The Influence of Thermal Cycle on the Grain Structure of Friction-Stir-Welded Pure Copper Joint

2 mm thick pure copper plates were butt welded by friction-stir welding, and defect-free welded joints were obtained. The effect of thermal cycle on the grain structure of the welded joints was studied by optical microscopy and electron backscatter diffraction. The obtained results showed that with the increasing of the tool rotation rate, the strain rate increased, and the initial recrystallized grain size which caused by dynamic recrystallization decreased. Owing to the effects of the increased peak temperature and duration of high temperature, the tendency of grain growth increased, and the average grain size of the welded joint increased. Under the relative high heat input condition, the grains with Goss-type {110}<001> texture grew rapidly, resulting in the shear texture changed to recrystallized texture. The change of thermal cycle did not significantly affect the grain structure and textural type of the welded joint, but it significantly affected the final grain size and the proportion of high-angle grain boundaries. This microstructural stability was attributed to the fact that the grain refinement was dominated by continuous DRX throughout a wide range of heat input. The model proposed in this study can predict that an ultrafine grain structure can be prepared in the FSW pure copper welded joint by simultaneously increasing the tool rotation rate and the cooling rate.