Study on the microstructure and mechanical performance for integrated resistance element welded aluminum alloy/press hardened steel joints

Abstract

Resistance element welding (REW) is a recently developed hybrid joining process for aluminum (Al)/steel dissimilar materials. In this study, the microstructure and mechanical performance of the Al/press hardened steel (PHS) joints generated using an integrated REW process were investigated. The microstructure of fusion zone (FZ) between rivet and PHS was lath martensite, which was transformed from the fast cooling of austenite. The Al could be divided into four zones according to the microstructure and microhardness distribution: re-solidified zone (RZ), softening zone (SZ), transition zone (TZ) and base metal (BM). The dissolution and coarsening of the precipitates are responsible for the hardness reduction of Al sheet. The variation of mechanical performance was explained in light of the increasing FZ size and the softening Al sheet as heat input rising. Moreover, four block shear models were introduced to predict the peak load of REW joints using the average hardness of SZ in Al sheet, among which the models from Architectural Institute of Japan (AIJ) provided a relatively good prediction. Considering both the average hardness of SZ and sheet thickness, an analytical model was established to predict the variation of critical nugget sizes and explain the failure mode transition.