1. Introduction
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 alloys3 ,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.