Numerical and experimental investigations on the enhancement of the tensile shear strength for resistance spot welded TWIP steel

Abstract

The microstructure manipulation by adjusting the content of alloy element plays a significant role in the material fabrication, and resistance spot welded joint has a crucial effect on the stable and safe operation of the vehicles. In the present study, a special method by simultaneously inserting the interlayers of Al foils and interstitial-free steel (IFS) is proposed to directly manipulate the nugget structure and enhance the tensile shear strength (TSS) owning to the change of heat and mass transfer. The welded joint in the original state (TWIP-A) possesses the TSS of 15.2 kN on average with an interfacial fracture (IF) mode while the TSS can reach 26.5 kN for the microstructure-manipulated specimen (TWIP-B) with a button fracture (BF) mode. The existence of the Mn element segregation and austenite with the columnar structure triggered the formation of IF mode for specimen TWIP-A. After adding the Al foils and IFS, the segregation of element Mn is reduced. The high welding current and long-term duration at high temperature are beneficial for the formation of the equiaxed grain zone (EGZ) in the nugget for specimen TWIP-B. The change of the structure morphology, strengthening effect by element Al and the relief of Mn segregation in the nugget for specimen TWIP-B can hinder the crack to propagate along the centerline, causing the failure mode transition to BF. The present study clearly shows the correlation between the microstructure and TSS and the strategy can guide the optimization of the RSW schedule of high strength steels and even their manufacturing process.