Hydroforming of anisotropic aluminum tubes: Part I experiments

Hydroforming of aluminum tubes, despite its appeal in weight-sensitive applications, presents challenges such as the reduced ductility of Al in comparison to steel and its more complex constitutive behavior. This two-part series of papers details a combined experimental and analytical study of the process and its limits. Part I presents a custom laboratory-scale facility used to conduct a series of hydroforming experiments on relatively long Al-6260-T4 tubes. The initially circular tubes are inflated against a square die with rounded corners while simultaneously they are axially compressed in order to delay wall thinning and burst. A 2D numerical model was used to optimize the loading histories considered. Despite careful design of the process, burst proved to be a limiting factor as friction prevented uniform material feeding to the expected levels. Furthermore, prediction of burst was found to require the calibration and implementation of non-quadratic, anisotropic yield functions in the constitutive modeling and the use of numerical models that include all 3D effects of the setup. These models and their performance in predicting all aspects of the experimental results are discussed in Part II.     

直缝焊管液压成形极限理论预测模型

直缝焊管广泛应用于汽车车身管状零件液压成形中,焊接区影响着焊管塑性变形规律,准确评价焊管缩颈或破裂现象是工程上倍受关注的问题。基于金相分析法和显微硬度测量法分析高频感应焊管的结构特征,并根据液压成形条件下高频感应焊管的变形特点,提出一种用于计算直缝焊管液压成形极限的理论方法。基于该方法,选用Swift硬化方程和Hill屈服准则推导出直缝焊管液压成形极限理论预测模型,在已知焊管(包含焊接区和基体区)材料性能参数条件下可获得直缝焊管液压成形极限图。运用此理论预测模型,计算出QSTE340高频感应焊管的液压成形极限图。成形极限的计算结果与试验对比表明,二者吻合较好,这证明所建立的直缝焊管液压成形极限的理论预测模型是正确的。     

Overview on constitutive modeling for hydroforming with the existence of through-thickness normal stress

The hydroforming process is used widely across many industrial fields. High applied pressure during hydroforming makes it necessary to consider the influence of the through-thickness normal stress, while traditional approaches based upon a plane-stress assumption are not appropriate in such cases. Reliable constitutive models that consider the through-thickness normal stress are summarized in this paper, which focuses on the state of the art in the following several aspects: determine the flow stress curve with proper experimental methods and choose the measurement and computational methods to minimize the error as much as possible; select the proper three-dimensional anisotropic yield criterion for the specific material; Define the forming limit model and construct corresponding experimental verification method. The review of existing work has revealed several gaps in current knowledge of the hydroforming process accounting for the through-thickness normal stress. Conclusions are drawn concerning some critical issues and potential future developments in hydroforming modeling.     

国产2.25Cr-1 Mo钢板热处理工艺研究

介绍了国产 2 2 5Cr 1Mo钢板首次用于制造临氢重整反应器所进行的热处理工艺性能评定试验 ,通过对工艺评定试验结果和实际生产中钢板热加工性能的分析和研究 ,探讨了最佳热处理规范     

脉动液压成形技术与设备

介绍脉动液压成形技术的成形原理、工艺特点及应用领域。从工艺和材料两方面对脉动液压成形提高材料成形能力的机理进行研究,试验证明脉动载荷一方面能够促进管材液压成形时的补料、降低摩擦力的阻碍作用并利用成形过程中小褶皱的出现与消失提高变形的均匀性;另一方面,对于奥氏体不锈钢,脉动载荷可以增强形变过程中的相变增塑效应,从而提高其成形性。自主设计并研制出能实现工业化生产需求的自动化程度高的脉动液压成形设备,为该项技术在汽车、航空及航天制造领域中的推广和应用提供重要的理论指导和实践探索。     

Numerical simulation of various cross sectional workpieces using conventional deep drawing and hydroforming technologies

This study focuses on the determination of optimum sheet metal forming process and process parameters for various cross sectional workpieces by comparing the numerical results of high-pressure sheet metal forming, hydro-mechanical deep drawing (DD) and conventional DD simulations. Within the range of each cross section, depth, characteristic dimensions ratio and fillet radius have been altered systematically. Steel of types St14 and DC04 have been used as the specimen material in the numerical analyses and the experimental verification throughout the study. All numerical simulations have been carried out by using a dynamic–explicit commercial finite element code and an elasto-plastic material model. During the analyses each workpiece was simulated by the three competing processes. The results of analyses, such as sheet thickness distribution, necking, forming of radii etc., are used for assessing the success of each forming process alternative. The analyses revealed that depending on the workpiece geometry and dimensional properties certain processes are preferable for obtaining more satisfactory products. Working windows for each process have been established based on the analyzed parameters of the circular, elliptic, rectangular and square cross sectional product geometries. This data is expected to be useful for selecting the appropriate production process for a given workpiece geometry and understand the limits of each sheet metal forming processes.     

