Numerical Studies Of Al-Steel Resistance Spot Welding Process With Multi-Ring Domed/Ballnose Electrodes

Researchers in General Motors have designed a new electrode called Multi-Ring Domed(MRD)electrode to conduct Al-steel resistance spot welding(RSW).This electrode is obtained by cutting out five concentric rings on the surface of ballnose electrode.It can break the oxide film on the surface of Al more easily and decrease the contact resistance at the interface.Since there is little research about resistance spot welding process using MRD electrode,it has theoretical value to study the physical phenomena during the process of RSW with MRD electrode by numerical simulation.The finite element model was developed by ANSYS APDL to simulate the welding process of Al-steel RSW with MRD / ballnose electrode in the study.MRD electrode was used at Al side and ballnose electrode was used at steel side.The model has optimized meshing and geometric profile of the rings,selected reasonable material properties,and considered the effect of MRD electrode on contact resistance by breaking the oxide film on the surface of Al.The modeling results of welding voltage,central thickness of sheets and Al nugget diameter were close to the experimental measurements,which validate the accuracy of the modeling results.The mechanical field and electric-thermal field were analyzed based on the modeling results.The relation between hardness and thermal history of the Al heat affected zone(HAZ)was studied and the thickness of intermetallic compounds(IMC)at Al-steel interface was predicted using the thermal history at the interface.The results showed that the two rings close to the center had the most deformation and the most obvious cutting effect to break the Al oxide while the outermost ring nearly had no influence.The heat generation was dominated by contact resistance at start and then became to be dominated by bulk resistance with increase of time.The weld schedule used in the study was well designed to keep the temperature at the Al-steel interface stable and low during the welding stage,which could prevent over-growth of IMC.In Al HAZ,temperature decreased while hardness increased from inner to outer area.The modeling thermal history at Al-steel interface was used to predict the thickness of IMC and the predictions were close to experimental measurements.