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<p>In this paper, friction stir spot welding (FSSW) is employed to join ultra-thin SS400 steel (0.8&#xa0;mm thick) and 6061-t6 aluminum (Al) alloy (3&#xa0;mm thick). The correlation between the evolution of intermetallic compounds (IMCs) and the tensile-shear performance of FSSW joints is systematically investigated by varying the tool rotational speeds. The FSSW joint cross section is divided into the pin influence zone (PIZ) and shoulder influence zone (SIZ). Microstructural characterizations reveal that phase-specific IMC layers form at the Al/steel interface in the PIZ and SIZ. The PIZ at 500&#xa0;rpm is dominated by possible brittle Al-rich Fe₂Al₅, while possible Fe-rich phases (FeAl) become predominant at 700 and 900&#xa0;rpm due to increased heat input. Tensile-shear tests show that the FSSW joint strength first increases and then decreases with tool rotational speed, reaching a maximum of 4986 ± 85 N at 700&#xa0;rpm. Correspondingly, the failure mode transforms from interfacial failure (500&#xa0;rpm) to button pullout failure (700 and 900&#xa0;rpm). The optimal performance of the FSSW joint at 700&#xa0;rpm is attributed to the synergistic effect of a robust Fe-rich IMC interface and moderate softening of the AA6061-t6 in the PIZ. This study clarifies the IMC evolution mechanism of ultra-thin steel-Al FSSW joints, providing a theoretical basis for parameter optimization in automotive lightweighting.</p>

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Interfacial Evolution and Fracture Mode Analysis of Dissimilar SS400 Steel/6061-t6 Aluminum Alloys via Friction Stir Spot Welding

  • 期刊论文

  • 发表日期：

    2026年7月18日

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  • Yuanbo Chu

摘要

In this paper, friction stir spot welding (FSSW) is employed to join ultra-thin SS400 steel (0.8 mm thick) and 6061-t6 aluminum (Al) alloy (3 mm thick). The correlation between the evolution of intermetallic compounds (IMCs) and the tensile-shear performance of FSSW joints is systematically investigated by varying the tool rotational speeds. The FSSW joint cross section is divided into the pin influence zone (PIZ) and shoulder influence zone (SIZ). Microstructural characterizations reveal that phase-specific IMC layers form at the Al/steel interface in the PIZ and SIZ. The PIZ at 500 rpm is dominated by possible brittle Al-rich Fe₂Al₅, while possible Fe-rich phases (FeAl) become predominant at 700 and 900 rpm due to increased heat input. Tensile-shear tests show that the FSSW joint strength first increases and then decreases with tool rotational speed, reaching a maximum of 4986 ± 85 N at 700 rpm. Correspondingly, the failure mode transforms from interfacial failure (500 rpm) to button pullout failure (700 and 900 rpm). The optimal performance of the FSSW joint at 700 rpm is attributed to the synergistic effect of a robust Fe-rich IMC interface and moderate softening of the AA6061-t6 in the PIZ. This study clarifies the IMC evolution mechanism of ultra-thin steel-Al FSSW joints, providing a theoretical basis for parameter optimization in automotive lightweighting.