In analyzing resistance spot welding, it is not too much to say that the main topic up to date is the realization of Joule heating and contact resistance. These researches resulted in the temperature distribution and the nugget size of spot welded structures. However, they were hardly concerned with the tensile residual stress and distortion increasing the maximum stress of spot welded structure, when enforced by the external load. This thesis represents the correlation between residual stress and welding conditions, and shows the sufficient conditions minimizing the deformation of spot welded structure.
Tensile residual stress of resistance spot welded structures increases the maximum stress and tends to reduce the fatigue strength. Due to these reasons, it is necessary that the welding process is optimized to minimize the tensile residual stress. This thesis contains the residual stress obtained from electrical-thermal-structural analysis and the parameters that have effects on the residual stress. The various properties of materials were considered dependent on temperature, and the contact resistance was given in the function of temperature. Firstly, assuming the bilinear isotropic material in elasto-plasticity, the simulated residual stresses have good agreements with the measured data when the hardening tangent, as a parameter, is supposed to be one percent of the elastic modulus. As another result, the contour of residual stress around a spot weld can be drawn within the weld lobe, and the residual stress can be represented with welding conditions on the three dimensional regression plane. After all, the equations for the residual stress are developed in terms of welding conditions, and used to find the welding conditions that minimize the residual stress. The simulated results of residual stresses were compared with the data measured by XRD, and investigated on the factors that affected the residual stress, for example, welding current, welding time, electrode force, and hardening tangent.
The deformation due to spot welding can be minimized by controlling the welding conditions and the positions of spot welds. A new method is proposed, analyzing the deformation due to spot welding, and the minimization of deformation on the spot welded structure is introduced. In order to analyze the spot welding process, the finite elements are axisymmetrically modeled, and the results from them are used for analyzing the deformation of spot welded structure. The deformation of multi-spot welded structure is obtained by the combination of results from analysis of single-spot welded structures. The positions of spot welds are used as parameters to minimize the maximum deformation of spot welded panel. It can be represented that the optimized positions of spot welds minimize the deformation due to spot welding.
