Joining relatively thick stringer sections using the Friction Stir Welding technique is beneficial due to the potential strength gain and weight saving over fusion welding techniques.  Optimised welding parameters have been used to produce 20mm Friction Stir Welds which have subsequently undergone microstructural analysis using SEM, TEM and SAXS techniques.  Friction Stir Welding has been carried out within the AA7449 ageing sequence, the alloy initially being in a temper for age forming prior to welding and subsequently (post weld) overaged to a T7 in service condition.  Each of the processing steps including the parent material, post welding and after post weld heat treatment have undergone microstructural characterisation.  This provides a detailed description of the effect of the processing on the microstructure and relates this to the strength of the joint.  Quantification has been carried out in terms of the welding zones, nugget, TMAZ, HAZ and also the parent (unaffected) material.  Hardness maps have been produced for each of the weld conditions and are linked with maps of particle radius relating strength to the microstructure throughout the weld.  A simple hardness model has been developed and linked to an existing thermal model, which together provide a reasonable prediction of hardness across the weld zone.  In contrast to most sheet applications the thermal histories present in thick plate Friction Stir Welds have been found to promote extensive strengthening phase precipitation in the nugget region but conversely reducing post weld natural ageing.  Subsequent heat treatment to the T7 condition reduces the strength of nugget zone and parent material but has little effect on the HAZ minimum and location.