基于三维成形极限应力图的AA6061挤压管材成形性能分析

为解决AA6061挤压管材成形性能极差的问题,建立了“固溶水淬+颗粒介质胀形+人工时效”的铝合金管件成形工艺流程。研究了固溶水淬和人工时效工艺参数对材料性能影响规律,并建立了考虑厚向应力的三维成形极限应力图;建立了管件颗粒介质胀形有限元仿真模型,分析管件变形特征质点的运动轨迹和应力应变状态,并应用理论成形极限图对管件破裂失稳点和胀形极限进行分析和预判。四方截面管件胀形工艺试验结果表明,颗粒介质胀形工艺与合适的热处理工艺相结合能够有效地解决AA6061挤压管材的成形问题;考虑厚向应力的三维成形极限应力图可作为铝合金管件胀形工艺方案制定的破裂失稳判据。     

盒形件固体颗粒介质板材成形工艺研究

固体颗粒介质板材成形工艺是采用固体颗粒微珠代替刚性凸(凹)模(或弹性体、液体)的作用对板材拉深成形的新工艺。选用非金属固体颗粒介质——GM颗粒作为研究对象,以固体颗粒介质在高应力水平下的体积压缩试验和摩擦强度试验为基础,应用散体力学理论中扩展的Drucker-Prager线性模型构建固体颗粒介质有限元材料模型。以具有非轴对称性的方盒形件为代表,进行固体颗粒介质成形工艺的有限元模拟,研究成形过程中板材的流动特征和壁厚分布规律。工艺试验成功得到方盒形零件,将加载曲线、成形过程变形特征和壁厚分布曲线与数值模拟结果比对较为吻合。分析表明,采用以散体力学为基础建立的固体颗粒介质材料模型进行工艺模拟,能够得到与试验较为接近的变形特征和力能参数,可以应用于制定工艺方案的依据,为该技术在板材成形中的应用起到指导和借鉴作用。     

Q235覆管对AA5052铝合金基管颗粒介质胀形行为的影响

采用AA5052铝合金挤压管作内层基管、Q235碳素结构钢卷焊管作外层覆管的钢铝复合管对复合管颗粒介质胀形行为进行研究。通过塑性理论分析胀形过程中管间切向摩擦力及法向压力对基管应力大小的影响;利用数值模拟分析管间摩擦因数和覆管各向异性对基管的应变成形极限的综合影响,并给出单管、复合管胀形时的壁厚减薄情况和基管的应力、应变分布;通过管材颗粒介质内高压胀形试验,对比单管和复合管胀形条件下铝合金管的极限胀形比,分析复合管的变形协调性。结果表明：通过施加Q235碳素结构钢覆管,减小了AA5052基管胀形区中间截面处的双向拉应力,基管胀形区壁厚减薄变小,胀形比提高了22%,复合管下基管最大减薄率为17.5 %,成形性能显著提高。     

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.     

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.     

Analytical model for planar tube hydroforming: Prediction of formed shape, corner fill, wall thinning, and forming pressure

An analytical model for planar tube hydroforming based on deformation theory has been developed. This analytical model can be used to predict hydroformed shape, corner fill, wall thinning, and forming pressure. As the model is based on a mechanistic approach with bending effects included, local strain and stress distribution across the wall thickness can be determined. This includes strain and stress distributions for the outer layer, inside layer, and middle layer. The model is validated using finite element analysis and tube hydroforming experiments on irregular triangular, irregular quadrilateral, and pentagonal hydroformed shapes.     

A procedure for designing initial thickness variation for superplastic free inflation

A procedure is proposed for determining the required initial thickness distribution in order to produce a given design specification by the superplastic blow forming method. Analytical solutions are first derived to predict the thickness variation during a free inflation process. The equation is then used to obtain the thickness distribution prior to inflation from a prescribed final thickness as though the deformation is reversed. The commercial finite element program, ABAQUS, is adopted to simulate the superplastic blow forming process of the acquired preform. Although the inflated thickness is compared fairly close to the desired result, errors do exist due to the simplified assumptions made in the analytical method. In order to overcome the difficulty, a model describing the thickness variation of the inflated sheet is first obtained from curve-fitting the solutions of finite element simulation instead of analytical derivation. The initial thickness determined from this model is shown to give the intended final thickness from finite element simulation. Experiments have also been performed to verify the results.     

