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

Kabirmohammadi, M ; Sharif University of Technology | 2024

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  1. Type of Document: Article
  2. DOI: 10.1016/j.jmrt.2024.10.092
  3. Publisher: 2024
  4. Abstract:
  5. 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. © 2024
  6. Keywords:
  7. Bainite ; Crack propagation ; Friction stir welding ; Heat affected zone ; Microstructural evolution ; Advanced high strength steel ; Bainitic steel ; Carbide-free ultrafine bainitic steel ; Dwell time ; Friction stir spot welding ; Mechanical performance ; Micro-structural ; Tensile shears ; Ultra-fines ; Ultrafine ; Spot welding
  8. Source: Journal of Materials Research and Technology ; Volume 33 , 2024 , Pages 4033-4046 ; 22387854 (ISSN)
  9. URL: https://www.sciencedirect.com/science/article/pii/S2238785424023627