The friction stir welding process has been used to join 13mm gauge 2024-T351 aluminium alloy plates together. A detailed microstructural study of the resulting weld was carried out using Differential Scanning Calorimetry (DSC), hardness testing, Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD). DSC was used to explain the hardness results at a number of regions across the weld in terms of co-cluster dissolution and reformation and S phase formation, coarsening and dissolution. The "onion rings" structure found in the nugget weld was shown to be the result of a combination of the slight grain size variations and a change in nature and size of the particles present (i.e. intragranular versus intergranular). The variation in corrosion properties and hardness of the rings is discussed in terms of the local microstructure and quench sensitivities.

Abstract. Fatigue failure characteristics of friction stir welds in 13mm gauge 2024-T351 plate have been assessed. Failure occurred from either the weld region (nugget/flow arm) or from the material immediately surrounding the weld. Fatigue failure from the surrounding material was essentially conventional, initiating from large S-phase intermetallic particles and growing in a macroscopic mode I manner. Corresponding fatigue lives were seen to be comparable to parent plate and results previously reported for similar welds in thinner plate. Failure over the weld region was identified with discontinuities in the macroscopic flow pattern of the weld flow arm. Subsequent crack growth showed pronounced macroscopic crack deflection around the 'onion ring' structure of the weld nugget. The bands making up the onion rings were identified with variations in local hardness levels, consistent with a mechanical contribution to the crack deflection process.