Inhomogeneous Gradient Microstructure and Mechanical Properties of Thick Copper Plate via Friction Stir Welding

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 mu m. 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.