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.

在本研究中，研究了 SAF2507 SDSS FSW 接头中沿 SZ 厚度的微观结构不均匀性及其对机械性能和耐腐蚀性的影响。结果表明，沿 SZ 厚度存在明显的温度梯度，在 700 rpm 时，顶部和底部之间的最大峰值温差达到 446.9 °C。无论转速如何，在 350 和 700 rpm 下，厚度分别为 20 μm 和 38 μm 的顶部 SZ 均发生了铁素体到奥氏体的意外相变，这证实了顶部的最高耐腐蚀性，同时显着降低了性能抗应力腐蚀开裂的能力。SZ 中各组成相沿厚度方向的不均匀晶粒尺寸和晶粒取向差由动态再结晶和随后的晶粒生长协同决定。晶粒尺寸分布与相变相结合导致了垂直 SZ 显微硬度的变化。纵向拉伸试验结果表明，对于源自整个关节的中间部分。这样的结果可以归因于均匀、平衡和细化的微观结构，在 SZ 中部具有合理的晶粒尺寸，