【作者】 梁彩平；

【导师】 林忠钦；

【作者基本信息】 上海交通大学 ， 车辆工程， 2008， 博士

【摘要】 汽车轻量化是目前汽车设计制造的发展方向之一。液压成形管(Tubular Hydroforming)作为一种新的结构工艺零部件,以其低零件数、低组装成本和高结构完整性等优点在轻量化车身结构中逐渐被广泛应用。然而,在车身制造装配过程中,如何将不规则的液压成形管件与车身薄板进行有效连接并保证接头强度是亟待解决的关键问题。电阻点焊工艺由于其高效率、低成本的突出优点一直是轿车车身制造与装配中的首选连接方法。管板单面电阻点焊因其焊接过程中双面加压单面成核的特点,是实现液压成形管件与车身薄板连接的新方法。然而,管板点焊过程中由于管的刚度较小,在气动焊枪恒定压力作用下,其形成的焊接质量很难保证。目前,针对管板单面电阻点焊熔核形成机理及焊点强度预测等研究,仍旧缺乏有效的研究手段和焊点质量控制方法。针对上述问题,本文首先对管板单面电阻点焊与传统板板点焊的特点进行了对比分析,指出采用伺服焊枪更有利于管板之间的焊接,并建立了伺服焊枪管板单面电阻焊试验系统;通过建立多场耦合作用下管板单面电阻点焊熔核形成过程的有限元仿真模型,揭示管板焊接过程环形熔核形成机理;采用拉丁正交超立方法试验方法,建立管板焊接环形熔核拉剪强度预测模型;在此基础上进行理论分析和试验研究,获得不同焊接工艺参数对焊点强度的影响规律;针对管板焊接结构特点及工艺参数影响规律,利用伺服焊枪变电极压力控制特性,提出通过减小焊接过程中的电极压力来增加管板焊焊点强度的方法。论文的研究成果为提高管板点焊接头质量和实际应用提供理论基础和参考。具体开展了以下几个方面的研究工作:(1)管板单面电阻点焊物理过程和试验系统在回顾传统板板点焊过程的基础上,分析了管板单面电阻点焊的特点,由于管的刚度较小,在焊接过程中存在大变形,将直接影响到熔核的生成过程和熔核形态,探讨利用伺服焊枪的柔性控制特点进行管板单面点焊的可行性,并建立集成伺服焊枪、机器人和焊接控制器的焊接试验系统;设计专门的柔性的焊接夹具用于管板焊接中焊接件的固定;采用激光位移传感器,并通过采集卡和串口与电脑连接,建立了电极位移采集系统,用于焊接过程电极位移的采集。(2)管板单面电阻点焊环形熔核形成过程多场耦合分析建立电、热及结构耦合的管板单面电阻点焊熔核形成过程有限元模型,对焊接过程的预压与通电过程进行仿真计算。研究发现,预压阶段,在电极和板以及板和管之间已形成了环形的接触区,电极和板间的压力较集中,压力值大,管和板间压力区域相对较宽,压力值较小,同时在该接触面的远离轴线处存在一个压力值很小的“副接触区”。焊接通电初始阶段,在电极端面边缘和板的接触处首先出现一个高温区,这将直接加重管板焊接中的电极磨损;通电过程中,电极和板以及板和管的接触部分存在向远离轴线方向的偏移过程,而温度场的高温区也随着接触区的变化动态地改变着,两者相互作用,相互影响,使熔核的形成较困难,焊接所需通电时间相对较长;“副接触区”导致通电过程中一直存在分流现象,使板上表面将产生局部高温区,容易发生飞溅;在管和板的接触面上最终形成环形熔核,和传统的熔核相比,其焊透率较小。(3)管板单面电阻点焊环形熔核力学性能预测研究由于管板焊熔核形状的特殊性,传统焊点质量评判的公式已经不能适用该焊核。文中建立了单次拉剪试验有限元模型,采用拉丁正交超立方方法设计试验,对仿真拉剪试验的结果进行回归分析,研究了试件的材料属性、几何尺寸及形成的熔核尺寸对焊点强度的贡献程度,建立了焊点强度和熔核尺寸之间的关系式,为焊后通过环形熔核的外表特征判断熔核是否合格提供依据。(4)焊接规范参数对管板单面电阻点焊焊点质量影响研究基于前两章的模型和研究结果,理论上分析了焊接过程中点焊的工艺参数(包括焊接电流、通电时间和焊接压力)对熔核形成过程的影响,并通过试验研究了焊接参数等对焊点质量的影响规律;在此基础上,研究了不同结构尺寸(包括管剖面宽度和管厚度)的焊接件焊接时对工艺参数的要求,为管板焊实践中选择焊接参数提供理论依据。(5)基于变电极压力的管板单面点焊焊点质量控制方法基于上述仿真分析和试验研究,利用伺服焊枪变电极压力控制特性,提出通过改变焊接过程中的电极压力来提高管板焊接质量的方法。分别对焊接过程不同阶段的电极压力进行了变化,并对压力改变后获得的焊点质量进行对比分析。结果表明,焊接通电过程的电极力减小可明显提高管板焊焊点的拉剪强度,减小焊接过程焊接件的变形,改善焊后焊件表面的质量,提高管板焊的可焊性。本文研究以数值模拟结合试验进行分析,对管板焊接过程、焊后熔核的力学特征及质量影响因素进行了系统的研究,研究结果对管板焊接过程的进一步认识及生产线上广泛的应用具有重要的借鉴和指导作用。因此,本文具有重要的理论价值和实践意义。更多还原

