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 structure [2 ]; (2) High strength alloys [3 ,4 ]; (3) Material joining, e.g. self-piercing riveting (SPR) [5 ], electric resistance welding (ERW) [6 ] and friction stir welding (FSW) [7 ].
FSW has achieved outstanding welding properties, but the key-holes within the joint will bring in a weak bonding interface [8 ] and poor mechanical stability [9 ]. To overcome the above challenges, refill friction stir spot welding (RFSSW) was proposed and developed [10 ], which works on similar Al-Al and dissimilar Al-steel metallic joints [7 ], as well as metal-polymer composites [11 ]. However, the fatigue behavior of the similar and dissimilar material joints fabricated by RFSSW are seldom examined. 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.