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

• Published in: Journal of materials research and technology Vol. 33; pp. 4033 - 4046
• Main Authors: Kabirmohammadi, Maryam, Yazdani, Sasan, Saeid, Tohid, Pouranvari, Majid
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-2024
• Subjects: Advanced high-strength steel; Carbide-free ultrafine bainitic steel; Dwell time; Friction stir spot welding; Tensile-shear; Advanced high-strength steel; Friction stir spot welding; Tensile-shear; Carbide-free ultrafine bainitic steel; Dwell time
• ISSN: 2238-7854
• DOI: 10.1016/j.jmrt.2024.10.092

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.