Evolution mechanism of geometry morphology for metallic bump assisted resistance spot welded (MBaRSW) joints

Abstract

Abstract Successful joining between aluminum alloy and bare steel was achieved using metallic bump assisted resistance spot welding (MBaRSW) process. Moreover, the MBaRSW welds owned much different nuggets, e.g., geometry morphology and size, from the traditional RSW welds. To reveal the different mechanism in nugget morphology, a fully coupled electrical-thermal-mechanical finite element (FE) model was established on the commercial ANSYS/Multiphysics (Education edition) platform. The uniaxial tension testing was conducted at different temperatures for each material to be welded, and the interfacial contact resistance was measured at room temperature and at different pressures. Moreover, the interfacial contact resistance at elevated temperatures under different pressures were calculated. Based on the experimental results, the FE model was validated in terms of bump morphology after squeezing stage and weld morphology after welding stage, respectively. Results showed that the FE model in this study owned relatively high accuracy in predicting the morphology of Al-steel RSW welds and MBaRSW welds, and the simulated nugget, expressed by the temperatures higher than the melting point of aluminum alloy, owned the similar variation law to the experimental nuggets. Compared to the RSW weld, the much smaller melting zone of both the Al and steel sheets could be observed in the MBaRSW weld, indicating that the heat affected zone (HAZ) in the MBaRSW process was obviously decreased, contributing the welded joints to maintaining the original properties of base metal. At early welding stage, the current density in the MBaRSW process was obviously lower than that in the RSW process with larger conduction path, resulted from its obviously higher contact resistance after introducing the metallic bump, lowering heat input and delaying nugget initiation. With increasing welding time, the increasing heat input softened the workpieces, causing very low interfacial contact resistance. The smaller conduction path in the MBaRSW process caused larger current density, resulting in larger nugget growth rate and thus the larger nugget after welding. As a result, the initiation of the MBaRSW nugget was delayed by adding the metallic bump, its growth was promoted, however, in the later welding stage.