管材胀形中的圆角成形

提出考虑管材和模具间滑动摩擦的力学模型,来研究在方形模具中的管材胀形的塑性变形行为。通过对无摩擦条件下的滑动模型的研究,在理论上首次证明成形圆角半径存在最小极限值,并给出理论计算公式。采用提出的方法确定的最小极限半径可用于确定管材成形工件形状及其成形压力。最后采用自行设计的胀形装置进行管材在方形模具中的胀形试验,并与滑动模型计算结果进行比较。试验结果与计算结果对比表明,试验结果介于摩擦滑动模型和无摩擦滑动模型之间,这说明无摩擦滑动模型和滑动摩擦滑动模型的分析是正确的。     

内高压成形矩形断面圆角应力分析

采用力学分析和数值模拟方法对圆角部位在内高压成形过程中的应力分布和变形机理进行分析,揭示出圆角和直边过渡区的减薄以及开裂的力学机理。提出内高压成形中圆角和直边的过渡区材料最易满足塑性屈服条件, 并发生厚度方向的压缩应变,因此在过渡区易发生剧烈的减薄变形;并且当管材壁厚与圆角半径的比值大于0．5 时,直边区和圆角区的屈服条件差别较大,引起变形不均匀性增大,更易导致过渡区的过度减薄和开裂。分析结果有助于指导内高压成形零件设计和工艺设计,改善矩形断面零件内高压成形件产品质量。     

凸环管件颗粒介质内高压成形工艺理论解析

离散体颗粒介质使颗粒介质内高压成形工艺中的传压具有非均匀性、颗粒介质与管件之间摩擦作用显著等特征,基于此,建立了颗粒介质非均匀载荷传压模型,对凸环管件胀形工艺过程进行了理论推导和数值解析,探讨了内压状况和摩擦条件对管件成形性能的影响,并通过工艺试验对理论分析结果进行了验证。分析结果表明,颗粒介质内高压成形工艺所具有的内压非均匀性、介质与管坯摩擦作用显著两大特征可有效减小胀形过程中的壁厚减薄和成形压力。对比试验与理论分析结果表明,壁厚分布和成形压力的理论计算结果与试验结果一致,颗粒介质非均匀载荷传压模型的构建策略可用于管件成形的预测和分析。     

Improvement of formability for the incremental sheet metal forming process

In order to obtain competitiveness in the field of industrial manufacture, a reduction in the development period for the small batch manufacture of products is required. In order to meet these requirements, an incremental sheet metal forming process has been developed. In this process, a small local region of a sheet blank deforms incrementally by moving a hemispherical head tool over an arbitrary surface. In this work, an incremental sheet metal forming process controlled three dimensionally by a computer has been accomplished. It has been shown by the experiments that a sheet blank is mainly subject to shear-dominant deformation. Therefore, the final thickness strain can be predicted. The uniformity of thickness throughout the deformed region is one of the key factors to improve the formability in the sheet metal forming processes. Using the predicted thickness strain distribution, the intermediate geometry is decided in the manner that a shear deformation is restrained in the highly shear-deformed region and vice versa. This double-pass forming method is found to be very effective so that the thickness strain distribution of a final shape can be made more uniform.     

