Multimodal analysis of aluminum-steel dissimilar joint interface: Strength improvement mechanism through Ni addition

Abstract

To reduce vehicle weight and CO₂ emissions, multi-material automotive structures combining Al alloys with steel are in high demand. However, dissimilar joining of Al alloys to steel is still challenging due to the formation of a brittle intermetallic layer (IML) at the joint interface, which deteriorates joint strength. In this study, the effect of Ni on the joint strength, microstructure, and local mechanical properties of the IML of Al/steel joints was examined. Although the Ni increased the IML thickness, it significantly increased the joint strength by 138%. Multimodal analysis revealed that Fe (α) incorporated into the IML had a greater effect on joint strength than on the local IML properties: the presence of α promoted the formation of voids in the IML, deteriorating joint strength. FIB-SEM serial sectioning showed that Ni addition resulted in the formation of a dense IML with minimal α content, thereby suppressing void formation and improving joint strength. Local mechanical properties of the IML were unaffected by Ni, indicating that strength improvement was not due to changes in intrinsic mechanical properties. The findings suggest that Ni improved joint strength by modifying IML morphology rather than reducing its thickness, providing insights for designing robust Al/steel joints.