Numerical Simulation Of Dissimilar Al-Steel Resistance Spot Welding And Studies On Its Interfacial Behavior

With more and more introduction of aluminum sheet components into body-in-white to enhance lightweight of automobiles,the technology of join Al and steel has been a focus of research.Compared with some novel technologies of spot welding,resistance spot welding?RSW?has a couple of advantages such as high depth of automation,low cost and high efficiency,besides,it enjoys all of the developed and available equipment,which makes it the first option to join Al and steel sheet components for the manufacturing of car body.However,as the result of sharp differences in physical properties and mutual metallurgical incompatibility of Al and steel,there exists a layer of hard and brittle intermetallic compound?IMC?at Al-steel interface along with multiple defects near the interfacial area,which could largely deteriorate mechanical performance of the joint.Thus,to systematically analyze various physical phenomena involved in the process Al-steel RSW;to study the initiation and growth mechanism and dynamics of IMC,which is one of the most important factors determining final mechanical performance of the joint and the effect of welding parameters on IMC thickness distribution could offer some instructions to future parameters optimization or electrode design.In this study,a numerical model,based on APDL language,to simulation the RSW process of 6022-T4 Al alloy and HDG DC07 steel has been established on ANSYS.The model sequential couples the mechanical and electro-thermal model and takes into consideration the effects of temperature,pressure,coating and oxide on the value of interfacial electrical and thermal contact resistance.The numerical model at last is validated in the aspect of energy input,process of nugget growth and joint deformation,and it is found out that it predicts very well in the above-mentioned aspects.Some important physical phenomena in the stages of initial squeeze,preheat,welding and hold have been demonstrated and analyzed,including the pressure distribution along three interfaces in initial squeeze,temperature distribution along Al-steel interface in preheat,Al nugget growth in welding and factors determining final solidification microstructures in hold stage.Besides,energy generation and redistribution of the whole process has been calculated and analyzed,and it is found that steel plays as“heat source”in the welding assembly and it keeps providing energy to Al through welding and hold process,which facilitates Al nugget and temperature gradient within it to form.Finally,some research has been done on the behavior of Al-steel interface.Two kinds of IMC which have distinct characteristics have been discovered at the Al-steel interface of the joint by welding time of 800ms and current of 13kA.The first kind,as the result of reaction between of liquid Al and solid steel,is composed of thicker tongue-like Fe₂Al₅ and thinner serrate-like FeAl₃,while the second one consists of a mixture of needle-like FeAl₃ and Al and it grows with the reaction diffusion of solid Al and steel.Based on the interfacial temperature history at Al-steel interface and mathematical model for the growth of Fe₂Al₅,IMC thickness distributions by different welding parameters along the interface have been predicted well,and it has also been revealed why IMC thickness distribution tends to be“M”shaped with the increase of welding time.