Prediction And Control Of Welding Deformation Of Sheet Structures

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.