Microstructural evolution and mechanical performance of friction stir spot welded ultrafine carbide-free bainitic steel: Role of dwell time

This study focuses on unraveling the role of dwell time on microstructural evolution, mechanical performance, and failure characteristics of friction stir spot welded ultrafine carbide-free bainitic steel. Results clarify that the microstructures of the different weld zones exhibit a receptive behavior to extended dwell time, by which not only coarser martensite is acquired within the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and fine-grain heat affected zone (FGHAZ) but also sub-critical heat affected zone (SCHAZ) experiences a more pronounced tempering. Applying a dwell time of 3 s is sufficient to make an ample bonding width containing refined martensite, being effective in retarding the crack propagation, and in turn, delivering a joint with the highest load-bearing capacity. However, enlarging the bonding width at dwell times beyond 3 s fails to provide higher peak loads owing to the formation of coarse martensite within the SZ. The failure modes of the welds are classified into two distinct modes: interfacial and partial pullout. The reduced upper sheet thickness (∼685 μm) paves the way for crack propagation along the thickness direction, allowing for partial pullout failure to occur.