Micromechanical aspects of fatigue in a MIG welded aluminium airframe alloy: Part 1. Microstructural characterization

Fusion welding of high strength aluminium alloys is being widely considered within the commercial aircraft industry as an alternative to established mechanical fastening methods. The objective of the present article is to characterise the microstructural features of metal inert gas (MIG) welded plate of the conventional damage tolerant alloy, 2024-T351. Micromechanical aspects of the fatigue performance of this welded material are then addressed in a companion article. The general microstructure of the MIG weld was studied using optical microscopy and scanning electron microscopy, in association with electron back-scattered diffraction mapping. Columnar dendritic structures at the edge of the fusion zone are seen, with fairly uniform equiaxed dendritic grain structure dominating in the weld centre. Local microstructural conditions of the different elements of the weld have been assessed via micro-hardness and differential scanning calorimetry, identifying the balance between aging, overaging, re-solutionising and re-precipitation occurring across the weld region. Residual stress measurements by laboratory X-ray and synchrotron X-ray diffraction are also discussed, showing stress relaxation and redistribution occurring under loads representative of smooth specimen fatigue testing. The fusion zone is seen to present a tensile residual stress field, with peak longitudinal stress occurring towards the interface with the fusion zone.