| Energy-saving and emission-reduction are the keynote of the automobile industry,and automobile lightweight is of great importance for the reduction of the vehicle weight.Currently,the application of unequal thickness sheets and multi-material sheets are the major ways for automobile lightweight.Meanwhile,resistance spot welding(RSW)is the main joining method in vehicle industry.However,the nugget shifting that happens during the welding of unequal thickness & dissimilar material sheets(general designation as dissimilar sheets)would be inimical to the joining quality of welds,which would lead to safety hidden hazard in car body.In this thesis,electromagnetic stirring is used to offset the nugget shifting in the spot welding of dissimilar sheets,and external magnet was asymmetrically arranged to form an asymmetric stirring inside the nugget.The nugget profile is modified under the magnetic fluid movement driven by the external magnetic force,which would offset the nugget shifting and improve the welds’ quality.In order to realize the magnetically assisted resistance spot welding(MA-RSW),following work have been launched in this thesis:1)The experimental study of the RSW process between the dissimilar sheets.The experimental system is established to study the welds of dissimilar sheets.The effects of thickness and material variation on the nugget morphology and mechanical performance have been studied,and based on this,the impact of nugget shifting on mechanical performance of welds have been studied.The results shows that the nugget shifting degree would increase as the difference of the thickness and material increase,besides,the peak load and failure energy of welds are in proportion to the nugget diameter and nugget penetration of the thinner(lower resistivity)sheet respectively.2)The mechanism of nugget shifting in RSW between dissimilar sheets.Finite element modeling has been established to study the nugget forming process of spot welding between dissimilar sheets,and the model is been validated through comparison of simulation results with experimental results.Hereafter,the mechanism of nugget shifting is been studied using this FEA modeling.The results show that the nugget shifting could be attributed to heat imbalance in the workpiece.Further,the essential reason is the thermal radiation intensity difference caused by unequal thickness and the heat generation difference caused by dissimilar material.3)The mechanism and key factors of MA-RSW.The external magnetic field distribution has been analyzed and compared,and the results show that the magnetic field which is axisymmetric about the central axis of the electrode arm and radial towards the center could generate circumferential magnetic force.A three dimensional electromagnetic FEA modeling had been constructed to calculate the external magnetic force,through which the key factors that affect the distribution of the external magnetic flux and force were revealed.The results says that the external magnetic force would increase as the working distance decreases or the wall thickness of the magnet increases when the height of the magnet is constant.Additionally,the strongest external magnetic force would be generated by the magnet with the height between 8~12mm.4)The improvement of MA-RSW on nugget profile,microstructure and mechanical performance.The results suggested that under external magnetic field,the nugget diameter increased by 7%~12% and the nugget penetration of the thinner sheet increased by 40%~80%.Moreover,the grain inside the nugget had been refined and the microhardness distribution was much more even;compared with traditional,the peak load of the MA-RSW had increased by 7%,and the failure energy of the MA-RSW had increased by 25%.Additionally,the MA-RSW welds tend to exhibit pullout failure rather than interfacial failure under the same welding parameter. |
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