Failure of friction stir spot welds in carbide-free bainitic steel: effects of physical and metallurgical weld attributes

Friction stir spot welding (FSSW) is a promising joining method for advanced high-strength steels. Understanding the factors influencing the failure behaviour of FSSW joints is crucial for using this method in the automotive industry. This study addresses the role of rotational speed on the microstructure and hardness across the weld, as well as on the geometrical features and failure behaviour of ultrafine carbide-free bainitic steel FSSW joints. Increasing the rotational speed from 600 to 2000 rpm is identified as effective in eliminating the hook feature. A small effective thickness (320 μm), accompanied by a sharply upward hook, renders the weld susceptible to partial pullout failure at 600 rpm, beyond which interfacial failure emerges as the prevailing failure mode. A rotational speed of 1000 rpm is sufficient to develop an adequate bonding area with fine martensite, resulting in the maximum failure energy.