Evaluation of the effect of underwater friction stir welding on microstructure and tribological properties of copper and its alloy

Amorphous/ultrafine grained production method using friction stir processing under water is a novel method which uses a rotating tool to knead the base metal, and immediately after that, it is cooled by water to alter the crystalline structure in the next crystallization and change it into amorphous/ultrafine grained structure. In the present study, the applied metal was copper with purity of 99.8 percent. The properties of crystal structure before and after the test were examined. Examinations using optic microscope (OM) showed that the friction stir processing with higher rounds would decrease the tunnel defects and heterogeneity and cavities, and grain size of sample was reduced. Moreover, hardness test was conducted on samples in cross section and perpendicular to the process line. Results showed that making amorphous using friction stir processing can increase the sample hardness to 45 percent compared with the base metal. The structural properties of samples were examined using x-ray diffusion. Results showed that the peaks presented in diffusion pattern were decreased and their width were increased because of a severe change in plastic form and conducting the process under water which prevent the growth of grains and complete re-crystallization of microstructures. The main reason for reduction in peak intensities and increase in their height was the presence of ultrafine grained phase and extra fine granules. Moreover, the friction behavior of samples were examined using pin on disk method, and a reduction in the friction coefficient was found which was 48 percent compared with the base metal.