A hybrid shoulder to achieve a significant improvement in tensile strength and fatigue performance of friction stir welded joints for Al–Mg–Si alloy

In this paper, a hybrid shoulder friction stir welding (HS-FSW) process was developed combining the advantages of rotational shoulder friction stir welding (RS-FSW) and non-rotational shoulder friction stir welding (NRS-FSW). Both static and fatigue performance of the HS-FSW Al–Mg–Si alloy joints were examined and compared with those of the RS-FSW and NRS-FSW joints. The ultimate tensile strengths of the joints welded by HS-FSW are increased by 17.3 % and 6.7 %, compared with those of RS and NRS FSW, respectively. Besides, fatigue strengths of HS-FSW joints grow by respective 25.4 % and 1.8 %. For a better understanding of the significant improvements, the fatigue crack propagation (FCP) test and residual stress measurement were carried out. FCP test indicates that the advancing side (AS) is the most sensitive zone for FCP. The residual stress measurements suggest that the HS-FSW joint has the lowest distribution amplitude of residual stress compared with that of the RS-FSW and NRS-FSW joints. Based on above evolution, an assessment is firstly conducted, which reveals that the axial force and the lower heat input to mitigate the amplitude and the peak value of tensile stress during welding, jointly contribute to the superior imprisonment of the tensile strength and fatigue strength of the HS-FSW joints.