diagram of bursting failure with respect to axial feeding and hydraulic pressure are presented. In addition, the influences of material properties such as anisotropy parameter, strain hardening exponent and strength coefficient on plastic instability and bursting pressure are investigated. As a result of the above approach, the hydroforming limit with respect to bursting failure is verified with experimental results.     

Study of Hydroforming Lubricants with Overbased Sulfonates and Friction Modifiers

Due to the high cost of hydroforming equipment, attempts have been made to model this process using bench testing. While tensile and twist compression tests have yielded promising results, they do not simulate any single metal forming process. In this study we investigated the use of expansion zone, guiding zone, and twist-compression tests to simulate hydroforming processes and evaluated various overbased sulfonates and organic friction modifiers performance. The amorphous overbased magnesium sulfonate improved the performance of the hydroforming oil in the expansion zone and provided no harm in the guiding zone, while the crystalline overbased calcium sulfonate provided no harm in the expansion zone and improved performance in the guiding zone. It was found that the friction modifiers tested display an antagonism, where, as the expansion zone performance increased, the guiding zone performance decreased. Additionally, we found in twist-compression testing that the initial coefficient of friction displayed a small correlation with the guiding zone test (from r = 0.26–0.47), while the failure time displayed a small correlation with the expansion zone test (r = 0.36–0.40).     

The effect of tube material, microstructure, and heat treatment on process responses of tube hydroforming without axial force

In this paper, the influence of tube material, microstructure, and heat treatment on process responses of tube hydroforming has been studied. One of the most important parameters in performing a successful tube hydroforming process is the selection of appropriate material for tubes. In the analysis section, effective parameters for the selection of an appropriate tube material for the hydroforming process have been investigated; it was concluded that higher strain hardening exponent (n), elasticity modulus (E), and anisotropy index (R) can enhance formability in this process; and the effects of microstructure and heat treatment on the formability of ASTM C11000 copper and ASTM AA1050 aluminum have been investigated. Consequently, four different heat treatment processes, which had different heating temperatures and durations, were selected, in addition to different cooling methods for each of the materials. In the experimental tests, the effects of these heat treatment methods on maximum bulging height, thickness strains, and final forming pressures were scrutinized. The effects of heat treatment on copper microstructure were also studied through metallographic tests; on the other hand, the effects of microstructure on tube hydroforming process were justified. As a result of these analyses, two heat treatment methods, namely, heating to 450 and 350 °C for 15 min and cooling in water, were recommended for copper and aluminum, respectively. Using these methods and due to their consequent fine and homogenous microstructure, higher mechanical strength and increase in material formability was achieved by attaining higher thickness strain and bulging height values. Finally, after extracting the mechanical properties of the two materials and comparing them with each other, parameters of strength coefficient and strain hardening exponent were reported as two effective factors that would improve tube deformation by tube hydroforming process.     

管件液压成形技术在汽车制造中的应用研究

阐述了管律液压成形的基本概念及其优点，介绍了在汽车制造领域中的应用，综述了管件液压成形技术的研究成果，重点介绍材料、塑性失稳、加载路径优化、预成形等问题；讨论了应用研究中进一步的发展方向。     

A technology to improve formability for rectangular cross section component hydroforming

In order to overcome difficulties in non-uniform thickness distribution and cracking failure during rectangular tube quasi-static hydroforming, a new forming technology, named as electromagnetically assisted hydroforming, is put forward. Both experiment and finite element method were conducted to investigate corner deformability and deformation pattern and its effect mechanisms. Results indicate that both corner deformability and thickness distribution are improved greatly under electromagnetic-assisted hydroforming. The reason is that deformation behavior changed after electromagnetic force application. As electromagnetic force is applied, tine petal cross sections are periodically produced and flattened. Thus, petal-like preform continues to generate and play a useful role in corner filling. Such deformation pattern overcomes friction holding back defect and results in stress state going over from tensile stress to compressive stress, which helps to avoid cracking failure and greatly improve thickness uniformity. At the same time, it also contributes to improve surface quality and decrease forming pressure simultaneously.     

Analysis and design of hydroformed thin-walled tubes using enhanced one-step method

This paper deals with the analysis and design of tube hydroforming parameters in order to reduce defects which may occur at the end of the forming process, such as necking and wrinkling. We propose a specific methodology based on the coupling between an enhanced one-step method for the rapid simulation of tube hydroforming process and a surrogate model based on a metamodeling technique. The basic formulation of the one-step method has been modified and adapted for the modeling of 3D tube hydroforming problems in which the initial geometry is a circular tube expanded by internal pressure and submitted to axial feeding. In the surrogate model, approximate responses are built using moving least squares method and constructed within a moving region of interest which moves across a predefined discrete grid of authorized experimental designs. Two applications of tube hydroforming of aluminum alloy 6061-T6 have been utilized to validate our methodology. The final design is validated using experiments together with the classical explicit dynamic incremental approach using ABAQUS? commercial code.