Prediction of necking of high strength steel tubes during hydroforming—multi-axial loading

This article studies tubular hydroforming of high strength low alloy (HSLA) and dual phase (DP600) straight tubes under the action of end feeding loads. Experiments demonstrate that higher end feed loads enhance the formability of the tubes and increase the internal fluid pressure for onset of necking and bursting. Because of the action of the internal pressure and the axial compressive load, the onset of localization (necking) is due to a complex three-dimensional state of stress. Using free expansion experiments, approximate upper and lower bound strain-based forming limit curves are determined for the tube materials. These limit curves, in turn, are used to derive upper and lower bound extended stress-based forming limit curves [Simha et al., Prediction of necking in tubular hydroforming using an extended stress-based FLC. Transactions of the ASME Journal of Engineering Materials and Technology 2007;129(1): 36-47]. In conjunction with finite element computations that use solid elements to model the tube, these stress-based limit curves are used to predict upper and lower bound necking pressures under the action of end feed loading. These predictions of necking pressures, when an appropriate coefficient of tube-die friction is used, are found to bracket the experimentally measured necking pressures. Computations using plane stress shell elements to model the tubes are shown to give erroneous results, since the plane stress approximation is not valid when tubes are hydroformed in a die.     

A simple experimental tooling with internal pressure source used for evaluation of material formability in tube hydroforming

Material properties have powerful impact on the tube hydroforming (THF) process and the quality of the deformed tube, so it is important to select proper materials and evaluate the material formability prior to conducting the process. A simplified and applied tooling, which has no use for any external hydraulic pressure source but internal one, was designed for charactering the material formability in THF. A pressurized-fluid supplier is automatically established to provide the internal pressure and axial load synchronously required for THF, and the ratio of the two loads is achieved by proper design of the supplier. As a stand-alone hydraulic bulging fixture, the tooling can be worked on a conventional press, even on a single action press. Free bulge forming (FBF), bulge forming with axial loading (BFAL), free and restrained bulge forming (free and fixed ends) can be fulfilled by the tooling, and furthermore, bulge forming with proportional loading to some extend can be realized. Comparative bulge forming experiments under various forming conditions were carried out with the tooling to validate this project and the results suggest that restrained conditions on the tube ends highly affect the FBF, while the ratio of the two loads dominates the BFAL.     

Design optimization of extruded preform for hydroforming processes based on ideal forming design theory

The conventional practice to predict preform shapes in hydroformimg processes based on finite-element analysis and/or experiment is an iterative procedure and requires many trials. In this paper, a computationally efficient direct design method, which effectively improves the design procedure, was introduced. The direct design method based on ideal forming theory, which was successfully applied for the design of flat blanks for stamping processes, was extended for the design of non-flat preform for tube hydroforming processes. A preform optimization methodology for non-flat blank solutions was proposed based on the penalty constraint method for the cross-sectional shape and length of a tube. The hybrid membrane/shell method was employed to capture thickness effect while maintaining membrane formulation in the ideal forming theory. Several classes of examples were analyzed to verify the current formulation.     

Improvement of Formability by Oscillation of Internal Pressure in Pulsating Hydroforming of Tube

A new history of internal pressure in the hydroforming processes of tubes is developed to attain high formability. The effect of improvement of formability by the oscillation of internal pressure in a pulsating hydroforming process of tubes is examined using both finite element simulation and experiment. The deformation behaviour during the hydroforming is greatly affected by the oscillation of internal pressure. For a monotonic history of small internal pressure, the wrinkling was caused by insufficient bulging, whereas the necking and bursting occurred for a monotonic history of large internal pressure. The occurrence of these defects can be avoided by oscillating the internal pressure in the pulsating hydroforming. The improvement of formability in the pulsating hydroforming is due to both low pressure and oscillation of pressure. The effects of the amplitude and cycle number of pressure in the pulsating hydroforming on the deformation behaviour are investigated. It is found the oscillation of internal pressure is effective in preventing the occurrence of defects.