Micrographs showing different regions of the weld for AA 7075 are shown in Fig. 2. The microstructure of the welds consisted of a nugget zone (NZ), a thermo-mechanically affected zone (TMAZ) and a heat affected zone (HAZ). The weld nugget is wider at the top of the weld due to the upper surface contact with the tool shoulder. The grain structure within the nugget is fine and equiaxed and the grain size (2–7 μm) is significantly smaller than in the parent material due to the higher temperature and extensive plastic deformation. The transition region between the HAZ and the nugget was much smaller on the advancing side than on the retreating side. The grain structure within the TMAZ is elongated and deformed in an upward flowing patter around the NZ due to the mechanical action from the welding tool. The HAZ is mechanically unaffected by the welding tool, and retains the same grain structure as the base material (BM). Generally, the microstructure had significant effect on mechanical properties for metallic materials, so inhomogeneous fatigue resistance across the welds can be inferred from the microstructural changes in various zones.