| Contact resistance is one of the main factors which influence the quality of weldment, the burning loss degree and life of electrode, etc. The models of contact resistance built as yet are difficult to describe the effect of surface morphology of workpiece and electrode, the physical properties of welding materials and the welding parameters on the contact resistance. To build a contact resistance model including the factors mentioned above, a series of studies around the contact resistance are performed. The main results obtained in this thesis are listed as follows.At first, the revised Weierstrass-Mandelbrot fractal function is utilized to characterize the surface morphology of electrode and workpiece, according to the fractal theory, the elastoplastic contact model at the early stage of resistance spot welding of aluminum alloy is setup, based on which the microcontact behaviors at the early stage of resistance spot welding of aluminum are analysized, but also the influences of all kinds of material physical parameters on the microcontact behavior are studied. It is found that the parameters of fractal dimension and fractal characteristic length characterized the surface morphologies of workpiece and electrode produced by different processing methods. The surfaces of electrode and workpieces are multifractal. For given load and interface, when fractal dimension changes from 1.30 to 1.90, the contact rate reaches the least value at 1.50. For aluminum alloy, the contact rate of Electrode/Workpiece is less than that of Workpiece / Workpiece.Secondly, based on the results of contact analysis, the network of contact resistance without oxide film on the surface is constructed, the Cantor set is applied to the crack of oxide film, the Cantor crack model of oxide film is presented, the fractal contact resistance model of aluminum under the contact load in which the effect of oxide film is considered is setup, the contact resistance formula with the oxide film is derived. The fractal contact resistance model is verified with the mono-plate and bi-plate methods, the results show that the resistance curve of this model agrees well with that of the experiment, and it reflects the negative exponent relationship between contact resistance and contact load. After detailed analysis, it is found that the model is the general case of two-level and three-level contact models built before, the model reflects a general law. The electrical contact resistance built combined with the contact analysis are utilized to describe the formation of nugget, it is found that the nugget initializes on many contact spots at the same.Finally, with the contact resistance caculated, the temperature field and the size of nugget are simulated using finite element method, the relationships between welding parameters, nugget size and the tension-shear strength are studied by artifical neural network. The results show that the nugget grows radially at first, and then expands in the direction of thickness. The spline fitting method can't reflect the nonlinear relationship between the welding parameters, size of nugget and the tension-shear strength of nugget, but the BP artifical neural network can. The tension-shear strength is predicted using two BP neural networks with three-layers, the predictive error is about 8%, it means that the predictive result is reasonable, and the BP network can be used to predict the tension-shear strength and the size of the spot nugget.
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