Numerical Simulation Of Steel-Al Resistance Spot Welding And Studies On Its Contact Surface Micro-mechanism During Pre-Cleaning Schedule

Steel-aluminum hybrid structure car body is the top priority of auto lightweight technology.Resistance spot welding(RSW)has an irreplaceable position in the automotive industry because of its advantages of high efficiency,low cost,high automation and good adaptability.It is one of the key technologies to accelerate auto lightweight process by using RSW technology to obtain a stable and reliable steel-aluminum joint.Due to the great difference of physical properties between steel and aluminum,coupling with the electrical,thermal and force transient non-linear characteristics of RSW,the steel-aluminum RSW process is extremely complex.Each welding parameter not only affects the formation and growth of intermetallic compounds(IMC),but also affects the stability of the welding process,which has a direct effect on the performance of the welding join.It is of great research value and prospect value to promote the wide application of steel-aluminum RSW by studying the physical phenomenon during RSW,analyzing and mastering the effects and the change rules of various factors in RSW,exploring the effective means of controlling welding process stability and improving joint performance.In this research,we has established a more precise numerical model of steel-aluminum RSW,which changed the welding parameters and the top electrode surface morphology according to the latest experimental conditions and did comprehensive improvements and fine calibration in model operation mode,faying interface setting,material performance parameters and boundary conditions on the basis of existing models.The new improved numerical model can solve many problems in the original model from aspects of convergence,adaptability and accuracy,which makes the analysis from simulative data more reliable.Based on the improved model,we analyzed the important physical phenomenon in pre-pressing,pre-cleaning,welding and maintaining process and studied the influences of the factors such as cooling medium,electrode grinding degree,electrode wear degree,welding ambient temperature,electrode pressure and pre-cleaning parameters respectively.At last,we found a dynamic negative feedback mechanism in steel-aluminum RSW and described the influences of electrode force and pre-cleaning parameters on welding process.In this paper,we systematically studied the pre-cleaning process of steel-aluminum RSW by numerical simulation method and revealed its micro-mechanism for the first time.We also proposed a "high current-multipulses" pre-cleaning mode for suppressing the Al-Steel faying interface temperature and preventing excessive thickening of IMC after doing the research in ’zinc-exclude’ mechanism.Through numerical simulation and practical welding experiment,we found the "high current-multipulses" pre-cleaning mode not only facilitates the zinc-exclude occurring in the pre-cleaning process,enlarges the Al-Steel contact area,improves the micro-contact behavior,effectively controls the heat generation of Al-Steel contact surface during pre-cleaning process,but also makes the internal electric field distribution of steel and aluminum plates more uniform during the subsequent welding process,effectively controls the heat generation inside the steel plate,suppresses the excessive increase of the faying interface temperature and the IMC thickness.