Microstructural evolution, strength-ductility synergy, and corrosion behavior of the UNS N08825 Ni-based superalloy/UNS S32750 super duplex stainless steel friction stir welds

Fusion welding of Ni-based superalloys (NBSs) and duplex stainless steels is challenging due to the difficulty of achieving optimal performance, primarily caused by the formation of an unbalanced phase ratio and detrimental phases in the weld. This research elucidates the microstructural evolution in various regions of the friction stir weldment (FSW) of UNS N08825 NBS and UNS S32750 super duplex stainless steel (SDSS) through an electron backscatter diffraction study. Additionally, the tensile behavior, strengthening mechanisms, and corrosion properties were examined. The results reveal that the activation of distinct recrystallization mechanisms in the stir zones (SZs) led to the formation of fine-grained austenite in the NBS SZ and ultrafine-grained austenite and ferrite in the SDSS SZ. Alternating layers of NBS and SDSS were also observed within the weld zone. This heterostructure in the weld zone led to an enhancement in the work hardening, strength, and particularly elongation (54%) compared to the SDSS (29%) and NBS (43%) base metals. The primary strengthening mechanism was identified as solid solution strengthening on the SDSS side (497 MPa) and grain boundary strengthening on the NBS side (190-266 MPa) and in the weld zone (262 MPa). Moreover, the weld zone exhibited superior oxide film resistance and enhanced pitting corrosion resistance relative to the NBS base metal. Overall, this research demonstrates that FSW is an effective technique for maintaining the phase balance in the SDSS SZ while minimizing the formation of detrimental phases and improving the microstructure and properties of the weldment compared to the base metals.