Correlation between microstructure and microhardness in a friction stir welded 2024 aluminium alloy

A 2024-T351 aluminium alloy has been friction stir welded and the microstructures investigated. An inner HAZ hardness minimum was a result of an overaged S phase, whereas an outer minimum was believed to be due to precipitate dissolution. An interjacent HAZ maximum was attributable to the presence of very fine S phase precipitates. The nugget zone contained ∼4 μm grains and complex dislocation structures.