The mechanical and fatigue properties of T-shaped samples made out of Al–Si–Mg aluminium plates assembled using metal inert gas welding were investigated. Samples with standard quality welds were fatigue tested until failure and analysed using the plastic hinge model. The importance of welding residual stresses on the fatigue life was shown. Samples with poor quality welds were also fatigue tested and compared with standard samples. The results showed that poor quality welds tremendously reduce the fatigue life of assemblies.