Double-sided friction stir spot welding of ultra-high strength C-Mn-Si martensitic steel by adjustable probes

• Published in: Journal of materials processing technology 2022-02, Vol.300, p.117422, Article 117422
• Main Authors: Wang, Xiaopei, Morisada, Yoshiaki, Ushioda, Kohsaku, Fujii, Hidetoshi
• Format: Article
• Language: English
• Subjects: Carbon manganese steels; Cooling rate; Dispersion; Friction stir spot welding; Friction stir welding; Grain boundary migration; High strength steels; Interface structure; Martensite; Martensitic stainless steels; Martensitic steel; Mechanical properties; Microstructure; Quenching and tempering; Spot welding; Welded joints
• ISSN: 0924-0136; 1873-4774
• DOI: 10.1016/j.jmatprotec.2021.117422

•High-quality joints of ultra-high strength C-Si-Mn martensitic steel were obtained.•Severe material flow vertical to the welding interface favored the welding interface strengthening.•A stable plug failure was obtained owing to the formation of high-quality welding interface.•Soundly welded high strength interface can be obtained by higher welding temperature and introducing strong material flow. High-quality joints of ultra-high strength C-Si-Mn quenched and tempered martensitic steel were successfully fabricated by double-sided friction stir spot welding (FSSW) with adjustable probes. We found that the mechanical properties of the welded joints increased with the rotation speed, due to the increase of hard phase and strengthened welding interface. In the surroundings of the shoulder/probe interface, a large amount of martensite was formed resulting from the higher austenitizing temperature and the higher cooling rate. In addition, severe material flow vertical to the welding interface and strong relative slip along the interface conspicuously favored the fragmentation and dispersion of the oxides, and the drive force for grain boundary migration introduced by severe material flow further promoted the dispersion of the oxides. Consequently, a high-quality welding interface was fabricated, and a high-strength welded joint with a stable plug failure was obtained. Furthermore, we pointed out that soundly welded high strength interface of ultra-high strength steel can be obtained by higher welding temperature and introducing strong material flow vertical to the interface.