Water Cooling Effects on the Microstructural Evolution and Mechanical Properties of Friction-Stir-Processed Al-6061 Alloy
Water cooling was applied during friction stir processing (FSP) of an Al-6061 alloy, and it was found that the strengthening efficiency induced by water cooling was varied, which was dependent on the initial base metal temper. As to the natural-aged Al-6061 alloy (6061-NA), the stir zone (SZ) was strengthened significantly by water cooling. Compared with the air-cooled FSP sample, the relatively high strength of the water-cooled FSP sample mainly arose from grain refinement, increased density of dislocations and large amounts of fine re-precipitated Guinier-Preston zones in the SZ. On the contrary, as to the annealed Al-6061 alloy (6061-O), the difference in the microstructural evolution between the FSP samples with and without water cooling was nonsignificant, where only finer grain morphology was observed in the water-cooled SZ, and thus the strengthening effects caused by water cooling became less.