Lightweight is one of the most important directions in the global development of new energy mobility ¹ . Several concepts were developed as: (1) Hybrid sandwich metal-plastic structure² ; (2) High strength/weight ratio alloys^{3 ,4} ; (3) Material joining, e.g. self-piercing riveting (SPR)⁵ , electric resistance welding (ERW)⁶ and friction stir welding (FSW)^{7 ,8} .

FSW has achieved outstanding welding properties, but the key-holes within the joint will bring in a weak bonding interface⁹ and poor mechanical stability¹⁰ . To overcome the above challenges, refill friction stir spot welding (RFSSW) was proposed and developed¹¹ , which works on similar Al-Al and dissimilar Al-steel metallic joints⁷ , as well as metal-polymer composites¹² . Owing to the welded metallic structure often suffers vibration and other damages during long-term service, the fatigue properties and failure mode of RFSSW joint needs to be careful examination before real application¹³ . Therefore, the fatigue testing and failure analysis shall be carried out. As the physical properties between Al and steel are different, the joint interface shall have different effects on the fatigue behavior. The present research shall accelerate the design of similar and dissimilar materials joining under RFSSW process.