| Aluminum alloy is an important material for lightweight vehicle body. The spot welding aluminum alloy has the features high thermal efficiency, high degree of automation and high degree of production flexibility, which has a wide range of applications in producing automotive, aviation and aerospace products. As the aluminum has such characters: high conductivity and high thermal conductivity, large thermal expansion coefficient, low melting point, active chemical properties, coated with high dense and high melting point oxide film which is almost non-conductive, easily reaction with the copper electrode, the technology of aluminum alloy spot welding of thin plate is seriously affected. In order to improve the welding technology and enhance the quality of the spot welding joint, the following work has been studied:Firstly, by using the numerical simulation methods, the microstructure of the welding joint, the nugget form and the distribution of the residual stress have been studied deeply. The microstructure distribution of the nugget, the shape of the nugget and the stress state are the inner factors of the mechanical properties of the spot welding joint. In this paper, the heat, electricity, force, metallurgical coupled model of SPW and the phase-change model of aluminum alloy are used on the numerical simulation on the microstructure of the nugget. Based on the results, the different between the nuggets of two main kinds of aluminum alloys are predicted. Several types of molten core are calculated and analyzed in this paper under the same condition. The stress states of all the models are studied. Based on the results of the simulation, the forming of these models is analys. Using the model of the SPW, the residual stress of the nugget and the heat affect zone is studied with welding parameter verified. All these work provide a theoretical basis for strengthen the welding nugget by means of welding process design.Secondly, by using the experiment test,the changes in the internal microstructure of nugget and changes in mechanical properties of the joints have been studied deeply. The mix distribution of dendritic structure and the Non-dendritic structure caused by the electromagnetic phenomena during the SPW in the nugget is found. And the test design and the tests are carried out for spot welding 6082 aluminum alloy. The changes of the microstructure in the nugget and the mechanical performance of the welding joint are studied. All these work provide a theoretical and experimental basis for strengthen the welding nugget by means of the physical methods.At last, by using the method of adding the large resistance medium in the welding process, the microstructure of the welding joint is improved, the strength results are studied and explained, and many mechanical properties tests carried out. Strengthen with alloy element is an effective method for strength the aluminum alloy, and it is also an important method for strength the welding nugget. In this paper, the stable distribution is firstly used to describe the distribution of the initial resistance between the electrodes. And based on the character of the stable distribution, the calculation method of the initial resistance between the electrodes is deduced when welded with the high resistance media.And many medium including Ti, C, Y, Er, CrAlY have been tested for studied the strength result to the nugget. All these work provide a theoretical and experimental basis for strengthen the welding nugget by means of the chemical methods.
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