Abstract
The 2024 aluminium alloy sheets in the T4 temper were welded successfully by the friction stir welding techniques. The AlCuMg (U-phase) of metastable precipitates was found to exist in the thermo-mechanically affected zone using TEM analysis. Moreover, the (U-phase is likely to induce the formation of two distinct microhardness minima in the stir zone of FSW weld. The AlCu₃ and Cu₂Mg particles were precipitated on the grain boundary of the thermo-mechanically affected zone and in the weld nugget region, respectively.

Key words
aluminium alloys, friction stir welding, microstructure, transmission electron microscopy

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