Abstract
In this work, friction stir processing of pure lead metal was performed with various processing parameters and with the varied environment (air and underwater) to investigate the effect of the processing on the microstructure, mechanical and thermal properties. The results showed that the underwater processed samples had a more refined grain structure than those processed in air. The least grain size of 5 μm was seen in the sample processed underwater with a rotational speed of 900 rpm (S09) which was the optimum condition. The optimum condition for samples processed in the air was the 650 rpm rotational speed (S03) which revealed the grain size of 12 μm. The tensile strength of the sample S09 increased by 141.94 % from the base metal value while it increased by 77.33 % for the sample S03. The thermal conductivity of the sample S09 decreased by 59.92 % from the base metal value while it decreased by 39.48 % for the sample S03.

Key words
underwater friction stir processing, lead, microstructure, tensile strength, thermal conductivity, Zener-Hollomon parameter

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