Friction stir welding (FSW) was performed at different tool rotation speeds of 1250 and 1600 rev min−1 on both annealed, and severely plastic deformed copper sheets via constrained groove pressing (CGP). Also, the effects of water cooling during FSW on weld properties of CGP-treated samples were investigated. It is found that the heat-affected zone (HAZ) has a lower hardness value than the corresponding base metal due to the large grain growth. While the stir zone (SZ) shows a finer grain size with a higher hardness. Moreover, the results proved that in-process water cooling can effectively reduce grain growth in HAZ of CGPed samples and improve the weld strength by increasing the minimum hardness to about SZ hardness of the annealed sample.

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