Research On Performance Characteristics And Welding Mechanism Of Resistance Spot Welding For Dual Phase Steel

Dual phase steel is a kind of new material developed to meet the needs of lightweight technology and safety of automotives. This paper researched the performance characteristics and welding mechanism of the resistance spot welding for DP600 steel with a thickness of 1.7mm.Firstly, the effects of electrode force, welding current and welding time on the welding nugget size, tensile shear load, failure energy and failure mode of spot welded DP600 joints were studied by static tensile-shear test. The relationship between welding nugget size and mechanical properties were established. The results showed that a larger welding nugget size resulted in better mechanical properties. For small and large welding nugget sizes, the tensile shear strength and the failure energy are more sensitive to the welding nugget size than for medium welding nugget sizes. Based on a more realistic "drum" nugget model, the quality evaluation model of resistance spot welding for dual phase steel was constructed to ensure the welding quality and reduce the welding cost.Secondly, the effects of electrode force, welding current and welding time on the microstructure, welding defects and micro-hardness of spot welded DP600 joints were studied by microscopic experiments. The results showed that the electrode force had a weak effect on the welding nugget micro-hardness, the welding time had the most significant effect on the welding nugget micro-hardness, and the welding current influenced the welding nugget micro-hardness moderately. Then a relationship between welding parameters, micro-performance and mechanical properties was established, which showed that the welding nugget size, micro-hardness and welding defects were controlling factors of the quality of resistance spot welded DP600 joints.In addition, based on the experimental results and two-dimensional axisymmetric model, the paper analyzed the pre-pressure contact behavior of resistance spot welding for dual phase steel, derived the finite element equation of pre-pressure contact, and determined the initial conductive region. The thermo-electricity transient and thermal elastic-plastic mechanism of resistance spot welding for DP600 were analyzed by indirect coupling method and thermal elastic-plastic theory. By analyzing electric field and temperature field, the wave curve of welding nugget size was obtained under 50Hz AC, which provided a theoretical basis for controlling the welding nugget size. And by analyzing the contact pressure, stress and strain distribution, the deformation mechanism was revealed. It was concluded that the workpiece surface indentation and the "wedge" deformation were caused by the plastic strain of heat affected zone, and the deformation of workpiece area away from the welding range was not obvious. This result provided a theoretical guidance for the welding deformation controlling of dual phase steel.The contrast between numerical simulation and experimental results showed that, the simulated nugget sizes were consistency with experimental values under appropriate current, and there were significant errors when the current was too large. And the simulated deformation shape also showed good consistency with the experimental results. These showed that the analysis results of welding mechanism of the RSW for DP600 has good accuracy.Finally, based on the results, the relationships of the performance characteristics and welding mechanism were established. This study has practical significance for comprehensive understanding of the mechanism of RSW process as well as the application of dual phase steel and the quality controlling in the spot welding of automotive body.