Nugget Formation and Characterization of Effect of Cover Plate on the Formation of Solidification Defects in Resistance Spot Welding of Dissimilar Al6016/HC420LA Steel

Herein, 6016 aluminum alloy is joined to HC420LA high‐strength steel by resistance spot welding technology with two kinds of cover plates. The morphology and microstructure of the joint are observed by optical microscopy and scanning electron microscopy. The formation mechanism and control methods of solidification defects such as shrinkage cavities and cracks are discussed. The results show that independent nuggets form on aluminum alloy and high‐strength steel side, respectively, when two kinds of cover plates are added. In some special conditions, the aluminum/steel interface presents a double‐peak shape because of the imbalance between the stress from molten aluminum and that from applied force. Meanwhile, a large shrinkage cavity and longitudinal crack originate from the shrinkage cavity form in the nugget on the aluminum side. All these defects generate in the process of solidification, and a model is established to explain the formation mechanism of solidification defects. Moreover, the control methods to prevent the solidification defects are discussed. Based on the model and control methods, a good joint without solidification defects is produced.