管材屈曲和起皱分析

研究了薄壁管在高压成形过程中的屈曲、起皱失稳现象,通过对所建立的弹塑性屈曲（全局屈曲）和起皱（局部屈曲）模型的分析计算,获得了管子屈曲、起皱的初始条件、临界载荷和临界应力计算公式。研究表明,屈曲和起皱的产生取决于几何参数,如相对半径r0/l0和相对厚度d0/r0等,且对长管而言,屈曲是主要的失稳形式,但对于短管,起皱则是主要的失稳形式,薄壁管内压对薄壁管起皱、屈曲没有影响。     

基于成形应力极限的管材液压成形缺陷预测

基于塑性应力应变关系及Hill79屈服准则,推导出极限应力与极限应变间转化关系,进而建立2008T4铝合金的成形应力极限图(Forming limit stress diagram,FLSD)。采用LS-DYNA软件对三通管液压胀形过程进行模拟,应用FLSD预测胀形过程中破裂的发生及成形压力极限,并与传统成形极限图(Forming limit diagram,FLD)结果进行了对比。研究表明,FLD与FLSD预测结果中破裂缺陷位置相同,但极限内压力值存在很大差别,而FLSD预测结果与物理试验结果较吻合。考虑到FLD受应变路径影响显著的因素,将FLSD作为管材液压成形等复杂应变路径下的成形极限的判据更加方便可靠。     

铝合金球壳液压胀形试验研究

研究了板厚1.5mm的3A21O铝合金板材和焊接接头的塑性成形能力,进行了铝合金球壳的整体液压胀形实验。实验结果表明:在保证焊接质量的前提下,铝合金球壳的胀形过程能够顺利进行。分析了壳体胀形过程中的变形和应力分布规律,从理论上给出了球壳的胀形屈服应力和开裂应力,最后采用动态显式有限元分析软件LS-DYNA给出了壳体的壁厚分布。     

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.     

Analytical approach to bursting in tube hydroforming using diffuse plastic instability

Analytical studies on onset of bursting failure in tube hydroforming under combined internal pressure and independent axial feeding are carried out. Bursting is irrecoverable phenomenon due to local instability under excessive tensile stress. In this paper, in order to predict the bursting failure diffuse plastic instability based on the Hill's quadratic plastic potential is introduced. The incremental theory of plasticity for anisotropic material is adopted and then the hydroforming limit and bursting failure diagram with respect to axial feeding and hydraulic pressure are presented. The influences of the plastic anisotropy on plastic instability, the limit stress and the bursting pressure are also investigated. Finally, the stress-based hydroforming limit diagram obtained from the above approach is verified with experimental results.     

SAE4130钢管热压成形工艺分析

以SAE4130钢管为研究对象,分别在不同温度下进行塑性成形试验,通过宏观和微观组织观察、硬度测试等方法,研究SAE4130钢管在不同温度条件下塑性成形的规律,得到较优的不锈钢管塑性成形温度条件。结果表明,SAE4130钢在加热条件下具有更好的塑性,能实现更大程度的塑性变形。随着加热温度的升高,马氏体含量降低,贝氏体含量升高,此时对管端进行塑性成形,得到的成形部位质量最好。     

AZ31变形镁合金等温变形力学特性的研究

针对AZ31镁合金,采用Gleeble1500热模拟试验机对其在不同温度和变形速率下的流变应力进行了实验研究,结果表明镁合金在进行高温压缩的情况下,变形温度和应变速率对流变应力有显著的影响,流变应力随应变速率的升高和变形温度的降低而升高。在此基础上,建立了流变应力的数学模型,其结果为镁合金的塑性成形工艺的制订提供理论依据。     

A contribution to determining the friction coefficient in hydroforming of tubes

Hydroforming is a relatively new technology, which enables complex shaped hollow parts to be produced efficiently. Compared to alternative methods, this technology is characterised by significant potential advantages, such as complex shape production, lightweight design, high accuracy, and process integration. The tribological aspects of hydroforming are very significant, as friction influences all the main process parameters practically in a direct or indirect way. However, there is a paucity of information relating to hydroforming tribology. In most reports on this subject only qualitative data regarding the friction coefficient are given. However, for finite‐element method simulation and optimal process and tool design, quantitative data are indispensable. This paper describes an attempt to determine the coefficient of friction in both the elastic and plastic states of a workpiece during tube hydroforming processes. Push‐through tests were carried out in order to determine the coefficient of friction in the elastic state, and tube upsetting tests were conducted for the plastic state. Various commercial lubricants were used in the experiments.     

小型桥壳液压胀形初始变形条件分析及成形试验

介绍了小型汽车桥壳的液压胀形工艺,提出了初始胀形内压的表达式,预测了初始胀形内压与轴向推力的匹配关系（即经向应力比的大小）对预胀形时各部分变形顺序的影响。在普通液压机上进行了两种加载路径下的液压胀形试验,在初始经向应力比小于零并保持恒内压的条件下,预胀形管坯先变形成两侧高、中部低的双鼓形,经增压后将中部胀起;在初始经向应力比大于零且内压恒定的条件下,预胀形管坯中部沿轴向胀裂;两种加载路径下,管坯扁锥体凸起与胀形区之间均产生了明显内凹缺陷。理论分析与试验结果均表明,初始变形条件对小型桥壳的预胀形有重要影响。     

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。