Inhomogeneous microstructural gradient of the friction stir welding (FSW) joint in T2 thick copper plates was studied in the present work. Microstructures along the thickness direction of the nugget zone (NZ) were systematically characterized via using electron back-scattering diffraction (EBSD) method. The corresponding mechanical properties of different layers in the NZ, involving tensile performance and microhardness, were examined as well. Through our characterization results of each layer in the NZ, it is found that a significant gradient microstructure is generated due to the temperature and strain gradient produced by FSW process. Refined equiaxed grains dominate the NZ’s lower areas, while coarse grains with a mass of twins occupy the upper regions. Along the thickness direction from the top to bottom, the mean grain size of the NZ gradually declines from 15.56 ± 8.79 to 5.74 ± 3.06 μm. Such refined microstructure in the lower layer of the NZ leads to the enhancement of mechanical properties of the specimens due to the Hall–Petch relationship. Particularly, the strength and hardness of the lowest layer are found to be comparable with the cold-rolled base metals. The result further uncovers the underlying mechanism about how the microstructural gradient forms on FSW and how it affects the mechanical performance of the weld joint. The work also inspires that, through controlling the heat input of the present FSW method, the gradient microstructure of NZ can be rationally adjusted to improve the welding quality.

在本工作中，研究了T2厚铜板中搅拌摩擦焊（FSW）接头的不均匀组织梯度。利用电子反向散射衍射（EBSD）系统地表征了沿熔核区（NZ）厚度方向的微观组织。还检查了NZ不同层的相应机械性能，包括拉伸性能和显微硬度。通过我们对NZ中每一层的表征结果，发现由于FSW过程产生的温度和应变梯度，产生了显着的梯度微观结构。细磨的等轴晶粒在NZ的下部区域占主导地位，而带有双胞胎质量的粗晶粒占据了上部区域。沿着厚度方向从上到下， μ微米。由于Hall-Petch关系，在NZ下层的这种精细的微观结构导致样品的机械性能增强。特别地，发现最低层的强度和硬度与冷轧贱金属相当。该结果进一步揭示了有关FSW上的微结构梯度如何形成以及如何影响焊接接头机械性能的潜在机理。这项工作还鼓舞了通过控制当前FSW方法的热量输入，可以合理地调整NZ的梯度微观结构，以提高焊接质量。