颤振冷挤压降载机理与仿真研究

针对金属冷挤压成形过程中金属变形抗力大、设备吨位高而普遍振动形式降载效果不理想的问题,将颤振激励技术引入金属冷挤压成形过程中。从表面效应角度分析了颤振激励下的金属冷挤压成形过程中摩擦力的变化规律,阐明了颤振降载的基本原理。以万向节轴套零件为例,分别讨论分析了传统冷挤压模式和颤振激励冷挤压模式下的挤压力数学模型,从理论上说明了颤振冷挤压的降载机理,进一步应用Deform-3D软件建立了其有限元分析模型,对万向节轴套零件进行了受载和模具寿命分析。仿真实验结果表明,在100 Hz频率0.03 mm振幅的周期性颤振激励下,万向节轴套零件的冷挤压过程最大成形压力下降8.3%,最终成形压力下降10.1%;冲头单次最大磨损量下降,但凹模单次最大磨损量有一定程度的增加。     

方矩形管连续辊弯成形过程的变形和应力场

应用三维大变形弹塑性有限元法,对方矩形管连续辊弯成形过程中金属流动规律以及应力分布进行了模拟分析.考虑了各道次之间的数据信息传递,保证了下一道次变形历史的延续,从而真实模拟了方矩形连续辊弯成形过程.获得的结果对连续辊弯成形中的产品质量的控制和孔型的优化有着重要意义.     

Finite element analysis on neck-spinning process of tube at elevated temperature

Tube spinning process is a metal forming process used in the manufacture of axisymmetric products and has been widely used in various applications. Finite element analysis has been successfully applied to the tube spinning processes, but no temperature effects have been considered on neck-spinning. For this reason, the aim of this research is to investigate numerically the neck-spinning process of a tube at elevated temperature. The commercial software Abaqus/Explicit was adopted in the simulation. For the construction of the material model, special uniaxial tensile tests were conducted at elevated temperature and various strain rates, since the material is sensitive to strain rates at high temperature. Comparisons between experimental and simulation results on thickness distribution and the outer contour of the spun tube are discussed. During the final stage, the average deviations between the simulation and experiment were 10.65% in thickness and 3.03% in outer contour. Good agreement was found between experimental and simulation results. The influence of the coefficient of friction, roller translation speeds, and the tip radius of the rollers were also investigated through numerical simulation.     

确定拉延筋约束阻力的一种数值模拟方法

利用率形式的虚功原理和考虑弯曲影响的 Mindlin曲壳单元 ,根据塑性变形体积不可压缩的假设 ,将大变形弹塑性有限元应用于板料成形过程中 ,建立了接触摩擦模型。在此基础上 ,利用自行开发的软件 Sheet Form模拟板料流经拉延筋的过程 ,确定板料成形数值模拟中建立等效拉延筋模型需要的拉延筋约束阻力、塑性厚向应变和最小压边力 3个边界条件 ,并与Nine等人的实验结果进行对比 ,证明了该方法的有效性     

Effect of die corner radius on the formability and dimensional accuracy of tube shear bending

Tube shear bending is a beneficial technique to realize considerable small bending radii. The authors have investigated the tube shear bending process of circular tubes experimentally. Moreover, an elastoplastic 3D finite element simulation has been conducted, aimed at clarifying the forming mechanism. Both the experiment and simulation results indicate that, in order to perform successful forming, the value of the applied pushing force on the tube must be appropriate. In this paper, the mechanism of defect generation was clarified. Two failure criteria were introduced and employed to recognize the occurrence of defects in the simulation. The effects of the die corner radius, as the main parameter, on the defect generation of circular A1050 aluminum tubes were investigated both by experiments and numerical simulation. From the results, the formability of tube on dies with different corner radii applying various pushing pressures was clarified. Moreover, the influence of the die radius on the dimensional accuracy of the deformed tube regarding cross-section ovality and thickness changes of the deformed tube was evaluated. The results of this study indicate that, whilst a small bending radius results in high cross-section ovality, increasing the die corner radius raises the wrinkling tendency of the tube. However, the die radius has a small effect on the suitable values of pushing pressure required for a successful shear bending deformation. Moreover, the effect of the die corner radius on the thickness strain of the deformed tube is insignificant.     

