The Effect of Initial Temper Condition and Tool Rotation Speed on the Corrosion Susceptibility of AA6061 Friction Stir Welded Joints

Potentiodynamic polarization testing in neutral NaCl solution and immersion testing in acidified NaCl solution were applied to investigate the corrosion behavior of friction stir welded (FSW) joints of aluminum alloy AA6061. The joints were produced at welding rotation speeds of 560, 900 and 1400 rpm from plates having different initial temper conditions. After a solution heat treatment process, the alloy plates were air cooled (AC), or water quenched (WQ) or water quenched with subsequent peak aging (PA). The results showed that the rotation speed controls the location of the most corrosion-susceptible zone whereas the initial temper condition determines the extent of corrosion attack. For the high welding rotation speeds, the most corrosion-susceptible zone of the joints is the heat affected zone (HAZ) while the main corrosion attack is found in the stir zone (SZ) for the slower rotation speed. The PA joint exhibited the highest corrosion resistance compared to the AC or WQ joints. The corrosion attack in the weld region occurred intergranularly with some pitting in the HAZ regions of the high rotation speed welds and in the SZ of the slow rotation speed welds. The results are discussed in terms of changes in the precipitation state in the different weld zones associated with the peak temperatures achieved across the weld joint due to the friction stir welding process.