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.