Characterization of intermetallic compound at the interfaces of Al-steel resistance spot welds

This study characterizes the interfaces of Al-steel joints generated by resistance spot welding (RSW) processes. Two types of intermetallic compound (IMC) layers at the interface are characterized and discussed. The first type, located in central area of the interface, is composed of tongue-like Fe2Al5 adjacent to the steel and serrated-like FeAl3 adjacent to the Al; the second type, a mixture of FeAl3 and Al, lies in the periphery of the joint interface. Formation mechanisms of the two types of IMC layer observed are proposed and discussed in the paper. The thickness distributions of the IMC layer generated by different welding parameters are predicted based on the dynamic interfacial temperature histories from Al-steel RSW process simulation. The predicted IMC thickness distributions are validated against physical measurements and show good agreement. A bimodal IMC thickness distribution is found when an adequately long welding time is applied. This is found to be related to lower interfacial temperature at the center of Al-steel interface than what in the surrounding area after about 200 ms of welding due to strong cooling effect by the electrode.