| Summary: | Underwater friction stir welding of AA6061-T6 was carried out in order to improve mechanical properties of fabricated joints in butt mode configuration. Full factorial experimental design was used to design the required experiments and finding effect of water cooling on various combinations of tool rotation speed and travel speed. It was found that by applying water cooling condition, the tensile strength and hardness of the joints can be improved about 16% and 12.5% respectively. Also, research findings showed that the elongation is deteriorated by applying cooling condition and reaches from average values of 10.46%–6.98%. From microstructure analysis, it was found that grain refining and prevention of heat affected zone from softening are main mechanisms for improving the welding strength and hardness. Also, residual stress distribution showed that applying watercooling to FSW process causes increase of magnitude and depth of compressive residual stress that significantly reduces the deflection of the sample after unclamping. Keywords: Friction stir welding, AA6061-T6, Watercooling, Mechanical properties, Microstructure, Residual stress
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