【Abstract】 Concerns about fuel economy, safety, reliability and performance are continuously driving auto designers to use new materials, structures and techniques in auto designs. Hydroforming tubes as new structures are being currently used to manufacture several automotive parts included in car body systems due to its lower weight and increased structural integrity. How to integrate the hydroformed tubes without flanges into vehicle body structure becomes a new challenge.At present, resistance spot welding (RSW) technology is the most dominating joining and assembly method in auto body manufacturing process. As a result, it is significative to join sheet to tube using RSW technology. An innovation welding method——single-sided spot welding (SSSW) is developed to perform the joining of sheet and tube. However, new challenging issues limit the use of SSSW in auto industry. The SSSW approach attaches a sheet to a closed tube without inside support. A bottom block as an outside support is used for force transmission and current flow. The weak structure is the clearest characteristic of the sheet-to-tube SSRSW. As a new type of resistance welding process, the nugget formation mechanisms and its intensities of SSSW are greatly different from that of conventional RSW. The quality of the ring nugget is difficult to be guranteed under constant electrode force using traditional pneumatic gun.Depending on the issues mentioned above, this dissertation, firstly, analysed the difference between the sheet-to-tube welding and conventional sheet-to-sheet welding. Based on the characteristics of SSSW, a welding system using servo gun is built. Secondly, a multi-physical coupled finite element (FE) model for SSSW is proposed to study the nugget formation process based on commercial FE code ANSYS. Thirdly, a set of tensile-shear tests were designed based on Latin hypercube sampling and were performed by FE method. The objective is to research the effects of the physical and geometrical factors on the strength of the ring nuggets. Forthly, the effects of the welding process parameters on the quality of nuggets were deduced based on above models and the correlation rules were revealed by experimental methods. Last, considering the forcing characteristic of servo gun and the progress parameter rules of SSSW, a new method that the quality of SSSW welds could be improved by verifying electrode force during welding stage was presented. All those are critical to the research and application of SSSW. The following five works were done mainly:(1) SSSW physical process and testing systemThe structural stiffness of SSSW is much smaller than that of the conventional RSW. Consequently,there exists large deformation during welding process, which influences directly the nugget formation process and nugget form. The smooth feature in applying electrode force of servo gun meets the need of SSSW. So, servo gun experimental system including servo gun, robot and welding controller was set up. In order to fix the weldments during SSSW, a special flexible clamping fixture was designed. In addition, an acquisition system of electrode displacement including laser sensor with high precision, acquisition processor, and computer was built to supply testing parameters for verifying the solution of simulation.(2) FE multi-physics analysis of SSSW ring nugget formation processA parametric electro-thermal-mechanical coupled FE model was built to research the squeeze and welding process. Results show that two ring contact regions come into being at the interface of both electrode/sheet and sheet/tube. Smaller contact area and greater pressure is obtained around the bottom of electrode than that between the sheet and tube. In addition, there exists an obvious minor contact region between sheet/tube along the faying surface far away from the main contact region and its pressure is much smaller than that of the main contact region. At the beginning of welding,there is a high temperature region between electrode and sheet, which may accelerate the wear of upper electrode. During the welding stage, the high temperature softens material and more fierce deformation occurs, which leads to the contact region and temperature zone at sheet/tube interface transferring outward slowly and makes it difficult to form nugget. Owing to the shunting by minor contact region and the dynamic change of deformation, expulsion exists. At the end, a ring nugget formed.(3) Investigation on the mechanical performance of ring nugget in sheet to tube joiningIn SSSW for sheet-to-tube, a ring nugget is obtained. The form and size of ring nugget are different from that of button nugget in two sheets joining. Conventional button nugget quality inspection is not preferred for ring nugget quality prediction. In this chapter, a detailed computer model of ring nugget formed in sheet-to-tube joining was presented and a computer design of experiments method was used for carrying out to evaluate the strength of spot weld. The regression model between weld quality and weld attributes were established, depending on which the ring nugget strength could be estimated.(4) Study on the influencing factors of weld quality for SSSWBased on the above models and results, the effects of process parameters including welding current, time, and electrode force on the nugget formation were analyzed theoretically and tentatively. Aimed to get good welds, the characteristic of process parameters are achieved for workpieces with different physical dimensions including the width of tube cross section and the thickness of tube. All those are critical for practical application of SSSW.(5)Weld performance optimization of sheet-to-tube single sided spot joining based on electrode pressure changeDepending on the characteristics of servo gun and the effect rules of weld conditions on the weld strength of sheet-to-tube joining, it is investigated that the weld performance can be increased by changing electrode pressure during welding progress. The comparations among the weld performances with different electrode pressure styles are carried out. The results indicate that the electrode pressure decreasing during current stage can increase the strength of sheet-to-tube weld clearly, decrease the deformation of the weldment during the weld stage, improve the surface quality of weldment, and upgrade the weldability of sheet to tube.The dissertation researched and revealed the nature rules during SSSW process, the mechanical properties of nugget in sheet to tube joining, and the influencing factors of weld quality on numerical analysis and some necessary experiments validation. The research results have significant guidance meaning in further understanding SSSW process and its widely application in auto production-line. Above all, the research fruits in this dissertation have very important value and practice significance.更多还原