Study On Nucleation Mechanism Of Unequal Thickness Resistance Spot Welding Based On Multi-field Coupling

Resistance spot welding(RSW)is a complex physical process involving multiple physical fields such as force,electricity,heat,magnetism,and flow.Through the interelectrode pressurization and the instantaneous energization,the workpiece contact surface instantaneously reaches a high temperature,and the metal is melted and cooled to solidify to form a joint.RSW is widely used in practical production due to its flexible and efficient characteristics.Electromagnetic stirring technology is a process of agitating the molten pool by magnetic force and reshaping the crystal nucleus.This technology has been successfully applied in the fields of continuous casting and arc welding.At the same time,electromagnetic stirring has gradually demonstrated its great potential in resistance spot welding in recent years.The quality control of the RSW head is directly related to the safe use of the joint,which is a key part of the spot welding research.In particular,in the spot welding of thick plate parts,it is a major research topic to improve the phenomenon of nugget shift.In the study of RSW problems,the invisibility of spot welding has caused enormous research obstacles.The development of numerical simulation provides an idea for studying the spot welding process.However,the traditional spot welding simulation model is difficult to predict the RSW problem involving magnetic fluid.In this paper,the ANSYS numerical modeling method is used to couple the five fields of force,electricity,heat,magnetism and flow in the magnetically-assisted RSW(MA-RSW),and the internal mechanism and relationship between each physical field change are analyzed.The accuracy of the model is verified by experiments,and theoretical support and technical guidance for the quality control of MA-RSW are proposed.Through the above research,this paper mainly produces the following results:(1)Modeling analysis is carried out by means of step-by-step calculation and fieldby-field coupling.The establishment of deformation field,electric heating field,magnetic field and heat flow field considers the inheritance before and after data.The analysis results show that the analysis error of the five-field coupled model is less than 5%,and in the MA-RSW simulation,the simulation accuracy is much higher than the traditional three-field coupling model.(2)By analyzing the whole physics in the unequal plate,it is found that the nugget offset in RSW of unequal plate is the result of the interaction of the flow field and the temperature field.In MA-RSW,the approximately mirror-symmetric flow field behavior mode pushes the high-temperature metal in the central region toward the edge,so the edge of the nugget has a tendency to grow toward the side with high magnetic field strength,and accordingly,by applying on the side of the thin plate a larger magnetic field strength distribution can adjust the offset of the nugget.(3)The performance of the MA-RSW joint is obviously superior to the traditional RSW.The agitation of the external magnetic field refines the joint grain and makes the joint component more uniform.The tensile strength of the joint increases with the increase of welding current.The diameter of the critical button fracture nugget is 4.62 mm,which is smaller than the traditional spot welding.The joint structure is a uniform austenite matrix and fine ferrite between them,and the hardness distribution is uniform,which exhibit excellent joint performance.