Inhomogeneous microstructure and properties along the thickness of stir zone in friction stir welded SAF 2507 super duplex stainless steel joint

• DOI: 10.1016/j.jmapro.2021.11.043
• Publication Date: 2021-11-29T22:02:54Z
• Authors: Fujun Cao; Guoqiang Huang; Wei Guan; Wentao Hou; Ruiyang Ni; Yifu Shen; Qijun Liu
• Keywords: Dynamic Recrystallization; Friction Stir Welding; Ductility (Earth science); Misorientation

Abstract In the present study, the microstructural inhomogeneity along the thickness of SZ in the SAF2507 SDSS FSW joint and its effect on the mechanical properties and corrosion resistance were investigated. The results suggested that a significant temperature gradient was existed along the thickness of SZ and the biggest peak temperature difference between the top and bottom reached up to 446.9 °C at 700 rpm. Whatever the rotational speed, the unexpected phase transformation of ferrite to austenite were both generated in the top SZ with thickness of 20 μm and 38 μm at 350 and 700 rpm, respectively, which confirmed the highest corrosion resistance on the top while significantly deteriorated the property of resistance to stress corrosion cracking. The inhomogeneous grain size and grain misorientation distributions of the constituent phases in the SZ along the thickness direction were synergistically determined by the dynamic recrystallization and subsequent grain growth. The grain size distribution combined with phase transformation contributed to the variation of microhardness in the vertical SZ. The longitudinal tensile test results showed that a good combination of strength and ductility with higher YS and UTS of 596 and 830 MPa respectively as well as slightly lower El of 31.5% comparable to that of the BM, was obtained for the longitudinal tensile specimen derived from the middle part of whole joint. Such results can be attributed to uniform, balanced and refined microstructure with reasonable grain size at the middle of SZ, eventually resulting in the corresponding transverse tensile sample fractured at BM.