ANALYTICAL SOLUTION OF FILLING AND EXHAUSTING PROCESS IN PNEUMATIC SYSTEM

The filling and exhausting processes in a pneumatic system are involved with many factors, and numerical solutions of many partial differential equations are always adapted in the study of those processes, which have been proved to be troublesome and less intuitive. Analytical solutions based on loss-less tube model and average friction tube model are found respectively by using fluid net theory, and they fit the experimental results well. The research work shows that: Fluid net theory can be used to solve the analytical solution of filling and exhausting processes of pneumatic system, and the result of loss-less tube model is close to that of average friction model, so loss-less rube model is recommended since it is simpler, and the difference between filling time and exhausting time is determined by initial and final pressures, the volume of container and the section area of tube, and has nothing to do with the length of the tube.     

Effects of forming media on hydrodynamic deep drawing

Apart from the punch and the die, a pressurized fluid (water or oil) is used in hydroforming. The presence of such pressure media is the main difference between hydroforming and conventional deep drawing. No comprehensive study has yet been conducted on the effect of forming media on the formation of cylindrical cups via hydrodynamic deep drawing assisted by radial pressure. This study investigated the formation of such cups through Finite element (FE) simulation and experiments. First, the process was modeled numerically using ABAQUS FE software. After simulation, copper and St14 sheets were formed with water and oil as the forming media. The effect of these forming media on thickness distribution and maximum punch force was investigated. By examining the thickness distribution curve of the hydroformed cup, a close agreement was found between experimental and numerical results. Using oil as the forming media reduced thinning at the corner radius zone of the punch and increased the maximum punch force. Changing the forming media does not significantly influence the maximum thickening at the cup wall region.

     

Tube hydroforming compression test for friction estimation—numerical inverse method, application, and analysis

Friction plays an important role in forming processes, in fact it influences the material flow and therefore it affects the process and part characteristics. In particular, friction is a very influencing factor in tube hydroforming (THF), where high die–part contact pressure and area make the material sliding very difficult. As a consequence, the material hardly flows to the expansion zones and the part formability can be compromised. To obtain sound parts, FEM models allow the study of the process and optimize its parameters, but they require the right definition of the friction at tube–die interface. For these reasons, friction represents a key-point in THF processes and its knowledge and prediction are very important even if, nowadays, a comprehensive friction test for THF is not available in literature. With this paper, the authors want to propose and evaluate a method to estimate friction for THF processes. In particular, a numerical inverse method allowing the estimation of the Coulombian friction coefficient combining experimental test and FE simulation results will be described. The method is based on the effects of friction on the tube final thickness distribution when it is pressurized and compressed by two punches under different lubrication conditions without expansion. In particular, how the use of few and fast FE simulations allows to estimate an analytical function that takes into account the process conditions and that can be used in combination with experimental results in order to estimate the friction coefficient in THF processes will be shown.     

Three-dimensional effects of the bend–stretch forming of aluminum tubes

Bend–stretch forming is commonly used to shape extruded tubular aluminum parts for automotive applications. The tubes are pre-stretched, pressurized and bent over rigid dies. Tension prevents buckling of the compressed side and significantly reduces springback during unloading. An unwanted byproduct of the process is distortion of the cross section. Small amounts of pressure applied during forming can reduce this distortion. A systematic study of how to select the appropriate amounts of tension and pressure for accurate forming with minimal distortion has been conducted. The problem was first studied experimentally using a custom forming facility. An efficient 2-D model of the process was previously developed which was shown capable of capturing the main deformation features of interest. Its efficiency made this model a useful design tool for optimally selecting the forming parameters. In this paper, a 3-D finite element model of the forming process is used to simulate the complete forming process. By using a specially calibrated non-quadratic yield function, the model accurately reproduces all aspects of the process. The model is used to study 3-D features of the problem such as variations of distortion and springback along the length, the effect of friction, the lifting of a section of the tube around the mid-span off of the die, the effect of post-tension, and forming over a variable radius die.