【作者】 侯志刚；

【导师】 陈传尧； 李春植； 王元勋；

【作者基本信息】 华中科技大学 ， 固体力学， 2005， 博士

【摘要】 薄板焊接结构在生产实践中有着广泛的应用,例如汽车的车架、车身等重要部件,都是由薄板型材或冲压件经过不同焊接方法制造而成的。焊接过程中产生的变形对这些部件甚至整车的质量都有重要的影响,因此,研究薄板结构焊接过程中的变形规律及其控制方法,对于提高产品质量,增强产品的市场竞争力,具有重要的意义。本文首先对薄壁箱形梁CO₂ 气体保护焊和薄板电阻点焊的焊接过程进行了有限元分析。在薄壁箱形梁的分析中,将断续焊接热输入简化为分段瞬时体热源,按照实际焊接顺序加载计算,得到了焊接过程中各时刻的温度分布。该温度场能够反映实际焊接过程的基本特征,可以作为焊接过程热弹塑性分析的依据。在薄板电阻点焊的分析中,采用热电耦合单元建立了二维轴对称有限元模型,将结合面和电极-工件界面的接触电阻简化为温度的函数,考虑随温度变化的材料热物理性能参数,分别对等厚度和不等厚度薄板点焊过程的瞬态温度场进行了分析。通过分析模拟了焊核的形成过程,并计算了焊核及热影响区的形状和尺寸。对不等厚度的薄板,首次通过仿真方法得到了因板厚不同造成焊核向厚板一侧偏移的结果,与实际的焊接过程相吻合。基于温度场分析的结果,对薄壁箱形梁和薄板电阻点焊的焊接过程进行了热弹塑性分析。在薄壁箱形梁的分析中,得到了构件的变形、应力和应变的分布及变化情况,并与实验结果进行了对比。对不同位置的焊缝中的残余塑性应变进行了详细分析,发现端部焊缝中的残余压缩塑性应变因受端部效应的影响,由端部向内部呈线性增加,根据该结果,可以对固有应变法的加载过程进行相应的修正。在薄板电阻点焊的分析中,得到了结合面和电极-工件界面上接触压力的分布和变化,以及点焊接头中的应力、应变和变形情况。对于等厚度薄板,由于结构是对称的,所以焊点局部的变形不会影响结构的整体变形。对于不等厚度的薄板,首次模拟了由于不对称造成结构向薄板一侧翘曲的结果,提出该翘曲变形是由不对称的残余塑性应变造成的,并详细分析了残余塑性应变的分布形式和产生原因,为采用固有应变法分析点焊变形奠定了基础。采用固有应变法对薄壁箱形梁和大型空间细长薄壁梁的焊接变形进行了分析。针对现有固有应变法的不足,提出了固有应变加载的温度载荷法,可以方便有效地实现分布式固有应变值的加载。利用单元坐标,实现了与整体坐标轴不平行的空间位置焊缝的固有应变加载。根据热弹塑性分析的结果,对端部焊缝中的固有应变值进行了修更多还原

【Abstract】 Welding sheet structures are widely used in many industries; for example, some important components such as automobile frame and body are produced by various welding methods with profiled sheet materials and stamping parts. The welding induced deformation has great influences on the quality of the components and even the whole automobile. Therefore, it is of great significance to research the law of welding deformation and its control method, in order to improve the quality of productions and its market competition capability. In this dissertation the transient thermal process of a thin-wall beam produced by CO₂ Gas Metal Arc Welding （GMAW） and sheet structures joined by Resistance Spot welding （RSW） were analyzed by Finite Element Method （FEM）. In the analysis of the thin-wall beam, the thermal input was simplified as transient section body heat sources and loaded as its actual sequence. The obtained transient temperature field can represent the basic characteristics of the real welding process and can be used as the foundation of thermal elastic-plastic analysis. In the analysis of the sheet RSW process, a 2D axisymmetric FEM model was developed using thermoelectric element. The contact electric resistance of the faying surface and workpiece-electrode interface was simplified as the function of temperature. Accounting for the temperature dependent material properties, the RSW processes of sheets with equal and unequal thickness were analyzed respectively. The growth of nugget was simulated and the geometry and size of the nugget and Heat Affected Zone （HAZ） were calculated. The simulation of the unequal thickness sheets showed that the nugget offset to the thicker sheet due to the different thickness, and the result was consistent with the actual RSW process. Based on the temperature field, thermal elastic-plastic FEM analyses were performed on the thin-wall beam and the RSW sheet structure. In the analysis of the beam, the distribution and change of the welding deformation, stress and strain were obtained and compared with the experiment results. Detailed studies were conducted on the residual plastic strain of welding seams at different locations, and it was found that the value of the welding seam at the ends of beam was linear increased. According to this result, an improvement could be made on the inherent strain method. In the analysis of the sheet structure, the distribution and change of the contact pressure at the faying surface and workpiece-electrode interface as well as the deformation, stress and strain in the joint were obtained. For the sheets of equal thickness, the local displacement had little influence on the structure’s global deformation. However, for the sheets of unequal thickness, the simulation result revealed that the structure distort to the thinner side due to the unsymmetry. It was assumed that the distortion is induced by the unsymmetric distribution of the residual compress plastic strain, which was analyzed carefully. This assumption was the precondition of the possibility to introduce the inherent strain method into the analysis of RSW process. Using the inherent strain method, the welding deformation of the thin-wall beam and a large-scale spatial beam were calculated respectively. A temperature loading method was developed to load the variable inherent strain value expediently. The loading of inherent strain value on spatial welding seam that was unparallel to the global coordinate axis was achieved with the application of element coordinate system. According to the results of thermal elastic-plastic analysis, a modification was made on the inherent strain value of the welding seams at the beam ends. Comparison with the experiment results showed that this modification could improve the calculation precision effectively. The inherent strain method was firstly introduced into the deformation analysis of RSW structure. According to the residual compress plastic strain results of thermal elastic-plastic analysis, the 2D axisymmetric FEM model was extended to 3D model step by step. Using the 3D model, the deformation of a RSW structure with three weld points was calculated. The result showed that the global deformation of the structure is not the linear accumulation of the single point, whereas it was significantly amplified. Having the capacity of taking into account the production and assembly errors, the inherent strain method provided a new technique for the deformation analysis of RSW structure. An experimental study was conducted on the CO2 GMAW process of the thin-wall beam. The welding deformation was measured and taken as the comparison of the theoretical analysis. The creation of inherent strain during the welding process was investigated using one dimension model under elastic and plastic constrain conditions. A criterion of eliminating welding deformation by reverse deformation method was presented: the stress resluted from the reverse deformation should reach the elastic limit of the material at the welding seam. Based on the criterion a formulation was established to calculate the reverse deformation value of beam. According to the formulation, an experiment was performed on the large-scale spatial beam to control its welding deformation. The results showed that the formulation could be effectively used in adjusting the value of reverse deformation, and the welding deformation of some locations could be reduced near to the tolerance after one adjustment.更多还原