【作者】 金春凤；

【导师】 刘渝；

【作者基本信息】 西华大学 ， 机械制造及其自动化， 2009， 硕士

【摘要】 拉弯工艺是型材弯曲成形的重要方法,它可以有效减少回弹,达到提高成形精度的目的。因此,在飞机、汽车弯曲件成形中得到了广泛的应用。拉弯卸载后的回弹控制是影响成形精度的主要因素。拉弯成形过程中,材料受力状态复杂,同时受材料物性参数、摩擦条件等因素的影响,拉弯成形回弹预测很困难。在实际生产中,拉弯模修正量及工艺参数的确定主要以实际经验或通过试错法来解决,但此方法不能够一次拉弯精确成形型材,且周期长,成本高。本文以型材为研究对象,首先介绍了型材拉弯的基本概念,在此基础上讨论了型材拉弯工艺相对于其他弯曲工艺的优点、用途以及迫切需要解决的两个关键技术问题。总结了拉弯工艺过程中容易产生的五种成形缺陷,从型材力学性能、截面形状、工艺参数三个方面阐述目前国内外在试验和数值模拟这两方面的研究状况。介绍了弹塑性力学的初始屈服条件、基本法则,其中包括流动法则和强化法则。在此基础上以各向同性硬化法则的材料为例推导了应力应变增量关系,为后续的力学分析提供了理论基础。在介绍的理论基础上,根据拉弯成形力学分析的基本假设,建立了预拉阶段、拉弯弯曲阶段和拉弯补拉阶段三个阶段拉弯成形过程的力学模型。在拉弯弯曲阶段详细描述了模具与型材间的摩擦力计算方法;型材拉伸弯曲后应力应变分析和弯距的计算方法。介绍了拉弯成形回弹分析方法,推导了板材回弹半径与回弹角的计算公式;进一步在型材拉弯与板料压弯过程中应力分布差异的基础上推导了型材的回弹半径与回弹角的计算公式。在拉弯回弹理论分析基础上,形成了结合拉弯零件数字化模型来修正拉弯模模具轮廓的方法;给出了用圆弧样条表示的模具轮廓上点的计算方法;修正后的模具轮廓曲线沿整个轮廓曲线都是一阶连续的,曲率为分段常数。该方法可通过拉弯件轮廓曲线上布点的密度来控制模具轮廓曲线上曲率分段的多少和误差大小。本文还通过有限元分析方法计算了拉弯型材的回弹量,用于评估拉弯模轮廓面的回弹修正量,以及拉弯件的校形余量的减少情况。本文的工作为提高汽车和飞机拉弯件的质量和促进工装的数字化设计提供了一合理有效的方法。更多还原

【Abstract】 Stretch bending is an important method of bending of extrusions. Stretch bending can decrease the spring-back effectively and improve the forming precision, so the process of stretch bending was widely used in the forming of extrusions in the car and aircraft manufacture. The spring-back control after stretch bending forming is a major concern in improving extrusions parts accuracy. Otherwise stretch bending forming, with different non-linear part’s material parameters and complex stress state, friction conditions during forming, is very difficult to predict the spring back. So, experimental trial-and-error method, which results in the long cycles and the high cost, was used to select the process variables and to compensate die profile surface. But the experimental trial-and-error method can’t be used to obtain the precision profile surface before single stretch bending forming practice.In this dissertation, at first, the basic concepts of stretch bending and the two urgent key technical problems needed to solve on stretch bending technology were introduced. Five kinds of forming defects which were easily produced during forming process and the current international research status on stretch bending about mechanical properties and cross-section shape、process parameters in experiment and numerical simulation were summed. The initial yield condition and the basic law including the flowing law and the strengthen law in elastic-plastic mechanics were introduced. The incremental stress-strain relations, which provided a theoretical basis for the subsequent mechanical analysis, were derived for isotropic hardening material. Secondly, the engineering mechanical model used for pre-stretch and bending and post-stretch, was established on a series of basic assumptions. In bending stages friction unit model and stress-strain analysis and bending moment was detailed. The analysis of stretch bending spring-back was introduced and the formula for calculating sheet and extrusions spring-back radius and angle was derived. Thirdly, the theory analysis of bending spring-back was combined with the numerical model of parts to form an computation method, the related formula were derived in detail, to modify the die profile curve which is fitted by arc spline. The modified die profile curve was continuous and its curvature was constant in separate segment along the entire profile. This method through the point’s density distribution on the die profile could control fitting error of the curvature of the arc spline. Finally, elastic-plastic FEM is used to compute the magnitude of the spring-back of the stretch bending forming of the extrusion, and to evaluate the reasonableness of the die profile compensated by the method formed. This work in the dissertation may provide a rational and useful method for improving the quality of stretch bending forming of extrusions and a useful tool for promoting computer aided design of the tools.更多还原