Enhancing mechanical properties of friction stir welded 2219Al-T6 joints at high welding speed through water cooling and post-welding artificial ageing

The effect of water cooling and post-welding artificial ageing on the microstructure and mechanical properties of 5.6-mm thick friction stir welded (FSW) 2219Al-T6 joints was subjected to detailed investigations. Sound FSW joints could be produced at the investigated welding speeds of 100-800 mm/min under both water-cooling and air-cooling conditions. The FSW thermal cycle resulted in a low hardness zone (LHZ) on both the retreating side and the advancing side. Water cooling did not exert an obvious effect on the hardness value and location of the LHZs but slightly improved the tensile strength of the FSW 2219Al-T6 joints at welding speeds of 100 and 400 mm/min. After post-welding artificial ageing, the hardness of LHZs and tensile strength of FSW 2219Al-T6 joints were substantially enhanced for all welding speeds and the strength increment increased with increases in the welding speed from 100 to 800 mm/min, attributable to increased precipitation of Al2Cu phases in both the LHZs and the nugget zone. A combination of high welding speed and post-welding artificial ageing is proven to be the optimal path to improving the mechanical properties of FSW 2219Al-T6 joints, with a maximum joint efficiency of 91% obtained.