Residual stress fields may be rather frequently observed in several mechanical and structural parts, usually as a result of the manufacturing and joining techniques adopted. Their effects on material performances can be quite different, going from highly detrimental to beneficial, according to their distribution and to the acting material damage mechanisms (fatigue fracture, brittle or ductile failure, corrosion,...). Residual stresses are produced in welded structures by thermal expansion, plastic deformation and shrinkage during cooling. The amount of constraints determines the level of residual stress. Friction Stir Welding (FSW) is a quite new joining technique particularly for aluminium alloys difficult to fusion weld. Since conventional FSW showed its limits, Bobbin Tool technology has been developed. These both types of welds produce low-distortion of high quality (even no post-weld distortion) and relevant residual stresses. The residual stresses of the aerospace material 6056-T78 aluminium alloy Friction Stir Welded sheets have been analysed on various tempers in accordance with these two different technologies. The effects have been investigated parallel and orthogonal to the weld direction. By means of local or global heat treatments, the residual stress state can be generally reduced or changed from tensile to compressive stresses on surface; in that way, the different heat aging conditions T78 as-welded and post-welded are checked in this study. This paper presents the method used, the measurements of this fatigue damage and their consequences on the fatigue life performance of structural materials.