Effect of welding heat input and post-welding natural aging on hardness of stir zone for friction stir-welded 2024-T3 aluminum alloy thin-sheet

The hardness of stir zone (SZ) at as-weld state and post-welding natural aging (PWNA) state under different heat input conditions were investigated for friction stir-welded joints of 2024 aluminum alloy thin-sheets. Results show that welding heat input has obvious effect on the hardness of the SZ. Under the high welding heat input condition, the hardness of the SZ is lower than that of the base metal (BM). However, under low welding heat input condition, the hardness of the SZ exhibits strong dependency on the rotational speed of the stirring tool. With increasing the rotational speed of the stirring tool, the hardness of the SZ increases. The average hardness in the SZ of the joint welded at 1500 rpm and 1000 mm/min achieves the BM level. In comparison, the effect of the PWNA on the improvement of the hardness in the SZ is limited. The variation in the strengthening-phase particles plays a more important role than does grain size in the SZ for the improvement of hardness.