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In this study, the effects of tool rotational speed, tool traverse speed, and Zn content on the grain size and hardness of the friction-stir-welded (FSWed) Cu–Zn alloy joints were investigated. The microstructures of the joints were examined using optical microscope (OM) and scanning transmission electron microscope (STEM). Vickers hardness test was conducted to evaluate the hard-ness of the joints. In addition, the relationships between the process parameters, grain size, and hardness of the joints were established. The results show that the developed relationships predict the grain size and hardness of the joints accurately. The Zn content of the alloys is the most effective parameter on the grain size and hardness, where the tool traverse speed has the minimum effect. The rela-tionship between the hardness and grain size of the joints has a deviation from the Hall–Petch equation due to for-mation of high dislocation density inside the grains. At higher Zn amounts, the dislocation tangles with high den-sity form instead of dislocation cells, and hence, lower conformity with the Hall–Petch relationship is observed.