Abstract:Underwater friction stir processing was performed on commercially pure copper with a purity of 99.8% and a copper-zinc alloy (brass). The tool was made of tungsten carbide in the threaded cone form. Friction stir processing was performed at a tool rotational speed of 1800 r/min and a tool transverse speed of 4 mm/min while the samples were immersed in a water tank with a water circulation system. In order to evaluate the effect of the number of process passes on the microstructure and mechanical properties of the samples, this process was continued for up to 6 passes. Microscopic studies using light microscopy on commercially pure copper samples show significant decrease in grain size. Likewise, the hardness of the cross-sectional area shows an increase more than the base metal. The X-ray diffraction pattern of the underwater friction stir processed samples in comparison to that of the base metal exhibits shorter and wider peaks, while the background of the pattern is increased. The sum of these factors represents the formation of an amorphous/ultrafine grained structure. Also, the wear behavior of the samples was investigated by means of pin-on-disk method and the results show that the friction coefficient of processed samples is decreased compared to that of the base metal. The results of wear and hardness tests show that the underwater friction stir processing can significantly improve the wear resistance and hardness of commercially pure copper and brass.

Key words: copper alloy; friction stir processing; underwater; wear; microstructure